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Marine Biological Laboratory Library
Woods Hole, Mass.
Presented by
McGraw-Hill Book Co., Inc.
Septe 5, 1961
PS SS SS SS ES a SS eS ed
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Introduction to
Piatt iaGEOGRNAPELY
Other Books by the same Author :
Russian Waters, London: Arnold (1931)
The Isle of Auks, London: Arnold (1932)
Botany of the Canadian Eastern Arctic :
Part I, Pteridophyta and Spermatophyta, Ottawa: King’s
Printer (1940)
Part II, Thallophyta and Bryophyta (ed.), Ottawa: King’s
Printer (1947)
Part III, Vegetation and Ecology, Ottawa: King’s Printer (1948)
Arctic Unfolding, London, etc.: Hutchinson (1949)
Circumpolar Arctic Flora, Clarendon Press, Oxford (1959)
Arctic Botany (3 vols.), Clarendon Press, Oxford (in Press)
Editor of Plant Science Monographs, London : Leonard Hill; New
York: Interscience.
General Editor of World Crops Books, London: Leonard Hill; New
York: Interscience.
INTRODUCTION TO
PAIN PeGEhOGR APTLY
and Some Related Sciences
by
NICHOLAS POLUNIN
M.S. (Yale), M.A., D.Phil., D.Sc. (Oxon.)
Visiting Professor, University of Geneva; lately
Professor of Plant Ecology and Taxonomy, Head
of the Department of Botany, and Director of
the University Herbarium, etc., Faculty of Sctence,
Baghdad, Iraq. Formerly Fielding Curator and
Keeper of the University Herbaria, University
Demonstrator and Lecturer in Botany, and Senior
Research Fellow of New College, Oxford;
Macdonald Professor of Botany, McGill Univer-
sity; Research Associate, etc., of Harvard and
Yale Universities.
McGRAW-HILL BOOK COMPANY, INC.
New York ‘Toronto London
1960
© Nicholas Polunin 1960
First published 1960
Printed in Great Britain by Butler & Tanner Ltd., Frome and London
DEDICATED ‘To
THE AMERICAN
GEOGRAPHICAL SOCIETY
OF NEW YORK
who inspired and first
commissioned this book
CHAPTER
CONTENTS
LIST OF ILLUSTRATIONS
ACKNOWLEDGMENTS
J
HT.
ELL
WHAT IS PLANT GEOGRAPHY ?
Plan of the book
Geographical patterns .
Climate the master
The ideal plant
Plant sociology
The animal side . :
Some earlier works on plant geography
THE VARIOUS GROUPS OF PLANTS AND
HOW AND WHERE THEY “LIVE
Classification and nomenclature
Schizophyta
‘Thallophyta
Bryophyta .
Pteridophyta
Spermatophyta
Further consideration .
PHYSIOLOGICAL, ._REACTIONS,.. ©“ ADAPTA-
TIONS’, AND LIFE-FORMS
Physiological make-up .
Ecological limitation j ; ’ :
Structural ‘ adaptations’ of vegetative parts .
Classification by life-forms .
Further consideration .
DISPERSAL AND MIGRATION: AIDS AND
BARRIERS
Wind dispersal :
Dispersal by water and ICEL:
Dispersal by animals (apart from Man)
Dispersal by human agency.
Mechanical dispersal
Barriers :
Further consideration ;
EVOLUTIONARY DEVELOPMENT AND PAST
BISTORY:«.
Groups of fossil lower plants
Fossil Seed-plants
vil
ot ee
96
te
Vill
CHAPTER
WAR
VII.
V EEE
IX.
CONTENTS
Past ages and their plant life
Further consideration .
FOUNDATIONS OF MODERN
DISTRIBUTIONS
Some effects of relatively recent climatic changes .
Pleistocene persistence versus subsequent immigration
Continental drift, shifting Boles: land-bridges, etc..
Postglacial changes k 3
‘The genetical heritage.
Polyploids and their areas
Further consideration .
TYPES AND AREAS OF NATURA
DISTRIBUTIONS
‘ Continuous ” intercontinental ranges
Discontinuous ranges .
Relic areas .
Vicarious areas
Endemic areas
Polytopy and the incidence of areas
Intraneous, extraneous, and other elements
Major regions :
Further consideration .
MODIFICATION AND DISTRIBUTIONS OF
CROPS (AND WEEDS): .
Effects of cultivation
Naturalization and acclimatization.
Some herbaceous crops and their areas
Forestry and other woody ‘ crops’
Significance and distribution of weeds and plant diseases
Further consideration .
VITAL IMPORTANCE TO MANKIND .
Foods
Beverages and flavours
Medicinals and drugs .
Fatty oils and waxes ;
Smoking and chewing materials
Structural and sheltering materials
Industrial uses and extractives
Clothing materials and other fibres
Fuels (including fossil, etc.).
Latexes and exudates .
‘Tanning and dyeing materials
Essential oils and scents (perfumes)
Insecticides and herbicides .
Environmental and ecological
Aesthetic and ornamental
Microorganisms and miscellaneous
CHAPTER
x
XII.
XII.
XIV.
CONTENTS
Some nuisances . ;
Further consideration .
ENVIRONMENTAL FACTORS.
Climatic
Physiographic
Edaphic
Biotic. :
Further considerision :
MAIN HABITATS, SUCCESSIONS, AND
CLIMAXES
‘Terrestrial habitats
Aquatic habitats .
Microhabitats
Main successions
Main climaxes :
Further consideration .
VEGETATIONAL TYPES OF TEMPERATE
AND ADJACENT LANDS
Deciduous summer forests
Northern coniferous forests .
. Warm-temperate rain forests
- Sclerophyllous, etc., woodlands
Heathlands and grasslands
Semi-deserts and deserts
Salt-marshes
Seral communities :
Some physiographic effects .
Further consideration .
VEGETATIONAL TYPES OF POLAR LANDS
AND HIGH ALTITUDES
Arctic tundras
Arctic scrub and heathlandes
Arctic fell-fields and barrens
Seaside and other local types
Seral types.
High altitudes
Antarctic types
Further consideration .
VEGETATIONAL TYPES OF TROPICAL AND
ADJACENT LANDS
Tropical rain forests
‘Tropical forests with a seasonal rhy thm
Tropical and subtropical savannas and other grasslands .
Semi-desert scrubs é ¢ ;
‘Tropical and subtropical deserts .
Mangrove and other sea-shore vegetation
Further seral or edaphic communities .
XVI.
XVII.
eV TIT.
CONTENTS
Altitudinal effects
Further consideration .
VEGETATIONALY TYPES OF FRESH “ANS
INLAND SALINE WATERS .
Some features of the freshwater aquatic environment
Plankton ,
Cryophytic communities
Benthos :
Bogs and saline waters
Hydroseres . : :
Further consideration .
VEGETATIONAL, TYPES OF SEAS
Some features of the marine environment
Plankton
Benthic Sfbiemmens andl fee fomane
Benthos :
Aphotic bottoms .
Further consideration .
LANDSCAPES AND VEGETATION
Landscapes and component landforms .
Landforms and plant life
Interpretation and uses
Land-use classification.
Further consideration .
PLANT ADJUSTMENTS AND APPLICATIONS
‘ Adaptations’ of individuals
Man-made adjustments
Vegetational adaptation
Manipulation of vegetation .
Plant geographical study ;
Further applicational possibilities .
Additional reading
INDEX (including technical terms, which are usually defined
where first used in the text) .
PAGE
a
Q
COI AUR Dm
EST OF TEEVUSTRATIONS
Generalized land vegetation map of the world : . Facing page 1
PAGE
Some plant-like animals’. : : 2
Zoogeographical realms and floristic regions “of the world . 18-19
Various types of Bacteria . : : : : ; 26
Various Blue-green Algae (Cy anophyceae) ; f § ; é 28
Some forms of Green Algae Heer Se) : P : = 30-1
Diatoms (Bacillariophyceae) : ; ‘ ; : 33
Dinoflagellates (Dinophyceae) : ; ; : : : : 34
Some Brown Algae (Phaeophyceae) . : ; 5 : 3627
Various forms of Red Algae ee cea) ; : : , 40
Slime-moulds (Myxomycetes) : 3 : : 2 42
Some Fungi : : ; : . - 44-5
Various forms of Lichens (Lichenes) . : 4 : : ; = 48
Types of Liverworts ieee: 3 ‘ : 5 ; ; 50
Some Mosses (Musci) : f : : : . 52-3
Field Horsetail (Equisetum arvense age.) : : : 3 : 55
Types of living Lycopodineae_. : : : : =) 658,
Various types of Ferns (Filicineae) . : : . 60-1
Some examples of Gymnosperms (Gy mnospermae) : : . 64-7
Features of Angiosperms . d : . 7O-I
Features aiding water conserv ation or absorption . : : . 83-5
Features promoting aeration ; 86-7
Various adaptations for climbing, twining, scrambling, and running 88-9
Modifications for storing food or catching insects : : . Qgo-I
Diagrams illustrating some Raunkiaer life-forms . ; : 5 oy
Wind-dispersal mechanisms and disseminules ; : ; . 104-5
Water-dispersed fruits and other bodies ; ; : : = SL LO
Adaptations for dispersal by animals. . 13
Dispersal by extension of growth or by mechanical propulsion, ete. 122-3
Some primitive plant fossils : i : : ; : . 130-1
Some Psilophytineae . 5 : : : 5 ; : 5 134.
Fossil Equisetineae_. : 3 : : : : : 1) 230
Fossil Lycopodineae . ; : : ‘ ‘ t » 137
Pteridosperms (reconstructed) : SE eIiag
A reconstructed Cycadeoid and parts of livi ing and fossil Ginkgoales 140-1
Cordaitales (reconstructed) . 3 : : : P Wie
Fossil parts of Conifer and Angiosperm : 5 Ar43
Distribution of plant groups in geological time as ; far as known . 145
Restoration of a late Devonian forest of New York . : 2 PAG,
Generalized reconstruction of a Carboniferous forest . : . 148
Reconstructed ‘Triassic landscape ; 5 wee’
A reconstructed scene in Switzerland during Miocene times. 52
Known distribution of species of Liriodendron in Tertiary times and
nowadays ‘ : ; : els 7)
Past and present distributions of Redwoods. : 158
Map showing periods since when various areas of present- -day North
American land have supposedly been free from major ice- sheets, etc. 159
Maps illustrating ‘ Continental drift ’ and the begets of land-masses
as apparently affecting floristic richness . . 168-9
Map showing arctic circumpolar distribution as exemplified by
Edwards’s Eutrema. ; 5 Stig
Maps showing circumboreal and “circumaustral. distributions : . 186
x1
LSD OF TLC US TWAT TONS
Map showing pantropic distribution of the Palm family :
Map showing arctic-alpine distribution as rani by Saxifraga
PAGE
187
oppositifolia agg. . 189
Map showing range of Drooping Ladies’-tresses (North Atlantic,
etc., distribution) 190
Map showing range of Skunk- cabbage (North Pacific distribution) 190
Map showing North-South American distribution of Pitcher-plant
family 190
Map showing range of Cimicifuga ‘foetida (Europe- Asian distribution) 191
Map showing range of species of Platanus . 192
Map showing pantropical discontinuous range of the genus Buddleia
and the mainly neotropical range of the family Vochysiaceae 193
Map showing range of the genus Jovellana (South Pacific distribution) 194
Map showing range of the genus Asclepias (South Atlantic, etc.,
distribution) : 194
Map showing (Antarctic) range of the genus Nothofagus 195
Map showing (bipolar) range of the genus Empetrum 196
Map showing intracontinental discontinuous distribution in Australia
of the section Erythrorhiz sa of the genus Drosera 197
Map showing ‘ Lusitanian’ distribution of Mackay’s Heath . 197
Maps showing known localities of Low Sandwort : 198
Map showing recent and ‘ fossil’ stations of the Water- chestnut in
Scandinavia 199
‘Transatlantic vicariads 203
Map showing main vegetational- climatic regions of the world 213
Geographical distribution of world Rice production 225
World Wheat production ‘ 3 : 227
World Rye production 228
World Maize production : 5 5 ; : : 230
World Potato production ; also some Greenland vegetables . (232-3
World Flax-seed production : : ; 1-236
World Cotton production 237
World Peanut (Groundnut) production 238
World Tobacco production. 240
World distribution of annual Cane and Beet Sugar production 241
World production of Cocoa Beans 244
World Coffee production 245
Distribution of the world’s human population 2 256
Moss Campion flowering only on the south-facing sides and tops of
its domed tussocks near 80° N. in Spitsbergen . a) ZING
Annual mean, and mean temperature of the warmest month, in
different parts of the world . : . 286-7
Average annual precipitation in different parts of the world . 7 290
Effect of wind on trees. ; 292
Aspect effects in Colorado and Nepal . 295
Three soil profiles of comparable depth 299
Distribution of primary soil groups of the world. 300
Some important effects of grazing 308
Devastating results of overgrazing ; 309
Margin of tropical oligotrophic lake, with steep rocky sides and rapidly
deepening water, supporting few larger plants . 318
Lake of eutrophic type near Prout’s Neck, Maine 319
Diagram illustrating stages of hydrosere with deposition of peat 326
Sedge-meadow stage of hydrosere colonized by some hygrophytic
shrubs and trees, near Prout’s Neck, Maine ‘ ; 7 326
‘Telescoped stages of xerosere in Norwegian Lapland 327
Psammosere at Prout’s Neck, Maine, showing Marram Grass binding
sand above high-tide mark 328
Leafless condition of mixed deciduous s summer forest in nontheaeteend
United States—in winter
338
FIG.
95.
96.
97-
98.
99.
100.
IOI.
102.
103.
104.
105.
106.
107.
108.
109.
110.
IIl.
I1i2.
eee
II4.
IIS.
116.
117.
118.
iC
120.
21.
122.
TR
124.
125.
PS TeOE Snr SaivAv TONS
Mixed deciduous forest in northeastern United States—in summer
Open scrubby Birch ‘ forest ’ in Finmark, northern Norway. ;
Boreal coniferous forest surrounding lake-side bog in sheltered valley
in Troms, northern Norway
Outside the taiga in northern Ungava, Canada
‘ Lake-forest ’ of southeastern Canada, in summer
The same area of ‘ lake-forest’ but under winter conditions, with
snow covering the ground
Warm-temperate rain forest in southeastern United States
Bald-cypress swamp in southeastern United States : ;
Rocky area with patches of mixed scrub of ‘maquis’ type in the
Mediterranean island of Corsica : : ‘ :
Sierran chaparral climax, Santa Barbara, California : :
Sclerophyllous forest in ‘Australia, dominated by lofty Gum- trees .
The North American Prairie : mid- and short-grass communities.
Salt desert and salt-marsh of a warm-temperate region
Pine ‘ Krummholz’ at timber-line in the Rocky Mountains .
Tundra on Southampton Island, Hudson Bay .
Marshy tundra near the south shore of Hudson Strait, dominated by
Cotton-grasses, Sedges, and Grasses : :
Dry tundra on raised area overlooking Hudson Bay F
Mesophytic Sedge-grass tundra on the north coast of Alaska near
Point Barrow, overlooking the Arctic Ocean
Extensive area of damp ‘ hillock tundra’ on the coast of Spitsbergen
Discontinuous tundra-like tract of mixed Grasses, NorthernWood-rush,
forbs, and Polar Willow, in inland valley, West Spitsbergen, grazed
by a pair of wild Reindeer
Tangled Willow scrub in the low- arctic belt of the Northwest
‘Territories, Canada.
Patchy scrub of Glaucous Willow and Scrub Birch, up to ‘nearly
2 metres high, in southwestern Greenland
Dense low-arctic heath dominated by Arctic Blueberry i in northern-
most Quebec :
“ Snow-patch ’ darkened by Arctic Bell-heather, southern Baffin
Island . : : . ‘ : ;
Mixed middle-arctic heath with many light-coloured and other
Lichens . : ;
‘Polygons’ in northernmost Spitsbergen : : ‘
Fell-field on calcareous soil in exposed situation, northernmost
Labrador :
Lichen barrens in the ‘uplands of central Baffin Island, looking south
Monotonous tract of prairie-like fell-field in the vicinity of the
Magnetic North Pole, Prince of Wales Island, Canadian Arctic
Archipelago
Se beach bound by swarded Lyme- grass ‘ : : :
Looking down on a salt-marsh dominated by Pacific Silverweed and
Creeping Alkali-grass F , ‘ ‘ : 3 :
Luxuriant ‘ patchwork quilt’ of mixed and many-coloured Lichens
and Mosses developed near top of bird-cliff
Looking down on a luxuriant mossy mat on the manured periphery
of a wildfowl nesting-ground in Spitsbergen
Purple Saxifrage barrens on exposed ridge ov erlooking the sea in
northernmost Baffin Island
‘ Late-snow’ patch in the highlands of central Baffin Island
Looking down on the Herb-like Willow zone of the late-snow area
shown in Fig. 129. ; i 3 , :
Luxuriant lakeside marsh ‘dominated - by Water aes and ‘Tall
Cotton-grass
Fine bed of Scheuchzer’s Cotton- ~grass beside tarn in northern
Spitsbergen : : :
XIV
FIG.
Heee
134.
T25¢
136.
137.
138.
139.
140.
141.
142.
143.
144.
145.
146.
147.
148.
149.
150.
pe
LiS2:
53
154.
155-
156.
157.
158.
159.
160.
161.
162.
163.
164.
165.
166.
167.
168.
169.
LIST OF ILLUSTRATIONS
PAGE
Top of flower-slope below weathering crag in southern Baffin Island 408
Spitsbergen flower-slope dominated by Alpine Arnica 408
High in the mountains near the margin of the ice-sheet in southern
Greenland 411
Upland scrub of Dwarf Birch and silky- leafed Willows constituting
an altitudinal climax above tree-limit in northern Norway 411
High-alpine vegetation and flowering 412
Alpine puna-like formation near mountain summit in Colombia 414
Crustaceous and foliose Lichens on rocks near shore, Goudier Islet,
Antarctica : 416
Luxuriant growth of Mosses, broken chiefly by rocks bearing Lichens,
extending up snow-melt ‘oully in area cela by Penguins,
Deception Island, Antarctica ; a eS
Profile diagram of primary mixed tropical rain “forest, Moraballi
Creek, British Guiana 426
Profile diagram of climax ev ergreen forest in “Trinidad, British West
Indies 426
Tropical rain forest in the Philippine Islands 427
Another scene of tropical rain forest in the Philippines 429
Base of tree-trunk showing exaggeratedly buttressed roots in tropical
rain forest 429
An epiphytic Fern which has small humus- gathering leaves and
larger photosynthetic ones that also produce spores . 433
An epiphytic Bromeliad with a mass of fibrous roots investing the
branch of the ‘ host’ tree : % 3 488
Roots of Strangling Fig on a large tree-trunk 435
An old specimen of Strangling Fig in which the roots serve as trunks,
the original ‘ host’ having disappeared 436
Flower and buds of Rafflesia manillana, a true parasite on the roots of
a Cissus vine . 437
A tropical hemiparasitic Mistletoe, Viscum orientale, the root of
which forms a single haustorium 438
Palm-savanna in southern Florida 445
Savanna in Australia under rainfall of 25— 75 cm. “annually 446
Semi-desert ‘ bush-land’ in Australia . : 448
Arizona near-desert scene showing the giant Saguaro (or Sahuaro)
Cactus and bushy Ocotillo 5 ; : ; 451
Areas of desert in central Iraq 453
A typical Mangrove plant, Rhizophora candelaria, forming a char-
acteristic marginal ‘ mangrove’? and showing prominent prop-roots
below 455
Interior of Philippine mangrove- swamp forest at low tide 455
A Screw-pine and a subtropical estuarine salt-marsh 459
Coconut Palms along a tropical sea-coast 461
‘Two-storied montane rain forest at an altitude of 740 metres in the
Philippine Islands . 468
Epiphytes on trunk of tree near upper limit of montane rain forest i in
the Philippine Islands 469
Mossy elfin forest near summit of mountain, Fhilippine Islands 470
Vascular plants floating freely on fresh water ‘ .484-5
Diagrammatic representation of the plankton in a Wisconsin lake
during May to October . 486
Diagram indicating distribution in a European lake of a cy yanophy cean
(Glosotrichia echinulata), which is rendered buoyant by included
gas-vacuoles 487
Diagrammatic representation of a 1 ty pical lake- marginal, profile 493
Diagram of cross-section through a ‘ highmoor’ bog that has arisen
from a small lake 501
Leaves of Sacred Lotus projecting out of the water, and Pistia stratiotes
floating on the water, in the Philippine Islands 504
FIG.
170.
70s
172
17}
174.
175:
176.
GTI
178.
179.
180.
181.
182.
183.
184.
LIST On TLE UsSPRAT ONS
Diagrammatic representation of typical sea-marginal profile
Photomicrographs of marine phytoplanktonic communities
Postelsia palmaeformis .
Scene at low tide on a rocky sea- -shore of the eastern United States
A characteristic Kelp, Alaria dolichorhachis .
A giant Pacific Kelp, Macrocystis pyrifera ‘
An arctic foreshore photographed from near low -tide mark .
Old sand-dune colonized by shrubs and Pitch Pine after stabilization
by Marram Grass . ‘ F
An example of Class VII land
An example of Class VIII land . 2
Illustration of the eight land-use capability classes
Part of a maze of gullies which crosses more than an entire county
in the southern United States.
Destructive water-erosional euy in heavily overgrazed pasture in
Illinois .
The same as Fig. 182, two years later
Map showing positions of meteorological stations of the Ukraine,
with indications of their climatic analogues in the United States
575
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45
ACKNOWLEDGMENTS
To one who considers that personal friendship and generosity
in sharing the fruits of scholarship are among the very best things
of life, it gives great pleasure to acknowledge indebtedness to the
many savants who have contributed of their knowledge or store of
illustrations to the benefit of this work. ‘They must not, however,
be held responsible for any shortcomings it may have—such as,
perhaps, in some views, omission of discussion of certain con-
troversial issues which seemed best by-passed at least at the time
of writing. Notable among these are Dr. C. W. Thornthwaite’s
work on evapotranspiration and the classification of climates,
Professor Eric Hultén’s views on the history of arctic and boreal
species, and various ideas about the places of origin of plant forms
and the possibilities of their being multiple (polytopic).
The book owes its inception to the foresight of Dr. George H. T.
Kimble, who, when Director of the American Geographical Society,
was instrumental in my being invited and generously commissioned
to prepare it for a new series of ‘ readers ’ on geographical subjects.
Early on, the general plan was improved from time to time
following discussion with colleagues at Oxford, Yale, and Harvard
Universities, in its near-final form being approved by a seminar at the
last-named. ‘The plan also derived benefit from many individuals
elsewhere ; among these my former teacher and chief, the late
Professor Sir Arthur G. 'Tansley, and my former pupils, Professor
John H. Burnett and Dr. John Warren Wilson, were particularly
helpful. Yet others who made valuable suggestions, most of which
were gladly adopted, include Professor Hugh M. Raup of Harvard
University, Professor Paul B. Sears of Yale University, Professor
Joseph Ewan of Tulane University, and Drs. Raymond F. Fosberg
and Henry K. Svenson, both of Washington, D.C. The then
Directors of the two main botanical gardens of the United Kingdom,
the late Professor Sir William Wright Smith of Edinburgh and Sir
Edward J. Salisbury of Kew, also gave freely of their advice, as did
the former Director of the New York Botanical Garden and of the
Arnold Arboretum, the late Professor Elmer D. Merrill. The book,
moreover, derives much from the able (and direct) teaching of two
XVi11
XVI ACKNOWLEDGMENTS
others who are unfortunately no longer with us, namely the late
Professors George E. Nichols of Yale (in ecology) and Merritt L.
Fernald of Harvard (in taxonomy).
Colleagues or friends who have been kind enough to read and
give helpful advice about particular chapters or groups of chapters
have included Professors G. E. Hutchinson, Harold J. Lutz, Paul
B. Sears, and Mr. Albert F. Burke, all of Yale University, Pro-
fessors Kenneth V. ‘Thimann and Hugh M. Raup, and Drs. A. F.
Hill and Richard E. Schultes, all of Harvard University, Drs. W. O.
James, F.R.S., and F. H. Whitehead, both of Oxford University,
Drs. H. Hamshaw Thomas, F.R.S., and Harry Godwin, F.R.S.,
both of Cambridge University, the late Professor Sir Arthur G.
Tansley, F.R.S., of Grantchester, Cambridge, Professor Paul W.
Richards of the University College of North Wales, Bangor, Dr.
John Hutchinson, F.R.S., of the Royal Botanic Gardens, Kew, Mr.
F. 'T. Walker of the Institute of Seaweed Research, Inveresk,
Musselburgh, Scotland, Messrs. Robert Ross and W. 'T. Stearn, both
of the British Museum (Natural History), Dr. Richard S. Cowan of
the New York Botanical Garden, Professor G. W. Prescott of
Michigan State University, Professor George L. Church of Brown
University, Professor Valentine J. Chapman of Auckland University
College, New Zealand, Professor G. Einar Du Rietz, of Uppsala,
Professors Gunnar Erdtman of Stockholm and Karl H. Rechinger
of Vienna (while Visiting Professors in my department at Baghdad,
Iraq), Professor Thorvald Sorensen, of Copenhagen, Professor John
H. Burnett, now of the University of St. Andrews, Dr. Frank E.
Egler of Aton Forest, Norfolk, Conn., and Dr. Edward H. Graham,
Director of Plant Technology in the Soil Conservation Service of
the United States Department of Agriculture, Washington, D.C.
Whereas the choice of these kind mentors was naturally governed
largely by their specialist interests, to mention who ‘ passed’ what
might leave them open to being held responsible for errors of com-
mission or omission which are in fact my own. Most of the sub-
stance of this book was earlier presented in a full-year graduate
course at Yale University—a circumstance which, at the instance of
some senior participating students, has led to further constructive
comment and, surely, improvement.
In the matter of illustration, so vitally important to a work of
this kind, the greatest debt is to Ginn and Company, of Boston,
Massachusetts, and Mrs. William H. Brown, for their loan of, and
permission to use freely, so many of the fine drawings and photo-
ACKNOWLEDGMENTS XIX
graphic prints made for the late Professor William H. Brown’s The
Plant Kingdom, published in 1935. ‘This was not only a great
convenience but also a great privilege, these illustrations being often
of unsurpassed excellence. Acknowledgment is also due to the
National Museum of Canada for permission to reproduce many of
my photographs of arctic regions that are now in their possession.
Other sources of illustrations, where not contributed by myself, are
acknowledged individually.
NICHOLAS POLUNIN
Faculty of Science,
Baghdad, Iraq
Spring, 19571
1 Since this was written it has not been possible to incorporate extensively any
new ideas or to consider subsequent works, though some details of publication
have been brought up to date. The proofs have kindly been read by Professor
John H. Burnett and Dr. A. D. Q. Agnew (now of my Department in Baghdad),
while in connection with their correction warm thanks are due to my secretary,
Miss Christine Wright. Acknowledgment is also made to Dr. B. Barnes for his
valued part in the preparation of the Index.
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Generalized Land Vegetation Map of the World
Specially prepared by A. W. Kiichler (1958);
northern forest and arctic limits by Nicholas Polunin
It must not be expected that these
often physiognomic designations
necessarily correspond to the
vegetation-types described in the text.
40°
LEGEND
Broadleaf evergreen forest
Broadleaf evergreen shrubs
Scattered broadleaf evergreen shrubs
Broadleaf evergreen dwarf shrubs
Open stands of broadleaf evergreen dwarf shrubs
Broadleaf evergreen dwarf shrubs and grasses
Barren, or only sparsely vegetated (fell-field etc.)
Broadleaf deciduous forest I20"W__ Equator
Open broadleaf deciduous forest
Broadleaf deciduous shrubs
Open broadleaf deciduous shrubs
Scattered broadleaf deciduous shrubs
Broadleaf deciduous shrubs and grasses
Scattered broadleaf deciduous dwarf shrubs
Broadleaf deciduous forest with broadleaf evergreen shrubs
Broadleaf deciduous trees and grasses
Needleleaf evergreen forest
\Trepie of Capricorn
Scattered needleleaf evergreen trees
Grassland ,
Patches of grassland
Grassland with scattered broadleaf evergreen trees (Savanna)
GBsp Grassland with scattered broadleaf evergreen shrubs (Bush Savanna)
GDp Grassland with scattered broadleaf deciduous trees (Savanna)
GDsp Grassland with scattered broadleaf deciduous shrubs (Bush Savanna)
GDzp Grassland with scattered broadleaf deciduous dwarf shrubs (Bush Savanna)
GSp Grassland with scattered broadleaf deciduous and evergreen trees (Savanna)
GSsp Grassland with scattered broadleaf deciduous and evergreen shrubs (Bush Savanna)
L _Lichens and mosses; some vascular plants
Grassy etc, tundra with lichens & mosses
Broadleaf deciduous and needieleaf evergreen forest
Needleleaf deciduous forest Approximate northern limit), | /
of forest \ ¥ Tes : 3000 Miles
Broadleaf evergreen and deciduous forest 2000 3000 4000
Broadleaf late and decid hrubs Northern boundary 'of Goode’s Homolosine Equal-Area Projection Kilometres
‘oadleaf ev A ssl ircti m4 = Dp ,
2 ciduous shrul low-arctic belt (tentative) Copyright by the University of Chicago’ Pr (True distances on mid-meridians and parallels O° to 40")
Broadleaf evergreen, deciduous and needleleaf evergreen forest ?
=.-.— Northern boundary of
Broadleaf evergreen and deciduous trees and grasses middle-arctic belt (tentative)
Cartography by A.W. Gatrell
LEGEND
Broadleaf evergreen forest
Broadleaf evergreen shrubs
Scattered broadleaf evergreen shrubs
Broadleaf evergreen dwarf shrubs
Open stands of broadleaf evergreen dwarf shrubs
Broadleaf evergreen dwarf shrubs and grasses
Barren, or only sparsely vegetated (fell-field etc.)
Broadleaf deciduous forest
Open broadleaf deciduous forest
Broadleaf deciduous shrubs
Open broadleaf deciduous shrubs
Scattered broadleaf deciduous shrubs
Broadleaf deciduous shrubs and grasses
Scattered broadleaf deciduous dwarf shrubs
Broadleaf deciduous forest with broadleaf evergreen shrubs
Broadleaf deciduous trees and grasses
Needleleaf evergreen forest
Scattered needleleaf evergreen trees
Grassland
Patches of grassland
Grassland with scattered broadleaf evergreen trees (Savanna)
Grassland with scattered broadleaf evergreen shrubs (Bush Savanna)
Grassland with scattered broadleaf deciduous trees (Savanna)
Grassland with scattered broadleaf deciduous shrubs (Bush Savanna)
Grassland with scattered broadleaf deciduous dwarf shrubs (Bush Savan)
Grassland with scattered broadleaf deciduous and evergreen trees (Sav
Grassland with scattered broadleaf deciduous and evergreen shrubs
Lichens and mosses; some vascular plants
Grassy etc. tundra with lichens & mosses
Broadleaf deciduous and needleleaf evergreen forest
Needleleaf deciduous forest
Broadleaf evergreen and deciduous forest
Broadleaf evergreen and deciduous shrubs
Broadleaf evergreen, deciduous and needleleaf evergreen forest
Broadleaf evergreen and deciduous trees and grasses
(
40
-meridians and parallels 0!
CHAPTER I
Wo EAdas TS seal AN LAG EOGRAPHY?
Let us begin with a few basic definitions and follow them with
some general explanations.
Biology is the science of life, the study of living things, and it
has two main branches—botany, which deals with plants, and
zoology, which deals with animals. But whereas every one of us
must surely be clear about the differences between the typical plant
(which is static, green, and does not ingest solid food) and the
typical animal (which is motile, not green, and ingests elaborated
food), there remain many ‘border-line cases’ that are apt to be
claimed by both botanists and zoologists. Indeed, each of the
characteristics mentioned for one of these primary groups (king-
doms) of living organisms is exhibited by some members of the
other, which prevents the drawing of any hard and fast line between
all animals and all plants. And even if we add the stipulation that
the greenness of plants shall be due to chlorophyll, and that they
shall contain the carbohydrate cellulose, there remain many organisms
which possess neither feature but still in other ways seem to be
plants, and are usually treated as such.
Consequently it seems best in this case not to attempt precise
definition but rather to visualize the typical plant as a living organism
that is fixed, possessed of cellulose cell-walls, and dependent for its
main food-supply upon simple, gaseous or liquid substances (princip-
ally carbon dioxide and water). With the aid of chlorophyll in the
light, the organism builds up these substances into sugars and other
complex materials. The green plant is thus responsible for the
fundamental chain of reactions on which almost all life depends.
But this partial description excludes many organisms (such as Yeasts
and other small Fungi) which are commonly considered to be plants.
These ‘ exceptions ’ often form major groups although, as we shall
see in the next chapter, they may exhibit none of the stipulated
main plant characteristics. ‘The description also leaves behind a
basic ‘ hub’ of organisms, chiefly of microscopic types, that seem
to belong almost as much to one kingdom as to the other. Among
I
2 INTRODUCTION TORE ANG © GEOGRAPHY, [CHAP.
the more important of these are the Bacteria, which cause so many
of our worst diseases but in other instances benefit us greatly.
These and other ‘ border-line cases’, which include many of the
most primitive organisms living today, will be considered as within
our immediate sphere of interest.
Fig. 1 illustrates some cases of plant-like animals; _ several
animal-like plants will be described and illustrated in the next
Fic. 1.—Some plant-like animals. A, Hydra (= 20); B, Obelia ( about 12);
C,.a Sponge (x about 4); DD; a Coral (x 4):
chapter. Defining and classifying such nebulous groups is one of
the trials and at the same time one of the fascinations of biology.
Geography is the study and description of the differentiation
and distribution of earthly phenomena, embracing all that composes
or affects the earth’s surface—including its physical features, climates,
and products whether living or inert. A major branch is biological
geography, or biogeography, which for practical purposes is usually
subdivided along the main line of division of living things into two
kingdoms, so yielding plant geography and animal geography. Our
main subject, plant geography, also called phytogeography (from
1] WHAT IS PLANT GEOGRAPHY ? 3
the Greek word gvror, a plant), accordingly deals with the plant
cover of the world—with its composition, its local productivity, and
particularly its distribution. ‘This matter of distribution should be
tackled both on the separate basis of individual species, etc., and
collectively by dealing with their various and complex assemblages
that make up vegetation. Our object will be to describe and inter-
pret all we can of the manifestations of plant geography, paying
special attention to the differences and similarities existing between
the various floras and vegetations of the world. The continued
increase in total human population makes such a study vitally
significant, Man being dependent on plants for the very where-
withal of his existence.
The economic importance of our subject stems from the fact that
green plants alone, on any substantial scale, are able to build, from
simple raw materials and energy derived from sunlight, the complex
substances on which animals as well as the plants themselves all
depend for food. ‘This food constitutes the main source of material
used in body-building, and in it is locked the energy required for
the various processes of life. Animals, with their usually active
existence, commonly need this energy in abundance. In them, as
in plants, it is liberated by the process of respiration, which is a
kind of slow burning that takes place in living matter and gives to
Mammals and Birds their bodily heat. Green plants provide food
for us directly, when we eat them or their products, or indirectly,
when we eat animals that fed on plants or were at least ultimately
dependent upon some form of plant life, as indeed all are.
Plants also provide us with much of our clothing and housing as
well as industrial raw materials, while in the world as a whole they
largely condition our environment—forests, for example, being clearly
different to live in from grassy plains or desert oases. Indeed,
many of the major migrations of Man and other animals have been
primarily due to plant distribution. Plants constitute for mankind
the main inexhaustible source of fuel and industrial supplies and
are of fundamental importance in many different branches of
industry: in drug production, brewing, pulp and paper making ;
in lumbering, in the textile industries, in tanning, dyeing and curing ;
in the production of plastics, animal feedstuffs, scent, oil, rubber,
resin, gum, wax and fibres; and, of course, in the wider fields of
agriculture, horticulture, forestry, fish-culture, and the direct uses
of their innumerable products.
It can be seen from the contents of the earlier works listed at
4 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
the end of this chapter that authorities have differed greatly in the
past as to the bounds and, in their view, legitimate content of the
science of plant geography. In the present introductory work it
will be interpreted in a much wider sense than usual, as including
not only all geographical manifestations of plants whether single or
collective (and hence a good deal of economic and some morpho-
logical botany), but also the reasons behind these manifestations.
This will presuppose some consideration of the bases of distributions
in space and time, and consequently of the relationship to environ-
ment (ecology), of the classification and systematic arrangement of
different kinds of plants particularly through their external form
(taxonomy and systematy), of the study of their internal workings
(physiology), of their economic importance, and of other disciplines
that are not normally thought of as plant geographical—hence in
part the reference to ‘some related sciences’ in the sub-title of
this book, to certain of which in some modest degree it may also
serve as a general introduction.
The ultimate purpose of geography is the study of the differences
in the areas which make up the world. Yet when the plant popula-
tions are taken into consideration it comes as no surprise, in view
of their extreme variability, to find that one of the main results of
such a study is the realization that each area is unique. Once we
leave the ‘ systematic’ study of particular phenomena, such as the
relationship of individual kinds of plants to different areas, and
enter the ‘ regional’ sphere of correlation of the various manifesta-
tions which point to these differences in area, the problem of
organizing our study becomes almost overwhelming. With any set
of phenomena as infinitely variable as vegetation (in both time and
space, as we shall see), the areal integration desired in their geography
is rendered practicable only by ignoring variations within the smaller
unit-areas, which may then be studied together and ‘ lumped’ into
larger ones.
Plant geography attempts to integrate these floristic and vegeta-
tional features as far as possible on a world basis, and for recording
and illustration makes use of maps as one of its main tools. But the
very construction of these maps presupposes the utmost care in the
comparison of the entities whose ranges they indicate. Lack of
such care is one of the greatest limitations with which the plant
geographer is faced. Another is the still fragmentary state of Man’s
knowledge of the distribution of the vast majority of the many
hundreds of thousands of different kinds of plants inhabiting the
1| WHAT IS PLANT GEOGRAPHY? 5
world—not to mention their proper delimitation and description.
Yet another limitation is the extreme difficulty of collating such
complex and often distant ‘entities’ as vegetation-types, with all
their infinite variation and intricate intergradation. Even so, when
no more research than has already been accomplished along these
lines is brought together, we have a very impressive volume of
material from which it seems permissible to make some useful
generalizations, and on which we can build further our edifice of
plant geography.
PLAN OF THE BOOK
As plants are our chief concern, we shall, after the present intro-
ductory chapter, first describe the main groups into which the
myriad forms comprising the plant kingdom (in the wide sense) are
classified. For each group we shall give some account of how its
members live and reproduce, with mention of their habitats, dis-
tributions, and individual importance, and illustrations of examples.
Then we shall have at least some conception of what we are dealing
with, and, if previously uninitiated, have an opportunity of becoming
familiar with our tools.
Our other main concern being with geographical phenomena and
particularly with area, we shall deal next with the physiological
attributes and external features that enable particular plants to grow,
or prevent them from living, in particular circumstances. In this
third chapter we shall also touch on the subject of plant classification
by means of the various ‘ life-forms ’ which are brought about largely
by the nature of the environment. ‘This is particularly important
because the reactions of plants to the environments in which they
exist (see pp. 8-9) constitute one of the main ‘keys’ to their
geographical ranges. In the next chapter we shall consider the means
by which plants disperse themselves and migrate—with the kinds
of aids they employ and, incidentally, some of the hindrances they
meet in attaining their present-day distributions. ‘The following
chapter, our fifth, will deal with the early evolutionary development
of plants, and the sixth will be concerned particularly with those
developments in recent geological ages which have most profoundly
influenced plant distributions as we see them nowadays.
In Chapter VII we shall go on to consider examples of the main
types of distribution and consequent areas recognized today, where
possible interpreting them in the light of information contained in
6 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
the earlier chapters. ‘This consideration of ‘ natural’ distributions
will be followed by a chapter on man-made ones—both intentional
(of crops) and unintentional (of weeds, etc.). And as the crops, or
potential crops, of the various parts of the world introduce some
of the greatest problems of mankind today, the next chapter will
emphasize the economic, and basic, significance of plant life. For
the geographical ranges of plants important to Man are often largely
determined by him, on whom their very existence may depend.
In these initial chapters we will be dealing chiefly with special
kinds or systematic groups of plants and their distributions. This
is little more than a prelude to consideration of the natural group-
ings, the complex and variable assemblages of different plant com-
munities each composed of more or less numerous and diverse kinds
of plants, that make up collectively what we term vegetation. Before
actually beginning our study of vegetation we must consider the
environmental conditions (the ecological factors) which largely
control its distribution and form: such consideration will occupy
Chapter X. Some attention will also be given to physiological
make-up, which primarily determines the reaction of a plant to its
environment.
The ecological factors at a point collectively constitute the habitat,
or ‘ habitat conditions’. ‘The habitat, the place where an organism,
or commonly many organisms, live, may vary greatly from place
to place but tends to recur in at least comparable form in many
different places. Particular habitats are often relatively uniform
over considerable areas, as in the cases of salt-marshes, shallow
ponds, and sandy plains. Moreover, when a bare or disturbed area
is left alone, the vegetation inhabiting it tends to change, exhibiting
a series of vegetational types ranging from the first lowly colonists
to a relatively stable community which is ultimately the highest
the area can support, the advancing series being called a * succession ’
or ‘sere’, and its outcome the ‘climax’. Chapter XI will deal
in a general way with the main types of plant habitats, successions,
and climaxes to be distinguished.
The next five chapters will outline and illustrate the chief vegeta-
tional types of the world, starting with those to be observed in
temperate and adjacent lands as being most familiar and compre-
hensible to the greatest number of us. Following an account of
the vegetational types of polar lands and high altitudes elsewhere,
will be a chapter on tropical and adjacent lands, and thereafter one
on the plant communities of fresh and inland saline waters, wherever
r] WHAT IS PLANT GEOGRAPHY? 5
they may be located, and another (Chapter XVI) on the communities
of the oceans and seas.
Landscapes are often notable for peculiarities which concern the
vegetation more than the actual surface of the land, and in Chapter
XVII we will consider how landforms, which make up landscapes,
tend to be characterized in this way. ‘The study of vegetation,
especially of the more established communities, generally gives a
better indication of the combined action of environmental factors
than all manner of measurements. Consequently the plant geo-
graphical and ecological evidence afforded by an area can be of
the greatest practical value in interpreting local conditions and in
planning the best use of land—particularly for agriculture and
afforestation.
The concluding chapter deals with (1) some natural adaptations
and (2) man-made adjustments, both in (a) individual plants and
(b) vegetation. ‘This consideration gives us by cross-inference four
sets of topics, all of great interest and importance. Examples of
these are (1a) evolution and its mechanisms, (15) successional
change in vegetation, (2a) plant-breeding, and (2b) combating
erosion (itself usually brought on by Man’s desecration of vegeta-
tion). ‘The final paragraphs survey some of the more useful methods
of study of plant geography, and give further indication of the
values and future possibilities of the subject—both academically and
in the service of Man. Despite vast advances in recent decades,
there remain whole hosts of unsolved problems ; and, indeed, it is
to be questioned whether this last chapter can ever be brought to
a satisfactory closure. For such is biological science—an unending
frontier.
Most chapters conclude with some indication, in smaller type, of
how further pertinent information may be obtained through recom-
mended books which are cited for the purpose. Shorter contribu-
tions are ignored in this connection as being too numerous and
difficult to select, as well as usually unavailable to the layman,
although naturally much of the material presented in this book has
been drawn from such specialist ‘ papers ’.
GEOGRAPHICAL PATTERNS
The botanical aspects of what may be termed geographical or
areal patterns constitute much of plant geography. Such plant-
distributional patterns are partly based on physiological reaction to
8 INTRODUCTION LO VPLANTD GEOG RAP E Y [CHAP.
ecological factors and, consequently, to a considerable extent on
climate (see next section). An understanding of them is fundamental
to our main subject, so some consideration of them seems desirable
at this stage.
Just as the land-masses of the world, for example, make up a
definite (if seemingly unorganized) pattern on the surface of the
globe, so do other features, that are likewise definable in area, make
up their own special patterns. Such pattern-forming features
include the various factors of the environment, with which we shall
deal in Chapter X. ‘Thus, certain ranges of temperature, for
example, obtain only within certain areas, and the same is true
especially of other climatic features (see next section).
In the simplest case it might be supposed that a particular kind
of land-plant, needing land to live on, could occupy all of the water~
and ice-free land of the globe. But in actual fact quite numerous,
often interdependent and overlapping, environmental and other
factors prevent this, and no known kind of plant, however wide its
habitat tolerance, occupies more than a very small proportion of the
world’s surface. At the other extreme are the numerous species
which appear to inhabit only one limited tract of the globe or even
a single spot. Each and every species has its particular area, its
geographical distribution, whether this be small or large, and
whether continuous or broken up into a more complicated pattern.
And the pattern will be related to some particular factor or factors
of the environment, to some migrational ability the plant may
possess, and/or to evolutionary and geological or more recent
history.
These migrational tendencies, together with the historical aspects
of distribution, will be discussed in Chapters IV, V, and VI, and
it will be found that such aspects may greatly affect the areas at
present occupied by particular plants. But the factors of the
environment are apt immediately to limit and circumscribe the area
that can be occupied by a plant and, although treated in fair detail
in a later chapter, require some explanation here before we can
proceed.
Any condition of the habitat, whether climatic, physiographic,
edaphic (concerned with the soil), or biotic (concerned with living
organisms), may limit the area occupied by a plant, and usually
many of these conditions do come into play. A simple instance is
that a tropical plant requires warm conditions—or at least, it cannot
grow in the cold. Usually, however, matters are far more com-
1| WHAT IS PLANT GEOGRAPHY? 9
plicated than this, in that such a plant commonly requires also
environmental conditions within a certain range of moisture, light,
soil type, etc. As the world affords, usually over considerable areas,
practically every conceivable combination of habitat factors, the
area occupied by a particular plant will, :mter alia, depend upon its
physiological make-up and reaction to the component factors.
Herein lies the close connection between particular plants and their
favoured habitats.
Whereas probably no two situations or even areas of a seemingly
uniform habitat are exactly identical, we must in practice accept
them as being alike, even as they appear to be so accepted by plants.
Accordingly the similar habitats of the world may be grouped together
to form recognizable patterns ; and, looking at things the other way,
we find that there usually is a pattern of areas occupied by each
plant. Often a single habitat factor lies behind such a pattern of
plant distribution and may readily be recognized as doing so. But,
unfortunately for those who crave simple monistic explanations, the
area potentially inhabitable by a particular kind of plant, as pre-
scribed by suitable habitat conditions, and that area which it actually
occupies in the world today, are rarely if ever identical or even
similar. Yet although the area which might be occupied is of both
interest and importance to the scientist and to mankind, the plant
geographer’s immediate concern is with the fact, 7.e. the actual area
occupied.
In the same way, the many different types of vegetation form
geographical patterns of their own, but here again the plant geo-
grapher is concerned more with the effects, 7.e. the actual patterns,
than with the causes which properly belong to the historical side of
his studies. ‘lhe vegetation pattern is to a considerable extent the
sum of the overlapping distributions of the component plants ; but
it commonly has a form of its own, for in biology the sum total
of the components does not necessarily constitute the expected
whole. ‘The organisms’ interrelationships and reactions add much
that is new to the system and form an integral part of the end result.
CLIMATE THE MASTER
As climate tends to supply the most important over-all factors
determining plant distribution, it behoves us to give at this stage
some outline of the main ‘ world’ types of climate and related
vegetational features. Further details on climatic factors will be
1o INTRODUCTION WO RLAN DT GEROGRAP EH Y¥ [CHAP.
found in Chapter X, with accompanying figures indicating tempera-
ture and precipitation in different parts of the globe, and in such
recognized works as W. G. Kendrew’s Climatology, second edition
(Clarendon Press, Oxford, pp. xv + 400, 1957). ‘The main vegeta-
tional types occurring on land are dealt with particularly in Chapters
XII, XIII, and XIV.
Climate is the most far-reaching of the natural ‘ elements’
controlling plant life, and its study, climatology, is accordingly
fundamental to plant geography and related disciplines. In the
words of Kendrew (/.c.), ‘ ““Climate”’ is a composite idea, a generaliza-
tion of the manifold weather conditions from day to day throughout
the year... . In the study of climatology the primary interest
lies in the facts of the climates of the earth in themselves, and as
elements in the natural environment of life. To the phrase
‘throughout the year’ the ‘ historical’ plant geographer might wish
to add ‘and through the ages ’.
Climatology deals with the atmospheric conditions which affect
life—particularly light, temperature, precipitation, evaporating power,
and wind. Additional factors include radiation, cloudiness, and
storms. ‘hese components are often interdependent, their various
combinations giving us the characteristic climates of different parts of
the world which for our purposes may be divided broadly into three.
These are the polar, temperate, and tropical regions, and they
are primarily temperature zones. For convenience, the temperate
areas lying north and south of the equator are considered together,
as are the north and south polar areas in their turn. In this book
the temperate regions are purposely treated first, for reasons already
mentioned, and are followed by the polar regions. Besides these
three primary categories there are the more localized climates of high
altitudes (whose land vegetation, being largely comparable, we shall
consider with that of the polar regions), of ‘ monsoon’ and ‘ Mediter-
ranean’ types with warm and damp seasons alternating with dry
ones, and of equable ‘ oceanic’ and widely-extreme ‘ continental ’
types (see pp. 11-12). ‘To the three primary climatic groupings the
main vegetational belts of the world largely correspond, with local
variations engendered by localized climatic and other features.
The climates of temperate and adjacent lands are mostly fairly
warm and moist, at least in the favourable periods. ‘They exhibit
rather marked seasonal and diurnal fluctuations, and also vary greatly
from place to place. ‘The mean of the warmest month each year
is normally above 10° C. (50° F.) and the annual precipitation is
1| WHAT IS PLANT GEOGRAPHY ? II
widely more than 762 mm. (30 inches). ‘There is a marked difference
between winter and summer light-climates and temperatures, and
often, precipitation. ‘Ihe vegetation tends to be fairly luxuriant at
least in favourable situations, with trees and shrubs widely dominat-
ing but herbaceous plants usually exceeding them in number and
variety. Most areas having a * Mediterranean’ type of climate,
with hot and dry summers but with other seasons that are damp
and not too cold for plant growth, are included here, their vegetation
being often dominated by leathery-leafed shrubs but including
many bulbous and ephemeral herbs. ‘The main vegetation-types of
temperate and adjacent lands are dealt with in Chapter XII.
The climates of polar lands and high altitudes are mostly rigorous,
with the mean of the warmest month usually below 10° C. Pre-
cipitation is mostly in the form of snow and widely less than 254 mm.
(10 inches) per annum, though owing to the prevailingly low tem-
peratures the relative humidity may be high and the evaporating
power low. ‘There are wide seasonal fluctuations in most polar
regions and wide diurnal ones in most alpine areas. In the higher
latitudes there is continuous light in summer and darkness in
winter. ‘The vegetation is mostly low and scant—of dwarf shrubs,
herbs (including many of grass habit), Lichens and Mosses. The
main vegetational types of polar lands and high altitudes are dealt
with in Chapter XIII.
The climates of tropical and adjacent lands are warm and widely
humid, with the mean temperature of the coldest month usually
above 17°8° C. (64° F.) and the rainfall often heavy (e.g. 200-400 cm.).
Frost and snow are usually unknown, the conditions being torrid
and widely equable, with often little or no seasonal variation. The
vegetation ranges from the world’s most luxuriant rain forest to
various scrub, grassland, and desert communities as the available
water decreases. Most ‘ monsoon’ areas of alternating wet and dry
seasons, commonly dominated by deciduous trees and shrubs which
lose their leaves to conserve water during dry periods, are included
here. ‘The main vegetational types of tropical and adjacent lands
are dealt with in Chapter XIV.
It should be noted that the distinction between even ‘ oceanic’
(‘ maritime’, or ‘ insular’) and uneven ‘ continental’ climates is
largely one of degree, being irrespective of latitude or temperature-
relationships and consequently found in all of the above three primary
climatic groupings. In general the oceanic extreme occurs on land
where the prevailing winds come off the sea and are consequently
12 EN DR ODUCLLON =] LO EWAN GEO GRA EY [CHAP.
moist and cloudy ; its areas tend to be well vegetated, often with
broad-leafed forests or verdant pastures. ‘Ihe continental extreme,
on the other hand, is usually found far inland from the ocean and
tends to have low relative humidity and precipitation, though
exhibiting wide seasonal and daily fluctuations especially of tempera-
ture. ‘The summer here is commonly sunny and warm but dry,
the winter being relatively cold, so that vegetation tends to be
limited, often consisting of drought-resistant Grasses, Heaths, or
desert plants.
THE IDEAL PLANT
At this point will be given a brief account of the structure and
adaptation of a multicellular ‘ higher’ plant, such as a member of
the Angiosperms which top the ‘ evolutionary tree’ and are dealt
with at the end of the next chapter. Such flowering plants make
up most of the bulk of modern vegetation, give us very many of
our foods and other necessities of life, and consequently loom largest
in our plant geographical and allied studies.
Our ideal plant, as we may thus conceive it, will consist of (1)
roots for anchoring in the ground and absorption from it of water
and soluble nutrients, (2) a stem to hold the leaves and reproductive
parts aloft, (3) green leaves to manufacture food substances in the
light, and (4) flowers to produce seeds and so effect reproduction.
Such features are too familiar to require illustration.
Each main portion of a higher plant is composed of ‘ cells’, which
are minute and often box-like structural units that are variously
adapted to cover different needs. Cells of one kind are commonly
aggregated together to form ‘ tissues ’ of particular form and function.
Thus some cells are for conduction—particularly of water and
dissolved salts upwards from the roots and of elaborated materials
downwards from the leaves—and are consequently elongated and
often pipe-like. Other cells have greatly thickened walls and give
tensile strength to roots and rigidity to aerial parts of the plant—
especially in the latter instance when aggregates of them are situated
near the periphery, as they commonly are in stems. Many cells on
the other hand remain thin-walled and serve the purpose of aeration,
food-storage, or mere ‘ packing’, while some may perform more
than one function either concurrently or consecutively. All kinds
of cells are produced from undifferentiated thin-walled ‘ meriste-
matic ’ ones which divide actively, for example in the growing-points
1] WHAT IS PLANT GEOGRAPHY? 13
(meristems) situated near the apices of stems and roots. Fig. 19 (B
and C) shows stem-sections of higher plants with the main types
of tissues and examples of their disposition.
The above references are chiefly to the more or less solid walls of
plant cells. But all these cells are alive, at least in youth—for they
contain a viscous and very heterogeneous fluid known as protoplasm,
which is the living matter of the plant. It is in the protoplasm that
occur the extremely complex sequences of events which integrate
into what we know as life, and which include the processes enabling
the protoplasm to increase itself. ‘This increase forms the basis of
growth, which normally involves increase in size of the cell until
it reaches a maximum and thereupon divides into two daughter
cells. ‘The daughters then repeat the process, and as a result of
numerous repetitions of this activity the plant as a whole grows in
size. Another activity going on in all living cells is the slow oxidative
‘burning ’ known as respiration, which gives to living organisms the
energy required for their life-processes.
Besides the apical meristems by which plant organs grow in
length, there is, in the stems and roots of many long-lived higher
plants, a layer of actively dividing cells (the ‘ cambium ’) which add
daughters radially on either side and so lead to growth in girth.
When this takes place year after year in regions of fluctuating
climate, where cells of different sizes are produced at different
seasons, annual ‘ growth-rings’ are formed which may easily be
seen in most timbers. In addition there are meristems in buds
whose behaviour—varying from dormancy to active elongation—
greatly affects the ultimate shape of plants. ‘These and other growth
phenomena are largely controlled by special chemical substances
produced by the plant, and in ways which are only nowadays being
elucidated. ‘These plant growth substances, for example, may
stimulate the elongation of cells in some tracts while inhibiting that of
others—resulting in curvature of an organ in relation to a directional
stimulus, such as light, which itself affects the production or
availability of the chemical stimulant. Other substances inhibit
growth, an example being produced by many terminal buds ;
accordingly it is only when such inhibitors are removed that the
lateral buds grow out actively (hence the sprouting of a hedge after
clipping, and of pasturage after close grazing). For a general survey
of this fascinating and important subject, see Professor L. J. Audus’s
Plant Growth Substances, second edition (Leonard Hill, London,
Pp. xxii + 553, 1959).
B
14 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
Plants cannot exist without water, though different kinds require
it in very different amounts. Our ideal plant must be well adapted
in its water economy to the prevailing conditions ; thus if water is
scarce, it must have some means of keeping down the loss which
takes place continuously from its aerial parts in the process known
as transpiration. ‘This economy may be effected by such devices
as a thick and impervious bark or waxy or hairy covering, by pro-
tection of the ‘ breathing pores ’, or by reduction of the total surface.
Often more than one method is employed by a plant, which at the
same time will have to be adapted to other factors of the environment.
Through long processes of evolution, involving among other things
the elimination of unsuitable features, different kinds of plants have
become adapted to different environments, and this, as we shall see
for example in Chapter III, is one of the most fundamental bases
of their distribution and consequently of plant geography.
PLANT SOCIOLOGY
Although opinions vary as to what constitutes a species (broadly
speaking, a kind), we all have some conception of how similar
individuals, whether plants or animals, make up such an entity.
‘The numerous individuals comprising a particular species, while by
no means all exactly identical, nevertheless are closely comparable
in most respects, and normally have the appearance of belonging
to the same kind. We have already observed that different plant
species and other entities, often of many and various groups, become
associated together in nature to compose what we term vegetation.
‘This is made up of more or less definite plant communities, related
at least in part to local conditions. Each of these communities is
characterized by its own particular form (physiognomy), and in
most cases also by one or more predominant species.
Plant sociology, also called phytosociology, is, strictly speaking,
the study of the plant communities that make up vegetation—
including their inception and formation, their structure, and, above
all, their composition. Accordingly some parts of this subject, and
particularly the composition of plant communities, are of vital
interest to the plant geographer, even as the distribution of these
communities forms an important part of his study. But in spite
of a wide overlap of material, students of the two disciplines
approach their problems and subjects from different angles of
interest, and so it is not proposed to consider plant sociology
r] WHAT IS PLANT GEOGRAPHY? 15
here, except in so far as it may help us to understand our own
problems.
THE ANIMAL SIDE
As animals are so largely dependent upon plants for food, shelter,
and other requisites of life, their geography and ecology tend to
be less fundamental than those of plants, or at all events less closely
related to the physical environment. Nevertheless the animal side
of the picture of life (and in particular, Man’s influence) must be
vividly borne in mind by students of plant geography. ‘Thus we
shall see in Chapter IV how numerous plants depend upon animals,
in many and various ways, for the dispersal of their seeds and
fruits. Later on, in the chapters on vegetation-types, we shall be
repeatedly reminded of how animals modify vegetation during their
feeding and other activities, often favouring the growth, or very
existence, of one species while discouraging that of another, and
profoundly affecting the vegetation locally. In these and other ways,
Man is apt to have the greatest influence of all. Many plants,
too, depend on animals for pollination and hence fertilization of
their flowers ; here, at least in the absence of vegetative means of
propagation, reproduction will not normally take place without
animal aid. All of these features can, and frequently do, affect
the spreading and ultimate distribution of plant species. It is
therefore not surprising that many areas, such as Australia and
South Africa, have both a floristic and faunistic character and
identity of their own, their (often peculiar) plants and animals going
hand in hand, so to speak. Furthermore, animal geography often
corroborates the conclusions of plant geography, and offers splendid
evidence of evolutionary trends in its fossil record. It also appears
to corroborate migrational tendencies in its suggestion of certain
‘land-bridges’ and ‘ refuges’.
In view of the closeness with which the two are linked in nature,
there is much to be said for the study, which has increased in
popularity in recent decades, of plants and animals as they exist
together in joint ‘ biotic’ communities. But whereas the particular
physical conditions in an area are more or less vividly expressed in
the local plant cover, which forms, as it were, a living framework,
1 Interested readers are referred to the standard work on the subject by Dr. J
Braun-Blanquet (Plant Sociology, McGraw-Hill, New York & London, pp.
xviii + 439, 1932), or the second German edition (Pflanzensoziologie : Grundsziige
der Vegetationskunde, Springer, Wien, pp. xi + 631, 1951).
16 INTRODUCTION TO PLANT CEOGRAPH ¥ [CHAP.
it is only secondarily that this in turn largely conditions the animal
population—which thus becomes a subordinate characteristic of the
locality and is usually less evident and immediately significant than
the vegetation, at least on land. Indeed, where there are no suitable
plants there can be no animals living normally. As M. D. Haviland
puts it in the work cited at the end of Chapter XVII,
‘It is the faithful correlation of plant growth with the physical environ-
ment, especially to the important factor, or complex of factors, called
“climate”, that leads us naturally to define the main types of land
environment in terms of plant life as Woodland, Grassland and Desert
. . . for vegetation is the apparel of scenery. As Darwin wrote: “A
traveller should be a botanist, for in all views plants form the chief
embellishment.” But when the zoologist, forsaking botanical terms,
tries to classify environments in the language of his own science, he
cannot construct a workable scheme . . . he finds that he must fall
back on the language of the botanist or geologist.’
In general, zoologists have not been very successful in recognizing
definite animal communities of a complex nature, and their study
in individual species of adaptive response to particular environments
tends to be of less immediate significance than that of botanists
with plants. ‘hus, whereas the marked dwarfing of many arctic
and alpine plants is related directly to exposure to harsh physical
environments, many similarly striking animal characteristics, such
as broad teeth for grinding seeds and special organs for climbing
trees, are related to the climatic conditions only indirectly through
plant response. Nevertheless, as pointed out by Professor G. E.
Hutchinson (7 /itt.), there are a number of known cases of warm-
blooded animals responding directly to climate. For example,
boreal Mammals not only have under-fur but also are of larger
absolute size, and have shorter ears and tails, than their southern
counterparts, while almost all desert Mammals and Birds are pale
even if nocturnal, and insular races of Birds have relatively large
beaks and feet. But in spite of such exceptions, and others which
act in the opposite direction (such as the striking adaptations of
many flowers to insects in relation to pollination), plant response
to climate is usually direct whereas that of animals tends to be
indirect. ‘Then again, animals are usually mobile, and individuals
may wander or migrate vast distances. Consequently, apart from
such connections as those mentioned above, animals tend to be of
less geographical significance than plants, in the sense that they do
1] WHAT IS PLANT GEOGRAPHY? 17
not characterize areas to the same extent, and for our present purpose
seem best considered as a mere factor of the environment.
This recognition of the more fundamental role of plants does
not seem to be weakened by the realization that, often, plants and
animals have evolved together and are necessary for one another’s
existence. For even in the case of flesh-eating animals, sooner or
later, as we trace back the food-chain, we come to the ultimate point
of dependence upon green plants. Furthermore, the animal geo-
grapher is not necessarily of much help to us, for the areas and
boundaries which he recognizes (e.g. Fig. 2, A) are often very different
from ours (e.g. Fig. 2, B), and he is prone to take for granted that
the vegetation (which he considers simply as part of the environ-
ment) is a mere response to local conditions. Yet a plant community,
quite apart from its historical implications, gives us many clues to
the nature of the environment because its component members
exhibit recognizable responses to physical features. No such general
virtue is displayed by animal communities, if indeed these can be
satisfactorily recognized.
Recent books on animal geography, with useful bibliographies
suggesting further reading, include R. Hesse, W. C. Allee, & K. P.
Schmidt’s Ecological Animal Geography, second edition (Wiley,
New York, pp. xiii + 715, 1951), F. L. de Beaufort’s Zoogeography
of the Land and Inland Waters (Sidgwick & Jackson, London, pp.
vill + 208, 1951), Sven Ekman’s Zoogeography of the Sea, translated
by Elizabeth Palmer (Sidgwick & Jackson, London, pp. xiv + 417,
1953), and Philip J. Darlington’s Zoogeography : the Geographical
Distribution of Animals (Wiley, New York, pp. xiii + 675, 1957).
SOME EARLIER WorRKS ON PLANT GEOGRAPHY
In English :
Anonymous and other early works include The Geography of Plants (The
Religious Tract Society, London, pp. vi-+ 7-192, undated), J.
Barton’s A Lecture on the Geography of Plants (Harvey & Darton,
London, pp. 1-94 and index, etc., 1827), and R. B. Hinds’s The
Regions of Vegetation ; being an analysis of the distribution of vegetable
forms over the surface of the globe in connection with climate and
physical agents (Palmer, London, pp. 1-140, 1843).
MeyeEN, F. J. F. (1846): Outlines of the Geography of Plants : with
particular enquiries concerning the native country, the culture, and the
uses of the principal cultivated plants on which the prosperity of nations
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20 IN GRO DUC PION LOS EiaAIN AS GE OGRA EL Yi [CHAP.
is based, translated by M. Johnston (Ray Society, London, pp.
x + 422). Follows the German edition, published in Berlin in 1836.
Of historical interest as indicating the teachings of the day, including
Man’s dependence upon plants, but with over-emphasis on latitude
as the limiting factor in plant distribution.
DAUBENY, CHARLES, ed. (1855): Popular Geography of Plants ; or, a
botanical excursion around the world (Lovell Reeve, London, pp.
xl + 370). A very readable illustrated account following Meyen’s
arrangement and, although often unreliable, of historical interest as
indicating the type of work apparently favoured by the intelligent
layman of a century ago: by ‘E.M.C.’, with a well-written and
penetrating preface by its eminent editor.
PICKERING, CHARLES (1876): The Geographical Distribution of Animals
and Plants. Part II. Plants in Their Wild State (Naturalists’
Agency, Salem, Mass., pp. 1-524 and additional maps). A sumptuous
but evidently rare publication of some interest and foresight.
ScHimpER, A. F. W. (1903): Plant-geography upon a Physiological Basis,
translated by W. R. Fisher, revised and edited by Percy Groom and
I. B. Balfour (Clarendon Press, Oxford, pp. xxx + 839 and 4
additional maps). Still the great reference book on the subject in
English, profusely illustrated and a commendable feat for its time,
though in places unreliable and now largely outdated. A detailed
up-to-date work in English, planned along modern lines and executed
in the light of the latest knowledge, is badly needed to supersede it.
WarMING, E., et. al. (1909) : O0ecology of Plants (Clarendon Press, Oxford,
pp. xi + 422). For many years a standard source-book mainly on
the ecological side but also describing the main vegetation-types.
Harpy, M. E. (1913): A Jumor Plant Geography (Clarendon Press,
Oxford, pp. 1-192). A light-weight but useful discourse on some
aspects of the subject—like the next work, illustrated and readable
though outdated and not always reliable. Some copies have been
seen entitled ‘ An Introduction to Plant Geography ’.
Harpy, M. E. (1920): The Geography of Plants (Clarendon Press,
Oxford, pp. xii + 327). Consists chiefly of a discursive account of
the more obvious vegetational features of the main land-masses and
climatic regions (cf. above), largely ignoring aquatic habitats.
Reprinted up to 1952.
CaMPBELL, D. H. (1926): An Outline of Plant Geography (Macmillan,
London [and New York], pp. ix + 392). Illustrated and readable :
concerned chiefly with climatic zones and areas and their land flora
and vegetational characteristics, but loosely written and frequently
inaccurate, and omitting many topics which might with advantage
have been treated.
NewsIGIn, M. I. (1936): Plant and Animal Geography (Methuen,
London, pp. xv + 298). See also the practically identical ‘ second
T| WHAT IS PLANT GEOGRAPHY? 2.
edition’ (Dutton, New York, pp. xv + 298, 1948). A stimulating
and usually reliable outline of many aspects of biogeography. The
so-called second edition, although recent, is practically a reprint that,
unfortunately, fails to remedy some misconceptions and to correct
errors particularly in those chapters for which the original author
was not responsible.
Wutrr, E. V. (1943): An Introduction to Historical Plant Geography,
translated by E. Brissenden (Chronica Botanica, Waltham, Mass.,
pp. xv + 223). Useful in elucidating the origin and development
of floras as opposed to their composition, ecology, and other aspects.
A succeeding volume was published later in Russia (see below).
_ Carn, 8. A. (1944): Foundations of Plant Geography (Harper, New York
& London, pp. xiv + 556). An important though rather technical
survey of the history and interpretation of many phenomena of
vascular plant distribution on land. Does not deal with aquatic
habitats or lower groups of plants, and is professedly not a descriptive
plant geography.
Croizat, L. (1952): Manual of Phytogeography (Junk, The Hague, pp.
vill -+ 587 and 106 additional illustrations). Considers plant geo-
graphy simply the study of plant dispersal, being ‘ that branch of
botany which integrates plant-migrations in time and space’. A
large part (pp. 68-399) is occupied by treatment of the ‘ intercon-
tinental dispersal ’ of various (mainly tropical) Angiosperms. Often
opinionated and sometimes crotchety : nor is the coverage in accord-
ance with the subtitle which claims the work to be ‘an account of
plant-dispersal throughout the world’.
Goop, RONALD (1953): The Geography of the Flowering Plants, second
edition (Longmans, London etc., pp. xiv + 452). An illustrated
manual of the distributions of flowering plants and the factors
controlling them; does not deal with vegetation or with lower
plants. Nevertheless a valuable work, and widely considered the
standard one on the floristic side of the subject. "The second edition
should be used rather than the first, which was published in 1947.
‘TuRRILL, W. B. (1953): Pioneer Plant Geography : the phytogeographical
researches of Sir Foseph Dalton Hooker (Nijhoff, ‘The Hague, pp.
xii + 267). Readable and instructive, with up-to-date comments,
as well as historically interesting.
In other languages :
Humpo.pt, A., & A. BoNnpLAND (1805): Essar sur la Géographie des
Plantes ; accompagné d’un tableau physique des régions équinoxiales
(Paris, pp. i-xii + 13-155 and map). One of the main foundations
of our subject, followed by a German edition in 1807, and also by
kindred works in various languages ; of great historical interest.
Scuouw, J. F. (1822): Grundtraek til en almindelig Plantegeographie
22 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
(Kjobenhavn, pp. x + 463 and 4 additional illustrations). Fol-
lowed the next year by a German edition entitled Grundziige einer
allgemeinen Pflanzengeographie (Berlin, pp. xix +- 528 and 4 additional
illustrations). Of historical interest.
Rupotpu, L. (1853): Die Pflanzendecke der Erde (Berlin, pp. xiv + 416
and additional plates ; also ‘ Supplementheft ’ of 34 pages, published
in Berlin in 1859). An early semi-popular treatment.
DECANDOLLE, ALPHONSE (1855): Géographie Botanique Raisonnée ou
exposition des faits principaux et des lois concernant la distribution
géographique des plantes de l’époque actuelle (Paris & Geneve, vol. I,
pp. xxxli + 606 and additional maps, and vol. II, pp. 607~—1366).
An early detailed synthesis of much of our subject, now chiefly of
historical interest.
GRrISEBACH, A. (1877-8): La Végétation du Globe d’apreés la disposition
suivant les climats . . ., translated by P. de Tchihatchef (Bailliére,
Paris, vol. I, pp. xvi + 765 and additional map, 1877, and vol. II,
pp. vi + 905, 1878). Discursive but interesting, at least historically.
ENGLER, A. (1879-82) : Versuch einer Entwicklungsgeschichte der Pflanzen-
welt, insbesondere der Florengebiete seit der Tertiarperiode (Engelmann,
Leipzig, vol. I, pp. xviii + 202 and additional map, vol. II, pp.
xiv + 386 and additional map). Includes suggested explanations of
plant distribution but was soon in part superseded. Vol. I, published
in 1879, deals with the extratropical regions of the northern hemi-
sphere, and vol. II, published in 1882, with the tropical regions and
the remainder of the southern hemisphere.
GRISEBACH, A. (1880): Gesammelte Abhandlungen und kleinere Schriften
sur Pflanzengeographie (Engelmann, Leipzig, pp. vii + 628). Various
contributions including the general and lengthy ‘ Berichte iiber die
Fortschritte in der Geographie der Pflanzen’ (pp. 335-556).
ConrTEJEAN, C. (1881): Géographie Botanique ; influence du terrain sur
la végétation (Bailliere, Paris, pp. 1-144). Deals chiefly with habitat
differences and their effect on local flora.
Gorze, E. (1882): Pflanzengeographie fiir Gartner und Freunde des
Gartenbaues (Ulmer, Stuttgart, pp. xiv + 476). <A general treat-
ment, primarily but by no means exclusively for horticulturists.
Drupe, O. (1890): Handbuch der Pflanzengeographie (Engelhorn, Stutt-
gart, pp. xvi + 582 and additional map). A general manual, illustrated
chiefly by maps, of the subject as then developed, by a renowned
investigator of the time. Followed in 1897 by a French edition
entitled Manuel de Géographie Botanique (Klincksieck, Paris, pp.
Xxill + 552).
Sotms-LaupacH, H. zu (1905): Die leitenden Gesichtspunkte einer
allgemeinen Pflanzengeographie (Felix, Leipzig, pp. ix + 243). A
briefer, unillustrated account of much of the subject, with some
novel ideas.
1] WHAT IS PLANT GEOGRAPHY? 23
GRAEBNER, P. (1910): Lehrbuch der allgemeinen Pflanzengeographie nach
entwicklungsgeschichtlichen und phystologisch-dkologischen Gesichts-
punkten mit Beitrdégen von Paul Ascherson (Quelle & Meyer, Leipzig,
pp. viii + 303). A general, illustrated account of the history and
composition of various floras and vegetation-types, paying due regard
to their ecological bases.
Hayek, A. (1926): Allgemeine Pflanzengeographie (Borntraeger, Berlin,
pp. viii + 409 and 2 additional maps). A more modern account
along similar general lines to the last, but illustrated by only a very
few diagrams and maps.
Diets, L. (1929): Pflanzengeographie, third edition (Grunter, Berlin &
Leipzig, pp. 1-159 and additional map). Useful as presenting a
largely modern account in outline of many aspects of the subject
in handy pocket form. Later editions (not seen) have since appeared.
RuseL, E. (1930): Pflanzengesellschaften der Erde (Huber, Bern-Berlin,
pp. viii -+ 464 and map). Describes and illustrates the main plant
communities and vegetation-types of the world.
WarMino, E., & P. GRAEBNER (1933): Lehrbuch der dkologischen Pflanzen-
geographie, fourth edition (Borntraeger, Berlin, pp. vill + 1158). A
valuable illustrated work dealing particularly with ecological and
vegetational aspects.
Scuimmper, A. F. W. (1935): Pflanzengeographie auf physiologischer
Grundlage, ‘ third’ edition, revised by F. C. von Faber (Fischer, Jena,
vol. I, pp. xx + 588, and vol. II, pp. xvi + 589-1613 and 3 additional
maps). An illustrated, extensive work weighing about 10 Ib. and
dealing mainly with the natural flora and vegetation of different
zones and regions, after some consideration of ecological factors and
principles. Generally reliable in those aspects of our subject with
which it deals. Although called (in German) the third edition, this
was in reality the second, as the so-called second edition was
merely a reprint of the first edition.
ALEKHIN, V. V. (1944): [Geography of Plants], in Russian only (State
Publisher, Moscow, pp. 1-455 and 2 maps). An illustrated account
of many of the distributional and ecological aspects of the subject.
Wutrr, E. V. (1944): [Historical Plant Geography : history of the floras
of the world], in Russian only (Akademiya Nauk SSSR, Moscow-
Leningrad, pp. xix + 546). ‘This is the second volume, mainly on
the origins of the floras of the different regions of the world, of a
projected three-volume work of which the first was translated into
English (see above) and the third was apparently never completed,
the author being killed in 1941 during the siege of Leningrad.
Gaussen, H.: Géographie des Plantes, second edition (Colin, Paris, pp.
1-224, 1954). Gives a brief but useful account of many aspects of
the subject.
CHAPTER II
THE VARIOUS GROUPS OF (PLAN Psa
HOW AND WHERE HEY E 1g
CLASSIFICATION AND NOMENCLATURE
To enable us to name and deal effectively with the almost infinite
variety of plants inhabiting the world, it is necessary to sort into
groups those which seem to have the closest affinity or, at least,
the greatest outward similarity. ‘These groups in turn have to be
aggregated into larger groupings, and so on, to create a hierarchical
system of classification which we also like to think bears a close
relation to evolutionary history. ‘Thus the members of a group
which look closely alike probably bear a ‘ blood relationship’ in
being descended from a common ancestor at no very remote period
of geological time ; indeed in some instances such a relationship
has been experimentally demonstrated. Biologists, and this includes
botanists, may in some cases disagree about the definitions, names,
and limits of this hierarchy of groups, but for general purposes
(and in descending order, from large to small) these groupings! may
be listed as follows :
Divisions or phyla (sing. phylum) : the major (highest) groupings
used in classifying plants, with names normally ending in -phyta,
those commonly recognized being the Schizophyta, Thallophyta,
Bryophyta, Pteridophyta, and Spermatophyta, and each consisting
of one or more
Classes: the next commonly recognized units, plentifully
exemplified below, and each consisting of one or more
Orders : each of which has its name ending in -ales, and in turn
consists of one or more
Families ; these, except in a few long-established instances, have
their names ending in -aceae. Usually the members of a family all
have some recognizable characteristic or characteristics, some com-
mon ‘stamp’; they are grouped into one or more
' Also called taxa (singular taxon), regardless of rank.
24
DHE VARIOUS GCROUPS OF PEANTS 25
Genera (sing. genus) : the members of each genus usually look
alike in a number of features and constitute one or more
Species: these represent the smallest unit of classification in
general use, being those whose members show a broad similarity.
Biologists differ in their conception of what constitutes a species, and
indeed the term is scarcely capable of satisfactory definition. How-
ever, for the great majority of animals and many plants a species is,
roughly speaking, constituted by all those individuals which are able
to interbreed among themselves but are unable to breed, at least at
all freely, with members of other groups. ‘he individuals of a
species are by no means identical but form a more or less variable
population in which some entities are often recognizable as subspecies
(written subsp. or ssp.), or as still more subordinate varieties (written
var.) or formae (written f.).
Even as species are divisible into subspecies, so are the major
groups often divided into subphyla, subclasses, etc. ‘he individual
is the ultimate unit, but inasmuch as no two individual plants can
be exactly identical, any more than two individual persons can be,
the smallest recognizable unit of classification is the biotype, con-
sisting of all those individuals which have the same genetical make-up.
Thus most species consist of a large number of biotypes that differ
slightly in their inheritance.
The scientific name of each species is normally made up of two
Latin or latinized words of which the first is the name of the genus
to which it belongs and the second is its own specific epithet, usually
having some descriptive or historical connotation. ‘The initial letter
of the first, or generic, name is always capitalized, that of the specific
epithet nowadays being customarily left ‘small’. Unfortunately,
English or other ‘ popular’ names are too unreliable to employ at
all widely, particularly because the same name is apt to be used for
different plants in different places or by different people, and also
because it is undesirable to have the same plant known under different
names in different places or sometimes even in the same place.
Moreover, it is confusing to have more than one combination of
names in use for a single entity, and so the Latin one is agreed upon
and employed internationally by scientists.
We will now consider briefly what seem for our purpose to be
the main classes of the plant kingdom, each being treated under the
primary heading of the phylum (division) to which it belongs. ‘The
sequence followed is probably indicative of evolutionary history in
broad outline. After a brief general account of the characters of
26 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
each of our chosen classes, we will deal with its modes of nutrition
and reproduction and also, in the broadest terms, with its main
habitats, distribution, and importance—both economically and as
a component of natural vegetation.
SCHIZOPHYTA
Bacteria: ‘These are very simple, exceedingly minute, and
virtually ubiquitous organisms. Most types consist of single
spherical, rod-shaped, branched, or variously curved cells, with or
without delicate superficial thread-like processes known as flagella
(sing. flagellum), through the action of which they may attain a fair
SM N e)
es ag
Cp se 22 d
=D @ 9
S ii U
Fic. 3.—Various types of Bacteria. Among those causing serious human diseases
are B, anthrax; H, typhoid fever; N, cholera; Q, tuberculosis; R, leprosy;
S, diphtheria; T, meningitis; U, pneumonia; V, dysentery; X, tetanus. (Mostly
1000, but Q and V considerably more magnified.)
degree of motility in liquid media. In other types the cells may
adhere together in small groups or chains, or remain attached by
the ends to form regular filaments. Various types of Bacteria are
shown in Fig. 3, most of them being magnified about 1,000 times.
Bacteria are found in vast numbers almost everywhere: in soils
and the atmosphere, in fresh and salt waters, and in many of the
most unlikely and unsavoury ‘ habitats’. Normal soils with a fair
percentage of organic matter contain on the average from 2,000,000
to 200,000,000 Bacteria per gram, while manure and sewage may
contain greater numbers still.
Bacteria multiply principally by simple cell division, sometimes
2| THE VARIOUS GROUPS OF PLANTS 27
as frequently as every twenty minutes—but only for a time, the
chief limitation being their food supply. ‘They may also form
spores which in some cases are highly resistant. Recently, convinc-
ing evidence of a sexual process has been obtained in some Bacteria,
but it is not known how frequent such a process may be in nature.
In their modes of nutrition Bacteria vary greatly: for although
commonly they are either (1) saprophytic, deriving their energy and
materials for life and growth from dead and usually decaying organic
matter, or (2) parasitic, depending similarly on living organisms;
there are also (3) many forms which can build up their bodies and
live from carbon dioxide obtained from the air or water and energy
liberated in the oxidation of inorganic compounds or even elements.
Such organisms are said to be chemosynthetic, and examples of the
substances oxidized by them are sulphur, hydrogen sulphide, nitrites,
ammonia, hydrogen, and, apparently, iron and manganese com-
pounds. ‘The members of one interesting group of sulphur-oxidizing
Bacteria, known as the Purple Bacteria, contain pigments enabling
them to absorb radiant energy from light, and they appear to practise
some kind of photosynthetic process which may be a prototype of that
occurring in ‘ normal’ green plants. Other types that seem properly
referable to the Bacteria are green, through the inclusion of chloro-
phyll of a kind. But it seems improbable that the earliest living
organisms possessed real chlorophyll or obtained their energy
through such an elaborate series of reactions as are involved in
photosynthesis (cf. p. 32). Rather is it considered likely that some
of these peculiar Bacteria indicate means by which elementary
organisms obtained their energy and other requisites of life before
either chlorophyll or any form of photosynthesis was evolved.
Consequently it seems most reasonable to start our sequence with
this group.
Certain Bacteria are of major importance in causing diseases—
particularly of animals, and including some of those most deadly
to Man—while various other Bacteria cause the decay and breakdown
of dead matter, or make available food-substances for higher plants,
or produce chemical ions of many kinds. With their infinitesimal
size and often resistant spores, they are among the most widespread
of living organisms, being carried by air or water currents or in the
bodies of animals practically everywhere in the world and its sur-
rounding atmosphere. Nevertheless, as they are so minute, they
play only a very minor direct role as components of most types of
vegetation. Exceptions are afforded by some aquatic muds, in which
28 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
Bacteria may dominate, and also, of course, in diseased or decaying
systems. ‘hey may also be the pioneers in the colonization of some
bare areas.
CYANOPHYCEAE: ‘These are the so-called ‘ Blue-green Algae’, or
Schizophyceae, but they seem to have their closest relationship with
the Bacteria, which they resemble particularly in their lack of a
et)
CS.
bap
ae ~
ea Bae
S280 ae 8e
A
Fic. 4.—Various Blue-green Algae (Cyanophyceae). A, Rivularia (< about 600) -
B, Aphanothece (* about 1500); C, Merismopedia (< about 200); D, Oscillatoria
(= about 800); E, a species of Nostoc, consisting of filaments embedded in a
gelatinous matrix (* about 300); F, Gloeocapsa: a single-celled individual and
colonies of two, three, and four cells ( 1285).
typically organized nucleus in the cell, in their method of cell
division, and in the obscurity at all events of any sexual reproduction.
They consist either of single cells, of cells joined end-to-end to form
2 | THE VARIOUS GROUPS OF PLANTS 29
filaments, or of colonies in which either individual cells or filaments
are held together in gelatinous masses. Examples, variously
magnified, are shown in Fig. 4; but whereas the individual cells
are microscopic and often exceedingly minute, the colonies may be
many centimetres in diameter and of considerable bulk.
The Cyanophyceae contain chlorophyll of a sort and mostly live
by photosynthesis although some appear to be at least partly sapro-
phytic. The chlorophyll is diffused through the outer layers of
protoplasm instead of being accumulated in special bodies as it is
in higher plants. Cyanophyceae also contain bluish and/or reddish
pigments, the former of which, with the chlorophyll, gives to many
a blue-green coloration—hence their popular name. Others, how-
ever, are very differently coloured. Multiplication is by simple cell
division or the formation of spores. ‘The filaments or parts of
filaments of many exhibit motions of various but characteristic kinds,
including glidings and oscillations, the mechanisms of which are
not understood.
Cyanophyceae are very widespread, occurring especially in a range
of freshwater and damp to marshy habitats. However, though often
abundant, they make relatively little showing as components of
vegetation—except sometimes in lakes or ponds, where they may
form a ‘bloom’, or on stones in fairly rapid streams where they
provide a mucilaginous mat, in which other organisms such as
Diatoms thrive. ‘They tend to be important in arctic regions where
they often form dark colonies on damp soil, in marshes, and about
the margins of tarns. Ecologically they may be of some importance
as the initial colonists (pioneers) on bare rock and other surfaces.
They have little economic significance except as nuisances in fouling
water supplies—to which they may impart a disagreeable odour and
taste, sometimes killing fish or even cattle.
"THALLOPHYTA
CHLOROPHYCEAE: These are a large and diverse group of Algae
(seaweeds, aquatic ‘ slimes ’, etc.) having chlorophyll located in well-
defined bodies called chloroplasts and not normally masked by other
pigments. Consequently the plants are usually green in colour and
the group is called the Green Algae. Characteristically they have
cellulose cell-walls and store food in the form of starch. What
appear to be the more primitive forms are microscopic, unicellular,
and either motile by flagella or non-motile, the cells occurring singly
30
INTRODUCTION TO PLANT GEOGRAPHY
[CHAP.
2] THE VARIOUS GROUPS OF PLANTS 31
eh AY
A or BY
Pil ia IM
Bi 4 4 Y,
i Was ae
Yh ‘ Tal ‘\\ et
mi He i Nia
WI MAB A \
0"
Ni YT g DNS
dag y) why
Fic. 5.—Some forms of Green Algae (Chlorophyceae). A, Pleurococcus—unicel-
lular, non-motile (* 2470); B, Chlamydomonas—aunicellular, motile ( about
500); C, Pleodorina, colonial, motile (* 250); D, cells from a filament of Ulothrix
( 462): 1D, Enteromorpha intestinalis, a relative of Ulva (see G) (x 4); F, Chara,
a highly organized Stonewort ( 1); G, Ulva lactuca, a Sea-lettuce (< about 2);
H, various Desmids (e220):
or grouped into colonies. Advanced types are commonly attached to
some object and consist of filaments or more substantial branched
structures, or have the form of a flattened thallus (simple vegetative
body lacking differentiation into true root, stem, and leaf) that may
be several inches in diameter. Fig. 5 shows a range of different
Chlorophyceae ; they are not merely extremely diverse but also
appear to have undergone evolution along a number of different lines,
most of which have proved to be ‘ dead ends’. Here are included
the Desmids, which are freshwater forms consisting usually of a
single cell that is sharply marked off into two symmetrical (and often
32 INTRODUCTION TO PLANT GEOGRAPHY [| CHAP.
complicatedly lobed) halves by a constriction around the centre.
Also included are a wide range of freshwater and marine slimes and
scums, the sometimes bulky Sea-lettuces, and, according to most
students, the peculiar and fir-like Stoneworts.
Different kinds of Green Algae are to be found in a vast array of
habitats including damp soil, the surfaces of rocks, and the bark of
trees. Here and on such objects as posts and palings they may
form a green investment most typically on the pole-facing side,
which tends to be less dried by the sun. But mainly they are
aquatic, being especially abundant in freshwater lakes and streams,
although numerous types occur in the sea. Many Green Algae are
among the most widespread of plants, the group as a whole being
virtually ubiquitous. ‘Their nutrition is mainly by photosynthesis
—that fundamental series of reactions on which they and practically
all other forms of life depend, directly or indirectly. In this vital
process, chlorophyll absorbs radiant energy from light and catalyzes
the building up of simple materials into complicated carbohydrates
in which the energy is locked—there to remain stored until it is
liberated, for example by burning or the slower process of respira-
tion. Such carbohydrates made by green plants constitute the main
basic food materials of the world. Although some simulate higher
plants, Algae do not need (or have) roots or other special absorbing
organs, but take in the raw materials they require (chiefly water
with certain salts and gases in solution) more or less all over their
surfaces.
The Green Algae reproduce by various methods, several of which
may be practised by the selfsame species. ‘The chief types are
asexual reproduction by cell division, fragmentation of the thallus,
or the liberation of spores—which may swim actively by means of
flagella. Or there may be sexual reproduction following conjuga-
tion or, more often, the fusion of special bodies (gametes) which are
frequently differentiated into large female and small male ones. In
some cases both of the gametes and in others only the male ones
are motile. In spite of their great diversity and virtual ubiquity,
the Green Algae are of rather little importance except as food
for aquatic animals; but they are of great interest in indicating
some of the lines along which evolution to higher plants may have
taken place. ‘They are widely important constituents of aquatic
vegetation and are often dominant in freshwater pools, though as
constituents of human or domestic animals’ food they are at best
minor.
2] THE VARIOUS GROUPS OF PEANTS 33
BACILLARIOPHYCEAE: ‘These are the Diatoms, familiar to all
microscopists, and comprise a large and important class of Algae that
are all unicellular and microscopic, occurring singly or attached in
filaments or chains, or grouped into colonies. ‘The form is extremely
various, as may be seen from Fig. 6, Th
wi ip
|
Pye
which shows a range of different types. aw VE
T
The cell-wall is composed of two \\(!
‘valves’, overlapping one another like WF
the halves of a pill-box, and is impreg- Ni,
nated with silica. Its surface is finely
and often beautifully sculptured, with
extraordinary regularity and precision.
The chloroplasts contain a brown pig-
ment in addition to the all-important
chlorophyll, and accordingly the colour
of Diatoms both individually and en
masse is usually a shade of brown or
olive-green.
The nutrition of Diatoms is primarily
Fic. 6.—Diatoms (Bacillariophyceae). A, various forms (variously magnified) ;
B, a colonial type, Licmophora flabellata (™ 70).
by photosynthesis, food being stored in the form of oil. ‘Their
reproduction is normally by cell division, though occasionally sexual
conjugation takes place, followed by the production of special
‘auxospores’, or these latter may be produced apomictically ;
alternatively, small flagellated gametes or spores may, be formed.
Diatoms are abundant in both marine and fresh waters in practically
all climates, forming a significant, and often the main, constituent
34 INTRODUCTION TO PLANT GEOGRAPHY [CHAP,
of the plankton—the more or less passively floating or drifting plant
and animal population of seas and lakes—and as such are of vital
importance as the ultimate source of food of many fishes and other
sea and freshwater animals. ‘They are virtually world-wide in dis-
tribution, being found, for example, on and in damp soil and upon
as well as under the sea-ice even about the North Pole. When
they decompose or are digested by animals, their siliceous valves
usually do not decay but sink in considerable quantities to the
bottom of the body of water, often forming extensive deposits of
‘diatomaceous earth’. ‘This is widely used for scouring, filtering,
insulation, and other purposes.
DrinopHycEAE: ‘These are the Peridinians or Dinoflagellates—
usually motile, microscopic unicellular organisms of yellowish or
brownish colour and sometimes of marked luminescence. A few
are naked but the vast majority have cellulose walls, which are
often composed of several sculptured plates. Fig. 7 shows three
flagellated, motile examples ; but even those types which are non-
motile and filamentous reproduce by motile spores (zoospores) that
have the form of typical Dinophyceae. ‘These
have a particularly characteristic feature—two
grooves at right-angles, one of which encircles
|
|
by
A B G
Fic. 7.—Dinoflagellates (Dinophyceae). A, Gymnodinium, a type without plates
(* about 1560); B, Goniaulax, a type armoured with plates (x about 1200);
C, Ceratium, with plates and form-resistance ( about 300).
|| THE VARIOUS GROUPS OF PLANTS 35
the cell transversely while the other runs longitudinally along
one side. ‘Two flagella are inserted where the grooves cross each
other—an undulating one which lies in the transverse groove and
appears to be largely responsible for the rotation of the organism,
and a more normal looking one running down the posterior portion
of the longitudinal groove and effecting movement forward.
Nutrition is mainly by photosynthesis, food being stored as either
starch or oil. Not only is a reddish eye-spot frequently present,
but some types have colourless bodies and are saprophytic, while a
few, at least, ingest solid food and so are animal-like in their feeding.
As these are among the organisms that are on the border-line between
animals and plants, they are liable to be claimed also by zoologists.
Reproduction is chiefly effected asexually by the division of an
individual into two dissimilar halves (the cells are often markedly
asymmetric at first), after which each half regenerates the missing half.
Peridinians are very widely distributed in both fresh and salt
waters and may be especially abundant in the ocean. Thus in
arctic seas they tend at some times of the year to outnumber even
the Diatoms and, temporarily, to form the main constituent of the
plankton. Consequently they are an important source of food for
marine animals—including, ultimately, Fishes, Seals, and even the
greatest Whales.
PHAEOPHYCEAE: ‘This large group, commonly called the Brown
Algae or Brown Seaweeds, are characterized by their brown or
olive-green colour which is due to the chloroplasts containing a
special brown pigment in addition to chlorophyll. ‘They are practic-
ally all marine, being among the most abundant and familiar sea-
weeds of temperate and more austral (southern) as well as boreal
(northern) coasts. The thallus is multicellular and usually attached,
but shows a very wide range of different forms, some examples
of which are shown in Fig. 8. Though sometimes slender and
filamentous, the thallus is more often complex. Frequently it is
relatively massive, being differentiated into a disk- or root-like organ
of attachment to tidal rocks or sea-bed objects, and a stem-like part
of varying length and thickness bearing a ribbon- or leaf-like portion.
This last may be branched or unbranched and is usually elongated
and flexible, streaming easily with the current or, in shallow water,
often floating. Such buoyancy is commonly aided by the inclusion
of air bladders, which are usually conspicuous and large enough to
‘pop ’ when trodden upon—as in the familiar Bladder Wrack (Fucus
eee
C3
Lar
36
LEE VNR OUS: GROUPS ORS PLANTS 3
~I
Fic. 8.—Some Brown Algae (Phaeophyceae). A, Ectocarpus, a filamentous type
(X 45); B, Dictyota (x 4); C, Fucus (x 4); D, Agarum (xX vs); E, Chorda
x 4); F, Alaria (x 5); G, Ulopteryx (x is); H, Sargassum (x 3).
vesiculosus). ‘Vypes of Brown Algae living between tide-marks, as
many do, are usually whippy and tough enough to remain uninjured
by the waves. In some of the giant Kelps the ‘fronds’ have a
relatively complex internal structure and may be around 200 feet
in length. (Reports of much greater lengths do not appear to have
been authentic.)
The Brown Algae obtain their food for body-building, growth,
and energy by means of photosynthesis, the raw materials for this,
carbon dioxide and water, being absorbed over the entire surface,
as are also the needed salts, etc., in solution. Reproduction is
effected asexually by fragmentation of the thallus or by liberation
of motile spores (zoospores), or, sexually, by the fusion either of
two similar motile gametes or of dissimilar gametes. When there
are dissimilar gametes one, the male, is motile and small while the
other, the female, is non-motile and relatively large. ‘Thus sexuality
38 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
may here be comparable in several ways with that of most animals.
Many types of Brown Algae have an alternation of sexual and asexual
generations—usually of strikingly different sizes and forms, as in
the case of Pteridophytes (see pp. 55 e¢ seq.).
Although a few simple types occur in fresh water, the vast majority
of Brown Algae live in salt seas or in brackish lagoons and estuaries.
They are abundant in the tropics, but tend to be still more prominent
in colder waters, even persisting to the northernmost arctic shores.
Being often of considerable size, they are commonly the most
conspicuous features of many northern rocky shores between tide-
marks and for some distance below, forming extensive and often
almost pure “ beds’. ‘These may cover and obscure the rocks or
boulders, to which the plants are attached by holdfasts so tough that
they are normally detached only during severe storms. ‘Thereafter
they may be cast up in large piles upon the beach, or float until
they die and ultimately disintegrate. A few special kinds of Brown
Algae, however, seem able to live indefinitely in a detached floating
state, forming extensive masses; of these masses the largest and
most notable characterizes the Sargasso Sea in the western Atlantic
Ocean.
The distribution of the Brown Algae seems to be world-wide
wherever suitable sea-shores are found. Nor is their economic
significance negligible. ‘hus some are extensively harvested for
human food or animal fodder, particularly in eastern Asia, while
their use as manure after being cast up during storms is widespread.
‘The ash obtained by burning certain Kelps and Wracks, particularly,
is still in some places an important source of iodine and potassium.
Finally, owing to their peculiar food-storing and other biochemical
activities, Brown Algae are nowadays an important source of often
unique organic chemicais. An example of these is ‘algin’, the
production of which runs into about a thousand tons annually in
the United States alone.
RHODOPHYCEAE: ‘These are the Red Algae, or Red Seaweeds,
which tend to be more prolific in different species but are generally
less bulky and abundant as individuals than the Brown Algae. ‘This
is especially the case in the seas of temperate and boreal regions,
where Red Algae may be little in evidence. They are characteristic-
ally red or purplish owing to the presence in the chloroplasts of
special pigments besides chlorophyll, although some may be greenish,
bluish, olive, or brown. ‘The thallus is again very various in form
2| THE VARIOUS GROUPS OF PLANTS 39
in the different genera or even species, ranging from filamentous
(very rarely unicellular) to densely branched and coral-like (owing
to encrustation with ‘lime’), and from a flat blackish disk to a ribbon-
shaped or widely expanded ‘frond’. Attachment to the substratum
is by disk-like holdfasts or special filaments ; or the whole plant
body may form a close investment. Many Red Algae are very
intricate and beautiful in form: a range of examples is shown in
Fig. g. ‘The internal structure is peculiar and relatively complex,
though none of the Red Algae approaches in size the larger Brown
Algae.
The nutrition of the Red Algae is much like that of the Phaeo-
phyceae, except that many of the chemical products of photosynthesis
and subsequent metabolism are different ; these include the food-
storage materials, of which a unique starch-like substance is the
chief. ‘The reproduction is remarkable in lacking any self-propelled,
flagellate stage. Sexual reproduction is effected by fertilization in
situ of a large and fixed female cell by a small male one or its dis-
charged contents—in either case carried along, aimlessly as it were,
by an impinging water current. Instead of resulting in the formation
of a new, “ daughter ’ plant, fertilization leads to further development
which results in the formation of special asexual spores called
carpospores. ‘These in the simpler Red Algae germinate to produce
sexual plants ; but in the vast majority of types they give rise instead
to asexual plants producing another kind of asexual spores, called
tetraspores, which in their turn germinate to produce sexual plants.
In such cases there is a regular alternation of a sexual generation
with two asexual ones of which the second is on a separate plant.
The Red Algae are very widespread. Not only do they occur in
fair numbers in the habitats occupied by Brown Algae—with which
they are commonly interspersed even to the extent of frequently
growing superficially on their bodies, as epiphytes—but there are
also a number and range of forms inhabiting cool streams and other
freshwater habitats. Many of the marine types tend to grow in
deeper water than the Brown or Green Algae, being supposedly
adapted through their special pigments to photosynthesize far under
the surface of the water by absorbing the shorter wave-lengths of
light which penetrate to relatively great depths. Red Algae also
tend to be more numerous in warm than in cold seas, although not
a few occur well north in the Arctic. At their best they may dominate
the deeper layers, especially, of marine coastal vegetation. Of the
Red Algae, again, the carbohydrates and carbohydrate derivatives
INTRODUCTION DO PLANT CHOGRAPIIY [CHAP.
D E F
Fic. 9.—Various forms of Red Algae (Rhodophyceae). A, Phyllophora (< about
(x
4): B, Batrachospermum ( 5); C, Grinnellia (x about 4); D, Chondrus about
4); E, Corallopsis (* about 4); F, Polysiphonia ( about $).
2| THE VARIOUS GROUPS OF PLANTS 4I
are used commercially in the production of colloidal substances that
are widely employed for food and in industry. Instances are
‘Carrageen’ or ‘ Irish-moss’, used for food, and agar, which is
of great importance in bacteriological and allied work, though it is
even more extensively used in other connections.
Myxomycetes: These are the Slime-moulds, or Mycetozoa, which,
as the latter name implies, exhibit animal as well as plant character-
istics, being indeed near the border-line of the two kingdoms, though
widely considered as Thallophyta. ‘They are simple organisms
which in the vegetative condition (1.e., when not reproducing)
consist of naked, multinucleate masses of protoplasm termed
‘plasmodia’. ‘These show the animal characteristics of slowly
creeping movement and ingestion of food, and the plant character-
istics of reproduction by spores (which in some genera have cellulose
walls) formed in a special spore-producing organ (sporangium).
The vegetative plasmodium tends to shun the light and to be shape-
less and often several inches in diameter, growing as long as food
is available, though when food is scarce it may form a mere starved
network of living strands. Its outer layer is less liquid than the
inner portion and is usually devoid of nuclei ; the commonly slimy
appearance has led to the name of Slime-moulds. Although chloro-
phyll is lacking, the plasmodium may be variously and often brightly
coloured—most frequently yellow or brown, but sometimes orange,
red, black, or even greenish.
Nutrition of Slime-moulds is primarily saprophytic, the plasmo-
dium living upon a variety of organic materials such as rotting wood
and dead leaves, apparently ingesting tiny particles of these and break-
ing down the complicated carbohydrates in them to simple sugars
which are used as food. Frequently, living bodies such as Bacteria
and fungal spores are ingested ; indeed, Slime-moulds can be grown
experimentally on an exclusive diet of appropriate Bacteria. Fruit-
ing bodies (sporangia) may be made when food becomes scarce ;
these are very various in form in different types, as indicated in
Fig. 10. Often they are gracefully stalked, with rounded or
elongated sporangia consisting of an outer membrane enclosing a
mass of very small uninucleate spores and, frequently, a system of
ramifying tubes. Sometimes almost the whole mass of protoplasm
becomes a single, large, spore-producing structure. ‘Ihe spores are
eventually released by rupture of the outer membrane and are
scattered by the wind. ‘They germinate in water, each spore
42 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
producing one to a few flagellated swarm-cells. ‘These, following a
period of swimming and often of division, become Amoeba-like, and
after feeding and further division behave as gametes, fusing in pairs
to form zygotes. The zygotes may grow each into a single plas-
modium, or numerous zygotes or plasmodia may fuse together, or
one plasmodium may divide into two or more.
Fic. 1o.—Slime-moulds (Myxomycetes). A, Plasmodium of Didymium ( 1);
B, Sporangia of Hemitrichia (* 15); C, Comatricha (* 20); D, Trichamphora
(@<*ro)!
Whereas for spore-formation the plasmodium will usually creep
to a light and airy place, in general Slime-moulds are found on
decaying vegetable matter in moist and shady situations. ‘Thus they
are very widespread in damp woods and thickets, though scarcely
ever forming any appreciable feature of local vegetation. If the
2] THE VARIOUS GROUPS OF PLANTS 43
group be taken in the wide sense as including also those organisms
which cause such diseases as club-root of Cabbages and allied plants
and wasting disease of Eel-grass, it has a considerable economic
nuisance-value. Otherwise its members, however interesting, can
hardly be regarded as doing more than a very minor amount of
scavenging.
Funct: These are the Mushrooms, ‘Toadstools, Moulds, Rusts,
Smuts, Yeasts, etc., and comprise, with the Bacteria, the main
ultimate scavengers of the organic world, besides causing many of
the worst plant diseases. ‘The Fungi are a large and diverse group
of relatively simply organized plants. ‘They are usually composed
of branching tubular filaments (‘ hyphae’, collectively forming the
so-called ‘ mycelium ’) and always lack chlorophyll, though occasion-
ally they may be green in colour. Some are unicellular, and many
others are microscopic though filamentous ; commonly, however,
the filaments are sufficiently numerous or massed to be evident to
the naked eye—usually as a soft whitish investment. ‘hey contain
numerous tiny nuclei and may be divided internally by cross-walls
(septa), or, alternatively, be non-septate. When reproduction is
taking place Fungi may be variously, even very brightly, coloured ;
this is especially the case with some of the larger and often highly
characteristic fruiting bodies, such as (those of) ‘Toadstools and
Puffballs, which can reach a considerable size. Fig. 11 shows some
of these fruiting bodies and other reproductive structures of Fungi,
which exhibit a great diversity of form. In these fruiting bodies the
masses of filaments, instead of being soft and cobwebby as in the
Moulds, are so closely interwoven as to form a solid or even hard
structure of definite organization.
As they lack photosynthetic pigments and are not, alternatively,
chemosynthetic, Fungi have to obtain their food for energy and
body-building by living either parasitically (on or in other living
organisms) or saprophytically (by the breakdown of dead organic
materials). As parasites they are the cause of numerous and often
devastating diseases, especially of plants, and as saprophytes they
cause widespread decay and effect a large proportion of the breaking
down of elaborated materials such as leaf-mould. Without such
breakdown and return of the raw materials into circulation, life on
earth would be brought ultimately to a virtual standstill, or even
cease altogether—hence the vital significance of Fungi and Bacteria
as scavengers. Animal characteristics exhibited by Fungi include
44
DHE VARTOUS GROUPS: OF PLANTS 45
Wi ! VD. \
WN
Fic. 11.—Some Fungi. A, cells of Yeast (Saccharomyces) budding actively
Cx 960); B, Diagram of Mucor mucedo, a common Mould, showing the mycelium
growing symmetrically from a central spore, and dev eloping sporangia, successive
stages of which are marked a, b, c, (* about 20); C, Puftballs (Lycoperdon sp.)
which have opened at the top (x 3); D, Stereum affine (x 14); E, a Stinkhorn,
Ithyphallus (< 4); F, Morel (Morchella) (* %); G, Auricularia (* 1); H, the
Deadly Amanita (Amanita phalloides), a Gill Fungus (* 4); I, Boletus, a stalked
Pore Fungus (»
the commonly chitinous cell-walls and the storage of food mainly
as glycogen. ‘Their reproduction may be effected vegetatively by
separation and subsequent development of part of the mass of
filaments, and asexually by spores which are usually of very small
size and produced in enormous numbers (sometimes millions of
millions by a single fruiting body). In some of the simpler Fungi
these asexual spores are swimming zoospores ; usually, however,
they are non-motile and are enclosed in a more or less resistant
wall. Various sexual processes occur, usually involving the fusion
of unlike gametes, gametangia, or hyphae, and resulting in the
production of resting or airborne spores. ‘The ‘ budding’ practised
by Yeasts is another effective mode of vegetative propagation (see
Fig. 11, A, above ; the formation of spores internally is seen below).
Fungi find habitats for existence almost everywhere there are
either living organisms to parasitize or dead and decaying organic
materials to attack. Many are aquatic, including marine, and some
grow actively even in the absence of free oxygen. ‘The other
familiar habitats are soils, dung, and various foods, fabrics, and
wooden or other structures, which Fungi frequently cause to rot.
Thus they occur throughout the world wherever life is possible
and they can find materials upon (or in) which to grow. Yet as
c
40 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
actual components of vegetation their role is usually minor, unless
it be very locally and temporarily when food supplies are plentiful.
However, in economic connections their importance is vast and
multifarious : the frequency with which they cause diseases, especi-
ally of plants, has already been referred to. Such diseases result in
many hundreds of millions of dollars’ worth of damage to crops
yearly in North America alone. Also explained above is their
significance as scavengers, returning the products of organic break-
down to the air and soil in simple forms that can be absorbed and
used again by green plants. Many other saprophytic Fungi are
outstanding nuisances to Man in causing spoilage of food and
destruction of fabrics, timber, and so forth. Yet others are valuable
in positive ways, examples being the activities of Yeasts, which are
employed in the making of wines, beer, and bread, and the use
of Mushrooms and ‘Toadstools as human food. ‘There are also the
‘industrial’ Fungi that provide valuable sources of certain food
proteins and vitamins, and the ‘ medicinal’ ones that provide such
antibiotics as penicillin. Finally, Fungi play a significant role in
the nutrition of many higher plants—including forest trees, in or
upon whose roots they live and form mycorrhizas.!
The above nine groups, which we have treated as classes, are
considered by many authorities to be subdivisions (subphyla) or,
especially in some instance, full divisions (phyla). ‘The same is true
of a few other, smaller groups which are of very little importance
phytogeographically, or as components of vegetation, and which we
are accordingly ignoring.
LicHeNrs: ‘These are the Lichens—peculiar dual organisms
produced by the intimate association of two plants, a Fungus and
an Alga or a Schizophyte, and accordingly belonging to different
groups. ‘They seem best treated as a separate class or subdivision
of the ‘Thallophyta. Such a ‘ living together” for mutual benefit
is termed a symbiosis, and of this Lichens afford the great example,
though mycorrhizas (see above) are another. In the formation of
Lichens the Fungus usually forms a tough, often leathery, invest-
ment, the algal or schizophyte cells or filaments being interspersed
or grouped within, most typically forming a layer near the upper
surface. Lichens are often luxuriant and may live for centuries, the
symbiosis being evidently a mutually beneficial relationship in that
' For an up-to-date account of this intriguing subject, see Dr. J. L. Harley’s
The Biology of Mycorrhiza (Leonard Hill, London, pp. xiv + 233, 1959).
2| THE VARIOUS GROUPS OF PLANTS 47
the ‘algal’ element is enabled, by the protection afforded by the
fungal envelope, to live in dry and exposed situations where other-
wise it could not exist, while the Fungus derives food which results
from the photosynthetic activity of its partner in places where
otherwise none would be available.
The ‘ algal’ elements in Lichens may be members of either the
Chlorophyceae or the Cyanophyceae and, like the Fungi concerned,
are usually definitely identifiable. With the varying combinations
involved, as well as differing heritages, habitats, and growth
tendencies, a vast array of different forms of Lichens result, though
their growth tends to be very slow. In size, Lichens vary from
minute to some which are whole metres in diameter. In colour
they may be of almost every conceivable shade, being often of
brilliant hue ; on the other hand a great many are a dull greenish-
grey, as a result of combination of the colours of the components.
The main groups of forms are (1) ‘ crustose’ (crustaceous), forming
a thin crust over (or sometimes mainly beneath the surface of) the
rock or other material on which they grow, (2) ‘ foliose’, being
more or less prostrate and flat, with leaf-like lobes, and (3) ‘ fruticose ’,
being upgrowing, branched, and often bush-like, or pendulous from
the branches of trees. A range of types is shown in Fig. 12.
The nutrition of Lichens is primarily by the photosynthesis of
their ‘ algal’ components, in which connection the fungal element
may in a sense be considered parasitic. Vegetative reproduction
is by fragmentation of the plant body, especially when this becomes
old and decrepit and liable to break down, or by special structures
termed soredia, which are groups of fungal filaments interspersed
with ‘algal’ cells. ‘These soredia become separated from the
parent, being often produced in large numbers, and blow about
easily. Sexual reproduction is confined to the fungous partner and
follows its particular pattern, usually involving the formation of
spores in a special structure, following a fusion of gametes. On
germination of these spores, new lichen plants are formed only if
some fragment of the appropriate Alga or Schizophyte is present.
Lichens occur plentifully in a great variety of habitats, usually
of dryish nature. Thus they favour the trunks and branches of
trees, exposed rocks, and bare ground provided the surface is stable.
In these and other situations they are to be found practically every-
where on land, though shunning large cities owing to their sensitivity
to fumes. Although particularly characteristic of high mountain
peaks and of arctic and antarctic regions, where they may dominate
48 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
or virtually constitute the vegetation over considerable areas of the
drier habitats, they are also found as minor—but sometimes con-
spicuous—constituents of the vegetation in temperate and even
tropical regions. ‘They are especially significant as pioneer colonists
on bare areas, and in boreal regions where they afford much of the
winter food of wild Reindeer and Caribou. Economically they are
important chiefly in the feeding of domesticated Reindeer, on which
Fic. 12.—Various forms of Lichens (Lichenes). A, Usnea barbata, a branched
epiphytic type (= 1); B, Haematomma puniceum, a crustaceous type (x 1); C,
Cladoma verticillata, a terrestrial fruticose type (= 4); D, Lobaria pulmonaria,
a foliose type ( 4).
2| THE VARIOUS GROUPS OF PLANTS 49
whole tribes of northern peoples depend for the wherewithal of life.
Their plentiful storage of a starch-like carbohydrate also makes some
of them, such as ‘ Iceland-moss ’, useful for human food, though
most are highly unpalatable. ‘The use of certain Lichens as sources
of attractive dyes has greatly diminished with the chemical advances
of recent years, but some dyes, such as litmus, are still widely
obtained from them.
BRYOPHYTA
HeEpATICAE: These, the Liverworts, are a smallish class of usually
green photosynthetic plants of relatively small size, though always
multicellular and visible to the unaided eye. ‘There are two main
forms: the thalloid, having a thin and more or less flat, prostrate
plant bedy which tends to branch frequently and equally, and the
‘leafy ’, consisting of a creeping central axis up to a few inches
long, provided with delicate leaf-like expansions. ‘These last are
only one cell thick and lack a midrib ; they are usually arranged in
two rows, lying on either side of the often prostrate axis, with
commonly a third row of smaller lobes lying along the under surface.
Noticeable on the lower surface, especially of the thalloid types,
are numerous thin root-like ‘ rhizoids ’, primarily serving the pur-
pose of attachment to the ground or other material on which the
plant grows. Often there are air-chambers or other special features
on the upper surface. ‘The main photosynthesizing plants are the
gametophytes, so termed because they produce the gametes ; they
comprise the gametophytic generation which alternates regularly
with the spore-producing (sporophytic) one to complete the life-cycle.
The gametes are formed in minute male and female organs, the
male spermatozoids swimming freely to fertilize the passive and
well-protected female ‘eggs’, from which, after fertilization, the
sporophytes develop. In most types these last consist of an absorb-
ing foot, a more’ or less elongated stalk, and a roundish capsule
(sporangium) in which the microscopic spores are produced in
considerable numbers: ‘The foot is embedded in the tissue of the
gametophyte, from which it absorbs nourishment. ‘This is passed
on to the rest of the sporophyte, which in most types lacks chlorophyll
and is thus parasitic on the gametophyte. In some simple forms
there is no foot or stalk, the sporangium being embedded in the
gametophyte, and in one group the sporophyte is photosynthetic
and grows continuously from near the base. But in any case there
50 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
is a regular alternation of gametophytic and sporophytic generations,
such as we shall see later in all normal higher plants. ‘Though the
gametophyte tends to be more evident and ‘dominant’, these
generations in the Liverworts have no independent existence, and
consequently their dual significance is somewhat difficult to grasp.
Both generations are depicted in each of the different types of
Liverworts shown in Fig. 13.
Fic. 13.—Types of Liverworts (Hepaticae). A, Marchantia, a thalloid type,
showing female plant on left and male on right (* 14); B, C, leafy Liverworts
(B, x 4; C, Xv.
2] THE VARIOUS GROUPS OF PLANTS SI
The spores, given suitable conditions after liberation, can germinate
to form fresh gametophytes, so completing the life-cycle. They
usually afford the main means of multiplication, although vegetative
methods, such as fragmentation of the gametophyte or formation by
it of special bud-like bodies called gemmae, may also be effective in
some cases. Liverworts chiefly inhabit damp places, such as
stream-banks, sheltered nooks, and the boles of trees and decaying
fallen branches in shady forests. ‘They also grow on moist soil and
in tufts of Mosses, etc., and are practically world-wide in distribution
on land. Many grow on the leaves of other plants in the tropics,
and some in freshwater habitats. With the exception of a very few
which are saprophytic, the nutrition of the gametophyte is primarily
by photosynthesis ; on this the sporophyte is, as we have seen, in
most instances parasitically dependent. Although they are some-
times important as pioneers on bare ground and may even form
more or less ‘ pure’ patches some yards in extent, Liverworts in
general play only a minor role as ‘fillers’ in higher vegetation.
Nor have they any particular value for Man, except sometimes as
aides in the binding and consolidation of eroding surfaces.
Musc1: ‘These are the Mosses, which, in accordance with their
near relationship, are in many ways closely comparable with Liver-
worts. ‘lhe Mosses are all rather small plants in which the gameto-
phyte, during the greater part (but by no means all) of its life,
consists of a more or less upright stem bearing small leaves. ‘These
leaves, unlike their counterparts in leafy Liverworts, usually have
midribs and are spirally arranged on the stem, which may vary
from a fraction of an inch to perhaps a foot in length. The midribs
contain elongated cells, and a central strand in the stem usually
contains similar elongated cells which are supposed to conduct water
and nutrients. ‘True roots are absent, but the base of the stem in
most types is plentifully supplied with anchoring rhizoids. In one
characteristic and important group, known as Bog-mosses or Peat-
mosses, the leaf is not only peculiar in lacking a midrib but unique
in consisting of a network of small living cells separating large dead
ones which are transparent and perforated, soaking up and holding
water with extraordinary efficiency—hence the water-retaining
capacity of many bogs which are largely formed by such plants.
On the gametophyte are borne the minute male and female organs,
commonly in groups that are made evident by the modification of
the surrounding leaves, and either on the same (hermaphrodite)
52 INTRODUCTION TO CP MAND 1G BO GRAPE ys [ CHAP.
plant or, more often, on separate (male and female) individuals.
Fertilization is again by a motile spermatozoid which, when water
is present, swims to the passive and protected egg. ‘The body
formed by this sexual fusion develops into the sporophyte which,
when mature, consists of an absorptive foot, a usually long stalk,
and a more or less complicated and characteristic capsule. Fig. 14
shows both the gametophyte and sporophyte of different types of
Mosses, for here again the two generations are unseparated, forming
one continuous plant body, the sporophyte being at first parasitic
on the gametophyte but later becoming at least partly self-supporting
through its possession of chlorophyll.
The spores, formed in the capsule and liberated often by some
complicated mechanism that works only in dry air (which is more
beneficial than moist air for their further dispersal), are again the
main mode of multiplication. But instead of growing directly into
a typical new gametophyte as in the Liverworts, they develop in the
Mosses, on germination, into an extra stage known as a ‘ protonema’.
This is an independent, cellular plant containing chlorophyll and
manufacturing its own food. It is filamentous and branched in
most types but in some it is thalloid, like the gametophyte of many
Liverworts. On the protonema develop lateral buds which grow
2| LES VARTOUS GROURS OR) PbAN TS 53
We
(a
} i/ PB
Fic. 14.—Some Mosses (Musci). A, gametophyte of a Moss, showing a group
of male and female organs at the top ( 7); B, a typical species of Sphagnum,
the Bog-mosses ( about 4); C, Atrichum (Catharinea), showing at the base the
masses of rhizoids and protonemal filaments from which grow the leafy gameto-
phtyes bearing, above, the sporophytes ( 2).
into the main, leafy gametophytes ; thus is the life-cycle completed.
Mosses can also propagate by gemmae and multiply by fragmenta-
tion, and they are remarkable for their power to remain alive after
long periods of desiccation. ‘Their nutrition is again primarily by
photosynthesis, starch being stored, although some appear to be
partially saprophytic.
54 INTRODUCTION HO VP VAN T \GEOG RAR HY [CHAP.
Mosses grow on a wide variety of exposed surfaces—particularly,
but by no means entirely, in damp situations. ‘Thus they occur
plentifully on the ground, on tree-trunks in moist woodlands, on
decaying wood, on old brick and stone structures, on rocks and
boulders, and in both still and running fresh water. ‘They are also
common as subsidiary forms in higher vegetation. Mosses are
more numerous in species and individuals than Liverworts, and
tend to cover considerably larger areas and to be far more conspicuous
—especially in arctic and boreal regions, and high up on mountains.
They are relatively important as components of natural vegetation
and frequently dominate substantial areas especially of bogs, whose
water-level they often raise. In so doing they may even destroy
tracts of forest and make terrain difficult to traverse, thus affecting
the economy of Man. On the positive side they are important as
producers of peat, which often consists largely of the remains of
Bog-mosses, and as stabilizers of sand-dunes and other erosive
systems whose surfaces they help to bind. Peat is used extensively
as fuel and in the improvement of soils. Owing to their insulating
properties when dry, Bog-mosses are also used in construction work
and packaging, and, owing to their absorptive and water-retaining
powers, for surgical dressings and the transport of living plants.
The above groups all belong to the non-vascular cryptogams and
lead up to the vascular plants (Vasculares, or 'Tracheophytes), to
which all the remaining groups belong. ‘The vascular plants are
those possessing a vascular system, and include all the most advanced,
or evolutionarily ‘ higher’, types; these are generally the largest
and most dominant on land. A vascular system consists mainly of
special tracts (bundles) of elongated wood (xylem) and ‘bast’
(phloem) cells forming a continuous system linking all the main
parts of the plant. Its chief manifestations are such bundles in the
stem and veins in the leaf. ‘The primary purpose of such a vascular
system is the conduction of water, mineral salts, and elaborated food
materials to portions of the plant where they are needed, so that it
is partly comparable with the blood and lymphatic systems of higher
animals. Its secondary function is to give mechanical support,
especially in the ‘ secondarily thickened’ older stems of perennial
plants which consist largely of vascular tissues and have the whole
crown to support. An account of the general make-up of a vascular
plant was given in Chapter I, with some indication of how ‘ The
Ideal Plant’ lives and grows (pp. 12-14).
2] THE VARIOUS GROUPS OF PLANTS 55
PTERIDOPHYTA
EQUISETINEAE: ‘These are the Horsetails, of which the living
examples are mere depauperated relics of a group which was much
more important in earlier geological ages, when it included larger
tree-like forms. ‘Those remaining belong to a single genus of
perennial, herbaceous plants consisting of an underground stem
(rhizome) beset with fibrous roots, and sending up usually erect and
SN ye
. WA,
;
Ni
Y
B
Fic. 15.—Field Horsetail (Equisetum arvense agg.). A, sporophyte with fertile
branch (on right) and two young sterile branches ( 3); B, mature sterile branch
(Ca):
stiff, grooved aerial stems that are generally slender but hollow
and bear at the nodes (parts of the stem where leaves arise) close
whorls of rudimentary scale-leaves. ‘The stem is commonly green
and photosynthetic, rarely more than a few feet high, and either
unbranched or, more often, bears whorls of slender branches (which
may themselves be much-branched) in the axils of the scale-leaves.
Fig. 15 shows a characteristic modern Horsetail. Such plants
56 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
practise the mode of nutrition of normal vascular types, namely,
photosynthesis together with absorption of the necessary elements
(usually in simple compounds) from the air and soil, or, when the
plants are aquatic, in solution with the water in which they live.
In common with almost all of the plants remaining to be described,
the main food-storage substance is starch.
These relatively large plants are the sporophytes, which comprise
the main, dominant generation in this and all the remaining groups.
The spores are produced in special organs (sporangia) developed on
short outgrowths which are compacted into distinct, cone-like
‘strobili’ that are commonly developed at the tops of the stems.
The spores are all alike but unique in having attached to them at
one point four slender processes that bend or straighten rapidly with
changes in atmospheric humidity, consequently often causing the
spores to move. On germination the spores produce a gametophytic
body called a prothallus. ‘This is small and possessed of rhizoids,
irregularly branched, green and photosynthetic, and usually each
one produces organs of only a single sex. After fertilization by the
peculiar spiral, multiflagellate swimming spermatozoid, the egg
develops 7m situ into a young sporophyte plant which soon becomes
independent, so completing the life-cycle.
In spite of the limited number and variational range of living
forms, Horsetails occupy a considerable array of land and freshwater
habitats and are geographically very widespread, extending from the
tropics to the highest latitudes of land. ‘They especially favour
marshes, lakesides, and damp sand or silt, which they may colonize
aggressively ; but as components of more mature vegetation they
are very minor. The outer part of the stem is often heavily
impregnated with silica, which has led to some species being widely
used for scouring pots and pans—hence their alternative name of
Scouring-rushes.
LycopopINEaE: ‘These include the living Club-mosses (Ground-
pines), Spike-mosses, and Quillworts, as well as numerous huge
trees of earlier geological ages that are now known only as fossils.
The present-day representatives are lowly and herbaceous, differ-
entiated into stem, roots, and leaves, the last being numerous and
usually small as well as simple. The stems are rarely more than a
matter of inches in height or feet in length. Most species are
evergreen, overwintering without the aerial parts dying back. ‘The
stems are trailing or upright and usually branched, or, in the Quill-
2] THE VARIOUS GROUPS OF PLANTS 57
worts, unbranched and extremely short ; these last, peculiar types,
also have relatively large, elongated leaves. In them and some other
members, the leaf has a characteristic tongue-like appendage (ligule)
near its base. Fig. 16, showing examples of some types from this
group, indicates the range of existing forms. ‘These are the main,
sporophytic plants, and they produce spores in sporangia borne
singly in the axils of special and usually modified leaves (sporophylls)
that either occur in groups at intervals along the stem or, more
often, form terminal cones. In some types the spores are all of
one kind, but, in others, different sporangia produce numerous
small ‘microspores’ or relatively few (occasionally one) large
‘ megaspores ’.
On germination the spores produce prothalli, representing the
gametophyte generation. ‘These are always small and relatively
obscure bodies. However, they vary in different types from lobed
photosynthetic ones or underground non-green tuberous ones living
saprophytically with the aid of mycorrhizas (in either case usually
producing both male and female organs on the same prothallus), to
limited growths largely enclosed within the old spore-wall in those
instances where spores of two different sizes occur. In such instances
the megaspores each produce a few female organs and the tiny
microspores only a single male organ, the ‘ prothalli ’ being dependent
upon the food stored earlier in the spore by the sporophyte. Follow-
ing fertilization by the spirally-shaped, swimming spermatozoid, the
egg develops im situ into a new sporophyte plant which is photo-
synthetic and independent from an early stage, so reversing the
situation met in Bryophyta. Some Club-mosses produce bulbils or
gemmae which are effective in multiplying the sporophyte. More-
over, vegetative propagation following fragmentation of large old
plants often occurs, at least among the longer trailing types.
‘The Lycopodineae are fairly numerous in species and wide in their
habitat tolerance. ‘Though most characteristic of shady woods from
tropical to boreal regions, or, in the case of Quillworts, of the beds
of freshwater lakes, they also occur in more exposed heaths and
marshes and, in such situations, range far north in the Arctic.
Nevertheless, as components of vegetation, except very locally and
then usually far beneath the dominants, or occasionally in deserts
where little else grows, they are so very minor as to be almost
negligible. Very different was the position of some of their fossil
relatives, which, as we shall see in Chapter V, apparently dominated
whole forests in much earlier geological ages, and greatly contributed
SEE
WEES
SS
Fic. 16.—Types of living Lycopodineae.
k), all Club-mosses (Lycopodium species)
58
,
A (>
1D) ((
E (= about 3), a Quillwort (Jsoetes).
3), BO
about 4), C (> about
1), a Spike-moss (Selaginella) ;
THE VARIOUS GROUPS OF PLANTS 59
to the formation of coal. Otherwise their economic significance is
very limited : some warmth-loving species are used as pot-plants,
and trailing Club-mosses are made into Christmas-wreaths (hence
another name, ‘ Christmas-greens ’), while the minute spores of
members of the same genus are so highly inflammable owing to
stored oil that they can be used to produce ‘ stage lightning’. ‘They
are still employed in dusting operations where a fine powder is
required, and for demonstrating sound-waves in physics.
FILICINEAE: ‘These are the Ferns, and include the majority of
living Pteridophyta as well as some extinct forms. ‘They are
perennial plants, sometimes small and moss-like but usually of at
least substantial size. ‘The stems range from creeping and slender
to erect and stout, and from subterranean to aerial or occasionally
aquatic ; in ‘Tree-ferns, the often massive, erect aerial stems may
be several yards high. ‘There are usually plentiful fibrous roots
below, and, above, leaves (fronds) that characteristically are large
and compound, composed of more or less numerous segments.
Occasionally, however, the leaves are small and simple ; indeed the
Ferns are very varied in form, as may be seen from Fig. 17, which
shows a range of different types. ‘These, of course, are all sporo-
phytes, the gametophytes being always small and insignificant.
The spores are formed in sporangia which are commonly borne
in groups upon or partly within the lower surface of the ordinary
leaves, though in many cases the spore-producing leaves, or parts
of leaves, are modified—sometimes so drastically that they are
scarcely recognizable as leaf members. In most types the spores are
all of one kind and produce on germination a filamentous, or more
often a flat, green prothallus rather like a small unbranched thalloid
Liverwort, anchored to the ground by rhizoids and bearing both
male and female organs, though a few types have a subterranean
and saprophytic prothallus. However, in the small and usually
aquatic group known as Water-ferns, which have slender stems and
small and sometimes very simple leaves, separate microspores and
megaspores are formed, which produce male and female organs,
respectively, on germination. Following fertilization by the swim-
ming, corkscrew-like spermatozoid, the egg develops into a fresh
sporophyte plant which soon becomes photosynthetic and inde-
pendent. This is its primary mode of nutrition. ‘Thus the life-
cycle, involving as usual in these vascular land-plants an alternation
of sexual and asexual generations, is completed. ‘The sporophytes
THE VARIOUS GROUPS OF PLANTS 61
\ /
Sy / Y/ If
— tA ) ips
\ AA \
~ aN BAI
7 WAL
5
Fic. 17.—Various types of Ferns (Filicineae). A, Shield-fern (Dryopteris)
(< 3); B, a group of Tree-ferns, Cyathea (scale indicated by man in foreground);
C, Pteris longifolia (= +s); D, Moonwort (Botrychium) (< about 3); E, Common
Adder’s-tongue (Ophioglossum vulgatum) ( about 3); F, a Water-fern (Marsilea)
(x about 3).
of many Ferns also propagate vegetatively—for example, when the
old parts of types with branching rootstocks die off and fragmentation
results, or through the growth, after detachment, of special bulbils
or plantlets that develop on the leaves of some species.
62 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
Although mainly favouring shady and humid situations, Ferns
occupy a wide array of habitats ranging from dryish heaths and
crevices of rocks to wet mud and open fresh water, and from forest
floors to quite lofty branches or crutches of trees. ‘They are plentiful
especially in the damper tropical and temperate regions and reach
the southern portion of the Arctic in fair array, while a very few
species persist northwards to almost the highest latitudes of land.
Vegetationally they are chiefly of importance as subsidiaries in humid
habitats from the tropics northwards and southwards to temperate
regions, though arborescent types may contribute substantially to
the forest, for example in New Zealand. North of the temperate
regions in the northern hemisphere they tend to become scarcer,
being often absent from dry areas and almost negligible as com-
ponents of vegetation in most boreal and arctic regions. ‘Their
economic significance is chiefly aesthetic and horticultural ; many
are among the most beautiful of living things, and consequently
fern-growing is a popular hobby. Some are minor constituents of
animal fodder or may be employed as food by humans or cut and
dried for litter, but probably far outweighing these uses is the
nuisance-value of others—particularly the common Bracken, which
is a pestilential weed that widely overgrows pastures and young
tree plantations.
SPERMATOPHYTA
GYMNOSPERMAE: ‘This, the more primitive of the two classes (by
some considered subdivisions) of the Spermatophyta (Seed-plants),
includes the Conifers, Cycads, and Gnetales among living groups,
and many extinct fossil representatives that were of great importance
as components of vegetation in earlier geological ages (see Chapter V).
The Seed-plants are the main phylum existing on land today,
providing the vast majority of dominant species and the great
preponderance of vegetation in most situations. ‘They are, of
course, vascular plants, being, briefly speaking, those which bear
seeds. A seed is an organ peculiar to this ‘ highest’ division of the
plant kingdom and is the product of a fertilized ovule, consisting
of an embryo that is often embedded in a nutritive tissue and is
normally enclosed by one or two protective seed-coats. The large,
complicated visible plant is always the sporophyte generation, the
usually microscopic female gametophyte being embedded within it
and never having a separate existence, while the male gametophyte
is even more reduced.
2| THE VARIOUS GROUPS OF PLANTS 63
The Gymnosperms are distinguished by having ovules which are
borne ‘naked’ on leaf-like organs (cone-scales). Although not
enclosed in an ovary, the ovules are often well protected by the
mutual contact of the cone-scales or by the development of other
special structures. For fertilization a small gap is left or an opening
occurs, and for liberation of the ripe seeds the cone-scales or other
protective structures simply spread apart. ‘The living Gymnosperms
are all perennial woody plants, ranging from small shrubs to the
very largest of trees. ‘Their internal structure is generally different
from, and more primitive than, that of the other, remaining group ;
and they are far less numerous in species and individuals, tending
to be less widely dominant. ‘They show, however, effective internal
differentiation and external division of labour, allowing conduction
to the tops of the world’s tallest trees (Coastal Redwoods, which
reach 364 feet in height!), and, externally, involving highly specialized
roots, stems, leaves, and intricate organs of reproduction. ‘The
roots are generally much-branched and fibrous, the stems essentially
columnar at least below (though they may vary from tuberous in
some Cycads to much-branched and shrubby, especially in Ephedras),
while the leaves vary from very large and subdivided in Cycads to
small and needle-like in many Conifers. ‘The plants most commonly
grow as trees, the leaves being usually evergreen, lasting for several
years. Examples of Gymnosperms are shown on pages 64-67.
The structures that ultimately produce the male and female
gametes are borne on modified leaves, usually aggregated into terminal
cones of one sex and developed either on the same (hermaphrodite)
or on different (male and female) plants. "The female gametophyte
consists of a mass of cells developing within the megaspore and,
like it, remaining hidden in the sporophyte. In this gametophyte
develops the egg, which, with the immediately surrounding envelopes
comprising the ovule, forms the seed after fertilization. Sexual
fusion is effected by male gametes which may be motile spermato-
zoids or merely passive nuclei, and which are normally enclosed
within a pollen tube growing towards the egg; these gametes are
formed by a microscopic and vestigial male prothallus developed on
germination of the microspore. ‘This last is the pollen grain,
produced in great numbers in special organs, the pollen sacs, and
each with some infinitesimal chance of being carried by the wind
to the vicinity of a receptive ovule. The seed contains the embryo
sporophyte and, after liberation from the parent and given suitable
1 This goes for living trees, though some Eucalypts in south-eastern Australia
appear to have exceeded this figure in the recent past—see note on p. 379.
64 IN LRODUE TLLON: TO} 2yAN NG OIG IRAP Eine [CHAP,
conditions, grows into the new generation, so completing the life-
cycle. ‘Thus, in such sexual reproduction-cycles, the chances of
dispersal of individuals are limited to the seed, although the pollen
grains can transport heritable characters. Nor do Gymnosperms
normally possess effective modes of asexual reproduction, except for
such vegetative methods as ‘ layering’, which involves the rooting
ml Ga,
PS
(See p. 67.)
2| THE VARIOUS GROUPS OF PLANTS 65
of lateral shoots or branches and their separate growth on segregation
from, or death of, the parent.
Among the Gymnosperms, the usually stocky and unbranched
Cycads, with their palm-like crown of huge compound leaves, are
chiefly characteristic of the drier areas of the tropics and subtropics,
as are the much-branched, bushy Ephedras, though their broad-
leafed relatives, the Gnetums, favour moist tropical habitats. Far
more numerous, important, and widespread, however, are the
Conifers, different members of which occupy almost the complete
range of land habitats from swamps to dry sands. ‘They dominate
vast areas of temperate and boreal forests, many of which they
virtually compose, and constitute the northern limit of arborescent
growth practically around the top of the globe, as well as, often,
(See p. 67.)
66 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
the altitudinal limit on mountains. Altogether they probably play
a role in constituting higher vegetation on land, which is second only
to that of the other, last remaining group which we shall discuss
next. Moreover, their economic importance is in keeping with such
a position, for besides affording shelter and greatly affecting Man’s
environment, they provide him with a large proportion of his timber,
pulpwood, turpentine, firewood, and numerous other commodities,
besides minor foods and other items of everyday or local life too
numerous to mention.
(See p. 67.)
2] THE VARIOUS GROUPS OF PLANTS 67
Fic. 18.—Some examples of Gymnosperms (Gymnospermae). A, a Cycad
(Cycas rumphiz) (x 26)} B, male twig of Ephedra (< 3), a member of the Gnetales;
C, twig of typical Conifer (Pinus insularis), showing female cones of the three
latest years (x 4); D, Coastal Redwood (Sequoia sempervirens), another Conifer
(scale indicated by standing man). (Phot. W. 5S. Cooper.)
ANGIOSPERMAE: "These, the flowering plants, are evolutionarily
the highest, and vegetationally and economically the most important,
of all groups in the world today. They are seed-plants, but dis-
tinguished from Gymnosperms by having their seeds enclosed in an
ovary—a variously shaped, but commonly roundish, vessel formed by
the enclosing ‘ carpel’ (or ‘ fused’ group of two or more carpels)
68 INTRODUCTION TO PLANT GEOGRAPHY [CHAP
which produces the seed or seeds internally. After fertilization, the
ovary becomes the fruit. ‘The Angiosperms are also generally to
be distinguished from the Gymnosperms by their internal structure
and by their possession of flowers, which are specialized short
reproductive shoots bearing typically four different sets of organs
in close proximity. ‘These are (1) on the outside the sepals, which
are usually leaf-like and protective in the bud stage, and inside of
which come (2) the petals, which are commonly attractive in colour,
form, and odour ; then (3) the stamens, producing the pollen grains
(microspores), and finally (4) the one or more carpels lying in the
centre and producing the ovule or ovules.
Various types of Angiosperms are so entirely familiar to us all
that it would be superfluous to illustrate them here. Instead, Fig. 19
(pp. 70-71) shows a diagrammatic representation of a dicotyledon-
ous (see p. 73) Angiosperm flower, and, in addition, sections of
stems of monocotyledonous (see p. 73) and dicotyledonous plants
to indicate the disposition and something of the appearance of the
vascular bundles when magnified. Many examples of Angiosperms
will be found illustrated in the chapters on vegetation (especially
Chapters XII-XIV), and of their fruits and seeds there are accounts
in Chapter IV, whilst the two main groups of them, Monocotyledons
and Dicotyledons, are distinguished in the last paragraph of the
present chapter. Familiar Angiosperms include all of our common
agricultural and garden crops, all Grasses and other flowering herbs
whether annual or perennial, and almost all broad-leafed trees and
shrubs such as Oaks, Elms, Beeches, Maples, Birches, Poplars, and
Willows. ‘There is consequently no need to emphasize that each
consists primarily of roots, stem or stems, and leaves, nor to describe
the form of these organs.
Sexual reproduction is the object of the flowers and with it the
stamens and carpels are particularly concerned. Frequently the
stamens and carpels are in different flowers or even on different
plants, in which event the individuals are unisexual. In any case
the function of the stamens is to produce the pollen grains, usually
in large numbers, for transference (by such agencies as wind, animals,
or water) to the stigma, which is the receptive part of the carpel.
Within this last the ovules are formed, each containing a female
gamete. ‘The pollen grains germinate on the stigma, sending out
pollen tubes which come to contain the male gametes and usually
grow through the underlying tissue, deriving nourishment as they
go. ‘Vhis growth of pollen tubes normally goes on until the tip of
2| THE VARTOUS, GROUPS OF PLANTS 69
one reaches the immediate proximity of the female gamete in the
ovule, there discharging the male gametes, one of which effects sexual
fusion. From the fertilized ovule the seed develops, contained in
the ovary which commonly becomes the fruit. Seeds and fruits,
though often alike in appearance, are technically very different and
should always be distinguished. Many fruits are attractive to
animals, or are winged or plumed to be caught in the wind, or
buoyant to float on water, being dispersed by these agents with the
seed inside. However, in those fruits which contain more than one
seed, it is more effective to liberate the seeds and have these individu-
ally attractive or appendaged for separate transportation. In any
case the embryo within, if still alive and given the right conditions,
germinates to form a new sporophyte plant which soon becomes
independent of any stored food-reserve and so completes the life-
cycle. Such is the general story—though there are all manner of
variations and even exceptions—in which it should be noted that
the gametophytes, both male and female, are microscopic, vestigial,
and entirely dependent on the sporophyte for food, the female being
so embedded therein that to all appearances there is only the one,
sporophyte generation.
Asexual reproduction is extremely common and widespread in
Angiosperms. Not only are there numerous species which are
habitually parthenogenetic, the ovules developing successfully with-
out fertilization, but there is a wide array of vegetative means of
propagation in nature quite apart from those commonly practised
by Man. Familiar examples are the underground stems (rhizomes
and rootstocks) as well as suckers and overground runners and stolons
of many plants which, rooting at the nodes, constitute daughter
individuals on severance from or death of the parent. Also familiar
is the fragmentation of many water or colonial plants, as well as
separation of bulbs and tubers, while the production of bulbils or
young plantlets in place of flowers in many species, or in the axils
or even on the margins of leaves in others, affords further ready
means of vegetative propagation. Indeed, so common and effective
are these or other asexual methods, that many plants resort to them
habitually, and frequently are enabled by employing them to
reproduce and live indefinitely in regions where climatic or other
conditions prevent the ripening of fruit or even successful flowering.
The mode of nutrition of most flowering plants is primarily by
their own photosynthetic activity, which takes place mainly in their
green leaves. Here, with the aid of chlorophyll in the light, they
7O
INTRODUCTION TO PLANT GEOGRAPHY
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Fic. 19.—Features of Angiosperms. A, diagram of section of flower at time of
fertilization, showing pollen grains (greatly magnified) germinating on the stigma
and pollen tubes growing down towards ovules (seen in centre). Ranged around
are the stamens, the large attractive petals, and the sepals which protected them
when in bud. B, cross section of stem of a monocotyledonous Angiosperm (Zea
mays) ( about 30); the dark oval areas are sections of vascular bundles which
are characteristically scattered (instead of being disposed in a more or less peri-
pheral ring, as in most Dicotyledons). C, diagram of portion of stem of a dicoty-
ledonous Angiosperm dissected in cross, radial, and tangential sections to show
the various tissues of which it is composed ( about 80, but not all parts to precisely
same scale).
71
72 IN GRODUC TION LO EAN A GEO GRAPE, [CHAP.
build up simple raw materials to complex carbohydrates. But
besides carbon dioxide and water they also require, like the lower
plants, certain mineral elements which are usually obtained from the
soil or water in which they grow ; alternatively, some other materials
may be objectionable or even poisonous to them. ‘The likes and
dislikes, as weil as requirements and inabilities, of different plants
in the matter of water or soil constituents, often complicate their
distribution patterns, and constitute, as we shall see later, one of
the many sets of factors determining their actual or potential areas
on earth. Not a few of the Angiosperms which are primarily
photosynthetic appear to be aided in their nutrition by mycorrhizal
associations with Fungi in their roots—examples being the Heaths,
Orchids, and many forest trees. It has even been suggested that
the majority of vascular plants may be so aided. In addition there
are entire groups of Angiosperms that are either wholly or partially
parasitic on other plants, or saprophytic on a variety of decaying
substrata—again largely with the aid of mycorrhizas.
As for their habitats, Angiosperms tend to be plentiful almost
everywhere anything can grow on land or in shallow fresh water,
though they may be scarce in, or even absent from, some of the most
inhospitable situations such as rock faces, deserts, or mountain
summits where nevertheless some lower plants may exist. Except
in warm regions where free-floating types flourish, they do not
normally occur in deep open water; nor do they grow directly on
snow or ice, as some lower organisms can. Relatively few live in
the sea, and these appear to be limited to rather shallow water and
in no instance to extend northwards beyond the low-arctic zone.
But, in general, Angiosperms are practically ubiquitous in anything
approaching orthodox situations for plant growth, and a recital of
their habitats would be practically that of plants in general as outlined
in Chapter XI. Within the limits stated they are also virtually
cosmopolitan, extending from the tropics to the farthest north land,
which many attain, and also to the Antarctic Continent, which does
not appear to be reached by any other vascular plants nowadays.
Moreover, it seems likely that in the matter of number of species
they may be the largest of all plant groups, the order of 250,000
being currently suggested, though of course a great deal depends
on what precisely is understood by a species.
Finally, in the matter of vegetational and economic significance,
Angiosperms are of paramount importance in the world today,
affording the main dominants of most plant communities on land
2] THE VARIOUS GROUPS OF PLANTS 73
and of many in the water, and comprising almost all agricultural
and horticultural crops as well as the majority of forestral products.
To one or other of these aspects the remainder of this book will
bear such abundant testimony that it would be superfluous to give
details here, though the reader may be referred especially to Chapters
XIJ-XVI for vegetational aspects and to Chapter IX for economic
ones. It is by the distribution and growth potentialities of angio-
spermic plants, more than any other group, that human migrations
have been affected in the past, and civilizations have been caused
to wax and wane.
In view of their general importance it seems desirable here to
point out the two main groups (subclasses if the Angiosperms be
considered a class) into which the latter are usually divided (though
the individual criteria are not infallible). ‘These are the Mono-
cotyledones (Monocotyledons), characterized by having a single
seed-leaf (cotyledon), and the more numerous Dicotyledones
(Dicotyledons), characterized by having two seed-leaves. In addi-
tion, the Monocotyledons usually have (a) narrow leaves with
parallel veins, and (b) the vascular bundles in the stem loosely
scattered and unable to extend ; also (c) rarely any woody develop-
ment, and (d@) the flower-parts most often in whorls of three. ‘The
Dicotyledons, on the other hand, usually have (a) broad foliage
leaves with net-like veins, and (5) the vascular bundles disposed in a
ring in the stem and commonly able to extend indefinitely ; also
(c) often extensive woody development to form shrubs or trees, and
(d) the flower-parts most commonly in fours or fives. Examples
of Monocotyledons are the Grasses, Sedges, Aroids, Orchids, Palms,
and Lilies; of the Dicotyledons, most forest trees (other than
Conifers, Palms, etc.), members of the Pea family, and such crops
as Beets, Cabbages, Tomatoes, and Cucumbers, in addition to the
majority of broad-leafed herbs and shrubs.
FURTHER CONSIDERATION
Many more details and illustrations of each of the main systematic
groups of plants may be found in almost any modern textbook of general
botany, such as R. D. Gibbs’s Botazy ; an Evolutionary Approach (Blakis-
ton, Philadelphia & Toronto, pp. xiii + 554, 1950), or R. C. McLean &
W. R. Ivimey-Cook’s Textbook of Theoretical Botany, vol. 1 (Longmans,
London etc., pp. xv + 1069, 1951)—or, for the predominant Angiosperms,
vol. II (ibid., pp. xiii + 1071-2201, 1956). It should, however, be
74 INTRODUCTION TO PLANT GEOGRAPHY
remembered that authors rarely agree as to the status and disposition,
or even the limits, of every group.
An attempt to cover all the groups is made in A. Engler & K. Prantl’s
Die natiirlichen Pflanzenfamilien, second edition (Engelmann, Leipzig,
or latterly Duncker & Humblot, Berlin, numerous volumes from 1924),
and, in greater detail, in A. Engler’s very incomplete Das Pflanzenreich
(formerly published by Engelmann, Leipzig). Also primarily under
Engler’s name are published from time to time revised editions of the
handy Syllabus der Pflanzenfamilien (Borntraeger, Berlin), giving an out-
line of the entire plant kingdom.
For further details the following treatments of the various groups
should be consulted :
C. E. Crirton. Introduction to the Bacteria, second edition (McGraw-
Hill, New York etc., pp. xiv + 558, 1958).
K. V. Turann. The Life of Bacteria : their Growth, Metabolism, and
Relationships (Macmillan, New York, pp. xvitl + 775, 1955).
F. E. Frirscu. The Structure and Reproduction of the Algae (Cambridge
University Press, Cambridge, Eng., vol. I, pp. xvii + 791, 1935,
and vol. II, pp. xiv + 939 and 2 additional maps, 1945).
G. M. Smitu (ed.). Manual of Phycology (Chronica Botanica, Waltham,
Mass., pp. xii + 375, 1951); also Algae.
E. A. GAUMANN & F. L. Wynp. The Fungi (Hafner, New York &
London, pp. 1-420, 1952).
C. J. ALExopouLos. Introductory Mycology (Wiley, New York, pp.
xill + 482, 1952); also Fungi.
A. L. Smiru. Lichens (Cambridge University Press, Cambridge, Eng.,
pp. xxvill + 464, 1921).
F. VERDOORN (ed.). Manual of Bryology (Nijhoff, The Hague, pp.
ix + 486, 1932); Bryophytes.
F. VERDOORN (ed.). Manual of Pteridology (Nijhoff, The Hague, pp.
xx + 640, 1938); Pteridophytes.
G. M. Smiru. Cryptogamic Botany, voi. I1, Bryophytes and Pteridophytes,
second edition (McGraw-Hill, London etc., pp. vil + 399, 1955).
C. J. CHAMBERLAIN. Gymunosperms : Structure and Evolution (University
of Chicago Press, Chicago, Ill., pp. xi + 484, 1935).
G. H. M. Lawrence. Taxonomy of Vascular Plants (Macmillan, New
York, pp. xili+823, 1951); Angiosperms, etc.
CHAPTER III
Rives LOMO CEE Ae RICA CL LON S;
ADAPTATIONS: AND (LIFE=FORMS
Plants can grow only where the conditions are reasonably suitable
for them, and different species have often entirely different needs.
It follows that local conditions are a primary factor in limiting the
distribution of any particular kind of plant. ‘Tropical plants cannot
survive in arctic conditions, nor aquatic plants in a desert. ‘The
same holds good to varying degrees in less obvious instances, down
to examples where the balance is so fine that the difference between
success and failure, or actual life and death, is struck by some
barely perceptible difference in local conditions. Very often a com-
plex of interacting factors will be found operating, whose differences
may be extremely small but nevertheless sufficient to determine
whether or not a particular plant can grow successfully in a given
situation.
What actually determines the reactions of a plant to the conditions
making up the environment in which it finds itself ? Fundamentally
it is the general physiological make-up of the kind of plant involved,
although the state of development of the individual and its degree
of adaptation to local conditions may also come into play. Plant
physiology deals primarily with the internal workings of plants,
whether biological, chemical, or physical. ‘The main physiological
characteristics that are found in a particular species are usually
inherited and, taken together, largely determine the conditions under
which it can grow and the places where it can survive.
PHYSIOLOGICAL MAKE-UP
Later in this chapter we shall give an account of special features
which enable plants to offset or at least limit the effects of unfavour-
able conditions. Such so-called ‘ adaptations ’ include physiological
acclimatization, and are commonly responses to external conditions.
When not inherited they seem best considered as mere temporary
modifications in make-up. ‘Their real value to the plant can be
75
76 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
seen from such obvious examples as the tall stems of forest plants
which reach for light, or the long roots of desert plants which seek
out water.
Before considering these adaptive modifications, we must deal
with those physiological attributes which are most significant in
affecting plant distribution, for they are fundamental to plant
geography.
Water is essential for the life and growth of plants, being a con-
stituent of their bodies and necessary for many of their life-processes.
Consequently its availability is among the most important factors
of a plant’s environment. Where there is no water, plants cannot
long persist in an active state; although seeds may survive bone-
dry for many years, they need water to germinate as the plants do
to grow. Between a desertic lack and an aquatic superabundance
of water, which are extremes that only suitably adapted plants can
withstand, there are various degrees of water availability to which
particular species are accustomed and often limited. ‘The need for
water largely determines the distribution of plants on the face of
the earth, as we may see when passing from any lastingly dry area
to a wet one, when the flora and vegetation will change drastically.
The actual effect of available water may be complicated by con-
ditions, such as temperature and atmospheric humidity, that affect
its utilization within the plant—for example through controlling
absorption by the roots, movement in the stem, or loss from the
leaves, etc. Particularly susceptible to atmospheric changes are the
microscopic pores (stomata) through which most water-vapour and
other gaseous exchange takes place between the internal tissues of
higher plants and the atmosphere at large. Consequently the all-
important water economy of the plant is affected by conditions in
the surrounding air as well as by the availability of water in the
soil.
‘Temperature is another of the most important factors of the
plant’s environment. Particular plants require particular tempera-
ture-ranges for their life-processes and normal development, and,
different temperatures being characteristic of different climates, such
requirements widely limit the geographical distribution of plants.
And along with plants, of course, go the vegetation-types which
they make up.
Under otherwise constant conditions each plant has an optimum
temperature at which it does best, and, on either side of this, a
range extending to maximum and minimum temperatures beyond
3] PHYSIOLOGICAL REACTIONS Ha
which it cannot grow normally and many even be killed. However,
for most plants, in the words of one specialist correspondent,
‘temperature requirements depend on illumination ; in low light
[intensities] the optimum is cooler than in high light’. In nature,
temperatures fluctuate more or less markedly and affect different
life-processes differently, so that the optimum must take into
consideration such natural fluctuations on one hand and the optima
for different life-processes on the other. Even the maxima and
minima, outside of which death may result, often vary with other
physical factors and with the recent experience as well as evolutionary
history of the plant in question. ‘They may also vary with the
time of exposure as well as with the state of the plant structure or
its stage of development. ‘Thus resting seeds and other reproductive
bodies are, in general, far more resistant to extremes than are adult
plants or, particularly, tender young parts: whereas the killing of
young shoots and blossoms by even the slightest frosts is an all-too-
common experience in temperate regions, more mature parts of the
selfsame plants often survive. Indeed there are numerous known
instances, involving all the main groups of plants, of such resistant
bodies as spores and seeds surviving much lower temperatures in
laboratories than are ever found in nature—including those of liquid
hydrogen or even of liquid helium near absolute zero.
Far from all vital activity ceasing at the freezing point of water
(32° F. = o° C.), there are known instances of such physiological
functions as photosynthesis and respiration proceeding at tempera-
tures below this point in higher plants, while some Bacteria and
Fungi are capable of growth at temperatures as low as 16° F.
(— 889° C.). It has even been claimed, in Russia, that flagellate
Algae have been observed swimming in drops of brine cooled
artificially to — 15° C. On the other hand, whereas most plant
bodies are killed by heat at much lower temperatures than the boiling
point of water at sea level (100° C.), some bacterial spores are merely
stimulated to germinate by being so boiled (though of course
the actual germination only takes place subsequently, at lower
temperatures).
The responses of plants to night temperatures have recently been
demonstrated to have considerable significance in connection with
their geographical distribution. ‘Thus the Big Bluegrass (Poa
ampla) of western North America flowers equally well at day tempera-
tures of 20, 23, and 30° C.—but only when the night temperature
is below 14° C., for at 17° C. right temperature it does not flower
78 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
at all, even though vegetative development is good. Again, the
English Daisy (Bellis perennis) dies when grown continuously in a
warm greenhouse: with a day-temperature of 26° C. the plants
survive only at night temperatures below 10° C., and flower
abundantly only at still lower night temperatures. Accordingly
such plants are unable to reproduce normally in consistently warm
climates. In other instances, closely related strains may differ
markedly in their night-temperature requirements for flowering ;
these requirements may be decisive in determining which strains,
if any, can flourish in a particular area. ‘This is true of ‘Tomatoes,
where fruit-set is dependent upon a very narrow range of tempera-
tures—a phenomenon which is reflected in very large differences in
yield in varying circumstances and with different strains having
even slight deviations in optimal requirements. Numerous instances
are now known in which, for these or other reasons, slight differences
in the temperature-response of plants will exert a controlling influence
on their local survival and consequently on their distribution.
In the many parts of the world that have markedly varying seasons,
one of the main concerns of their plants is to tide over unfavourable
periods—usually of cold or drought. ‘To this end is expended a
good deal of what might be called evolutionary ingenuity, and also
much physiological effort—for example, in storing food for the
adverse period and for subsequent development. Among the most
successful methods employed is the annual habit, in which the
adverse period is evaded by being passed over in the form of a
resistant seed or fruit, the parent having meanwhile died. Numerous
common weeds, such as Chickweeds (Ste//aria spp.) and Shepherd’s-
purse (Capsella bursa-pastoris), practise this method, as do many of
the diminutive ‘ ephemerals’ which blossom so pleasingly after rain
in the less extreme deserts. In the Arctic and some other rigorous
regions, however, the growing-season, though fairly regular, is too
short and cool to allow full development—from seed through seedling
and adult to flower and seed again—in a single season. Accordingly
almost all the plants there are perennial, passing the adverse winter
period in a more or less resistant and dormant state—often after
dying down (in the case of herbs) or losing their leaves (in the case
of deciduous shrubs and trees). Most Mosses and some other plants
have the fortunate capacity to endure drought by drying up almost
entirely without ill effect, resuming normal life again when moistened.
All these, as well as any growth-responses they involve, are physio-
logical activities (or inactivities) and, in a sense, adaptations to
3] PHYSIOLOGICAL REACTIONS 79
environmental conditions. On a plant’s capacity for them may
depend its geographical range.
The production of reproductive bodies involves various physio-
logical activities that are closely correlated and indeed wonderfully
integrated, yet may be affected by environmental conditions in a
unique way. Although some trees may live for more than 2,000
years, there is no known instance of life being really permanent in
any individual. So in order to persist a plant must reproduce, and
any condition which prevents it from doing so in a particular area
will preclude that area from its normal range (that is, in the absence
of persistent immigration). Many conditions—climatic, nutritional
or otherwise—can and frequently do prevent the normal reproduction
of certain plants, so limiting the geographical areas they occupy.
Some plants circumvent this either by separating off parts of their
bodies for ‘ vegetative’ reproduction or by the development of
special organs for the same purpose, thereby enabling themselves
to persist in areas where seed etc. cannot be produced. ‘This is
true of many plants living under extreme conditions, for example
in the Arctic.
Although on land there is almost everywhere sufficient light to
enable plants to grow, the effect of light-climate on their reproductive
processes affords another instance of range-limitation. For many
plants require a day-length within particular limits before they can
flower successfully, and, in latitudes where the length of day during
their flowering period is outside these limits, are unable to reproduce
sexually. ‘This appears to be one reason why many southern
species fail to flower in the north, and vice versa. However, such
reactions are by no means immutable, but tend to vary with other
conditions, and may also be changed by treatment with certain
chemicals.
The ranges of particular species may be limited by chemical
‘antagonism’ (i.e. active opposition to growth, etc.), nutritional
conditions, and other factors bound up with the soil. Familiar
instances are afforded by some plants which require much ‘ lime ’
(actually, calctum carbonate) in the soil, and others which avoid it.
Examples of the former category are Yellow Mountain Saxifrage
(Saxifraga aizoides agg.) and Salad Burnet (Poterium sanguisorba),
and, of the latter, most Heaths (Ericaceae). Often the merest trace
of a particular compound or element, such as boron, can have a
profound effect in encouraging or precluding particular species.
Deficiency diseases, due to lack or insufficiency of particular
fe) INTRODUCTION TO PLANT GEOGRAPHY [| CHAP.
substances, are common. ‘These diseases, with the ones produced by
attacks of various Fungi, Bacteria, Viruses, and Nematode Worms,
and the browsing of lower animals such as Locusts and of higher
ones such as Goats, may drastically limit plant distributions. Often
the very presence of a plant species in a spot is dependent upon the
absence there of serious pests and predators.
The areas of parasites and saprophytes are naturally limited to
ones where suitable hosts or elaborated materials, respectively, are
available for attack. ‘Thus, for example, the devastating Late-
blight of Potatoes and ‘Tomatoes, caused by the Fungus Phytophthora
infestans, is limited to the areas of those crops and of some other
members of the family (Solanaceae) to which they belong. Again,
the deadly White Pine Blister-rust, Cronartium ribicola, is virtually
limited to areas supporting both of the hosts that are necessary for
the completion of its life-cycle, namely, five-needled Pines and
species of Ribes (Currants and Gooseberries).
As regards physiological antagonisms due to poisonous residues
and excretions, it seems that these may be important in some circum-
stances, such as ‘ fairy rings’ and the avoidance by some plants of
the shade of certain trees. ‘Thus the roots of Black Walnut (Fuglans
migra) have long been known to excrete a toxic substance, juglone,
that inhibits the growth of many other plants and can even kill
Apple trees. ‘There are also the cases of the western North American
members of the Daisy family (Compositae), Parthenium argentatum
and Encelia farinosa, which are known to poison other plants by
minute amounts of chemical excretions, thereby reducing competi-
tion. Is it possible that this may be one of the factors lying behind
the notorious success of this family as colonists ? We do not know,
and indeed our information in such fields of study is still only
fragmentary. Also undetermined but pregnant with possibilities for
research, is the extent to which antibiotic substances may be effective
In nature.
ECOLOGICAL LIMITATION
The realms of physiology range imperceptibly into those of
ecology, which in part may be looked upon as the application of
physiology to ‘ field’ conditions. The ecological requirements of
different plants are widely various and, as we have seen, commonly
limit their geographical areas. This limitation is actually to those
regions where appropriate ‘ habitats’ (7.e. living places) exhibiting
3] PHYSIOLOGICAL REACTIONS 81
suitable conditions are found, and, within such regions, naturally to
those habitats themselves. Consequently plants in nature are limited
not only to areas of particular climate but more precisely to special
habitats within these areas, the final limitation being ecological. In
such cases as oases in a desert or islands in an ocean, this limitation
may be extreme.
The subject of modification by, or adaptation to, various conditions
is dealt with in the next section. Here we should mention the
manner in which, quite apart from any special antagonism, sheer
physical competition among plants for the requisites of life may
limit the habitat and actual range of a particular species or even
strain. Especially may root-competition for water and aerial com-
petition for light prove veritable struggles for existence in which
the weaker individuals succumb. Generally speaking, the closer any
two types are in their ecological requirements, the keener will be
the competition between them: in such even contests the slightest
advantage to one competitor can swing the balance in its favour.
As most plants living on land need soil in which to root and some
well-lit space in which to grow, it is particularly in ‘ open’ areas
not yet covered with higher vegetation that competition is least and
plants can enter and establish themselves successfully. Most such
areas tend to be colonized by successive waves of plants that usually
start with primitive or other lowly types but in favourable regions
normally lead up to forest. ‘This progressive colonization is called
‘succession’, and is described in Chapter XI. The farther it
proceeds, the less space there is left for new colonists and the more
tendency there is for former colonists to be ousted by coarser
competitors. Meanwhile animals, including Man, are continually
opening up new habitats and abandoning old ones—often after
destroying the natural vegetation, and rarely without disturbing it.
For these and other reasons the geographical areas of plants and
plant communities are rarely if ever static.
STRUCTURAL ‘ ADAPTATIONS’ OF VEGETATIVE PARTS
Numerous features help plants to offset the effects of unfavourable
conditions and consequently widen their potential ranges. Having
noted already such functional modifications as acclimatization of
various sorts, and physiological ‘ adaptations’ such as the ability of
many plants to evade unfavourable periods of cold or drought, we
shall deal here with changes of form that appear to be developed in
82 IN TRODUG ELON LTO, PLANT GEOGRAP Ey
relation to the needs of plants to combat adverse conditions.
Through such structural changes they may be enabled to maintain
their geographical areas and even to extend them. ‘Those modifica-
tions of reproductive bodies that are helpful in dispersal will be
dealt with in the next chapter, the present section being concerned
primarily with the ‘ vegetative ’ parts—comprising, in higher plants,
the stems, roots, and leaves.
‘The water relationships of plants often involve strikingly ‘ adap-
tive’ features—particularly ones that are helpful in tiding over
periods of water deficiency, for example by increasing absorption
or decreasing loss, or by storage against times of need. Instances
are seen in the deep roots of many plants of deserts or semi-deserts,
allowing the tapping of underground reserves, and in the matted
turf of the Grasses of semi-arid regions, which aids retention of such
water as becomes available from atmospheric sources. Actually, as
pointed out by Professor Kenneth V. Thimann (in Utt.), ‘ roots
elongate when aerated ; hence in dry soils (which are therefore full
of air) they grow longer. . . . I should call [this] a simple response
to external conditions. Low nitrogen also favors elongation of
roots, with obvious ecological advantages in nitrogen-poor soil.’
The aerial parts of a wide range of plants are modified to reduce
water-loss, often to the slightest proportions in times of shortage.
This may be done, for example, by protection of the stomata in grooves
or among a mass of hairs, by general reduction of the ‘ evaporating
surface ’, and by covering with wax or hairs, etc., even those areas
that remain. Often the leaves are reduced to spines or scales, their
normal functions being taken over by green stems. In addition
many plants, such as the more massive succulents of the Cactus,
Spurge, and some other families, store water extensively in special
stem or other structures which are modified into reservoirs. ‘There
may also be one or more layers of large water-storing cells in leaves
and other green parts. ‘lhe development of some of these features,
such as the tall stems of many trees in dense forests, may depend
upon the conditions under which an individual grows, whereas in
other cases the features may develop regularly, irrespective of local
conditions, as part of the normal form of the plant. But in either
instance the ‘ ability’ has to be present, else the plant could not
develop the desirable adaptation and benefit accordingly. Fig. 20
shows some examples of features that help land plants to conserve
or obtain water ; conversely, many water plants have special tissues
or growths that enable them to float or otherwise improve their
84 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
(See p. 85.)
3] PHYSIOLOGICAL REACTIONS 85
BLACK GRAMA
|
1
|
17
PW
¢
mt nn
como
t
aN
A
act. a
PROPERLY GRAZED \
F
Fic. 20.—Features aiding water conservation or absorption. A, branch of a
desert plant, Hakea, with the leaves modified as spines ( 3); B, stems cf
Euphorbia tirucalli, specialized for photosynthesis and water storage (< about 4);
C, Arizona desert with large Cacti (phot. F. Shreve); D, cut bank on Jornada
Experimental Range, New Mexico, showing deep rooting of low desert plants—
in particular a Mesquite bush about 12 inches (30 cm.) in diameter and only
6 inches high but with roots about 8 feet (nearly 2} metres) deep (phot. U.S.
Forest Service); E, ‘ bisect ’ diagram of above- and below-ground parts of forbs
and Grasses in the Palouse prairie grassland association of western central Idaho,
U.S.A. (courtesy of U.S. Soil Conservation Service) ; F, BlackGrama Grass
(Bouteloua eriopoda) grown under three degrees of grazing, showing effect on root
system (courtesy of U.S. Soil Conservation Service).
86
INTRODUCTION TO PLANT GEOGRAPHY
[CHAP.
aeration, three examples being shown in Fig. 21. Among these
last the Water-hyacinth affords an example of how floating may
aid in dispersal without involving special reproductive bodies, for
individuals may be transported considerable distances by water cur-
Ny) iy on
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3] PHYSIOLOGICAL REACTIONS 87
Fic. 21.—Features promoting aeration. <A, Jussiaea repens, a rooting or floating
aquatic with numerous inflated roots which project upwards into the air and
contain a great development of air spaces through which air can pass to submerged
organs (* 4); B, Water-hyacinth (Eichhornia crassipes), with leaf-stalks modified
for buoyancy, the whole plant floating freely (> 4); C, cross section of leaf-
stalk of a Water-lily (Nymphaea stellata), showing large air-passages ( 30).
rents, and, having so migrated, often multiply to cover large areas
of water.
Also significant in enabling plants to grow in many situations
where otherwise they could not exist, are modifications for climbing,
twining, scrambling, and running. Examples are shown in Fig. 22.
Further modifications apparently playing a similar role in plant
geography include those for catching insects to supplement the food
supply, and those for storing food to tide over the adverse period
of winter. Examples of carnivorous plants and of food-storage in
special underground organs are shown in Fig. 23 ; included in the
latter category are Potatoes and many bulbs and other structures
that are, besides, reproductive in function.
The giving off of water-vapour from the aerial parts of plants
helps to keep them cool, and many are further protected from
intense sunlight by their structure or covering, so that ‘ scalding ’
and other injury may be averted even in very hot and sunny deserts.
The structural changes which restrict or accelerate the rate of water
loss are in general either hereditary and consequently characteristic
of the race, or are acquired by an individual plant or part of a plant
88 INTRODUCTION TO PLANT GEOGRAPHY [ CHAP.
3| PHYSIOLOGICAL REACTIONS 89
Fic. 22.—Various adaptations for climbing, twining, scrambling, and running.
A (x 4), leaf-tendrils of Common Pea (Pisum sativum, left) and Clematis (Clematis
sp., right); B, branches of Bougainvillaea modified as spines used in scrambling
( ); C, Dodder (Cuscuta), a parasitic twiner that sends haustoria into the host-
plant (x 1); D, a‘ Walking’ Fern (Adiantum caudatum) ( 4); E, Ivy (Hedera),
showing climbing roots (x 4).
in response to the particular conditions under which it has grown.
Thus in the latter instance we may even get large but thin * shade ’
leaves and small but thick ‘sun’ leaves on the selfsame branch of
a tree, whereas no matter under what conditions most compact
desert plants are grown they will not become tall and lax, the char-
acteristic of compactness being in such instances usually hereditary
[CHAP.
INTRODUCTION TO PLANT GEOGRAPHY
go
3] PHYSIOLOGICAL REACTIONS gi
Fic. 23.—Modifications for storing food or catching insects. A, expanded
storage-root of Turnip (Brassica campestris) (* 4); B, Ginger (Zingiber) plant
with enlarged storage rhizomes (Xx 4); C, bulbs of Lily (Lilium sp., left) and Onion
(Allium cepa, right) (x %); D, Sarracenia, a Pitcher-plant, showing flowers and
pitcher leaves (x 4); E, Sundew (Drosera), a carnivorous plant (x 3).
and ‘ fixed’ through long evolutionary history.'| Of such a deep-
seated and lasting nature are most of the vegetative and reproductive
features which go to make up a plant species, giving it its special
form or morphology. By this we classify it as part of a systematic
hierarchy in the manner explained at the beginning of Chapter II.
1 Often the same character-manifestation is hereditary in one group of plants
and due to direct environmental impress in another—an example of the latter
being the compact form of many alpine plants as opposed to those characteristic
of deserts. In such instances special cultivation may be necessary to determine
to which category a feature belongs.
Q2 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
CLASSIFICATION BY LIFE-FORMS
The ‘ life-form’ or ‘ growth-form’ of a plant is the form which
its vegetative body produces as a result of all the life-processes,
including those that are affected by the environment within the
plant’s life-time and are not heritable. Although a plant’s life-form
is among its most striking characteristics, it may be of a rather fickle
nature. ‘Thus different individuals of the same species can some-
times belong to different life-forms, for example when they have
been grown in different environments ; for under any particular
life-form are merely grouped together those plants which, in their
entirety, show similar morphological adjustments. Life-forms may
accordingly give a fair indication of environmental impress, or at
least tell us something about local conditions.
Although the description of vegetation in terms of life-forms is
widely imprecise, and classification by them is inadequate for our
ultimate purpose, nevertheless it is a part of common parlance and
can be of some value. Its use goes back at least to the times of
the ancient Greeks, who classified plants into trees, shrubs, herbs,
etc., which are among the most obviously differing life-forms. Even
nowadays to the general geographer or other non-biologist the species,
etc., making up plant communities are often less significant than the
prevalent life-forms. ‘These last may yet be of importance in two
allied biological fields, namely, plant sociology, where consideration
of life-forms may help in the description of the structure of the
communities that are the main subject of study, and ecology, where
mention of the predominant life-form is often sufficient to give some
idea of the local environment.
In spite of the limitations mentioned above, there is one particular
system of life-forms which as plant geographers we may find useful,
although it suffers from rather difficult Greek terminology. As
originally elaborated by the late Professor C. Raunkiaer of Denmark
and usefully modified by, among others, Dr. J. Braun-Blanquet of
Montpellier, this system lays stress primarily on the adjustment of
the plant to the unfavourable season, and particularly employs the
position of the perennating! buds relative to the soil surface in
attempting to classify together plants of similar habit. ‘The result is a
series of life-forms that is especially interesting to the more statistic-
ally minded among us, the main categories of which are as follows :
* Perennation is the act of tiding over an unfavourable period, such as a cold
winter or a dry summer.
3] PHYSIOLOGICAL REACTIONS 93
(a) Phanerophytes (tall aerial plants). Perennials, mostly trees or
shrubs, with their renewal buds on shoots at least 25 cm. (about
10 inches) above the surface of the ground, and hence exposed to
unfavourable weather. Phanerophytes are especially numerous in
moist areas of the tropics and subtropics, where they tend to pre-
dominate in the matter of numbers of species as well as individuals.
Elsewhere the species are usually few, even if the numbers of
individuals are great and their dominance is overwhelming.
(b) Chamaephytes (surface plants). Perennial herbs and some
undershrubs with renewal buds between ground-level and a height
of 25 cm.—hence usually enjoying only such protection as may be
afforded by the plant itself or by snow, and consequently plentiful
in boreal and alpine regions.
(c) Hemicryptophytes (half-earth plants). ‘These have perennial
shoots and buds at ground-level or within the surface layer of soil,
etc., and hence protected by the habitat. Such plants are particularly
preponderant in high alpine and arctic regions but are also plentiful
in the temperate zone.
(d) Geophytes (earth plants). ‘These have the perennating organs
(such as bulbs, tubers, or rhizomes) well buried in the soil and there-
fore not exposed in unfavourable seasons. ‘They tend to be com-
monest in temperate regions but also persist in fair numbers farther
north and south.
(e) Hydrophytes (water plants). ‘These include all water plants,
whether anchored or not, apart from microscopic free-floating or
swimming types which form the main basis of the separate category
known as ‘plankton’. ‘This group of hydrophytes tends to cut
across the other main ones and so is often omitted from ‘ spectra’
(see pp. 94-5).
(f) Therophytes (annuals). Plants which complete their life-cycle,
from germination to ripe seed, within a single limited vegetative
period, surviving the unfavourable times as seeds, spores, or other
special (usually resistant) reproductive bodies. ‘They are especially
abundant in deserts where the unfavourable period may be par-
ticularly severe and prolonged, but are largely lacking in arctic
regions where the growing-season is too short or the warmth is
insufficient to allow them to complete development before winter
comes again.
Examples of (5), (c), (d) and (f) are illustrated in Fig. 24. Almost
all trees and tall shrubs belong to (a), while examples of (e) were
illustrated in Fig. 21.
94 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
Further categories may, if desired, be added to the above system
—such as epiphytes growing on trees etc. Moreover, refinements
may be used such as the subdivision of phanerophytes into nano-
phanerophytes (shrubs) in which the renewal buds lie less than 2 metres
above ground, microphanerophytes (small trees) in which they lie
at a height of from 2 to 8 metres, the taller mesophanerophytes
(8-30 metres), and the still taller megaphanerophytes (above 30 metres) ;
also phanerophyta scandentia (lianes) which are woody climbing
plants whose renewal buds pass the unfavourable season high above
the ground.
Fic. 24.—Diagrams illustrating some Raunkiaer life-forms. A, a _ creeping
chamaephyte; B, a rosette hemicryptophyte; C, a tufted hemicryptophyte; D,
a bulb geophyte; E, a rhizome geophyte; F, a therophyte. (After Braun-
Blanquet.)
The values of this system are relative, its applications limited.
Being based on wide life-form categories, it certainly cannot take
the place of detailed description of vegetation including naming of
the main species concerned, which alone will indicate to the qualified
reader the precise nature of each named species and, through these,
reveal much concerning the community itself. Its use is, moreover,
limited in arctic and alpine regions where the success of a particular
plant in life is apt to depend not so much on its adaptation to a
rigorous winter as on its adjustment to the very short and cool
summer. Nevertheless, in the hands of the student who is statistic-
ally but perhaps not taxonomically minded and trained, not wanting
or able to name specifically the plants concerned, this system is
useful in giving a fair analysis of the components of a community
or flora in terms of the representation of each life-form.
Such an analysis is usually expressed as a ‘ biological spectrum ’,
indicating the percentage of the total flora belonging to each of the
life-forms involved. Considering only vascular plants and excluding
3] PHYSIOLOGICAL REACTIONS 95
hydrophytes, examples from areas in the main climatic belts whose
land-vegetation is described in Chapters XII-XIV are as follows,
in round figures :
(i) Temperate—phanerophytes 15, chamaephytes 2, hemicrypto-
phytes 49, geophytes 22, therophytes 12 ;
(ii) Arctic—phanerophytes 1, chamaephytes 22, hemicryptophytes
61, geophytes 15, therophytes 1 ;
(111) Tropical (moist)—phanerophytes 61, chamaephytes 6, hemi-
cryptophytes 12, geophytes 5, therophytes 16 ;
(iv) Tropical (arid)—phanerophytes 9, chamaephytes 14, hemi-
cryptophytes 19, geophytes 8, therophytes 5o.
With the above it is interesting to compare the ‘ normal’ spectrum
for the world as a whole, which is claimed to be: phanerophytes
46, chamaephytes 9, hemicryptophytes 26, geophytes 6, therophytes
13.
Altogether it may be concluded that such life-form spectra can
give a useful if generalized impression of the biological effects of
climatic features and hence help characterize the various phytogeo-
graphical regions. But dealing as they do with wide categories,
and with flora rather than vegetation (that is, with the different
kinds of plants inhabiting an area regardless of their abundance and
relative importance), they are no adequate substitute for more
thorough description with structural details, precise naming, and,
wherever possible, good illustration. ‘Thus, for example, a small
group of species or even a single species may dominate and largely
characterize a plant community or sometimes a whole region, and
yet scarcely ‘tell’ in the spectrum. ‘This, however, is an objection
to the spectrum method of counting species rather than to the life-
form classification itself.
FURTHER CONSIDERATION
The principles of plant physiology can readily be acquired from W. O.
James’s An Introduction to Plant Physiology, fifth edition (Clarendon
Press, Oxford, viii + 303, 1955) or, in more detail, from such a text as
B. S. Meyer & D. B. Anderson’s Plant Physiology, second edition (Van
Nostrand, New York etc., pp. vill + 784, 1952).
More details and examples of structural ‘ adaptations’ that apparently
enable plants to maintain or extend their geographical ranges, may be
gained from almost any good modern work on structural botany, or from
G. Haberlandt’s classic Physiological Plant Anatomy, translated by M.
Drummond (Macmillan, London, pp. xv + 777, 1914, reprinted 1928).
96 INTRODUCTION TO PLANT GEOGRAPHY
The system of life-forms outlined above is clearly elaborated in Chapter
XII of J. Braun-Blanquet’s Plant Sociology (McGraw-Hill, New York &
London, pp. xvill + 439, 1932); there are some refinements in the second
German edition, Pflanzensoziologie : Grundziige der Vegetationskunde
(Springer, Wien, pp. x1 + 631, 1951). However, for a detailed account
of the development and application of this system, the interested student
should refer to the volume of collected papers of the late C. Raunkiaer,
entitled The Life Forms of Plants and Statistical Plant Geography (Claren-
don Press, Oxford, pp. xvi + 632, 1934). A briefer account is given in
the same author’s Plant Life Forms, translated by H. Gilbert-Carter
(Clarendon Press, Oxford, pp. vii + 104, 1937).
Any walk in the country, or even in a garden or public park, with due
contemplation of the seemingly endless variety of plants encountered—
the Lichens or green powdery algal cells on the bark of many trees are
just as truly plants as the giants on which they grow—should convince
even the most sceptical layman of the need for classification. ‘The more
intelligent and interested will almost inevitably find themselves comparing
similar plants and mentally putting them into groups, which may be
either systematic or life-form ones. It may be noted in the course of
such observations that the life-forms chiefly give some indication of the
physiognomy of the vegetation. ‘This is largely dependent on local
environmental conditions and may look alike even where quite different
kinds of plants are involved. On the other hand, systematic relationships
(e.g. following the lines indicated in Chapter II) depend also considerably
on past and present geographical connections and barriers, so that only
an account including floristic determinations and details of frequency
etc. can give the more complete picture for which we strive.
CHAPTER IV
DISPERSAL AND MIGRATION:
ALD SVAND BARRIERS
Having stated our objectives and familiarized ourselves with the
main groups of plants, we must consider the methods by which
different plants increase their areas, at least potentially, by special
‘adaptations’ of the reproductive bodies and by seizing such
opportunities for their transport as may be offered. ‘These adapta-
tions are of the nature of beneficial structural modifications (see
Chapter III). ‘The areas attained are the mainstay of our plant
geographical studies, and although they are liable to be profoundly
affected by past history (as we shall see in the next two chapters)
and are further greatly limited by the physiology of the plants
themselves (as we have already seen in Chapter III), these areas
must to a large extent be a function of the plants’ own aptitudes.
In the final analysis, areal spreading is often limited by the ecological
reactions of the plant to a new environment which may, for example,
be too cold or too dry for its successful establishment. Such
reactions are primarily physiological, and, though their outcome is
often capable of modification, as we have already seen, they com-
monly determine the potential or ultimate area which a species can
occupy when there is fully effective dispersal. ‘The actual areas
within the physiologically circumscribed potential ones are largely
determined by barriers to successful migration.
It should be noted that dispersal and migration, although closely
connected, are different activities. Dispersal merely involves dis-
semination from the parent and distribution (in the dynamic sense)
to a new spot, whereas migration implies also successful growth and
establishment (ecesis). Thus dispersal is a necessary forerunner of
migration, which is actually accomplished only on establishment in
a new place. In nature only a small proportion of the plant bodies
which become dispersed, and which may conveniently be termed
disseminules (diaspores), actually become established and effect
migration. Not only do many of them die prematurely or fall on
97
98 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
‘barren ground’, or come to rest where they cannot even start a
new life, or fail to survive the struggle with stronger competitors, but
the ecological conditions and physiological reactions have to lie
within often narrow limits for ultimate success. In any case, there-
fore, the vast majority of disseminules are doomed.
These disseminules, the actual bodies moved, are most often
reproductive structures such as spores, seeds, or fruits. In numer-
ous instances, however, they are special structures of a vegetative
nature, or unmodified parts of plants, whole plants, or even groups
of plants—though in the last instance usually effective only by
chance. An example of a whole plant being transported was the
Water-hyacinth mentioned in the last chapter and shown in Fig. 21, B.
Often the same plant species or individual will produce more than
one type of disseminule, thereby increasing its chances of effective
migration. ‘Thus, whereas the majority of our familiar north-
temperate forest trees, such as Oaks and Spruces, normally repro-
duce by seed, they may also do so by means of suckering, layering,
or other vegetative activity. Moreover, many of the plants that
are most successful in colonizing vast areas, resort to more than one
means of dispersal. ‘Thus the Common Reed (Phragmites com-
munis agg.), which is often claimed to be the most widely distributed
vascular plant species in the world, has the multiple advantages of
a wind-dispersed, plumed fruit, and a water-dispersed, more or less
buoyant rhizome—besides considerable variability in form, and an
ability to occupy a wide range of moist to aquatic habitats. ‘These
it colonizes so aggressively and holds so strongly that its ‘ beds’
form a formidable barrier against immigration by other plants. On
the other hand, one of the numerous unsolved problems of plant
geography is that of why many plants with seemingly excellent
advantages in dispersal are not widely distributed. Yet another
major question is posed by the number of groups and even species
that are widespread without seeming to have any adequate means
of dispersal. ‘That precisely the same type of plant should have
evolved separately in several different places is almost unthinkable
to most students, and so it is widely assumed that the areas currently
occupied by particular plants are due to dispersal and effective
migration. Now that we have explained the distinction between
these terms, they need not henceforth be separated. Rather will
we refer to dispersal when the question of establishment can be
ignored, and to migration when such establishment is to be
emphasized.
4] DISPERSAL AND MIGRATION 99
WIND DISPERSAL
A walk in the woods and fields of a north-temperate region on
a boisterous autumn day should convince any sceptic that air currents
of one kind or another are important in the dispersal of many
different plants. Not only do winds blow leaves, and sometimes
small branches, about—and with them adhering parasites or sapro-
phytes, for example—but they obviously transport some seeds and
fruits for considerable distances. ‘The more efficiently adapted of
these, whose bodies are so light or whose ‘ form-resistance ’ is such
that they sink only slowly in still air and float almost indefinitely
in a light breeze, may be transported far from the parent. This
undoubtedly happens with such plumed seeds as those of Milkweeds
(Asclepias spp.) and Fireweed (Epilobium angustifolium agg.), or with
such ‘ parachute’ fruits as those of Dandelions (Taraxacum spp.)
and many other members of the Daisy family (Compositae).
Even more effective is the dispersal by air currents—including
upward eddies that carry them into the upper atmosphere—of
microscopic spores, especially of Fungi and Bacteria. Such dis-
persal may take place over distances that in numerous instances
have been proved to run into many hundreds of miles. ‘The present
writer has studied this subject for years and is convinced that these
smaller ‘ botanical particles’ or ‘spora’ can be (and often are)
carried thousands of miles in the atmosphere, frequently without
losing their power to resume active life on regaining suitable con-
ditions. ‘Thus he has trapped some spora in the immediate vicinity
of the North Pole under both winter and summer conditions, as well
as elsewhere at vast distances from their nearest conceivable point
of origin.
Quite apart from disseminules which are specially modified for
transportation by winds, and others which are so minute that they
need not be so modified to be transported, there are many recorded
instances of large and heavy bodies being blown for considerable
distances by hurricanes, etc., on special occasions. After the
devastating tornado in and around Worcester, Massachusetts, in
June, 1953, abundant shingles and often bulkier roofing materials
and sizeable living branches of trees were to be seen littering the
ground fully 20 miles nearer the coast than the closest point at which
the ‘twister’ had struck. There are also records of windfalls of
uprooted plants scattered over wide areas. It need scarcely be
remarked that, as successful transportation and growth of a single
TOO IN DRODUCLVONG TO ePZAIN Aa G Ey OG RASPES [CHAP.
plant or disseminule is sufficient for its establishment in a new region,
even extremely rare occurrences may be important and involve
quite unexpected species and circumstances. Instances in point
include exceptional winds in various regions, and the blowing of
seeds, fruits, or whole plants over the ice or compacted snow in
arctic regions in winter. Not only may such blowing over ice be
effective from time to time, in the case of higher plants, but, more
often as they tend to remain longer alive, it may also result in the
dispersal of parasitic or saprophytic Fungi, etc., growing upon or
within the bodies of these higher plants. When we recall that, not
very many thousands of years ago, ice covered vast tracts of what
are now among the most populous parts of the northern hemisphere,
as well as, doubtless, the adjacent seas, we can imagine that such
dispersal may have been of great importance in the migrational
history of plants in areas far south of the present-day Arctic.
It is instructive to consider briefly the main organs or methods
of wind-dispersal, and particularly those plant bodies which are
especially modified for the purpose. For this, there should be
recalled the distinction between seeds and fruits which was explained
on page 6g, and the very different origin and ‘nature of spores in
different cases.
- (a) Spores. ‘These, as we saw in Chapter II, are the main dis-
seminules of most of the groups of plants up to and including the
Ferns, and are often produced in fantastically great numbers. ‘Thus
a single specimen of the Pasture Mushroom (Agzricus (Psalliota)
campestris) has been estimated to produce 1,800,000,000 spores,
while a large specimen of the Shaggy-mane Mushroom (Coprinus
comatus) may produce 5,240,000,000 spores, and some Pufftballs
many times that number! Although extremely variable in size
and form, spores are commonly minute and easily blown about by
the wind—being frequently borne by upward air-eddies rising from
warm plains and carried into the upper atmosphere where they may
be transported vast distances. Indeed, like volcanic dust, they are
probably sometimes blown around the world without settling to
earth. Bacterial and some other minute cells may belong to the
same category as spores in the matter of size and aerial buoyancy.
The spores are often extremely resistant to low temperatures and
desiccation which in fact appear to prolong their life, so that many
caught in the most remote situations are alive, able to germinate
when given suitable conditions, and, as we say, ‘viable’. ‘They
may live for many years and, apparently, often withstand the radia-
4] DISPERSAL AND MIGRATION IOI
tion effects of high altitudes. According to Ridley, whose monu-
mental work on plant dispersal is cited at the end of this chapter,
‘There is no part of the world where some are not present, and
there appears to be a constant rain of the more minute kinds falling
everywhere.’ With little doubt this easy wind-dispersal of many
of the Bacteria, Fungi, and other so-called spore-plants is the
primary reason for their extremely widespread distribution; a
secondary reason is their often wide tolerance of conditions and
modest requirements for life.
(b) Dust seeds (and minute fruits). ‘The seeds of many plants,
such as the members of the Orchid family (Orchidaceae), and the
one-seeded fruits (for example) of some of the mainly tropical
parasitic family Balanophoraceae, are also minute and extremely
light, as well as sometimes winged, and so tend to be blown away
and about in much the same manner as spores.
(c) Plumed seeds. ‘These usually bear a light tuft of silky hairs
at one end and are liberated from a capsular fruit which, on splitting,
only releases them gradually, often one by one. ‘The plants involved
are usually herbs or climbers, good examples being species of
Willow-herb (Epilobium) and Milkweed (Asclepias), and they gener-
ally occur in open situations, in or from which they can travel for
hundreds of miles.
(d) Plumed fruits. ‘These include the familiar ‘ parachutes’ of
Dandelions (Taraxacum spp.), the long feathery fruits of species
of Avens (Geum), and the silky-haired ones of Cotton-grasses
(Eriophorum spp.). ‘Their appendages cause them to be detached by
the wind and floated away, often for very considerable distances.
The plants concerned are usually herbaceous, and include many
Grasses. An extreme case is that of some disseminules of Grasses
which have been trapped in the air several thousands of feet above
the ground, and in view of the highly fortuitous nature of such
observation it would seem likely that they may reach the upper
air currents quite frequently.
(e) Winged seeds. In these it is usually a thin portion of the
seed-coat which forms a wing that catches in the wind when they
are liberated, often in considerable numbers, e.g. by splitting of
the containing fruit-wall. They chiefly occur on trees, shrubs,
and lianes (woody climbers), and so are liberated some distance
above the ground—which is just as well, for their dispersal mechanism
tends to be much less efficient than those of the categories mentioned
above. Good examples are afforded by members of the Bignonia
102 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
family (Bignoniaceae), and by Pines and Spruces and many other
Conifers.
(f) Winged fruits. Again chiefly occurring on trees and shrubs,
these are so modified as to cause the fruit, on detachment by the
wind, to be borne at least out of the immediate sphere of influence
of the parent plant, or to trundle along as is the case with many
bladder-fruits. Often the flight is a spinning one and, though
spectacular, not very efficient in terms of distance. Each fruit (as
n the Birches, Betula spp.), or half of a separating fruit (as in most
Maples, Acer spp.), is usually one-seeded—functionally, at least.
(g) Long-haired seeds and fruits. ‘These are sufficiently alike to
be considered together, while also approaching (c) and (d), their
main feature being that the surface is covered with long silky or
woolly hairs. Such disseminules tend to be less efficient than
plumed ones but are nevertheless capable of travelling for some
miles. ‘The plants, as in categories (e) and (f), are most commonly
trees or shrubs. Examples of seeds of this nature are those of
Cotton (Gossypium), Willows (Salix spp.) and Poplars (Populus
spp.); and of fruits, those of some Anemones (Anemone spp.).
That this mode of dispersal is abundantly effective, at least so far as
transport of the seeds is concerned, has been frequently and
strikingly demonstrated to the writer when he has looked out from
his laboratory windows in the ancient Botanic Garden at Oxford
and thought a snow-storm was raging, the ‘ flakes’ being masses of
hairy seeds blown from pollarded Willows mostly hundreds of yards
away.
(h) ‘Tumble-weeds. Such plants, or detached portions bearing
the seeds, tend to roll before the wind or be blown across open
country, usually scattering their seeds or fruits as they go. ‘They
are commonly short-lived herbs that branch densely and stiffly from
a central stem and have a rounded form. Normally they break off
easily near ground level and have the seeds or fruits so loose that
they are lost as the aerial part trundles along. ‘Tumble-weeds occur
chiefly in deserts or arid prairies or steppes. Examples include the
so-called Russian-thistle (Salsola pestifer) in North America and
Eryngium sp. on the northern border of the Sahara in Egypt (R. W.
Haines voce). In less ideally displayed form, all manner of plants
or parts of plants can, in special circumstances, act fortuitously as
tumble-weeds—including the so-called Rose of Jericho (Anastatica
hierochuntina), and some Lichens and Mosses in the Arctic.
(1) Other organs or methods. ‘These include pieces of such
4| DISPERSAL AND MIGRATION 103
epiphytes (plants growing on other plants) as Spanish-moss (Til-
landsia usneoides) which get blown to new situations on the trees on
which they grow, often abundantly ; seeds which fortuitously stick
to or get curled up in dead leaves and are transported with them
for considerable distances ; small seeds or fruits which adhere to
sticky stalks (for example of Catch-flies, Lychnis spp.) that are blown
about after detachment ; and soredia of Lichens as well as bulbils,
for example of such Grasses as Poa alpina, that may be usefully
scattered by the wind.
(j) Jactitation. ‘This is the slinging of seeds out of fruits such
as the capsules of Poppies (Papaver spp.) or Mulleins (Verbascum
spp.), which are held aloft on long stalks that are liable to be bent
before the wind or jolted by passing animals—often springing back
subsequently to jerk out some more of the contents in the opposite
direction. Such a‘ censer mechanism’ is commonly feeble, barely
(or even not at all) removing the seed from the immediate sphere
of influence of the parent ; but given the good fortune of a strong
wind to carry the seed farther, or a favourable slope down which it
can bounce and roll, jactitation may occasionally be quite effective.
Fig. 25 shows a wide range of wind-dispersed disseminules.
Here it seems reasonable to suggest that the primary objective
of a disseminule, so far as transportation is concerned, is to get
away from the immediate parental influence and possible competition
of seedlings developing from its brothers, which for many small
plants is effected by displacement of merely a matter of centimetres.
Most dispersal is probably of this relatively minor nature, the long-
distance ‘ saltatory’ dispersal (that may drastically extend the area
and ultimately increase the importance of a race) being supposedly
much rarer.
Before proceeding to the next main topic we should give some
consideration to the barriers and deterrents to wind dispersal,
remembering that it often includes blowing about on the surface of
water whose currents may, moreover, carry originally airborne
disseminules much farther. Wind dispersal operates chiefly on free,
air-buoyant spores etc. in open places—so that it is not unexpected
to find that in treeless, high-alpine and arctic regions an unusually
large proportion of the native plants have wind-borne disseminules,
whereas in dense forests and other sheltered areas wind is little
effective. A great deal of wind dispersal, at least of the larger fruits
and seeds, is discontinuous, bodies being blown up by a gust of
wind and soon alighting to await another gust, the process in some
104
TLR ODMCH LONT f OFFAL BIN I
GEOGRAPHY
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Vi wn, My TURAN
GG aye bain tis Ms
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Ly iy Mbit yp»
i) Yin} OM i
44/4, Min tail
[CHAP.
4] DISPERSAL AND MIGRATION 105
Fic. 25.—Wind-dispersal mechanisms and disseminules. A, capsule of an
Orchid (Cymbidium), open, with minute seeds being scattered by the wind ( 3);
B, fruit of Milkweed (Asclepias), showing liberation of the effectively plumed
seeds (* 4); C, ‘ parachute’ fruit of Dandelion (Taraxacum) ( 2); D, flattened
seed of Macrozanonia with large papery wing ( vs); E, pollen grain of a Pine
(Pinus), with inflated, bladder-like ‘ wings’ making it buoyant in air (x 390);
F, flattened fruits of an Ash (Fraxinus) (* 1%); G, fruit of Maple (Acer) with
flattened wings ( 1); H, fruit of Linden (Lime-tree, Tilia), adapted for wind
dispersal by being attached to a specialized leaf (bract) (< 3); I, capsule of Poppy
(Papaver), from which the seeds are liberated only on violent shaking (* 23).
instances being repeated again and again. When such bodies alight
on even small tracts of water, these are apt to constitute insuperable
barriers to disseminules which cannot float for a protracted period.
Thus it has been observed that plants depending on winged seeds
or fruits for their dispersal are rare on oceanic islands. Dense
forests may have an effect similar to oceans, though of a less finite
nature. However, the fact that many disseminules await a parti-
cularly strong gust of wind before becoming detached from the parent,
is obviously advantageous in that such stronger winds are the more
likely to carry them afar.
Mountain ranges also prove a barrier in many instances—though
the lighter disseminules are easily blown up and over them—as,
to a lesser degree, do cliffs, walls, and fences. ‘This is evidenced
by the fact that beneath such obstacles a wide range of wind-borne
seeds and fruits are often to be found germinating, having been
stopped in their flight and fallen down. Pits and other depressions
have much the same effect in providing a barrier against the heavier
106 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
disseminules. ‘The lightest and most effective of these bodies, on
the other hand, come to grief chiefly through the action of moisture
which clogs their ‘ flying apparatus’, or condenses on them and
weighs them down. In this connection rain is extraordinarily
effective in removing, often during a single shower, practically all
of even the lightest spores, pollen grains, etc., from the atmosphere
through which it falls. For this reason, and because the strongest
and most lasting winds are chiefly at high altitudes, it is mainly
those botanical particles which reach the upper air which travel
really great distances. ‘The fact that many do so appears to be
primarily due to the upward air currents resultant on the warming
of dark land-surfaces by radiant energy absorbed from sunlight.
Finally, for effective migration the plant has to become established,
and to that requisite any lack of suitable climatic or edaphic or
other conditions constitutes an insuperable barrier.
DISPERSAL BY WATER AND ICE
The earliest forms of plant life were probably aquatic and water-
dispersed, and water still plays a very important part in the dispersal
of plants—particularly of those that live in or near it. But although
modifications that appear to be for water dispersal are found in a
wide variety of land plants, they are not so striking, or so widely
necessary, as those for wind dispersal. For practically any light
disseminule may be effectively dispersed by water up to the limit
of its ability to float and retain the power of germination—that is,
until it becomes waterlogged and sinks or decays, or until it is
killed, or, having begun to germinate, has failed to reach a suitable
habitat. Hence the main requirements for water dispersal are
sufficient buoyancy and impermeability, their degree of development
in a particular disseminule being often the most important factor
determining its success.
Among Algae and many higher plants (such as the Canadian
Water-weed, Elodea canadensis) which normally live submerged in
water, there is no need for impermeability: the plant or special
disseminule merely drifts with any water current, sometimes attached
to floating logs, etc. Such drifting appears to be the main mode
of distribution of most seaweeds. Free-floating plants such as
Duckweeds (Lemna spp.) or Water Crowfoots are widely dispersed
as they float on the surface of the water, though they may sink to
the bottom to perennate. A fine tropical example is the Water-
4] DISPERSAL AND MIGRATION 107
hyacinth (Eichhornia crassipes), whose dilated leaf-stalks act as floats,
as illustrated in Fig. 21, B. It is not, however, by any means neces-
sary to float on or drift in the body of water to be water-dispersed.
Thus some seeds or fruits sink at first but rise to the surface on
germination, to drift until they become stranded—perhaps under
conditions ideal for further growth—while many are carried short
distances by rainwash or sudden rushes of water over the ground
or frozen surface, for example during snow-melt in alpine and arctic
regions. Severe floods may dislodge and transport whole trees,
as well as innumerable seeds and fruits that are deposited on the
wet flood-plain when the water ultimately recedes. Also apt to
transport living materials are islands of drifting branches etc., ice-
bergs, drifting ice-floes, and the still larger and more lasting ice-
islands. ‘These are largely fortuitous and probably capable of
involving almost any category of plant from time to time, whereas
the regularly water-dispersed plants normally live in or near the
water and are modified accordingly.
The main modes of water dispersal may now be considered :
(a) Sea currents. ‘These can cause very effective long-distance
dispersal of suitably modified disseminules, in some known cases
for over 1,000 miles. For this the body must normally be able to
float for a long time without becoming waterlogged and must also
belong to a littoral species that can establish itself under saline con-
ditions on a sandy, muddy, or other sea-shore. Coconuts are sc
dispersed, even if there is some doubt as to whether actual migration
is thereby effected ; and among familiar plants of north-temperate
and boreal shores that evidently migrate in this manner may be
mentioned the Oysterleaf (Mertensia maritima agg.) and Sea-beach
Sandwort (Arenaria (Honckenya) peploides agg.). Excellent tropical
examples are afforded by the characteristic dominants of mangrove
swamps, such as species of Rhizophora and Avicennia, the seedlings
of which float widely. Also normally dispersed by sea currents in
the manner of seaweeds are further herbaceous maritime Angio-
sperms—most often as whole plants, parts of plants, or asexual
propagules. On the other hand, the vast numbers of seeds and
fruits of freshwater and normal land plants, and of course individuals
themselves, that are blown into the sea or carried thereto by rivers,
in general perish.
(b) Rivers and streams. ‘These commonly transport fruits, seeds,
and other parts of plants—sometimes as far as from their sources
right down to the sea.__In other cases they may help with the seeding
108 INTRODUCTION, LOVER LEAN TG BOG RAE EY; [CHAP.
of inundated areas. Such dispersal is, however, virtually limited
to the direction of the current and to the particular land-mass con-
cerned, the disseminules of other than marine and strand plants
rarely surviving protracted flotation in the ocean. ‘Thus all manner
of seeds, fruits, and living fragments of aquatic or river-bank plants
are to be seen among the ‘ flotsam’ of debris floating downstream,
often to be left stranded in situations suitable for growth and
establishment, while in tidal estuaries migration is often away from
the mouth of a river, aided by tides which run upstream as well as
down. ‘The ebb of a high tide where the water is brimming widely
is particularly effective in the deposition of floating materials.
Examples of flowering plants regularly dispersed by freshwater
streams are many of the Pondweeds (Potamogeton spp.) whose small
fruits in some instances can float for months on end, and the Yellow
Water-lily (Nuphar lutea), the pulpy fruit of which floats for a few
days before disintegrating and releasing the seeds, which sink and
later germinate. An example of a species whose seeds, as such, are
commonly distributed by water, is the Summer Snowflake (Leucojum
aestivum). Casually, almost any plant or its disseminules may be
transported downstream by flotation, striking examples being the
alpine species that are often to be found in open streamside habitats
in the lowlands. Familiar instances in the boreal regions are such
‘open-soil’ types as Mountain Sorrel (Oxyria digyna), Moss
Campion (Silene acaulis agg.), and various Saxifrages.
(c) Rainwash, floods, and lakes. Rain not only splashes out the
seeds or spores from open organs but, when forming a wash, may
carry them much farther than other agencies commonly do—
especially when it develops into a flush or extensive run-off, perhaps
in time to form a rivulet or even to join a major stream. A con-
siderable run-off may be noted in boreal regions when the snow
melts in spring but the ground remains frozen and impervious.
Often it is not necessary that disseminules, in order to be washed
away, should be able to float, though to reap the benefit of wider
dispersal by ordinary floods they should do so, as of course they must
normally do to be blown about on lakes. Almost any plant or part
of a plant may in certain circumstances be dispersed by drastic
floods, involving as they do the uprooting of trees and the carriage
of all manner of debris, sometimes for considerable distances—per-
haps to be deposited in a silty flood-plain well suited to the establish-
ment of migrant plants. In lakes the methods and plants involved
are in general similar to those in streams, but there is more limitation
4] DISPERSAL AND MIGRATION 10g
of effective dispersal to aquatic and semi-aquatic types, and the
distances of dispersal are usually small. Most often, partially corky
or other air-containing tissues cause the body involved to float, or
buoyant vegetative parts are detached by feeding Mammals or
wildfowl.
Fig. 26 shows a range of water-dispersed bodies.
(d) Icebergs, ice-floes, etc. The ‘rafting’ of all manner of
material, including living plants and their disseminules, after blow-
ing, falling, or spring-time washing on to fast-ice near the shore
or on to glaciers which later ‘ calve’ to form icebergs, has been
widely recognized in arctic and subarctic regions. ‘There can be no
doubt that by this means much material is transported out to sea
and often far away before the ice melts and releases it, though it
seems unlikely that the disseminules of land plants find their way
back to suitable habitats at all frequently. Probably more important,
and certainly more frequent, is the dispersal of Diatoms, particularly,
which grow upon the ice-floes and may in time travel hundreds or
even thousands of miles with them, or of such strand-plants as
Creeping Alkali-grass (Puccinellia phryganodes agg.) which are ‘ picked
up’ after being frozen solid in ice that forms about the shores on
which they grow. It may be presumed that these occurrences were
more widespread during the Ice Ages, though there are instances
occurring even well south nowadays—e.g. in the estuaries of the
Atlantic seaboard of the United States. Ice floating down rivers or
blown about lakes may also be of significance in carrying disseminules
that do not float. The present writer has investigated the plant
materials collected on a large ice-island in the vicinity of the North
Pole, that had drifted many hundreds of miles from the point
where they were washed or blown down from the land on which
they grew. Almost all of these materials were dead, but those
collected when the ice-island had drifted at the very least 3,000 miles,
and quite possibly several times that distance, included an extensive
though thin tussock of the Moss Hygrohypnum polare which was
found to be still alive.
Charles Darwin, in The Origin of Species (6th edn. 1873, p. 326),
after noting that the natives of the coral islands in the Pacific procure
stones for their tools solely from the roots of drifted trees, remarked
that these roots also frequently enclose small parcels of earth * so
perfectly that not a particle could be washed away during the longest
transport : out of one small portion of earth thus completely enclosed
by the roots of an oak about 50 years old, three dicotyledonous plants
E
110 INTRODUCTION TO PLANT GEOGRAPHY [| CHAP.
E
Fic. 26.—Water-dispersed fruits and other bodies. A, sectional view of fruit
of Coconut (Cocos nucifera) showing the thick fibrous outer husk which encloses
much air and enables it to float protractedly ( 3°s); B, germinating seedling of
a Mangrove (Rhizophora) projecting from a fruit that is still attached to the tree
(many such seedlings on detachment can float in the sea for weeks on end) (* 4);
C, inflated capsules of Cardiospermum, the one on the right having been cut through
to show the contained seeds (such fruits may be blown about as well as float)
(« %); D, fruit of Heritiera Iittoralis, adapted for water dispersal by its thick
fibrous husk enclosing an air-cavity (seen in the half-specimen below) (< 3);
E, seeds of Macuna gigantea, adapted for water dispersal by having an impervious
coat and contained air-cavity surrounding the embryo (seen in the half-specimen
on right) ( 1); F, fruits of Lotus (Nelumbo nucifera), embedded in top of enlarged
receptacle (both fruits and receptacle are buoyant) (x 4).
h
| rol
4| DISPERSAL AND MIGRATION ae
germinated : I am certain of the accuracy of this observation. Again,
I can show that the carcases of birds, when floating on the sea,
sometimes escape being immediately devoured : and many kinds of
seeds in the crops of floating birds long retain their vitality: peas
and vetches, for instance, are killed by even a few days’ immersion
in sea-water ; but some taken out of the crop of a pigeon, which
had floated on artificial sea-water for 30 days, to my surprise nearly
all germinated.’ Darwin had already made a conservative estimate
that the seeds of about one in every ten ‘ plants of a flora, after having
been dried, could be floated across a space of sea goo miles in width,
and would then germinate’. Although in the light of modern
knowledge this would seem a rather optimistic guess, at least so far
as practical opportunities are concerned, there is no reason to doubt
that odd instances of such off-chance, accidental long-distance
dispersal do occur from time to time.
As for the barriers and deterrents to water- and ice-dispersal or
effective migration, these obviously include any absence of water,
any obstacle to its movement, or, temporarily, any freezing ‘ solid’
to the bottom. ‘There also seems to be extremely little effective
interchange between salt and fresh water, while a wide ocean or
even lake may constitute a barrier to disseminules which cannot float
and live long enough to cross it; so may, in addition, a different
climate which proves unsuitable for the establishment of a trans-
ported plant.
DISPERSAL BY ANIMALS (APART FROM Man)
With their obvious mobility and life among plants on which they
are largely dependent for food and in other ways, many animals are
important agents of dispersal. Although there are numerous re-
finements in the method of carriage of the disseminules, there are
two main categories—those that are carried externally, by adhesion
to the surface of the animal’s body (the so-called ‘ ectozoic’ or
“epizoic ’ form of transportation), and those that are carried intern-
ally, after swallowing (‘ endozoic’ transportation). For this latter
type of dispersal the seed, fruit, or other disseminule (or container
of disseminules) is commonly modified by being attractive in appear-
ance and particularly as food, for example by its bright colour and
palatable flesh. This should commonly be sweet and juicy when
ripe, as in Peaches, Figs, Raspberries, and Plums. In addition,
the embryo or other vital part should be protected from digestion
IZ INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
by a resistant covering, in which case germination is often hastened
by passage through an animal. For ectozoic dispersal the dis-
seminule is commonly adhesive by means of a sticky surface or,
more often, by its possession of hooks or other devices by which it
catches on to the fur etc. Anyone who has tried to extract the
fruits of Burdocks (Arctium spp.) or Beggar-ticks (Bidens spp.)
from woolly garments will be aware of the effectiveness of such
adhesion.
Some examples of disseminules modified for dispersal by animals
are shown in Fig. 27.
In addition to such * official ’ types of dispersal there is the frequent
‘pecking apart’ by birds: for example, of the seeds or fruitlets
contained in Apples and Rose-hips. ‘There is also the transport
of materials for nest-building, and the still more fortuitous adhesion
of disseminules to the feet, etc., of animals in mud and clay or by
freezing to their fur or feathers. ‘Thus, for example, Darwin (op.
cit., Pp. 328) mentions removing a considerable amount of clayey
earth from the feet of Partridges, reporting that in one instance
around the wounded leg and foot there was ‘a ball of hard earth
adhering . . . weighing six and a half ounces .. . but when...
broken, watered and placed under a bell glass, no less than 82 plants
sprung from it. . . . With such facts before us, can we doubt that
the many birds which are annually blown by gales across great
spaces of ocean, and which annually migrate—for instance, the
millions of quails across the Mediterranean—must occasionally
transport a few seeds embedded in dirt adhering to their feet or
beaks ?’ Quite apart from this, Rabbits, etc., will often drag twigs
for some distance when these are attached to their fur, and Water-
fowl have frequently been observed carrying sizeable pieces of
Pondweeds (Potamogeton spp.) on their backs or around their necks
—even when in flight.
(a) Birds. On account of their abundance almost everywhere in
the world, of the very great distances which many regularly fly,
and of their consequent power to cross wide expanses of water,
Birds tend to be the most important group of animals from the point
of view of plant dispersal. Although it has been contended by some
authors that Birds ‘ fly clean ’ on migration, this does not seem to be
always the case ; indeed, according to Ridley (op. cit. p. 444), it is
‘strongly negatived by much evidence’. Moreover, they are apt to
‘neglect their toilet’ when unwell, and similarly can have materials
sticking or frozen to their beaks, feet, or feathers when flushed or
4] DISPERSAL AND MIGRATION 113
Fic. 27.—Adaptations for dispersal by animals. A, fruits (x 25) of Elephantopus
(left), Cosmos (centre), and Beggar-ticks (Bidens, right), which catch on to animals;
B, fruit of Triumfetta (x 1), with hooks causing adhesion; C, sectional view of
fruit of Peach (Prunus persica), ( 4), showing attractive flesh, protective ‘ stone ’,
and central seed; D, fruit of Strawberry (Fragaria) (x 1), showing superficial
resistant ‘ pips’ enclosing embryos; E, ripe fruit of Nutmeg (Myristica), splitting
to show seed adorned with attractive coloured ‘ aril’ (x 4); F, fruit of Chinese
Forget-me-not (Cynoglossum amabile) ( 4), bearing sticky, hook-like appendages.
114 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
blown out to sea in a gale—or during repeated shorter * hops’ which
sooner or later may amount to considerable traverses. Ridley cites
numerous instances of aquatics, etc., being evidently dispersed to
isolated ponds and marshes by Water-fowl. Kerner (see the work
cited at the end of this chapter), like Darwin, secured ‘a sufficiently
striking result’ with fertile seeds in ‘the mud obtained from the
beaks, feet, and feathers of swallows, snipe, wagtails, and jackdaws
. . and when it is remembered that pigeons and cranes traverse
from 60 to 70 kilometres in an hour, whilst swallows and peregrine
falcons cover as much as 180 kilometres, it is clear that fruits and
seeds affixed to these birds may be carried in a very short time over
several degrees of latitude’.
An interesting case in point seems to be furnished by the sub-
antarctic Macquarie Island, situated approximately 650 km. from
the nearest other land, and supporting thirty-five known species of
vascular plants. It has recently been contended that all of these
could well have been, and indeed probably were, brought in by Sea-
birds since the end of the Pleistocene glaciation. Of these Sea-birds,
vast numbers inhabit the island and many are known to make long
flights to South America, New Zealand, and to other subantarctic
islands. Moreover, many of the habitats on the island, as on
mountains and in the Arctic, are conveniently ‘ open’ for the growth
of immigrants. Unidentified seeds, apparently not belonging to
any of the local species, have been found on Macquarie Island on
Black-browed Albatrosses, adhering to the feet and so coated with
regurgitate that they ‘ could be carried almost indefinitely in flight
and could withstand immersion in sea water if the bird alighted to
rest, yet on landing the seeds would be easily rubbed off’ (Ecology,
Vol 35, pe570, Octabers1954):
As for endozoic transportation, the effectiveness of this will
depend not only on resistance to digestion but also on times of
retention within the Bird’s body. Sometimes, especially after
gorging, seeds may be regurgitated at a distance, without passing
through the alimentary canal. Kerner found that whereas many
types of Birds have in their excreta ‘ under ordinary conditions’ no
seed capable of germination, some others may void unharmed up
to 88 per cent. of the small and smooth seeds or fruits eaten, though
retention in such cases is commonly for only some two or three
hours. But Ridley quotes a report of Pigeons being shot at Albany,
N.Y., ‘with green rice in their crops, which it is thought must
have been growing, a very few hours before, at a distance of 700
4] DISPERSAL AND MIGRATION 115
or 800 miles’; he also gives this distance as the one up to which
he believes frugivorous Birds have visited very many islands, carrying
germinable seeds in their viscera and consequently stocking these
islands with plants.
Earlier, Darwin had similarly remarked (op. cit., pp. 326-7) :
‘after a bird has found and devoured a large supply of food, it is
positively asserted that all the grains do not pass into the gizzard for
twelve or even eighteen hours. A bird in this interval might easily
be blown to the distance of 500 miles, and hawks are known to look
out for tired birds, and the contents of their torn crops might thus
readily get scattered. Some hawks and owls bolt their prey whole,
and, after an interval of from twelve to twenty hours, disgorge pellets,
which, as I know from experiments made in the Zoological Gardens,
include seeds capable of germination. Some seeds of the oat, wheat,
millet, canary, hemp, clover, and beet germinated after having been
from twelve to twenty-one hours in the stomachs of different birds of
prey ; and two seeds of beet grew after having been thus retained for
two days and fourteen hours.’
What a distance they could have gone in a migrating Peregrine
Falcon !
(6) Mammals. ‘These, among animals, stand next in importance
to Birds as disseminaters of plants. Except in the case of Fruit-bats,
which can transport seeds, etc., over stretches of sea much as Birds
do, their disseminative powers are confined to individual land-masses
—apart, of course, from traversable shallow or very narrow waters
or sea-ice in arctic regions (there are no land mammals in Antarctica).
The Mammals are important dispersal agents of many herbaceous
plants with small seeds, which they swallow with the foliage, etc., of
the plants they consume, and are also the main transporters of plants
with adhesive disseminules. Even though many _ herbivorous
Mammals effect such thorough digestion that the vast majority of
seeds and fruits which they take into their bodies are incapable of
germination after voiding, there are nevertheless plentiful instances
of disseminules being excreted unharmed, and we should always
remember the odd animal that dies suddenly, or is killed and eaten
by a predator. Ridley (op. cit., p. 336) remarks that, in the case of
stone-fruits, ‘ almost invariably the seeds pass through the intestines
of the animal, not only unharmed, but much benefited by the treat-
ment. Seeds so passed are known to germinate more quickly and
produce stronger plants than those which have not been swallowed
by bird or animal and acted on by the gastric or intestinal fluids.’
116 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
Fruits destined to have their contained seeds disseminated by
Mammals tend to be less conspicuous than those primarily intended
to attract Birds, e.g. when flying. And even as Birds will devour
the attractive part of a fruit and scatter the seeds without ingestion,
so will many Mammals do to large fruits. Arboreal Mammals, such
as Monkeys, commonly do not eat fruits when they gather them,
but quietly remove them to a distance—apparently to avoid being
robbed. If they drop a fruit they do not pick it up, but go on to
another. Moreover, such types as Squirrels make large winter
caches that may involve extensive transportation and frequently are
not eaten in the end. ‘hese and many other activities of Mammals
can help plant dispersal within continental confines.
Because of their frequently furry coats, Mammals tend to be more
commonly effective than Birds in the ectozoic transportation of
adhesive fruits, such as those with hooks or other devices for attach-
ment. Many fur-coated Mammals wander extensively, or travel far
on migration, some even crossing wide tracts of sea-ice in the Arctic.
Apart from being furnished with obviously effective hooks or spines,
some seeds and fruits adhere to animals by viscid glands, gummy
exudations, or owing to their wholly sticky nature, while the spikelets
of many Grasses do so by jagged parts or minutely toothed awns.
Many other seeds and fruits which are normally wind-dispersed,
will adhere to animals by entanglement or sticking of their hairs or
plumes especially when wet. ‘There is, indeed, no lack of means or
instances of such dispersal, as inspection of one’s clothes at the end
of an autumn walk in a temperate woodland will show. Moreover,
it should be remembered that animals, like plants, are selective of
habitat, and tend to keep, as Birds tend to alight, within a single
habitat-range—so increasing the chances a disseminule would have
of coming to rest in a place suitable for germination and successful
establishment. For instance, the rocky ridges and ravines in boreal
regions that are inhabited by such birds as Snow Buntings and
Ptarmigan, which commonly ingest seeds and migrate from one to
another such area, afford numerous habitats for open-soil Saxifrages
and Sandworts which may be lacking in intermediate areas.
(c) Lower animals. Although most of the Reptiles of the present
era are carnivorous, some feed on fruits and may disseminate them.
More important in this respect are freshwater Fishes, many of which
are vegetable feeders that swallow the seeds of aquatics and semi-
aquatics, and some of which can migrate overland, usually through
wet Grass. Of a wide range of seeds or fruits of aquatic plants, such
4] DISPERSAL AND MIGRATION 117
as Bog-bean (Menyanthes trifoliata) and Pondweeds, that have been
fed to Perch and Roach, nearly all germinated after being retained in
the viscera for one to three days before being passed naturally. ‘These
fish are liable to be eaten by such predators as Fishing Eagles,
Herons, and Pelicans, which, after an interval of many hours, either
reject any contained seeds in pellets or pass them in excreta—often
still in a viable condition, as was shown by Darwin. ‘The same
doubtless happens to many Algae, aquatic Fungi, etc. By the time
such plant material is ejected, the carrying bird may have flown
many miles. Some of the larger aquatic Crustacea and Mollusca as
well as, of course, Reptilia, obviously play a part in the dispersal of
Algae which grow epizoically upon them or their shells ; while on
land, Snails and Slugs disperse seeds and spores that adhere to their
bodies or have been swallowed. Indeed, it is said that the spores
of some Fungi will only germinate after passing through a Slug ; and
when the latter is eaten by a Toad, Bird, or other predator, the
possibility occurs of far more extensive dispersal.
Insects are probably the most important of the groups of lower
animals in the matter of plant dispersal, especially of very small
bodies such as fungal spores. ‘Transport is commonly by swallowing
and * passing ’ in the excreta, by carrying to their nests for food, and
by adhesion. Locusts are said to afford examples of the first method,
sometimes over considerable distances, and ants frequently transport
seeds with edible appendages, while flies and many other insects
often carry spores of cryptogams adhering to their bodies—especially
when the latter are densely hairy. Further instances are the well-
known transmission of Fungi- and Bacteria-engendered diseases by
insects, as well as important viruses (such as those of Potatoes)
having aphid vectors.
It seems desirable here to treat briefly the subject of pollen.
As we saw in Chapter II, pollen is composed of vast numbers of
microscopic ‘grains’. These, though capable of producing on
germination only a tiny particle of plant, and hence scarcely to be
considered as true disseminules, nevertheless carry the potential
male gametes and, in them, the genes introducing hereditary
characters. As it is now known that transport of pollen can in some
circumstances take place naturally over many hundreds of miles,
and that given suitable conditions some detached pollens can live
for many months, it seems conceivable that by this means heritable
characters may be transported vast distances. To be sure, the
grain has to find its way to a receptive female stigma to have any
118 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
chance of effective survival. But when it is recalled that pollen
grains are formed each year in trillions of trillions, and that a single
pollen ‘ bullet’ finding its stigmatic billet in a millennium might
suffice to carry thither the genes of any subspecific characters it may
possess, the possibility can scarcely be denied of what has facetiously
been termed ‘ absent-treatment hybridization’. Wind and insects
are the chief transmitting agents of pollen, though carriage is also
effected by some other animals (especially small Birds) and by
water. Most of the strikingly beautiful features of flowers, as well
as their possession of nectar and scent, are adaptations to attract
insects to gather pollen for the purpose of cross-fertilization, and so
it is to be expected that this is very commonly effected, though
chiefly over rather short distances.
DISPERSAL BY HUMAN AGENCY
There can scarcely be any question that Man is the most active
agent of vegetational change—including plant dispersal—of modern
times. He is the greatest despoiler of forests and causer of erosion,
dispersing weeds as well as growing crops. As he travels about the
world in greater and greater numbers and with ever-increasing speed
and ease, he is always transporting the disseminules (or sometimes
transplanting whole individuals or groups) of plants either intention-
ally or unwittingly. Also of vast importance is his indirect effect,
through the pasturing of his domestic animals or his disturbance
of natural communities of herbivorous animals. As a result, there
are few parts of the world where the vegetation and its component
flora do not bear the stamp of Man’s interference, and quite a few
areas, for example in Hawaii and Ceylon, where the native plants
have been largely ousted by alien ones. In general, however, unless
there is some drastic disturbance of the natural vegetation, recently
introduced plants fail to compete successfully with the native
dominants and, consequently, take only a minor part in the con-
stitution of most plant communities. Often these aliens are limited
almost entirely to burned-over or otherwise cleared areas—such as
waysides and abandoned fields, which characteristically support
hosts of weeds.
Between the extremes of those plants, such as many horticultural
strains, which are restricted to gardens and need constant tending,
and those which, following introduction, have so thoroughly estab-
lished themselves by natural agency that they are distinguishable
4] DISPERSAL AND MIGRATION 11g
from the indigenous flora only by their known history, we see all
manner of degrees of success in establishment. Some aliens flourish
for a time and then disappear, while others, after many years of
restriction to one locality, suddenly burst forth all over a countryside.
A notable example of the latter category is the Oxford Ragwort
(Senecio squalidus), which was introduced into the Oxford Botanic
Garden late in the seventeenth century but scarcely spread at all
until late in the nineteenth century, when it started migrating along
the railways. ‘Thereafter, migration proceeded so extensively that by
the nineteen-twenties it became known from the vicinity of railways
in other counties, and when, in the nineteen-thirties and -forties, the
present writer was in charge of the botanical collections at Oxford,
it was apt to be sent in from quite remote districts of Great Britain
as a curiosity or for identification.
It will be sufficient—without going into detailed examples which
could fill whole chapters—to indicate here some of the main methods
by which Man introduces plants to new areas and, often, new
countries and even continents (for it is said that the majority of
alien plants in Australia and New Zealand come from Europe).
In addition to intentional transport of desirable plants for agri-
cultural, horticultural, forestral, medicinal, or other purposes, weeds
are often dispersed unwittingly with the seeds of vegetables, cereals,
and garden flowers, as well as with pot plants and in making trans-
plants. All manner of disseminules and whole plants are dispersed
accidentally (but quite commonly) by land or water traffic, garbage
removal, and in baggage and soil transportation, while admixture
in animal fodder, litter, and manure are other extensive means of
transport. Dispersal used to be widely effected in ships’ ballast and
still is in many packaging materials, often to the far corners of the
earth, as it is also in bird-seed and building material, or as algal
growth attached to ships’ hulls. Other sources of dispersed dis-
seminules are timber and drug and spice materials—such as Caraway
seeds imported by the Danes to Greenland for flavouring bread, with
the result that the plant, Carum carvi, is now common around many
of the settlements. Indeed, very many kinds of commercial export—
import traffic must involve the carriage of disseminules, some of
which evidently lead to fresh introductions ; the same is true of
personal travel, for people often carry a considerable range of seeds
and fruits about their clothing, and, doubtless, greater numbers of
microscopic spores.
In this connection air travel may be particularly effective, for one
120 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
steps on to an aircraft in one continent and off it in another, often
with little movement meanwhile to brush off adhering seeds and
fruits. Much the same may be true of transported animals, which
are always apt to carry seeds and fruits in their wool and fur, or
otherwise about or in their bodies, and evidently account for many
plant introductions. Moreover, there is practically no limit to the
number and diversity of seeds and fruits that adhere to men and
women when they fall or merely walk in mud and clay, to be brushed
or picked off later, or are transported by them for food or as curios—
often to be discarded at a distance. And to the abilities of some
seeds to pass through the human digestive tract unharmed, the
‘spontaneous’ growth of ‘Tomato plants in sewage farms bears
ample testimony. Finally, with their hosts are frequently carried
parasitic (and also saprophytic) species. ‘hat such spread of plant
diseases can be very serious is indicated by the rigid restrictive
measures adopted by many governments against the importation of
living plants.
The great differences often observed in the actual migration of
thus ‘artificially’ introduced plants are, however, probably due
more to the adaptability of the species to local environments than
to the dissemination itself, essential though this is. As we shall see
in Chapter VI and elsewhere, plants tend to be adaptable when they
are variable—in habitat requirements as well as in form. Accord-
ingly, some familiar European weeds, such as Shepherd’s-purse
(Capsella bursa-pastoris), Common Chickweed (Stellaria media agg.),
and the little grass Poa annua, have become practically world-wide
without having any special adaptations for long-distance dispersal,
whereas other plants the disseminules of which are doubtless more
commonly, and sometimes more widely, carried, are still relatively
restricted in their geographical area. Often the climatic, soil, or
other local conditions are unsuitable ; or the competition of native
plants is so severe that ‘ open’ habitats have to be found—especially
by weeds. Such open habitats are commonly due to Man’s acti-
vities, soon becoming closed over with vegetation when abandoned,
so that the colonizing aliens become restricted or often ousted. In
time the signs of human interference may virtually disappear,
though, as has already been emphasized, such interference is nowa-
days so widespread and drastic in various ways as to constitute the
most active agent of vegetational change in the world. Moreover,
as compared with earlier times, the barriers to dispersal by human
agency are greatly diminished now that men can (and frequently do)
4] DISPERSAL AND MIGRATION L2T
travel to almost all parts of the world in a matter of days, and
traverse vast distances in a very few hours.
MECHANICAL DISPERSAL
Although it is usually effective over only short distances, mech-
anical propulsion or even extensive growth can be of distinct
advantage in migration. ‘Thus plants which shoot out their dis-
seminules can thereby launch them into a goodly wind or on to a
passing animal that will carry them for miles. And often it is
agitation by wind or an animal which sets off the explosive mech-
anism. Furthermore, the aggressive growth of overground runners
(Fig. 28, B) and underground stems (rhizomes, see Fig. 28, A) often
give plants a distinct advantage in competition over their neigh-
bours, so that when, as is often the case, the peripheral growth is
detached as a separate plant, for example by the death of the parent,
it may be established at an appreciable distance ; and such distances
mount up usefully through the generations. As examples, Ground-
ivy (Glechoma hederacea) can trail a distance of 20 feet (about
6 metres) along the ground, and Elms can reproduce by suckers from
underground roots at fully 50 yards (about 46 metres) from the
parent tree. Even such growth as that of the Walking Fern shown in
Fig. 22 leads, in due course, to a worthwhile amount of dispersal.
Particularly effective are the explosive spore-discharging mech-
anisms of some Fungi, which, usually on sudden rupture to relieve
stresses, may shoot their spores or spore-producing organs in some
instances as much as 15 feet. However, in the case of spores, a
tiny distance to take them into the free air is often sufficient to
launch them in atmospheric currents that may carry them practically
anywhere. Also capable of being shot out for distances of as much
as 3 feet are the bulbils of some Club-mosses (Lycopodium spp.).
Better known, however, are the explosive mechanisms of some fruits,
of which examples are shown in Fig. 28 and more may be cited.
The ‘records’ seem to be held by species of a genus of small
parasitic Mistletoes (Arceuthobium), followed by tropical American
trees of the Spurge family (Euphorbiaceae), particularly the Sand-box
Tree (Hura crepitans), which can throw its seeds more than forty
feet, and Para Rubber (Hevea brasiliensis), whose performance is
nearly as good. The explosion of a Hura fruit is spoken of as a
‘regular detonation’. In a similar manner, on drying of the fruits,
even small herbaceous members of the Spurge family often shoot
out their seeds for a distance of a dozen or more feet. In the case of
DISPERSAL AND MIGRATION ; 128
E F
Fic. 28.—Dispersal by extension of growth or by mechanical propulsion, ete.
A, horizontal rhizome of a Grass (the sand-binding Marram Grass, Ammophila
arenaria (* about 4); B, Strawberry (Fragaria) runner establishing daughter
plant (= 4); C, fruit of a Balsam (Jmpatiens balsamina), which explodes and
scatters the seeds ( 1); D, seed dispersal in the Squirting Cucumber (Ecballium
elaterium (* about $); E, ripe fruit of Pansy (Viola sp.), showing seeds ready
to be, and being, shot out (* 2); F, over-ripe fruit of Geranium (Geranium
sp.), showing slings which have thrown out the seeds (* about 24).
one species of Arceuthobium the tiny bullet-shaped seeds have been
reported to travel over 66 feet (about 20 metres) from a point 8 feet
above the ground, and in one instance large numbers were collected
from the roof of a cabin one-quarter of a mile (about 402 metres)
from the point of liberation—presumably after transportation by
wind, though the seeds are also viscous and apt to be carried by birds.
Such a combination of explosion and adhesion is utilized also by
the Squirting Cucumber (Ecballium elaterium), shown in Fig. 28, D.
When the fruit is ripe, it breaks from the stalk, and through the hole
thus left the internal pressure is relieved by the seeds being ejected
with an abundance of mucilage and with such force that they com-
monly fly for several feet through the air. A slight touch will send
off the ripe fruit, so that a passing animal is liable to receive a broad-
side—and, incidentally, carry the adhering seeds much farther.
There are many varieties of this type of turgor-engendered explosion,
another being exhibited by certain Cresses (Cardamine spp.) that
have explosive pods of which the narrow valves, on being touched
when ripe, suddenly curl outwards with some violence, shooting out
the seeds—sometimes for more than 2 feet. In the case of such
diminutive plants, this is ample to take the seeds away from the
parental sphere of influence. Notable among the rather many and
diverse plants which do this sort of thing are the Balsams (/mpatiens
spp.), in which the wall of the fruit is made of three layers of which
124 INTRODVICTION LO, PLANT GEOGRAPHY [CHAP.
the innermost consists of large turgid cells. When ripe, especially
if touched, the wall of the fruit suddenly separates into five segments
that curl inwards violently (cf. Fig. 28, C), shooting out the seeds
—in some species for fully 20 feet. In the Wood-sorrels (Oxalis
spp.) the ripe fruit suddenly splits lengthwise or by lateral slits
when touched, shooting out the mucilage-covered seeds.
In many fruits the explosion that leads to a forcible ejection of
seeds is caused by stresses set up on drying. ‘The audible cracking
of the pods of some members of the Pea family (Leguminosae) 1s of
this nature, the two valves of the pods (for example, when drying
in the sun) suddenly separating with often a violent spiral twisting,
and forcibly ejecting the hard and smooth seeds. Familiar European
examples of this are furnished by the Gorses (Ulex spp.). ‘The
action is due to a hard layer of strongly thickened, elongate cells
lying transversely, and to which the softer tissues offer little resist-
ance. ‘he distances to which seeds are shot by these means vary
greatly, but in some instances are said to exceed 40 feet and at
least rival the ejections of Hura and Hevea. ‘Thus the turgor- and
drying-induced methods may be about equally effective.
Many species of the familiar genus Vzola, including some wild
Violets, in which the fruit splits into three boat-shaped valves
(Fig. 28, E), shoot out their seeds for up to 15 feet as a result of unequal
drying of the layers of the fruit wall. ‘This drying causes a curving
of the sides of the valves and the consequent pressing of the glossy
seeds together—until they ‘ pip out’, one after another, being often
further dispersed by rainwash. Also dispersed on explosion of the
hard ripe fruit are the seeds of almost all members of the family
Acanthaceae—sometimes to nearly 30 feet from the parent—and
those of Claytonia, Montia, some Phloxes, and the Witch-hazels
(Hamamelis spp.). In the last instance the drying fruits may exert
such pressure on the seeds that these are discharged, like miniature
bullets, to distances of up to 40 feet—again rivalling Hura and Hevea.
In many members of the Geranium and Stork’s-bill family
(Geraniaceae) the fruit suddenly splits into strips which curl up and
act as slings (Fig. 28, F) to throw out the seeds, which may travel
as much as 20 feet. Some fruits are also effectively dispersed by
mechanical propulsion, including those of Flat-figs (Dorstenia spp.)
which are embedded in the large fleshy receptacle that shrinks on
drying, setting up pressures which lead to the tiny fruits being
forcibly ejected. ‘The spores of many Ferns are well known to
be discharged forcibly into the air by the springing backwards of
4] DISPERSAL AND MIGRATION 125
part of the capsule when it has dehisced and attained a certain degree
of desiccation, while the movements of teeth of Mosses are hygro-
scopic, curving backwards to open the capsule and disseminate the
spores. ‘These actions take place chiefly in dry weather when
conditions are best for dispersal. Also able to move as a result of
hygroscopic changes are many awned fruits, etc. Finally, it should
be recalled that the spores or even the whole bodies of many of the
lower cryptogams are actively mobile, swimming by means of flagella
being particularly common among them,
BARRIERS
When we reflect that in many species of flowering plants, such as
Flixweed (Sisymbrium sophia) and Pigweed (Amaranthus retroflexus).'
a single individual may produce a million or more seeds in one
summer, and that some cryptogams, such as the Giant Puffball
(Lycoperdon (Calvatia) giganteum), may produce as many as several
million million spores, and yet none overruns the world,? it is obvious
that only an infinitesimally small proportion of the plant disseminules
produced ever attain their real biological raison d’étre. ‘To realize
its full potentiality, a propagule must develop into an adult which
in turn reproduces. ‘This stupendous mortality is due to the action
of various types of barriers—either to dispersal or to actual survival
—of some of which we have already seen examples as applied to
particular agents of dispersal. ‘They are of four main types :
(1) Physiographic, due to features of the earth’s surface. ‘The
most obvious of these for terrestrial plants are expanses of water, and,
for aquatic plants, bodies of land. Another physiographic barrier
is afforded by mountains—both directly by constituting a mechanical
impediment, and indirectly by changing climatic and allied conditions
such as air temperatures and currents. Many local winds are
caused by a combination of physiographic and climatic factors, and
constitute virtual barriers to dispersal in one direction even as they
may aid it in another.
1 An individual of this annual species has been known to produce an estimated
2,350,000 seeds.
*It is said that an individual Giant Puffball can produce 7,000,000,000,000
spores, and it was calculated by the late Professor A. H. R. Buller (Researches
on Fungt, vol. III, 1924) ‘ that, if every spore of this puff-ball had germinated
and given rise to a puff-ball like its parent, and if every spore of the second-
generation puff-balls had likewise germinated and given rise to a puff-ball like its
parent, then, at the end of these two filial generations only, there would have
come into existence a mass of puff-ball matter equal to 800 globes the size of
the planet on which we live!’
126 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
(2) Chmatic, involving different temperature, humidity, light, and
other conditions. Owing to the close dependence of plants on
climatic conditions, zones of vegetation and climate tend to cor-
respond with one another, the climate commonly determining the
general limits of a plant’s distribution. A change of climate, such
as a migrating plant is apt to find in a new land, often constitutes a
very real or insuperable barrier—not only as a whole but, very
often, as to one or other of the climate’s component factors, which
may react in a particular way on the plant’s physiological make-up or
a vital part thereof. Moreover any condition, including lapse of time,
which proves lethal to disseminules may constitute a major barrier,
lack of viability being an important factor militating against migration.
(3) Edaphic, due to features of the soil. ‘These are again various,
involving physical structure, chemical composition, moisture con-
tent, temperature conditions, or even content of living organisms,
any one of which alone can prevent a disseminule from establishing
a plant in a new area, even if it germinates quite successfully.
Either in combination or separately, edaphic conditions tend to
limit the distribution of plants (and, of course, vegetation) rather
drastically within the main climatic belts—commonly to particular
habitats, which may be narrowly prescribed in their type and of
very limited extent. Absence of the suitable habitat, or at least of
the required conditions, is apt to constitute an insuperable barrier
to successful migration.
(4) Biotic, due to living organisms, including other plants. ‘The
competition for space, light, water, etc., of other plants already
established in an area and growing in reasonable equilibrium with
local conditions, is apt to constitute an insuperable barrier to the
successful establishment of newcomers, as is grazing or other dis-
turbance by animals (including Man). Consequently widespread
immigration is largely limited to more or less “ open’ areas, such as
cliffs, sands, and disturbed ground—where other sets of barriers
come into play. If they did not, virtually every scrap of soil or ray
of light would probably be utilized ; for the struggle for existence
is a very real and desperate one, taking place chiefly between organ-
isms where the general conditions for life are good, and predominantly
with the physical factors of the environment where conditions are bad.
The magnificent perseverence and virtual ubiquity of plant life
are vividly exemplified to the author by the myriad Diatoms which
may so impressively if variously colour ice-floes on arctic seas.
During long flights over the North Polar Basin and elsewhere he
4| DISPERSAL AND MIGRATION 127
has observed such ‘ dirty’ floes in many places up to almost the
highest latitudes (the ice immediately around the North Pole appears
to be ‘clean’). From the air, some of the browns and yellows of
these floes seem not far removed in colour-effect from some barren
arctic limestones—as the author and his pilot had occasion to remark
once in 1946 when flying over Foxe Basin and sighting an unexpected
island of limestone which turned out to be some go miles long and
nearly as wide. It was officially ‘ discovered’ two years later by
the Royal Canadian Air Force and added to the world’s map as
‘Prince Charles Island ’, being, with its neighbours which were also
noted on that 1946 occasion, evidently the last major land discovery
or confirmation to be made in the world. Subsequent exploration
failed to reveal any unexpected features of plant life. Until the
advent of general air travel not so long ago, many areas even in
comparatively low latitudes were difficult or at least tedious to visit.
But now in superb antithesis we are looking to other planets for
explorational opportunities, and the prospects of space travel are
advancing so rapidly that the author is prompted to guard himself
by remarking that the present volume is concerned purely with
vital phenomena as we know them on the Earth and in its immediately
surrounding atmosphere—regardless of any possibilities elsewhere.
FURTHER CONSIDERATION
H. N. Ripiey. The Dispersal of Plants throughout the World (Reeve,
Ashford, Kent, pp. xx + 744, 1930). A monument of usually
authoritative information on the methods and effectiveness of plant
dispersal, covering almost all aspects of the subject. Its author died
recently at well over 100 years of age, maintaining that such studies,
of which there are still not nearly enough, are fine preservers of life.
CuHaRLES Darwin. The Origin of Species by means of Natural Selection,
sixth edition, with additions and corrections (Murray, London,
pp. xxi + 458, 1873).
H. B. Guppy. Plants, Seeds, and Currents in the West Indies and Azores
(Williams & Norgate, London, pp. xi + 531, 1917). A work of
wider implication than its title suggests.
A. KERNER (von Maritaun), & F. W. Oxiver. The Natural History of
Plants, vol. U1, pp. 790 et seg. (Blackie, London, 1895).
Sir E. J. Sarissury. The Reproductive Capacity of Plants (Bell, London,
pp. Xi + 244, 1942).
L. V. Barton. Seed Preservation and Longevity (Leonard Hill, London
—in press).
C. T. Incotp. Dispersal in Fungi (Clarendon Press, Oxford, pp.
vill + 197, 1953).
CHAPTER V
EVOLUTIONARY DEVELOPMENT AND
Poet iS Oren
In Chapter II we gave a brief general account of each of the main
classes of the plant kingdom now living, mentioning that the sequence
used was probably indicative of evolutionary history at least in broad
outline. ‘The classes treated were usually those most important as
components of vegetation at the present time, regardless of their
significance in the past. ‘Those omitted even included some that
had been vastly important in earlier ages but had subsequently
become extinct or nearly so.
Evolution is a continuous process, that started with the earliest
forms of life and still goes on abundantly today. And even as the
numerical representation and importance of a particular group of
organisms may go down as well as up, so may evolution manifest
itself in simplification or decline as well as advance. ‘This is parti-
cularly evident in many parasitic forms of both plants and animals.
Nevertheless the general trend is towards advancement in complexity
if not always in size, economy in the use of material being also
important ; with these tendencies and the need for adaptation to the
environment and other circumstances constantly in mind, we can
place the forms known to us in a sequence that seems most likely
to be the actual one of their own evolution. ‘This will be done
briefly below for those groups that are important as fossils, regard-
less of the significance of relatives living today, but with close
reference to these in order to link relationships in the mind’s eye.
It is necessary here to give an outline account of the geological
time-sequence. In this, four major eras are defined, the last three
of which are divided into several periods or epochs each. It is now
believed that the earth had its beginnings about 4,500,000,000 years
ago. ‘The latest measurements indicate the age of the oldest known
rocks to be about 3,300,000,000 years, and the era from that time
to about 550,000,000 years ago constitutes the pre-Cambrian. ‘This
era has often been divided into two—the ‘ older’ Archaeozoic (of
metamorphic and igneous rocks) and the ‘ younger’ Proterozoic (of
128
EVOLUTIONARY DEVELOPMENT AND PAST HISTORY 129
sedimentary rocks). At least in the latter time, relatively simple
Algae and Sponges and apparently also Fungi and Bacteria were
widespread. ‘The pre-Cambrian was followed by the Palaeozoic era,
of invertebrates and Fishes and large Pteridophytes. It extended
for about 360,000,000 years from the Cambrian period through the
Ordovician, Silurian, Devonian, and Carboniferous (Mississippian
and Pennsylvanian) periods to the Permian period, which ended
about 180,000,000 years ago. Next came the Mesozoic era, which
extended for some 130,000,000 years through the ‘Triassic, Jurassic,
and Cretaceous periods and was the great era of Reptiles and Gymno-
sperms. Finally, extending over the last 60,000,000 or so years, has
been the Cainozoic (Cenozoic) era, of Angiosperms and Mammals.
This is commonly considered to consist of two periods, the Tertiary
(made up of the Paleocene, Eocene, Oligocene, Miocene, and Pliocene
epochs) and the Quaternary. In the higher latitudes and altitudes
this last period consisted of alternating glacial and interglacial times
and may be referred to as the Pleistocene epoch, the ‘ recent’ being
the time since the last ice recession took place, although some con-
sider this a mere interglacial. ‘The Quaternary period has extended
over perhaps the last 1,000,000 or so years! and has seen the advent
and ascendancy of Man. A chart showing the eras and main periods
ete; is given in Fig. 37 (p. 145).
Groups OF Fosstt LOWER PLANTS
We can only guess at the form of the first living organisms, which
were probably not distinct as either plants or animals, but must have
possessed the powers of deriving energy from outside sources and of
sustaining themselves. Presumably they were microscopic bits of
naked protoplasm far simpler than any organisms of which fossils
are known. From such a source sprang the ‘tree of life’, near the
bottom of which the Bacteria appear to remain. ‘These organisms
play such essential roles as agents of decomposition that it is difficult
to conceive of the balance of nature being maintained without them,
and indeed there is evidence that they were in existence in very
1 According to F. E. Zeuner’s Dating the Past : an Introduction to Geochronology,
third edition (Methuen, London, pp. xx + 495 and 24 additional plates, 1952).
Other modern estimates range from one-half to double this total, a difficulty being
to decide at what point in time the Quaternary began. Very recently, Professor
Zeuner has suggested (in litt. 1957) that ‘an estimate of 600,000 years for the
period from the First [Pleistocene] Glaciation onwards is a reasonable one ’.
130 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
early geological times. ‘Thus in rocks far back in the pre-Cam-
brian! are found supposed signs of Bacteria in the form of filamentous
chains and minute spherical bodies, often associated with slender
branching filaments and other remains believed to be of Blue-
green Algae (cf. Fig. 29, C). What appear to be the oldest known
structurally preserved organisms that clearly exhibit cellular differ-
entiation and original carbon complexes are in pre-Cambrian sedi-
ments of southern Ontario and represent Blue-green Algae and simple
forms of Fungi or possibly Algae (see Fig. 29, D). It seems probable
that their age exceeds 1,000,000,000 years, and it may quite likely
be nearer to 2,000,000,000 years. From later on, and at least
beginning with the Devonian around the middle of the Palaeozoic
era,! there are plentiful indications that Bacteria were practically
ubiquitous, as indeed they are today. Nor is there evidence of major
evolutionary change in their structure through those many millions
of years.
Algae, which among living plants probably rank next in importance
to vascular ones in the formation of contemporary vegetation, are
also significant for the roles they appear to have played in the
formation of petroleum and limestone. As a result of Algae obtain-
ing carbon dioxide for photosynthesis from (soluble) calcium
bicarbonate, the relatively insoluble calcium carbonate was left as a
1 See previous page and Fig. 37 concerning the geological eras, etc., with their
dominant forms of life and supposed ages. Fig. 37 also indicates the approximate
relative development of the main plant groups at different times. It is now thought
that the earth may be of the order of 4,500,000,000 years old, and that life may
have begun on it about half-way along the time to the present.
5] EVOL ULLONARY DEVEL ORMENT AND! PAST HISTORY 127
E F
Fic. 29.—Some primitive plant fossils. A, Collenia undosa, a Proterozoic fossil
believed to have been formed by the action of one or more Blue-green Algae
(after Walcott) (= 4); B, Newlandia concentrica, a Proterozoic fossil apparently
formed by the action of a Blue-green Alga (after Walcott) ( #); C,a Proterozoic
fossil, apparently a colonial Blue-green Alga consisting of aggregations of filaments
in globose sheaths (courtesy of E. S. Barghoorn and Science) ( about 200); D,
a Proterozoic fossil of fungal or possibly algal type, showing spores and non-
septate hyphae (courtesy of E. S. Barghoorn and Science) ( about 400); E, a
Cambrian Alga, Dalyia racemata (after Walcott) ( about 3); F, a Cambrian
Alga, Marpolia aequalis (after Walcott) (Xx about 2).
132 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
residue and became deposited as limestone. Limestones believed
to have been formed in this manner by Algae occur in extremely
early rock formations, and apparently such deposition of calcium
carbonate as a result of algal activity has been going on ever since.
However, owing to such features as their generally soft bodies, Algae
often leave no traces of their original cellular structure, and so as
fossils they are difficult to recognize with certainty. In any case
there seems no doubt that, well before the end of the Cambrian
period, not only an abundance of Blue-green but also large numbers
of Green and some Red and probably Brown Algae had evolved.
Certainly all of the first three groups were plentiful in the Ordovician.
‘wo examples of fossil Algae of the Cambrian period are shown in
Fig. 29, Eand F. Although Desmids and even Stoneworts are now
known to go back into the Palaeozoic, being present in the Devonian
period, the Diatoms appear to be of more recent origin, none being
known before the Jurassic period in the middle of the Mesozoic era.
It may well be that many groups of Algae have remained evolution-
arily static throughout the time since their remote ancestors laid
down some of the earliest fossils of which we have knowledge.
It was indicated above that the Fungi as a group are also very old,
and there is no reason to doubt that they have acted as scavengers
throughout their long geological past, even as they act today. Indeed
it appears that fossils were chiefly formed when deposition of plant
material took place under conditions unfavourable to fungal growth,
so that the usual destructive activity of Fungi was evaded. Well-
preserved fungal mycelia and spores have been found in the tissues
of vascular plants as far back as the Devonian, and Fungi apparently
occur in sedimentary deposits of much earlier date (see p. 130).
Such discoveries have not, however, shed any clear light on the
origin of the Fungi, which have long been believed to have evolved
from Algae through the loss of chlorophyll. However, some authori-
ties now hold that Fungi were derived from a distinct group of
primitive organisms, the similarities with Algae being due to parallel
evolution, and it may well be that, unlike the various members of
most other groups, Fungi had no common starting-point but origi-
nated at different times and in various groups. Fossils of higher
Fungi’ do not appear with certainty until the Cretaceous, and no
indubitable early fossils of Lichens are known.
‘The Nematophytales comprise an extinct group of spore-producing
Silurian and Devonian plants of uncertain relationship, in which the
plant body was composed of a system of interlacing tubes. It may
5] EVOLUTIONARY DEVELOPMENT AND PAST HISTORY 133
be that their affinity is with the Algae, and it even seems conceivable
that they may represent the long-sought link between that group
and the lowest land-plants. A further link may be forged by
certain cellular forms producing firm-walled spores, which forms,
like some Nematophytales, have a covering cuticle resembling that of
land- plants, but which seem to belong to a relatively advanced group
without actually being typical land-plants in other respects.
Fossil remains of Bryophyta are not common, probably owing to
the fragile nature of the plant body. Some Liverworts are known
from as early as the Carboniferous towards the close of the Palaeozoic
era ; traces of Mosses have also been found in rocks laid down before
the end of the Carboniferous. From the Triassic period onwards,
fossil Bryophytes tend to become less rare, so that a considerable
number are known from the Pleistocene. It now seems that fossil
Bryophytes throw little if any light on the problem of the origin of
vascular plants, and that there is no justification for thinking them
ever to have served as intermediate stages in the evolution of
higher plants. Instead, these presumably evolved from Algae, as
also with little doubt did the Bryophytes—but in this latter instance
without going much ahead. Consequently they are little changed
to this day, although they are now numerous and often ecologically
successful within the limits prescribed by their relative diminutive-
ness.
By the close of the Silurian period there were undoubted land-
plants, the primitive aquatic or semi-aquatic Algae (perhaps through
more advanced types such as the Nematophytales or others of which
we have no knowledge) having apparently come out on land and
given rise to vascular forms. As we saw in Chapter II, these last
are characterized by the possession of a conducting system com-
posed essentially of wood and bast elements ; nor is it by any means
impossible that such a system was developed among marine ‘Thallo-
phytes, the more adaptable of which may gradually have become
transformed to withstand permanent life on land. At the same time
their holdfasts could have developed into rhizome-like structures
bearing rhizoids. For such are the earliest known land-plants, and
so may the great ‘ subaerial transmigration’ have taken place.
The earliest known plants that were clearly adapted to life on land
belong to the class Psilophytineae, which was probably more primitive
than any of the other Pteridophytes, and of which some reconstructed
examples are shown in Fig. 30, A and B. ‘They range from the
middle Silurian to the upper Devonian periods of the Palaeozoic era,
Fic. 30.—Some Psilophytineae. A, two species of Rhynia, showing the sporangia
at the ends of the branches (after Kidston & Lang) (= 3); B, Psilophyton princeps,
showing rhizomes below and, above, young branches uncurling at left and branches
bearing sporangia at right (after Dawson) (x probably about io); ©, Psilotum
triquetrum, showing the habit (> 3) and, enlarged, on left, part of a branch with
sporangia. A and B are reconstructed fossils, C is drawn from life.
134
EVOLUTIONARY DEVELOPMENT AND PAST HISTORY 135
and appear to be represented by a very few allies living today. Of
these the best known is Psilotum (Fig. 30, C), which is widespread
in warm regions. Both extinct and living members are branched,
with naked or spiny stems having cuticle and stomata on the surface,
and sometimes small simple leaves. ‘They form spores of one size,
produced in characteristic sporangia. Further details regarding the
Psilophytineae and also some other groups of ancient vascular plants
of often obscure relationship, may be obtained from such works on
fossil botany as are cited at the end of this chapter, though com-
parisons will show how difficult it sometimes is for authorities to
agree. Thus the present group are sometimes given the status of
a division, as the Psilophyta (or Psilopsida).
The Equisetineae (Horsetails) have also a very ancient history, the
present-day representatives, already dealt with in Chapter II, being
mere depauperated relics of a once large and important group that
flourished at least as far back as the Devonian. With their extinct
fossil representatives they are sometimes given the rank of a division,
under the name of Arthrophyta (or Sphenopsida). ‘They show
plentiful adventitious roots and small whorled leaves, and sometimes
secondary thickening. Of this major group there are five subsidiary
groups or orders, of which the most important are: (1) the lowly
Sphenophyllales, which had slender reclining stems and expanded,
wedge-shaped or lacerate leaves usually less than 2 cm. in length ;
(2) the Calamitales, which were like giant Horsetails, attaining heights
of some tens of feet and with their jointed, hollow stems sometimes
exceeding 20 cm. in diameter ; and (3) the Equisetales, which were
characterized by their slender, jointed stems and were altogether very
like the representatives living today. Although the Equisetales
appeared only in the Carboniferous, they were and are closely allied
to the earlier Calamitales, and consequently Equisetum may be
regarded as the oldest living type of vascular plant ; indeed some
authorities maintain the Calamites and Equiseta in the same order.
Probably both groups arose from some common source, the
Equisetales lingering on to the present day without major changes
and representing the end of a once virile line whose ecological
aggressiveness still saves it from extinction. It should be noted
that in some Calamitales there was differentiation into large mega-
spores and small microspores. A living Equisetum is shown in
Fig. 15, and in Fig. 31 may be seen fossils of typical members of
the other two main groups.
Living representatives of the Lycopodineae (Club-mosses and
136 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
their allies) are described in Chapter II and illustrated in Fig. 16.
They, too, have a long fossil history, both as the small herbaceous
types which we know today and as huge trees that flourished chiefly
in the Carboniferous period, when they evidently composed a large
part of the vegetation, at least in coal-forming swamps. Subse-
quently, near the beginning of the Mesozoic era, the tree types
appear to have become extinct, much as in the case of the giant
Fic. 31.—Fossil Equisetineae. A, a Calamite, Calamites suckowi (x about 4);
B, a Sphenophyll, Sphenophyllum emarginatum ( about 1). (Both after Zeiller.)
5] EVOLUTIONARY DEVELOPMENT AND PAST HISTORY 137
Horsetails. Outstanding examples are Lepidodendron and Sigillaria,
shown in Fig. 32, which were characterized by extensive rooting
systems of a unique kind, spores of two sizes, and usually tall and
straight, woody trunks covered with the scars left by the spirally-
arranged leaves. ‘These last were narrow and grass-like but ligulate,
Fic. 32.—Fossil Lycopodineae. A, Lepidodendron, two figures on the left, and
Sigillaria, five figures on the right, showing also the characteristic rooting system
(after Grand’Eury) (the tallest is about 16 metres in height); B, Lepidodendron
lycopodioides, showing the small leaves and leaf scars (after Zeiller) ( about 4).
and in Sigillaria are said occasionally to have exceeded 50 cm. in
length. Some of the earlier groups quite likely go back to Silurian
times, and probably evolved from the psilophytinean complex ;_ later
ones became highly specialized trees, sometimes with seed-like
138 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
organs. But, having become structurally modified for life in swamps,
with extensive tissues for gaseous exchange and very little for water
conduction, they were apparently unable to adapt themselves to the
more adverse climates following the Carboniferous. Meanwhile
the small herbaceous types survived and gave rise to those persisting
at the present time, even though they apparently represent another
evolutionary end-line. Like the Psilophytineae and Equisetineae,
this group is sometimes given the rank of a division, as the Lycopsida ;
for it is now realized that these groups of so-called ‘ fern-allies ’
represent separate lines of development as far back as it is possible
to trace them.
Of Filicineae, the Ferns, characterized by large complicated leaves
and gaps in the vascular cylinder, there are plentiful fossils—which
go well back into the Devonian in the case of the long-extinct
Coenopteridales. ‘lhe earliest of these ‘ Primofilices’ were only
partly distinct from their presumable psilophytinean forebears, but
others soon became characteristic and prominent elements of the
flora. Further groups arose towards the end of the Palaeozoic era
and persist to the present day—in the case of the relatively primitive
ones such as the Marattiales apparently in decreasing numbers,
but in the case of the more orthodox and modern Filicales still
plentifully. Many even in early times were much like those, living
nowadays, that are described in Chapter II and illustrated in Fig. 17.
Fossit SEED-PLANTS
Probably more important than true Ferns in the Carboniferous,
and certainly accounting for a large proportion of the fern-like
foliage of that and the following periods up to the mid-Jurassic, were
the Pteridosperms or Seed-ferns, also called Cycadofilicales. As
their names imply, these were fern-like (often tree-fern-like, but with
secondary thickening) in some respects. But they bore seeds, as may
be seen from Fig. 33, which shows a reconstructed plant and part
of a frond of different types. ‘The seeds were more or less naked,
this group belonging to the Gymnosperms ; but although there are
abundant ditferences, there remain sufficient deep-seated similarities
to suggest that they may have given rise to the groups dealt with in
the next paragraph, even as they themselves probably arose in pre-
Carboniferous times from Ferns or fern-like stock that had not
advanced far beyond the psilophytinean stage. Apparently allied
or belonging to this group are the Caytoniales, which in some respects
5] EVOLUTIONARY DEVELOPMENT AND PAST HISTORY 139
suggest primitive Angiosperms. ‘The Gnetales may perhaps have
sprung from the same stock ; but such fossils of them as are known
are relatively modern, and so the origin and relationships of this
group remain obscure (in Fig. 37 it is left near the Angiosperms).
The Mesozoic has often been called the era of Cycads, owing to
the presence in its deposits of some Cycads and of many more
—.
Ss"
TSS
Ez
SSS
\
\
A B
Fic. 33.—Pteridosperms (reconstructed). A, Lyginopteris oldhamia (after Berry)
(x about 36); B, Sphenopteris tenuis, portion of leaf with seeds (after Halle)
(x #).
Cycad-like Gymnosperms (Cycadeoidales) which between them com-
prise the Cycadophytes. ‘The living Cycads (an example is illus-
trated in Fig. 18, A) are apparently the relics of a once more important
and widespread group, whereas the Cycadeoidales have long been
extinct. By many authors the Cycadeoidales are kept as a separate
group, also called the Bennettitales, on account of their remarkable
reproductive structures that were apt to look more like flowers than
cones (a reconstructed example is shown in Fig. 34, A); others were
slender and branching but probably never very big. ‘Vhe Cycado-
phytes presumably arose from pteridospermous ancestors during the
late Palaeozoic, the living Cycads supposedly representing the end
of a line that in most respects has changed little since the early
140 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
‘\\ \
We
5] EVOLUTIONARY DEVELOPMENT AND PAST HISTORY I41
Fic. 34.—A reconstructed Cycadeoid and parts of living and fossil Ginkgoales.
A, Cycadeoidea, showing the flower-like strobili on the squat stem ( about 4's);
B, branch of Ginkgo with seeds ( #); C, leaves of two species of Baiera, Mesozoic
Ginkgoales (* about 2).
Mesozoic, when its members were far more (probably very) wide-
spread.
Of comparable antiquity, extensive development in the Mesozoic,
and bare persistence to the present day are the Ginkgoales, repre-
sented among living types by only the Maidenhair-tree, Ginkgo
biloba. ‘his is very restricted in anything like a wild state, but
is widely familiar in cultivation ; a female sprig is shown in Fig. 34, B,
as also are leaves of relatives which were almost world-wide in the
Mesozoic. Likethe Cycads, Ginkgo has motile sperms ; its obviously
primitive characters have led to its being called a ‘living fossil’.
An earlier, long-extinct and apparently quite distinct group of
Gymnosperms, the Cordaitales, flourished chiefly in the later part
of the Palaeozoic, constituting with the Pteridosperms the bulk of
the seed-plants of the Carboniferous coal-forests. Their origin
is obscure, the earliest known members apparently being already far
advanced. ‘The Cordaitales were mostly large trees with sizeable
flattened leaves and bearing their pollen and seeds in slender strobili
as indicated in Fig. 35.
The other major group of fossil Gymnosperms is the Conifers,
whose surviving representatives afford the vast majority of living
Gymnosperms. Examples of Conifers are illustrated in Fig. 18,
their general features being described in Chapter II. ‘They bear
some resemblance to Cordaitales, and as fossils went back at least to
the upper Carboniferous, an example from that time being shown
in Fig. 36, A. Subsequently they appeared to reach their develop-
mental climax in the Mesozoic, when several were similar in external
form to those surviving today. Before the end of that era they began
F
142 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
to tail off, some groups becoming less diverse and more restricted in
geographical range, though as a whole they remained fairly numerous
as well as various and ecologically important. It is widely supposed
that the Coniferales derived originally from cordaitalean stock,
(middle, right) (* about 3). (Both after Grand’Eury.)
though we cannot yet be certain of this. And in spite of the aggres-
sive nature of some familiar types—such as the Pines and Spruces
which still dominate huge tracts, especially of the colder lands—it
seems that many others are eking out a rather precarious existence.
5] EVOLUTIONARY DEVELOPMENT AND PAST HISTORY 143
‘Thus the group as a whole appears to be retreating before the angio-
spermic onslaught, from which it has suffered since before the dawn
of the Cainozoic era. Much as in the case of the so-called Pterido-
phytes, it may be that the Gymnosperms represent several largely
separate stocks as far back as we have yet any knowledge of them.
The Angiosperms, as we saw in Chapter II, are the most highly
evolved and successful group of plants at the present time, affording
most of the dominant species on land. After an apparently slow
start at least as early as the Jurassic, they gave rise before the end of
Fic. 36.—Fossil parts of Conifer and Angiosperm. A, branch of Lebachia
(Walchia) frondosa, one of the Palaeozoic Coniferales (after Renault) (< about 4);
B, leaves of a dicotyledonous Angiosperm ( about 4).
the Cretaceous to a vast assemblage of forms that between them
rapidly came to cover practically the whole surface of the earth,
comprising most of the land-vegetation we know today, and a good
deal of that developed in water. A considerable proportion of the
genera and even species which are familiar to us nowadays as common
shrubs or trees, particularly, seem to have persisted throughout the
Cainozoic—often with little if any apparent change, and sometimes
doubtless as dominants. Of the two main groups of Angiosperms,
the Monocotyledons are much less numerous as fossils, and especially
144 INTRODUCTION TO PLANT GEOGRAPHY
as early fossils, than the Dicotyledons. An example of these last is
shown in Fig. 36, B.
Altogether the Angiosperms seem to be still in their early stages
of expansion, with their fossils not yet expressing clear develop-
mental trends. As to their future evolution we can only conjecture,
and in regard to knowledge of their origin we are scarcely better off ;
for each and every one of the gymnospermous groups, and in addition
the Ferns, have been suggested by different authorities as having been
their precursors. ‘The outcome has been to stress our ignorance and
leave the question unanswered, as we know of no series of fossil
forms connecting the flowering plants with more primitive groups.
Yet it does seem probable that they evolved from some primitive
unspecialized group rather than from any modern and familiar one.
At present it appears that if precursors of the Angiosperms are ever
to be found, it will most likely be in the early Mesozoic or just possibly
the late Palaeozoic, and that the Pteridosperms afford the most likely
source of such stock as, conceivably, may in due course have evolved
into the Angiosperms we know. ‘This speculation arises from the
facts that the Pteridosperms do not seem to constitute such a ‘ dead
end ’ as the other groups of Gymnosperms, whether living or extinct,
and that some of their members or allies (the Caytoniales) have their
seeds largely enclosed in a recurved cupule which is suggestive of the
arrangement in the angiospermous fruit. As pointed out by Dr.
H. Hamshaw Thomas (zn Uitt.), the possible connection of the Angio-
sperms with the Pteridosperms is further suggested by the similar-
ities of structure in (a) wood anatomy (homologous types), (b) leaf
form, for example in Glossopteris and Gigantopteris, (c) male flowers,
and (d) seeds, especially as regards Angiosperms with integumentary
bundles.
Past AGES AND THEIR PLANT LIFE
Fig. 37 aims to show the distribution of the main plant groups in
geological time, with some suggestion of their relative abundance as
far as this is known and can be indicated by varying the thickness of
the figures representing the respective groups. It also indicates the
geological eras and main periods, etc., mentions in sequence the
dominant forms of life, and gives a series of supposed ages (cf.
pp. 128-9).
We will now proceed to a brief review of the main floras of the
past, having familiarized ourselves with the sequence of geological
time and with the chief groups of plants concerned.
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146 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
The pre-Cambrian and early Palaeozoic preceding the Devonian
together represent probably more than nine-tenths of known geologic
time and constituted the age of Schizophytes, ‘Thallophytes, and
invertebrate animals. Bacteria, Blue-green Algae, Fungi, and true
Algae of various groups appear to have been the main plants living
through most of this time, although towards the end of it, in the
Silurian, there seem to have been added the first land-plants in the
form of some primitive Psilopsida and Lycopsida, and the still more
problematical Nematophytales.
The Devonian and Carboniferous periods constitute the real age
of Pteridophytes and Fishes, even if these groups had an earlier
origin. ‘Thus in the Devonian the primitive groups already men-
tioned apparently continued to flourish, as have Bacteria, Fungi, and
Algae to the present day, while there were added various Sphenop-
sida, etc. Apparently none of these early Pteridophytes has sur-
vived, at least at the generic or lower level, any more than have the
primitive Ferns which first appeared in that period, or possibly
the Gymnosperms which were well established before the end of the
Carboniferous. Although the early Devonian is often spoken of as
primarily psilophytinean, there appear to have been plentiful other
Pteridophytes living at the time. ‘These evidently became more
diverse and numerous in the upper Devonian, when the Psilophytales
seem to have died out, and by then to have included some of tree
dimensions. ‘Ihese may show what appear to be annual rings and
indicate marked seasonal changes. An attempted reconstruction of
a late Devonian forest is shown in Fig. 38.
The Carboniferous period was the culmination of the age of
Pteridophytes, including many of large tree form as indicated in the
diagrammatic reconstruction (Fig. 39). ‘To these the Lycopsida,
Sphenopsida, and Filicineae all contributed, and, in addition, the
early gymnospermous Pteridosperms and Cordaitales. ‘The Bryo-
phytes first appeared at this time, at least so far as known indubitable
fossils are concerned, as did several of the modern groups—including
the true Club-mosses, Horsetails, and some Ferns—while before its
end there existed some primitive Conifers. ‘The upper Carboni-
ferous or Pennsylvanian was the great Palaeozoic coal age, and
evidently a time of damp and widely favourable ‘ even’ climate in
both northern and southern hemispheres. The vegetation which
gave rise to the immense coal deposits of the northern hemisphere
seems to have been one of the most widely luxuriant of all time,
though the coals of the southern hemisphere appear to have been
HISTORY 147
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148 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
formed from the remains of a flora which was relatively poor in
species and almost certainly belonged to a later age.
Of late Carboniferous time the early Permian was essentially a
continuation, though, later, arid conditions became widespread, the
Fic. 39.—Generalized reconstruction of a Carboniferous forest. The tall, much-
branched trees on the left are Lepidodendron. Below these are Sigillaria and leaves of
Lyginopteris. In the centre foreground is a slender type of Calamite and in the back-
ground are much-branched tree types. ‘To the right of the water in the foreground
are more Sigillarias and a Tree-fern. On the extreme right are lofty Cordaitales,
and, in the undergrowth, several Pteriodosperms, among which is seen a Sphenophyllum.
big Pteridophytes and the Pteridosperms tending to decline and
become replaced by other groups—including the drought-resisting
Cycadophytes and more Conifers. At the same time the character-
istic if limited Glossopteris flora existed under the relatively cold
conditions of the south, in the areas of India, Africa, South America,
Australasia and Antarctica that were supposedly occupied by the
ancient ‘Gondwanaland’. For the Permian was a period of active
mountain building and rearrangement of large areas of land and sea,
with severe glaciation at least in the South. It may be considered as
starting the real age of higher Gymnosperms, which held sway
throughout the Mesozoic era. In contrast with the generally uniform
growth of trees in the coal age, those which grew during and immedi-
5] EVOLUTIONARY DEVELOPMENT AND PAST HISTORY 149
ately following the Permian glaciation show strongly developed
‘annual’ rings.
The early part of the Triassic period, at the beginning of the
Mesozoic, tended to have an arid and generally unfavourable climate
in the northern hemisphere, the fossil record being, moreover,
fragmentary. Pteridosperms, Cycadophytes, Ginkgoales and Coni-
fers appear to have been plentiful, as well as numerous Ferns and a
lesser number of Lycopods and Horsetails. In Argentina, South
Africa, and much of southeastern Asia, the climate was humid at
that time, and supported a rich flora. It became drier and probably
arid in these regions towards the close of the Triassic, when to the
north conditions became much as in the Jurassic. Nevertheless the
wide occurrence of similar floras indicated that comparable con-
ditions extended over an extremely wide area. Fig. 40 shows an
attempted reconstruction of a Triassic landscape, with plentiful
Cycadophytes, etc., and some large Horsetails.
Jurassic floras were apparently developed under warm and moist
conditions as indicated by fossil deposits, and were widely distributed.
Indeed, their composition appears to have been fairly uniform all
over the world, involving (with relatively minor variations) such far-
flung lands as continental Europe, Spitsbergen, Greenland, tem-
perate North America, Mexico, India, Japan, Australia and New
Zealand. ‘They included numerous Cycadophytes, Ginkgoales, and
Conifers, besides representatives of most of the modern groups of
Pteridophytes, though the giant Lycopods and Horsetails had long
since disappeared, as had the Cordaitales. During this period the
Pteridosperms declined and, according to some authorities, the first
indubitable Angiosperms appeared.
The Cretaceous period witnessed the latest transformation of the
plant world, in which the Angiosperms really came into their own.
Conversely, most of the other groups of vascular plants were on the
decline. It is interesting to note, however, that the lower cryp-
togams had meanwhile often held their own, as many do to this day—
presumably because they do not normally compete with Angiosperms,
though actually they have not been widely dominant on land since
early Palaeozoic times. ‘Thus, of the widespread upper Cretaceous
‘Dakota flora’ of North America, more than go per cent. of the
species were Angiosperms, most of them belonging to familiar woody
genera of Dicotyledons, although the presence of a few Palms
indicated a fairly warm climate. Cycads were reduced to 2 per cent.
and the Conifers were only slightly better represented. Whereas
150 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
apparently still more warmth-loving floras appeared during the
Cretaceous even in the continental United States, the existence at
that time in Greenland and other arctic regions of floras of temperate
or warmer type suggests that the climate was widely genial, and again
probably of comparable nature over most of the world.
a ge Bed
Fic. 40.—Reconstructed Triassic landscape (after Heer).
In the early part of the Cainozoic, individual species of plants
tended to be more widely distributed than they are today. Of the
first five parts of the era, making up the Tertiary, the second,! or
Eocene (by many considered the first that is really distinguishable),
affords widely distributed floras indicating still favourable conditions.
1'The Paleocene is nowadays frequently distinguished as the first, especially
America.
5] EVOLULIONARY DEVELOPMENT AND PAST HISTORY 15%
Thus in North America and across Eurasia, conditions were sub-
stantially more favourable than at the present time, with Palms
extending plentifully into Canada and England. Already the flora
had a largely modern aspect, though herbaceous species in general
and Monocotyledons in particular appear to have been far less
abundant relatively to woody ones than at present.
There may have been some deterioration of climate in the northern
hemisphere in the next age, the Oligocene, but it appears to have
been slight, with plentiful large trees prevailing well north. In
any case in the southern hemisphere the climate seems to have
remained more comparable with that of today than it did in the
northern hemisphere.
The Miocene was a time of widespread volcanic activity in North
America, when uplifting of the Cascade Range deprived the area to
the east of much of its accustomed rainfall, so that increasing aridity
prevailed—and probably lower temperatures, although the climate
was still warm over wide areas. Fig. 41 shows a reconstructed scene
in Miocene times, with Palms and Cycads growing in what is now a
cool-temperate region.
By the Pliocene, conditions in America east of the Cascade Range
had become generally unfavourable for the growth of dense forests
and for the preservation of their remains, trees apparently occurring
chiefly along the streams. As a result of the general cooling in the
North, the vegetation became more like that of today, and actually in
North America and eastern Asia the Pliocene deposits contain a
large proportion of the species still found living in the same regions.
On the other hand, many of the wide-ranging species then occurring
in Europe have since disappeared therefrom.
In spite of all these changes in various parts of the Cainozoic, the
same major plant groups appear to have persisted through it to the
present day, even if the species and often higher taxa have changed ;
particularly striking is the dwindling importance of some of the
Conifers.
The Pleistocene and Recent together make up the Quaternary and
are now estimated to involve only the last million or fewer years
(cf. p. 129), of which the Recent or post-Pleistocene occupies per-
haps one-hundredth part. Remains of Pleistocene vegetation are
preserved chiefly in unconsolidated lake and stream deposits, in
peat bogs, or in a frozen condition, while postglacial peat deposits are
still accumulating. In general only modern species are represented,
most of these being still living and familiar; but in Pleistocene
152 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
deposits they often extend far (and in post-Pleistocene ones sometimes
considerably) beyond their present limits, suggesting substantial
changes 1n climate since they were laid down. Indeed, such changes
we know to have taken place in the northern parts of Eurasia and
Fic. 41.—A reconstructed scene in Switzerland during
Miocene times (after Heer).
America, where several periods of advancing and declining glaciation
widely affected the climate, and where further changes are indicated
by such evidence as that afforded by sub-fossil pollens and other
remains of contemporary plants preserved in bogs. These remains,
5] EVOLUTIONARY DEVELOPMENT AND PAST HISTORY 153
when identified with plants of known climatic requirements, afford
a valuable indication of local conditions. And as a further instance
of the significance of climatic change, we shall see in the next
chapter how the great diversity of woody plants in North America
and eastern Asia is attributable to the fact that in these regions
such plants were able to migrate far south before the extending ice
and return north after its margin had receded, whereas in Europe
they were supposedly forced against the southern mountains or sea
and exterminated. For such reasons we must examine in the next
chapter the historical bases of modern geographical ranges before
we can deal in an understanding way with the distributions we
actually find.
FURTHER CONSIDERATION
The facts and theories advanced in this chapter, as in the case of most
others, have usually been gleaned from various specialist and often highly
technical sources which the general reader will scarcely wish to consult
even if they are available to him. However, further details (and some-
times other opinions) may readily be obtained from one or more of the
following generalized or introductory books in English :
Sir A. C. Sewarp. Plant Life Through the Ages (Cambridge University
Press, Cambridge, Eng., pp. xxi + 601, 1931).
C. A. ArNoLtp. An Introduction to Paleobotany (McGraw-Hill, New
York & London, pp. xi + 433, 1947).
H. N. Anprews. Ancient Plants and the World They Lived in (Comstock,
Ithaca, N.Y., pp. ix + 279, 1947).
JoHN Watton. An Introduction to the Study of Fossil Plants, second
edition (Black, London, pp. x + 201, 1953).
CHAPTER VI
FOUNDATIONS OF MODERN DISTRIBUTIONS
The distribution of each kind of plant making up a modern (and
presumably any other) flora is apt to depend upon (1) the history
of the plant in geological and recent times, (2) what may be called
its migrational ability, and (3) its adaptability in physiological and
other ways to the conditions of such new environments as it may
reach. ‘lo a considerable extent migrational ability depends upon
the efficiency of dispersal (as described in Chapter IV), and adapta-
bility on plasticity of form or function (as treated in Chapter II1).
This leaves the ‘ fossil history ’ and recent vicissitudes to be dealt
with next, at least in such aspects as are best known or most pertinent.
Whereas we have seen that, apart from such periods of local or
regional change as occurred most notably in the Permian, the flora
and vegetation of the aerial parts of the globe tended to be widely
comparable in different regions during the earlier geological ages up
to and including the Mesozoic, this relative uniformity was not
maintained through the Cainozoic. ‘There is plentiful evidence to
show that still later than the end of the Mesozoic, during early
Tertiary times, the forests tended to be more widespread and more
uniform than at present—practically throughout the land of the
northern hemisphere, including what are now high-arctic regions.
And although climatic belts doubtless existed during these and earlier
times, they can scarcely have been as marked in favourable periods
as those we know nowadays ; for, with luxuriant vegetation flourish-
ing within a few degrees of both the North and South Poles, the
tropics would have been too hot for normal life—at least if the land
of the world had at all the conformity it has today. But particularly
from the Miocene onwards there were marked local changes in
conditions, and so these latest fossil and sub-fossil floras are of great
significance to us when considering the problem of the origin of the
flora existing today.
154
FOUNDATIONS OF MODERN DISTRIBUTIONS 155
SoME EFFECTS OF RELATIVELY RECENT CLIMATIC CHANGES
At the close of the Tertiary or beginning of the Quaternary,
although the vegetation of the tropical and adjacent zones continued
in considerable luxuriance, there was a marked lowering of tempera-
ture in most other regions, that led to the covering of some by
glaciers and to complete changes in the floras of others. Climate
being the primary controller of vegetation, the flora in favourable
warm areas probably continued, as it does to the present day, much
as it had done at least through the middle Tertiary. But in the
less favoured belts to the north and south, a lowering of temperature
and decrease in precipitation—for instance in much of the Mediter-
ranean Basin, in eastern Europe, and in northern and central Asia
—led to the widespread replacement of the warmth-loving floras
by hardier types. It may be presumed that there was at the same
time a reduction in numbers of species and individuals, and in the
general luxuriance of the vegetation. And whereas in the boreal
regions, and even in the Arctic, there had formerly flourished (and
according to some authorities evolved) a vast array of warmth-loving
or at least mesothermic (7.e. liking moderate temperatures) types,
these were in time pushed far south or, in some cases, doubtless
exterminated.
Although evolution in general is a gradual process, it seems to
be accelerated by sudden changes in habitat conditions. ‘Thus the
violent upheavals of the earth’s crust or direct changes of climate
appear to have induced sudden inherent and hereditary changes
involving the abrupt creation of new races and, in time, of new
species. Contemporaneously and for similar reasons, floras have
been caused to migrate. ‘These variations of conditions over long
periods of time have probably led to a far more intricately adjusted
and highly evolved general flora and vegetation than would have
obtained if conditions had remained as they were, for example, in
the Carboniferous. Indeed, some authorities have pictured a world
of gigantic Reptiles instead of Mammals, and of giant Club-mosses
and Horsetails instead of Angiosperms, as continuing today if the
Carboniferous climatic and other conditions had persisted.
Besides direct climatic changes and others depending on such
geological revolutions as the thrusting up of mountain ranges, there
have evidently been changes in the conformation of land and sea
—eyen, according to some authorities, in the positions of the con-
tinents with regard to one another and in relation to the geographical
156 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
poles. All such changes would inevitably lead to alterations in the
distributions of plants and of the animals that are dependent upon
them, profoundly affecting also their evolutionary development.
Thus, whereas in the northern hemisphere the distribution of the
climatic zones favoured the development of relatively luxuriant
vegetation over wide areas up to nearly the end of the Tertiary, in
the southern hemisphere conditions appear to have been much more
disturbed. Particularly did the great Permian glaciation, whose
principal centre of development apparently lay in South Africa, play
havoc with the flora and fauna; to it is attributed in some degree
the poverty to this day of the flora of tropical Africa as compared
with the floras of South America and Asia. It is supposed that
relatively arid conditions prevailed subsequently over much of the
southern hemisphere, which may have been further diversified by the
separation of the continents beginning as far back as the Mesozoic.
Thus tropical Africa is supposed to have been cut off from the rich
vegetation of Eurasia by a wide sea occupying the position of the
Sahara during the Cretaceous, and, in later times, by the Sahara
Desert as we know it today—both sea and sand being effective in
barring the migration, it has been suggested, of the majority of
species from the North.
The climatic deterioration that began well back in the ‘Tertiary
led ultimately to widespread glaciation in the northern hemisphere.
Thus, early in the Quaternary, vast areas of North America and
northern Eurasia became enveloped in ice which may be compared,
in extent and continuity if not in thickness, with that covering most
of Greenland and Antarctica today. ‘This Pleistocene ice reached a
maximum extent and receded probably four times, the limits reached
in each extension being different. ‘lhe intervening, ‘ interglacial ’
periods were protracted and relatively favourable to plant life, being
apparently for long periods at least as warm as the present day.
The ice largely ousted the previously rich floras of the North, the
plants being forced to migrate before its advance or, if they could
not do so successfully, being exterminated—apart perhaps from some
which persisted on mountains or other ice-free refugia, a subject
to be discussed in the next section.
The poverty of the present western and central European flora
is commonly ascribed to the fact that the Tertiary components were
largely destroyed shortly before or during the Ice Age, by being
driven into the sea or against high mountains. For post-Pleistocene
restocking across the Mediterranean, over the Alps or Pyrenees, or
6] FOUNDATIONS OF MODERN DISTRIBUTIONS 157
from the floristically poor regions lying immediately to the east, was
problematical to say the least, and from the west was wellnigh
impossible as the Atlantic was now indubitably in existence. On
the other hand, in North America and eastern Asia the warmth-
loving plants among others were free—and evidently often able—
to migrate far south in the lowlands or along mountain ranges,
subsequently to return when the ice retreated and suitable conditions
were reinstated in the North. Much the same appears to have
happened in the Balkan region. ‘To such considerations is attributed
the presence of many living woody plants, such as members of the
Magnolia family (Magnoliaceae), in eastern North America and
Fic. 42.—Known distribution of species of Liriodendron in Tertiary times (black)
and nowadays (hatched). (After Good).
southeastern Asia but, on the other hand, their absence from Europe,
though their remains in fossil deposits indicate that they were once.
widely distributed and locally plentiful in all three of these continents
Fig. 42 shows the known distribution of Tulip-trees (Liriodendron
spp., Magnoliaceae) in Tertiary times (black) and living today
(hatched), and Fig. 43 shows the even more drastic restriction of
the giant Redwoods. Fig. 44 indicates the periods during which
the various parts of North America are supposed to have been free
from major ice-sheets. Even in the last of the interglacial periods,
many plants persisted much farther north than they grow today—
most notably in Europe—and it has been suggested that we have
still not emerged permanently from the Ice Age.
Whereas it is thought that considerable expanses of territory in
158 INTRODUCTION LO] PLANT GEOGRAPREH ¥
northern Asia and smaller ones in northern continental North
America, as well as the northernmost insular tracts, were free from
major ice-sheets throughout the Pleistocene, owing for example to
their ‘continental’ climate and especially low precipitation, it is
believed that ice at one or more stages of the Pleistocene engulfed
Fic. 43.—Past and present distributions of Redwoods. Above, known localities
of fossil Redwoods (after Chaney); below, modern (relic) areas of Coastal Redwood
(Sequoia sempervirens) on left and of Sierra Redwood or Big-tree (Sequoiadendron
giganteum) on right. (Modified from Cain: Foundations of Plant Geography, copy-
right 1944, Harper & Brothers.)
much of northern North America (see Fig. 44) and virtually the
whole of western and central Europe as far south as the Thames
Valley in the west and the Carpathians in the east. Nor could such
extensive glaciation of northern territories obtain without being
reflected in a lowering of temperatures to the south and even in
the tropics, where some mountains became glaciated, and where
Uns riiitite
Exposed after disappearance of PLEISTOCENE ice.
Driftless and nunatak areas wholly or partly exposed during PLEISTOCENE.
: a Areas of mountain and valley glaciation during the PLEISTOCENE.
<u.
Gz Atlantic coastal plain and Pacific areas exposed during the QUATERNARY.
QQ Probable land area available since close of the TERTIARY.
(UT Probable land area available since close of the CRETACEOUS.
A Probable land area available since close of the JURASSIC.
eel Probable land area available since close of the PALAEOZIC,or unknown.
Fic. 44.—Map showing periods since when various areas of present-day North
American land have supposedly been free from major ice-sheets, etc. (Harvard
University handout, modified after Flint and others.)
159
160 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
warm lowlands (at least in the southern hemisphere) experienced
‘pluvial’ periods of increased precipitation. Simultaneously there
was more extensive glaciation in some regions of the South, notably
the uplands of South America and southern Australasia. All these
and allied changes were gradual, the advance and retreat of the ice
each time occupying many thousands of years, and allowing plants
to migrate before its face in accordance with climatic changes.
Numerous living genera, especially of woody plants, are known
once to have been far more widely distributed than they are today
(striking examples are shown in Figs. 42 and 43). Many of our
species and even vegetational associations are of similar nature to,
but geographically more restricted than, those developed as far back
as the Miocene. It seems that by this time most of the major
land-masses of today had been formed, at least in outline, and that
thereafter the deterioration in climate was marked, leading to a
considerable reduction of the areas of many species of plants and
animals—quite apart from the restriction effected by or through the
Pleistocene glaciation. Already in the Pliocene the floras had
tended to be markedly impoverished as compared with those of the
Miocene, considerably less than 1,000 species of plants being known
from the Pliocene in contrast with over 6,000 from the Miocene.
The analogy of animals would suggest that over 80 per cent. of
Pliocene plant species are still living today—some of them, e.g. the
Bog-cypress (Taxodium distichum) and Black Oak (Quercus nigra)
in Alabama, in the selfsame region. In other instances they are
not now known to live in a wild state within thousands of miles of
their old haunts—as in the case of the Water-chestnut (7rapa), fruits
of which are widespread in deposits in North America up to the
Pliocene, though it appears to be no longer a native of the New
World. An interesting set of examples is afforded by a middle
Pliocene flora of western Europe whose closest agreement is to be
found with some areas of southeastern Asia; on the other hand,
the flora yielded by the overlying late Pliocene beds finds its closest
relationship with the existing flora of central Europe and indicates
an already much cooler climate.
It is supposed that the Miocene and early Pliocene floras were
practically circumpolar in distribution and also widespread latitudin-
ally. ‘hen the increasingly colder conditions in the North, often
accentuated locally by the uprising of mountain ranges which
furthermore caused aridity in their rain-shadows, forced the plants
to migrate southward. It has been suggested that in this pre-
6] FOUNDATIONS OF MODERN DISTRIBUTIONS 161
Pleistocene southward migration there were three principal avenues
of escape, determined primarily by the prevailing direction of the
mountain systems : (1) along the lowlands of eastern Asia, (2) along
the mountain ranges of North America, and (3) down the Scan-
dinavian Peninsula and adjacent areas into western and central
Europe. The first route may have led to intermixture of northern
species with the original native flora and so may largely explain
the richness of the existing flora of China. ‘The second route may
have had much the same effect in America and would, moreover,
explain the similarity between the floras of eastern North America
and eastern Asia. In the third instance, however, the great European
mountain ranges with axes lying east and west would have prevented
further southward migration, with resulting impoverishment both
currently and as regards possible recolonizing elements even after
the final Pleistocene recession, as we have already seen. However,
there has been some opposition to this hypothesis, and serious
doubts expressed, for example, as to whether southwestern France
was cold enough at this time to cause the postulated extermination,
though this might still have been effected by the persistence there
of closed communities, which are among the toughest barriers for
a migrant to cross. Nor is it known whether the great extinction
of plants and animals in Europe took place before the end of the
Pliocene or during the Pleistocene. Probably it was a gradual pro-
cess, each advance of rigorous conditions leading to the loss by the
flora of some of its less hardy elements, so that potential waves of
recolonizing vegetation became successively more impoverished.
PLEISTOCENE PERSISTENCE versus SUBSEQUENT IMMIGRATION
The Pleistocene itself, as we have seen, was a climatically unsettled
period of cold spells with extensive glaciations, which were separated
by relatively long warm intervals (interglacials). In general the
plants living during the Pleistocene were specifically identical with
those of the present time, the chief differences exhibited by them
being in spatial distribution. During some at least of the inter-
glacials the climate appears to have been similar to that of the
present day—which indeed has led to the suggestion that we may
nowadays be merely in another interglacial. Thus more than
70 per cent. of the species whose remains have been identified from
an interglacial deposit in Germany live in the same district at present,
and so do most of those in a deposit near Toronto in southern
162 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
Canada, though a few are now restricted to areas slightly farther
south.
The climate near the margin of the ice during the glacial stages
was probably rather like that of southern Greenland today, where
arborescent growth is to be found within a few miles of the terminal
face of the ice-cap; even more striking is the situation in New
Zealand, where subtropical 'Tree-ferns may be seen growing a bare
mile from the end of a glacier. ‘Thus the Pleistocene tundra (treeless
plain) in North America probably formed only a narrow belt around
the margin of the ice-sheet, with conditions becoming rapidly more
equable away from it. In Europe the ice-free zone appears to have
been relatively wide, at least during the later glacial maxima. In
central Eurasia the tundra was apparently bordered by steppes, while
in North America extensive deposits of loess (wind-transported
sediment) suggest a drier climate than now prevails. ‘To the south
of the tundra and steppe belts lay more or less broken forest, although
it seems likely that in some places the advancing ice impinged
directly on the forests. When precipitation decreased or the climate
became warmer, the ice retreated and the whole series of vegetation
zones followed its margin northwards—only to be pushed south
again when conditions deteriorated and the ice advanced once
more.
It is often supposed that in Alaska and eastern Asia the glaciation
was chiefly of the local, mountain type, considerable areas being left
free or at least not covered by extensive ice-sheets. It also seems
that, because the precipitation was locally insufficient for extensive
accumulation of ice, the northernmost parts of Greenland and
the Canadian Arctic Archipelago remained largely unglaciated.
There is no reason to doubt that many plants persisted in these
unglaciated areas—in some cases probably throughout the whole of
the Pleistocene, in others at least through its latest glacial maximum
(which was by no means everywhere the greatest in extent as com-
pared with its predecessors).
But whereas it is an observation of today, and a supposition for
similar phenomena of the past, that many plants can and do grow
on ice-free areas even when these are surrounded by ice, it is another
matter to explain apparent anomalies of modern distribution in terms
of such survival throughout long periods of intense glaciation. Yet
this has been so widely assumed that it seems necessary to point
out the seemingly overwhelming counter-arguments to this ‘ nunatak ’
hypothesis—so called from the Eskimo word for a mountain sticking
6] FOUNDATIONS OF MODERN DISTRIBUTIONS 163
out of an ice-cap, for on such mountains much perglacial persistence
is supposed by some students to have taken place.
To begin with, much of the supposed evidence brought forward
for survival, such as the ‘relict’ nature of particular plants in
particular areas, has proved to be either capable of other interpretation
or actually erroneous. ‘Thus, geologists have insisted that several
of the areas claimed by the advocates of persistence to have been
‘refugia’’ during the Pleistocene, were in fact glaciated. Moreover,
latterly there have been numerous instances of plants of supposed
isolation or disrupted distribution (which were claimed to indicate
such refugia) being found in intermediate positions—sometimes even
on islands that have only recently risen out of the sea. Nor does
endemism (restriction to particular geographical areas) necessarily
indicate isolation and persistence ; it sometimes actually suggests
the opposite, as in some cases of hybrid origin !
It is noticeable that many of the isolated or restricted (endemic)
species, which are made so much of by advocates of the nunatak
hypothesis, are characteristic of, or sometimes restricted to, cal-
careous soils. It has been strongly counter-claimed that their spotty
or localized occurrence can best be correlated with soil characteristics,
the presence or absence of signs of recent glaciation being of little
or no consequence. And when we remember that many of these
are far-northern plants which favour ‘ open’ soils where competition
is lacking, and that in boreal regions calcareous areas are notable
for their poor vegetation but diversified flora, other possibilities
spring to mind and the nunatak hypothesis becomes less and less
attractive.
There are also objections to the nunatak hypothesis on the ground
of far easier and wider dispersal than its adherents will admit—for
example by Birds, and as regards characters of higher plants trans-
ported in pollen, and through airborne disseminules of lower plants.
Thus in the manner of plants, Birds also have their habitat pre-
ferences, often traversing great distances from one to another similar
spot. Although normally they are supposed to ‘ fly clean’ when
on migration, they must surely (as already mentioned in Chapter IV)
sometimes carry materials frozen or otherwise stuck to their plumage,
etc.—especially if flushed unexpectedly or unwell, when they are
apt to ‘ neglect their toilet’. Pungent instances seem to be afforded
by the seeds, coated in protective regurgitate, that have been found
adhering to migrant Birds on the subantarctic Macquarie Island,
but apparently do not belong to any of the local species, as noted
164 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
already on p. 114. And in the final analysis it must be remembered
that a single disseminule in many millennia may be sufficient to
‘plant’ a species.
But whereas the idea of certain plants persisting in tiny areas for
hundreds of thousands of years, unchanged and unaffected by the
coming and going of ice-ages and hordes of other migrants, is scarcely
in accordance with our saner biological reflection, some persistence
on ice-free tracts, for example through the last glacial maximum,
may well have taken place. And restocking may indeed have been
helped by plants surviving on temporary sheltered ‘ islands’ sur-
rounded by ice, or on ice-free coastal strips—thence to recolonize
surrounding areas on release from the bondage of glaciation. On
the other hand, the plants surviving on mountain nunataks would
be mainly arctic or high-alpine ones accustomed to rigorous con-
ditions and unlikely to recolonize lastingly the surrounding plains
on recession of the ice and marked amelioration of conditions. ‘Thus,
as was pointed out by Professor G. E. Du Rietz (i litt.), ‘ Scan-
dinavian botanists have believed more in glacial survival in coastal
areas than on nunataks ’, and such an ‘ hypothesis of glacial survival ’
would seem more reasonable ; nor are the criticisms of the nunatak
hypothesis which apply to North America necessarily valid in
northern Europe where conditions tend to be different. Many
persisting plants, however, appear to become depleted in biotypes
and, owing to a consequent narrowness of ecological amplitude,
rather passive; some become reactivated by cross-breeding fol-
lowing release from glaciation, aggressively recolonizing surrounding
areas from their vegetated ‘islands’. ‘Thus, although the nunatak
hypothesis in its strictest form seems unsound and indeed unneces-
sary, some residue of it may well be valid and still found useful,
in spite of the fact that the recolonization of deglaciated areas
appears in general to have taken place by immigration of plants
that tided over the inimical period in distant (usually southern)
areas. ‘Thence the plants subsequently migrated, for the most part
by gradual stages.
Persistence through the Pleistocene, or at least through its latest
phases, may also help to explain the existence of ‘ arctic’ plants on
mountains far to the south, although in some cases migrating birds
are probably responsible. In other instances, the similarity to arctic
conditions evidently enabled properly acclimatized plants to retreat
to the mountain tops, even as they were able to persist near the
margin of the ice and follow it north. For in both these situations
6| FOUNDATIONS OF MODERN DISTRIBUTIONS 165
the plants were able to get away from the competition of ranker
(but often less hardy) types.
Altogether it seems most reasonable to consider the post-Pleisto-
cene flora of those territories of the northern hemisphere which
were freed from the last major glaciation as being made up of :
(1) some elements which had with little doubt persisted in at least
recent unglaciated ‘ refugia’, particularly in sheltered coastal areas,
(2) elements spreading from sheltered refuges afforded particularly
by mountain systems where the Ice Age did not have such catastrophic
consequences as to destroy the characteristic Tertiary flora even if
it did cause impoverishment, (3) probably numerous elements which
migrated northwards from the territories bordering the southern
extremity of the ice after it retreated, and (4) recent immigrants
from afar (or at least from regions not drastically affected by the
Pleistocene), whose establishment has been favoured, in areas recently
freed from glaciation, by the local lack of competition from already
closed (7.e. continuous) vegetation. ‘These recent migrants were
aided by natural means—wind, water, or animals—or by Man,
through intentional or accidental importation, and many such migra-
tions are still going on plentifully all the time. Examples are afforded
by the considerable numbers of weeds that have recently become
established in ‘open’ areas, including inhabited parts of West
Greenland, and the westward advance of Asian species into Europe.
CONTINENTAL DRIFT, SHIFTING POLES, LAND-BRIDGES, ETC.
The present distribution of any given plant species is in part a
reflection of the geological revolutions and climatic changes that
have occurred in the world during the period of its existence as a
species. In former sections of this chapter we considered the
effects of climatic change in the past and particularly the significance
of the Pleistocene Ice Age. It is now time to examine some other
leading theories that have been advanced in an attempt to explain
the current distribution of particular plants.
Perhaps the most promising and plausible (though still highly
controversial) theory in this connection is that of ‘ continental drift ’,
which is often identified with the name of its principal proponent
of recent times, the late Dr. Alfred Wegener. ‘This * displacement
hypothesis’ is based on the assumption that the present-day con-
tinents once formed part of a single land-mass (Pangaea), or, accord-
ing to a recent modification, two land-masses, whose continents
166 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
started breaking and drifting apart during the Mesozoic era and
went on doing so until they came to reach the positions they now
occupy. According to some advocates of the theory, this drifting
is still proceeding and actually demonstrable in the case of Green-
land. Such recent drifting might conceivably have left Europe and
North America in contact with one another until the early Quaternary,
and likewise Antarctica and South America may be presumed to
have remained long in contact. ‘The theory also accounts for
considerable rearrangement of the climatic zones on land, as the
drifting of the continents was supposed to involve changes in their
several positions relative to the North and South Poles. ‘This
in turn would allow marked changes in the distributions of living
organisms, and bring plants and even entire floras to regions whose
present climates are barely adequate for their growth. Still more
spectacularly, it would allow fossil remains to drift with their
enclosing land-masses to distant parts of the globe—and hence into
entirely different climatic belts—and this could explain, for ex-
ample, the presence of fossils of warmth-loving plants in the Far
North.
Although it is rejected by some geologists and, especially, geo-
physicists, the theory of continental drift has won the ardent support
of others—even if only for earlier geological times than those which
allow most help to the biogeographer in theorizing about the anomalies
he finds. ‘To many of the botanists among these biogeographers, it
seems to constitute the most plausible working hypothesis upon
which they can base their suppositions as to the history of plant
ranges. In one connection or another, the controversy rages back
and forth and probably will continue to do so for a long time to
come, though it may be noted meanwhile that the maps produced
by proponents of the theory are extraordinarily suggestive, and
would, one imagines, if analyzed statistically, indicate a very high
degree of probability (cf. Fig. 45, A). However, geologists and geo-
physicists are prone to put back the time of possible rift and major
displacement of continents well before the beginning of the Mesozoic,
when it would be of little if any help to plant geographers in their
search for explanations of striking similarities of flora in certain areas
that are now widely separated by oceans. For it is obvious that,
as ditferences between species commonly involve the establishment
of various genetically independent mutations (sudden changes), the
chances that two isolated populations will evolve in exactly the same
way are incalculably low, while convergence in every respect of
6] FOUNDATIONS OF MODERN DISTRIBUTIONS 167
previously dissimilar types is even more improbable. Accordingly,
similarities across wide stretches of water are more popularly
explained by suppositions of previous proximity or even contiguity,
though, as we have already suggested, dispersal may be more effective
than is commonly supposed, and, conceivably, responsible for some
at least of the apparent anomalies of distribution.
If it were accepted for Mesozoic and later times, the hypothesis
of continental separation and drift would make unnecessary, or at
all events less necessary, several of the following and other theories
that are of interest in plant geographical considerations—though it
may be confidently assumed that it will not be so accepted without
far more conclusive evidence than has yet been presented. Mean-
while it should be noted that this theory is supported by, or is in
accordance with, a great many data on the migrations and inter-
relations of different floras and plant groups in past geological ages,
besides explaining many anomalies of plant distribution of the
present day. On the other hand it offers no explanation of the
similarities of the floras of eastern Asia and North America, or of
the floras of islands in the Pacific which suggest trans-Pacific con-
nections—or at least migrations. Indeed, according to Professor
G. E. Du Rietz (voce), who yet believes it possible that the same
taxonomic entity may have arisen independently in more than one
area, the flora of New Zealand is contrary to the hypothesis of
continental drift, bearing similar relationships to both its east and
its west.
The theory of polar oscillations or ‘ shifting of poles’, sometimes
called the “ pendulum theory ’, also aims at explaining, on the basis
of incidental earlier climatic changes, some phenomena of plant
distribution that are far out of line with climatic zones as they now
exist. It is presumed that changes in climatic zones, as indicated
inter alia by the distributions of fossil plants, were caused by changed
location of the continents in relation to the sun’s orientation, and
this theory of polar oscillations attempts to explain these phenomena
by assuming that periodic changes have occurred, owing to the
position of the geographical poles oscillating back and forth like a
pendulum, or at least ‘ wandering’ quite widely. ‘This must not
be confused with the now well-known movement of the North
Magnetic Pole. It should be noted that continental drift and
(geographical) polar wandering are considered to have quite possibly
taken place together, in view of the plasticity implied by the former,
and that proponents of the latter are prone to put an earlier position
168 INTRODUCTION TO PLANT GEOGRAPHY [CHAP
Le ESS
EE
a ge
6] FOUNDATIONS OF MODERN DISTRIBUTIONS 169
STATISTICS OF SOME ISLAND FLORA's
GREENLAND (827,300 sq. miles)
Flowering Plants 420 species (less than 1% endemic)
Preridophytes 20 species (none endemic)
BRITISH ISLES (130,800 sq. miles)
Flowering Plants about 1,490 species
Preridophytes 6 species
Endemic element very low (at most 90 microspecies many of which
may yet be found on the Continent of Europe), CEYLON (25,300 sq miles)
CEYLON (25,300 sq, miles) ete Rerein Plants about 2,300 species (avout 34% endemic)
Flowering Plants about 2,300 species (about 34% endemic) s ee Preridophytes about 250 species (endemic % uncertain)
Preridophytes about 250 species (endemic % uncertain) Sy ie
JAMAICA (4,400 sq. miles) J)
Flowering Plants about 2500 species (about 20% endemic) =
Preridophytes about 523 species (abour 12% endemic)
SAO TOME GULF OF GUINEA (390 sq. miles)
Flowering Plants 573 species (194% endemic)
Preridophyces 117 species (abouc 5% endemic)
Preridophyces
66 species
found on the Continent of Europe). ~~
ak mre Fa <
0 20. 40 60 80 100 Miles
Gee SES
B
Fic. 45.—Maps illustrating ‘ Continental drift’ and the proximity of land-masses
as apparently affecting floristic richness. A, reconstructions of the map of the
world at three stages according to the theory of continental drift, the thinly stippled
areas being shallow seas, while present-day rivers, etc., are shown merely for pur-
poses of orientation (after Wegener); B, comparison of areas and floras of islands
in relation to the proximity of major land-masses and their floristic richness
(prepared by W. T. Stearn).
170 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
of the North Pole in the North Pacific, the implied change in the
earth’s axis bringing the South Pole into the South Pacific.
If the theory of polar oscillations were accepted, it would in turn
make unnecessary any separate theory of land-bridges, as changes
in level of the sea induced by oscillations of the earth (that are pre-
supposed to have caused those of the poles) would sufhce to cause
the joining together or separation of different parts of the earth’s
land surface. However, in spite of some recent attempts at resur-
rection, and continuing discussion of the possible instability of the
earth’s axis and of the phytogeographical implications this might
have,! this theory of polar oscillations is said to possess little geo-
physical foundation or geological support. Moreover, if it were
made to explain some anomalies of plant distribution, it would
apparently merely precipitate others !
The theory of land-bridges has long been popular in some quarters,
and indeed there are few seas or even deep oceans that have not
been hypothetically bridged by one or another over-enthusiastic
author to explain present-day anomalies in plant or animal distribu-
tion. And certainly the simplest way to explain similarities (some-
times only supposed) of plant and animal life between such areas
as Europe and eastern North America, or Australia and South
America, is to assume that they were once connected by a bridge
of land or a ‘ lost continent’, though the bridging and hence pos-
sibility of migration across the present-day ocean need not necessarily
have been continuous at any one time. But altogether the theory
strikes the sceptic as being too artificial and ‘ convenient ’ for reality,
and again there are contrary geophysical and geological arguments.
It was advanced chiefly as a concession to some aspects of bio-
geography, while leaving others unexplained. ‘Thus it leaves
unclarified why plants in former geological ages flourished in regions
whose climates are now far removed from those to which their
modern counterparts are accustomed, and it also gives no satisfactory
explanation of discontinuous areas of distribution. For if the
floristic similarities of two distant areas having alike plants were to
be explained by their having once been joined by a ‘ bridge’, it
would be necessary to assume that the entire extent of that bridge
offered similar ecological conditions suitable for the migration of
these plants. ‘This requires more imagination than to visualize
recent dispersal by the means discussed in Chapter IV, or, perhaps,
1 See, for example, Nature, vol. 175, p. 526, 1955, vol. 176, p. 349, 1955, and
WO, THOS jy Ce, LORS
6] FOUNDATIONS OF MODERN DISTRIBUTIONS 171
continental drift involving the ‘ like’ areas being once together but
subsequently separated !
It seems, however, that the theory of land-bridges is justified to
the extent that such phenomena come and go nowadays on a small
scale—for example with the emergence and submergence of isthmuses
with changes in the relative level of water, and with the throwing
up or destruction of beaches by the sea. Also, there can be little
doubt that such ‘ bridges ’ have existed on a bigger scale in the past,
at least to the extent of once linking together some present-day
islands with their adjacent mainland over an area of continental
shelf, and joining in continuity such close land-masses as Alaska
and easternmost Siberia. Nor, according to some authorities, can
certain distribution-patterns in the southern hemisphere well be
explained without the supposition of an Antarctic land-bridge.
This may have joined South America to Australasia, to which two
regions such restricted genera as Nothofagus (Southern Beeches) and
Fitzroya are common, while also occurring as fossils on the Antarctic
Continent.
The theory of permanence of oceans and continents, which grew
out of objections to, and largely opposes, those of land-bridges and
continental drift, presupposes the land-masses to have occupied their
present positions from pre-Cambrian times. However, it still leaves
unexplained many biogeographical phenomena—particularly those
that suggest the continents to have been connected at some period
—and is unable to cope with the high-arctic flourishing of luxuriant
vegetation in earlier ages. For although it has been suggested, and
may yet be maintained, that plants might have changed their climatic
requirements in the past, they can scarcely have done this sufficiently
to account for such extreme cases.
This brings us to the theory of the polar origin of floras, which
in one or another form has some eminent advocates to this day.
When it was believed that climatic conditions were uniform through-
out the world in early geological ages, with differentiation into
climatic zones not taking place until the end of the Cretaceous or
beginning of the Tertiary, it was widely thought that the floras of
the world spread rapidly from a single centre lying in the north
polar region—through Europe into Africa, through eastern Asia into
Malaysia and Australia, and through North America into South
America. This was the arctic or monoboreal theory of the origin
of floras. Subsequently such discoveries as that of fossil floras in
Antarctica indicated that a similarly drastic reduction in temperature
172 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
has taken place there, and suggested that the lands of the Far South
had once been connected with one another by way of the Antarctic
Continent. ‘This provided a basis for the presumption that there
had probably been a centre of species-formation there too, and that
life originated in the lands encircling both geographical poles.
However, neither theory can well be accepted, for it is now recognized
that climatic zones have existed as long as there has been life on
earth, and that ice-ages occurred previously to the Pleistocene and
in parts of the world other than the present polar areas. Moreover,
other centres of plant development have been strongly advocated
—particularly in the tropics.
An interesting example of the importance of historical and other
factors in the diversity and constitution of present-day island floras
is illustrated in Fig. 45, B, kindly contributed by Mr. W. 'T. Stearn,
of the British Museum (Natural History). ‘This shows that whereas
the flora of Jamaica is large and diverse although its area is small,
the flora of Ceylon is smaller although its area is larger than that
of Jamaica, while the flora of the many times larger British Isles
is smaller still. Each island having a comparable degree of topo-
graphic variation, and the climates of Jamaica and Ceylon being
similarly favourable and that of the British Isles not unfavourable,
it seems reasonable to presume that the greater floristic diversity of
Jamaica is due to its proximity to the Central American region of
continuous evolution from early geological times. Ceylon, on the
other hand, although also tropical and humid and likely to be capable
of supporting a large flora, is near the geologically much younger
and floristically poorer southern portions of India, and consequently
has had a less diverse flora to draw upon. In the British Isles, the
flora and proportion and also degree of endemism are all smaller
than in Jamaica and Ceylon, owing in part to relatively recent
glaciation, in part to the prevailingly less favourable climate, and
in part to the proximity of (and only recent separation from) the
mainland of Europe. Greenland, most of whose still greater area
is covered by ice, has an even smaller flora, owing, it seems clear,
to the widely inimical climate and severe Pleistocene glaciation. Its
about 475 species of vascular plants (including well-established
introductions and 31 Pteridophytes) show an extremely low propor-
tion and degree of endemism, with none at all among the Pterido-
phytes. (These are the latest statistics which have become available
since the preparation of Fig. 45, B.)
6] FOUNDATIONS OF MODERN DISTRIBUTIONS 178
POSTGLACIAL! CHANGES
Climatic change has affected plant distribution right up to the
present time, and indeed such changes and their effects on plant
life are still going on and presumably will always continue. The
postglacial sequence of changes is best known and perhaps most
marked in temperate Europe, where the sequence has been briefly
as follows :
(1) the earliest deposits following the last glacial recession show
evidence of an arctic-subarctic—arctic sequence of vegetation-types
characterized by Dwarf Birch (Betula nana agg.), shrubby Willows
(Sahx spp.), and Mountain Avens (Dryas octopetala s.1.),? developed
under probably rather dry as well as cold conditions except around
the middle of the period, which persisted for example in the British
Isles until about 11,000 years ago. ‘This ‘ Subarctic’ period was
followed about a millennium later by
(2) a ‘ Pre-Boreal’ period of variable but milder climate, char-
acterized by Scots Pine (Pinus sylvestris), Birch (Betula pubescens s.1.),
and Elm (Ul/mus), with Spruce (Picea) dominant in some eastern
regions. ‘This was in turn followed by
(3) a ‘ Boreal’ period of relatively warm and dry ‘ continental’
climate which towards its end supported mixed hardwood forest—
particularly of Oak (Quercus) with abundant associated Hazel
(Corylus) in the temperate belt, and persisting there until probably
seven or eight thousand years ago. ‘Thereafter followed
(4) a still warmer but wet ‘ Atlantic’ period characterized by
mixed Oak and Lime (Tilia) forest, constituting the so-called
‘climatic optimum ’ (z.e. for northwestern Europe) that lasted until
5,000 or fewer years ago. ‘This was in turn followed by
(5) the more continental, drier ‘ Sub-Boreal’, which lasted until
about 2,500 years ago and in which there occurred a reduction of
bog growth but an increase of Conifers and the entry of Beech
1Tt is said that we are still not out of the Pleistocene epoch and that, con-
sequently, we should not speak of the Present or Recent or post-Pleistocene ;
from our point of view, and although because of variability in different places it
cannot be satisfactorily defined, the time since the last great glacial recession is
all postglacial, and seems best so termed (informally, with a small ‘p’). While
suggesting this course, Professor R. F. Flint confirms (voce) that the terms Recent
and Holocene, which are often used for postglacial time, have also not been
properly defined.
2 Also, Professor Gunnar Erdtman informs me (voce), by such ‘ pioneer ” plants
as species of Artemisia, Helianthemum, members of the Chenopodiaceae, and
even Ephedra.
G
174 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
(Fagus) and Hornbeam (Carpinus). In early historical times the
climate tended to be cool and moist, yielding
(6) the ‘ Sub-Atlantic’’ period which was characterized by con-
siderable bog formation. ‘This appears to have given way during
the last millennium, and particularly in recent decades, to a warmer
and drier period. Meanwhile the forests have been gradually
destroyed by Man.
So far as has been determined, and indeed as might be expected,
details of the above changes have varied considerably in different
areas, so that in some more, but in others fewer, phases have been
recognized. Nor, according to Dr. H. Godwin (in Iitt.), were the
periods as certain and defined as is commonly believed. In general,
the tendency has been to find less favourable temperatures to the
north, and especially in the Arctic, where at best conditions
approximating those of the present-day Subarctic have prevailed.
Moreover, different workers have placed different interpretations on
the ‘ sub-fossil’ and other evidence available, only the simplest
generalization being applicable to the majority of the drastically
affected parts of the world. However, there has clearly been a
climatic succession consisting of three main phases, viz. (1) increasing
temperature, (2) culmination of warmth-loving trees, and (3) decrease
of warmth-loving trees and appearance of those predominating today.
Thus in temperate America there are scarcely any profiles known
to yield such tundra plants as occurred in the Subarctic of Europe,
but the other periods are closely comparable with the European,
comprising a cool Pre-Boreal (Spruce—Fir), a warm and dry Boreal
(Pine), a warm and moist Atlantic (Oak—Hemlock—Beech, etc.), a
warm and dry Sub-Boreal (Oak—Hickory), and a cool and moist
Sub-Atlantic (Spruce—Hemlock, Oak—Beech, etc.). Age-checks by
such methods as ‘ carbon-14’ have recently indicated that these
climatic changes on the two sides of the Atlantic approximately
coincided in time.
This hypothesis of climatic change suggests that, of the plants
which may be segregated into groups because of their preponderance
during one or another of these periods, the Atlantic and Sub-
Atlantic species favour regions having an oceanic climate and the
plants of the earliest or Subarctic period do not avoid coastal regions
nowadays, whereas the Boreal and Sub-Boreal ones favour inland
areas of continental climate that tend to be drier and warmer though
often given to extremes. Sometimes the species of one category
will be so grouped together in an area as to suggest a particular
6] FOUNDATIONS OF MODERN DISTRIBUTIONS 75
postglacial history or origin, such considerations again being
important as a basis of present-day distribution. So here again we
see instances of the significance of historical causes for an under-
standing of the present distribution of species and of their groupings
in vegetation.
THE GENETICAL HERITAGE
The remaining aspect to be considered as part of the historical
background of plant distributions is the genetico-evolutionary one,
an outstanding example being afforded by polyploids, treated in the
next section. It seems reasonable to suppose that the physiological
tendencies and habitat preferences of particular plant entities have
long been much as they are today, as have, doubtless, many of the
habitats themselves, and that morphological (that is, of general
form) and anatomical (of internal structure) indications of ecological
relationships that hold nowadays are also largely applicable to plants
which lived in earlier ages. Indeed such assumptions are behind
many of our contentions regarding climatic and other changes in
bygone ages. Nevertheless, evolution has doubtless proceeded at
various speeds and with varying results throughout the period in
which there have been advanced forms of life on earth, and among
the characters affected have surely been such ones as migrational
abilities, acclimatization potentialities, and habitat preferences.
Consequently, we should consider in broad outline the evolutionary
tendencies that manifest themselves in these characters, and some
facts and fallacies of resultant areal indication, before proceeding
with the more practical parts of this treatise.
Just as most obvious manifestations of form are inherited by each
generation from the last, and this process is repeated virtually ad
infinitum, so are different functions, different physiological attributes,
usually so inherited—including those which control the migrational
abilities, acclimatization potentialities, and habitat preferences of
different plants. Indeed these last three groups of factors are best
considered as one, being in any case all dependent on inherent
tendencies that can scarcely be separated. An outcome of this
inheritance, generation after generation, in particular lines or strains
of plants, is the obvious suitability of certain plants for certain areas,
the converse holding often more strongly—namely, that certain
other plants are unfitted for growth in these areas. Just as evolution
of form results from the action and often interaction of one or more
176 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
processes such as mutation (including chromosomal multiplication
or other change), hybridization (producing new combinations of
genic materials), isolation, and natural selection, so does evolution
regardless of form result in particular physiological make-up. Here
are included those physiological factors on which depend the suit-
abilities of particular plants to grow in particular areas.
By such means is delimited the potential area of a species, outside
of which it cannot grow naturally. ‘The ultimate limits are accord-
ingly genetically controlled, being restricted primarily to areas of a
particular climatic range and secondarily, within these, to areas of
particular sorts of soils, etc. And even though acclimatization
allows some latitude, this can only be within genetically fixed limits
—unless, of course, there is some fundamental evolutionary change
in the race. ‘This sometimes happens, for example through muta-
tion or hybridization. Also effective in this direction is some change
in the population, such as can take place through isolation and
selection of the forms best adapted to withstand particular local
conditions. ‘The latter type of instance is not so much genetically
controlled as genetically allowed, for, as we have already seen, a
population even of a single species or lower entity usually consists
of more or less numerous biotypes differing slightly in their inherit-
ance. Owing to different groupings of biotypes occurring in
different local populations, or at all events to varying selection,
different geographical races of a species often appear that have
different habitat preferences and, consequently, ranges. And geo-
graphical isolation promotes evolutionary divergence—not merely
because of differing selection-pressures and variational tendencies,
but also because mutations appearing subsequently cannot be shared
around by interbreeding ; in time, new species may result.
In spite of this relative ease and speed of evolution in some
instances, it appears to be extremely slow in others. ‘Thus it is
supposed that most at least of the woody species of temperate
regions date well back into the Tertiary, having existed for five
million or more years. ‘That, as we have seen, was a time when
equable climates extended much farther towards the poles than they
do nowadays, and woody species of temperate regions tended to be
much more widespread. One of the most active principles tending
to blur the immediate effects of evolution is introgression, which
is the gradual infiltration of the germ-plasm (and hence inculcation
of facets of the character) of one species into that of another as a
result of hybridization and repeated back-crossing with the original
6] FOUNDATIONS OF MODERN DISTRIBUTIONS 177
parental lines. Disturbance of the habitat is known to favour
hybridization, and when such disturbance is so common and wide-
spread as in the Arctic, with the frequent frost-heaving of open
habitats where there is little competition, we may expect ready gene-
flow—hence perhaps in part the notorious ‘ plasticity’ of arctic
plants. For related populations can be lastingly sympatric (that is,
coexist in the same territory) only if they are reproductively isolated.
Length of life may be an important factor controlling hybridization
and introgression, especially where uniformity is maintained by a
preponderance of vegetative propagation, with consequent restriction
of gene-flow and limitation of genetical recombination. In such
circumstances, the ill-adapted offspring tend to be easily eliminated
by competition. On the other hand, with free gene-flow and a fair
amount of mutation in rapidly succeeding generations, much new
“raw material’ is provided for natural selection to work upon, and
evolution may be expected to proceed with some dispatch.
The distribution patterns of organisms, like the external appear-
ance and genetic constitution of the component individuals them-
selves, are the end result of the interaction of evolutionary processes
and climatic, pedological, and other changes over long periods of
time. Now similarity of distribution patterns suggests similarity of
general background including evolutionary history, and this may
give some possibility of divining the history of an organism well
represented in the fossil record but unknown genetically, provided
we have others of comparable distribution that are well known
in this last respect. Such elucidations, and indeed the broader
ones of plant geography, must, however, be indulged in only
on the basis of all known facts, and then only with the utmost
caution.
In the light of modern knowledge some old assumptions should
be discarded or at least greatly modified—for example, that the
diversity of a group is dependent upon its age, which may be deter-
mined, at all events relatively, by counting the number of members
now living, and that the age of a species or other taxon is directly
related to the size of its area of distribution. Such are the main
tenets of the hypothesis of ‘ Age and Area’, discussed further on
pp. 182-3, 209. And although ideally there may be some basic truth
in these assumptions, some aggregate responsible effect in that diver-
sity may come with time and increase colonization potentialities, even
as time itself may increase the chances of dispersal, in actual fact
evolution and migration have proceeded at very different rates in
178 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
different groups and at different periods, both because of inherent
differences and of being rarely unimpeded.
Again, such generalizations as those which seek to give directions
for determining the place of origin of a particular plant group are
apt to be dangerous, as for example the supposition that the original
home of a group is the place in which the largest number of its
representatives exist. ‘Thus old groups have frequently survived
great alterations of climate and have died out in major regions where
they formerly flourished—sometimes, with little doubt, including
those in which they originated—and have apparently found secondary
centres of diversity in favourable areas where, it may be expected,
conditions for evolution are different. And it should be noted that
for genera and higher taxa of Mammals, where the fossil record is
far more nearly complete than with plants, the evidence is largely
contradictory to this hypothesis of diversity indicating the centre
of origin. Nor is the newer generalization, that the centre of origin
is the area in which the most advanced species are found, any less
dangerous—especially with its stated corollary that the most primi-
tive species will be those remote from this centre. Indeed, in many
groups of plants, the more advanced members differ from the
primitive ones in being more effectively specialized for dispersal and
more genetically ‘ open’ for migration, so that they may be expected
to overtake their ancestors in colonizing the earth.
Finally, even the common assumptions of a single (poar or
tropical) region of origin and differentiation of the groups of lhigher
plants, and of a simple basis for their migration from one continent
to another, are presumptuous in view of the present meagre state
of our knowledge and the assertion that the main centres of mam-
malian differentiation have been at middle latitudes in the interiors
of the large land-masses. For, as we have seen, evolutionary change
and migration are fundamental activities that seem to go on practically
all the time everywhere, though at very different rates in different
instances and places.
POLYPLOIDS AND THEIR AREAS
The plant geographical implications of polyploids (organisms
whose body-cell nuclei contain more than two haploid or single
chromosome sets) have been so much discussed in recent years that,
although most conclusions still remain tentative, the subject needs
to be mentioned here. Polyploids are found in most of the major
6] FOUNDATIONS OF MODERN DISTRIBUTIONS 179
groups of plants and, in the Angiosperms, are particularly plentiful
among perennial herbs. ‘They are often more vigorous than their
diploid (that is, with body-cells having double the number of
chromosomes basic to the species or group and characteristic of the
reproductive cells) relatives even of the same species, and, in addition
to anatomical differences such as larger cells and pollen grains, may
show morphological deviations such as usually larger flowers and
coarser stems. Nevertheless it is customary, unless these differences
of form are striking, to keep related diploids and polyploids in the
same (major) species. Polyploids also exhibit a greater tendency
to adopt vegetative or asexual means of reproduction than related
diploids. Even more important from our point of view is the fact
that they may show very different ecological preferences and geo-
graphical distributions from diploids, though no definite rules can
safely be formulated to govern their behaviour in this respect.
It has been widely contended that polyploids are more hardy
and consequently more northerly (in the northern hemisphere) and
high-alpine in distribution than the diploids from which they have
been derived. About this there is, however, no unanimity of
opinion—largely because there are numerous exceptions to what still
appears to be a distinct tendency. Suggestions that polyploids are
unusually prevalent in hot and dry regions and that they favour
coastal rather than inland areas, seem to be based on less factual
evidence and, indeed, to be without adequate foundation when the
situation is viewed on a sufficiently broad basis. ‘There does, how-
ever, appear to be some tendency for polyploids, especially when
they have arisen through hybridization (allopolyploids), to have a
wider geographic range than diploids: thus of 100 examples
assembled by Professor G. L. Stebbins as recounted by him in his
book Variation and Evolution in Plants, 60 polyploids were more
widely and 33 less widely distributed than their diploid relatives.
It is thought that the proportions of polyploids showing wider
distributions would be higher if the examples were limited to closely
related pairs of entities, such as polyploids and their more immediately
ancestral diploid progenitors. ‘There are also indications that poly-
ploids may be more prevalent in regions that were glaciated in the
Pleistocene than in those which were not, and in the peripheral
areas or near the ecological boundaries rather than towards the
centres of distribution of particular plant groups. ‘This tendency
evidently goes hand in hand with variation, and results from the
fact that polyploids have changed reaction norms. As Professor
180 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
S. A. Cain writes in his Foundations of Plant Geography : ‘ Ecological
advantages may arise from the competitive ability of the polyploids
that allows them to associate favorably with or even to replace their
progenitors, or from the capacity of the polyploids to occupy new
climatic or edaphic situations, and hence areas in which they are
not confronted with competition from their close relatives.’
Altogether it seems that the phenomenon of polyploidy may have
considerable significance in ecological and geographical connections.
Thus some of the changes which are apt to accompany polyploidy,
such as alterations in plant stature and leaf-size, in the frequency
and size of stomata, and in hairiness, may affect transpiration and
hence the water economy of the plant. Although some of these
characteristics are evidently beneficial to polyploids, others, such as
their commonly observed retarded rate of development and lateness
of flowering, may militate against their own good, weakening their
competitive ability. Various physiological changes have also been
observed to be associated with polyploidy, including changed cold-
and perhaps drought-resistance which may have great survival and
hence phytogeographical significance. ‘The outcome, however,
apparently varies with circumstances and with the particular case
under consideration. ‘The same is true of life-form changes,
perennials being often polyploid in contrast with their annual
relatives. ‘These and other features affect the adaptability and com-
petitive power of the plant and hence its ecological amplitude and,
consequently, geographical distribution. Moreover, any tendency
it may have to dominate is thereby affected, and, where dominance
is concerned, so is the habitat and, ultimately, the distribution of
other species. In view of the ease with which polyploidy may now
be induced in plants by various laboratory practices, it may be that
this tendency will become of even greater importance in the future
than it is today—for example in the production of larger and better
crop plants. On the other hand, it should be recalled that some
species which lack the benefit of chromosomal races ‘ do’ just as
well as those with polyploids, showing great ecological and geo-
graphical amplitude owing probably to a richness in biotypes or
larger genetic entities.
FURTHER CONSIDERATION
E. V. Wutrr. An Introduction to Historical Plant Geography (Chronica
Botanica, Waltham, Mass., pp. xv + 223, 1943); for various
palaeobotanical and allied aspects.
6] FOUNDATIONS OF MODERN DISTRIBUTIONS 181
W. C. Darran. Principles of Paleobotany (Chronica Botanica, Leiden,
|
Holland, pp. [vi +] 239, 1939).
R. F. Frrntr. Glacial Geology and the Pleistocene Epoch (Wiley, New
York, pp. xviii + 589 & maps, 1947).
R. F. Fiint. Glacial and Pleistocene Geology (Wiley, New York, pp.
xilil + 553 and 5 additional maps, 1957).
S. A. Cain. Foundations of Plant Geography (Harper, New York &
London, pp. xiv + 556, 1944); for a philosophical discussion of
the origin and history of plant types and areas.
G. L. Stessins. Variation and Evolution in Plants (Columbia University
Press, New York, pp. xx + 643, 1950).
NicHoLas PoLuNIN. Arctic Botany, vol. I: Exploration, Taxonomy,
Phytogeography (Oxford University Press, London etc., in press) ;
for application to the northern regions of the world.
H. Gopwin. The History of the British Flora (Cambridge University
Press, Cambridge, Eng., pp. vii +- 384 and additional table, 1956) ;
for application of recent stages to a more limited area.
An interesting instance of protracted persistence after introduction was
noted by the present writer in 1936 in southwestern Greenland, where he
discovered living descendants of plants which had evidently been intro-
duced from North America by the Norsemen whose Greenland settle-
ments are known to have died out several centuries ago. As certain of
these plants are of known but restricted (in two instances barely over-
lapping) distribution on the eastern North American seaboard, they give
a clear indication of where their ancestors probably came from, and where,
accordingly, Viking relics should be sought which would prove once and
for all that North America was known to Europeans long before the birth
of Columbus.
In connection with the wide acceptance of sub-fossil pollen grains as
evidence of former climates, the author cannot forget that through much
of the summer of 1950 he found the most plentiful pollen in the air near
the ground in West Spitsbergen to be that of Pinus sylvestris, the nearest
trees of which were growing on the Scandinavian mainland several
hundreds of miles away to the south. This indicates the need for caution
in interpretation—including the desirability of statistical comparisons
and, above all, avoidance of any tacit assumption that a small deposit or
reasonable amount of an airborne pollen was necessarily produced locally.
CHAPTER VII
LY PES AND, ARE AS: Or IN Aa OaRe Age
DISTRIBUTIONS
In the last chapter we considered what lies behind the geographical
distributions of plants as we see them today. We must now concern
ourselves with those distributions that appear to be natural, leaving
until the next chapter the ‘ artificial’ ones which have obviously
been made or modified by introduction or other interference by Man.
Each different kind of plant has its own particular distribution or
range, which is dependent, as we have seen, on its history, migrational
ability, and adaptability. Indeed, it 1s doubtful whether any two
of the hundreds of thousands of different kinds of plants known to
science have precisely the same distribution; and in any case
distributions are changing all the time. It is consequently impractic-
able, and wellnigh impossible, to consider such matters in detail ;
yet when a broad view is taken many interesting facts stand out, and
generalization may be valuable. For whereas any hard and fast
system of classifying ranges would be artificial, because it would not
reflect the natural diversity observed, some useful categories can,
and for practical purposes should, be widely recognized.
The term area (or range) in plant geography is applied to the
entire region of occurrence of a particular taxonomic entity (taxon,
plural taxa) or vegetational unit (econ, plural eca). Within this
range it is often necessary to consider the local distribution, some-
times called ‘ topography ’, which at best is no more nearly continuous
than are suitable habitats for the entity or unit in question. For
whereas climatic limits usually constitute the chief boundaries of
plants, local topographic, edaphic, and biotic factors are all apt to
have their effect—as will be explained and illustrated further in
Chapter X. This, albeit secondary, effect is usually considerable,
often drastically limiting the areas of plants, within the bounds
prescribed by climate, to those offering otherwise favourable
conditions.
Mention should be made here of the hypothesis of ‘ Age and
182
TYPES AND AREAS OF NATURAL DISTRIBUTIONS 183
Area’, which claims that the area occupied by a species is propor-
tionate to its age (7.e. the time it has existed). In spite of what
has just been said, this is often true and is indeed somewhat axiomatic,
as reconsideration of dispersal and migrational ‘ mechanics’ would
lead us toexpect. For if two or more species with identical capacities
in these respects begin their migrations at different times, the earliest
starter will be found, at any particular time, to have extended the
farthest. Yet actually there are so many superimposed factors
causing complications, and such numerous and often striking excep-
tions, that the hypothesis is of very doubtful value, and at best may
be considered a mere generalized working one—cf. p. 209.
Before proceeding to the main topics to be considered in this
chapter, we should explain the additional taxonomic concepts of
ecads, ecotypes, and clines. An ecad as understood in this work
is a plant type or form produced within the life-time of the individual
in response to a particular habitat factor. An ecotype is a distinct
race resulting from the impress or selective action of a particular
environment. A cline is a geographical or ecological gradient in
phenotypic characters (i.e. physical make-up). ‘These entities are
below or outside the usual specific bounds but should be borne in
mind as exhibiting much the same geographical characteristics as
do the usually higher taxa which we are more prone to consider.
*“ CONTINUOUS’? INTERCONTINENTAL RANGES
Except perhaps if it is very limited, the area of a taxon or of a
vegetational feature is never really continuous ; in reality all manner
of interruptions occur, resulting in some characteristic topography
(using this term as implying local distribution-pattern). _Neverthe-
less we tend to refer to those distributions which involve spreading
over a whole territory as ‘ continuous ’, at least provided the various
stations are not more widely separated than the normal dispersal-
capacity of the plants concerned.
Among the most frequent causes of interruption is the lack of
suitable habitats, which indeed may themselves be widely separated
or sparsely distributed. In such circumstances it is a matter of
proportion, and consequently of opinion, as to whether a particular
range should be considered continuous or otherwise. ‘Thus whereas
the Sea-beach Sandwort (Arenaria peploides agg.) is found on almost
all sea-shores of temperate and boreal regions, where its distribution
may in the wide sense be considered virtually continuous, it is
184 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
usually absent inland, and accordingly in the floras of many individual
regions it is either lacking or actually of disjunct distribution.
Again, a ‘continuous’ area may have ribbon-like prolongations
extending beyond its main boundaries and even lack continuity in
these prolongations, especially when they are narrow, as for example
along river valleys which are interrupted by narrow gorges.
Of continuous intercontinental ranges we may consider four main
types: the cosmopolitan, the circumpolar, the circumboreal (or,
alternatively, circumaustral), and the pantropic.
(1) Cosmopolitan—distributed all over the globe. In reality no
species is truly so, or, probably, found in all edaphically similar and
hence potentially suitable habitats. ‘Thus even without the funda-
mental effect of climate and the common interference of other living
organisms, it seems unlikely that any one kind of plant can be really
cosmopolitan. ‘Those which most closely approach being so are
the ones which are least exacting in their habitat requirements,
tending to be ubiquitous. ‘These wide-ranging plants which tend
to be indifferent to environmental conditions are called ‘ cosmo-
polites ’ or ‘ pan-endemics’ ; in view of the merely relative nature
of the condition, the newer term ‘ semi-cosmopolite ’ seems more
accurately descriptive of them. ‘They should at least occur in all
of the six widely inhabited continents. Actually, outside of weeds
of cultivation that have followed Man, they seem to be confined to
the lower groups of cryptogams.
(2) Circumpolar—distributed around the North or South Pole.
This term, again, has been used far too commonly and loosely. It
seems desirable to apply it only to plants which reach the arctic or
antarctic ‘ polar’ regions, wherever else they may occur, and pre-
ferable, at least in the present writer’s opinion, to accept as ‘ arctic
circumpolar” only those plants which occur at least in all of the
ten sectors into which he has divided the Arctic for such purposes.!
For these plants are truly ranged around the North Pole. Whether
such criteria can be used in the case of the Antarctic has not yet
become clear. Even if the limits of the Arctic are rather narrowly
set, so as to exclude for example the whole of continental Scandinavia
and Iceland, there are rather numerous arctic circumpolar species
already known among the higher plants, and many more among the
lower cryptogams which tend to be relatively easily dispersed and
less exacting in their habitat requirements. Still others will, clearly,
1 Cf. Circumpolar Arctic Flora (Clarendon Press, Oxford, pp. xxviii + 514,
1959).
7] TYPES AND AREAS OF NATURAL DISTRIBUTIONS 185
Fic. 46.—Map showing arctic circumpolar distribution as exemplified by Edwards’s
Eutrema (Eutrema edwardsii). (From data kindly supplied by E. Hultén.) The
broken line indicates the southern -oundary of the Arctic as proposed by the
present author, and the 10 sectors (given Roman numerals I—X) into which
the region is divided are those he employs in phytogeographical citations.
186 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
be added with further exploration ; meanwhile familiar examples
of flowering plants belonging to this category are the Purple Saxifrage
(Saxifraga oppositifolia agg.) and Edwards’s Eutrema (Eutrema
edwardsu (see Fig. 46).
Fic. 47.—Maps showing circumboreal and circumaustral distributions. A, Ribes
spp. (Currants and Gooseberries), circumboreal (after Hutchinson), but omitting
some arctic stations; B, southern species of Danthonia (Poverty-grasses and Wild
Oat-grasses), circumaustral (after Fernald) ; also northern range, but omitting a
station in southwestern Greenland.
(3) Circumboreal (or circumaustral)—distributed around the top
(or bottom) of the world in the boreal (or austral') zone. It seems
desirable to separate this category from the circumpolar, though
clearly a plant can belong to both, as in the case of the Purple
' Beware confusion with the other uses of this word in biogeography.
ll TYPES AND AREAS OF NATURAL DISTRIBUTIONS 187
Saxifrage, which is also alpine (cf. Fig. 49). Indeed, most circum-
polar plants are at the same time circumboreal, though the converse
is by no means true. ‘The boreal and austral zones lie next to the
arctic and antarctic ones, and seem best considered as extending to
the border of the subtropics. Examples of groups having such
distributions are shown in Fig. 47, the circumboreal being the genus
Ribes (Currants and Gooseberries) and the circumaustral the southern
species of Danthonia (Poverty-grasses and Wild Oat-grasses).
ZO Oh (Shee AD,
ka OO GED»
Zp GD GOLD
LOL ee (ty
LLL ES LO,
LEE OS LATTE BELLE UMA, 4
ya
WEN
CPF Lge NE SP
Fic. 48.—Map showing pantropic distribution of the Palm family (Palmae).
(After Good.)
(4) Pantropic—extending practically throughout the tropics and
subtropics, or at least widespread in the tropical regions of Asia,
Africa, and America. <A fine example is the Palm family (Palmae),
as indicated in Fig. 48. Most, but by no means all, pantropic
species appear to have been introduced by Man through much of
their range.
It may be noted in the above that when a very wide view is main-
tained, mere outliers can be overlooked and even oceans practically
ignored, distributions across them being considered continuous.
Moreover, as we proceed from the poles to the tropics and the
distances involved expand, there is a tendency for fewer and fewer
minor taxa to be circumglobal. Indeed, in the tropics, subtropics,
and adjacent warm-temperate regions, it is not uncommon for whole
genera and even larger groups to be limited to closely adjacent or
even single land-masses.
188 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
DISCONTINUOUS RANGES
In discontinuous or disjunct ranges the plants are separated by
wider gaps than the dispersal capacity of their propagules would
normally bridge. Sometimes the distinction from the so-called
continuous ranges is doubtful, in being a mere matter of degree, but
often a taxon will inhabit two or more widely separated areas whose
elucidation may be a difficult matter. In many cases, however,
areas once thought to be entirely distinct have been found to be
otherwise on exploration of intervening tracts, in which the plant
or plants in question have appeared, and consequently supposed
gaps have been closed (cf. Fig. 62). In yet other cases, ranges may
if desired be considered continuous provided there is an absence
of any suitable habitat between the colonized areas, though here
again a discreet sense of proportion must be exercised and such
major barriers as oceans and ice-caps often recognized.
The above discussion affords instances emphasizing the need for
caution in describing the distribution of a particular plant—especially
if it is of a small or insignificant nature, or is adapted to a limited
range of habitat conditions and consequently has a ‘ fragmented ’
and complex topography. But provided these warnings are borne
in mind and no inflated body of theorizing is based on unwarranted
supposition, the concept of discontinuity of area is a very real one
and has greatly stimulated research and philosophical speculation
in plant geographical and allied fields.
As for the main causes of discontinuity (apart from the controversial
extremes of sudden long-distance dispersal or ‘ historical’ wiping
out in intermediate areas, both of which have obviously taken place
in the past), they are usually environmental in being due to particular
topographic, climatic, edaphic, or biotic characteristics which lead
to areas being separated from each other by tracts of different
character. ‘This sets aside for the time being the possibility of
polytopic origin (see pp. 206 et seq.).
We should mention some of the general types of discontinuity,
regardless of cause. An area is described as (1) diffuse when it is
broken up into small, more or less numerous and equal parts ;
(2) bipartite when it is composed of only two separate parts in the
same hemisphere, one of which is extensive and forms the main
part and the other of which is subordinate ; (3) bipolar when it is
composed of two parts widely separated in the northern and southern
nemispheres ; (4) altitudinal when it is composed of one part
7] TYPES AND AREAS OF NATURAL DISTRIBUTIONS 189
Fic. 49.—Map showing arctic-alpine distribution as exemplified by Saxifraga
oppositifolia agg. (from data kindly supplied by E. Hultén).
f
BiG7 50:
Map showing range of Drooping Ladies’-tresses (Spiranthes roman-
z2ffiana) (North Atlantic, ete., distribution). (After Fernald, emended according
to directions given by E. Hultén.)
Pacific distribution). (After Fernald.)
Fic. 52.—Map showing North-South American distribution of Pitcher-plant
family (Sarraceniaceae). (After Hutchinson, emended.)
vl DYPIES AN DPAREAS OF INATURAL, DISTRIBUTIONS Ig!
Fic. 53.—Map showing range of Cimicifuga foetida (Europe-Asian distribution).
(Contributed by E. Hultén.)
192 INTRODUCTION TO PLANT GEOGRAPHY
situated in one altitudinal zone and another in another zone not
directly adjoining ; and (5) when diffuse, bipartite, or otherwise,
and populated by identical forms, it is said to be homogeneous—as
opposed to the heterogeneous discontinuity that involves related or
vicarious forms occupying different component parts of the range.
As for more specific types of discontinuous ranges, we may
mention the following as being among the most familiar and
important :
(1) Arctic-Alpine—distributed in the arctic region and in moun-
tain systems of temperate or even warmer zones ; examples are the
Herb-like Willow (Salix herbacea) and the Purple Saxifrage (Saxifraga
oppositifolia agg.), see Fig. 49.
AAS
L'9 sy
: a Se occidentalis |
Re
Fic. 54.—Map showing range of species of Platanus (Plane-trees, or Buttonwoods).
(After Fernald, emended.)
(2) North Atlantic—distributed in North America and Europe,
and sometimes also locally in Asia ; examples are the familiar
Bog Club-moss (Lycopodium inundatum) and the Hooded or Droop-
ing Ladies’-tresses (Spiranthes romanz zoffiana), see Fig. 50.
(3) North Pacific—distributed chiefly in North America and
Eastern Asia, though sometimes elsewhere ; examples are afforded
by different species of Torrey Pine (Torreya) and by the Skunk-
cabbage (Symplocarpus foetidus), which is one of that remarkable
group of species common to eastern Asia and eastern North America
but wanting in the regions lying between—see Fig. 51.
(4) North-South American—distributed in North and South
America but lacking continuity between ; an example is afforded
by the members of the Pitcher-plant family (Sarraceniaceae), see
igs (52.
(5) Europe-Astan—distributed in Europe and Asia but lacking
continuity between; examples are Leontice altaica and Cimicifuga
foetida, see Fig. 53.
(‘uosUIYyoINFYT Jay) “(eUul, Uayorq) seveoRIsAYyIOA ATIUIe} JY} Jo aBue4
jesido1jo9u Ayureur ay} pue (SUIT PI[Os) (9vI0vTURSO'T) Yiajppng snudas ay} Jo aBuvi snonuuoosip jeoidosued Surmoys deyy—"S$ ‘OIY
n03
194 INTRODUCTION TO PLANT GEOGRAPHY
(6) Mediterranean—various types including the European and
African shores of the Mediterranean Sea, or the Mediterranean
basin and some distant continent—as in the species of Platanus
(Plane-trees, or Buttonwoods), indicated in Fig. 54.
Fic. 56.—Map showing range of the genus Jovellana (Scrophulariaceae) (South
Pacific distribution). (After Hutchinson.)
aN Stee ee ree Antarctic Circle sie meee OU
Fic. 57.—Map showing range of the genus Asclepias (Milkweeds, or Silkweeds)
(South Atlantic, etc., distribution). (After Good.)
(7) Tropical—distributed in two or more separate tropical regions
such as occur within the Old World (palaeotropical discontinuity)
or the New World (neotropical discontinuity) or both (pantropical
discontinuity). Among the various minor types, two are illustrated
in Fig. 55, one being of a genus and the other of a family.
(8) South Pacific—distributed at least in South America and New
Zealand, as in the case of the genus Fovellana illustrated in Fig. 56,
and often also in other Pacific islands and in Australia.
(‘voraury YyNoG aysoddo sSutA] vorjDIvJUW JO 10}99s 9Y} WOOF S9ARIT PUB POOAA Y}Oq JO Sp1ODII [ISSOJ OS][R a1B d19Y,],)
(uayny “gy Aq painqgiju0D) ‘(saysse9gq UtoYy NOG) suspfoyJoAy SNUaS ay} Jo aBueI (OTJDIeJUY) Surmoys depy—'gs ‘oT
190 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
(9) South Atlantic—distributed at least in South America and
Africa (often including Madagascar), as in the case of rather numerous
genera including Asclepias (Milkweeds, or Silkweeds), see Fig. 57.
(10) Antarctic—distributed on the antarctic mainland (usually
as fossils) and in the southern parts of South America, New Zealand,
or on some other austral island or islands, as in the case of the
genus Nothofagus (the so-called ‘ Beeches’ of the southern hemi-
sphere), see Fig. 58.
These ten major types do not by any means cover the detailed
diversity of discontinuous areas even of an intercontinental nature.
LL
LEE Ze Lei tbs EZ MZ. LILLE " 7
Fic. 59.—Map showing (bipolar) range of the genus Empetrum (Crowberries).
(After Good, emended.)
Thus in the general category there is the ‘ bipolar’ type mentioned
earlier, an example of which is afforded by the genus Empetrum—
see Fig. 59. Other well-known examples are afforded by certain
Sedges. Also involving parts of more than one continent is the
‘Gondwana’ type which, in conformity with the palaeogeographical
basis of ‘ Gondwanaland ’, tends to embrace Africa, Madagascar,
India, and Australia.
Even more numerous than the above are the types of discontinuous
range of an intracontinental nature, as for example in Australia
where the section Erythrorhiza of the genus Drosera (Sundews)
affords a striking example as indicated in Fig. 60, or in southwestern
Europe with the ‘ Lusitanian’ element in the British flora. This
last element is composed ideally of plants which grow in oceanic
7A IDMPES AND VAREASTOF NATURAL DISTRIBUTIONS 197
western Ireland and only reappear, at least so far as natural occur-
rence is concerned, in some such distant region as the Iberian
Peninsula, an example being afforded by Mackay’s Heath (Erica
Fic. 61.—Map_ showing
“ Lusitanian ’ distribution
of Mackay’s Heath (Erica
mackatana.)
Fic. 60.—Map showing intracontinental discontinuous dis-
tribution in Australia of the section Erythrorhiza of the
genus Drosera (Sundews). (After Diels.)
mackaiana), see Fig. 61. It has been suggested that the northern
stations in such instances may be relics of a postglacial warm period
or, in the case of some still more widely disrupted ranges involving
the tropics, of the interglacial ‘ pluvials ’.
ReLic AREAS
These, as supposedly in the cases just mentioned, are areas
occupied by relic species (often called ‘relicts’) which in the
phytogeographical sense are remnants of an earlier flora that have
been ‘left behind’ when surrounding areas have been vacated.
Thus relic areas themselves normally constitute remnants of once-
extensive areas, being usually isolated and often contracting.
The determination of whether a species is really a relic is not
always an easy and definitive matter, and the same consequently
applies to relic areas. Even when the usual criteria are applied we
may go astray, as in the case of some of the plants whose apparently
disrupted ranges were formerly supposed to indicate that they had
persisted on unglaciated ‘ nunataks’ through the Pleistocene glacia-
tions; yet when intermediate areas were properly explored the
Fic. 62.—Maps showing known localities of Low Sandwort (Arenaria humifusa).
A, as published by Nordhagen (1935); B, as published by Polunin (1943); C,
as published by Porsild (1957) for the New World only, including areas that have
been heavily glaciated and islands that have only recently emerged from the sea.
Still further localities are now known.
198
TYPES AND AREAS OF NATURAL DISTRIBUTIONS 199
plants were found there too, the ranges being in fact almost as nearly
continuous as habitat suitability allowed. Fig. 62 illustrates this
point. Ideally the finding of fossil remains of the plant in question,
in surrounding areas where it does not now grow, will demonstrate
its relic nature and indicate from what period of time its local
| Trapa natans L.
( Water- chestnut )
| e Recent
° Fossil
Fic. 63.—Map showing recent (dots) and ‘ fossil’ (circles) stations of the Water-
chestnut (Trapa natans) in Scandinavia. (After Hultén.)
occupation dates. ‘This is illustrated by the present-day and ‘ fossil ’
stations of the Water-chestnut (7rapa natans) in Scandinavia
(shown in Fig. 63) and of the giant Redwoods in the northern
hemisphere in general (indicated in Fig. 43): In this connection
it is interesting to note that whereas 7rapa is now known in North
America only as an introduced weed that is apt to be aggressive,
200 IND RODUCT TON, tO} PAL AWN iG EO GRAVE Tay [CHAP.
there is much fossil evidence that it was formerly widespread there
in the wild state.
In general the main, non-relic part of an area either occupies its
original territory, any outlying parts having become separated owing
to habitat changes, or results from secondary dispersal, in which
case the relic part may actually lie within the main area. An example
of this latter type is afforded by the Scots Pine (Pinus sylvestris),
which has relic portions of its area in the mountains of Europe,
whereas its extensive occupation of surrounding sandy lowlands is
the result of secondary colonization. Such colonization may even
take place from a relic area, for example when climatic conditions,
whose deterioration in surrounding areas had led to isolation,
subsequently improve so that suitable plants can recolonize around
the isolated area. Much the same often occurs with the removal
of heavy grazing which had caused the restriction of some plants
to limited areas (that in a sense were then relic ones), whence they
spread again when the animals moved off or died.
A species which occupies a relic area throughout its range may
be termed an ‘ absolute relic’, while one of which only an isolated
part of the area is relic is known as a ‘local relic’. An ‘endemic
relic ’ is one that is restricted to a single region. ‘Those relics which
achieve a secondary distribution by the occupation of suitable
habitats are known as ‘migrant relics’, the more recently occupied
habitats being termed ‘ pseudo-relic ’ and the plants * pseudo-relics ’
(or ‘ -relicts ’), though these terms may also be applied in cases where
a plant acquires the apparent character of a relic without actually
being one. Most of these principles may be applied equally to
single species or other taxa, groups of species (as in relic colonies),
or entire floras.
Many relics, having long been in partial disharmony with present-
day habitat conditions, have become depauperated in biotypes ; the
consequent loss of the capacity for variation and adaptation has led
to their being considered conservative. Such plants have often been
called ‘ senescent’. Being commonly restricted to narrowly specific
environmental conditions, they may fail to retain even their limited
area, in extreme cases becoming extinct. However, if favourable
conditions should reappear and, for example, allow the approach of
formerly segregated variants so that hybridization can take place and
new forms thereby arise, the species may take on its former vigour
or even exceed it and become aggressive.
Leaving aside the so-called ‘ anthropogenic relics’ whose areas
7] TYPES AND AREAS OF NATURAL DISTRIBUTIONS 201
have become drastically reduced through the activities of Man, and
the ‘ cultivated relics ’ whose sown area, owing to their low economic
value, has been reduced to small compass and few localities, we
may usefully distinguish three main classes of relics on the basis
of the type of natural habitat-change which has isolated them.
(1) Formation relics that occupy limited areas within the boundaries
of major plant communities (formations) which have undergone
considerable changes in composition. Striking examples are the
residual wooded tracts that are sometimes found in some extensive
grasslands.
(2) Geomorphological relics that are connected in their habitat
preferences with particular ecological conditions but that, owing to
edaphic or allied changes, are no longer provided with the conditions
of growth to which they are accustomed. F’amiliar examples include
marine plants inhabiting freshwater lakes, and shore plants growing
along the edges of dried-up gulfs.
(3) Climatic relics that give evidence of having originated and
formerly flourished under other climatic conditions than those in
which they now grow. Examples are the mesothermic plants to
be found in some boreal areas that have cooled at least since the
‘postglacial optimum’ when such plants presumably migrated to
these areas.
While it is scarcely possible to distinguish further classes of
‘biotic relics’ resultant on grazing, etc., as distinct from those
engendered by Man or his animals, or on plant competition with
our present limited knowledge thereof, there is another basis on
which relics may be classified, namely, their age and origin, the
main such classes being: (1) pre-Tertiary, (2) Tertiary, (3) glacial,
(4) interglacial, and (5) postglacial relics.
VICARIOUS AREAS
These are areas belonging to closely-related taxa (vicariads)
derived from the same common ancestor and tending to be mutually
exclusive of one another in naturally (7.e. without human interference)
occupying separate areas. Sometimes, and especially when their
ecological requirements differ only very slightly, vicariads may be
mutually exclusive through being closely competitive. ‘This is the
case with many subspecies and closely-related species ; on the other
hand, with higher groupings—and even families and whole com-
munities may in a sense be vicarious —there 1s less reason to suppose
202 INTRODUCTION TO PLANT GEOGRAPHY
that their mutual exclusiveness is due to competition. From their
very nature, ecotypes often tend to be vicarious, as do the extreme
‘ends’ of clines. Inany event, the process of genesis of geographical
races seems to be the main basis of the formation of ‘ vicarious
areas’, at least among the lower taxa, and this and any subsequent
segregation tends to take place towards the periphery of the range
of the ‘ parent’ species, where the latter is in general least happily
adapted to the environmental conditions.
Examples of vicariads are to be found in almost any modern
taxonomic monograph in which the series of closely-related entities
inhabiting independent geographical areas are commonly recognized
as subspecies or even species. ‘These tend to be mutually exclusive,
although sometimes their areas may show some overlapping ; and
it is often a matter of opinion (as well as, of course, the degree of
difference they exhibit) whether they should be termed subspecies
or raised to the rank of species. ‘This is one of the most persistent
sources of controversy between the ‘ lumpers’ and the ‘ splitters ’
(of species). Numerous instances are afforded by major species that
are represented by different minor species or varieties (which most
often seem best considered as subspecies) on the two sides of the
North Atlantic—as, for example, the European Royal Fern, Osmunda
regalis, and its North American subsp. spectabilis (see Fig. 64).
Polyploids, dealt with towards the end of the preceding chapter,
are often vicariads (see also p. 204).
Here it may be well to quote Jordan’s * Law of Geminate Species’ :
‘Given any species in any region, the nearest related species is not
likely to be found in the same region, nor in a remote region, but
in a neighbouring district separated from the first by a barrier of
some sort or at least by a belt of country the breadth of which
gives the effect of a barrier.’ Such pairs of twin or ‘ geminate ’
species (or subspecies) actually constitute vicariads, differing in only
minor characteristics that are of later origin than their common
characters.
True vicariads (which have arisen from a common stock) should
be distinguished from false ones which have not this close genetic
relationship. Frequently these last are members of different sec-
tions of a genus that have developed similar life-forms through
‘convergent evolution’. ‘True vicariads (and consequently the
areas they demarcate) may be classified according to the manner of
their separation from one another into (1) horizontal (geographical),
(z) altitudinal (physiographic), (3) habitat (ecological), and (4)
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VA Le
204 INTRODUCTION TO PLANT CEOGRAPHY [CHAP.
seasonal (exhibiting seasonal dimorphism, as in the case of closely
related forms differing in their times of development).
Most ‘systematic’ vicariads, consisting of pairs or sets of the
higher taxa which are vicarious, belong to the first or geographical
category, a good example being afforded by the various races of
Bracken (Pteridium aquilinum agg.) inhabiting different parts of the
world. ‘There are also plentiful examples of physiographic vicariads
inhabiting lower and higher altitudes, respectively—for example the
Wood and Alpine Forget-me-nots (Myosotis sylvatica and M.
alpestris agg.), and the Common and Alpine ‘Timothys (Phleum
pratense and P. alpinum s.1.). As examples of ecological vicariads
growing in different habitats and characterizing different communi-
ties, we may cite: in fresh and mainly salt marshes, respectively,
the Bulrush and Glaucous Bulrush (Scirpus lacustris and S. tabernae-
montanit); in soils with high and low available water, respectively,
the Water and Wood Avens (Geum rivale and G. urbanum) ; and in
calcium-rich and calcium-poor soils, respectively, the Yellow Moun-
tain and Brook Saxifrages (Saxifraga aizoides agg. and S. rivularis
agg.). Such vicariads are commonly intraregional, not infrequently
growing as closely together as their habitat requirements permit,
whereas the geographical ones tend to be more widely regional.
We have already indicated that vicariads, at least of the lower
taxonomic orders, tend to evolve chiefly about the periphery of a
migrating ‘parent’ taxon. Here new characters are particularly
prone to arise by mutation or other chromosomal change, and help
the better-adapted offspring to survive under conditions which are
less favourable to the parent. ‘Thus autopolyploidy, the pheno-
menon of multiplication of a plant’s own chromosome set, may result
from extreme habitat conditions and be accompanied by evident
changes in the plants involved, autopolyploids often having very
definite geographical ranges differing from those of their ancestors
which possessed the normal (diploid) number of chromosomes.
When the changes accompanying polyploidy are very marked, a new
species may be constituted, which is often a vicarious one. Vicariads
may also arise as a result of hybridization, which is frequently accom-
panied by a multiplication of chromosomes (allopolyploidy) ; or they
may be a consequence of mere local differentiation resultant on
changes in climatic or other habitat conditions in some part or parts
of a plant’s range. As a result, the initial species may ‘ break up’
into a number of vicarious ones, or of subspecies with distinct
geographical ranges.
7] TYPES AND AREAS OF NATURAL DISTRIBUTIONS 205
ENDEMIC AREAS
In contrast with the plants exhibiting various types of discon-
tinuous range, and which may be widely scattered or at all events
polyendemic (polytopic—see next section), are those whose range in
each case is confined to a single restricted area, not extending beyond
some one region, island, or other circumscribed tract. Such plants
are called endemics, although this term again is largely relative. An
endemic area is the area of a species or other taxon that, in its dis-
tribution, is limited to some single natural region or habitat, the
history or conditions of which mark it off from others. Islands and
mountain massifs are particularly pertinent in this connection.
Of endemics there are two main types. ‘There are the old ones
whose range was once far more extensive than it is today, and which,
being remnants or survivors of former floras, may be called relic
endemics or epibiotics. ‘These may make up a large proportion of
the species of ancient islands or mountain massifs, being said to
involve 72 per cent. of the thousand or so native vascular species
of New Zealand and 85 per cent. of those of St. Helena. A good
example of this type of endemism is furnished by the giant Redwoods
of the western United States, which used to be extremely wide-
spread in the northern hemisphere (cf. Fig. 43). Their drastic
contraction in range seems the more remarkable when we recall
that they include what are probably the oldest individual living
organisms today, some being reported to exceed 3,000 years in age.
The other main type of endemic is made up of the relatively young
taxa, usually below the rank of species, and then usefully termed
micro-endemics, which are characteristic of newer portions of the
earth’s surface. ‘Thus, when ecological conditions change within
the limits of some natural region, there is a tendency for new forms
to evolve, and these may be closely bound to the region owing to
its special habitat conditions or because they are physically unable to
spread beyond its confines. Such plants may be called neo-endemics.
The determination of the proportion of these main types of
endemism in a particular flora is an important factor in its analysis,
capable of telling us much about its age and history. Relic
endemics are particularly useful in indicating antiquity, isolation,
and diversification of habitats, for these factors all tend to produce
additional endemics and help in their survival, as probably do also
suitable conditions for the development of vegetation. Such
endemics tend to be deficient in biotypes and are usually recognized
H
206 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
by their relic character and geographical isolation, their small
amplitude of variation and narrow restriction to particular ecological
conditions, their relatively small chromosome number, and their
generally retrogressive nature. On the other hand, neo-endemics,
being secondarily derived types, commonly have larger chromosome
numbers ; they also tend often to be relatively aggressive. ‘This is
especially the case when they are rich in biotypes, for example owing
to hybridization.
Apart from the so-called ‘ pseudo-endemics’ that have been
encountered only in one place and appear to be mutants, etc., and
unlikely to persist, there are the ecological endemics which have arisen
in relation to particular habitat conditions.
Some endemics are confined to very limited areas, such as a single
small island or mountain peak, and may be called local endemics.
Such restriction is usually due (1) to their recent origin (so that
dispersal has only just begun), or (2) to their antiquity (so that
the area is a contracted one or even a ‘last remnant’), or (3) to
their high specificity with regard to habitat conditions (which
prevail only at a given spot within the area that can be reached by
viable disseminules), or (4) to the impossibility of expansion (owing
to physico-geographical obstacles).
It has been mentioned that isolated islands are often particularly
rich in endemics ; this is especially the case with those which are
at least some hundreds of miles from the nearest major land-mass,
and which may accordingly be termed oceanic. Although the dis-
tinction is far from definite, it is sometimes useful to think of other
islands, whose flora bears a closer relation to an adjacent land-mass,
and which are usually within at most a few hundred miles’ distance
from it, as continental. ‘These islands are phytogeographically like
fragments of continents or larger islands and are usually inhabited
by larger numbers of species than are comparable oceanic islands,
containing as they do both plants and animals for which transoceanic
transport seems virtually impossible. Some remote islands, such
as those surrounding Antarctica, are apt to be considered relics of
a formerly more extensive continent and hence scarcely oceanic.
POLYTOPY AND THE INCIDENCE OF AREAS
Polytopy is the occurrence of a species or other taxon in two
or more separate areas, such species being termed polytopic or
polyendemic. ‘Vhe discontinuous ranges involving disjunct areas
7| TYPES AND AREAS OF NATURAL DISTRIBUTIONS 207
which we dealt with earlier in this chapter are examples of polytopy,
which we must now consider briefly from the point of view of the
origin of the areas involved.
Excluding the old conception of special creation, it is yet believed
by some that approximately polytopic forms may have had an
independent origin in their existing plurality of areas whose popula-
tions are similar because of parallel descent from a commen ancestor.
This would explain discontinuity on the basis of the species con-
cerned having evolved independently in two or more separate areas.
But whereas this seems possible where a common near-ancestry and
minor taxa are concerned (for example, through natural selection
acting on a similar set of mutants), it scarcely seems conceivable
for members of major species whose common ancestry was remote
and whose distinctive characters are numerous. Largely separate
is the contention of the differentiation hypothesis that different
species of the same genus may have ‘ crystallized out’ from an
ancestral complex in two or more areas independently. Some
students even hold that the polytopic populations in different areas
have had an independent origin from taxonomically different
ancestors and have arrived at their present similarity through con-
vergent evolution. However, in the words of Cain (in the work
cited at the end of the last chapter) such polyphylesis “for most
students of evolution, genetics, and taxonomy, represents only the
result of inadequate knowledge, or the forming of groups (genera,
families) for practical convenience, except in cases of hybrid
descent...’
More widely accepted is the hypothesis that polytopic forms are
immediately related, the intervening tracts having been ‘ bridged’
in the past either by a continuous series of populations or by long-
distance dispersal. However, as in other biological instances, it
seems likely that different explanations apply in different cases.
Thus concerning dispersal, it has been calculated that even if the
probability that some member of a population will cross a barrier
in any one year is virtually nil, during the course of a million years
the event will be probable, and in ten million years almost certain.
With regard to the disruption of areas that were previously con-
tinuous, though not necessarily at one and the same time, we have
already discussed, especially in the last chapter, such possible causes
as continental drift, land-bridges, and climatic and other change.
We have also seen how isolated centres of survival may become
centres of dispersal, upon the return of more favourable conditions
208 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
—at least if the biotype depauperization of the relic community
has not been too extreme.
The centre of origin of an area of other than a lower taxon, at
least in the absence of extensive palaeobotanical data, is apt to be so
difficult and hazardous of determination that little will be said about
it here. For its indication about a dozen criteria are commonly
employed, though sometimes a fair one may be given by zsoflors,
which are lines delimiting regions supporting equal numbers of
species, e.g. belonging to a single genus. From the generic centre
outwards the number of species may be expected to decrease regularly,
and to assume a pattern which suggests the tracts of past migration,
which conversely may be followed backwards and found to converge
upon the generic centre. Even here, palaeobotanical confirmation
is wellnigh essential. Groups also have their single or multiple
centres of variation, where there are concentrated the greatest
diversity and wealth of forms (also called the mass centre), and their
centres of frequency, where there are accumulated the greatest
numbers of individuals or stations.
With single species or lower taxa the situation may be far simpler.
Thus the tendency to decrease in the number of individuals of a
species towards the periphery of its area, is closely connected with
adaptability to definite habitat conditions. For whereas in the
centre of its area the habitat conditions most nearly approximate to
the optimum—so that the species can grow under fairly diverse
conditions, as it often does on different types of soil—nearer the
periphery of its area anything approaching this optimum is of
increasingly rare occurrence, and there is often lacking even the
minimum of conditions required for its normal existence. ‘Thus
the European Beech (Fagus sylvatica), which ordinarily is capable
of growing on a variety of soils, is largely confined near the western
(moist) periphery of its range to the drier calcareous ones.
As regards an area itself, its shape is best indicated on maps by
connecting all the peripheral points of its distribution. The shape
generally depends primarily on the physico-geographical conditions
of the country, and secondarily on the biological peculiarities of the
taxon involved. In the frigid and temperate zones the diameter of
most specific areas is much greater from west to east than it is from
north to south, whereas in the torrid zone species tend to have a
relatively larger latitudinal amplitude than elsewhere.
An area of a species or subspecies usually comes into being largely
through migration and the operation of barriers thereto, the parent
7] TYPES AND AREAS OF NATURAL DISTRIBUTIONS 209
species during its dispersal often running into climatic and/or
edaphic habitat conditions to which it is unaccustomed and which
in time may lead to modification of the incipient area. From some
such beginnings further migration usually leads to the current area.
However, sometimes drastic change may lead to evolution in situ.
Especially in the cases of young species and of those that have had
their ranges reduced by relatively recent catastrophes, the area at
present occupied is only a part, and often only a very small part,
of what it might be. For, as we shall see in the next chapter, each
species tends to have, besides its actual area, a potential area which
may be demonstrated by artificial introduction and is often of great
practical importance in the regional allocation of crops. It can also
be of significance with regard to the nuisance caused by weeds.
Indeed, it seems probable that the majority of present-day ranges
are by no means complete so far as the occupation of areas of suitable
climate and soil conditions are concerned. Sometimes this incom-
pleteness is due, as implied above, either to an insufficient lapse of
time since the entity evolved, or to the basic inefficiency of its dis-
persal—or a combination of youth and inefficiency. But probably
it is more often due to historical changes such as glaciation, or to
the operation of boundaries set by physical barriers such as seas or
mountains or deserts, by ecological conditions, or by competition
with other species.
Although there is a natural tendency for the areas occupied by
many plants to increase with age, which can be an important factor
in biogeographical considerations, the relationship of area to age is
by no means as direct as has sometimes been supposed. ‘To be
sure, with some genera and species, especially in certain tropical
regions, the area of spread is roughly proportional to the age (as
was suggested by the now unpopular hypothesis of *‘ Age and Area’
—see pp. 182-3), but in others this is so far from being the case that
the area at present occupied gives little or no indication of age. ‘This
is true, for example, where ancient fossils indicate a much wider
distribution than now obtains, as in the case of the giant Redwoods
(cf. Fig. 43). For actually, at least outside of some favoured regions,
there have been so many, often drastic disturbances that the general
situation appears to be that the size of an area occupied by a species
depends less on the age of the species than on other factors. ‘These
include its adaptability and competition-rigour, the circumstances of
its genesis, and whether or not ecological conditions and any dispersal
mechanism or mechanisms have favoured successful migration.
210 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
INTRANEOUS, EXTRANEOUS, AND OTHER ELEMENTS
It is sometimes useful to classify the forms growing in a particular
territory according to whether in each case the occurrence is well
within the area of the form (intraneous) or near its periphery
(extraneous). For instance, the disjunct arctic species occurring in
the White Mountains of New Hampshire are extraneous there but
intraneous in their characteristic region of habitation, namely, the
Arctic.
The components of such groupings form special elements, which
are severally recognizable in most floras. ‘Thus we may have
intraneous and extraneous elements in a flora, a preponderance of
either characterizing certain areas. ‘This leads us naturally to con-
sideration of specific phytogeographic or floral elements, which are
closely related to migration. Ideally each such element is the
floristic expression of a territory of limited extent, in that it involves
the taxa and phytogeographic groupings characteristic of a given
phytogeographic area—such as the Mediterranean region, giving rise
to a ‘ Mediterranean element’. Often, however, it seems preferable
to extend this concept of floral elements to include other and much
wider applications.
Before a flora can be divided into its main general elements we
must eliminate all aliens and ‘ wides’ (also called ‘ polychores ’, 7.e.
species having such an extensive range that they embrace several
phytogeographic regions). ‘Then the endemics should be studied
and, as far as possible, assigned to their various categories. ‘There-
after the remaining species may be divided into groups according
to the geographical character of their areas, with the object of
determining the regions whence these groups originated, and so
perhaps establishing the genesis of the flora. For this grouping,
five main principles should be followed and elements sought (apart
from those already accounted for) :
(1) Geographical elements—grouped according to the types of their
total areas, their altitudinal ranges, or their distributions within the
region concerned. ‘This is, however, often insufficient to determine
the origin of a flora that is not a migration one, for relic and endemic
elements so grouped do not reflect the genesis of a flora. Even
with a marked arctic-alpine element it is often doubtful which way
the components have spread—whether they are arctic types which
have migrated southward into the mountainous regions offering
somewhat comparable conditions, or vice versa. All that can be
7] TYPES AND AREAS OF NATURAL DISTRIBUTIONS 211
done is to divide the species into apparently arctic and apparently
alpine groups; and much the same applies to the subarctic or, as
it is sometimes called, subarctic-mountain element. Other geo-
graphical elements are fortunately apt to be less vague, examples
being the Mediterranean and the Atlantic ones. ‘The broader types
of ecological groupings may also be included here when they char-
acterize geographical regions—as, for example, certain life-zones and
formations (or, better, biomes, which are climax formations of plants
and animals considered together, such as the characteristic Spruce-
Moose biome of most of the continental regions of Canada). Often
it is possible to decide on the geographical element to which a
species most likely belongs by locating its “mass centre’ (of maximum
variation), for that is the part of its area which is most likely to be
basic.
(2) Genetic elements—grouped according to their region of origin
and accordingly reflecting the genesis of the flora. For this, detailed
monographic study of the groups involved is necessary, and so at
best it is usually possible to classify in this manner only a few chosen
species. ‘These first two types of general element are considered
by some students to be the most important bases for floristic
analysis.
(3) Migration elements—grouped according to the routes by which
they migrated to the region concerned. Examples of migration
routes are particular mountain passes, river valleys, and suitable
coasts. Unfortunately, species are apt to reach the domain of a
given flora by more than one route, so that the establishment of
particular migration elements is difficult or futile—though often well
worth attempting, as such elements may provide valuable clues to
the history of a flora.
(4) Historical elements—grouped according to the time (such as
the postglacial climatic optimum period) when they became a part
of the flora concerned. Further examples are the so-called arctic-
Tertiary element of evergreen and deciduous trees, and the boreal-
Tertiary one which included such southerly members as Palms ;
these examples represent the inhabitants of the arctic and boreal,
respectively, regions in Tertiary times, while to their south, stretching
from southern England to Japan in those far-off days, lay the tropical
region of megatherms, 7.e. plants adapted to high temperatures.
(5) Ecological elements—grouped according to their immediate
habitat preferences. Most significant are the oceanic and continental
elements, embracing, respectively, those species which are adapted
212 INTRODUCTION TO PLANT GEOGRAP ERY
to a humid maritime climate and those which prefer an arid con-
tinental one of marked temperature extremes. ‘The oceanic elements
are generally considered to be the more ancient, the initial land
flora having been evolved from an aquatic one and further evolution
having been along lines of emancipation from dependence on water
and, accordingly, adaptation to more continental habitats. Such
ecological elements can be of great significance in elucidating the
history of a particular flora and any major vagaries of climate to
which it may have been subjected. It should be remembered,
however, that within the limits of a country or natural region there
may be found tracts, such as mountain massifs, in which particular
conditions predominate and which accordingly give refuge to ‘ alien ’
plants. ‘These may be termed inclusions, in contradistinction to the
basic element of types properly belonging to the floral region, and
the more general penetrants from outside.
Mayor REGIONS
These will be considered here only in the broadest outline, as
several subsequent chapters are devoted to them. Vegetational
regions, being based on life-form rather than on taxonomic proximity,
may cut across the ranges of systematic units and also differ greatly
from zoogeographical realms characterized by particular animal!
communities. Fig. 65 indicates the main vegetational-climatic
regions of the world in highly generalized form, and is the basis
of the division followed in Chapters XII et seg. It may be noted
that the western Old World desert region, which is sometimes
separated as a special one, is here included in the tropical region, as
the Mediterranean is in the temperate region. This gives us a
central tropical belt and, to both the north and the south, two others.
These are the temperate (in the wide sense) and polar belts, the
former ranging approximately from the polar tree-line and including
the subarctic and warm-temperate zones, while the tropical belt con-
veniently includes the subtropics. Each of these broadest of regions
is itself complex, tending to show latitudinal gradation—so much so
that accurate detailed maps are scarcely conceivable, at least in our
present state of frequent ignorance of local features.
FURTHER CONSIDERATION
Most of the subjects dealt with in this chapter are further discussed
by Wulff and Cain in their works cited at the end of the preceding chapter.
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214 INTRODUCTION TO PLANT GEOGRAPHY
For useful examples in several instances, reference may be made to
R. Good’s The Geography of the Flowering Plants, second edition (Long-
mans, London etc., pp. xiv + 452, 1953), and for a comparison with
zoogeographical areas to M. I. Newbigin’s Plant and Animal Geography
(Methuen, London, pp. xv -+ 298, 1936).
A valuable example of detailed mapping of the ranges of individual
species in a well-known region (northwestern Europe) is E. Hultén’s
Atlas éver Vaxternas Utbredning 1 Norden (Stockholm, pp. 119 + 512,
1950). A somewhat similar project is afoot for the British Isles, and it
is to be hoped that, in time, more and more regions will be covered in
this manner. For the ranges as known to date of many different species
and larger groups, see Die Pflanzenareale (Fischer, Jena, vols. I-V,
1926-40).
The floras of different areas are dealt with in numerous works to which
S. F. Blake & A. C. Atwood’s Geographical Guide to Floras of the
World affords the most comprehensive introduction and selective biblio-
graphy. Part I, covering ‘ Africa, Australia, North America, South
America, and islands of the Atlantic, Pacific, and Indian Oceans’, was
published in 1942 by the United States Government Printing Office,
Washington, D.C. (U.S. Dept. Agric. Misc. Publ. No. 401, pp. 1-336),
while Part II, treating western Europe, is to be published by the same
agency. It is contemplated that a third part will cover the rest of the
world.
CHAPTER VIII
MODIFICATION AND DISTRIBUTIONS
OF CROPS (AND WEEDS)
It is now time to deal with the so-called ‘ artificial’ changes
wrought by Man, whether intentionally or accidentally, in the
distribution of plants. In this connection Man seems during recent
centuries to have been the most potent factor in the world; and
as his activity increases and more and more barriers are broken
down by transport, his effectiveness as a distributor grows ever
greater. ‘his transportation is quite apart from the changes Man
brings about incidentally in the course of his ever-extending activities
of husbandry or desecration.
From what was said in the last chapter it should be clear that,
whereas plants have their own distribution patterns, and particular
taxa have particular areas which they are capable of occupying, it
is rarely if ever that a vascular plant taxon will occupy anything
like the whole of the geographical area or areas where the climate
is suitable for it. Usually, numerous unsuitable habitats will inter-
vene, and even then there are commonly left areas of suitable
habitat which the plant in question has failed to reach, or in which,
if it has arrived, it has failed to establish itself and survive. In
other words, the present areas occupied by particular plants tend
to fall far short of the maximum which they are capable of occupy-
ing: artificial introduction of a plant outside its present natural
area will frequently demonstrate its ability to grow in a wider range
of situations both geographically and ecologically. ‘Thus, besides
its own natural area of distribution, each species has, at least in
most instances, a wider potential area which, if we include places
where it can be grown in cultivation or otherwise in the virtual
absence of competition, is often very much more extensive. ‘This
principle is of great significance in connection with the introduction
and production of crops on which Man largely depends.
The dispersal of plants by Man was considered in a special section
of Chapter IV. Here we must deal with the results of such transport
so far as the all-important crops of field and forest and the recognized
215
216 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
weeds of cultivation are concerned, not forgetting the more devastat-
ing diseases of those crops. We must also consider the effects of
cultivation—and of the removal of Man’s protection, whether such
protection had been intentional, for crops, or unintentional, for
weeds.
EFFECTS OF CULTIVATION
It is a common experience of farmers and gardeners that cultivated
plants and the weeds infesting them or their ground tend to lack
the ability to spread independently of Man and, in many cases,
even to maintain themselves unaided. ‘This inability to hold their
own in the face of natural forces—including, in particular, competi-
tion from other plants—may even be observed within the natural
region and habitat range of the more immediate ancestors of the
cultivated plant or weed. ‘The reason is that Man’s influence on
a plant under cultivation is apt so to change its genetical make-up,
structure, and physiological capabilities that it is deprived of ‘ key ’
advantages in the general struggle for existence. ‘These advantages
will have been acquired, often by the rigours of natural selection,
in preceding periods of the plant’s evolutionary history, but may
be lost overnight, as it were, by artificial selection or, more gradually,
by the ‘ protection ’ afforded by numerous generations of cultivation.
We may here note a ‘round dozen’ of the many and diverse types
of changes wrought by Man in the make-up and structure of cul-
tivated (and infesting) plants.
(1) Genetical changes—involving the loss of characters that are
obviously beneficial and often needed for the plants’ survival in
natural conditions. Besides the examples indicated among the
following categories, and plentiful others resulting from such
activities as artificial hybridization and the induction of polyploidy,
there are the numerous instances of physiological or otherwise less
structurally obvious but fundamental hereditary changes.
(2) Physiological changes—which are commonly hereditary and
hence included in the above category, but which in other instances
manifest themselves in the life of a single generation and cause it
to ‘ pay the price’. ‘Thus, for example, plants that have not been
suitably hardened may succumb on transplantation to a less favour-
able habitat than the one in which they originally developed and
to which they had become accustomed.
(3) Structural changes—often bound up with physiological ones,
8] MODIFICATION AND DISTRIBUTIONS OF CROPS 217
and, although commonly hereditary, nevertheless often the result
of environmental impress during the life of the individual. Seed-
lings rendered weak by competition with their fellows under other-
wise favourable conditions of cultivation may not so much as survive
on transplantation ; or again, thin and delicate ‘ shade leaves’ of
some trees are liable to shrivel on exposure. ‘Though drastic, these
are examples of mere ontogenetic change during a single plant’s
life-time.
(4) Loss of adaptations for dispersal—or even for the initial act of
dissemination. ‘The fruits of cultivated Flax and of the Opium
Poppy do not dehisce when ripe, whereas those of their wild relatives
do. Again, among weeds, the inflorescences of some noxious
Brome-grasses break up less effectively than those of their wild
counterparts, which also tend to have longer awns.
(5) Loss of protective coverings and sturdiness—for example in
cereals whose fruits are deprived of the usual outer husks, and in
the pods of many cultivated members of the Pea family (Leguminosae)
which lack the fibrous lining characteristic of their wild relatives.
Presence of the fibrous lining also causes the valves to curl up and
thus helps dissemination of the seeds. ‘The commonly lesser
development of fibrous tissue in crop plants is apparently connected
with their growth in close stands—often protected from winds and
under conditions of favourable humidity, nutrition, and shading,
which all tend to promote rapid growth. Similarly, in a dense
forest the trees usually have tall and slender trunks and weakly
developed crowns, so that individuals left isolated on removal of
their neighbours are liable to be blown down, whereas in the open
the same species tend to be far more sturdy.
(6) Increase in size of seeds and fruits—usually accompanied by a
decrease in their number. ‘This tends to reduce their chances of
dispersal while at the same time reducing the plants’ opportunities
for propagation. Moreover, the production of unnecessarily large
seeds and fruits is wasteful so far as the plants’ economy is concerned.
How much more economical are the fruits of Fireweeds than of
Pumpkins, and how much more successful as colonists are the
former plants !
(7) Improvement of flavour—of seeds and fruits, which is a com-
mon objective of cultivation, tends to cause animals to eat them
more voraciously and completely, and so militates against effective
dispersal.
(8) Conversion of perennials into annuals—is common in the
218 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
domestication of plants, as for example among the cereals. This
is advantageous from Man’s point of view in speeding up crop-
production, and may favour the plants’ own chances of survival in
‘open’ habitats such as those prepared for cultivation; but it
places them at a great disadvantage in competition with natural
vegetation, which in most undisturbed land habitats is dominated
by perennials.
(9) Absence of successful fruiting—for example due to atrophy of
the sexual organs, to absence of pollinators far from the plants’
native habitat, or to the desecrations of Man—results in such plants
being incapable of self-perpetuation by the usual means.
(10) Seedless fruits—which are often an objective of plant breeders
for cultivation, likewise render a plant incapable of independent
existence unless it has some effective means of vegetative propagation,
in which case it will still lose the benefits of sexual reproduction
(such as hybrid vigour and the exchange of genic material).
(11) Double flowers—involving for example the ‘ conversion’ of
stamens into petals—again render the plant incapable of self-
perpetuation by the normal means.
(12) Loss of defensive adaptations—such as spines, thorns, hairiness,
and hardness—renders the plant defenceless against animal grazing
and, often, more susceptible to injury from excessive loss of moisture.
The above features may occur already among wild plants as
abnormalities, but in cultivated strains they tend to become intensified
by Man’s conscious or unconscious selection and, often, perpetuated
through his propagation and protection. When no longer cul-
tivated, or, in the case of weeds, enjoying the benefits of cultivation,
such horticultural or agricultural strains tend to disappear. Having
been modified by Man in ways most likely to suit his needs (but
at the same time harmful to the chances of persistence of the plant
as an independent organism), they are no longer able to help even
in maintaining the area of the species to which they belong, at least
in many cases in the absence of Man’s influence.
The weeds most notably modified through cultivation are those
that constantly accompany particular crops, thanks to which they
have long been involuntarily cultivated by Man. Some of them—
such as, apparently, the cultivated Rye—have become so transformed
as to be now themselves objects of cultivation. For in weeds, just
as in intentionally cultivated plants, there tend to be such changes
as increase in size of seeds at the expense of their number, and loss
by fruits of their protective coverings and abilities to disseminate.
8] MODIFICATION AND DISTRIBUTIONS OF CROPS 219
A good example of weeds closely associated with particular crops
is afforded by the so-called ‘ linicolous’ plants that accompany Flax
(Linum). ‘These appear to lack the normal adaptations for accom-
modating their development to seasonal changes, and may even be
dependent for the completion of their life-cycle upon being gathered
with the Flax crop when their seeds are ripe, kept in a storehouse
through the winter, and sown on open soil the following spring.
In extreme instances the plant has become so modified through
long association with the crop that its wild progenitors are unknown,
as is the case also with some crop plants. Examples of such weeds
of uncertain ancestry infesting cultivated Flax (as indeed their
specific epithets indicate) are a Campion, Silene linicola, and a
Dodder, Cuscuta epilinum. ‘There is thus not merely a very close
association but also a tendency to parallel variation between many
crops and some of their more commonly accompanying weeds, e.g.
through their disseminules being difficult, or mechanically impossible,
to separate from those of the crop itself.
NATURALIZATION AND ACCLIMATIZATION
Although, in general, weeds tend to be hardy and to have a very
wide range of tolerance to differing environmental conditions, so
that they can spread far and rapidly, at least in ‘ disturbed ’ areas,
crops are often fastidious in their habitat requirements. Both have
accompanied Man in his migrations over the world, however, and
from time to time have given rise to ‘ escapes’ or, more rarely, have
become established as naturalized aliens. But it is one thing to
escape from cultivation or a cultivated area into adjoining terrain,
perhaps repeatedly and under the beneficial influence of Man, and
quite a different problem to become sufficiently acclimatized to hold
sway in a fully wild state in undisturbed habitats among the local
natives. ‘This latter is a relatively rare feat, as we shall see. Indeed
for the most part not only crops but also weeds are limited to areas
that are, or recently have been, in some way disturbed by Man.
It is sometimes useful when dealing with plants transported out
of their normal areas to distinguish between naturalization, in which
they grow under natural conditions that are similar to those to which
they have been accustomed, and acclimatization, in which they are
adapted to new environmental conditions differing markedly from
those of their native habitat or habitats. Although instances of at
least some degree of the former are common and widespread, there
220 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
are relatively few of the latter unless it be among weeds. For
acclimatization, except in unusually hardy and tolerant plants,
involves adaptation to different habitat conditions which is so very
gradual that the time-span required would be likely to exceed that
during which Man has been a potent factor in plant distribution.
It may be expected to involve the natural selection of suitable biotypes
or more extreme mutants, towards which naturalization is no more
than a step.
That such naturalization, at least, is going on widely in the world
today, is a further indication, if any were needed, that the various
regions of the globe do not support by any means all of the species
which could thrive there—at least in the absence of competition.
However, special studies indicate that, quite apart from the effects
of competition, plants transferred to regions of seemingly comparable
habitat may have serious obstacles to overcome before they can be
considered fully naturalized. ‘hese obstacles may be introduced by
climatic or other environmental conditions which, although they
appeared similar, are actually significantly different from those of
the plants’ original habitats (as in minor variations of soil com-
position), or are not commonly recognized as important (as in the
case of some light and temperature effects). ‘This frequent difficulty
of naturalization is one reason for the rather small percentage of
alien species that actually enter into the composition of most wild
floras in undisturbed tracts. Even of the numbers that may be able
to propagate successfully and remain year after year in one spot,
or sometimes increase their area by aggressive extension, few are
known definitely to be permanent and able to persist in the absence
of Man. Most lengthening of the floristic lists by aliens is probably
only temporary.
This brings us to the other main reason, namely the need for Man’s
continued protection, behind the rarity of fully naturalized alien
plants even relatively to the number of aspirants. A plant which
has escaped from cultivation or a cultivated area on to some adjoining
rubbish dump or otherwise disturbed tract, even if it manages to
perpetuate itself there for years as many do, is far from attaining
the status of full naturalization. In between mere escape and
complete naturalization lie the various stages of success—including
capabilities of spread, colonization, and possibly even the attainment
of a dominant position. This last is often accomplished locally or
sometimes extensively by plants which are effectively dispersed and
rank in growth, as in the case of Fireweed (Epilobium angustifolium
8] MODIFICATION AND DISTRIBUTIONS OF CROPS 221
agg.). But that is merely on territory where the native vegetation
has been disturbed or destroyed: if this native vegetation is left
alone to develop naturally, the aliens, however rank and aggressive
they may once have been, will in most instances disappear within a
very few years. Likewise have the plants which used to be trans-
ported in ships’ ballast largely disappeared, since the discontinuation
of the dumping of such ballast, from the stations in which they
formerly grew as aliens. ‘The discontinuation of a road or railway-
line is apt to have a similar effect, and even those aliens which have
managed to spread from the immediate vicinity of the travelled track
usually disappear when Man’s influence is removed and the sur-
rounding vegetation comes back into its own.
All this does not mean that Man’s influence in changing the dis-
tribution of plants is other than enormous, but rather that it is in
many instances merely temporary, as the plants involved have
become only incompletely naturalized and certainly not lastingly
acclimatized. It should also be remembered that, with Man’s
increasing mobility, for every disappearance of a plant from an area
there is probably on the average at least one new introduction else-
where, though in this connection particular plants tend to have their
ups and downs. Even in those parts of the world, such as New
Zealand and Hawaii, where the native plants have been largely ousted
over considerable tracts by adventive aliens, this has happened only
following disturbance of the native plant communities as well as
importation of the aliens by Man or his domestic animals. ‘There
it is widely contended that removal of Man’s influence would lead
to a reversal of the situation through return of the natives whose
stronger competition would ultimately oust the alien colonists.
This matter of strength of competition is one of the most important
in the life of organisms, and is often the key to the present-day
distribution of plants as well as to their potential ranges. Crop
plants, sheltered and pampered as they are (and have usually long
been accustomed to being), are notoriously weak in competition,
and consequently rarely to be found in a truly naturalized state.
The adaptations of plants to particular habitat conditions are
varied and sometimes so precise as to remain unnoticed, yet sufficient
to prevent the leading of an independent life. Often a mere slight
change in environmental conditions will threaten the very existence of
aspecies. For example, a Mexican species of Birthwort (Aristolochia)
when transplanted to Java flowered abundantly but failed to bear
fruit—not because of any lack of pollinators but because the climate
222, INDRODMCLLON ZOeP WANT GE OIGIVAE Heys [ CHAP.
there was too humid for its normal biological development, the
pistillate stage of each flower being over by the time it opened. As
has been pointed out by Wulff in the work cited at the end of this
chapter, if to such precise needs ‘we add the unceasing struggle
for existence and the competition with the indigenous vegetation,
we should not be surprised at the relatively small number of those
species introduced by man for cultivation or accompanying him in
his migrations that became fully naturalized components of the local
flora’. As an outstanding example it may be mentioned that, of
the nearly one thousand alien species in the flora of Madagascar,
it is claimed that only one, which is particularly easily dispersed,
has gained a foothold in plant communities undisturbed by Man.
Only such territory may be considered as ecologically fully occupied.
For cultivation, land has in general to be cleared of native vegeta-
tion and otherwise specially prepared. Such ousting of the native
flora gives the adventives a chance to spread; so does less complete
destruction of the vegetation, for example by domesticated animals.
But once the cultivation or other disturbance is discontinued, there
ensues a struggle between the alien and native plants which usually
ends in victory for the latter and return to approximately the original
condition. ‘This is particularly noticeable and rapidly effected in
the more favourable forested regions, whereas in some others, such
as the drier grasslands, the breaking of the sod or other disturbance
may so upset the ecological balance as to make its return extremely
slow or even problematical. Another interesting example of this
appears to be afforded in some of the most favourable situations in
southwestern Greenland. Here the clumps of Willows in many
valleys are nowadays separated by grassy tracts (cf. Fig. 116) much
as they presumably were at the time of the extinction of the Viking
colonies and their pasturing Sheep several centuries ago: at least,
the present writer has been unable, during hundreds of miles of
wandering in those now uninhabited regions, to think of any other
explanation of a remarkable phenomenon. Nor have the tree
Birches returned at all widely, either in those parts of Greenland
or in Iceland, since the ‘ forests ’ were decimated in the early centuries
of the present millennium.
The majority of really widespread weeds, such as those which
qualify as semi-cosmopolites, tend to be collective species (such as
the Common Dandelion, Taraxacum officinale s.1.) or to consist of
numerous races (as in the Couch-grass, Agropyron repens) adapted
to diverse habitat conditions. ‘The distinction between these two
8] MODIFICATION AND DISTRIBUTIONS OF CROPS 223
categories is largely a matter of degree and hence of opinion, the
important feature from our point of view being that such ‘ poly-
morphs’ are able to occupy a wide range of situations and hence,
often, of regions. ‘This is, however, chiefly where Man has dis-
turbed the native vegetation. At the other extreme we have the
highly specialized ‘ monomorphs’, such as most cultivated strains,
which for successful growth have to be given conditions within a
very narrow range of amplitude. In such circumstances they may
grow well enough year after year and seemingly indefinitely. But
once Man’s influence is removed and the coarser local indigenes
are allowed to return to the area which is their normal heritage,
such pampered cultivates will disappear with surprising rapidity and
even aggressive weeds will usually fail within a very few years. ‘Thus
in the famous Broadbalk Wilderness of Rothamsted Experimental
Station in southern England, according to Sir William Ogg (in itt.
et incl.), ‘'The Wheat plants on the strip . . . which was allowed
to run wild survived for only four years’, being by then reduced to
‘a few stunted plants . . . barely recognizable as cultivated Wheat ’.
Subsequently ‘ a dense growth of bushes and young trees ’ developed,
which soon ceased to include even the hardier wheat-field weeds.
SoME HERBACEOUS CROPS AND THEIR AREAS
Most of the important plant products on which Man’s sustenance
depends come from field or other herbaceous crops of short duration.
The plants involved are usually special domesticated strains that
have been so highly selected and long cultivated that they are unable
to compete with natural vegetation—perhaps anywhere in the world
—but, with Man’s aid, they fortunately flourish sufficiently to enable
him to maintain his position of supremacy. Human civilizations
have largely developed in relation to the availability of suitable crops,
in particular cereals, and there is altogether widespread inter-
dependence between crops and Man. Communities living outside
the cereal belts are often backward to this day.
Whereas each of the various crops commonly had a single region
of origin—as indicated, for example, in the works of DeCandolle
and Vavilov cited at the end of this chapter—the main ones have
usually become important through having their areas spread by Man
into other regions. It has even been said that ‘no world crop
originated in the area of its modern commercial importance’. Not
only are these regions, the present-day areas of particular crop
224 INTRODUCTION TO PLANT GEOGRAPHY
plants, often virtually as extensive as climatic possibilities allow, but
by special breeding and cultivation techniques Man is always
endeavouring to extend the potential areas into new regions. ‘This
is notably true in the case of Wheat, the northern limit of which
has been pushed farther and farther towards the Arctic in recent
decades. It is chiefly with the currently attained areas of the most
important herbaceous crops (as opposed to ‘ woody’ ones, treated
afterwards) that the present section will be concerned. And whereas
the major vegetational belts and hence natural plant distributions
may to some extent have conditioned human migration in the past,
it is largely the crop-growing potentialities of different regions that
determine the density of human population today. ‘This we shall
see in the next chapter, though with the modern ease and efficiency
of transport, particularly, this general conclusion tends to become
less and less applicable in some areas of intense industrial or mining
productivity.
We will now indicate briefly on a world-wide basis the significance
and chief areas of cultivation of some of the more important and
familiar herbaceous crops ; ornamental ‘ flowers’ are apt to be even
more widespread owing to the special care, often including develop-
ment under greenhouse or other highly artificial conditions, that is
lavished upon them. ‘The chosen examples will be treated under
eight main headings.
(1) Grains—The principal grains occupy about one-half of the
world’s croplands and of them Rice (Oryza sativa) is probably the
most generally important, being ‘ an indispensable food of over half
the population of the world’. It replaces the other cereals as the
staff of life in many tropical and subtropical countries, and in several
of the most densely populated of these its cultivation is the chief
agricultural industry. Although g5 per cent. of the Rice cultivation
of the world is in the Orient, where the crop presumably had its
origin far back in antiquity, Rice is now cultivated practically
wherever in the tropics its usual needs of abundant moisture can
be economically satisfied, its distribution affording a good example
of that of a warm-climate crop (see Fig. 66). For the many types
of lowland Rice, which have to be flooded during part of their
development, are the ones grown almost exclusively ; relatively
unimportant is ‘upland’ or ‘hill’ Rice, which can be cultivated
in drier situations much like those favoured by other cereals.
Wheats (7 riticum vulgare and other species) constitute the chief
cereals of temperate regions and the ones most important to the
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white race nowadays. ‘Their areas of origin are doubtful but were
evidently diverse as regards the different forms, some of which
appear to have been cultivated for at least 6,000 years. Nevertheless
the predominance of Wheats is relatively modern, other cereals, or
mixtures comprising maslin, having been previously more widely
used for bread. Wheats were probably developed by selection from
weedy types and hybridization with other Grasses, but are still being
improved today. ‘They are grown under a wide variety of climatic
conditions, including some tropical ones (in winter), and, like
polymorphic weeds, are the more widespread because of their
diversity. Nevertheless the general distribution of Wheats is mainly
temperate, as indicated in Fig. 67.
Less widespread and important are Barley (Hordeum vulgare s.1.),
Oats (Avena sativa and other species), and Rye (Secale cereale),
though the first of these is probably the oldest of our major cereals
and possibly of all currently cultivated plants. It was widespread
already in Neolithic times and was used for bread even before Wheat.
Oats probably had a long history as a weed in fields of primitive
Wheat before becoming a crop in its own right, while Rye, which
was unknown before the Iron Age but is now the world’s second
most important bread crop, apparently originated as a grain-field
weed in Asia Minor. It can be grown on poorer soils than other
cereals. Owing to its greater winter hardiness and ability also to
mature grain under less generally favourable conditions than the
other cereals mentioned, Rye tends to be cultivated chiefly in
mountainous regions and about the northern limit of the Wheat
belt, being important chiefly in the cool-temperate parts of the
northern hemisphere—cf. Fig. 68. However, Barley is able to
mature in a shorter summer than the other cereals, and so is the
only one which the writer has seen being grown successfully for
grain north of the 7oth parallel of latitude in Norway.
Maize (Zea mays) is the largest of the cereals. According to that
foremost student of its history, Professor Paul C. Mangelsdorf of
Harvard University (7 litt. 1957), “ No wild ancestor is known with
certainty, but fossil pollen believed to be that of wild Maize has
been found at a depth of more than seventy meters below the present
site of Mexico City. Other evidence points to cultivated forms of
Maize originating on the eastern slopes of the Andes in South
America. Maize cultivation goes far back in prehistoric times.
Grains found in burial sites in Peru already represent several different
varieties, indicating that the plant had been grown for many centuries
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MODIFICATION AND DISTRIBUTIONS OF CROPS 229
before the period of the Inca civilization. Radiocarbon determina-
tions of primitive cobs found in Bat Cave in New Mexico, indicate
that this material was between 5,000 and 5,600 years old.’ In spite
of its numerous forms, Maize is a crop mainly of rather exacting
requirements of considerable summer moisture in warm countries.
It does not ripen far north; indeed not many regions have the
right combination of environmental conditions for the raising of
Maize on a large scale. Most notable is the eastern half of the
United States, which produces about half the world’s crop, although
there are other considerable centres of production in South America,
southern Europe, and eastern Asia, as indicated in Fig. 69. Maize
is used principally for feeding Hogs and other domesticated animals,
but is also favoured as a vegetable.
The various types of Millets, belonging to several different genera
of Grasses, and the Sorghums, belonging to the genus Sorghum, should
also be mentioned as widely grown for forage, grain, and many
other purposes. ‘The Sorghums have been cultivated in Asia and
Africa since very early times, constituting a staple food for millions
of native peoples. Latterly they have come to be grown in other
tropical and warm-temperate regions, being particularly useful
because of their ability to grow under dry conditions and actually
withstand droughts.
(2) ‘ Root’ Crops—Of these the Irish or White Potato (Solanum
tuberosum) is the most widely important, having no rival as an
efficient producer of food, especially in relatively moist and cool
countries. Although the Potato’s origin lay in the mountainous
portions of South America, over go per cent. of world production
is now in Europe, whose population has increased substantially as
a result of its cultivation. In comparison with the other leading
food-crops, the average annual world production during 1934-38,'
expressed in millions of metric tons, has been estimated as approxi-
mately 233 for Potatoes, 167 for Wheat, 152 for Rice, 115 for Maize,
65 for Oats, 52 for Barley, and 47 for Rye. Actually these figures,
although interesting, are only fragmentary for some crops, and are
moreover misleading in that Potatoes contain at least 78 per cent.
of water, against an average of only about 13 per cent. for cereals.
Consequently, the actual dry-weight food production of Potatoes in
that period was only about 51 million tons, whereas for Wheat it
was approximately 145 million tons, and for Rice and Maize also
1 The last period for which the F.A.O. Yearbook (vol. IX, part 1, 1955) gives
pertinent statistics for the U.S.S.R.
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MODIFICATION AND DISTRIBUTIONS OF CROPS 231
considerably in excess of that of the Potato. Yet under suitable
conditions of moist climate and rich but light soil, the Potato is
able to supply considerably more human food per unit area than
any of the cereals, the world crop during 1934-38 being produced
by about 22 million hectares as opposed to an estimated 168 million
hectares under Wheat. Relatively to the following two so-called
‘root’ crops, the Potato is hardy, especially in some of its numerous
forms, Fig. 70, A, indicating most of its cultivated range in the world.
In addition the present writer has eaten quite large home-grown
Potatoes in central Alaska and far down the Mackenzie Valley, and
small ones in southern Greenland and in northernmost Scandinavia
near 71° N. lat. He has even eaten tiny ones much farther north
in Spitsbergen, grown on the pyre of a burned-out hut. In the
White Potato the storage tuber is really an underground stem
structure bearing buds (the so-called ‘ eyes’).
The Sweet Potato ([pomoea batatas), an ancient crop of tropical
America, is now widely cultivated in the warm parts of the world,
being in fact a standard article of food in practically all tropical
and subtropical regions. It requires a sandy soil and a moist climate
for successful growth. Another very important tropical food plant
of this nature (though actually shrubby) is the Cassava (Manihot
esculenta), which originated in South America in prehistoric times.
It can be cultivated in hot, seasonally arid climates where cereals,
etc., will not grow. Its many varieties now furnish the basic food
for millions of people, particularly in Central and South America,
and also supply the world with tapioca.
Other important ‘ root ’ crops which are widely used as vegetables
include various types of Yams (Dioscorea spp.) in warm regions, and
Turnips and Rutabagas (Swedes) which are also used for animal
feed particularly in temperate regions. ‘There are also Beets (Beta
vulgaris) and Carrots (Daucus carota), which both succeed under a
wide range of climatic and soil conditions. Beets were domesticated
first as a leaf vegetable, then as root crops, and finally as a source
of sugar (cf. upper part of Fig. 75); they are probably derived
from one variable species that is native in the Mediterranean region.
Carrots are likewise of ancient origin, various form, and now very
widespread cultivation.
(3) Other Vegetables—This somewhat vague category includes
some structures (such as Tomatoes and the pods of Beans) which
technically are fruits. Examples are the Broad Bean (Vicia faba),
which is one of the world’s commonest and most important beans
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MODIFICATION AND DISTRIBUTIONS OF CROPS 2228
Fic. 70, B.—Bed of young Kale, with, behind, tall Rhubarb, in Lichtenau Fiord,
southwestern Greenland. ‘The native family are standing on a raised path in
front of the old mission house from which the garden slopes downwards.
and was the only edible one known in Europe before the time of
Columbus ; the Common or Garden or Kidney Bean (Phaseolus
vulgaris), which has long been cultivated in the New World where
it probably originated ; and the Soybean (Glycine max), which is
of great antiquity in the Orient, where over one thousand varieties
are grown. Soybean, particularly, has a very wide range of uses,
the seed, containing about 20 per cent. of oil and 30-45 per cent.
of protein, being the richest natural vegetable-food known. ‘The
climatic and soil requirements ideally are much like those for Maize,
and the crop is becoming more and more extensively cultivated in
temperate regions—including the United States, where it is grown
chiefly as a source of oil and stock feed.
Other important legumes are the Common Pea (Pisum sativum),
which is now very extensively cultivated, and the Chick Pea (Cicer
arietinum), which is an important food plant—particularly in India
and other parts of Asia, in Africa, and in Central America. Both
these plants appear to be natives of southern Europe or adjacent
regions, where they have been grown from early days and are still
extensively cultivated, and neither is known in the wild state. ‘The
234 INTRODUCTION LO PVAN TD iGE OG RAP TY [CHAP.
Common Pea needs plentiful moisture but thrives in cool regions,
whereas the Chick Pea is well adapted to dry conditions. Another
widespread and important crop plant of this general affinity is the
Lentil (Lens esculenta), which has been cultivated since Neolithic
times and is thought to have originated in southwestern Asia.
Mustards are extensively cultivated for their oil and use as greens
in Asia, as are the related Cabbages and Kales and their allies
(Brassica oleracea) in Europe and elsewhere for human and domestic
animal consumption (though originally for their oily seeds). ‘The
Cabbages, etc., were evidently developed far back in antiquity from
a variable Mediterranean species exhibiting numerous local races ;
now they are grown practically around the world, occupying a wide
variety of soils and climates ranging from the low-Arctic to the sub-
tropics. Fig. 70, B, shows a fine bed of young Kale growing in
southern Greenland, with, behind, tall Rhubarb.
Finally we should mention a few ‘ fruit vegetables’ such as the
Squashes and Cucumbers and their allies, many of which have been
extensively cultivated from early times, and the widely important
Tomato (Lycopersicum esculentum), which springs from a group of
small-berried weedy natives of Peru. Most of these types do best
in warm and moist regions, to which some are practically confined.
(4) Forage Plants—While several of the above-mentioned crops
may be used in part for forage, there are some more specific forage
plants to be mentioned primarily in this connection. Foremost
among these are various Grasses, of which the Bluegrass or Meadow-
grass (Poa pratensis s.l.) is an outstanding example. Cytotaxonomi-
cally it is one of the most complex mixtures of polyploid hybrids-cum-
apomicts known, appearing in numerous forms whose origin 1s often
obscure. Geographically it is widespread, particularly in the cooler
regions of the world. Ecologically it is unexacting and aggressive,
frequently forming a major constituent of pastures whether or not
it has been sown. Another important and widely cultivated forage
plant is Alfalfa or Lucerne (Medicago sativa), which was probably
the earliest forage crop to be developed—apparently in southwestern
Asia. Alfalfa prefers a deep, well-drained soil but is grown under
a wide range of moisture as well as temperature conditions. It
belongs to the Pea family as do also the Clovers and Vetches, which
are themselves of considerable significance in pasturage and hay.
(5) fibre and Oil Plants—In this extensive category the Cottons
(various species of Gossypium), Flax (Linum usitatissinum), Hemp
(Cannabis sativa), Jute (species of Corchorus), and Peanut or Ground-
8] MODIFICATION AND DISTRIBUTIONS OF CROPS 235
nut (Arachis hypogaea) are outstanding in importance and widely
cultivated. ‘The first three are used chiefly for fibre and oil, Jute
for fibre, and Peanuts for oil and food. All are plants primarily
of warm regions, Flax alone being successful far to the north (ef.
Fig. 71). Cotton as a whole is often claimed as the world’s greatest
industrial crop and chief source of fibre; its multiple origin is
shrouded in the mists of time. ‘The world distribution of Cotton
production is indicated in Fig. 72. ‘The others, too, are of ancient
and often uncertain origin—mostly in the Old World, but the Peanut
very likely in South America, though it is now extremely widespread
(see Fig. 73).
Other important vegetable fibres are Ramie or China-grass
(Boehmeria nivea, widely cultivated in Asia), Sunn-hemp (the Asiatic
Crotalaria juncea), the chiefly Philippine Abaca or Manila-hemp
(Musa spp.), Sisal and other Agave types, cultivated in Africa and
North and Central America, and filling fibres such as Kapok (the
floss from the seeds of the now widespread tropical tree Ceiba
pentandra). Of oils there are the essential or volatile types used
particularly in perfumery, and the fatty or fixed types which include
the drying (e.g. Tung, from species of Aleurites, native to China),
semi-drying (e.g. the Asian Sesame), and non-drying (e.g. Olive)
categories. . Olives are cultivated chiefly in the Mediterranean lands
‘but to some extent also in the United States, South Africa, and
Australia. In addition there are the vegetable fats such as palm oil
(obtained from the African Oil Palm, Elaeis guineensis) and coconut
oil (obtained from the Coconut—see pp. 242, 266). Of the sources
mentioned above, cottonseed oil, obtained from Cotton, is the most
important semi-drying oil, linseed oil, obtained from Flax, is an
important drying oil, and hempseed oil, obtained from Hemp, is
another, while peanut oil is a non-drying oil.
(6) Fruits—Whereas, technically, many of the above-mentioned
products are fruits or derived from fruits, the term is used here in
the popular sense. Most of our main fruits are borne by trees or
shrubs and will be dealt with in the next major section. ‘lhe
Pineapple (Ananas comosus) and Melon (Cucumis melo), and, for all its
appearance, the Banana (Musa paradisiaca s.1.), are, however, strictly
herbaceous, as are Strawberries and some other favourites. The
cultivated Strawberry (Fragaria grandiflora) is, according to Pro-
fessor Edgar Anderson, ‘ the one crop of world importance to have
originated in modern times ’—actually in the eighteenth century as
a true-breeding polyploid hybrid from artificial crosses between wild
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MODIFICATION AND DISTRIBUTIONS OF CROPS 239
North and South American types. Other Strawberries have long
been grown elsewhere, the fruit as a whole being a favourite in all
temperate countries. In contrast, Pineapples, Bananas, and Melons
are mainly tropical types of ancient origin. ‘The Malay Peninsula
appears to have been the chief centre of origin of cultivated Bananas,
whereas South America gave us the Pineapple, and Africa or southern
Asia various Melons ; but all three types are now grown practically
around the globe.
(7) Other Crops—Especially important among these are Tobacco
(Nicotiana tabacum) and Sugar Cane (Saccharum officinarum), while
the Hop (Humulus lupulus) and various Buckwheats (Fagopyrum
spp.) are of no mean significance in some areas. "Tobacco is
apparently a true-breeding polyploid hybrid between two weedy
inhabitants of South America, where it probably arose in cultivation
in early pre-Columbian times. Now it is grown extensively in
various of the sufficiently summer-warm regions around the globe,
as indicated in Fig. 74, and is an important commodity throughout
the inhabited world. Sugar Cane, a vigorous-growing perennial
Grass, is the chief source of sugar at present, although at times in
the past it has been rivalled by Sugar Beets. Sugar Cane probably
originated in southeastern Asia and comprises an assemblage of
forms that are unknown in the wild state but are now cultivated in
practically all moist tropical and subtropical regions, the main areas
of production of sugar from it and from Sugar Beets being indicated
in Fig. 75. Cane sugar probably constitutes the greatest export
crop of the tropics.
(8) Raw Materials for Industry—A large proportion of these are
afforded by plants in limitless supply. ‘This category largely cuts
across the others, which in most instances contribute familiar
examples to it, and so we need scarcely add details. Suffice it to
say that industrially important raw materials include not only the
examples already mentioned, such as various grains, roots, fibres,
oils, carbohydrates and their derivatives, but also a wide range of
forest products including rubber and pulp. ‘The field-crops involved
are grown in the main crop-producing parts of the world and the
forest products are obtained mostly in major forested regions.
Further information about the sources of these all-important raw
materials supplied by plants will be found in the books cited at the
ends of this chapter and the succeeding one which stresses further
their vital economic significance.
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242 INTRODUCDION TO PLANT GEOGRAEE Y [CHAP.
FORESTRY AND OTHER Woopy ‘ Crops’
The practices of forestry, being largely directed towards more
effective utilization of the forested regions of the world, are to a
large degree concerned with cropping. Especially when artificial
planting of trees or shrubs is involved—often with marked effect
on their natural ranges, and sometimes with the maintenance of
special strains—do the results demand some consideration here.
Woody plants as a whole greatly extend the category of fruits (No. 6)
introduced above, add one of beverages, and above all contribute
their own vast one of timbers and cognate products with which we
can deal only in brief outline.
Trees and other woody plants that are extensively cultivated for
edible fruits of importance to mankind include the Coconut (Cocos
nucifera), Breadfruit (Artocarpus altilis), Olive (Olea europaea), Date
Palm (Phoenix dactylifera), Fig (Ficus carica), Citrus fruits (Citrus
species), Grape (Vitis vinifera), Currants and Gooseberries (Ribes
species), Mango (Mangifera indica), Papaya (Carica papaya), Plum
(Prunus domestica and other species), Peach (Prunus persica), Apple
(Pyrus malus), and Pear (Pyrus communis). ‘The first three are
scarcely fruits in the lay sense.
The Coconut is sometimes claimed to be the most important or
at all events thoroughly exploited of cultivated plants, being used
also as a source of timber, thatch, fibre, and many other things—
especially by primitive peoples, who may be almost wholly dependent
upon it and use all parts. A native of southeastern Asia, where
the wild trees are still cropped, it has been carried to practically all
tropical and subtropical shores, being very extensively planted.
Another important Palm is the Date, which is widely cultivated in
the tropics and subtropics where it can be grown with less water
than any other crop. It is one of the oldest of crops and is supposed
to have originated in southwestern Asia, though it is unknown in
the wild state. ‘The Breadfruit, a native of Malaya that is now
widespread in the tropics, having been cultivated from early times,
is another very important food fruit (in the botanical sense). A
Man is said to be able to live throughout the year on the products of
a single tree. Most of the remaining fruits mentioned are attractive
and familiar cultivates that have long been widespread in the climatic
belts which they favour, ranging from the cool-temperate Currants
and Apples to the tropical Mango and Papaya. Here we might add
the Mangosteen (Garcinia mangostana), which is regarded by some
as the world’s most delectable fruit, | |
8] MODIFICATION AND DISTRIBUTIONS OF CROPS 243
Important beverages obtained from woody plants include Cacao
(Theobroma cacao), the source of cocoa and chocolate, Coffee (various
species of Coffea), and ‘Tea (Camellia sinensis). ‘The Cacao tree is
a native of tropical America; the others originated in the warm
parts of the Old World. All are now extensively cultivated in the
tropics, and, in the case of ‘Tea, in other warm regions. Fig. 76
indicates the main centres of production of Cocoa beans and
exemplifies a tropical crop of restricted origin that is now widespread.
Fig. 77 gives similar indications for Coftee, over half of which comes
from Brazil, whose economy is still bound up with this single crop
to an economically unhealthy degree.
Passing over further categories such as nut-bearing, rubber, and
drug plants, whose important products are often obtained from wild
sources, we come to the last great one of timbers and cognate forest
products. For details of these, reference may be made to such
works as that of Zon & Sparhawk or, for the New World, of Record
& Hess, both of which are cited at the end of this chapter.
Besides the timbers employed almost all over the world for con-
struction, fuel, and other purposes, important forest products include
tanning and dyeing materials and a great assortment of useful gums,
resins, oils, preservatives, cork, and latex products—to name only
a few. Many of these are taken with fair regularity as a kind of
crop, sometimes from planted trees. And whereas in the tropics
the vast array of generally mixed timber trees are usually of rather
restricted distribution, the relatively few types occurring in temperate
and boreal regions are often widely transported and cultivated.
Good examples are found among the Conifers that are successfully
planted in Europe, which is deficient in native trees for reasons that
were discussed in Chapter VI. Such Conifers have often been
transported from North America (as in the case of the Douglas
Fir, Pseudotsuga taxifolia) or Asia (whence come especially numerous
ornamental types, though admittedly these scarcely constitute crops),
but do quite well at least as long as Man’s influence prevails. Apart
from such artificial introduction, there are very few large woody
species common to both sides of the Atlantic—in contrast to the
situation with numerous herbaceous species especially in the boreal
and arctic regions. The tree genera, however, are commonly the
same in Europe and the temperate parts of North America and eastern
Asia, though some have disappeared from Europe in recent geological
ages.
Forests occupy about one-quarter of the total land-area of the
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246 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
world, as indicated in Fig. 65, which also suggests that a roughly
similar proportion is occupied by each of the other three main types
of landscape, namely, grassland (with savanna), desert or semi-desert,
and tundra (with fell-field, etc.). It has been estimated that South
America has about 44 per cent. of its land area forested, Europe
about 31 per cent., North America about 27 per cent., Asia about
22 per cent., Africa about 11 per cent., Australia about 6 per cent.
(although New Guinea has 80 per cent.), while Antarctica has no
forests at all. In many countries the forests were formerly much
more widespread than they are today, the reduction being due
primarily to interference by Man, but in some of the more civilized
lands extensive reforestation is now being undertaken. This
planting is often of exotics introduced from distant regions of com-
parable climate, the plantations representing a kind of crop whose
range is thereby greatly extended.
The characteristics of the main types of forest will be described
below in the appropriate chapters on vegetation. Here it will suffice
to mention a few of the more important timber trees which in most
instances are widely planted and tended (and to that extent, as well
as in their regular use by Man, qualify as crops). In North America
nowadays the Yellow Pines, Douglas Fir, Hemlocks, White Pine,
Cypress, and Spruces tend to be the most important softwoods,
with Oaks, Red Gum, Maples, Birches, and Poplars leading the
hardwoods, of which the area occupied and the annual ‘ cut’ are
much smaller than in the case of softwoods.
In Europe, 74 per cent. of the forests are classed as coniferous,
and such forests, as in America and Asia, are particularly char-
acteristic of the northern portions. ‘lhe principal European Conifers,
which are frequently grown in special plantations, are the Scots
Pine (Pinus sylvestris), Norway Spruce (Picea abies), and Larch
(Larix decidua), though the American Douglas Fir and certain other
Pines are extensively planted. ‘The most important European hard-
woods tend to be certain Oaks, but Beech (Fagus sylvatica), Ash
(Fraxinus excelsior), and some Birches and Elms are also prominent.
The genera are thus much the same as in North America although
the native species are different, and this situation continues over
much of northern Asia. Here, in the west, European species are
found, but these tend to give way to Asiatic species of the same
genera farther east. Conifers comprise an estimated 42 per cent.
of the forest area of Asia, and temperate hardwoods 27 per cent.,
the remainder being made up of tropical hardwoods which in many
8] MODIFICATION AND DISTRIBUTIONS OF CROPS 247
countries south of the Himalayas comprise nearly 100 per cent. of
the trees. ‘The species of tropical hardwoods in an area are often
very numerous, India, for example, being estimated to have fully
2,000. ‘The dominance tends to be intricately mixed, although ‘Teak
(Tectona grandis) and various members of the Dipterocarp and Pea
families are often prominent. Important commercially, if not
always ecologically, are Ebony (various plants including Diospyros
ebenum), Satinwood (Chloroxylon swietenia), and Burmese Rosewood
(Pterocarpus indicus).
South America, as we have seen, bears a greater proportion of
forested area than remains on any other continent. Nearly go per
cent. of its forest is tropical hardwood—mainly dense rain forest
which characterizes the great river basins and tends to be very
luxuriant and intricately mixed (there are said to be over 2,500
different tree species in the Amazonian forests alone). In some
drier areas an open deciduous type of tropical forest occurs, and on
the high mountains are mixed forests of Conifers and temperate
hardwoods. Important woods of tropical America include Balsa
(Ochroma lagopus s.1., the lightest of commercial timbers), Spanish-
cedar (Cedrela odorata s.l., forms of which are native in some areas
but introduced in others), Greenheart (Ocotea rodioe:), Lignum-
vitae (species of Guaiacum), Locust (Hymenaea courbaril), and
Mahogany (chiefly Swietenia macrophylla and S. mahagont, of which
the latter has been widely introduced).
In Africa, contrary to popular conception, forests cover only about
11 per cent. of the land area. ‘Tropical hardwoods predominate,
comprising some 97 per cent. of the forests. Here again there are
two main types, of which the dense and much-mixed rain forest is
the more extensive but is replaced by an open, park-like type where
the rainfall amounts to only 30-40 inches per annum. Of the woods
that have so far been exploited, an outstanding example is the
African Mahogany (Khaya senegalensis), which is widely exported.
Although in Australia forests cover only a very small proportion
of the land area, in New Zealand the percentage is about 26 and in
Oceania 71. In Australia tropical hardwoods predominate—par-
ticularly species of Eucalyptus and Acacia—and in Oceania they make
up the entire forest. On the other hand in New Zealand 68 per
cent. of the forests are coniferous and the remainder temperate
hardwoods, though the genera tend to be different from those pre-
dominating on other continents,
248 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
SIGNIFICANCE AND DISTRIBUTION OF WEEDS AND PLANT DISEASES
Although there are many tens of thousands of species of higher
plants in the world, only a few dozens of these are really troublesome
weeds which are able to reproduce and thrive in the presence of
cultivation and other interfering human activities. More numerous
by far are the weedy plants and ‘ escapes’, both herbaceous and
woody, which for the present purpose may be considered with the
more noxious weeds, and which link the latter with the categories
of cultivated plants. Yet the annual loss due to weeds is enormous,
often amounting to millions of dollars in a relatively small area.
Weeds are injurious to agriculture, e.g. by robbing crops of
needed water and nutrients, by crowding them out through root-
competition or overgrowing, by choking and pulling them down in
the case of (sometimes parasitic) climbers, by having seeds or fruits
so similar to those of the crop that they are difficult to separate and
so adulterate it and reduce its value, by harbouring undesirable
insects or plant diseases, by being poisonous or injurious to stock,
by tainting milk, and so on. ‘The nuisances caused by Bindweeds
and Couch-grass are all too familiar to almost every gardener as
well as farmer in the temperate belt ; the Prickly-pears (species of
Opuntia) which were introduced into South Africa and Australia as
a stock feed now usurp the ground; Barberries and Currants
harbour (as alternative ‘ hosts’) the devastating Wheat Stem-rust
and White Pine Blister-rust, respectively (see pp. 251-2).
Regardless of the common ‘ effects of cultivation’ listed near the
beginning of this chapter, annual weeds often produce numerous
seeds whose germination may be distributed over many years—for
example, after being buried for decades.1 Moreover, the seeds or
fruits of many weeds, such as Thistles and Dandelions, are provided
with efficient means of dispersal. Otherwise their wide distribution
seems to be largely due to Man’s transportation activities—such as,
for example, the shipment of commercial seeds and grain. ‘These
are often of much the same size and shape as the disseminules of
‘'The longevity of seeds and fruits comprises an interesting study for which
more and more authentic data are needed. Discounting claims of longer periods
of burying in marshes etc. which are not fully authenticated, and stories of
“mummy Wheats’ which are clearly bogus, the record seems to be held by a
“seed’ reputedly of Nelumbo nucifera (syn. Nelumbium speciosum, the Sacred or
Indian Lotus) which was germinated in the British Museum (Natural History),
South Kensington, London, during the bombing of 1940, apparently about 250
years after it had been collected.
8] MODIFICATION AND DISTRIBUTIONS OF CROPS 249
the weeds which adulterate them, and which are apt all too easily
to be shipped and sown with them. It is apparently largely in this
manner—though further means were listed in Chapter IV—that,
among others, the following pernicious European weeds have become
dispersed to temperate America, South Africa, New Zealand, and
temperate Australia: Couch-grass (Agropyron repens), Crab-grasses
(species of Digitaria), Russian-thistle (Salsola kali var. tenutfolia),
Bindweeds (species of Convolvulus), Sheep Sorrel (Rumex acetosella
agg.), Dodders (species of Cuscuta), Plantains (species of Plantago),
Wild Carrot (Daucus carota), Prickly Lettuce (Lactuca scariola), and
Sow-thistles (species of Sonchus). Europe is not, however, by any
means the only source of weeds: although it has contributed some
500 to North America alone—includ:ng all present-day Americn
representatives of Lamium, Melilotus, Medicago, Malva, and some
other familiar genera—many weeds have been dispersed in the
opposite direction. Some have even gone much farther afield than
across the North Atlantic—e.g. Canadian Fleabane (Erigeron
canadensis) and Gallant Soldier (Galinsoga parviflora, a native of
South America).
These and others among the most widespread of weeds are the
‘semi-cosmopolites ’ mentioned in the last chapter, and it is notice-
able that some of them lack special means of dispersal—commerce
having gradually distributed them to all the major regions where
they can thrive. ‘This is notably the case with such other types as
Shepherd’s-purse, Common Chickweed, Annual Meadow-grass, and
Lamb’s-quarters (Chenopodium album s.|.), which are among the
most widespread of all flowering plants because, primarily, of Man’s
unwitting transport and, secondarily, of their variability which
enables them to grow in a wide range of different habitats. How-
ever, as we have already seen, this is usually manifest only so long
as Man continues to interfere by clearing areas or at least keeping
the native vegetation at bay; competition tends to be too much
for weeds which lack Man’s help. ‘This help often extends to
breaking up underground parts from which vigorous regeneration
can take place—for example in the case of the rhizomes of Couch-
grass and Bracken, and the roots of Dandelions. Such weeds are
particularly difficult to eradicate.
Although the semi-cosmopolitan weeds tend to have such a wide
tolerance to environmental conditions that they are capable of
invading almost any agricultural or otherwise disturbed area, at least
within their normal climatic limits, other weeds are far more exacting
a
250 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
and restricted. Consequently some regions have a characteristic
weed-flora of their own, to which many of the component species
are largely limited. "Thus whereas many weeds have nowadays the
type of distribution exhibited by some crops, that is, practically
world-wide within climatically limited bounds, others are restricted
by special conditions to particular areas for which they are suitable.
In the former case peripheral portions of the distribution are often
very temporary besides being artificial, Man having extended the
area far beyond its natural bounds. Moreover, as we might expect
from examination of the ranges of wild plants, weeds in areas outside
those of their climatic optimum tend to be restricted to particular
habitats where such factors as soil type or microclimate compensate
for unfavourable climatic conditions. Here even vigorous weeds
may be only sporadic in appearance, easy to control, and liable to
disappear quickly in the absence of human interference. Neverthe-
less, many of the weeds which follow particular crops are almost as
widespread as the crops they accompany.
Modern plant pathology, the scientific study of plant diseases, is
a large and important subject in its own right—as may be gathered
for example by perusal of such works as those of Butler & Jones,
Walker, and Boyce cited at the end of this chapter. Only a few
selected distributional and allied items can be touched upon here,
for almost any abnormal state of plants, such as may depress the
yield of a crop, is liable to be considered as a disease.
Plant diseases may conveniently be classified into three primary
groups: (1) non-parasitic, incited primarily by such physical or
chemical factors as low or high temperatures, unfavourable oxygen
or soil-moisture relations, atmospheric impurities, lightning, and
mineral and other excesses or deficiencies ; (2) parasitic, incited by
Bacteria, various groups of Fungi and their allies, Angiosperms,
and animals such as insects and Nematode worms ; and (3) virus
diseases. Examples of most types can be lethal—sometimes to all
the plants belonging to a particular species throughout a tract of
country. ‘They are therefore of great importance to plant distribu-
tion ; for they are apt in some cases to attack any part of the area
of a particular species or even wider taxon, and consequently to be
of the utmost significance to mankind whose crops they so frequently
affect adversely or ruin or even basically destroy.
Whereas very numerous and often serious diseases are known of
wild plants throughout the world, it is chiefly among cultivated ones
that the worst ravages are caused or at all events noted. Here
8] MODIFICATION AND DISTRIBUTIONS OF CROPS 251
epidemics are of frequent occurrence and sometimes vast propor-
tions. For by growing crops in close plantations, Man offers to
parasites a great opportunity for rapid growth and reproduction,
while non-parasitic diseases which affect a particular crop plant may
be strikingly evident owing to the absence of other species which
might mask the effect.
Although Bacteria and viruses cause many serious plant diseases,
by far the most important group in this connection are the Fungi.
They may range from very local to very widespread in distribution,
sometimes covering virtually the whole area occupied by the plant
attacked (commonly called the ‘ host’). Indeed it seems quite likely
that particular diseases have been responsible for the complete
extermination of some plants in the past, even as they can nowadays
cause the disappearance of particular plants from considerable areas.
This can obviously be of vast significance in plant geography. As
an example we may cite the notorious Chestnut Blight which in recent
decades has almost exterminated Sweet Chestnut trees from the
United States, where formerly they were of major importance both
ecologically and economically. Another example of an important
plant disease is Late-blight of Potato, which led to the great Irish
famine of the eighteen-forties that resulted in the deaths of hundreds
of thousands of people and started the wholesale Irish peasant
migration to the United States. Yet another is Wheat Stem-rust
that has caused an estimated loss in western Canada alone of as
much as $200,000,000 in a single year. In the tropics, another
Rust Fungus caused the disappearance from Ceylon of the Coffee
industry which had long been the mainstay of its prosperity, and
further instances could be cited of such epidemic plant diseases,
often introduced from afar, profoundly influencing the economic
development of a country, or causing enormous loss or acute distress
over considerable areas.
The examples mentioned are all of airborne pathogens, dispersed,
in part at least, as minute spores which are blown by the wind often
for considerable distances. Accordingly such diseases as the cereal
Rusts and similarly airborne Smuts are present in all cereal-producing
countries. The chief means of combating them is by the breeding
and cultivation of resistant or immune strains, or, in the case of the
more intensive crops, by poisonous sprays, etc., which are lethal
to the infecting organism. In other instances suitable treatment of
contaminated soil or seed, or eradication of infected plants, will
destroy-the parasite. Or again, the imposition of strict quarantine
252 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
barriers can be effective, while with some virus and other diseases
which are spread by insects, the killing of these vectors, or growth
of the crop in areas where they do not occur, should suffice for
successful control. Some plant pathogens have an alternative host
in which part of the life-cycle is spent. In such cases eradication
of this alternative host is effective, an example being afforded by
the White Pine Blister-rust, which threatens the life of all five-
needled Pines in North America, and whose alternative host is the
genus Ribes (Currants and Gooseberries). It should be noted,
however, that such systematic eradication may not merely affect the
distribution of the species concerned but also the local ecological
balance, which may be seriously upset. Similarly, DD'T spraying
against harmful insects may at the same time kill off all the bees
which are necessary for pollination!
FURTHER CONSIDERATION
E. V. Wuirr. An Introduction to Historical Plant Geography (Chronica
Botanica, Waltham, Mass., pp. xv + 223, 1943); for further details
about many of the topics discussed in the early part of this chapter.
Origins of Crops :
ALPHONSE DECANDOLLE. Origin of Cultivated Plants (Kegan Paul &
Trench, London, pp. ix + 468, 1884); still useful.
N. I. Vavitov. The Origin, Variation, Immunity and Breeding of
Cultivated Plants, translated by K. S. Chester (Chronica Botanica,
Waltham, Mass., vol. 13, Nos. 1-6, pp. xviii + 364, 1951); see also
below.
EpGaR ANDERSON. Plants, Man and Life (Little, Brown, Boston, Mass.,
pp. [vii] + 245, 1952); stimulating.
E. D. Merritt. The Botany of Cook’s Voyages (Chronica Botanica,
Waltham, Mass., vol. 14, Nos. 5-6, pp. i-iv + 161-384, 1954); for
additional information, with a master’s pungent criticisms of some
previous contentions.
For Details about Herbaceous Crops :
E. E. STaNrorp. Economic Plants (Appleton-Century-Crofts, New York,
pp. xxll1-++ 571, 1934).
K. H. W. Kiaces. Ecological Crop Geography (Macmillan, New York,
pp. xviii + 615, 1942).
A. F. Hitt. Economic Botany, second edition (McGraw-Hill, New York
etc., pp. xi1-+ 560, 1952).
8] MODIFICATION AND DISTRIBUTIONS OF CROPS 253
L. H. Battey et al. Manual of Cultivated Plants, revised edition (Mac-
millan, New York, pp. 1-1116, 1949).
For Details about Forest Products :
R. W. Scuery. Plants for Man (Prentice-Hall, New York, pp. viii + 564,
1952).
R. Zon & W. N. SPARHAWK. Forest Resources of the World (McGraw-
Hill, New York & London, 2 vols, pp. xiv + 1-493 and vii + 495-
997, 1923).
S. J. Recorp & R. W. Hess. Timbers of the New World (Yale University
Press, New Haven, Conn., pp. xv + 640, 1943).
S. HapEn-GugesT, J. K. Wricut, & E. M. Tecrarr (ed.). A World
Geography of Forest Resources (Ronald Press, New York, pp.
xvili + 736, 1956).
Weeds and their Control :
L. J. Kinc. World Encyclopaedia of Weeds and their Control (Leonard
Hill, London, in Press).
W. C. Muenscuer. Weeds, second edition (Macmillan, New York, pp.
xvi + 560, 1955); mainly northern United States and southern
Canada.
W. W. Rossins, A. S. Crarts, & R. N. Raynor. Weed Control, second
edition (McGraw-Hill, New York etc., pp. xi + 503, 1952).
Plant Diseases :
Sir E. J. Burter & S. G. Jones. Plant Pathology (Macmillan, London,
pp. Xil + 979, 1949).
J. C. Waker. Plant Pathology, second edition (McGraw-Hill, New
York etc., pp. xi + 707, 1957).
J. S. Boyce. Forest Pathology, second edition (McGraw-Hill, New York
etc., pp. xi + 550, 1948).
In conclusion it may be interesting to speculate as to the main centres
of origin of our crop plants. It has long been thought that the cultivation
of plants by Man began independently in each of the three main centres
of ancient civilization, viz., the eastern Mediterranean, the Oriental of
southeastern Asia, and the American of southwestern North America
and at least the northwestern portions of South America. Nowadays
there is a tendency to extend or multiply these to include other areas of
supposed early cultivation. Vavilov (0p. cit.) visualized eight major
centres, which he found to be those of greatest diversity of cultivated
plants, as follows: (i) Chinese, (ii) Indian (with a suggested separate
Indo-Malayan area), (iii) central Asiatic, (iv) near-Eastern, (v) Mediter-
ranean, (vi) Abyssinian, (vii) south Mexican and Central American, and
254 INTRODUCTION TO PLANT GEOGRAPHY
(viii) South American (Peruvian-Ecuadorian-Bolivian and two minor
areas). Much further extensive as well as intensive investigation should,
however, be carried out before (if ever) broad generalizations may be
indulged in where so many intangibles are involved ; definite information
is still largely lacking. ‘This is why such a fascinating and potentially
important subject does not receive more consideration in this book.
But it does seem that cultivated plants mostly originated in warm regions,
if often in their upland areas.
As this book is in the press, there comes news of the discovery in Jericho
of a Neolithic culture considerably older than any formerly known. All
possibility of observing signs of the ancient cultivation which must have
existed around the site has been destroyed by modern agriculture. But
the large extent of the area of sedentary occupation constitutes reasonably
certain proof that agriculture of some sort must have been prosecuted
at that time, which on the evidence of carbon-14 dating was around
7000 B.c. (K. M. Kenyon voce). Among other plants, Linum appears to
have been cultivated in the plains of Iraq as early as around 5000 B.c.
(H. Helbaek voce).
CHAPTER IX
Vii Awe Vine Oia: AUN Cis TOU. MeA NREL ND
Early in this work we indicated that the green plant is the only
satisfactory mechanism for transforming the energy of the sun into
organic compounds on which, as an animal, Man is dependent for
food and other requisites of life. In the last chapter we mentioned
some instances of such dependence, chiefly in connection with the
distribution of leading crops. It is the object of the present chapter
to give a systematic account, with chosen examples, of the multi-
farious and often vital ways in which plants and plant products are
important to mankind. For Man is unable to synthesize, at all
events economically and in useful bulk, most of the materials which
he needs in such great quantities—often for his very existence.
Even though he can, for example, convert starch into alcohol and
the latter in turn into all manner of useful products, he needs the
Wheat or some other plant to make the starch for him. In such
food materials is locked radiant energy from sunlight, which can
then be liberated by the process of respiration that goes on in all
living bodies and is rapid in warm-blooded animals. ‘This process
usually requires oxygen in large quantities and consequently is again
dependent upon green plants as, during photosynthesis, they liberate
this vital gas and return it to the air, so purifying and maintaining
the atmosphere.
Not only, as we shall see, do plants afford for mankind, either
directly or indirectly, his food and many other requisites of life,
but they largely condition his environment. ‘Thus, for example,
forests are very different to live in from grassy plains, and deserts
and areas of arctic tundra are again widely different. Many present-
day grasslands and treeless cultivated areas are, however, due to
Man’s clearance of forests, and although he shows a natural tendency
to avoid desert and tundra areas, the correlation of forest or grass-
land (see Fig. 65) with dense human population (Fig. 78) is not
always close. Rather has Man wandered and settled where he most
conveniently could, having in mind his need for subsistence, which
meant in large measure the finding or growing of plants—or of
255
Tropic of Cancer
Equator
i Tropic of Capricorn
2
a
65 -256
Over 256 inhabitants per square mile
g - 64 ” > »
= Less than. 8 inhabitants per square mile 2
ull
Fic. 78.—Distribution of the world’s human population.
VITAL IMPORTANCE TO MANKIND 257
animals which are dependent upon them. ‘Thus the migrations of
Man have been dependent in considerable degree on plant distribu-
tion, even as his present population-density is conditioned by the
crop-growing potentialities of different areas.
From the point of view of what they are used (or, occasionally,
to be avoided) for, plants and plant products may be classified into
seventeen main (or often multiple) categories whose consideration
will occupy the remainder of this chapter. As each is apt to be a
large subject, the accounts will be brief or in mere outline ; it should
also be noted that the categories are neither hard and fast nor
mutually exclusive. Indeed a good deal of repetition is inevitable.
Many of the more important plants are dealt with elsewhere in this
work, though usually in other connections, and some are mentioned
in more than one category—though without cross-referencing, as this
can be done through the index. Nor, in this primarily ‘ economic ’
chapter, will technical botanical usages and names be maintained
in the manner in which they usually are elsewhere in this book,
and practically have to be for the use of scientists.
Foops
Though whole volumes can be—and have been—written about
food plants and Man’s dependence upon them, this theme is too
obvious to require detailed treatment here. It has latterly been
extended to include vitamins, of which plants are the main primary
source. Suffice it to say that practically every item of food of all
animals comes from plants. For even if one animal eats another,
and it in turn consumes yet another, when we follow back the food-
chain we come, sooner or later, to a point of dependence upon green
plants, as these alone are able economically to build up complex
food substances from simple inorganic materials. ‘This is true not
only on land but also in fresh and salt waters, where the ‘ producer ’
plants are often microscopic, larger and larger ‘ consumer ’ animals
succeeding one another to constitute the later stages in the food-
chain. Any exceptions are insufficient in scale to have serious effect.
The most important plants used directly for food by Man (as
opposed to those used indirectly through pasturage of his domestic
animals) are those affording abundant carbohydrates and other
energy-producing materials. Outstanding are the cereal and other
‘ grains ’ (such as Wheat, Rice, Maize, Barley, Rye, Oats, Sorghums,
Millets, Quinoa, and Buckwheat) and ‘ roots’ (such as Potatoes,
258 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
Sweet Potatoes, Yams, and Cassava), on one or other of which
practically the whole human population of the world is primarily
dependent. Useful additional ‘vegetables’ include Beets and
Chard, Salsify, Carrots, Parsnips, Radishes, Swedes, ‘Turnips,
Jerusalem Artichokes, ‘Taros, Dasheens, Onions and their allies,
Artichokes, Asparagus, Cabbages and Kales and their allies, Celery,
Chicory, Endive, Lettuces, Rhubarb, Spinach, Dandelions, Water-
cress, Avocado, Breadfruit, Jack-fruit, Cucumber, Pumpkins and
Squashes and their allies, Yautias, Chayote, Egg-plant, Okra, Tomato,
etc. ‘There is no need to point out that the staples among the above
constitute the mainstay of human life on earth, different ones in
different regions affording the chief (and often almost sole) food of
teeming millions.
Other important groups of foods are legumes (the fruits of members
of the Pea family, Leguminosae) and nuts (the term being used in
the layman’s sense). ‘The former category includes Peas of various
kinds, Chick Peas, Pigeon Peas, Cowpeas, Beans of various kinds
(produced by several different genera), Soybeans, Peanuts, Lentils,
Lablab, Algaroba and other Mesquites, Carob and other Locusts.
The nuts include some with a high carbohydrate content, such as
Chestnuts and Acorns, others with a high protein content, such as
Almonds, Beechnuts, and Pistachio-nuts, and many more with a
high fat content, such as Coconuts, Brazil-nuts, Cashew-nuts,
Hazel-nuts, Macademia-nuts, Hickory-nuts and Pecans, Walnuts
and their allies, Pili-nuts, and Pine-nuts.
Fruits used regularly by Man for food, and commonly cultivated,
are very numerous as well as various in their botanical significance.
Those produced mainly in temperate regions include such ‘ stone’
fruits as Plums and Prunes, Cherries, Peaches and Apricots, such
‘“pome’ fruits as Apples, Pears, and Quinces, such ‘ gourd’ fruits
as Melons and Watermelons, and such ‘ berries ’ as the various kinds
of Grapes, Blackberries and Raspberries, Blueberries and Huckle-
berries, Cranberries, Currants and Gooseberries, Strawberries, and
Mulberries. ‘The fruits produced in warm regions or often only in
the tropics include the diverse types of Citrus fruits such as various
Oranges, Lemon, Grapefruit, Lime, Citron, Tangerine, Kumquat,
and products of their hybridization, Bananas of various kinds,
Custard-apples and their allies, Dates, Durian, Figs of various kinds,
Guavas and their allies, Granadillas of various kinds, Jujube, Litchi,
Loquat, Mamey, Mangoes and their allies, Mangosteen, Olive,
Papaya, various Persimmons, Pineapple, Pomegranate, Sapodilla and
9] VITAL IMPORTANCE TO MANKIND 259
its allies, ‘Tamarind, and many more. Jams and other preserves
are chiefly made from fruits and sugar, often with the addition of
some flavouring, preservative, and/or stiffening principle.
The above outline, which serves to indicate the range and variety
as well as importance of plant foods for Man, is exclusive of beverages
and such adjuncts as spices and other flavourings, which will be
dealt with in the next section. ‘The dependence, on green plants,
of practically all other forms of life either directly or indirectly for
food, goes of course for the animals which are used extensively by
Man for his own sustenance, and so we should here recall this
further dependence of Man on the natural vegetation or cropping-
possibilities of each region. Many of the above-mentioned cereal
Grasses, food Legumes, and ‘ vegetables’ such as Kales and Turnips
are grown partly for domestic animal fodder, as are, in addition,
numerous Grasses such as Timothy, Sudan-grass, Johnson-grass,
Orchard-grass, Redtop, Bluegrass, etc. Furthermore there is an
important class of non-grassy forage crops which, like some of the
above-mentioned cereal and other plants when green, are often used
for silage. ‘These include Mangel-wurzels and such leguminous
plants as Alfalfa, various Clovers and Vetches, Kudzu, and
Lespedezas, in addition to Peanut, Soybean, Cowpea, and others
among those already mentioned in different connections. ‘These
and other Legumes, together with various Grasses, largely make up
hay. From the utilitarian point of view, forage plants may be
looked upon as a means of turning plant carbohydrate and protein
into meat and dairy products.
A number of usually minor foods are afforded by the lower plants.
Thus the use of Mushrooms, Truffles, Morels and other Fungi is
ancient and familiar, Mushrooms in particular being widely cultivated.
Food Yeast is another important fungal product. Although the use
of Lichens for human food has largely died out except in times of
severe shortage in northern regions, they are still important in the
feeding of the Reindeer on which whole cultures of boreal peoples
are largely centred. More widely used for human food nowadays
are marine Algae, which in the Orient and some Pacific Islands
constitute a major article of diet, many being cultivated, especially
in Japan. In Europe and North America the only Algae that are
at all extensively used in food are Carrageen or Irish-moss, Dulse,
Murlins, those that give agar and some other products, and various
Lavers. But extensive research is in progress on the possibility of
using cultures of freshwater Algae, such as Chlorella, for food.
260 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
BEVERAGES AND FLAVOURS
The major non-alcoholic beverages, although not as vitally essential
as the major foods, are yet so familiar that their significance is in
no need of explanation. ‘They are tea, which is used by fully one-
half of the population of the world, coffee, which is used by almost
as many people, and cocoa—the ‘beans’ affording this last also
yield chocolate and cocoa butter. Other beverages include mate or
Paraguay-tea, obtained from the leaves of various species of Holly ;
guarana, from the seeds of an Amazonian climber ; cola, obtained
by powdering Cola seeds; khat, a tea-like drink of northeastern
Africa, and cassine, a rather similar beverage of North America ;
also yoco, which is made from the bark of a South American tree.
All these beverages contain some caffeine and consequently have a
stimulatory and refreshing effect ; and numerous others are, or once
were, widely prepared from parts of various plants.
Other non-alcoholic beverages are the so-called ‘ soft drinks’
which include a vast array of preparations that tend to rise and fall
in popularity—more, perhaps, with the amount of advertising
lavished by their producers than with their inherent value, though
most contain a fair amount of sugar and so are a source of energy.
Many contain plant flavourings, etc., such as ginger, sarsaparilla,
malted Barley, wintergreen, cola, or fruit juices—the last constitut-
ing many popular and valuable drinks.
Of alcoholic beverages there are two main groups: the fermented
ones in which the alcohol is formed by the fermentation of sugar,
and the distilled ones obtained by distillation of some alcoholic
liquor. ‘The sugar is either present naturally, as in most fruit juices,
or is formed by transformation of starch—for example in cereals or
potatoes. Wines, of which there are almost endless types of varying
delectability and to suit different palates and pockets, are the oldest
and most important of the fermented beverages. ‘They are mostly
formed by fermentation of sugar in the juice of grapes through the
activity of wild Yeasts present on the skins of the fruit, though a wide
range of other plants and their products may be similarly employed.
The agreeable aroma and flavour are due to the presence of various
aromatic substances, though the characteristic ‘ bouquet ’ develops
only after some years or even decades of ageing.
Beer, ale, and the like make up the other most important group
of fermented beverages. In their production, cereal starch (usually
in Barley) is transformed into sugar in the process of malting, and
g| VITAL IMPORTANCE TO MANKIND 261
this sugar in turn is dissolved out and the resulting product flavoured
by boiling with Hops, after which Yeast is added to bring about
alcoholic fermentation in the main process of brewing. In addition
there are numerous relatively minor fermented alcoholic beverages
including cider, made from the juice of Apples; perry, from Pear
juice ; mead, from honey and water ; sake, from Rice ; Palm wine,
from the juice of Palm inflorescences ; chicha, from Maize; and
various so-called beers made from infusions of various roots and
barks, with the addition of sugar and yeast. Furthermore, acetic
acid fermentation by Bacteria leads to the formation of vinegar,
another widely used product.
The chief distilled alcoholic beverages (‘spirits’) are made by
successive distillations of fermented mashes or wines. ‘Thus whisky
is obtained from malted or unmalted cereals or potatoes and, after
distillation, has to be aged to eradicate unpalatable principles.
Vodka on the other hand is bottled directly after distillation. Brandy
is distilled from wine, or, in the case of fruit brandies, from fermented
fruit juices. Good gins are obtained from a mixed mash of barley-
malt and rye, the flavour being due to added oil of juniper or other
aromatic essential oils. ‘The numerous liqueurs and cordials con-
sist mainly of sugar and alcohol or spirits flavoured with various
essential oils, being often blended according to some secret formula.
Spices, condiments, and other food adjuncts are almost innumer-
able, so only the most important can be mentioned here. Man’s
craving for spices has done much to change the course of history
and affect international and inter-racial relations. ‘The value of
spices and condiments lies in their ability to increase the attractive-
ness of food, etc., usually owing to the presence of essential oils.
Besides their use as food adjuncts, spices are employed in various
industries, including perfumery, drug and soap manufacture, dyeing,
and inthe arts. ‘The vast majority are still obtained from the tropics,
chiefly from Asia.
Most of these flavouring materials originate in seeds and fruits.
They include allspice or pimento, obtained from a small tree of
tropical America ; capsicum or red pepper (including chilis, paprikas,
and sweet peppers), from several plants now widely cultivated ;
black and white and some other peppers, from weak climbing or
trailing shrubs that are widely cultivated in the tropics ; cardamon
and grains of paradise; the various mustards (black, white, and
Indian), from allies of the Cabbages growing often in cool climates ;
nutmeg and mace, from the Nutmeg tree which is now cultivated
262 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
principally in the East and West Indies ; anise and star anise, cara-
way, coriander, dill, fennel, vanilla and its substitute tonka beans,
and many others.
Spices obtained from flowers or flower-buds include cloves,
capers, and saffron, while from leaves are obtained such familiar
ones as peppermint, balm, basil, marjoram, sage, the savouries,
spearmint or mint, bay, thyme, lemon thyme, parsley, wintergreen,
tansy, and tarragon, most of which can be (and commonly are)
grown in gardens in temperate regions. Spices or flavourings
obtained from barks include cinnamon and cassia, from various
species of Cinnamon trees grown chiefly in southeastern Asia, and
sassafras, from the familiar North American tree of that name.
Important spices obtained from roots and rhizomes are angelica
(other parts of the Angelica plant are also used), now cultivated
chiefly in Germany ; ginger, from the Ginger plant which is widely
cultivated in the tropics ; horse-radish, widely grown in temperate
regions ; sarsaparilla, from several tropical Catbriars ; and turmeric,
cultivated in various tropical regions and employed to colour as well
as flavour curries, etc.
MEDICINALS AND DRUGS
The history of the medicinal use of plants is long and intricate,
being largely bound up with the beliefs of primitive peoples—for
example, that disease was due to the presence of evil spirits in the
body, which could be driven out by the use of unpleasant substances.
After the Dark Ages came the herbalists with their compilations of
what was known or supposed about the medicinal value and folk-
lore of plants, and the ‘ doctrine of signatures ’ according to which
plants were supposed to possess some sign indicating the use for
which they were intended. ‘Thus the Maidenhair Fern, so called
from the black hair-like ‘ stalks’ of the leaflets, was considered a
specific for baldness, and plants with heart-shaped leaves were
believed to be valuable for use against heart ailments. Plants were
believed to have been placed in the world for Man’s use, and to
have clear indications of their particular usefulness provided by
the Almighty.
From such crude beginnings developed modern pharmacognosy,
which is concerned with the knowledge and commerce of crude
drugs and their sources, and pharmacology, the study of the action
of drugs. ‘The medicinal value of drugs is due to the presence in
g| VITAL IMPORTANCE TO MANKIND 263
them of special substances having a particular physiological action
on the human body : commonly such substances are alkaloids, some
of which are powerful poisons if administered unwisely, while others
are dangerously habit-forming. Yet in small quantities skilfully
administered, even the most poisonous or dangerous drugs can be
of value to human health and well-being.
Throughout the world there are used for medicinal purposes some
thousands of different plants or plant products——many of them only
locally by savage peoples. ‘The use of others has been rendered
obsolete by synthesis of their active principles. Some are widely
cultivated, but many more are gathered chiefly or entirely in the
wild state and are still important commercially. A few of the most
significant, classified according to the part of the plant from which
they come, are the following:
Obtained from fruits and seeds: chaulmoogra oil, from a south-
east Asian tree, containing principles effective in the treatment of
leprosy ; colocynth, from a widespread perennial vine now cultivated
in the Mediterranean region, serving as a powerful purgative, as
does also croton oil, obtained from a shrub or small tree of south-
eastern Asia ; nux vomica, obtained from a tree ranging from India
to Australia, used as a stimulant in small quantities as it contains
strychnine ; strychnine itself, obtained from the same and allied
plants ; opium, obtained as an exudation from the injured fruits of
the widely cultivated Opium Poppy, employed to relieve pain but
flagrantly misused as a narcotic in the Orient; psyllium, from
Plantains, used chiefly as a laxative; strophanthus, from two
African lianes, used as a heart stimulant ; and wormseed, a native
of the warm parts of the New World, used in the treatment of
hookworm infections.
From flowers are obtained chamomile, which is rather widely
cultivated and used for a variety of purposes; hops, extensively
cultivated in temperate regions and used for their sedative and tonic
properties as well as in brewing; and santonin, one of the best
remedies for intestinal worms.
The vegetative parts of plants contribute numerous drugs. From
leaves are derived, for example, aloe, from African and American
Aloes, used as purgatives ; belladonna, from the Deadly Nightshade
of Europe, etc., but now extensively cultivated, used for the local
relief of pain and for a variety of other commendable purposes ;
cocaine, from the leaves of the South American Coca shrub that is
now extensively cultivated in the tropics, used chiefly as a local
264 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
anaesthetic ; digitalin, from the leaves of the European Foxglove,
almost indispensable in the treatment of heart ailments ; eucalyptus,
from various Blue Gum-trees of Australia but now widely cultivated
elsewhere, extensively used in medicine, for example in the treat-
ment of nose and throat disorders; hamamelis, from the North
American Witch-hazel, used as an astringent; henbane, from the
widespread weedy herb of that name, used as a sedative and hypnotic ;
stramonium, from the widespread and weedy Thorn-apple, used as
a narcotic and in the treatment of asthma; and many others.
From stems, etc., come ephedrine, from Asiatic Ephedras, used
in medical treatment e.g. of colds and hay-fever ; guaiacum, used
as a stimulant and laxative; and quassia, used as a tonic and in
the treatment of malaria. From barks are obtained cascara, from
the American Western Buckthorn, used as a tonic and laxative ;
curare, from a variety of South American plants, a very powerful
poison used also in medicine and surgery ; slippery elm, made from
the familiar North American tree, useful for its soothing effect ;
and, above all, quinine, the great anti-malarial drug, obtained from
several allied trees native to South America and now cultivated
especially in southern Asia.
From roots and other underground parts come an important series
of drugs including aconite, from the Eurasian Monkshood, used
particularly to relieve pain ; colchicum, from the Meadow Saffron,
whose active principle is colchicine, used in the treatment of
rheumatism and gout and also important in the plant sciences as it
produces doubling of chromosomes ; goldenseal, from the North
American plant of that name, used as a tonic and in the treatment
of catarrh; ipecac, from forest-floor plants of tropical America,
almost indispensable in the treatment of amoebic dysentery and
pyorrhoea ; liquorice, from the Liquorice plant now much cultivated
in Eurasia, used as a flavouring, etc. ; squills and senega, used as
expectorants and stimulants ; ginseng, considered a virtual cure-all
in the Orient ; and valerian, from Garden Valerian, used to relieve
nervous afHictions.
From various parts particularly of the Camphor Tree come
camphor and safrole, which have a wide variety of industrial and
medicinal uses. Among Pteridophytes, the tiny spores of some
Club-mosses are variously used for covering pills as well as in
industry and even warfare, while aspidium, obtained from certain
Ferns of the north-temperate regions, has long been employed to
expel ‘Tapeworms.
9] VITAL IMPORTANCE TO MANKIND 265
The lower plants come notably into their own as sources of drugs :
penicillin, streptomycin, aureomycin, chloromycetin, terramycin,
and others recently developed from Fungi and their allies are being
found extremely valuable in the treatment of some of the most
severe diseases. Ergot, produced from the common fungal disease
of cereals bearing the same name, has long been known and is used
in the treatment of haemorrhages. Agar, widely obtained from a
number of Red Algae, is employed medicinally to prevent constipa-
tion, as well as industrially in food, paper, and cosmetic manufacture,
and as a culture medium for Fungi and Bacteria. Algin, a product
of the larger Kelps, is used in a variety of cosmetics, foods, drugs,
and as a sizing for paper; these Brown Algae are also important
as sources of iodine and potash.
FaTTyY OILS AND WAXES
Fatty or fixed oils are those which, unlike the essential oils (see
Pp. 274-5), do not easily evaporate, and so cannot be distilled without
becoming decomposed. ‘Those which are liquid ‘ oils’ at ordinary
temperatures become solid ‘ fats’ on cooling, even as fats become
oils on sufficient warming. Like animal fats, they consist of glycerin
in combination with a fatty acid, and form soaps when boiled with
alkalis. Fatty oils (as they may in general be termed) are produced
in considerable quantities by many different plants, often being
stored in seeds for use in germination. Of them four main classes
may be recognized, as follows :
1. Drying oils, which on exposure dry into thin elastic films
and are of great importance in the paint and varnish industries.
Examples include linseed oil, obtained from Flax, and its substitute
tung oil, obtained from two Chinese trees ; soybean oil, which is
widely used in human foods as well as industry, etc. ; and such
oils as perilla, walnut, Niger seed, hempseed, poppy, and safHower.
2. Semi-drying oils, which form a soft film only after long exposure,
examples being cottonseed oil, obtained from Cotton, and used for
human food, animal fodder, fuel, and in a variety of industries ;
sunflower oil, obtained from the common Sunflower, used for the
same purposes as cottonseed oil and also in the paint, varnish, and
soap industries ; and such oils as corn, rape, and camelina.
3. Non-drying oils, which remain liquid at ordinary temperatures.
These include olive oil, obtained from the Olive and used principally
for food and in medicine (though inferior grades are employed in
soap-making and as lubricants); its widely-used substitute, peanut
266 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
oil, obtained from the Peanut; and castor oil, obtained from the
Castor-oil plant and formerly used chiefly in medicine but now much
more widely in industry.
4. Vegetable fats or tallows, which are more or less solid at ordinary
temperatures. ‘These include coconut oil, obtained from the ‘ meat ’
of the Coconut, and widely used for making margarines, candy-
bars and other sweets, soaps, cosmetics, stock-feed, and as an
illuminant ; palm oil, obtained from the African Oil Palm and used
for some of the same purposes as coconut oil; palm-kernel oil, the
Brazilian palm oils, cocoa butter, and many others.
Waxes are chemically allied to fats but tend to be harder. ‘They
usually occur as coverings of leaves, etc., and help to prevent too
great a loss of water by transpiration. Among those of value to
mankind are carnauba wax, obtained from a South American Palm,
widely used in the manufacture of candles, soaps, paints, varnishes,
ointments, and many other products ; cauassu wax, which can be
used for much the same purposes ; and the American candelilla,
myrtle, and jojoba waxes.
Lather-forming products of the cultivated Soapwort, of the
South American Soapbark tree, of the Soapberry, and of California
Soaproot, are used commercially as soap substitutes—as are many
others more locally.
SMOKING AND CHEWING MATERIALS
Of these ‘fumitories and masticatories’ the most universally
employed is tobacco, obtained principally from a tropical American
species that is now very widely cultivated, but to some extent also
from a North American species which was extensively grown by
the Indians before the time of Columbus. Important chewing
materials are betel, obtained from the widely cultivated Betelnut
Palm and said to be used by over 400,000,000 people ; cola nuts,
from the West African Cola tree which has now been widely intro-
duced elsewhere, whose use results in slight stimulation and tem-
porary increase in physical capacity ; and, in addition, chewing gum
and spruce gum.
In contrast to the above more or less harmless principles, the
true narcotics contain powerful alkaloids that make them gravely
detrimental to human health if used habitually, although they are
valuable in medicine in exceedingly small amounts. Among the
chief are opium and cocaine, obtained as already mentioned above
9| VITAL IMPORTANCE TO MANKIND 267
under drugs, and constituting important relievers of pain. ‘lhe
eating or smoking of opium, which at first produces alluring dreams
and pleasurable visions, may become an uncontrollable addiction
leading to delirium and death. ‘The chewing of the leaves of the
Coca shrub, containing cocaine, gives resistance to fatigue and
hunger and at the same time a feeling of exaltation; but if habitual,
it may in time lead to severe physical deterioration and even death.
Other important narcotics are cannabis, obtained from the Hemp
plant and widely used in medicine to relieve pain as well as in the
treatment of nervous disorders; fly agaric, obtained from the
poisonous Fungus of that name, which when chewed or added to
beverages has an intoxicating effect involving hallucinations and
finally unconsciousness ; peyote (mescal buttons), from an American
Cactus, mostly chewed for the feeling it produces of well-being
accompanied by hypnotic trances ; products from ‘Thorn-apples and
Henbane, which when smoked or eaten produce excitations, illusions,
and sometimes fanatical acts; and the Oceanian kavakava, whose
use as a beverage has a sedative and soporific action, bringing about
pleasant sensations. Cannabis consumption as hashish, marijuana,
etc., causes states of ecstasy and stupefaction and may have serious
results, as when it leads to fanatical acts by addicts.
STRUCTURAL AND SHELTERING MATERIALS
It can scarcely be questioned that wood is nowadays, and from
before the dawn of history has been, the most generally important
of all structural materials. Its uses in the construction of human
habitations and shelters, furniture and utensils, vehicles and boats,
tools and all manner of fittings and fencings, etc., are too universally
familiar to require detailed enumeration. If we reflect that until
a century ago ships were made almost entirely of wood, without
which the exploration and colonization and general development of
most of the world would accordingly have been impossible, we have
one impressive indication of its immense significance in human
affairs ; and to this day wood is the most widely used commodity,
apart from foodstuffs and, perhaps, clothing materials. It is also
highly versatile as a raw material (and one of the few that can per-
petually be renewed) for conversion into products as varied as paper
and textiles, Soap and lubricants, stock-feed and motor fuel, plastics and
disinfectants, explosives and preservatives (to mention only a few).
Quite apart from questions introduced by their local availability,
268 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
different woods vary markedly in mechanical and allied properties
and, consequently, in usages. Important features are general
strength, hardness, stiffness, toughness, fineness of grain, cleavability,
density, moisture content, the commonness of defects, and suscepti-
bility to insect damage and to decay. Expression of many of these
depends upon the age of the tree from which the wood was cut
and the treatment it has received since cutting, and all of them
naturally vary with the kind of tree involved. It is said that the
annual consumption of wood in the world amounts to some sixty
thousand million cubic feet, of which nearly half is used in North
America.
The two main types of timber are softwood, obtained from
coniferous trees such as Spruces, Pines, and Larches, and hardwood,
from Angiospermous trees such as Oaks, Maples, and Mahoganies.
Besides its use as a popular fuel and employment as a raw material
for conversion into diverse products whose origin is often unrecogniz-
able, wood is used, as such, in the form of structural timber for
buildings and bridges, as boarding and flooring, girders and rafters,
pit-props and railway ‘ sleepers ’, poles and posts, piling and cooper-
age, veneers and plywood, shingles and woodenware, parts or the
whole of ships and boats, furniture and vehicles, boxes and crates,
fences and hoardings, and for innumerable other purposes of which
some have already been mentioned.
Among specific sheltering materials other than wood, the leaves
of such woody plants as Palms, and herbaceous materials such as
straw, are widely used for shelters and thatches in different parts
of the world. Also extensively employed are various soft or com-
minuted plant products for insulating and packaging, familiar
examples being Bog-mosses and hay for insulation, and sawdust and
shavings for packaging.
INDUSTRIAL USES AND EXTRACTIVES
Many of the items discussed under other captions, such as fuels,
fats, and fibres, might be included here, but the chief classes to be
considered in this section are those whose chemurgical treatment or
processing makes them important sources of industrial derivatives
namely, sugars, starches, cellulose, and some distillation products.
To take this last item first, the destructive distillation of wood-
waste produces such valuable materials as charcoal, tar, oil, turpen-
tine, wood alcohol, acetic acid, and wood gas.
9] VITAL IMPORTANCE TO MANKIND 269
Sugar, as we have already seen, is obtained principally from the
Sugar Cane and the Sugar Beet, and though primarily used as food
it has also become an extremely important industrial chemical, with
thousands of different derivatives. About 35,000,000 tons are pro-
duced annually. When speaking of sugar we normally mean sucrose,
which is by far the most important and the one usually stored,
though other sugars have their places and uses. Further important
sources of sucrose are Maize, Sugar Maple, Sorghum, certain
Palms, and honey.
Starches constitute the chief type of food-reserve for most green
plants, being stored in the cells in the form of minute grains.
Starches are chemically complex but, being easily convertible into
sugars, are vastly important as human foods, the chief sources being
potatoes, various cereals, arrowroot, cassava, and sago. They are
also widely used in industry, for example in laundry and textile
work, in sizing, and as sources of glucose, dextrin, industrial alcohol,
and explosives.
Still more complex is cellulose, the chief constituent of the cell-
walls of most plants, which yields numerous textile fibres both
natural and artificial. ‘The chief sources nowadays are cotton, which
is almost pure cellulose, and wood, which by chemical and mechanical
treatment is made to yield pure cellulose. ‘This may be made into
fibres or plastics, or transformed into wood sugar, which in turn
may be made to nourish Yeast or yield alcohol and thus become
available for food or industrial use. In addition there is hemi-
cellulose, which forms the so-called ‘ vegetable ivory ’—obtained
from certain tropical Palms and useful as a substitute for ivory in
the manufacture of buttons and other small, hard objects.
While paper can be made from most fibrous materials, the chief
commercial sources are wood fibres, cotton, and linen. ‘The last
two, formerly the main source of paper, still yield the finest grades,
but wood fibres nowadays make up the vast bulk. ‘They are
obtained from a wide range of trees of which various Spruces,
Pines, Hemlocks, and Poplars are among the most important, while
sawmill waste is increasingly used. Other raw materials for paper-
making include papyrus, esparto, straw, mulberry, and various textile
fibres. By special processes the wood or other raw material is
pulped, after which a series of operations, including the addition
of rosin or other ‘ sizing’ of plant origin, lead to its manufacture
into one or another of the almost innumerable types and grades of
paper. ‘The coarser materials are often made into cardboard.
K
270 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
Cellulose is soluble in various solvents such as concentrated nitric
acid, and this has led to the development from it of ‘ plastics’ and
unbreakable glasses and many other important products including
guncotton, cordite, collodion, celluloid, cellulose acetate, viscoses
such as Cellophane, and various varnishes and fabrics including cloth
and leather substitutes. Photographic film is made chiefly from
cellulose nitrate or acetate coated with gelatin, while further breaking
down of cellulose yields various sugars which in turn yield alcohol
and, with appropriate treatment, ‘Torula Yeast or other foods. Alto-
gether cellulose products are countless in number and of great value
and usefulness ; moreover, being formed often from forest waste,
they seem endlessly replaceable.
CLOTHING MATERIALS AND OTHER FIBRES
Clothes, one of Man’s primary requisites, are made largely from
plant fibres or materials obtained from animals which are dependent
on plants for food. In addition, fibres and fabrics are used by
Man in innumerable other ways. Indeed, fibre-yielding plants are
probably second only to food plants in their influence on civilization
and their general usefulness to Man. But although there are many
hundreds of fibre-yielding plants known, only a few are of com-
mercial importance.
From the point of view of their utilization, fibres produced by
plants or from plant materials may conveniently be classified into
seven groups: (1) Paper-making fibres, which were discussed in
the last section dealt with above. (2) Artificial fibres, whose pro-
duction is nowadays a great and expanding industry. ‘The main
raw materials are (a) cellulose, derived from wood pulp or cotton
linters, whence are made the various so-called ‘ rayons’, and (6)
soft coal, whence are produced the various nylons and related
materials which between them have now some hundreds of important
uses. (3) Textile fibres, used as fabrics, netting, and cordage.
‘These include cotton of various kinds, flax, hemp, jute, ramie,
Manila hemp, sisal, coir, and many others. (4) Brush fibres, which
are tough and stiff, the chief being the piassavas and their allies,
obtained from certain Palms, and Grasses such as Broomcorn,
Broomroot, and Spartina. (5) Plaiting and tough-weaving fibres,
employed for making straw hats, baskets, chair-seats, matting,
wickerwork, etc., for which the stems of various Palms, Grasses, and
grass-like Sedges and Rushes are used. (6) Filling fibres, used for
g| VITAL IMPORTANCE TO MANKIND 271
upholstery, stiffening, packaging, and caulking, the outstanding
example being kapok (see p. 235) and its various substitutes, which
include the silky hairs on the seeds of the familiar Milkweeds.
There is also Spanish-moss, an excellent substitute for horsehair.
(7) Natural fabrics, etc., consisting of tough interlacing fibres that
can be extracted from bark in layers or sheets and used as a sub-
stitute for cloth. Examples include the Polynesian and Oriental
tapa cloth and the Jamaican lace-bark, as well as such fibrous pro-
ducts as the so-called vegetable sponges or luffas which are used
for making hats, for scouring, for filtering, and as substitutes for
bath sponges and body scrapers.
FUELS (INCLUDING FOSSIL, ETC.)
Fuel, as a source particularly of heat, light, and power, is one of
the greatest necessities of human life, and in general consists of
plants or plant products whether modern or belonging to earlier
epochs. A few of the main groups of plant materials that are widely
used as fuels may be outlined: (1) wood, probably still used more
for fuel than for any other purpose, certain hardwoods being in
general better than other types, but almost all woods making useful
fuels when dry ; (2) vegetable oils, used principally in this connection
for illumination and for powering Diesel engines ; (3) peat, con-
sisting of compacted deposits of partially decomposed vegetable
matter, which is widely used for heating and cooking in northern
lands especially where wood is scarce; (4) manure, which is the
almost universal fuel of hundreds of millions of people in southern
Asia ; (5) coal, the compressed and fossilized remains of plants that
lived in much earlier geological epochs and are now largely decom-
posed and converted into carbon, being a valuable and widely used
source of fuel and power and also of gases which are employed
particularly for heating and illuminating ; (6) coke, which is left
when coal-gas is driven off from coal, and is nearly pure carbon,
forming an excellent fuel which burns without appreciable smoke
or flame ; (7) charcoal, which bears a similar relationship to wood,
and is the chief domestic fuel in many tropical countries ; (8) saw-
dust, etc., used principally in the form of briquettes; and (g)
petroleum, whose familiar products of distillation include the all-
important gasoline (petrol) as a source of power, and paraffin and
kerosene as sources particularly of heat and light. Although no
trace of plant structure remains in petroleum, it is generally supposed
272 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
to have had its origin in minute primitive forms of plant life that
flourished in much earlier geological times.
LATEXES AND EXUDATES
Of the products obtained from the milky juice (latex) of various
plants, rubber is by far the most important. In spite of the con-
siderable employment of synthetic forms, over a million tons of
natural rubber are used annually, most being produced in south-
eastern Asia. Over three-quarters of the crude rubber consumed
goes into tyres and inner tubes, while other important uses are in
footwear, packaging, waterproof clothing, road construction, tubing
and belting, electrical insulation, etc. Rubber is produced princip-
ally from various tropical woody plants of the Spurge, Mulberry,
and Periwinkle families, some of which are now cultivated. Besides
wild and plantation rubbers produced from the Para (Hevea) Rubber
tree, which is by far the most important source of rubber, there are
such other natural types, produced from different sources, as Assam,
castilla, ceara, guayule, and even dandelion rubbers, the last being
cultivable in the cool-temperate belt.
Other latex products include the non-elastic gutta-percha and
balata, obtained from tropical trees and used for insulation, piping,
golf-balls, telephone receivers, and many other purposes, and chicle,
obtained from the tropical American Sapodilla tree, which is the
basis of the chewing-gum industry.
The gums which exude from plant stems either naturally or in
response to wounding, and the resins which are usually secreted in
definite cavities or passages, may conveniently be classed together
as exudates. Gums are used principally as adhesives, as sizing for
paper, in medicine and polishing, in cosmetics and ice-cream, in
printing and finishing textiles, as a glazing for paintings, and in the
confectionery and paint industries. ‘The chief commercial varieties
are gum arabic, obtained from certain Acacias of arid northern
Africa, gum tragacanth, from certain Milk-vetches of arid south-
western Eurasia, and its substitute karaya, obtained from a tree in
India whence several million pounds are exported annually. The
related pectins are widely used to make foods jell, in pharmaceuticals
and cosmetics, in sizing and adhesives, fibres, films, and other
preparations. ‘They come chiefly from citrous and apple wastes,
but many other fruits, etc., afford potential sources.
Resins are more various and tend to be still more important than
9| VITAL IMPORTANCE TO MANKIND 273
gums, though usually tapping is necessary to obtain them in com-
mercial quantities, or they may be collected in the fossil state. For
the most part they are forest products. Some of the more valuable,
with their uses, are (1) the various copals, utilized as varnishes and
in making paints and linoleum; (2) amber, a fossil resin particularly
from an extinct species of Pine, which is used for beads, ornamental
carvings, and mouth-pieces of pipes, etc.; (3) damars, used princip-
ally in varnishes; (4) lacquer, a natural varnish which hardens on
exposure to air and affords remarkable protection even against acids
and alkalis; (5) shellac, excreted by a particular insect on twigs of
certain trees on which it feeds, widely used in insulation and decora-
tion and for making varnishes, sealing-wax, size, drawing inks,
gramophone records, and many other products; and (6) turpen-
tines, chiefly obtained by tapping coniferous trees and yielding on
distillation oil of turpentine and rosin. Oil of turpentine is of major
importance in the paint and varnish industry as a solvent and thinning
agent, and in chemical manufacture, while rosin is the chief sizing
material for paper and is also used in many manufactures as well as
in greases and lubricants.
Other noteworthy products in this general category include Canada
balsam, used in mounting microscope slides and as a cement for
lenses ; spruce gum, used as a masticatory; Venetian turpentine,
used in varnishes and veterinary work ; and various balsams, used
in medicine, adhesives, soaps, lotions, and cosmetics, as well as for
the flavouring of foods and as fixatives in the perfume industry.
There are also such products as ammoniacum, used in medicine
and perfumery; asafoetida, widely used in medical treatment ;
copaiba, used for making varnishes and lacquers, as a fixative of
perfumes in soap, and in photography and medicine ; elemi, used
in various artistic, cosmetic, and medical operations ; frankincense
used in incense, cosmetics, and fumigation ; and myrrh, used for
cosmetic and medicinal purposes as well as in incense and embalming.
TANNING AND DYEING MATERIALS
Tanning involves the reaction of strongly astringent tannins with
such proteins as are present in animal skins, thus forming the strong
and resistant, flexible commodity we know as leather. Although
tannins are very widespread in plants, relatively few species are
known to contain a sufficient proportion to be of commercial import-
ance, and these are in great demand. The sources occur mostly
274 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
in the wild state and include the woods of Quebracho and Sweet
Chestnut, the leaves of Sumac and Gambier, such fruit products as
myrobalan, tara, and valonia (from the ‘Turkish Oak), root materials
from Tanner’s Dock and Palmetto, and barks of Hemlock, various
Oaks, Mangroves, Eucalypts, Wattles (Acacias), Larches, Spruces,
Birches, and Willows. However, the tannin content in the last four
instances is too low to warrant their general use. ‘Tannin-inks are
the most important inks at the present time, their tannin being
derived largely from the insect galls formed in great abundance on
the twigs of the Aleppo Oak ; to these galls or an extract made from
them are added an iron salt, an agglutinant such as gum arabic, and
a colouring matter such as logwood (see below). Other inks, too,
are made substantially from plant products.
Of natural dyes and stains obtainable from plants there is a vast
array involving almost all colours, though latterly most have been
supplanted at least in part by the synthetic or aniline dyes obtained
from coal-tar products. Such ‘ artificial’ dyes tend to be brighter,
cheaper, and more lasting, and with them only a few vegetable dyes
compete nowadays. ‘These vegetable dyes are especially useful in
dyeing fabrics, but are also employed to colour a wide range of
other familiar products. Some of the more important of these
natural colouring matters, grouped according to the plant part from
which they come, are: from seeds and fruits—annatto, Persian berries,
and sap green; from flowers—safHower and saffron; from barks
—quercitron, lokao, and gamboge (an exuded gum resin); from
leaves—indigo, henna, woad, and chlorophyll (which is harmless
and consequently used in foods and toothpastes); from woods—
logwood or haematoxylin (of which many thousands of tons are
used annually to give colours ranging from reds to purples and
black), fustic, cutch, osage orange, sappanwood, and brazilwood ;
and from roots and tubers—alkanna, madder, andturmeric. Lichens
yield some fine dyes, among which archil and litmus still find
extensive use.
ESSENTIAL OILS AND SCENTS (PERFUMES)
Very different from the fatty oils already considered are the
so-called essential oils which have a pleasant taste and strong aromatic
odour, easily volatilizing in air. ‘They are complex in chemical
nature but tend to be readily removed—by distillation, expression,
or solvent-action—from the many and various plants that produce
9| VITAL IMPORTANCE TO MANKIND 275
them. ‘Their main uses are for scenting, flavouring, or medicinal
purposes—for example in the manufacture of perfumes, soaps, and
other toilet preparations, in cooking and the production of all
manner of foods and beverages, and for therapeutic, antiseptic, and
bactericidal purposes. Other uses are as clearing agents and solvents,
as insecticides and deodorants, and in such diverse products as
printer’s ink and toothpaste, library adhesives and chewing-gum,
shoe-polish and tobacco.
We have already dealt with the flavouring and medicinal sides,
and noted the industrial uses, of various essential oils, and shall be
concerned with insecticides, etc., in the next section; here we
must mention some of the more important ‘ essentials’ used in
perfumes (or scents, as they are called in Britain). These often
highly-priced products commonly consist of blends of the essential
oil or oils in alcohol, usually with a less volatile fixative. Examples
are rose oil or otto (attar) of roses, obtained principally from flowers
of the Damask Rose which are distilled without delay after being
picked in the early morning just as they are opening; orange
blossom oil (neroli), formerly obtained from citrous plants grown
for the purpose but nowadays often synthesized and used in cos-
metics, etc. ; lemon-grass oil, from the leaves of a particular Grass
and used in cosmetics and medicine ; oil of citronella, from another
member of the same genus and used in cheap perfumes and as a
deodorant and insect repellent ; geranium, distilled from the leaves
of various species of ‘ Pot Geraniums’ (Pelargoniums) and widely
used in making perfumes and soaps ; ylang-ylang, from the flowers
of an Asiatic tree and now said to be present in almost every perfume ;
cassie, from one of the Acacias; cedarwood oil, from the Eastern
Red Cedar; bergamot, from a type of Orange; bay rum, from a
West Indian tree; calamus, from the Sweet-flag; camphor (in
spite of its solid form); lavender and rosemary which are used
extensively in eau-de-cologne and soaps; and very many others.
Although numerous synthetic products are now available, most of
the best perfumes are still of botanical origin and in increasing
demand, the annual gathering of flowers for this purpose alone being
said to exceed 10,000,000,000 Ib. (over 44 thousand million kilos).
. INSECTICIDES AND HERBICIDES
Although more than twelve hundred species of plants have been
reported to have some insecticidal or at least insect-repellent
276 INTRODUCTION TO PLANT GEOGRAPHY [CHAP
properties, the vast majority are of little importance and even the
best tend to be overshadowed nowadays by such synthetic insecticides
as DDT. Nevertheless some plant products continue to be used
very extensively and even increasingly to combat insects and other
vermin, and accordingly to be of great service to Man. ‘The three
most important are: (1) nicotine, extracted from leaves of Tobacco
plants and used as a spray that is lethal to some of the worst insect
pests ; (2) rotenone, obtained from the roots of various tropical
trees or shrubs belonging to the Pea family, and long used as a fish-
poison as well as, latterly, in powdered form or spray to kill various
insect pests of crops and livestock ; and (3) pyrethrum or insect
flowers—likewise used as dusts or sprays that quickly paralyse
insects including pests afHicting Man, and obtained from the flower-
heads of certain members of the Daisy family that are widely cul-
tivated for the purpose.
There are also such repellents, etc., as camphor, which is obtained
principally from the Camphor tree but is also synthesized, cedarwood
oil, the Mexican ‘ Cockroach plant ’, and the Chinese ‘ Thunder-god
vine’. Most botanical insecticides are harmless to human beings
and other warm-blooded animals. However, red squill, obtained
from the bulbs of a small herb of the Mediterranean region, is an
important ‘raticide’, having little effect on animals other than Rats
and Mice.
Whereas the old-fashioned herbicides are usually more or less
caustic chemical compounds (‘ weed-killers ’) that kill off vegetation
indiscriminately, investigation of plant growth hormones in recent
decades has led to the discovery and use of ‘ hormonal’ herbicides
that are highly selective in their action in killing some plants while
leaving others, and animals, unharmed. Outstanding in this respect
is 2,4-D, which at suitable concentrations kills most dicotyledonous
plants without affecting most monocotyledonous ones. ‘Thus it
forms an effective lawn or wheat-field spray, killing most of the
(dicotyledonous) weeds without injuring the Grass or grain crop.
Plant growth substances are now synthesized in quantity and are
used in various horticultural practices such as the promotion of
rooting in cuttings.
ENVIRONMENTAL AND ECOLOGICAL
In spite of the increasing ease and effectiveness of transport in
the modern world, the availability of this or that plant product in
9| VITAL IMPORTANCE TO MANKIND 277
a particular place depends to a considerable extent on whether the
plant from which it comes can be cultivated locally—especially if it
is bulky and needed in large quantities or in a fresh state, as so many
foods, etc., are. And quite apart from this direct dependence of
mankind on the crop and other plant productivity of different areas,
we get very different environments created by different types of
vegetation—as was already pointed out in the second paragraph of
this chapter. Among many other things, trees give shade and
shelter, and when they are widely aggregated into forests, these
may regulate climate to a considerable extent, ‘damping down’
temperature and humidity fluctuations in their shade. In some
ways, in spite of their transpiration, trees may also help to conserve
water, for example by preventing run-off and floods, meanwhile
checking erosion. Forests moreover afford shelter and range for
livestock and wild animals, and recreation for human beings.
The widespread use of such ecological devices as sand-binding
Grasses or other plants and wind-breaking trees and shrubs, is further
testimony to the value of plant life to mankind. Particularly are
various surface-binding plants of importance in combating erosion,
which is one of the world’s worst scourges, as is further indicated
in our concluding two chapters.
Finally it should be noted here as well as in the next section that,
in the local environments which Man makes for himself, the import-
ance of gardens of one sort or another is enormous practically the
world over. And the supply and use of agricultural, forestral, and
horticultural implements such as harvesters, saws, and lawnmowers,
and the general tending of plants, involves vast industries almost
everywhere.
AESTHETIC AND ORNAMENTAL
In most landscape views, plants form the chief embellishment,
and landscapes would suffer greatly without them. Apart from the
vital needs they satisfy and the material benefits they bestow, plants
greatly enhance Man’s aesthetic appreciation of the world in which
he lives—both in their natural growth as vegetation, whether
arborescent or otherwise, and through their cultivation for orna-
mental purposes. Almost everywhere Man lives, gardens are
cultivated for recreational and other reasons, and the dustiest city
streets and drabbest homes are enlivened by greenery and pot or
cut flowers.
278 INTRODUCTION TO PLANT GEOGRAPHY [CHAP,
Floriculture, the branch of horticulture concerned with the com-
mercial production of flowers, is really a huge industry that 1s
important on an almost world-wide scale, but especially in the
temperate zone. ‘The actual growing is often scientifically regulated
in many ways, as in greenhouses, and the organization for transport
and selling is complex and vast. Lawn-growing and landscaping
are also of considerable importance in urban areas and elsewhere,
for recreational and ornamental purposes ; their primary function,
however, is the growing of plants. All this, moreover, involves
extensive trading in seeds, bulbs, etc., and sometimes in apparatus
for plant growth—for example in hydroponics, their cultivation
without soil.
MICROORGANISMS AND MISCELLANEOUS
We have already mentioned the importance of some micro-
organisms in affording drugs (such as penicillin) and foods (such
as Food Yeast), and, towards the end of the last chapter, in causing
plant diseases. Others are of vast importance in bringing about
desirable changes—for example the Yeasts in causing fermentation
of sugars to alcohols, and various Bacteria in effecting further
fermentation to vinegar as well as in the ‘ curing’ of tobaccos and
‘ripening’ of cheeses. On the positive side, further valuable
fermentation, fibre-retting, organic acid and vitamin production,
hide-dehairing, nitrogen-fixing, sewage-disposal, purification and
preservation, cooking and silage and all manner of other operations
are carried out only by or with the aid of microorganisms. On the
negative side, there are the numerous human and other animal
diseases which they cause, as well as loss by rotting, putrefaction,
and general decay. ‘These last activities involve the breakdown of
complex carbohydrate and other materials to simpler ones and
ultimately to the raw materials whence they came. Such ‘ degrada-
tion’ is essential to keep the world going, for without it the vital
raw materials such as carbon dioxide would all be used up sooner
or later and life would come to a standstill. Consequently non-
green microorganisms, which are almost entirely responsible for the
breakdown and return of the essentials to general circulation, are
the world’s great scavengers, and, as such, are of fundamental
importance to all life.
Nor, among microorganisms, must we overlook the smaller Algae,
which possess chlorophyll and so are able to build up complex
9] VITAL IMPORTANCE TO MANKIND 279
foods from simple beginnings. For they afford the ultimate source
of sustenance for most of our Fishes, Crustaceans, and other ‘ sea
food’, help form Food Yeast, and give us such useful products as
limestones and the diatomaceous earth (see Chapter II) which is
widely employed in toothpastes, abrasives, filters, and so forth.
On the negative side, Algae may be a nuisance in clogging and scum-
ming and even poisoning fresh waters.
Among miscellaneous items introduced largely by higher plants
are the various animal litters and ‘farmyard’ manures that form
such an important part of the farmer’s microcosm ; there are also
the ‘ green’ manures used, for example, in crop rotation, and the
organic mulches, etc., that are employed to conserve soil moisture
and improve soil texture.
Yet another important product of higher plants is cork, obtained
principally from the Cork Oak, a tree native to the Mediterranean
region. Commercial cork consists of the outer bark of the tree and
can be removed every few years without injury to the tree as it
grows. Being exceedingly light, compressible but resilient, a low
conductor of heat and sound, and above all resistant to the passage
of moisture, it has numerous uses in industry, either in its natural
form or after being moulded as ‘ composition’ cork. Among the
more familiar of these uses are: as stoppers, corkboard, tips of
cigarettes, and handles of various kinds, in mooring-buoys, lifebelts
and life-jackets, footwear, tropical helmets, and various sporting
equipment, and in linoleum and linotiles.
It should also be recalled that, as implied earlier, green plants
enable us to breathe by returning oxygen to the air during photo-
synthesis. They are also the primary source of most vitamins,
without which we would expire from a combination of deficiency
diseases. And all the time these same plants afford valuable research
materials on which many of the greatest scientific discoveries have
been made and important studies continue practically throughout the
world.
SOME NUISANCES
The significance of various lower plants, particularly, in causing
diseases of animals and other plants has already been referred to.
Thus, such human diseases as tuberculosis, brucellosis, tetanus,
typhoid and some other fevers, plague, cholera, diphtheria and many
more, are all due to Bacteria, while the Potato and Chestnut Blights
280 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
and Cereal Rusts and Smuts are due to Fungi, which alone cause
many hundreds of different plant diseases, often of devastating effect
and vast importance. Other plant maladies may spoil amenities, an
example being the Dutch Elm disease which threatens the various
Elms that form such an important feature of the landscapes in
temperate regions on both sides of the North Atlantic. Some higher
plants and Fungi, particularly, are dangerously poisonous; but
although large doses may be lethal, small ones are often beneficial
(as we saw when discussing drugs). Consequently such plants are
not unmitigated curses ; and indeed, provided they are known and
understood, they may be of real benefit. On the other hand, some
of the selective herbicides may, like disease-provoking organisms,
yet be turned to destructive use in biological warfare.
The saprophytic Fungi and Bacteria that cause harmful decay,
putrefaction, and often loss of food and fabrics, etc., have also been
mentioned above ; in many circumstances there would be no organic
breakdown without them. At all events the beneficial ‘ scavenging ’
which these organisms accomplish is so essential as immeasurably
to outweight such nuisances as they perpetrate, considerable though
the latter may be—especially with the deterioration and spoilage
that are so rapid and marked in the tropics.
Weeds are often defined as ‘ plants growing where they are not
wanted ’—which is a broader conception than we had in dealing
with them in the last chapter. However, the same plant that is
useful in one place may be obnoxious in another, in which its
necessary control or eradication becomes a laborious and costly
procedure ; in short, it has become a weed. Modern methods of
weed control include chemical spraying with such herbicides as
2,4-D or chlorates, mulching, and the biological use of ‘ smother
crops ’ which combat the nuisances by competition. Other methods
include the introduction of diseases of the pests involved, as well
as such time-honoured activities as weeding, hoeing, harrowing, and
the like. Also effective is prevention of the growth of weeds by
the use of ‘ clean’ seed, sterilization of the bed by heat or chemicals,
and removal of nearby sources of infection.
Besides being nuisances in the ways mentioned in the last chapter,
some weeds, such as the Ragweeds and many Grasses, have airborne
pollen to which people suffering from hay-fever are particularly
sensitive. Such plants as Poison-ivy are also a great nuisance to
some individuals. And finally, not only field and garden crops but
also forests and even waters have their weeds—in forests the often
9] VITAL IMPORTANCE TO MANKIND 281
valueless but fast-growing and strangling Birches and Cottonwoods,
and, in water, the Algae which foul ships’ bottoms and reservoirs,
clog irrigation ditches and navigation channels, and are troublesome
in many other ways.
FURTHER CONSIDERATION
If it is felt that the résumé of economic botany given in this chapter
is scarcely appropriate to a work on plant geography, however wide and
introductory this may be, it should be recalled that the essence of our
subject is distribution, and that not only the origin and supply but also
the availability where it is needed of a plant product (and hence its geo-
graphy) is of importance to Man and pertinent to our study. For Man
often ‘ shapes ’ plants and their distribution even as they, in turn, largely
qualify his life and so limit the whereabouts and size of his populations.
The importance of plants to mankind is stressed in the following works :
F. O. Bower. Plants and Man (Macmillan, London, pp. xii + 365
1925).
R. Goop. Plants and Human Economics (Cambridge University Press,
Cambridge, Eng., pp. xii + 202 and additional maps, 1933).
W. W. Roppins & F. Ramarey. Plants Useful to Man, second edition
(Blakiston, Philadelphia, pp. ix + 422, 1937).
C. J. HyLanper & O. B. Stantey. Plants and Man (Blakiston, Phila-
delphia, pp. x + 518, 1941).
J. Hurcutnson & R. MEtvitte. The Story of Plants and their Uses to
Man (Gawthorn, London, pp. xv + 334, 1948).
For further information about most economically important plants :
A. F. Hitt. Economic Botany, second edition (McGraw-Hill, New York
etc., pp. xi1-+ 560, 1952).
R. W. Scuery. Plants for Man (Prentice-Hall, New York, pp. vii + 564,
1952).
E. E. Sranrorp. Economic Plants (Appleton-Century-Crofts, New York,
pp. xxili+ 571, 1934).
K. H. W. Kraces. Ecological Crop Geography (Macmillan, New York,
pp. Xvili-+ 615, 1942).
R. Zon & W. N. SparHawk. Forest Resources of the World (McGraw-
Hill, New York & London, 2 vols., pp. xiv + 1-493 and vii + 495-
997, 1923). a
W. W. Rossins. The Botany of Crop Plants, third edition (Blakiston,
Philadelphia, pp. x + 639, 1931).
282 INTRODUCTION TO PLANT GEOGRAPHY
United States Department of Agriculture 1950-51 Yearbook. Crops in
Peace and War (U.S. Government Printing Office, Washington,
D.C., pp. [xviii +] 942, 1951). A veritable mine of information
inter alia on chemurgical uses and future possibilities.
Poisonous plants are commonly treated on a local basis :
L. H. Pammet. A Manual of Poisonous Plants : chiefly of eastern North
America... (‘Torch Press, Cedar Rapids, Iowa, Pt. I, pp. viii + 150,
and: Pt. Ilpp: 153-977, 1911):
J. W. HarsHpBercer. Textbook of Pastoral and Agricultural Botany
(Blakiston, Philadelphia, pp. xiii + 294, 1920). Gives some idea of
the magnitude of the subject on a wider basis.
As well as the above more general works, there are various books which
are devoted to particular crops or closely-allied groups of crops. Along
these lines, apart from individual works, there are three special series in
English of which (1) the Economic Crops series of Interscience Publishers,
New York, has so far published books on Apples, Bananas, Cherries,
Cocoa, and Sweet Corn, while in (2) Longmans Green & Company’s
Tropical Agriculture series there have appeared books on Cocoa, Rice,
Tea, and Bananas; in preparation are Coconuts, Rubber, Oil Seeds,
Sorghum, Oil Palms, Cotton, and Spices. Also extensive and uniform
will be (3) the World Crops Books series of Leonard Hill Limited,
London, in which the volumes so far in press or arranged include separate
ones on Alfalfa, Barley, Brassicas, Coffee, Cucurbits, Eucalyptus, Flax
and Linseed, Hops, Jute, Mangoes, Mushrooms and ‘Truffles, Oats,
Onions and their allies, Peanuts, Pineapple, Rubber, Rye, Sugar Cane,
Taros and their allies, ‘Tomatoes, ‘Tropical Cash Crops, Vegetable Fibres,
and Wheat. Each of these World Crops Books deals with the botany,
cultivation, and utilization throughout the world of the crop or group of
crops concerned, and each is profusely illustrated and fully documented.
An allied series, entitled Plant Science Monographs, also published by
Leonard Hill Limited, deals especially with research advances involving
inter alia many of these and other important crops.
CrarrernoxX
ENVIRONMENTAL FACTORS
We have now dealt sufficiently with special kinds and systematic
groups of plants and their distributions, and must proceed to
consider the results of their natural aggregation, namely, vegetation.
As a basis for this we must deal in the present chapter with ecological
factors and, in the next chapter, with the main habitats these factors
constitute. In addition there will be considered in Chapter XI
certain fundamental tendencies and attributes of vegetation whose
recognition is essential to the full understanding of vegetational
changes and types.
Ecology is the study of the mutual relations among organisms and
between them and their environment—environment being the aggre-
gate of all external conditions and influences affecting life and
development at a given spot. Ecological or environmental factors
are many and diverse, and often intricately mixed and interdependent.
Either singly or in combination, the various ecological factors may
affect the presence or absence, vigour or weakness, and relative
success or failure of various plant communities through their
component taxa. Although the subject is enormously complex, the
immediate vehicles of influence are very few, being chiefly food,
light, temperature, water, and dissolved substances; these are
affected by variations in the ecological factors which characterize
different habitats and consequently lead to the differentiation of
vegetational types.
The four main classes of ecological factors with which we shall
deal below—namely, climatic, physiographic (of topography, etc.),
edaphic (of soil), and biotic (due to living organisms)—are them-
selves commonly interrelated and intricately mixed. Often they
work through one another, acting and reacting together, as in the
case of physiographic changes which bring about local climates that
in turn may affect the soils and competition-impress. Accordingly
this classification, like so many other biological ones, is to some
extent artificial. Yet all categories affect the plant either directly
or indirectly through modification of its reactions, bringing about
283
284 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
functional responses or differences in growth or structure, although
different plants vary widely, of course, in the nature and degree of
such response.
CLIMATIC
The climatic factors comprise the general features of regiona
climate, often being rhythmic—exhibiting, for example, diurnal,
seasonal, or long-term cyclic fluctuations. ‘They may also vary
locally, to give local climates, and even do so in extremely restricted
confines, to give microclimates. Examples of local climates are
found on steep northern or southern hill-slopes, and of microclimates
on the leeside of boulders which protect the immediately neighbour-
ing plants and animals from wind and insolation. In general the
factors classed as climatic have a dominating influence. Neverthe-
less, so far as plants are concerned, these factors are often but
poorly expressed by meteorological records which, for example, are
usually taken at some standard height above the ground (rarely the
height of the plant) and fail to observe the often rapid fluctuations
which can be so important to a sensitive organism. ‘There are five
main climatic factors which we must now consider in turn: it is
their different combinations which largely prescribe vegetation-types,
and so they cannot well be placed in order of relative importance.
(1) Light. ‘This, as we have seen, is essential for photosynthesis,
though fortunately sufficient illumination for this purpose is present
almost everywhere on land and in surface waters ; it may also be
important for some reproductive processes. Fig. 79 shows the
Moss Campion (Silene acaulis agg.) flowering only on the top and
south-facing side of its domed tussocks in high-arctic Spitsbergen,
where it can constitute a reliable compass ; the effect is thought to
result from differing light intensities.
The light-climate at a spot depends on the duration, time-
distribution, intensity, and quality of the insolation there obtaining,
though so far as the plant is concerned the effective period may be
modified by cold or drought. Moreover, for successful flowering
many plants require a relatively long (or, alternatively, short) day,
apparently regardless of the intensity or quality of the light, and
consequently such plants are largely limited to high (or low) latitudes,
respectively. ‘The effect of light on photosynthesis depends largely
on intensity, which also influences growth. In the open, elongation
is checked and lateral organs enlarge, whereas with congested
10] ENVIRONMENTAL FACTORS 285
conditions, for example in forests, the form tends to be more elongated
and narrow. In temperate forests, too, the seasonal aspects are
apt to be important—in particular the prevernal (7.e. before spring)
one of herbs which flower before the shading tree-leaves expand.
Thereafter different levels or layers in the forest usually have
different light-climates.
The measurement of light tends to be unsatisfactory, for no instru-
ment indicates precisely the quality as well as intensity, much less
the total quantity ; usually only the intensity at a particular point
Fic. 79.—Moss Campion (Silene acaulis agg.) flowering only on the south-facing
sides and tops of its domed tussocks near 80° N. in Spitsbergen. The surrounding
terrain is a mixed ‘ half-barren ’.
in time and space is measured, and that is done only as far as the
measuring device employed is sensitive to the component wave-
lengths encountered. ‘The particular rays of the spectrum which
are most effective in the diverse plant-functions affected by light,
also tend to be different. Nevertheless photoelectric cells and
actinometers of sensitized paper are useful, especially for purposes
of comparison, and both are widely employed by ecologists in the
field.
(2) Temperature. ‘This factor is vitally important as it conditions
the speed of the chemical actions and activities comprising life.
The great world vegetational zones, like altitudinal ones, depend
primarily on temperature, and we find it convenient to distinguish
between megatherms (plants favouring warm habitats), microtherms
(plants favouring cold habitats), and the intermediate mesotherms.
286
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288 INTRODUCTION TO PLANT GEOGRAPHY | CHAP.
Different plants are variously adapted as to the minimum, optimum,
and maximum temperatures for their life as a whole as well as for
its component physiological functions, even though these actual
temperatures may change with variations in other conditions and
with the state of the plant (as well as, of course, differently with
different plants).
Winter is normally a resting period when activity is at a minimum
in temperate regions, though many plants are active at much lower
temperatures in the polar lands and waters—some even below o° C.
On the other hand, temperatures above the freezing point may
already be lethal to tropical plants; so may temperatures above
45° C. if evaporation does not cool and save them. However, there
can be few places that are naturally too hot or too cold for any
plants. Important indirectly are clouds and other influences reduc-
ing the amount of direct insolation, and relative humidity which
greatly affects the loss of water by evaporation. ‘The soil also has
a marked local effect, dry and dark types warming up much more
quickly than heavy waterlogged ones.
Temperatures vary markedly at different levels as well as at
different times, and meteorological means (i.e. averages) are there-
fore of little value to the ecologist. An annual mean well below
freezing point is found in some continental regions where forests
abound, and although monthly means, and especially monthly mean
maxima and minima, are of more value to the plant scientist than
annual means, a thermograph tracing showing the continuous change
on the spot is most desirable for ecological purposes. ‘To give any-
thing like a complete picture of the temperature-climate as it strikes
the plants, tracings should be obtained synchronously at each
different level or layer of vegetation and root-infested soil. In
intricate work involving, for example, the surfaces or the internal
tissues of leaves, thermocouples are employed instead of ther-
mometers, while for determining the approximate temperatures of
hard surfaces, etc., thin shavings of parafhins of different known
melting points are convenient. Fig. 80 indicates (A) the annual
mean temperature, and (B) the mean temperature of the warmest
month of the year, in different parts of the world.
(3) Precipitation. 'The amount of rain, especially, falling in an
area during the year constitutes a factor of outstanding importance,
as it often mainly determines the availability of water for growth
and other vital processes. ‘To this availability the local vegetation
largely corresponds ; and although the year’s total is apt to be the
10] ENVIRONMENTAL FACTORS 289
feature most important for trees, the season in which it falls may
matter a great deal to herbaceous plants and grasslands. ‘These
last are especially favoured by spring rainfall in regions of cold
winters. With hot and dry summers but winters warm enough
for growth, there may be a preponderance of sclerophyllous shrubs
(7.e. having rather small leathery leaves). With less and less rainfall
there tend to be still more xeromorphic plants (z.e. with features
aiding them to conserve water, as illustrated in Fig. 20, xerophytes
being in general plants which grow in dry places). At the other
extreme are hydrophytes, living more or less submerged in water
(Fig. 21), and hygrophytes, or marsh plants, while between xerophytes
and hydrophytes lie the so-called mesophytes, living in habitats that
usually show neither an excess nor a deficiency of water.
Rain is caused by the cooling of moisture-laden air. Rainfall is
usually reported in the form of monthly means, which are the
amounts falling in the various calendar months but averaged over a
period of years, though the number of rainy days in each month
gives a better indication of its distribution. Moreover, sudden
heavy rain is apt to be largely lost as run-off, and may cause bad
erosion. Because of the often marked local differences with physio-
graphic changes, it is desirable for an ecologist to have his own
automatic rain-gauge which, like his thermograph, needs tending
only once a week.
Other forms of precipitation are snow (which may lie on the
ground to form a valuable protective blanket and also a reservoir
of water, but is apt to limit the growing-season by its late melting),
hail (which may cause serious injury especially to young crops),
sleet, and dew (which is important in some deserts where it pro-
vides much of the surface water on which the ephemeral plants
depend). Fig. 81 indicates the average annual precipitation in
different parts of the world.
(4) Evaporating power. ‘The evaporating power of the air is a
factor of the utmost importance to the life of plants, as it directly
affects their transpiration. It is indicated approximately by the
relative humidity (the ratio of the water-vapour present in the
atmosphere to that necessary for saturation at a particular tempera-
ture), or, more accurately, by the ‘ saturation-deficit ’, which takes
account also of temperature, and determines the ‘ pull’ exerted by
the atmosphere on the water-economy of plants.
The relative humidity is commonly measured by means of a
‘wet and dry bulb’ hygrometer (psychrometer), the difference in
Average annual precipitation in different parts of the world (in centimetres).
Fic. 81.
ENVIRONMENTAL FACTORS 291
temperature of the wet and dry thermometer bulbs giving a measure
of the deficiency of water-vapour below saturation point in the air
tested, while the saturation-deficit can be calculated directly from
these different readings. Although this instrument is customarily
swung through the air to simulate wind and replace any saturated
layers that may form around the wet bulb, from our point of view
it is best kept stationary as plants are. Even the hygrometer gives
readings only at a particular time ; often it is more important to
know the effect over a given period, and this may be measured by
using an ‘atmometer’ (evaporimeter), which indicates the water-
loss by evaporation from a given area of porous pot. ‘This integrates
the water-vapour content of the air with temperature, wind, and the
time-factor, and may be compared with a plant which it is placed
beside. Or, for continuous recording, a tracing may be made by
an automatic hygrograph, such as those using strands of human
hair which is highly sensitive to changes in atmospheric humidity.
Batteries of such instruments will often show marked differences in
different strata of a forest, for example—indicating local variation
which may be of great importance in varying the ‘ pull’ exerted on
different plants, on the same plant at different times, or even on
different parts of the same plant at a particular time.
(5) Wind. Owing to the friction of the soil surface, rocks,
buildings, and above all major physiographic features and masses
of vegetation, winds tend to increase in velocity with height above
the ground. Wind commonly affects other ecological factors in a
given spot—for example, water content and temperature, through
its effect on evaporation—but can also have a direct influence on
vegetation, especially by uprooting trees or by breaking off branches
or other portions. It has a similar, usually drying, effect upon the
soil, or may occasionally act in the opposite direction by bringing
up moister air which reduces transpiration and evaporation, and
may actually lead to deposition or precipitation. Most widely
important to plants, however, is the manner in which wind increases
water-loss, by constantly bringing unsaturated air into contact with
leaves and young shoots. Mechanically, wind can also cause
erosion of soil and abrasion of vegetation through carriage of
particles, and physiologically it can decrease growth by replacing
damp by dry air, and consequently increasing the transpiration and
reducing the turgor of organs on which it impinges. ‘This explains
the frequent growth of trees and shrubs chiefly away from the
direction of the prevailing wind in exposed situations (cf. Fig. 82, A),
292 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
and their restriction to tangled dwarfs in sheltered depressions
(Fig. 82, B). In strong dry winds young parts of plants may even
become shrivelled and killed in a few hours, and surface soil may
become dried out. Such effects may be observed in the warm foehn
Fic. 82.—Effect of wind on trees. A, deformed outliers of Black Spruce (Picea
mariana) near Churchill, Hudson Bay. The prevailing winds are from the left.
Note in foreground the luxuriant prairie-like tundra of the arctic-subarctic ecotone.
"he lower branches of such Spruces tend to form a dense layered mat that is well
protected from desiccation in winter by drifted snow, the farthest outliers (on
left) often being limited to such growths. Upper branches (on right) ‘ trail
away to leeward. B, deciduous broad-leafed and evergreen coniferous tree
species forming a dwarfed tangle in sheltered depressions on exposed hill-side on
coast near North Berwick, Scotland. "The Grasses and other herbs reach the
height of the gnarled ‘ trees ’ which scarcely anywhere exceed the level of the sur-
rounding terrain.
10] ENVIRONMENTAL FACTORS 293
or chinook winds that, sweeping down from mountains, can raise
the temperature of the air locally by as much as 30° C. in a very
short time. For such reasons a good deal can be inferred about
the wind-climate of a habitat by direct observation of the vegetation,
etc. Wind velocity is measured by an anemometer, but its effect
is included in observations obtained from stationary hygrometers
and atmometers.
As we saw in Chapter IV, wind is an important agent of dispersal.
It may also be of significance phytogeographically in determining
the local distribution of species or communities of plants, some
types being markedly more wind-resistant than others, and many
being unable to flourish or even exist in exposed situations. ‘Thus
the tree-limit on the sides of mountains is apt to be due largely to
winds, as may be seen by the frequent persistence higher up of
groups of trees in sheltered pockets; such trees further reduce
exposure very locally and enable tender herbs to grow in their
company.
Ocean currents, such as the warm Gulf Stream or the cold East
Greenland Current, may have a considerable effect on temperature
both locally and on land at a distance—especially when the winds
are predominantly on-shore. By bringing in fresh materials, such
currents may also alter the conditions with regard to nutrient salts,
etc. Moreover, as we saw in Chapter IV, water currents can be
an important aid in plant dispersal.
PHYSIOGRAPHIC
The physiographic factors are those introduced by the structure,
conformity, and behaviour of the earth’s surface—e.g. by topographic
features such as elevation and slope, by geodynamic processes such
as silting and erosion, and consequently by local geology. Other
causes of physiographic change from place to place include the
blowing of sand or dust which in time or special circumstances can
assume vast proportions. It is in such connections that the various
landforms described in Chapter XVII tend to be most significant
to students of plant geography and ecology.
Physiographic factors act on local vegetation largely through
climatic or edaphic features which they engender. ‘They are
consequently sometimes classed with these other groups. Yet,
ecological factors being so widely interdependent in any case, it
seems more conducive to clear understanding to consider the
294 INTRODUCTION TO PLANT GEOGRAPHY
physiographic ones separately—especially as they are well marked
in their effect on vegetation in regions of drastic topography and
harsh climate. Strong topographical relief tends to produce marked
local climates, summits for example being very different in these
respects from sides of mountains, and narrow valleys from open
plains. Quite apart from the tendency to greater windiness and
exposure at higher altitudes, the air and soil temperatures tend to
get lower and the relative humidity greater as we ascend, with
atmospheric pressure decreasing and heat-radiation increasing in
intensity. Altogether, climatic variation becomes more and more
extreme and rapid with increasing altitude.
As an example of physiographic effects in arid regions, we may
rise from unproductive plains to fertile slopes and forests on moun-
tain sides, and at very high altitudes reach again an unproductive
zone of low absolute humidity and rigorous exposure. ‘The changes
are due largely to local climate but would not take place if it were
not for the physiography. Moreover, major topographic features
often affect the climate at a considerable distance—as, for example,
mountain ranges which may cause rainfall locally and decrease it
in their lee—while in Chapter IV we saw how such features, and
wide expanses of water, can act as barriers to plant migration.
Another important physiographic effect is aspect : in the northern
hemisphere, north-facing slopes tend to be more hygrophilous
(adjusted to moist conditions) than south-facing ones at similar
altitudes (Fig. 83, A). ‘This is owing to the effect of insolation on
air and soil temperatures, and consequently on relative humidity
and evaporation and, through them, on the local water situation
(even when precipitation is the same). For vegetational differences
due to topography are most often correlated with moisture, and
naturally tend to be marked chiefly where water is deficient and
consequently is a critical factor. Sometimes for this reason there
may be entirely different vegetations and even floras on the two
sides of a deep valley or steep mountain (Fig. 83, B), as in the dry
Mediterranean region, while if similar zones are developed they
tend to be at higher altitudes on the south- than on the north-facing
slope. ‘The western slopes of mountains may be noticeably warmer
and drier than the eastern ones, owing to the sun’s afternoon warmth.
Quite drastic effects may often be seen already on a microclimatic
scale, for example in the shelter of rocks or even stones on exposed
sea-cliffs and mountain summits. Here we may refer again to
Fig. 79, which shows a striking aspect effect in the Arctic.
B
Fic. 83.—Aspect effects in Colorado and Nepal. A, Colorado, U.S.A.: the
relatively damp and cool north-facing slope (on left) is covered by a stabilized
forest of Douglas Fir, whereas the south-facing slope supports mainly dry Oak
scrub and scattered Ponderosa Pines. (Phot. G. E. Nichols. By permission
from Plant Ecology, by Weaver & Clements, copyright date 1938, McGraw-Hill
Book Co.) B, Jumla, West Nepal, c. 3048 metres (10,000 feet) in the Himalayas:
the north-facing slope (on left) is forested with Abies spectabilis and Betula utilis,
and, lower down, some Pinus wallichiana, whereas the dry and sunny south-
facing slope supports chiefly herbs and Grasses, with some scattered low Juniperus
bushes. (Phot. O. Polunin.)
295
296 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
Important in itself is the steepness of a slope, for it largely deter-
mines the stability of the surface and retention of water, and also
the effect of aspect or exposure—especially in the higher latitudes.
Thus in the northern hemisphere a steep south-facing slope will
receive the strong midday sun’s rays more or less perpendicularly,
while a steep north-facing slope may receive only oblique and weak
morning and evening rays, or perhaps none at all. ‘These differences
often have a marked effect, especially on the water and temperature
conditions in the two places, and consequently on the vegetation.
For besides the intensity, the duration, quality, and quantity of
incident light are at the same time affected.
Slope can also greatly affect the character as well as the amount
of soil which accumulates. This, like the nature of the underlying
rock, is often reckoned as an edaphic factor ; but in so far as either
determines or results in topographic change, it is to be considered
as physiographic. Different textures and types of rocks will produce
different topographies which bring about local climatic differences
that are physiographically engendered. Such differences may also
affect the water conditions, including the level below which the
ground is waterlogged or frozen, and so again drastically affect
the habitat.
Geodynamic agencies are particularly active in mountain districts
and about coasts, causing all manner of changes in topography—
sometimes almost from day to day. Steep slopes and river banks
are constantly being eroded, the material sliding down as talus, etc.,
or being washed down and deposited elsewhere. Such activities
often cause the formation of new ‘open’ habitats, both in the
places whence the eroded material came and in the areas in which
it comes to rest. Frosts may help disintegration through their
heaving, splitting, and other erosive tendencies, while avalanches
often clear off the surface materials from considerable areas. And
even as mountain slopes change their surfaces, and river-beds alter
their outlines and vary courses, so do sea-coasts and cliffs vary in
their conformation, erosion being widely at work. In other places
there are silting salt-marshes or gradually moving sand-dunes or
shingle-banks—to mention only a few of the geodynamic sources of
physiographic change.
EDAPHIC
The edaphic factors are those which are dependent on the soil
as such—on its constitution, water and air content, inhabiting
10] ENVIRONMENTAL FACTORS 297
4
organisms, and so forth. We have seen how climatic factors, such
as temperature and precipitation, are of supreme importance in
determining the general character of the vegetation over wide areas ;
here and in the next section we shall deal with the edaphic and
biotic factors, respectively, which are apt locally to modify the
conditions and vegetations of these major climatic belts. Especially
has it long been recognized, and utilized in agricultural, horticultural,
and forestral practice, that differences in the soil are often largely
responsible for differences in vegetation within the same climatic
region: consequently they are of great significance in plant
geography.
Soil may be considered as the unconsolidated superficial material
of the earth’s crust, lying below any aerial vegetation and undecom-
posed litter, and extending down to the limits to which it affects
the plants growing about its surface. Beneath the soil lies the
subsoil or unaltered rock. ‘Though usually composed primarily of
material derived from the parent rock, the soil has come into being
largely through interaction of this ‘ substratum’ with climate and
living organisms. ‘Thus its texture may be dependent largely on
water- and frost-action and other ‘ weathering ’ tendencies, while its
content of humus (partially decomposed organic matter) results from
the contributions and activities of inhabiting plants and animals.
For soil is a veritable ‘ microcosm’ or little world—with its own
physical structure, chemical composition, atmosphere, flora, and
fauna. And characteristically it exhibits a perpetual series of actions
and reactions between organisms and environment. In it about
one-third (by volume) of the bodies of higher plants spend their
lives—influencing, and being influenced by, the particular conditions
obtaining in the soil. These conditions, through the subterranean
organs of plants, in turn influence their living aerial parts.
Soils that are undisturbed by agriculture and other factors com-
monly become stratified into layers or ‘horizons’ at different
depths, which often have very different compositions as well as
natures. Such ‘ profiles’ are, however, largely destroyed by cultiva-
tion. Normally, three main horizons or groups of horizons are
exhibited, namely, the upper or ‘ A’ zone of extraction of soluble
salts and fine-grained materials, the middle or ‘ B’ zone of their
concentration, and, below, the ‘C’ zone where neither extraction
nor accumulation has occurred at all extensively. ‘The characteristic
profiles so formed are toa considerable extent climatically engendered,
and so mature soils can be classified broadly into climatic types or
298 INTRODUCTION TO PLANT GEOGRAPTIY
‘world groups’, such as podzols (developed chiefly in cool regions
of high precipitation relatively to evaporation), brown earths (with
generally lower rainfall and higher temperatures), chernozems (with
low rainfall in continental regions), prairie soils (with higher rainfall),
chestnut-brown soils (in warmer and drier places), /aterites (with high
rainfall in the tropics), red loams (with lower rainfall in warm-
temperate regions), tundra soils (of polar regions where the subsoil
remains frozen and organic decomposition is retarded), and so on.
Fig. 84 shows three characteristic soil profiles and Fig. 85 indicates
the distribution of the primary soil groups of the world.
As we shall see towards the end of the next chapter when dealing
with plant succession, soil development and vegetational develop-
ment are intimately connected, both being largely controlled by
climate. Meanwhile the essential constituents of most soils may
conveniently be treated in five categories :
(1) Mineral fragments of various sizes resulting from the dis-
integration of rocky materials by physical and chemical weathering ;
the parent material may be either local or otherwise, as may be the
weathering. ‘These mineral constituents form the inorganic frame-
work and differ widely according to the physical and chemical
nature of the parent material. ‘They affect plants particularly by
bringing about variations in soil water and aeration, as water reten-
tion is much affected by mechanical composition (the relative
proportions of different-sized mineral particles present). ‘The
mineral constituents may also affect the composition of the soil
water in important ways (see below).
Soils are mechanically analyzed by separating into ‘ fractions ’ the
particles whose sizes lie within definite arbitrary limits, ranging from
larger stones and gravel (more than 2 mm. in diameter) down
through various sizes of sand and silt to clay (less than -oo2 mm.
in diameter). ‘The majority of mature soils consist largely of silica
and silicates which are relatively insoluble and form a more or less
permanent basis, any calcareous material tending to become dis-
solved (‘leached’) out of the surface layers and many of the finer
insoluble particles being carried down mechanically to lower levels
(this process is termed ‘eluviation’). Particularly important in
leaching and chemical weathering is the acid-forming carbon dioxide
dissolved in soil water. ‘The particles forming the soil’s inorganic
framework tend to be coated with colloidal material of very fine
clay or of organic origin, which may help cement them into com-
pound particles or grains and increase the “ crumb structure ’ of the
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ENVIRONMENTAL FACTORS 301
soil and its ability to hold water and salts. Sandy soils are porous,
‘light’ to work, easily penetrated by roots, and they dry readily ;
in contrast, clayey soils are retentive of water, heavy, poorly aerated
and sticky when wet, and hard when dry. For most working and
plant-growing purposes, mixed ‘loams’ are best.
(2) Soil water containing dissolved substances is also of funda-
mental importance, being commonly the chief source of water for
plants. Water is of course essential to plants as usually their main
constituent by weight, as the medium for physical and chemical
changes, and because large quantities must be absorbed to cover
the continual loss by transpiration from their surfaces. Except in
extremely dry soils and below the level of permanent ground-water,
the soil water mainly forms films around the component particles
of the soil. ‘The amount of such water and thickness of the film
depends on such factors as the soil’s mechanical constitution, on the
recency of precipitation and tendency to run-off, on subsequent
weather conditions, on humus content, on the covering of vegetation
and litter, etc., and on the effect of this covering on water loss
by transpiration and evaporation. Especially does water percolate
through and evaporate from coarse gravelly or sandy soils, and remain
suspended in fine clayey and humous ones with their high capillary
action and immense aggregate surface of microscopic or colloidal
particles. However, in such retentive soils much of the water may
be so strongly held that it cannot be abstracted and used by the plants,
so that it is often necessary to distinguish between water which is
available to plants (the so-called chresard, above the point of per-
manent wilting, though this may vary somewhat with different
species) and that which is so strongly held as to be unavailable
(echard). ‘The entire water content of the soil may conveniently
be termed the ‘ holard’. Water tends to promote the stratification
of soils, and the soil’s content of available water is often the chief
factor causing local differences between plant communities.
Another important factor affecting vegetation is the soil water's
content of dissolved inorganic salts, etc., which are derived from
the mineral matter present and from organic breakdown. Certain
of these salts’ constituent elements are essential to the plants’ con-
tinued well-being and even to their life, while others are apt to be
obnoxious or actually poisonous. Outstanding are the extremely
saline soils which are inhabited only by specially adapted plants
(halophytes). In these connections the various needs and abilities
of different plants lead to correlations with the chemical content of
L
302 INTRODUCTION TO PLANT GEOGRAPHY | CHAP.
the soil water, which frequently constitutes a determining factor in
plant geography. Examples occur in the cases of plants which seem
to require ‘lime’ (calcicoles or calciphytes) and those which appear to
avoid it (calcifuges or oxylophytes), though frequently such prefer-
ences are bound up rather with questions of basicity or acidity,
respectively. For the soil * reaction’ (hydrogen-ion concentration)
also can determine the presence or absence of particular species
locally.
Thus can the varying tolerances of many plant species, etc., to
different environmental factors come into play as_ particularly
important phytogeographically, though it should be remembered
that away from their optimum sphere plants tend to be more and
more susceptible to competition and other influences, and hence to
show less and less ability to persist. Consequently it is especially
towards the limits of their ranges that they are liable to be most
narrowly restricted to special habitats involving particular environ-
mental conditions.
(3) Soil atmosphere mainly occupies the interstices between the
soil particles or crumbs, with their covering films of water. It
tends to contain a slightly lower proportion of oxygen and a much
higher one of carbon dioxide than ordinary air, and to be normally
saturated with water-vapour. ‘This may not be the case in the
surface layers of very dry soils, while at the other extreme water-
logged ones are liable to be deficient in oxygen, their lack of aeration
making them unsuitable for most forms of plant and animal life.
Normally, plentiful oxygen in the soil is necessary for the life of
most of the microorganisms and other inhabitants and for the
respiration of the underground parts of higher plants growing in
it, the differences in composition from the free air being reflective
of the gaseous exchanges involved, namely, absorption of oxygen
and giving out of carbon dioxide. However, interchange with
the free air by diffusion and other agencies appears to be fairly
rapid.
(4) Organic matter, arising from the death of plants or parts of
plants, or of animals or added as manure, forms another highly
important constituent of most soils. Included are roots that decom-
pose im situ. Indeed all soils which bear vegetation (and according
to some authorities all true soils) contain dead organic matter,
usually more or less broken down to humus, though the amount
may vary from very little in fresh ‘ new’ soils to virtually 100 per
cent. of dry weight in peat and leaf-mould. In the dissemination
10] ENVIRONMENTAL FACTORS 303
and breakdown of the dead plant and other remains, Earthworms
may be important in dragging down and partly digesting the material.
After this, soil Fungi and Bacteria cause further disintegration and
ultimately decomposition into more or less simple salts (such as
ones containing nitrogen and phosphorus which are essential to
plants), carbon dioxide, and water. These are the fundamental
plant foods, and the organic matter is the chief seat of the activities
of the microorganisms liberating (and sometimes producing) them.
Such decomposition and disappearance takes place relatively rapidly
in warm, moist, and well-aerated soils, these conditions being
favourable to the activity of the ‘ scavenging’ organisms. On the
other hand, in cold and wet soils that are poor in salts and acidic
in reaction, the debris may long remain on the surface as scarcely
decomposed ‘raw humus’ (mor).
Ordinary humus, having become structurally unrecognizable and
largely colloidal, improves both heavy clay and light sandy soils,
lightening the former and giving consistency and water-holding
capacity to the latter, as well as adding plant food in each case. The
“reaction ’ of a soil—its degree of acidity or basicity—is also import-
ant to many plants and may affect their distribution, as particular
species are apt to show a distinct preference for soils whose reaction
lies within certain limits. ‘This reaction again is largely bound up
with humus content, humus being the main source of acidity in
soils.
(5) Living organisms, together with the roots and other under-
ground parts of aerial plants, comprise the other widely essential
constituent of the soil. ‘They include, as we have already seen,
the mainly microscopic soil flora and fauna whose life is usually
centred on the soil’s humus content and which are often very
sensitive to changes in the conditions of their limited environment.
They also include the main mixing and ‘ scavenging’ animals and
above all saprophytic Fungi and Bacteria, etc., which are so vitally
important in the maintenance of ecological balance and indeed of
life in the world. For higher plants, as we have already noted,
these organisms make available various essential food substances,
including nitrogen in usable form.
In highly acid soils, Fungi largely replace the decomposing and
other useful Bacteria—such replacement in itself involving significant
phytogeographical changes. Indeed these soil communities and
their component organisms have their distributions and other
geographical implications in much the same manner as higher types.
304 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
Besides various essential or beneficial substances including growth-
stimulating ones, harmful toxins may be produced in the soil by
living organisms or parts of organisms—with corresponding effects
on plant distribution. And then there are various mycorrhizal
associations, Algae, and other units in the microcosm, all of which
may have their importance to the vegetation and to plant geography,
though quite how is often not well understood. Nor should we
forget the importance of soil temperature, which can act in so many
and various ways—as, for example, through the soil’s living content.
BIOTIC
The biotic factors in the wide sense are those due to living
organisms, whether animal or plant—ranging, as it were, from Man
and the great herbivores and trees down to the lowly but often
vitally important soil microorganisms with which we have just dealt.
It is useful to visualize the total components of an immediate
environment or recognizable habitat as forming a self-contained
ecosystem, composed on one hand of the inorganic and dead parts
and, on the other, of the various organisms which live together in
it as a sociological unit and comprise the biota. A large, primary
biotic community in which the climax vegetation (see next chapter)
is more or less uniform is termed a biome. Our interest is primarily
in the living components, the inert ones being from our point of
view mere factors conditioning the existence, structure, and develop-
ment of the biome—chiefly through their effect on one or more of
its component organisms. ‘These are the individual species or other
taxa, divisible approximately into animals and plants. ‘The latter
form the plant community, which may be loosely defined as an
entire population of plants growing together and maintaining as a
whole a corporate individuality that is not the same as the sum
total of the separate manifestations and effects of its components.
The component plants of a community have many immediate
internal (‘ autogenic ’) effects upon one another and upon their own
habitat, as for example in competition and the deposition of humus,
and by producing various changes in the soil; they even have
‘ allogenic’ effects in sheltering other plants and in dispersing them-
selves outside the immediate community. It is convenient, however,
to treat these plant-engendered repercussions under succession (see
next chapter) and to regard separately as introducing the (collective)
biotic factor chiefly those animals which have a marked effect upon
10] ENVIRONMENTAL FACTORS 305
the community or any part thereof. ‘These are largely external in
origin. For practical purposes such factors manifest themselves in
the totality of direct and indirect effects of animals on plants. It
is, however, also convenient to include with the animals such plants
as the lowly scavengers and agents of chemical change, and, in
addition, those which are damagingly parasitic. Also mentionable
here are ‘ insectivorous ’ (carnivorous) plants, climbers, epiphytes,
and mycorrhizas—which last, like Lichens and other symbioses, are
concerned with the existence of more than one species at a point.
Apart from changes engendered by the main or subsidiary com-
ponent plants themselves, organisms of many other sorts may affect
a plant community in numerous and diverse ways. ‘There are the
soil Bacteria, causing all manner of important chemical changes ;
the Protozoa, which devour the Bacteria; the Earthworms which
help disintegrate organic matter and aerate the soil; the Fungi
which carry this breakdown further ; the Snails which eat plants ;
and the browsing Mammals which may have the most profound
effect—for example in turning forest into treeless pasture. Other
important effects may be produced by Man and his domestic animals
and fires, by the Beavers which fell trees and turn valleys into lakes,
by the caterpillars and Locusts which may devastate whole areas, by
the Insects and Birds which pollinate flowers and carry diseases,
by the various animals which are so important in dispersing seeds
and other disseminules, and by the parasitic Fungi, etc., which in
extreme cases may even kill the dominant plant and change the
whole aspect locally.
Parasitic lower plants (or sometimes invertebrate animals or
vascular plants) are, as we have already seen, very important to
crops whose mode of growth often encourages them or their ‘ carry-
ing’ insects. Less known is their effect on ‘ natural’ vegetation ;
seemingly it tends to be less drastic as equilibria are approached,
though we still do not know what organisms may be kept out by
potential predators, or perhaps so weakened in competition as to
fail to become established. Certainly the death-rate of seedlings
tends to be very high. If we had not plentiful records and even
recollections of the Sweet Chestnut forests of eastern North America,
how could we know that they had been devastated by blight and
that their absence nowadays is not due to some inimical climatic
or other factor ? And is it not possible that some other plants whose
remains tell us that they once flourished in areas where they do not
now grow, were ousted by parasites or other biotic impress rather
306 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
than by the climatic vagaries which are indubitably the usual cause
of such major distributional change or extinction? ‘The fan-shaped
and ‘ feathery ’’ Elms of New and Old England, respectively, which
are such an important feature of the landscape of those two delightful
parts of the world, are threatened by the so-called Dutch Elm disease
which has already considerably changed the local environment in
many areas. ‘lhe White Pine Blister-rust is apt to kill the dominant
species of some of the most widespread and important forests of
North America. These are but a very few instances of devastating
parasitic attacks on natural or semi-natural vegetation.
The small herbivores include Snails, Slugs, Locusts, and the larval
stages of Insects such as caterpillars (the larvae of Butterflies and
Moths). ‘These often cause considerable local damage and even
devastation—particularly to individual crops but sometimes to
native species or whole tracts of vegetation.' But their exclusion
even for experimental purposes is often difficult, taxing the ingenuity
of the investigator. ‘The effects of the smaller vertebrate animals,
such as Voles and Mice, tend to be most conspicuous where Man
has upset the balance of nature by destroying their enemies, such
as Hawks and Foxes. Rabbits may be especially troublesome, often
converting heaths or even forest into grassland by their gnawing and
prevention of regeneration of such dominant trees as Beech on chalk
in England. ‘This is easily demonstrated experimentally by effec-
tively wiring off areas (the netting must be sunk some inches in
the ground) which soon come to exhibit the early stages of forest
regeneration, whereas the pastured surrounding tracts remain close-
cropped and grassy. Care must be taken also to exclude small
Rodents and Birds which can destroy tree seedlings, for example,
by nipping off the tender young shoots. It will be interesting in
due course to observe the effect on the local vegetation of the virtual
(if only temporary ?) extermination of Rabbits by myxomatosis in
several European countries.
Of large browsing Mammals such as Buffaloes, which used to
inhabit the great grassy plains, the natural populations are now
widely destroyed. ‘They were probably important in the mainten-
ance of the grasslands, at least in the damper areas where trees are
able to grow, but are nowadays largely replaced by domestic herds,
‘It has even been suggested that a plague of caterpillars may have been a
leading cause of the dying out of the ancient Norse settlements in West Greenland
—by devouring the vegetation and hence starving the Sheep on which the Norse-
men were largely dependent.
10] ENVIRONMENTAL FACTORS 307
which have a similar effect upon the vegetation. ‘Thus the forest
generally tends to advance on the grassland with the removal of the
heavy pasturing which ordinarily keeps it in check. Commonly,
browsing cattle kill all the young trees, so preventing forest regenera-
tion, and by their trampling, grazing, and other activities lead to
the replacement of the characteristic forest-floor litter and vegeta-
tion by Grasses (Fig. 86). For the Grasses are hemicryptophytes,
having their buds protected within (or at least lying at) the surface
of the soil, and far from being killed by grazing, may actually be
stimulated by it. Certainly they are encouraged by the removal of
their broad-leafed and woody competitors, so that they will normally
extend their area if the factor or factors suppressing these competitors
are increased in intensity. Overgrazing may, however, lead to
replacement of the Grasses by unpalatable herbs such as the weeds
of open soils—perhaps followed by erosion if the rainfall is heavy,
so that a barren waste may result (Fig. 87).
These last biotic effects are commonly regulated by Man and tend
to be destructive, often getting far out of hand, as in bad cases of
erosion. However, animals are still often helpful to vegetation—
for example in distributing seeds, fruits, and spores, etc., in effecting
pollination, in loosening or compacting soil as well as in manuring
it, in trampling seeds into the soil (for example, of range Grasses)
and favouring regeneration, in keeping down injurious Rodents etc.
(in the case of carnivorous Mammals and predatory Birds), and,
in the instance of Man, in such activities as irrigation, the construc-
tion of wind-breaks, soil-improvement and cultivation of many sorts,
and the transplantation of useful plants and even the creation of
new ones.
Human activity is, indeed, the outstanding biotic factor in the
world today, at least if we include consideration of Man’s domestic
animals. Especially is Man the enemy of forests, whether he
realizes it or not. His ‘shifting cultivation’ in the tropics is
particularly damaging to vegetation, trees being girdled and the
forest burnt, after which the accumulated fertility of the soil is
exhausted in a very few years by cultivation; thereupon a move
is made, similarly to cultivate and desecrate a fresh area. Con-
sequently some secondary and usually inferior type of forest now
replaces the original one over vast areas. In other cases still more
devastating erosion may result from forest clearance as indicated
above, from over-pasturing, or from ‘ exsiccation ’ leading to exten-
sion of desert areas. In such instances of penetrating and serious
:
P4
Fic. 86.—Some important effects of grazing. A, Sugar Maple—Beech forest in
central Indiana, showing plentiful regeneration and litter; B, similar forest type
in same region but subjected to heavy grazing by cattle. In the latter instance
Grasses and a few unpalatable herbs have taken the place of the usual plants and
litter of the forest floor, while tree regeneration has been eliminated so that the
forest’s life is jeopardized. (Reprinted with permission from R. F. Daubenmire,
Plants and Environment, copyright date 1947, John Wiley & Sons, Inc.)
308
ENVIRONMENTAL FACTORS 309
disturbance, return to anything like the original vegetation is
problematical. Appropriate measures following proper study are,
however, nowadays leading to more and more effective conservation
—for example of vegetation with the object of maintaining water
supplies, of ranges against erosion, and of forests against fire.
Elsewhere by irrigation or drainage, damming, building, persistent
mowing, road and railway construction, mining and all manner of
other enterprises, Man alters the water and other conditions over
Fic. 87.—Devastating results of overgrazing. Rill, gully, and sheet erosion on
overgrazed slope in San Joaquin Valley, California. (Courtesy of U.S. Forest
Service.)
vast areas—as he sometimes does the biological balance by the
introduction or extermination of various animals and weeds or other
plants.
In many regions fire is an important factor which, because it is
nowadays usually caused and controlled by Man, seems best con-
sidered here. ‘The effect is rather like grazing or cutting in that
material is removed and if only small areas are affected they generally
return to something like the former state—especially if the fire is
a surface one that does not kill the larger thick-barked trees. How-
ever, big and severe ‘crown’ fires may destroy the humus and
disseminules as well as most or all plants, and result in a community
of colonists followed by a distinct ‘ burn succession’ of vegetation.
310 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
Repeated burnings usually change the dominants and entire com-
munities to ones which can regenerate after fires and, so to speak,
withstand them. <A few species, for example among the Grasses,
seem actually to be stimulated by exposure to fire, while some
Conifers, besides possessing thick and resistant bark, have cones
which are opened by fire to liberate the ripe seeds earlier than would
otherwise be the case.
Although it may leave survivors open to fungous and other attack
through unhealed burn-scars, fire tends to favour the more resistant
species by removing their less resistant competitors ; it also alters
many factors of the environment that favour some species rather
than others. ‘These principles apply to most types of vegetation,
including forests, heaths, and grasslands. And whereas the general
tendency of fire is towards vegetational degradation, the result is of
course influenced by climate and human agency quite apart from
the frequency and intensity of the burning. Indeed there are some
circumstances in which firing will benefit desirable species and
consequently may be intentionally practised by Man, as in pastures
in many parts of the world where the old growth is burned off
regularly.
In conclusion we should outline the principles of cultivation,
through which Man now largely controls the plant life of much of
the land surface of the world and of some shallow waters. Cultiva-
tion consists basically in preparing the surfaces of suitable substrata
for the reception of seed (using this term in the widest sense),
normally after the area has been cleared of any natural vegetation.
Accordingly the early stages of growth of the crop are favoured by
a minimum of competition, although this may in time develop with
weeds or between the plants of the crop. Consequently effective
cultivation will include proper control of weeds and also sowing at
such intervals of space as will reduce the ill-effect of competition
to a minimum. Effort may also have to be given to maintaining
other conditions favouring the growth of the crop.
In order to continue satisfactory cropping year after year in the
same area, it is commonly necessary to repeat for each crop such
a ‘ tillage’ operation as digging or ploughing, which turns over the
soil and helps to maintain it in a suitable state. Lime or peat or
clay may also be added to some soils to improve their texture.
Often there has to be also some form of manuring, from animal or
mineral or chemical sources, to maintain the fertility of the soil by
replacing the necessary substances which are removed in cropping.
10| ENVIRONMENTAL FACTORS 311
Moreover, in such arid lands as Mesopotamia, whence western
civilization apparently sprang, successful cultivation also involves
the artificial supply of water by irrigation. Conversely in many
excessively humid or waterlogged areas, drainage is necessary before
most crops can be sown.
As a particular crop will remove more and more of the same (often
necessary) substances from the soil, and perhaps add more and more
of the same undesirable ones, it is common either to leave land
fallow (7.e. cropless) for one year out of every two to four, or else
to practise a ‘ rotation’ of different crops grown successively on an
area. In such cases more or different weeds will be fostered, with
obvious plant-geographical implications. Indeed the practices of
and incidental to cultivation, such as removing natural or semi-
natural vegetation, establishment (however temporarily) of artificial
vegetation in the form of crops, the introduction of weeds and diseases
whether they are controlled or not, and the opening up of fresh
areas for plant colonization and succession, have a continuous and
very widespread effect on the distribution and luxuriance of flora
and vegetation in the world, and, accordingly, on local landscape
and amenities. With Man’s predominant position in modern times,
his biotic or, more precisely, ‘ anthropic’ influence has become
widely overwhelming.
FURTHER CONSIDERATION
Three small introductory books, treating iter alia the factors of the
environment, are :
Sir A. G. TansLey. Introduction to Plant Ecology (Allen & Unwin,
London, pp. 1-260, 1947).
Witiiam Leacu. Plant Ecology, fourth edition (Methuen, London,
pp. vil + 106, 1956).
H. Drassie. Plant Ecology (Arnold, London, pp. 1-142, 1937).
Further useful treatments, which are mostly more detailed but by no
means uniform in their groupings and terminology, include :
A. G. Tansey & T. F. Curpp. Aims and Methods in the Study of
Vegetation (Crown Agents for the Colonies, London, pp. xvi ++ 383,
1926).
lp aoe Plant Sociology (McGraw-Hill, New York &
London, translated, etc., pp. xvili + 439, 1932).
J. E. Weaver & F. E. CLements. Plant Ecology, second edition (McGraw-
Hill, New York & London, pp. xxii + 601, 1938).
312 INTRODUCTION TO PLANT GEOGRAPHY
R. F. DauBenmire. Plants and Environment (Wiley, New York, pp.
Xill + 424, 1947).
H. J. Oostinc. The Study of Plant Communities, second edition (Free-
man, San Francisco, Calif., pp. vill + 440, 1956).
W. B. McDouca.t. Plant Ecology, fourth edition (Kimpton, London,
Pp- 1-234, 1949). ;
Most of the above books give references to appropriate specialist ones,
e.g. on soils.
The following may be found pertinent in particular connections :
V. J. Cuapman. An Introduction to the Study of Algae (Cambridge
University Press, Cambridge, Eng., pp. x + 387, 1941). Deals
inter alia with the conditions in fresh and salt waters—see also our
Chapters XV and XVI.
V. J. Cuapman. Salt Marshes and Salt Deserts of the World (Leonard
Hill, London, pp. xvi + 352 + index, in Press). Describes the often
peculiar conditions introduced by excessive salinity.
H. U. Sverprup, M. W. Jounson, & R. H. FLeminc. The Oceans :
their Physics, Chemistry, and General Biology (Prentice-Hall, New
York, pp. x + 1087, 1946).
Useful recent works on the general ecology of plants and animals
considered together include :
E. P. Opum. Fundamentals of Ecology (Saunders, Philadelphia & London,
pp. Xi + 384, 1953).
GeorGE L. CrLarKe. Elements of Ecology (Wiley, New York, pp. xiv
+ 534, 1954):
A. M. Woopsury. Principles of General Ecology (Blakiston, New York
& Toronto, pp. vill + 503, 1954).
CHAPTER XI
Wie NAb Ar Ss. owe CES SlO NS,
AON Dis@E PVEIAXCES
Whereas originally the term ‘ habitat ’ referred simply to the place
(locality or station) in which an organism or community lived,
ecologists nowadays take it to mean rather the kind of place, involving
the sum of effective conditions (operative influences) characterizing a
particular type of area or inhabited by a particular species or com-
munity. ‘Thus as a scientific term it involves all the conditions
affecting an individual or community that are incidental to the place
in which that individual or community lives, and we have to dis-
tinguish between the general habitat of a community and the partial
habitats of its component species, etc. These partial habitats may
vary greatly within the orbit of a single example of a single general
habitat. ‘Thus with a Beech forest growing on a calcareous soil
under certain climatic conditions, there may be rather similar general
factors of soil and climate under which the dominant trees flourish
in different spots within the same area or even in different areas in
the same region. Very different may be, for example, the * micro-
habitats ’ of Mosses or Algae growing on their boles or branches, and
of herbs upon the forest floor. In the present treatment of the main
different types of plant habitats that are discernible, we shall have to
confine ourselves largely to general terms in showing how the factors
of the environment dealt with in the last chapter constitute the
habitats of the various types of vegetation to be treated in the next
five chapters.
The habitat is thus made up of the many and various environmental
factors having any kind of influence upon life within it, and them-
selves interacting complicatedly. ‘The sum of effective ecological
conditions has many widely different manifestations which range
for instance from dry land to open water : and indeed water con-
ditions seem to provide the best criterion for the primary subdivision
of habitats. But even as most biological pigeon-holing is notoriously
imprecise, involving many an arbitrary cut-off across a more or less
complete line of gradation, so is it with any of the main types of
313
314 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
habitat that seem recognizable as entities. Within each of these
categories are almost endless variations : for practically any change
of one or more of the various ecological factors dealt with in the last
chapter can cause marked variation in the habitat—leading in turn
to differences in the vegetation and, incidentally, often adding to
the vast assemblage of habitats on the face of the earth or in salt or
fresh waters.
Perhaps the usual identity of vegetation with habitat, and the ease
with which the former is classified at least into such broad categories
as forest, scrub, grassland, etc., is in itself a reason why no satis-
factory over-all classification or even inventory of habitats is available.
For our present purposes, however, we must attempt some general
enumeration of the main types of habitat. But owing to the diversity
and variability of the factors involved, their numerous possible
combinations, and the frequent overlapping, a clear and unequivocal
delimitation of habitats according to operative factors is scarcely
attainable. Except for broad outlines, such as those given next
below, we have to go to the plant communities and their component
species to obtain satisfactory criteria, and then find ourselves dealing
with vegetation rather than with the habitats which support it.
"TERRESTRIAL HABITATS
Even as these are separated from the other main category, aquatic
habitats, by water conditions, so are their main subdivisions often
dependent upon the relative availability of water locally. Leaving
aside as largely aquatic the sea- and lake-marginal ones whose in-
habitants all spend at least a substantial part of their time more than
half submerged, we nevertheless have the marginal habitats of the
sea-shore that are characterized by salt spray or occasional inundation,
and of lakesides and streams where the bed is so shallow and sheltered
that ‘ reed-swamp ” types can grow up in such a manner that most
of their bodies are aerial. In addition there are the various tropical
swamps and mangroves. Also abundantly supplied with water (but
in these cases not normally covered by it) are silty marshes (whether
salt or otherwise), base-rich fens, acidic bogs, and the gently sloping
and usually mossy ‘ mires’ through which water percolates.
Of less aqueous land habitats there are many, often of very various
types. Beginning with the ones which are reasonably moist and flat
or nearly so, we may mention those of a wide range of soils in regions
favourable for cultivation, and which still comprise habitats even
11] MAIN HABITATS, SUCCESSIONS, AND CLIMAXES 315
when they are prepared for, and further altered by, various forms of
husbandry. Here all manner of changes in climatic and edaphic
conditions from place to place give rise to a great variety of different
habitats—usually supporting different types of forest if undis-
turbed, or of arable or pasture land if used for agriculture. More-
over, in these agriculturally favourable regions more than most others,
Man is constantly changing old habitats or opening up new ones.
There is no need to dwell upon examples, which are familiar in
almost all areas ranging from the Subarctic to the tropics. With
drastic reduction in rainfall, the resulting habitats favour diverse
forms of ‘ parkland’, scrub, heath, or grassland vegetation.
In truly arctic regions various types of treeless ‘tundra’ and
‘barrens’ take the place of forests, etc., in most of the flatter areas,
with a tendency for less and less of the vegetation to be continuous
over the surface (7.e. ‘ closed ’) as we go farther north. The growing-
season is greatly reduced and frost-heaving and allied influences are
active and, frequently, disruptive. ‘These conditions are partly, but
by no means wholly, simulated on mountains elsewhere—in general,
at higher and higher levels as we travel towards the Equator—so
that even in the tropics we may get, at very high altitudes, whole
series of habitats and attendant communities reminiscent of those
Other Arctic (cf. Kig..137,.B, and. Pig. 138).
Concavities and convexities in the general surface of the earth
may lead to all manner of local variations— including marked changes
in water conditions—resulting in what amounts to a wide range of
different habitats. So may local cliffs and talus or gravel slides or
wave-washed banks, blow-out or wash-out or other erosion effects,
and many other types of surface phenomena, profoundly affect con-
ditions locally. In arctic and alpine regions, features leading to
deep drifting of snow in winter (see Chapter XIII) are also very
important in changing local conditions and resulting in whole series
of special habitats. Here the dynamic or other inimical forces of
nature may prevent vegetation from taking a hold and changing the
habitat and whole aspect as it usually does elsewhere. In general,
however, vegetation rings many changes before coming to a state
of relative equilibrium with the environment, by which time various
factors of the habitat have usually become altered quite drastically.
The connection with landscapes is treated in Chapter XVII. Change
in habitat conditions through the activity of vegetation is particularly
marked in forested areas ; but it may also be considerable where the
vegetation is less prolific, and even where it is entirely dwarfed. It
316 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
tends to be least where the vegetation forms least of a ‘ show’, and
especially where plants fail to stabilize the surface, as in the case of
many dunes and coastal or desert or high-arctic areas. Plants are
again relatively impotent in places of drastic topography and con-
sequently strong geodynamic influences and recurrent catastrophe,
for here the physical forces of nature usually rule, rather than the
vegetation, and our recognition tends to be of habitats rather than
of their inhabitants.
Deserts are areas where the water conditions are too unfavourable
(in the sense that the drought is lastingly too severe) to allow the
support of any extensive continuous development even of short
Grasses or scrub. ‘They cover wide areas of flattish or other topo-
graphy and in a sense are simulated on a small scale by areas of
porous sand, gravel, shingle, or rock, where arid conditions may
prevail even in regions of plentiful precipitation. ‘The so-called
cold deserts are the high-polar and high-alpine regions where frozen
conditions make water unavailable to plants during most of the
year. Even where the precipitation is extremely small in these
rigorous regions, as in some high-arctic areas, there is, however,
usually plentiful water available from melting snow for fair plant
growth in favourable situations, at least early in the growing-season ;
moreover there is normally frozen ground-water not far below the
surface, so the regional appellation of ‘ desert ’ seems inappropriate.
In passing, mention should also be made of the so-called ‘ aero-
plankton ’, consisting of spores, etc., which float freely and unharmed
in the air (although they can scarcely be considered as normally
living thus), the microscopic ‘ cryoplankton’ which really live in
and on the surface layers of snow or ice (see Chapter XV), and the
‘edaphon ’, the flora and fauna of the soil, which actually forms a
special habitat for numerous recognized soil organisms.
Aquatic HasiTaTs
Even when we divide these into the two main groups of saline and
freshwater habitats, there are left intermediate ‘ brackish’ ones
which seem best considered with salt waters. The degree of
salinity can, and frequently does, greatly affect the habitat and
attendant community. Freshwater habitats, apart from the mar-
ginal ones already considered, comprise those of lakes, tarns, and
ponds where the waters are relatively static, and those of rivers and
streams where they are more or less dynamic. But streams can
11] MAIN HABITATS, SUCCESSIONS, AND CLIMAXES 2.417)
be reduced to chains of pools in dry weather, and lakes can have
considerable convection and wind-engendered currents as well as
run-off streams, so that here again any distinction is not wholly
valid. Major variations in salinity found in different seas, estuaries,
salt-lakes, etc., and the light, temperature, tranquillity or shelter
from disturbance, size and depth of the body of water, possibility
of attachment, and content of dissolved substances, are all im-
portant factors leading to the existence of whole series of different
aquatic habitats. Further variable factors may be the ‘ reaction’
(acidity, neutrality, or basicity), aeration, and seasonal or tidal or
other changes in the level of the surface.
Let us briefly consider examples of these and some other factors
as they may affect aquatic habitats: further details are given in
Chapters XV and XVI. Light, being essential for photosynthesis,
severely limits to those depths to which a sufficiency penetrates the
possibilities for normal plant development, while towards the lower
limit at which photosynthesis is possible, this vital function is in-
sufficiently active to sustain life unless it be of specially adapted
organisms. Some Red Algae seem to be so adapted, for they can
grow at depths of nearly 200 metres in exceptionally clear seas, while
some phytoplanktonic organisms have been dredged from, and can
apparently live at, fully 200 metres. ‘The larger Brown Algae, on
the other hand, do not seem to be able to grow at any such depths,
while vascular plants in fresh water usually extend no deeper than
10 metres even in the clearest lakes, and in shallower water form
zones correlated with their light requirements. However, in the
Mediterranean Sea one flowering plant, Posdonia, is reported to
extend down to depths of 80 or even 100 metres.
Of the other factors, temperature differences frequently have much
the same effect in aquatic as in aerial habitats, although major bodies
of water will act as reservoirs militating against rapid changes in
temperature. Consequently, conditions in water tend to remain
more ‘ even’ than in the air, with the result that aquatic organisms
and communities are often surprisingly widespread. Shelter from
wave or ice action, and tranquillity from currents and tides, is another
important factor profoundly affecting the habitat and attendant
vegetation, macroscopic (7.e. visible to the naked eye, as opposed
to microscopic) plants often being limited to sheltered bays, etc.
This is often bound up with the size and depth of the body of water,
on which convection and wind-engendered or other currents
frequently depend, as does the degree (if any) of freezing. But
318 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
such matters of size, depth, and shelter also introduce factors of
their own, including light and temperature variations and the question
of whether rooted or otherwise attached plants can grow up sufficiently
to perform all their vital functions. ‘This also depends upon the
possibility of rooting or other attachment, which is usually dependent
upon a suitable ‘bed’ that of course varies for different types of
plants.
As regards the content of dissolved substances, this can range
from * ocean’ or even more extreme salinity down to varying degrees
Fic. 88.—Margin of tropical oligotrophic lake, with steep rocky sides and dota
deepening water, supporting few larger plants. The hill-top vegetation is a semi-
arid savanna with prominent Acacias. Lake Tanganyika, E. Africa. (Phot.
R. Ross.)
‘fresh’ water. Often in bodies of fresh water the acidity and
especially the nutritive salt content are of key significance for the
development of planktonic communities—at least, within particular
temperature ranges. In this connection it is often useful to dis-
tinguish three types of such bodies, of which the first may give rise
to the others: (1) oligotrophic, of waters poor in dissolved minerals,
typically with Desmids abundant but supporting at most a narrow
zone of rooted higher plants because of a hard rocky bottom and
rapidly deepening water (Fig. 88); (2) dystrophic, with waters also
poor in nutrients but rich in humus and acidic in reaction, often
coloured, containing Desmids and Bog-mosses ; and (3) eutrophic,
11] MAIN HABITATS, SUCCESSIONS, AND CLIMAXES 319
distinguished quantitatively from the oligotrophic type in bein ;
usually poorer in the numbers of species but richer in individuals
and poor in humus though commonly silted and shallow. The
eutrophic type is also relatively rich in combined nitrogen, phos-
phorus, and often calcium, typically contains plentiful Blue-green
Algae, and has a broad zone of rooted Pondweeds, etc., and a sur-
rounding one of luxuriant reed-swamp (cf. Fig. 89).
SAOINMB A A ‘i ui WY
Fic. 89.—Lake of eutrophic type near Prout’s Neck, Maine. It is silted and
shallow, with floating-leaf plants outside the broad marginal reed-swamp dominated
by tall Cattails.
In addition, the reaction (or ‘ pH level’, whether acidic or basic)
of a body of water is commonly important to many organisms,
while lack of oxygen may be a limiting factor deep down in sheltered
situations. Also often limiting are seasonal and other changes of
level in shallow places. Any substantial tidal activities are especially
significant, as may be the speed and flow of currents and the presence
and particular powers of various living organisms.
Even as the zoned vegetation of sea-shores indicates the existence
of different habitats at different levels, e.g. above and below normal
low-tide mark, so do zones exist at different depths around the
margins of deep lakes. Moreover, seas (such as the Sargasso) and
especially lakes of warm regions, may bear extensive macroscopic
320 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
floating vegetation. Yet, in major bodies of water, far more exten-
sively occupied plant habitats are usually provided by the surface
waters where sufficient light penetrates for photosynthesis. Here
develop various planktonic communities of free-floating or swimming
organisms, the vast majority of which are microscopic. ‘The habitat,
and consequently the community, may vary greatly with climatic
factors and the presence of solutes and suspensions in the water, the
whole being often subject to marked seasonal fluctuations including
exhaustion of nutrients when the population is around its ‘ peak’.
In the deeper layers of water and on deep ocean or lake floors where
light does not penetrate, there are still habitats—especially for
saprophytic plants living on the ‘ rain’ of sinking bodies. Indeed
it is here that Bacteria are often especially numerous.
MICROHABITATS
We have seen that the environmental and internal factors of living
organisms have intricate and highly complex interrelationships, be-
longing as they do to a plethora of variables and potentialities that
may be set in motion by all manner of ‘ master’ forces. Yet it 1s
only the ‘thin shell’ of environment directly impinging on, or
immediately adjacent to, the organism that is of primary causal
significance to it. So we get what in effect are ‘ partial habitats ’
(microhabitats), for example in areas of drastic relief or uneven
ground, or in different situations in a forest or other gross and complex
community. Consequently it is rare for the measurements recorded
by meteorological instruments to be actually those of the conditions
of the microhabitat affecting the growing plant or, more precisely,
the growing part of the plant.
Microclimates are really the ultimate multiple expression of the
local climatic effect which is so commonly and variously engendered
by physiographic change, and microhabitats are their environmental
result, though they may often be based on edaphic or other vari-
ations. Very commonly one factor will compensate for another so
far as some plants are concerned, but not in a manner satisfactory
to the requirements of other plants—so leading to a jumbling of
local communities—and this effect may be extended to microhabitats.
Moreover, an alteration in one factor may initiate whole series of
adjustments in others, often having far-reaching consequences.
Especially are such effects apt to be complex when concerned with
groups of factors that are closely related to one another—such as
11] MAIN HABITATS, SUCCESSIONS, AND CLIMAXES 321
light, heat, and moisture relations, which vary simultaneously with
every change in the intensity of insolation.
In such circumstances it is difficult or sometimes impossible to
segregate individual factors experimentally. Instead of physical
apparatus we may use ‘ phytometers ’, which are standard plants or
clumps of vegetation that have the advantage of integrating all effective
factors of the environment and expressing the result in their own
responses. ‘hey react only to changes that matter to the plant or
plants, whose protoplasm has the power of making adjustments.
But much as species are usually composed of more or less numerous
biotypes, so may general habitats or even single examples of them
be made up of biotopes, which are the ultimate expression of environ-
mental variation, being defined as the smallest natural area of space
that is characterized by a particular environment. The biotope (or
“ ecological niche ’ of some authors) is thus the primary‘ topographic ’
unit used in habitat classification. ‘The community of forms in-
habiting it is termed a biocoenosis, and the biotopes having similar
characters are united into larger divisions called biochores. Many of
these are apt to be represented in any small area, so whole series of
phytometers, covering at least the different micioclimates, may be
necessary to determine the impress of even a single example of a
general habitat ; moreover they should be accompanied by batteries
of instruments adequate for the establishment of quantitative re-
lationships between stimulus and response.
In addition, matters can change rapidly with time. ‘Thus even
under a canopy of vegetation, such items as the movement of leaves
by the wind, the changing angle of the sun, and various effects of
weather and season, cause variations in the movement of shadows and
sun-flecks across the ground. ‘This in turn causes drastic changes
in the amount of light-energy received at a given point—on which,
as we have already seen, much else may depend. Altogether it is
not surprising that experimental ecology has become an exacting
(though scarcely exact) science. A saving grace is the fact that,
apart from the grosser direct effects of animals and drastic physical
agents, the actual effects of environmental factors upon plants are
resolvable into a few physical and chemical processes. Examples
are those which underlie the influence of light on photosynthesis and
growth, the effect of temperature on chemical changes in the plant
body, the evaporating power of the air on the water in the plant,
and the effect of the soil solution on the absorbing organs and other
parts of the plant.
322 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
Drastic microhabitat development may take place at different levels
or other situations in a forest, or on different sides of a hillock or
even pebble. ‘Thus the conditions under which an Alga or Moss
lives on the bole of a tree are substantially different from those of
an epiphyte in a crutch or on a branch high up in the crown, or of
course from those of a herb on the sheltered forest floor. And again,
the shelter from wind and sun given by even a minor projection from
the ground, may enable a delicate plant or small community to grow
there which could not exist in the exposed surrounding areas. ‘The
effect may even extend to the soils and their biota, and affect the
plant habitat through them. Especially striking are the differences
of temperature (and consequently of important dependent factors)
on the north- and south-facing sides of tussocks at high latitudes in
summer, which may vary by more than 20° C. within a few centi-
metres, and allow active growth to take place in one spot when
adjacent areas are frozen solid. Such effects may be the key not only
to the micro-distributions of plants but also to their ranges over wide
areas. Consequently it is important that we recognize the concept
of microhabitat, for it is a very real and indeed fundamental one.
Finally it should be recalled that not only different (phylogenetic)
strains and even differently treated individuals may respond dif-
ferently to microhabitat or other vagaries, but that different stages
in the (ontogenetic) development of an individual may have critically
different reactions, young seedlings being in general relatively feeble.
Similarly, the tender young parts of older plants often differ greatly,
in their resistance, from the remaining portions of the same plants—
hence the familiar ‘ killing back’ of shoots by frosts in temperate
regions, the rest of the plant being commonly unharmed.
MaIN SUCCESSIONS
When dealing in the last chapter with environmental factors we
referred briefly to the competitive and other ‘internal’ ones en-
gendered by the plants themselves. ‘Thus weeds compete for space
and nutrients, some of those introduced to inhabited regions creating
major nuisances by choking waterways, destroying the habitats of
wildlife, or colonizing and rendering practically useless whole areas
of agricultural land. ‘The shade cast by dominant species, and the
shelter they give, affect all the plants within the community ; also
affected are the local atmospheric humidity and, often, soil structure
and development as well as composition,
11] MAIN HABITATS, SUCCESSIONS, AND CLIMAXES 323
It is a commonplace that units of vegetation, left to themselves,
tend to change in a particular direction—usually from less complex
communities of small plants to more complex ones dominated by
larger plants of higher life-form (or, at all events, greater competition-
impress). ‘The change is continuous, recognizable ‘ stages’ being
mere nodes of vegetational expression. Such is succession, the de-
velopmental series of communities constituting a sere and leading
up to a state of relative stability and permanence known as the climax.
It should, however, here be admitted that not all ecologists accept
the idea that vegetation can be widely interpreted in terms of develop-
ment and equilibrium, while some, such as Professor Hugh M. Raup
(in litt.), seem to doubt the validity of some of the basic assumptions
involved—at least for those parts of the world in which they have
themselves worked. Certainly, many of the beliefs involved are
mere presumptions, or true only in some degree : thus successions
may proceed only in relation to preceding and following stages, and
climaxes are only re/atively stable. ‘This is often expressed by saying
they are in ‘ dynamic equilibrium’. Nor is it for us to write into
Nature’s book meanings which she does not intend, or to attempt
to inculcate for our own convenience an orderliness of pattern which
does not exist. But if we deny the existence of seres and climaxes,
we do away with two of the most stimulating concepts and useful
tools of our trade, and so with this reservation it seems best to pro-
ceed to use them. In doing so we ought also to bear in mind that
many of the principles with which we are concerned have emerged
from work carried out in temperate regions, and that in the Arctic
(for example owing to frost action) and in the tropics (where there
is often no clear dominance) things may be very different.
We shall deal a little later with the typical stages of some charac-
teristic seres, and, in the next section, with the main types of climax.
With the reservations expressed in the last paragraph, some under-
standing of these and allied concepts seems essential for an appreci-
ation of the mosaic which is vegetation, and whose study, at least
in terms of distribution, is the mainstay of modern plant geography.
But first we should outline the component (often more or less con-
tinuous) actions of a sere, which normally may be considered as
follows: (1) nudation (the production of a bare area) 1s the initial
prerequisite, whether it be by emergence or submergence, glacial
recession, erosion, deposit, climatic change, or bioticagency. ‘There-
after follow (2) plant migration (including initial colonization) ; (3)
ecesis (successful establishment); (4) aggregation of germules to
324 INTRODUCTION LO PLANT GEOGRAPHY [CHAP.
form families (of a single species) or colonies (of two or more species) ;
(5) competition (virtually the struggle for existence) among the
colonists, particularly for space, light, water, and nutrients; (6)
invasion by other plants, usually from adjacent areas ; (7) reaction,
which essentially comprises the changes wrought in _ habitat
conditions by the plants themselves (e.g. in soil formation); (8)
coaction, the influence of organisms upon each other ; (9) stabiliza-
tion, which of course is only relative, change being inevitable in all
living organisms and aggregates thereof; and (10) attainment of a
climax, by which time competition has generally become so intense
that further invasion is problematical unless the community is
drastically disturbed.
The complete sere just indicated is a primary sere (prisere), be-
ginning on a bare substrate without organic material. ‘The chief
types of primary seres are those initiated (1) in fresh water (hydro-
seres), from which may be distinquished * haloseres ’ beginning in
saline water; (2) on damp aerial surtaces such as alluvial mud
(mesoseres) ; and (3) on dry materials (xeroseres), of which out-
standing examples are those starting on bare rock (lithoseres) and
on dry sand (psammoseres). Secondary seres (subseres) are merely
partial, beginning after the succession has been stopped, and thus
not going back to a purely inorganic substratum unaffected by plants.
They are distinguished as ‘ hydrarch’, ‘ mesarch’, or ‘ xerarch’,
according to whether their initiation is under damp, median, or
dry conditions, respectively.
The broad tendency of succession is from simplicity to complexity
of organization, and from dominance by lower to higher life-forms
which make more and more exacting demands on the habitat. Yet
sometimes we see ‘ retrogression ’ to dominance by a lower life-form,
for example when the habitat undergoes a change to less favourable
water conditions. An incidental change in normal successions is
from open to closed conditions, involving also an increase in the
intensity of competition and marked alteration of local climatic and
edaphic factors such as atmospheric humidity, wind, and the humous
content of the soil. Such ‘reactions’ are reciprocal, the plants
affecting the habitat, which in turn affects the plants ; indeed, many
of the higher life-forms only enter when the ground has been suitably
‘prepared’ for them by the dominants of earlier stages, which in
turn they ruthlessly overshadow and frequently oust.
We will now outline examples of the main types of priseres as
postulated for temperate forested regions ; although the tendencies
11] MAIN HABITATS, SUCCESSIONS, AND CLIMAXES 325
are generally similar, the outcome is apt to be different in polar and
tropical regions, seres in the former being often much mixed and
disturbed, and, in the latter, commonly retarded by lack of humus
accumulation. ‘These items are explained in the appropriate
chapters.
In temperate regions the typical hydrosere, after various non-
essential (proseral) stages of plankton, etc., starts in bodies of fresh
water whose beds where suitable are colonized by attached or other
benthic (7.e. bottom) aquatic vascular plants and Mosses besides Algae
as deep down as light conditions allow. ‘These plants often form
dense mats that collect silt and humus, there being frequently
insufhcient oxygen for rapid decay. ‘The bed is thus built up
gradually until, at a depth of some 1 to 3 metres, it can be invaded
by floating-leaf types such as Water-lilies (Nymphaea spp.) or certain
Pondweeds (Potamogeton spp.), which tend to shade out the sub-
merged plants. ‘The long stalks of these floating-leaf plants trap
silt and their coarse bodies after death become deposited as, ulti-
mately, humus—so that the bed is built up with relative rapidity until
the water is shallow enough for swamp plants to enter the community.
‘These typically form a reed-swamp whose dominants are only partly
submerged, building up the beds quickly and ousting the previous
types. As the level continues to rise owing to the deposition of
humus, etc., fully terrestrial invaders enter to characterize the sedge-
meadow stage, the reed-swamp plants disappearing in due course
as conditions are rendered unsuitable for them. With further rising
in level of the soil surface and relative depression of the water-table,
shrubs and ultimately trees enter and in time give rise to a hygro-
phytic woodland. ‘The Alders, Poplars, Willows, etc., which com-
monly constitute this, will, in their turn, shade out the lower types
and prepare for the climax forest which requires drier and more
favourable soil conditions. Fig. go gives a diagrammatic repre-
sentation of the stages of a hydrosere in section, a typical example,
extending from the floating-leaf stage, having been shown in Fig. 89,1
while Fig. g1 continues this to the sedge-meadow and early timbered
stages. A more detailed account of the early stages of some hydro-
seres 1S given in Chapter XV.
As a characteristic xerosere we will take a lithosere initiated on
bare rock. Such surfaces are apt to be extremely difficult to colonize
1This was of eutrophic (‘ good foods’) type, an example of the oligotropic
(‘ few foods ’, geologically young) type, which may be expected in time to develop
into a eutrophic lake, being shown in Fig. 88.
326 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
and consequently may long remain uninvaded unless it be by pro-
seral colonies of Bacteria, Blue-green Algae, etc. In time, however,
the extreme exposure and general lack of water and nutrients are
usually overcome by crustaceous Lichens or other hardy cryptogams
as the first essential stage. ‘They spread over the surface, helping
the weathering forces of nature by corrosively or otherwise ‘ eating
into’ the rock and adding plant material to form something of a
nidus (nest) for ecesis of foliose Lichens, etc. ‘These, attached at
ip ht AH 2
ANY Ly \ WA DAWA Sas
NAA a
ee
aH
(Sys zk aa
7 o
LL UL LLLLILLLLL ILE a,
ee c (oF The it} 0
EEE Bre ae ° “ £2 s Fipeser le ° Oo ° °
wee . _ ° ° zt 0 é 3
Fic. 90.—Diagram illustrating stages of hydrosere with deposition of peat. The
usual stages are discernible on the gently-sloping shore, being, from left to right,
submerged benthic, floating-leaf, reed-swamp, sedge-meadow (fen), hydrophytic
scrub, hygrophytic forest, and finally climax forest.
Fic. 91.—Sedge-meadow stage of hydrosere colonized by some hygrophytic shrubs
and, on right, trees, near Prout’s Neck, Maine. An extensive reed-swamp is
visible in the middle-distance, surrounding the lake shown in Fig. 89,
11 | MAIN HABITATS, SUCCESSIONS, AND CLIMAXES 327
a single point, overshadow the crustaceous types, holding water and
fragments more effectively than their predecessors, and preparing
the way for the moss stage whose components usually start entering
as soon as soil particles accumulate in crevices and depressions.
Such hardy Mosses are able to withstand prolonged desiccation and
sometimes pioneer on uncolonized rock surfaces, making the Lichens
unnecessary and ‘ proseral’. ‘The Mosses usually enter as spores,
and, among their closely aggregated axes and often densely matted
rhizoids, young soil accumulates rapidly. Sometimes in the larger
cushions this accumulation becomes quite thick (Fig. 92), forming
Fic. 92.—Telescoped stages of xerosere in Norwegian Lapland. Gnarled speci-
mens of the dominant Scots Pine have managed to grow in crevices of the rock
whose surface is often still colonized by crustaceous Lichens, although in other
places cushions of Mosses or ground-shrubs have accumulated some soil.
fine nests for the colonization and establishment of herbs-—especially
annuals of xerophytic tendency. ‘These contribute further to the
humus accumulation and soil-building, and are typically followed by
more exacting biennials and herbaceous perennials, which in their
turn replace members of former stages while accelerating the further
processes of succession. Often more important than colonization
of the open rock surfaces is extension from crevices, at which Mosses
and ground-shrubs are particularly adept. In time taller woody
plants enter, constituting a less enduring stage that tends to overtop
and oust the herbs but meanwhile to improve the soil and often
328 INTRODUCTION TO PLANT GEOGRAPHY [CHAP:
conserve moisture, so that, in due course, forest and ultimately some
kind of climax can develop (see below).
The psammosere usually proceeds much more quickly than the
lithosere, the initial problem being the ‘ binding’ of the surface sand.
This is often accomplished by coarse Grasses or other ‘ advanced ’
types (Fig. 93), while on gravel slides and talus slopes the pioneers
may be coarse herbs or even woody plants, and succession still more
rapid provided a reasonable degree of stability can be attained.
Often, and especially on damp substrata that are immediately suitable
FIG. 93.—Psammosere at Prout’s Neck, Maine, showing Marram Grass (Ammo-
phila arenaria) binding sand above high-tide mark. Some stabilized dunes and
coniferous forest are seen behind.
for colonization by advanced types, the phases of succession may be
telescoped more or less completely ; but still the general tendency
is evident.
It may be noted that in these seres there is a general convergence
of water conditions, the hydrosere becoming progressively drier and
the xerosere progressively moister—until a mean is reached that in
any given climatic region is approximately the same in the two cases.
‘Typically this mean is inhabited by mesophytes and is said to be
‘mesic ’, though relatively xeric (dry) and hydric (damp) exceptions
exist. It has been suggested that ultimately this mean should be
the same under particular climatic conditions whatever the initial
situation, but although this idea may be theoretically attractive it is
II] MAIN HABITATS, SUCCESSIONS, AND CLIMAXES 329
evident that, at least in the present state of the world, different areas
in the same climatic belt can support widely different climaxes.
MaINn CLIMAXES
Although competition is the chief key to succession, the final out-
come of this latter lies in the population best fitted (among those
naturally attainable locally) to take advantage of the relatively mesic
conditions brought about by past reactions. ‘This population is the
‘climax ’ (often more cautiously termed ‘ climax type ’, and compare
the reservation on page 323) and, being in close harmony with an
essentially stable environment, is more or less permanent. Though
by no means invariable in time and space, it shows a regularity of
physiognomy and floristic composition that is usually lacking in
successional stages. ‘Thus in over-all form it persists as long as the
climate remains unchanged—provided no new dominant enters or
retrogressive change sets in, e.g. through impoverishment of the soil
or accumulation of toxic substances. Dominance is due primarily
to control of some ‘ key’ factor or factors of the environment, the
dominant or co-dominants making of all plants present the greatest
demands on the habitat, and normally when the climax is reached
excluding invasion by any serious rival. Fluctuations in the com-
munity thereafter tend to be minor in the absence of any forceful
change.
The climax is thus an equilibrated state of community compo-
sition and productivity, that is adapted to maximum utilization of
local resources by plants and, normally, animals. ‘This maximum
utilization is sustained, the climax being self-maintaining, and its
efficiency is determined by the particular habitat as well as by the
average climax population, which in turn is determined by migra-
tional possibilities on one hand and, on the other, by all the factors
that make up the mature ecosystem. It is evident, however, that,
with changes in the environment, plant populations change from
one type of area to the next, the vegetation varying largely according
to local habitat. That was already indicated above. Consequently,
climax and allied vegetation forms a pattern of communities varying
with, and largely corresponding to, the pattern of environmental
differences and gradients.
In a general way climate determines the dominants and associates
that can be present, and their life-form in turn characterizes the
climax. This is really the mature stage of vegetation living in a
330 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
state of more or less dynamic equilibrium with the local environment,
though minor adjustments go on all the time. For life, as we have
seen, can never be static, and the climax is only relatively so when
compared with other stages of the succession. So besides the obvious
differences in space, which are often attributable to diversification
of the habitat, the climax inevitably shows some variation with time,
its state remaining dynamic to that extent. ‘This variation may be
no more than that which results from the death and decay of indi-
viduals especially of the dominant species—the disappearance of a
big tree, for example, leaving a gaping hole in the forest canopy—
followed by replenishment. If, on the other hand, there is a pro-
gressive change, then we will have a continuing succession.
The climax must at least be sufficiently stable and lasting to out-
live the life-span of the dominant species. It commonly consists
of patches or phases of different but related composition. However,
these are normally at most representative of cyclic changes comprising
upgrade and downgrade parts that nevertheless return to much the
same climax type. If they do not do so, then a succession or retro-
gression must be involved, examples of the latter being the coloniz-
ation of eroded heaths by Lichens and of coniferous forests by Bog-
mosses. It is supposed by some that even these changes represent
parts of a long-term cycle, but for such generalization the evidence
seems inconclusive. We cannot wait long enough to see the true
situation, which might take millennia to emerge !
In nature we expect to see some kind of ‘ regional’ or ‘ prevailing ’
climax developed in local-climatically suitable situations at least on
undisturbed tracts of the better soils of a region. But besides the
complications already mentioned, there may be more important local
variations of soil, biota, treatment, and so forth, causing the sere to
be arrested, after being deflected, at some stage before the climax,
and so constituting a subclimax. ‘This is an imperfect stage in which
the dominants are of lower life-form or competition-impress than
those of the climax, the vegetation being ‘ held back’ by artificial
or natural causes other than the climate. For the immediate site
or ecological peculiarity may largely determine the actual growth.
Examples are the subclimaxes due to such treatments as persistent
burning or grazing (often called disclimaxes, being due to disturbance,
or plagioclimaxes, owing to the deflection involved), or to marked
differences in the rocky or other substratum. ‘This last instance may
if desired be termed an ‘ edaphic climax ” by those who doubt whether
it will ever attain (or even if there is such a thing as) a ‘ regional
11] MAIN HABITATS, SUCCESSIONS, AND CLIMAXES 331
climatic climax’. Similarly the biotically engendered disclimaxes
may be termed ‘ biotic climaxes’, and those due to topographic
features ‘physiographic climaxes’. However, with removal of such
a “master factor’ as burning or grazing, the succession usually re-
turns gradually to the autogenic main sere—at all events approxi-
mately. And then there are instances in which an apparent climax
constitutes in reality a preclimax in that its dominant or co-dominants
are replaceable by one stage of others more advanced in life-form
or competition-impress. ‘This is really only a final type of sereclimax,
or community arrested and held at some relatively early stage, and
comprising the other form of subclimax with seral relationships
simpler than the disclimax. Again by some authorities the term
preclimax is used for one type of what we have here called sub-
climax, namely that developing under locally unfavourable con-
ditions. Finally there is the postclimax, of vegetation more advanced
than that of surrounding climax tracts, due to locally more favourable
conditions obtaining in its limited area. ‘This should be distin-
guished from the relict community or fragment of a community
that has survived some important change, whether this was towards
the improvement or detriment of the general environment.
Whether or not we believe in the ‘ monoclimax’ hypothesis of a
full regional climatic climax as not only the highest type which can
exist in a given climate but also as the one which will ultimately
develop more or less throughout the land area of that climate, the
concept has its attractions and adherents. It appears, however, that
soil (including water) and other conditions often prevent locally
such ‘ regional ’ development more or less permanently, and certainly
no observer can wait to the end of the geological age and consequent
termination of such an ‘ experiment’. Even if a general regional
climatic climax were developed, who is to say that, for example,
subsequent leaching or other effects might not lead to differential
retrogression. Consequently it seems best to admit the likelihood,
in any one region, of several different climax communities as repre-
senting what may then be termed a ‘ polyclimax’. Even examples
of the components of such a regional mosaic themselves frequently
vary from spot to spot as well as with time, for example as indi-
viduals in the dominant layer come and go and all manner of very
local fragmentary seres are engendered ; indeed such variations can
be so endless and perplexing that some ecologists, as has already
been stated, doubt the validity of the very concept of climax, let
alone its regional expression.
220 INTRODUCTION TO PLANT GEOGRAPHY [ CHAP.
What may well be the true situation is expressed by Professor
H. J. Oosting, in the work cited at the end of the last chapter, as
follows :
‘The Clementsian interpretation postulates that a climatic region has
but one potential climax; the most mesophytic community that the
climate can support. It will be found on sites with average or in-
termediate environmental conditions, particularly regarding moisture
relations . . . Given sufficient time, with accompanying stability of
climate and land surfaces, succession will have proceeded to such
terminal, relatively mesophytic communities over much of the area.
The stands will not be identical, yet they will have such a high degree
of similarity that they are obviously related . .. The concept of a
regional or climatically controlled climax necessarily includes recogni-
tion of the convergence of successional trends toward a similar end.
In its simplest statement, it implies that any habitat in a region, given
enough time, could ultimately support a community representative of
the formation. From this statement it might be inferred that a region
of fairly uniform climate would eventually have a continuous and equally
uniform vegetational cover throughout. Actually, this is never true
Locally, there are always edaphic or physiographic situations
whose complex of environmental factors differ to such a marked degree
from those of the general climate that they cannot support the regional
vegetation type and probably never will . . . Monoclimax theory does
not ignore these extreme situations but rather emphasizes that they are
to be expected.’
In any case the main, apparently climax, vegetational types of the
world are regional and climatic to the extent that they tend to recur
on favourable soils more or less throughout a region of particular
climate ; they are also apt to have their counterparts in regions of
different climate. ‘These main types of vegetation are characterized
by the life-form of the dominant or co-dominants and include :
(1) tropical rain forests, the most luxuriant vegetation of all; (2)
tropical forest with a seasonal rhythm, due for example to monsoons ;
(3) sclerophyllous forest, developed where there is a hot dry season
and a cooler moist one, often merging into various parklands and
savannas, which appear to belong rather with grasslands ; (4) warm-
temperate rain forests, of evergreens, where there are few if any
frosts ; (5) deciduous summer forest, with dominants losing their
leaves in winter ; (6) northern coniferous forests, dominated mostly
by evergreens; (7) heath, dominated by members of the Heath
family or heath-like plants such as Crowberry ; (8) tundra, the very
variable but more or less continuous, treeless vegetation typical of
11] MAIN HABITATS, SUCCESSIONS, AND CLIMAXES 333
many arctic and alpine regions ; (g) discontinuous ‘ fell-fields ’ and
sparser * barrens’, etc., characterizing still more frigid regions ;_ (10)
grassland, of various types dominated by Grasses and grass-like
plants such as Sedges, often with scattered trees or shrubs forming
a savanna; (11) semi-desert scrub; (12) desert, with scanty but
characteristic vegetation ; (13) mangrove ; (14) salt-marsh, whick:
like some mangrove seems capable of persisting in the absence of
disturbance ; (15) benthos, of submerged bottom aquatics ; (16)
plankton of free-floating Algae, etc., including those of snow and
ice ; and (17) the edaphon or soil communities including numerous
Algae, Fungi, and Bacteria. Most of these are major, climatically
determined vegetation-types (formations) of each of which various
different ‘ aspects’ exist. Several are, however, apt to be seral in
some instances—as are, of course, the many recognized stages in
successions, such as the moss stage in the lithosere and the reed-
swamp and bog or fen stages in the hydrosere. But strictly speaking
a formation should represent the local climax. Examples of most
of the above types and of some other (usually seral) ones, such as
various swamps and marshes, are described (and often illustrated)
in the next five chapters, which deal with the outstanding vegetational
features of the world.
Before describing the vegetational types of different regions and
media, we must outline, in descending order of ecological status, the
main classificatory units (eca) of vegetation which it seems practicable
to recognize :
(1) Formations. ‘These are the great climatic units or regional
climaxes such as desert, semi-desert scrub, tundra, deciduous forest,
coniferous forest, broad-leafed evergreen forest, and some others,
such as many heaths and grasslands which are determined par-
ticularly by edaphic or biotic conditions but are so distinctive as to
rank as formations. Each formation usually covers a wide area in-
volving various conditions and so consists of more or less numerous
(2) Associations. "These are climax units dominated by normally
more than one species having the life-form characterizing the for-
mation to which their association belongs. An association exists
under broadly uniform habitat conditions and is uniform in type so
far as the general characters of the dominants and main associates
are concerned. Such units become aggregated regionally to consti-
tute formations. Examples of associations include various of the
mixed deciduous forests of Old and New England, such as an
Oak—Beech association. ‘The developmental counterpart of the
M
334 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
association is called an associes. It is a more or less advanced seral
community dominated by more than one species, and is usually
on its way to becoming an association. Commonly each association,
having more than one dominant, 1s composed of two or more
(3) Faciations (or else Consociations—see below). A faciation is
a climax community with two or more, but less than the total number
of, associational dominants. ‘The seral counterpart of the faciation
is the facies. Another local variant of the association is the lociation,
which varies particularly in the composition of the important sub-
dominants and influents. When there is only one dominant to
each climax community we usually have
(4) Consociations. ‘These are smaller unit communities whose
single dominant still has the life-form characterizing the formation.
Such eca commonly occur on different soils, examples being the
separate Oak and Beech consociations which make up the European
Oak—Beech association. ‘They may conveniently be named by adding
-etum to the stem of the Latin name of the genus of the dominant,
e.g. Quercetum (a consociation dominated by an Oak, Quercus) or
Fagetum (dominated by a Beech, Fagus). ‘The seral counterpart of
a consociation, such as a reed-swamp dominated by a single species,
is the consocies. Commonly recognized within a consociation or
association are
(5) Societies. ‘These are minor (but still often apparently climax)
communities that are commonly recognized within major eca, and
usually owe their existence to local variations of habitat. ‘hey are
dominated by one or more species other than the association domin-
ants, and commonly are of lower life-form than these, being frequently
subdominants of the higher econ, as in aspect (seasonal) and layer
(stratal) societies. ‘Thus a society represents a dominance within
a dominance, whose dominant species is (or are) subordinate when
we consider the association or consociationas a whole. Examples are
the local and often very limited edaphic societies in many woodlands
of temperate regions. ‘The seral counterpart of the society is the
socies, which, if it consists merely of two or more invading species
without evident associates, may be called a colony. Within societies
etc., there may be
(6) Clans. 'Vhese represent the lowest climax unit, consisting
each of a smallaggregation of a single very locally but overwhelmingly
dominant species. ‘The seral equivalent is the family, derived from
the multiplication and gregarious growth of a single immigrant.
11] MAIN HABITATS, SUCCESSIONS, AND CLIMAXES 335
FURTHER CONSIDERATION
Most of the subjects treated in this chapter will be found discussed
—though often in a different light—in each of the books of Tansley,
Leach, Drabble, Weaver & Clements, Oosting, and McDougall cited at
the end of the preceding chapter.
More specialized works dealing with the aspects indicated by their
titles are:
F. E. CLemMents. Plant Succession : an Analysis of the Development of
Vegetation (Carnegie Institution of Washington, Publ. No. 242,
pp. xiii-++ 512, 1916).
F. E. CLemMeEnts, J. E. WeEAveR, & H. C. Hanson. Plant Competition :
an Analysis of Community Functions (Carnegie Institution of Wash-
ington, Publ. No. 398, pp. xvi + 340, 1929).
Sir E. J. Russeti. Soil Conditions and Plant Growth, eighth edition
edited by E. W. Russell (Longmans, London etc., pp. xvi + 635,
1950).
P. J. Kramer. Plant and Soil Water Relationships (McGraw-Hill, New
York etc., pp. xiii + 347, 1949).
R. Geicer. The Climate Near the Ground, translated by M. N. Stewart
and others (Harvard University Press, Cambridge, Mass., pp.
xxl -+- 482, 1950).
It may be noted that whereas ecologists are notoriously prone to
make and use technical terms (so that humorists say they will even
call a spade a ‘ geotome ’), of which not a few have been introduced
in the above chapter, these latter were in most cases selected either
for their precision-giving value or because they would be needed
elsewhere in this work, and particularly in the following chapters
dealing with the main vegetation-types of the world. Many of these
terms occur again and again, though in other cases such non-com-
mittal words as community (for any grouping of plants, or econ)
or ‘ ecotone ’ (denoting the transition zone between two communities)
are employed. Several ecological terms, such as association and
subclimax, are unfortunately liable to be used in entirely different
senses by members of different schools of ecological thought ; for
the present work the most widespread or generally appropriate use
has been chosen, others being commonly ignored in the interests of
simplicity. A definition or other indication of the sense employed
has usually been given on introduction of each technical term in
this book, and may be found through the index.
CHAPTER XII
VEGE DPA TIONAL AI PES .O8) aE MERI Rexaiee
AN D2 ADJACENT LANDS
We now come to what in some respects 1s our main objective—
the study and interpretation of the vegetation and its component
communities inhabiting different areas of the world. It should,
however, be remembered throughout the following treatment that
the various types of vegetation described are merely those which
we recognize, almost all being apt to intergrade with little or no
distinction or even characterization. ‘These intergradings and also
the relative positions of the main vegetational types will of necessity
be largely ignored in the following brief treatment, although the
geographical situation and main neighbouring types in each instance
can be noted in a general way from the map facing page 1 of the
text, which is a highly generalized vegetation map of the world.
We have seen that the systematic relationships of the flora of a
region depend to a considerable extent upon its geographical con-
nections or barriers, whether past or present; on the other hand
the physical characteristics of the vegetation are largely conditioned
by local environmental factors. ‘Thus when two areas have been
separated since far back in geological time by such barriers as wide
seas which cannot ordinarily be crossed by plants, their floras (of
component species, etc.) will often be very different, whereas if
their environmental conditions are similar their vegetation in closely
comparable habitats is likely to have the same general appearance.
This is because similar external conditions which make up particular
habitats tend to produce communities (and life-forms as regards
component plants) whose external physical features are much alike
—however dissimilar may be their more fundamental reproductive
and allied structures by which we usually classify them. For this
reason we expect—and generally find—in hot arid habitats succulent
plants belonging to very various families, in moist temperate regions
deciduous trees, and on high mountains dwarf shrubs and perennial
herbs of tussocky growth. Nor does it matter in this connection
whether or not the areas concerned are separated from one another
336
VEGETATIONAL TYPES OF TEMPERATE LANDS 337
by thousands of miles, or, within reason, wherever in the world
they may be. For whereas a single factor of the environment may
result in a characteristic vegetational feature, it usually takes the
entire habitat complex to stamp the community fully. Consequently,
to the extent that distant habitats may be comparable in supporting
similar vegetation-types, we may deal with them on a world basis.
In this the first division is climatic, followed by local differentiation
caused by edaphic or other differences: and as the vegetational
types of temperate and adjacent lands are the most familiar to the
greatest number of us, we may conveniently start with them.
DectbuoUS SUMMER FORESTS
Summer-green forests, dominated by broad-leafed trees which
lose their leaves for the unfavourable period of winter, constitute
the main climax formation over much of temperate Europe, eastern
Asia, and North America, reappearing in some comparable regions
of the southern hemisphere. From the physiological point of view
the cold winter tends to be a dry period, owing to the fact that low
temperatures often hinder absorption of water by the roots: this
is counterbalanced by the leafless condition during winter, for it
is chiefly from the leaves that loss of water takes place, and it is
mainly such loss which has to be made good by absorption from
the soil. If active transpiration continued when the resultant
water-deficiency could not be made good by absorption, owing for
example to warm weather when the soil remained frozen, serious
injury and even death might result. As it is, these deciduous broad-
leafed forests occupy many of the most populous regions of the
world, and although in such areas we may now see around us only
patches of anything approaching a climax, this situation is largely
due to human disturbance—to clearance for agricultural or other
purposes, or to the depredations of Man’s domestic animals. For
the conditions that have favoured the growth of such forests have
also favoured the development of the most prosperous agriculture
and grazing, including widespread cultivation of cereals, and con-
sequently of some of the highest stages of human civilization.
Owing to the deciduous habit of the main dominants and the char-
acteristic dying down of many of the associated plants, these forests
look entirely different in winter (Fig. 94) and summer (Fig. 95).
Deciduous summer forests have their main development (1) in
eastern North America in the temperate belt northwards to the
338 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
Great Lakes and the upper reaches of the Gulf of St. Lawrence
and westwards beyond the Mississippi; (2) in temperate western
Europe whence they extend eastwards to the Urals as a wedge
between the northern coniferous forests and the southern steppes,
reappearing in the Caucasus region; (3) in northern Japan and
adjacent parts of eastern continental Asia ; and (4) in the southern
Fic. 94.—Leafless condition of mixed deciduous summer forest in northeastern
United States—in winter.
hemisphere in limited parts of Patagonia, southern Chile, and Tierra
del Fuego. In all these regions there is a cool to severe winter
but otherwise temperate and moist climate, with some precipitation
all the year round, and a total of from 70 to 151 or more cm. (approxi-
mately 28 to 60 inches) annually.
In contrast to the situation in most tropical forests, the trees in
deciduous summer forests form only a single main stratum or storey,
12] VEGETATIONAL TYPES OF TEMPERATE LANDS 339
though there may be tall shrubs or some smaller (often young or
unsuccessful) trees forming a partial second stratum below. Such a
lower stratum is especially to be seen when the main tree storey is
not well developed, for when the latter is dense there is normally
but scanty development of tall plants below it, and not much even
of under-shrubs and herbs. ‘There are also few climbers in most
deciduous summer forests, while any epiphytes consist usually of
lowly cryptogams. Consequently such temperate forests tend to
Fic. 95.—Mlixed deciduous forest in northeastern United States—similar to that
shown in Fig. 94, but in summer. (Phot. G. E. Nichols.)
be far less luxuriant than most tropical ones, as may be seen on
comparing Figs. 94 and g5 with Figs. 143 and 144. Often there
is far less undergrowth in the temperate forests than is shown in
these photographs, so that the contrast with tropical rain forests is
still more striking.
In deciduous summer forests the winter buds burst forth and the
leaves of the dominants expand quickly, soon after the growing-
season starts with the advent of suitable temperatures. ‘The foliage
is thus fully developed quite early in the season, so that little time
is lost. Flowering also tends to be completed early, giving ample
340 INTRODUCTION TO PLANT CEOGRAPHY [CHAP.
time for the development and ripening of the fruit ; indeed in many
types the flowers open before the leaves expand, this ‘ spring flower-
ing habit’ allowing freer access of wind and the early insects for
pollination. Also often taking advantage of the brief period before
the leaves of the dominants or other tall plants expand and cut off
most of the sunlight, are the small perennial herbs with persistent
underground portions that send up flowering shoots and leaves very
early in spring, so constituting the ‘ prevernal aspect’. ‘They
flower and fruit rapidly and often die down soon afterwards, as in
he case of the Lesser Celandine (Ficaria verna) and Virginian Spring-
beauty (Claytonia virginiana). Somewhat later are the plants of
the ‘ vernal aspect’, such as the Wood-sorrel (Oxalis acetosella) and
Yellow Archangel (Galeobdolon luteum), which flower during the
bursting of the buds and expansion of the leaves of the overtopping
types. ‘Thus not only are the winter and summer aspects of such
forests strikingly different, but also often the spring and autumn
ones—for in this last season the brilliant galaxy of falling leaves,
including the reds of Maples, the yellows of Birches, and the oranges
of various others, makes it in the opinion of many enthusiasts the
most beautiful of all.
Although they naturally often intergrade as well as vary com-
plicatedly, some five main types of deciduous summer forests may
be recognized in various temperate parts of the world, as follows.
1. Oakwoods of western and central Europe, which tend to be
relatively open and light. ‘The dominants are the Pedunculate Oak
(Quercus robur) and/or the Sessile Oak (Q. petraea = O. sessiliflora),
«_ the most important associated trees including Ash (Fraxinus excelsior),
Poplars (Populus spp.), Birches (Betula spp.), Elms (U/mus spp.),
Alder (Alnus glutinosa), and Wild Cherry (Prunus avium). ‘These
change largely according to the nature of the ground. Smaller
trees and tall shrubs flourishing in the comparatively light shade
include Hazel (Corylus avellana), Holly (Ilex aquifolium), Hawthorn
(Crataegus monogyna), Field Maple (Acer campestre), Crab Apple
(Pyrus malus), Mountain-ash (Sorbus aucuparia), and Yew (Taxus
baccata). Ina still lower layer are found a great variety of under-
shrubs and coarse herbs and Grasses, while cryptogamic epiphytes
may flourish on the bark of the trees. In areas of prevailingly high
atmospheric humidity, some vascular plants may grow as more than
fortuitous epiphytes. Ivy (Hedera helix) and Honeysuckle (Lonicera
periclymenum) are common woody climbers in European oakwoods.
2. ‘The more varied and luxuriant mixed forests of eastern North
12] VEGETATIONAL TYPES OF TEMPERATE LANDS 341
America, eastern Asia, and southeastern Europe, which differ much
from the above in systematic composition, but nevertheless tend to
be roughly comparable with them in physical form. Thus various
different Oaks (Quercus spp.), Beeches (Fagus spp.), Birches (Betula
spp.), Hickories (Carya spp.), Walnuts (Juglans spp.), Maples (Acer
spp.), Basswoods (Tilia spp.), Elms (Ulmus spp.), Ashes (Fraxinus
spp.), Tulip-trees (Liriodendron sp. or spp.), Sweet Chestnuts
(Castanea spp.), Hornbeams (Carpinus spp.), and many others, here
may vie with Conifers such as Pines and Spruces and their allies
—often doubtless as a result of disturbance. ‘The undergrowth is
commonly luxuriant and various, as is the herb layer especialily
where light penetrates, while climbers are relatively plentiful. The
range and variety, not only between the different regions but even
within the main individual ones, is far too great for us to do justce
to here, much less to describe the types in any detail. Those of
eastern North America are well treated by Braun in the book cited
at the end of this chapter, and those of eastern Asia are described
by Wang Chi-wu in a work which it is hoped may soon be published
by the successors of Chronica Botanica. Examples from eastern
North America are shown in winter and summer aspects in Figs. 94
and g5, respectively. Here the main pertinent types to be recognized
include: (a) the mixed mesophytic type of moist but well-drained,
unglaciated plateaus, e.g. of the Appalachians, in which dominance
is shared by a number of species of trees, particularly American
Beech (Fagus grandifolia), ‘Vulip-tree (Liriodendron tulipifera), several
kinds of Basswood (Tilia spp.), Sugar Maple (Acer saccharum), Red
and White Oaks (Quercus rubra s.1. and O. alba), and Hemlock (Tsuga
canadensis); (b) the mixed Oak—Hickory ope of southern mid-
western uplands, extending northwards on to ‘ glaciated ’ territory
and southwards with the admixture of abundant Pines; (c) the
Oak—Chestnut type extending eastwards on to the coastal plain from
northern Virginia northwards and, now that the Chestnut has largely
disappeared as a tree owing to fungal ravage, dominated chiefly by
White Oak, Red Oak, Chestnut Oak (Quercus montana), and 'Tulip-
tree ; (d) the Beech—Maple type lying in the glaciated territory to
the north of the mixed mesophytic type, and dominated chiefly by
Beech and Sugar Maple, though many areas are youthful and still
seral ; and (e) the Maple-Basswood type centred on the driftless
area of Wisconsin, in which Sugar Maple and Basswood (Tila
americana) are the dominants of the climax, which usually contains
much associated Red Oak. In Japan and adjacent China, etc., much
342 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
the same genera (apart from Hickory) but different species are
usually involved in the deciduous forests, examples among the Ashes
being Fraxinus mandshurica and among the Birches Betula ermanit,
while the Beech Fagus crenata characterizes fine ‘ Buna’ forests.
3. The Beech forests which, especially in Europe, where Fagus
sylvatica is the species involved, form an almost uniform closed
canopy, intercepting the sunlight so effectively that few shrubs and
herbs can grow below. ‘The trunks are tall and slender, particularly
when the trees grow closely together, and few competitors of the
dominant Beeches are able to enter their preserve. A thick brown
mat of fallen leaves and leaf-mould covers the ground. It is chiefly
in early spring before the Beech leaves expand that small perennial
herbs such as Bluebell (Endymion (Scilla) non-scriptus) and Wood
Anemone (Anemone nemorosa) tend to flourish as a prevernal aspect,
often growing gregariously to form attractive carpets. Otherwise
herbs are characteristically few, sometimes being largely limited
to nongreen saprophytes, while the lower shrubs may consist of
no more than scraggy Brambles (Rubus spp.) in the more open areas.
Occasional Ash, Wild Cherry, White-beam (Sorbus aria s.l.), or
other trees may reach the height of the canopy, or Hollies or Yews
form a scraggy subordinate layer. ‘The commonest tall shrubs are
Elder (Sambucus nigra) and Field Maple, with Spindle-tree (Huony-
mus europaeus), Dogwood (Cornus sanguinea), and Wayfaring ‘Tree
(Viburnum lantana) occurring chiefly in openings.
4. Southern Beech (especially Nothofagus antarctica) forests,
usually with associated evergreens such as Drimys winteri, of southern
South America. ‘he trees are closely crowded, shrubs again being
relatively few. Ferns and Bryophytes, on the other hand, are
numerous and often extremely luxuriant, the latter sometimes
forming an almost continuous carpet over the ground and fallen
logs, etc., and extending well up the standing trunks.
5. The damper aspect of deciduous woodland developed especially
on marshy ground subject to inundation, and dominated by various
Alders (Alnus spp.), Willows (Salix spp.), Poplars, Birches, and the
like, with often a tangle of hygrophytic shrubs. Climbers and
epiphytes may be plentiful, and on the moist ground often flourish
coarse herbs and tussocks of tall Grasses, Sedges, and Ferns such as
the Royal Fern (Osmunda regalis agg.). ‘These marshy thickets are
plentiful in temperate regions on both sides of the North Atlantic,
and appear to represent late stages in the hydrosere.
In addition, the Sweet Chestnut forests of various parts of southern
12] VEGETATIONAL TYPES OF TEMPERATE LANDS 343
and eastern Europe are sometimes described as a further type, and
the drier, park-like woodland on limestone hills of central Europe
as yet another. Deciduous ‘ parklands’ also occur in the northern
prairies of North America, and though dominated by groups of Aspen
and other Poplars (Populus spp.) in a manner reminiscent of seral
stages, appear in Alberta and Saskatchewan to constitute ‘a forest
type in its own right’ (H.M. Raup mm Uitt.). Also characteristic, if
limited, are the deciduous forests of the Pacific coastal regions of
the northern United States, and the very luxuriant ones of the
western Caucasus where various kinds of Oaks, Beeches, Maples,
Horse-chestnuts (Aesculus spp.), Cherries, and Cherry-laurel (Prunus
laurocerasus) are commonly mixed with Conifers and a wealth of
shrubs and climbers. On the other hand, the open Birch forests of
northernmost Scandinavia, etc., in spite of their broad-leafed
deciduous nature, belong to the next group.
NORTHERN CONIFEROUS FORESTS
These are also known as ‘ boreal forests’, ‘ subarctic forests’, or
‘taiga’, although it seems preferable to reserve the last term for
their open, park-like northern tracts (see pp. 346-8).
The main dominants of these forests, instead of having broad
leaves which they shed in winter, typically solve the problem of
perennation through that unfavourable period by having narrow or
small, needle-like or sometimes scale-like leaves. Besides their size
and shape, these leaves usually have other xeromorphic characteristics
that help to reduce transpiration to very modest rates. Consequently
they can be retained in winter, most such trees being evergreen and
having the advantages over deciduous types of being able to photo-
synthesize whenever conditions allow, and meanwhile of saving the
wastage involved in complete annual leaf-fall. However, it is as
though at their northernmost extremity such trees were unable to
support winter transpiration, for the Larches (Larix spp.) among
these needle-leafed types are regularly deciduous, losing their leaves
every autumn, and in places persisting farther north than the ever-
green trees. Thus it is the Dahurian Larch (Larix dahurica) that
alone forms the farthest north ‘forest’ in the world (at about
728504 N.-and to5° FE. in Siberia).
Although the main dominants of these hardiest of forests are
needle-leafed Spruces, Pines, Firs, and other Conifers, they often
have associated broad-leafed deciduous Birches, Poplars, and the
344 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
like—indeed Birches actually form the northernmost (albeit scrubby
and open, see Fig. 96) ‘forests’ of much of Europe, and supply
the only native arborescent growth in Iceland and Greenland. The
groves of Aspen and Balsam Poplars (all Populus spp.) are usually
(but apparently not always—see p. 343) secondary, replacing conifer-
ous stands after felling or fires, while those of Alders (Alnus spp.)
are normally merely seral.
The undergrowth and ground-flora in well-developed examples
of these coniferous, etc., forests tend to be less dense and diverse
: ” ss ‘ o 7 #29; oe : =. A ie ie ea, <a
in 2!
Fic. 96.—Open scrubby Birch ‘ forest’ in Finmark, northern Norway (‘ Nor-
wegian Lapland’). Dark ground-shrubs and Lichens cover the ground, the
former being here locally predominant.
than in most broad-leafed deciduous ones. ‘The reasons are
apparently (a) that there is no season during which the lower layers
are not shaded by the leaves of the dominants, (6) that the thick
and dry carpet of slowly-decaying resinous leaves hinders the
establishment of seedlings, and (c) that the generally less favourable
regions offer fewer potentialities for growth. Nevertheless there
may be a fair shrub layer, especially in the damper situations where
the ground is commonly moss-covered ; on the other hand, in
drier areas and in the northernmost sparse forests, luxuriant Lichens
often form a continuous ground-investment over vast areas.
One or another type of northern coniferous forest (or, occasionally,
12] VEGETATIONAL TYPES OF TEMPERATE LANDS 345
its broad-leafed faciations of Birch or facies of Aspen) occupies
most of the northernmost belt of forested terrain around the cool-
temperate and boreal shoulder of the globe. Not only do they form
the northern, but in many places also the altitudinal, limits of tree
growth, extending southwards through often some 15-20 degrees of
latitude from this northern limit, with outliers or tongues still
farther south—for example in the eastern and western United States
of America. ‘There are also important outliers in the mountains
of central and southern Europe and Asia (cf. Fig. 83, B). In addition,
the Pines are highly developed in area as well as species in Mexico
and throughout much of the Caribbean region, and, with other
genera of Conifers, are important far south in Central America and
the Mediterranean region, though these vegetation-types scarcely
belong to the above series. For in general the most characteristic
and extensive needle-leafed coniferous forests are developed chiefly
between the 45th and 7oth parallels of north latitude. Nor, in view
of the very limited persistence of ice-free land at corresponding
latitudes in the southern hemisphere, is it surprising that such forests
are not paralleled there, the austral Conifers, although evergreen,
being commonly broader-leafed and more warmth-loving (or at least
far less cold-resistant).
Except when deciduous trees or shrubs are plentiful, there is
relatively little difference in the appearance of the northern coniferous
forest at different seasons—apart, of course, from lying snow which
characterizes much of this belt more or less throughout the winter.
The climate is cool, with the winter extremely cold in the continental
regions ; indeed most of the lowest temperatures ever recorded have
been within this belt in the interior of the great northern continents.
In view of this frigidity, the absolute humidity need not be high
for the climate to remain relatively moist, with fairly regular pre-
cipitation throughout the year. ‘The soil is generally poor and of
glacial origin. Ignoring most of the southern outliers, the following
five main types of northern coniferous and allied forests may be
recognized.
1. The usually mixed coniferous forests occupying most of the
boreal forested parts of Eurasia and North America, dominated by
various assortments (or occasionally a single species) of Spruce, Fir
(Abies spp.), Pine, and Larch. ‘The precipitation is usually between
25 and 100 cm. (approximately 10 and 40 inches) per annum and
the mean temperature of the warmest month above 10° C. (50° F.),
though the summer is relatively short. In eastern North America
346 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
and westwards to the Rocky Mountains the local dominants in the
northern or ‘ Hudsonian ’ belt are usually one or more of the follow-
ing: White Spruce (Picea glauca agg.), Black Spruce (P. mariana),
Tamarack (Larix laricina), Balsam Fir (Abies balsamea), Jack Pine
(Pinus banksiana), with or without associated broad-leafed Poplars,
Birches, or American Aspen (Populus tremuloides). Farther south,
other dominants enter and in time there is a grading into the
deciduous summer forest. ‘Tall shrubs such as Willows (Salix spp.),
Scrub-birches, and species of Dogwood (Cornus) and Pimbina etc.
(Viburnum) may be plentiful, especially in damp situations where
Mosses form a characteristic carpet. Herbs are, however, usually
little in evidence, the normal ground-flora being heathy, including
various species of Blueberry etc. (Vaccinium spp.), Labrador-tea
(Ledum spp.), Pale-laurel (Kalmia spp.), and the Crowberry
(Empetrum nigrum s.l.), typically alternating with or growing out of a
lichen-rich carpet in the drier situations. In the damper situations
the ground layer is contrastingly mossy. Farther west the Balsam
Fir and Jack Pine are replaced by other species, particularly by
Alpine Fir (Abies lasiocarpa) and Lodgepole Pine (Pinus contorta
var. latifolia), while in Eurasia the dominant species are different
again. ‘Thus in northern Europe the Scots Pine (Pinus sylvestris)
is often the sole dominant in the west, with Norway Spruce (Picea
abies) entering to the south and east, and, farther east still, Siberian
Spruce (Picea obovata), Siberian Larch (Larix sibirica s.1.), Siberian
Fir (Abies sibirica), and, ultimately, Siberian Stone-pine (Pinus
sibirica). Except for Norway Spruce these all persist at least well
into Siberia proper, or are confined thereto. In the centre and east
of Siberia, however, the Siberian Larch is replaced by the Dahurian
Larch (Larix dahurica) and the Siberian Stone-pine by the Siberian
Dwartf-pine (Pinus pumila). Apart from this last, which is shrubby,
most of these Conifers (including the Lapponian form of Scots
Pine) are short-branched, at least above, to give a conical shape.
Moreover they tend to be shallow-rooting, and consequently able to
grow in open canopy in areas where the subsoil is permanently
frozen. Examples are seen in Figs. 97 and 98, the ground-flora
almost everywhere being of the characteristic heathy type.
2. The open park-like ‘ taiga’ occurring towards the northern
limit of arborescent growth. ‘This is really only the product of
depauperation of the various faciations of northern coniferous forest
just described, but it is so striking in appearance as to warrant
separate mention. It is characterized by the rather sparsely and
12] VEGETATIONAL TYPES OF TEMPERATE LANDS 347
ae ae = =
Fic. 97.—Boreal coniferous forest surrounding lake-side bog in sheltered valley
in ‘Troms, far north of the Arctic Circle in Norway. ‘The dominant is Scots
Pine of the conical Lapponian form.: he ground-vegetation is heathy, but in
drier situations is apt to consist largely of light-coloured Lichens as in Fig. 98.
Fic. 98.—Outside the taiga in northern Ungava, Canada. The scattered domin-
ants of the taiga, seen in the middle-distance, are Black Spruce (Picea mariana)
and Tamarack (Larix laricina), the ground between, as in the foreground, being
largely occupied by swarded Lichens, though many lichen-covered boulders and
dark patches of Crowberry (Empetrum) and Mosses are visible. In centre is seen
a typical Spruce outlier consisting mainly of a lower deck that is protected by
snow in winter (cf. Fig. 82, A), but with some puny stems straggling above.
348 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
often evenly spaced dominants, poverty in associated vascular plants
(apart from xeromorphic ‘ heaths’, such as Crowberry and species
of Vaccinium, in the shelter of the dominants), and richness of the
lichen carpet at least in dry places (Fig. 98). ‘The Lichens are
usually intricately mixed and inclusive of so-called Reindeer-mosses
(Cladonia spp.) and Iceland-mosses (Cetraria spp.), being typically
aggregated into a pale but dense sward several centimetres thick.
In damp depressions and especially along the courses of rivers, there
occur faciations approaching the ordinary northern coniferous forest
of the region. ‘These may project as timbered tongues containing
well-grown trees, or even form outliers in the tundra—so con-
stituting the so-called ‘ forest-tundra’. Such better growth often
seems to be correlated with better aeration of the roots where there
is active drainage of water (as confirmed by Professor Harold J.
Lutz, voce). In general, however, the dominants of the open
‘taiga’ are of poor development, often gnarled and only a few feet
high though ancient,' conditions for growth being here largely
unfavourable. ‘These dominants are usually the one or more hardiest
tree types of the forest lying to the south—for example, White or
Black Spruce and/or ‘Tamarack across most of northern Canada,
Dahurian Larch in much of Siberia, and a scrubby Birch (Betula
odorata) in northernmost Scandinavia (Fig. 96).
3. The Pacific ‘ coast forest ’ of western North America, developed
chiefly from southern British Columbia to northern California, but
with some of the main dominants extending much farther north-
wards as well as southwards. ‘The region is one of equable climate
with high rainfall and atmospheric humidity, and supports the
densest coniferous forest of the world as well as some of the biggest
and tallest of all trees—e.g. Coastal Redwood (Sequoia sempervirens),
Big-tree (Sequoiadendron giganteum), and Douglas Fir (Pseudotsuga
taxifolia). ‘These may reach heights of around 100 metres, or, in
the case of the first-named, 110 metres, with trunk girths often over
20 metres in the first two cases. A forest of Coastal Redwood is
shown in Fig. 18, D. Various further Conifers belonging to several
different genera constitute the other main dominants, etc. Although
Ferns, including the common Bracken (Pteridium aquilinum agg.) and
Hard Fern (Blechnum spicant), are widespread, herbs in general are
little in evidence. However, many characteristic shrubs occur,
1 Near the northern limit of forest in the Northwest ‘Territories of Canada,
the author has counted more than roo growth-rings in Black Spruces with trunks
barely three inches in diameter at the base and five feet high.
12] VEGETATIONAL TYPES OF TEMPERATE LANDS 349
including kinds of Spicy-wintergreen (Gaultheria), Barberry (Ber-
beris), Currants etc. (Ribes), Rhododendron, Elder (Sambucus), and
Blueberries ete.
4. The so-called ‘lake-forest’ of the eastern half of North
America, lying between the general northern Hudsonian belt and
the deciduous summer forest to the south, and centred on the
northern portions of the Great Lakes. The region is one of moderate
precipitation (60 to 115 cm.) and considerable temperature extremes,
and the forest consists of a single association or associes dominated
Fic. 99.—‘ Lake-forest ’ of southeastern Canada, dominated by White Pine (Pinus
strobus) and Hemlock (Tsuga canadensis), in summer. ‘The undergrowth is mixed
and relatively sparse.
by White Pine (Pinus strobus), Red or Norway Pine (P. resinosa),
and Hemlock (Tsuga canadensis). Associated are various broad-
leafed deciduous trees of various ecological affinities, making this
forest all the more difficult to delimit ; indeed it is now often ques-
tioned whether it ought to continue to be recognized as a type.
For whether or not it is climax, it is in many respects transitional
between the boreal coniferous and southern deciduous forests, from
whose migrational buffetings and competition it has suffered. ‘This
is reflected in the undergrowth, which tends to be poorly developed
owing to the dense canopy but includes many Pteridophytes,
350 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
saprophytes, and under-shrubs. Fig. gg shows an example of this
type in summer, and Fig. too shows the selfsame area in winter.
5. Besides the above we should mention some other Conifer-
dominated types such as the montane and subalpine forests of
western North America which, as their names imply, are due in
part to local physiographic factors ; also the various * Pine-barrens ’
\
\
Fic. 100.—The same area of ‘ lake-forest’ as that shown in Fig. 99, but under
winter conditions, with snow covering the ground.
and other characteristic communities of eastern North America,
which are due in part at least to fire or other disturbance, and
consequently are of seral or subclimax nature. Somewhat com-
parable but apparently still more often edaphically engendered types
exist in various parts of Eurasia south of the boreal forest belt, or
as outliers for example in the Mediterranean region.
WaARM- TEMPERATE RAIN FORESTS
In warm-temperate as in subtropical regions where rainfall is
plentiful and well-distributed through the year, evergreen forests
are developed. ‘The total precipitation is usually between 150 and
300 cm. per annum and frosts are no more than occasional and slight.
‘Towards the tropics these hygrophilous (7.e. moisture-loving) forests
merge into the subtropical and finally the tropical types described
12] VEGETATIONAL TYPES OF TEMPERATE LANDS Bin
in Chapter XIV, but in cooler regions they partake more of the
characteristics of the deciduous summer forests described above.
In spite of the relative abundance of climbers and epiphytes, even
the human inhabitant of cool regions can scarcely consider many of
these warm-temperate rain forests as anything like tropical, and so
it seems desirable to treat them here and correspondingly reduce
the width and complexity of our tropical, etc., belt. At best these
temperate rain forests tend to be considerably less luxuriant as well
as usually lower than the tropical ones, and to have fewer climbers,
epiphytes, and other ‘ forest furnishings’. Nor are plank-buttresses
(see Fig. 145) normally found in them. Moreover, they often show
a fairly sharp distinction between winter and summer aspects, for
example through admixture of deciduous trees. Although the
co-dominants are often numerous and inclusive of Conifers, the
local dominance is usually less mixed than in the tropics. The
leaves tend to be smaller and more leathery, and the main canopy
less dense, so that in different places Tree-ferns, smallish Palms,
Bamboos, small trees, tall shrubs, etc., form a lower tier. Often
the undergrowth is very dense and intertwined with herbaceous
climbers, the ground and tree-trunks being covered with a mat of
cryptogams and small herbs, making the whole quite difficult to
traverse.
Warm-temperate rain forests are developed sporadically in the
southern portions of the United States bordering on the north shore
of the Caribbean, and more extensively in southern Japan and
adjacent Korea as well as westwards deep into China, in south-
western South America, in the extreme south of Africa, and in New
Zealand and some adjacent parts of Australia. Types approaching
them are also found in uplands in the tropics, for example of southern
Asia. In some places they merge into the sclerophyllous types that
are commonly developed in the less summer-humid of the warm-
temperate regions and will be considered under the next general
heading. Many different faciations characterize different regions,
particularly, and could be distinguished for example on the basis
of different dominants. ‘They may also be marked by characteristic
associates, as in the case of those of Australasia with their Tree-ferns
and those of the southeastern United States with their festoons of
Spanish-moss (Tillandsia usneoides—see Fig. 101). Four examples
from different quarters of the globe may be briefly described.
1. The rain forest of southern Japan which, where undisturbed,
is largely dominated by several species of lofty evergreen Oaks.
352 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
Associated are other trees, including members of the Laurel and
Magnolia families, and numerous shrubs forming a dense under-
growth. Woody climbers (lianes) are plentiful, as are epiphytic
Ferns and some Orchids.
2. The temperate evergreen forest of the southeastern United
States (but not southern Florida which is subtropical) where again
more or less evergreen Oaks may predominate—especially Live
Oak (Quercus virginiana) in the so-called ‘hammocks’. Here the
Evergreen Magnolia (Magnolia grandiflora) is often prominent.
Fic. 101.—Warm-temperate rain forest in southeastern United States. Live Oak
(Quercus virginiana) and other trees are festooned with Spanish-moss (Tillandsia
usneoides). Aquatic vegetation inhabits a sluggish stream in the foreground.
There are a few lianes and temperate Palms in the rich shrubby
undergrowth, and on the trees may be a fair range of herbaceous
epiphytes among which the so-called Spanish-moss_ frequently
dominates the landscape (Fig. 101). However, the true broad-leafed
forest is rather little represented because of edaphic and biotic and
especially fire influences. ‘These lead to subclimax areas occupied
respectively by Bald-cypress (T'axodium distichum—see Fig. 102) or
other summer-green swamps or, on dry sands, by evergreen Pine
forest or savanna. Prominent Pines in this connection are the
Loblolly (Pinus taeda), Longleaf (P. palustris), and Slash (P.
caribaea).
3. The rain forest in New Zealand, which is almost entirely
12] VEGETATIONAL TYPES OF TEMPERATE LANDS 353
temperate in nature, in spite of the prevalence of large Tree-ferns.
Stately Conifers such as the Kauri (Agathis australis) and various
species of Podocarpus, and huge dicotyledonous trees with leathery
leaves, are among the various and usually mixed dominants, with
species of the smallish-leafed Southern Beech (Nothofagus) especially
in the less luxuriant upland regions of the south. Although bright
Fic. 102.—Bald-cypress (Taxodium distichum) swamp in southeastern United
States. [he dominant trees have ‘ breathing’ roots growing up into the air.
Their branches are heavily festooned with Spanish-moss. In the foreground are
floating-leaf and other early stages of the hydrosere.
354 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
flowers are largely absent in this forest, the aspect is one of consider-
able luxuriance owing to the profusion of shrubs, epiphytes, and
climbers—as well as of Ferns and lower cryptogams.
4. The temperate rain forest of southern Chile, which may be
almost impenetrable owing to the density of the undergrowth. ‘The
dominant trees are again various and usually mixed, including some
small-leafed evergreen Southern Beeches and other types, and a few
Conifers. Bamboos of the genus Chusquea are often important in
the dense undergrowth, and a considerable variety of climbers and
epiphytes are again to be found.
Cn
SCLEROPHYLLOUS, ETC., WOODLANDS
In warm-temperate regions having a rather hot and dry summer
alternating with a cooler moist season, the dominant trees and shrubs
tend to be evergreen and to have small and hard, thickish leathery
leaves (sclerophylls). ‘This is characteristic of most of the shores
and hinterland of the Mediterranean, after which such climates are
commonly named, though they occur also in the southwestern
portions of Australia and South Africa, in central-southern and
southeastern Australia, in the extreme southwest of the United
States and adjacent Mexico, and in central Chile. Although they
sometimes abut on areas of warm-temperate rain forest, these
sclerophyllous forest areas tend to show greater daily and seasonal
temperature extremes, with snow and ice not infrequent around
midwinter. Moreover the precipitation, besides falling irregularly,
is usually much less plentiful than in rain-forest areas, typically
ranging from 50 to 100 cm. (approximately 20 to 40 inches) yearly,
while the quantity of moisture in the air varies greatly at different
times. ‘The plant communities tend to be rather drab through most
of the year but attractively decked at flowering time.
Although shaded uplands and areas of sufficient ground-moisture
may bear more luxuriant mixed or deciduous forests, the typical
dominants of sclerophyllous regions are lowish and gnarled, examples
being rounded or flat-crowned evergreen Oaks, Olives (Olea spp.),
or similar trees, and needle- or scale-leafed Conifers. ‘They usually
grow in more or less open, scattered formation, at least after dis-
turbance by Man, and when destroyed are commonly replaced by
a fairly dense scrub of mixed deciduous and evergreen bushes—
including members of their own undergrowth. Even the associated
herbs often have the form of shrub-like perennials, or store water
12| VEGELATIONAL DY PES OF TEMPERATE LANDS 355
in massive aerial tissues in the cases of Aloes, Agaves, and Cactus-
like or other succulents. Particularly characteristic are a host of
geophytes with underground food-stores in bulbs, tubers, etc., ready
to develop with the rains. ‘The examples mentioned above in five
different continents may be considered briefly.
1. The thin sclerophyllous woodlands of the Mediterranean and
southern Black Sea regions. ‘The chief dominants include evergreen
Oaks such as the Cork Oak (Quercus suber) and Holm Oak (OQ. ‘/ex),
and various Pines such as the Aleppo Pine (Pinus halepensis) and
Stone Pine (P. pinea). However, most areas have been so disturbed
by felling and grazing that only sparsely scattered, low and gnarled
trees remain, the prevailing vegetation being a pale scrub on lime-
stone terrain, known as ‘ garigue’, and a densex and taller one on
siliceous soils, known as ‘maquis’. ‘This last is often 3 metres or
so in height and in various forms and densities covers vast areas,
being composed of a bewildering variety of shrubs including the
subdominants of the original woodlands—such as Cistuses (Cistus
spp.), Olive (Olea europaea, extensively cultivated as a tree), Myrtle
(Myrtus communis), Rosemary (Rosmarinus officinalis), Lavender
(Lavandula latifolia), and tall Heaths (Erica spp.). Some Palms
‘and large Cactus-like succulent Euphorbias may also occur.
Epiphytes are generally absent and climbers few, but any open
ground tends to support numerous bulbous or tuberous Mono-
cotyledons, xerophilous Grasses, dicotyledonous herbs, and short-
lived spring annuals in great variety. Fig. 103 shows a rocky area
in open sclerophyllous woodland, with patches of subdominant
mixed scrub of maquis and garigue sorts. East of the Mediter-
ranean this type thins out with decreasing precipitation, although
some semblance of it is still to be seen on the lower slopes of the
mountains of northern Iraq, so invoking the interior of Asia.
2. The Cape region of South Africa of which it has been written :
‘There seems to be no doubt that [it] once possessed luxuriant
forests of the Mediterranean type, and that the same process of
destruction which gave origin to the European maquis largely also
transformed these forests into mere brushes ’ (Hardy, The Geography
of Plants, p.246). Now there remains chiefly a wealth of sclerophyl-
lous shrubs such as species of Protea and Leucadendron, with
numerous tall Heaths and other bushy perennials belonging to very
various families, and bulbous and tuberous subordinates.
3. The sclerophyllous woodland and scrubby ‘ chaparral’ com-
munities of western California and some adjacent regions. Here
£2 ca On ee ae ae
Fic. 103.—Rocky area with patches of mixed scrub of ‘ maquis’ type (‘ garigue’
where light-coloured), with herbs in open tracts and sparse sclerophyllous, etc.,
woodland in background, in the Mediterranean island of Corsica.
FIG. 104.—Sierran chaparral climax, Santa Barbara, California. (By permission
from Plant Ecology, by Weaver & Clements, copyright date 1938, McGraw-Hill
Book Co.)
356
VEGHRTATIONAL TYPES OF TEMPERATE LANDS 357
the dominant trees are often sparse and include Conifers and several
species of evergreen Oaks, which in very dry situations are reduced
to mere shrubs. ‘he main mass of vegetation is commonly a thick
and often tall scrub composed of representatives of very diverse
families, with some associated succulents and numerous bulbous
and tuberous herbs. Over some considerable areas trees are absent,
this being the characteristic chaparral (Fig. 104), while, in others,
only occasional isolated gnarled trees rise above the maquis-like
scrub. Even where they form a canopy, the trees are typically only
20-40 feet high.
Fic. 105.—Sclerophyllous forest in Australia, dominated by lofty Gum-trees
(Eucalyptus spp.).
358 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
4. Very similar conditions and vegetation-types exist near the
coast of central Chile, where maquis-like scrub is widespread and
gives way, inland, to slopes of sclerophyllous woodlands often
dominated by trees strongly resembling evergreen Oaks. ‘Their
systematic allegiance 1s, however, usually quite distinct, as is that
of the associated shrubs. In these woodlands, climbing plants as
well as tuberous and bulbous ones may be common.
5. ‘he sclerophyllous woodlands and scrublands of southwestern,
central-southern, and southeastern Australia are again in many
cases reminiscent of those of the northern hemisphere, though their
systematic allegiance is for the most part entirely different, usually
involving other genera or even families. ‘he most luxuriant expres-
sion of the woodlands is found in the majestic forests of Gum-trees
(Eucalyptus spp.), with an abundant undergrowth of hard-leafed
shrubs often having beautiful flowers, and including Acacias,
Mimosas, and Heath-like Epacridaceae. In many places Grasses
cover the ground, the trees being scattered or letting through
plentiful light owing to their leaves (or leaf-like members in the
case of the Acacias) having a tendency to being orientated parallel
to the rays of the sun. With less rainfall or more disturbance,
the vegetation may be reduced to maquis-like * mallee’ or other
monotonous, tangled scrub most often 1-3 metres high. In areas
of intermediate water conditions this may be interspersed with
stunted Eucalypts, Casuarinas, and various other trees. Fig. 105
shows such a sclerophyllous forest dominated by tall Gum-trees in
a region having 75-100 cm. of rainfall annually.
HEATHLANDS AND GRASSLANDS
Areas dominated by characteristic members of the Heath family
(Ericaceae), or by narrow-leafed Heath-like shrubs, are common in
temperate and adjacent lands, as well as in arctic and alpine regions
beyond the limits of arborescent growth. ‘Typically the com-
munities are dense and of the order of 25 cm. in height. In warm
lands they may merge into the scrubby types of sclerophyllous
vegetation, which often include large Heaths, etc. But their most
notable development is in cooler regions with a moist winter—
particularly in western and north-central Europe, where the Common
Heather or Ling (Calluna vulgaris) is often the chief constituent of
the vegetation over considerable areas, particularly on acidic soils.
Associated may be various Heaths (Erica spp.), Bilberries etc.
12] VEGETATIONAL TYPES OF TEMPERATE LANDS 359
(Vaccinium spp.), and Bearberries (Arctostaphylos spp.), with a
variety of Grasses and sometimes some taller evergreen shrubs.
Especially in the North, the heath-like Crowberry (Empetrum
nigrum s.l.) is apt to be important and similarly gregarious, as are
taller but still low and shrubby deciduous Birches and Willows. On
the other hand in the South, the taller shrubs are often evergreen
and characteristically have needle-like leaves (e.g. Juniper, funiperus
communis s.1.) or other photosynthesizing members (e.g. Gorse, Ulex
spp.). ‘The tendency to be gregarious and mycorrhizal is almost
general in these plants : most of the chief dominants at least in the
north being moreover evergreen, dwarfed, and richly branched
chamaephytes, a characteristic dense and dark mat up to a }-metre
in height commonly results.
Whereas most low-lying heathlands in temperate regions are
subclimaxes of the disclimax type, being due to intensive grazing
or recurrent fires that prevent trees from returning to their once-
forested areas, in some coastal tracts especially of northwestern
Europe these heathlands are evidently maintained through exposure
to winds. In such instances they will be in more delicately balanced
equilibrium with the environment and consequently more ‘ natural ’.
Much the same delicate balance and relative stability probably
obtains in many upland areas, where damper peaty ‘ moors’ are
especially common in cool regions—including the ‘ highmoors ’ (cf.
Fig. 168) developed on acidic soils inhabited by Bog-mosses
(Sphagnum spp.), and the ‘ meadow-moors’ developed on circum-
neutral (usually calcareous) soils.
Still more important and widespread are grasslands, which indeed
in their various forms constitute one of the main ‘ world’ types of
vegetation. Whereas in tropical and subtropical regions grasslands
typically take the form of savannas, with widely-spaced trees and/or
tall shrubs as described in Chapter XIV, in temperate lands they are
usually without trees or bushes except along watercourses. And
although many grasslands are due to interference by Man or his
domestic animals, many others, including extensive ones in temperate
and allied areas, seem to be entirely ‘ natural’. ‘Thus if they are
due to grazing this is, or at all events originally was, apparently by
wild animals.
What seem to be climax grasslands develop in temperate regions
chiefly in areas having an average yearly precipitation of between
25 and 75 cm. (approximately 10 and 30 inches), or rather more in
warm parts. ‘These grasslands occur especially in the interiors of
360 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
the great land-masses, the main examples in principally cool regions
being the North American ‘ prairies’ and the Russian and adjacent
‘steppes’, with, in somewhat warmer regions, the South African
‘veld’, South American ‘ pampas’, and southern Australian and
New Zealand grasslands. ‘These last three regional types frequently
bear isolated or sometimes aggregated low trees or shrubs and accord-
ingly constitute savannas ; indeed much of southern Australasia,
particularly, is often indicated as savanna on world vegetation-maps.
Nevertheless the real dominants are usually Grasses, so such types
seem best classed here. In addition, meadows and other grasslands
occur widely in temperate and allied regions as biotic plagioclimaxes.
All grasslands have this in common, that they are dominated over
at least most of their often vast area by Grasses—usually by various
and often mixed perennial species which are narrow-leafed hemi-
cryptophytes, many of them being gregarious and extremely hardy.
Characteristically, the rooting is shallow and the underground parts
form a matted turf which holds rainwater when and where it falls,
checking penetration to deeper layers and accordingly aiding the
grassland plants to prevail in their competition with trees in transi-
tional areas where the rainfall is barely sufficient for arborescent
growth. ‘The turf and sod of old dead leaves, etc., may also help
to check the successful establishment of tree seedlings. ‘The winter
in the so-called climatic grassland areas is often severe and dry, as
may be the later summer; but recurrent spring and early summer
rains that more than compensate for evaporation will favour the
Grasses, which can then vegetate actively.
‘The main types of grasslands occurring in temperate regions may
now be briefly described.
1. The prairies of central and western North America, ranging
from well north in Canada southwards into Mexico. ‘The dominant
Grasses mostly form clumps (‘ bunch Grasses ’) in the drier regions
or more extensive sodded swards in the less arid ones, being inter-
spersed with a large variety of subdominant forbs (herbs of other
than grass habit). These often give the prairie a distinctive tone
locally and fall into mixed societies flourishing for example at different
times of the year. In general, however, various greens prevail in
the early part of the growing-season, and yellows or greys later on,
sometimes followed by darker orange or other autumn tints. Woody
plants are usually few and unimportant, except in depressions or
as a response to overgrazing, which may also favour Cacti. Yet
differences from spot to spot tend to be so marked and unstable
12] VEGETATIONAL TYPES OF TEMPERATE LANDS 361
that the whole has been claimed to constitute ‘an open system’ in
which repeated readjustment to disturbance overrules any tendency
to equilibrium. In general correspondence with decreasing rainfall,
the Grasses fall into three groups based upon stature and known
respectively as ‘tall’, ‘mid’, and ‘short’ Grasses. They range
from the height of a Man, or taller, down to a few centimetres in the
cases of the most drought- or pasturing-resistant types. Fig. 106, A,
shows an area of true prairie in Nebraska, dominated principally
by mid Grasses, but with shrubs and even trees in some damp
depressions. Fig. 106, B, illustrates an area of short-grass prairie in
Colorado, while Fig. 106, C, shows a similar area that has been badly
overgrazed. In the prairies there is a long resting stage each year,
which is due to low temperatures in the North—the melting of winter
snow affording plentiful water in early spring—and to low rainfall
in the South and West. ‘The general unity of this grassland climax
is evidenced by some of the grass species occurring in nearly all
of the component associations, and by the large number of grass
genera—such as Agropyron, Bouteloua, Elymus, Poa, Sporobolus, and
Stipa—that afford dominants more or less throughout its great
range. Nevertheless, differing local conditions lead to preclimax
and postclimax communities and different treatments to biotic
plagioclimaxes, while near the forest border is an ecotone with trees
which elsewhere persist chiefly along watercourses. In such zones
of tension, each life-form takes advantage of the slightest variation
of edaphic or biotic impress which may be in its favour.
2. The steppes of the U.S.S.R. and adjacent lands, covering vast
areas south of the northern forests and north of the central deserts,
etc., and ranging from eastern Europe to eastern Asia, with outposts
farther west and south. ‘These steppes are similar to the North
American prairies in all essential respects, though they differ con-
siderably in the component genera and much more in the species.
The Grasses tend to be highly xerophilous and to form dense tufts
or cushions composed of the remnants of previous years’ growth
seated on stools of superficial but much-branched fibrous roots,
while the associated forbs are mostly hardy perennials or bulbous
geophytes. These associates and any annuals or woody plants are
mostly small, few exceeding half-a-metre in height. ‘The growth
of trees is almost everywhere prevented by the scarcity of water
and by the extremely severe winters, when strong and dry winds
blow over the frost-bound soil on which snow may afford protection
only for ground-vegetation. Between the northern forests and open
Fic. 106.—The North American Prairie: mid- and short-grass communities.
A, Nebraska prairie area dominated principally by ‘mid’ Grasses, with shrubs
and trees in depressions. (By permission from Plant Ecology, by Weaver &
Clements, copyright date 1938, McGraw-Hill Book Co.) B, short-grass com-
munity in Colorado. (Phot. U.S. Forest Service.) C, area similar to (B), but
overgrazed and in poor condition, with bad weeds, including Cacti. (Phot. U.S.
Forest Service.)
362
VEGETATIONAL TYPES OF TEMPERATE LANDS 363
steppes there is often a broad ecotone, rather incongruously known
as ‘forest steppe’, in which the two formations occur intermixed
in patches.
3. The Argentinian and other South American pampas and
steppes, again developed chiefly on flat or gently undulating plains,
and with most of the general characters and range of variation of
the above. Quite frequently some low trees or tall shrubs give
these grasslands the aspect of savannas.
4. The grasslands and savannas of South Africa and southern
Australasia—the grasslands being again closely comparable with the
northern steppes and prairies, whereas the savannas have also
scattered trees and/or bushes. But although the dominant Grasses
tend to be of familiar forms and even genera, the woody plants are
very different in different regions, often including unique types
(e.g. in Australia).
5. The subclimax meadows and other grasslands of various
temperate regions that except for biotic disturbance would support
vegetation of higher life-form. These probably include considerable
areas of present-day steppes and prairies especially towards their
forested margins, but are more notably represented by the verdant
meadows that form such a prominent feature of the dairy and other
pastured lands, for example, on either side of the North Atlantic.
Such meadows are typically due to clearance of the forests and
more or less long-continued mowing and/or pasturing by domestic
animals and wild herbivores such as Rabbits and Hares. ‘This
favours the Grasses and other hemicryptophytes that dominate such
areas, so that they tend to form a continuous sward with closely-
compacted turf which represents a biotic plagioclimax in that it is
deflected from the normal succession. However, with removal or
reduction of normal grazing, woody plants soon enter the system
and the subsere proceeds towards the climax. Much the same
Grasses and forbs with buds hidden in the surface layer of soil are
protected against fire, which often helps to maintain grasslands, as
does the tendency of the turf to retain water. In meadows, the
dominant Grasses are usually broader-leafed and less xerophilous
than in climatic grasslands, although a wide range of often similar
types and even genera are involved. Meadows, moreover, tend to
support a wider variety and often greater admixture of forbs. Most
of the meadow inhabitants are more or less hygrophilous, lacking
marked protective devices reducing transpiration, and, although
their flowering-axes die down, many remain uninterruptedly green in
364 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
winter. ‘The dominant Grasses are perennial, with axes on the
average about half-a-metre high, and typically involve such familiar
genera as Poa, Festuca, Lolium, Agrostis, Alopecurus, Phleum, and
many others. ‘lhe associated forbs are also mostly perennial, or,
less frequently, biennial, and are often rosette-chamaephytes. Like
the Grasses, they may be coarse and 1-2 metres high in favourable
circumstances especially in damp situations, but rise scarcely at all
above the surface of the soil when intensely grazed; such close
cropping cannot, however, be long withstood by most species.
Annuals occur chiefly in open patches that are due to overgrazing
or drought, while Mosses may form a weak ground-layer especially
in the damper situations. Woody plants, like geophytes, if present
are usually little in evidence—except where the biotic disturbance
is light and there is a tendency towards heath development or
reforestation.
SEMI-DESERTS AND DESERTS
Although what amount practically to desert conditions are com-
monly developed very locally on dry rock and sand or other porou
surfaces, these usually show evidence of at least incipient succes
sional change. On the other hand, extensive desert areas tend to
be among the most stable vegetationally, owing to the general lack
of water for more growth than that of the plants already present.
Intermediate in water-relations between true deserts and the drier
among the areas whose vegetation has been described earlier in this
chapter, and usually intermediate also in geographic position, are
various semi- or near-desert areas such as the Sage-brush ones of
western North America. ‘These are characterized by the Sage-brush
(Artemisia tridentata) and other low and often greyish shrubs having
herbaceous branches, that dominate a climax in regions having an
annual precipitation of 12-25 cm. (approximately 5-10 inches).
Such shrubs often belong to predominantly herbaceous families and
also thrive in areas of greater rainfall when the competition from
Grasses is eliminated. The scrub is low and often broken or of
more or less widely-spaced single bushes, the depauperated types
merging into the still more xerophytic ‘ desert-scrub’” developed
where the rainfall is limited to 7-12 cm. annually. In this latter
instance the dominants are bushy shrubs, particularly of Creosote-
bush (Larrea tridentata), usually 4-2 metres high and spaced on
the average 5-15 metres apart through a widely-spreading root
system.
12] VEGETATIONAL TYPES OF TEMPERATE LANDS 365
In the often comparable ‘ desert steppes’ of the U.S.S.R., which
constitute the transition between the more or less continuously
vegetated steppes and the deserts of predominantly bare spaces,
other Wormwoods (Artemisia spp.) of Sage-brush type tend to be
the most characteristic plants. Frequently they are halophytic, the
soils being salinized and supporting also such characteristic under-
shrubs as Salt-bush (Atriplex canum). Often prominent in addition
are sod-forming Grasses such as wiry Fescues (Festuca spp.) and
Feather-grasses (Stipa spp.), and dicotyledonous as well as mono-
cotyledonous annuals which develop rapidly with spring rains but
die down with the advent of hot weather, so leading an ephemeral
life. Examples of this last phenomenon are well seen in desert areas
pinlcag——cf. Fig 156:
Some Lichens may grow or lie on the surface of the earth, as may
Blue-green Algae including colonies of Nostoc, but arborescent
growth, for example of Willows and Poplars, is usually limited to
occasional damp depressions or the vicinity of watercourses. Such,
for example, are the oases of the Gobi Desert. Rather similar
conditions and attendant vegetation, with a dry and generally warm
climate subject to sharp contrasts of heat and cold, and one or two
brief vegetative seasons each of 1-3 months (in spring and/or
autumn) characterized by intermittent rainfall and favourable
temperatures, are widespread in temperate as well as in warmer
countries. ‘hus they occur on the plateaux of Asia Minor, in belts
around the deserts of Central Asia and Australia, in southern South
America and South Africa, and flanking the Sahara and Arabian
Deserts. Some of these areas support extensive thorny or other
scrub and are apt to be so designated on world vegetation maps.
These semi-desert regions of both the Old and the New Worlds
are typically made up of vast, flat or undulating expanses of bare
soil supporting hoary or dull-grey, often sticky and scented, ‘ heathy ’
or sage-like shrubs 4-2 metres high. Such shrubs may form a
continuous but thin or else broken brush, or be ‘ scattered’ over
otherwise naked tracts, though in particularly arid areas the spacing
is often more even and apparently due to root-competition. Al-
though there are no green grassy swards there may be bunches of
wiry Grasses, and although there are normally no trees there may be
tallish Cacti or giant Euphorbias or other large succulents, while
saline lagoons, dry most of the year, may be surrounded by fleshy
or bushy halophytes. The usually erect or ascending, switchy
dominant shrubs often have thickish woody stumps and deep roots.
N
366 INTRODUCTION TO PLANT GEOGRAPHY
Their leaves are commonly narrow, leathery and inrolled, less than
3 cm. long and often densely covered with hairs which give them a
greyish-white tint. Outside the rainy season, dependence is largely
on underground water. Except for those which are deciduous, the
dominants frequently show little contrast in appearance at different
times of the year—especially as few display at all large and bright
flowers. Matters are otherwise with the often plentiful associated
ephemerals which burst forth when rain allows, and form quite a
display with such succulents as Cacti, Mesembryanthemums, Aloes,
and Agaves, which contain considerable stores of water.
With still more precarious precipitation or other water supply,
deserts usually result. But as these usually belong to tropical or
subtropical regions, or at least extend well into them, they are treated
chiefly in Chapter XIV. Exceptions are afforded by the Trans-
caspian desert which is an open plain and the Gobi Desert which
is a plateau, both lying in temperate parts of Asia. ‘These are
mainly areas of prevailing drought and extreme temperature con-
ditions, and consequently are very little vegetated, such plant life
as exists being mainly the result of depauperation of the so-called
desert steppes described above. ‘Thus dunes may be partly fixed
by low and open, shrubby growth, but siliceous and clayey soils,
often containing loess, are apt to be virtually barren, as are gravelly
and talus-strewn areas over considerable tracts of country. Nor do
the frequent saline or ‘ alkali’ and gypsiferous areas afford much
relief to the abiding monotony. Nevertheless one seldom finds at
all extensive tracts even in the Gobi that are entirely devoid of some
kind of vegetation, and very commonly the transition to steppe or
at least desert-steppe is marked by a scattering of poor Grasses
ranging from some 25 cm. high in exposed situations to a metre in
height in depressions where water collects in the rainy season.
Besides Grasses and some Sedges, xeromorphic members of the
Daisy (Compositae), Goosefoot (Chenopodiaceae), and ‘Tamarisk
(T'amaricaceae) families tend to be prominent in these temperate
deserts, as do geophytic monocotyledons such as Tulipa uniflora, Iris
sisyrinchium, and species of Gagea. Desert-like ‘ bad-lands ’, often
characterized by lowly Cacti or brush-like shrubs, also occur more
locally in parts of temperate North America and southern South
America.
For examples of desert areas in warm-temperate regions we may
go to central Iraq. Fig. 156, A, shows a quadrat in a typically
gravelly area in which one small perennial tuft is visible but there
Fic. 107.—Salt desert and salt-marsh of a warm-temperate region. A, an area
of salt desert near Shithatha, in southern central Iraq. Clumps of the co-domin-
ants are up to 2 metres wide and 60 cm. high. Cracking of the surface encrustation
is noticeable in slight depressions. ‘The dots at the foot of the distant scarp are
grazing Camels. B, saline marshes outside the oasis of Shithatha, Iraq. The
vegetation is mostly closed and sedgy-grassy, with some Tamarisks (Tamarix spp.).
367
368 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
are numerous tiny seedlings springing up after heavy rain. Fig.
156, B, shows a close-up of a near-by less gravelly area when the
ephemerals have developed. Fig. 107, A, shows an area of salt
desert near the oasis of Shithatha, in southern central Iraq, where
higher vegetation occupied from one-eighth to one-half of the sur-
face, bushy Chenopodiaceae affording the main dominants. Soluble
salts totalled 7-5 per cent. of air-dry soil at the surface where tested
but decreased markedly below ; the pH was 8-3 at the surface and
scarcely varied from this below. Some widish and flat, white-
encrusted areas tended to be barren, though in places where water
could collect in the rainy season and Blue-green Algae grew, loose
mud ‘ medallions’ covered the surface.
SaLtT- MARSHES
Whereas various salts, such as nitrates, sulphates, and phosphates
of potassium, calcium, magnesium and iron are essential, at least
in the small concentrations that are usually present in soils, for the
normal development of land vegetation, excessive salinity (e.g. of
more than o-5 per cent.) is harmful to the growth of most plants.
Such high salinity, particularly due to sodium chloride, is most
commonly found around sea-shores, and is the most constant factor
leading to the replacement there of normal land-vegetation by plants
which habitually grow in very salty soils (halophytes) or at least
can grow in such soils (facultative halophytes). ‘Thus in maritime
salt-marshes, which are primarily determined by periodic immersion
in salt water and mostly lie between the levels reached by the higher
neap and ordinary spring tides, the components of the characteristic
vegetation are almost all peculiar to the habitat. And whereas, if
the tide is effectively excluded and there is adequate drainage, the
salt in time is washed out of the soil and non-halophilous species
colonize the area, ‘ There is no good evidence that salt marsh can
develop by the mere accumulation of silt or humus, without human
assistance, into a non-maritime vegetation’ (Tansley, Jntroduction
to Plant Ecology, p. 78). Consequently the salt-marshes of tem-
perate and allied regions seem best considered here among subclimax
or local climax types.
Salt-marshes are chiefly developed on mud-flats about sheltered
tidal estuaries. In them such Green Algae as Rhizoclonium and
Enteromorpha, or such halophilous vascular plants as succulent or
shrubby Glassworts or Saltworts (Salicornia spp.), are the normal
12| VEGETATIONAL TYPES OF TEMPERATE LANDS 369
pioneers, followed typically by Alkali-grasses (Puccinellia spp.) or
other halophytic Grasses. Any such growth tends to impede the
flow of water and increase the deposition of silt, helping to raise
the level of the bed. Higher up, on the ‘ flats ’ covered by most of
the spring tides, a mixed vegetation is usually developed. This is
the general salt-marsh community which, for example on both sides
of the North Atlantic, characteristically includes such types as Sea
Plantain (Plantago maritima s.|.), Sea Arrow-grass (Triglochin
maritima), Sea-blite (Suaeda maritima), and Sea-pink (Armeria
maritima s.|.), with shrubby Chenopodiaceae especially in warm-
temperate regions. Various Brown and other Algae, often of peculiar
habit and squalid mien, are commonly associated. Still higher up,
where the saline flats are covered only by the higher spring tides,
a more or less close turf usually develops with the help of grazing,
composed of halophilous Grasses with associated forbs. At the
uppermost levels reached only by the very highest spring tides or
storm-waves, less markedly halophilous types such as Red Fescue
(Festuca rubra s.|., a facultative halophyte) tends to predominate,
with often an admixture of ordinary land species that can tolerate
small amounts of sea-salt in the soil. At the higher levels in river
estuaries the water is alternately fresh and brackish with the ebb and
flow of the tide, so inhabiting plants must be physiologically special-
ized to withstand rather sudden and extreme changes in the osmotic
value of the inundating water.
Whereas such geodynamic factors as tidal action may cause the
maintenance of a condition of equilibrium between accretion and
erosion, and thus of a lasting type of vegetation in maritime salt-
marshes, it seems probable that in some instances at least there is
gradual continuation of accumulation and advance towards normal
land vegetation—even without human interference—as in the case
of sand-dunes and shingle beaches, and in spite of what was quoted
above. But at all events the types just described are commonly
both long-lasting and distinctive, and consequently had to be
treated as special entities.
In many arid regions, for example of interior Asia and western
North America, there occur salt lakes or smaller basins or seasonally
dry ‘ alkali pans’ exhibiting a similar range of conditions of varying
salinity which is, however, often more extreme than on sea coasts.
Inland, these saline areas result from excessive and prolonged
evaporation, changes in level and salinity in individual cases being
seasonal instead of tidal, though they may vary rapidly with the
370 INTRODUCTION TO PEANT GEOGRAPHY [CHAP.
weather. Still, the sequence of zones is often remarkably definite,
and in a general way reflects not only the prevailing degree of
salinity but also the poor aeration accompanying the increasing soil-
moisture. Here again there is no certainty that succession will ever
proceed to any proper climatic climax in the absence of biotic
disturbance, especially as the tendency is for the salinity to increase
rather than decrease with time. Consequently it seems best to
consider the marginal vegetation as forming a zoned series of sub-
climaxes so far as the general region is concerned (or edaphically
limited climaxes of their own immediate areas). In any case it
appears likely that the local allogenic conditions will have more
effect upon the future of these inland saline areas than will any
autogenic seral tendencies. Fig. 107, B, shows an area of saline
marsh outside the oasis of Shithatha, in southern central Iraq. ‘The
vegetation is mostly closed and sedgy-grassy, with some shrubby
T'amarisks, being dominated by such types as Scirpus maritimus,
Cyperus distachyos, Aeluropus littoralis, and Tamarix pentandra. ‘The
pH was 8-6 where tested and the area was said to remain damp
throughout the year, having some shallow pools of open water at
least in spring. Open mud ‘ polygons’ were bound by mixed
Oscillatoria, Lyngbya, and other primitive Algae.
Some hitherto productive cultivated areas in arid regions requiring
irrigation have become too saline for the growth of crops—owing
to prolonged evaporation leaving the salts of the ‘ raw’ irrigation
water behind when no drainage was provided. In other cases the
crops are limited to facultative halophytes such as Date Palms
(Phoenix dactylifera) or Barley. Much of central and southern Iraq
is of this nature, and as Man has not yet become proficient at
remedying such soil salinity on a wide scale, the increasing amount
of irrigation is worsening the situation.
SERAL COMMUNITIES
Besides subclimax communities, various other deflected or
‘ straight > successional ones have been described or implied above,
while many others are of such limited occurrence or importance
(as in the cases of salt-sprayed coastal areas or the ‘ serules ’ occurring
for example on fallen logs) that we can scarcely even mention them
in this brief outline of the main vegetational types of temperate
and allied lands. However, to complete our picture there remain
a number of (mostly seral) communities that should be cursorily
12] VEGETATIONAL TYPES OF TEMPERATE LANDS 371
elucidated, some of these being sufficiently widespread to demand
more attention.
Notable are the characteristic types afforded by the sand-dune
and shingle-beach successions which occur chiefly (but in the former
case by no means entirely) near sea coasts. ‘Thus sandy sea-shores
that are wetted by the highest tides are commonly inhabited in
sufficiently sheltered situations by a characteristic open community
of halophytic shore-plants such as Sea-rockets (Cakile spp.), Sea-
purslane (Arenaria (Honckenya) peploides agg.), and species of Orache
(Atriplex). ‘These tend to arrest any dry sand that is blown on to
them, and, consequently, to form the basis of hillocks through which
the shoots grow, the effect being cumulative in that the deeper
the sand comes to be piled up, the higher the plants will grow,
and vice versa. More important in this connection are the major
dune-formers—coarse Grasses such as, particularly, Marram Grass
(Ammophila arenaria) and Lyme-grass (Elymus arenarius s.1.) in north-
temperate regions. ‘These have similar powers of colonization and
sand-accumulation but grow chiefly farther back from the sea and
more extensively, binding the sand by the ramifications of their
rhizomes and roots. ‘his is illustrated in Fig. 93, which shows
Marram Grass doing the binding on the coast of Maine. Meanwhile
the motion of the surface particles is checked by the tall and usually
tufted aerial parts, and other species, which cannot colonize moving
sand, are enabled to establish themselves between the axes of the
main pioneers. ‘These secondary colonists typically include Lichens,
Mosses, and less coarse Grasses, which finally bind the surface and
consolidate the community. Maritime shrubs and in time climax
forest typically follow if the land is not severely pastured or turned
into golf links (for which old, grass-covered dunes are the traditional
sites). Meanwhile there will have been produced some characteristic,
though seral, vegetational tvpes whose non-halophytic counterparts
may occur far inland, as for example around the Great Lakes of
North America.
Shingle-beach vegetation has considerable affinity with that of
dunes, especially when sand is admixed and the habitats are thereby
rendered similar. Where sand is lacking, various Lichens may
colonize the surface of the stones inland of the usual ‘ storm-crest ’,
and over them may extend clumps of such maritime flowering plants
as the Sea-pea (Lathyrus maritimus agg.) and halophytic Grasses.
These often persist for a long time, the plants being rooted in the
crevices between the pebbles. Ultimately the crevices become filled
392 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
with sand and plant remains, and gradually the vegetation extends
to form a complete covering of the shingle, so that on old shingle
beaches away from the direct influence of the sea, inland types of
vegetation commonly develop, and in time scrub or even forest.
Except that, usually, only inland plants are involved and the early
stages tend to form long downward stabilized strips separated by
dynamic tracts, much the same sequence takes place on many talus
screes and heaps of detritus. ‘These, when reasonably stable and
including a fair amount of comminuted ‘ soil’, can, with the help
of their surface crevices, run the whole gamut of the xerosere with
relative speed and ease right up to a forest climax. Otherwise the
lithosere is usually very slow in progressing, especially in its early
stages. Although these stages may be represented by highly
characteristic cryptogamic or herbaceous communities, these norm-
ally occupy only very limited areas of rock, such as occur naturally
in temperate regions chiefly on cliffs and rocky outcrops.
The hydrosere also affords characteristic seral communities such
as floating-leaf, reed-swamp, sedge-meadow, and damp scrub ones
which were outlined in the last chapter and may severally cover
considerable areas. Although they vary widely in different instances
and especially in different regions within the temperate and allied
belts, these communities are mostly too familiar and obvious to
require description in any detail here. Dominants on both sides
of the North Atlantic include Water-lilies (Nymphaea spp.) in the
floating-leaf stage, Cattails or Reedmaces (Typha spp.) in reed-
swamp, various Sedges (Carex spp.) in the sedge-meadow, and
Alders (Alnus spp.) in the damp scrub. Examples of most of these
features are described and illustrated elsewhere in this chapter or
the immediately preceding one (stages of hydroseres being shown
especially in Figs. 89, 90, g1, and 102).
Besides the various natural or semi-natural scrublands mentioned
above, there are the often characteristic scrubs and thickets belonging
to subseres that follow cutting or burning of forests in temperate
and allied regions. ‘These unstable types may occupy considerable
areas, though usually not without more or less recent human dis-
turbance or intervention. Even the later and taller stages of these
secondary growths are frequently dominated by plants—such as
‘weedy’ Birches or Poplars—that do not occur in the climax
forest.
There remain to be mentioned among types naturally occupying
substantial areas in temperate and allied regions the various marshes,
12] VEGETATIONAL TYPES OF TEMPERATE LANDS 373
fens, bogs, mires, moors, and waterside zones that commonly
represent part of some hydrosere or other. As such, many have
been covered above, at least by general implication; in other
instances they seem to form subclimaxes, for example of plagioclimax
nature where a continuing “ master factor’ is involved, and, at least
when covering considerable areas, appear needful of some treatment.
Outstanding are the marshlands, fenlands, and boglands which may
be distinguished respectively according to whether the soil is formed
mainly of silt, of peat containing considerable quantities of lime
or other bases, or of peat very poor in such bases. With marshlands
of one sort or another we are already fairly familiar, while further
examples are mentioned in the following chapters.
Fenland occupies the alluvial borders of rivers and streams as
well as parts of old estuaries and the borders of certain lakes—
especially of those into which streams bring ‘ hard’ water rich in
lime. Its most characteristic manifestations represent stages in the
hydrosere—particularly reed-swamp, sedge-meadow, and, if the
latter is not regularly cut or pastured, damp scrub or woodland
‘carr’ dominated by Alders or sometimes Birches, or constituting
meadow-moors in exposed situations. ‘The vegetation in these
cases is largely calcicolous, whereas in the general run of alluvial
marshlands it is less exacting.
Bogs or ‘ mosses’, on the other hand, develop chiefly where the
water is very poor in calcium and other basic salts, and support
entirely different communities—for example around the shores of
lakes and tarns in areas where the rock is deficient in basic mineral
salts and the water is consequently ‘ soft’. ‘Typically the growth
consists largely of Bog-mosses (‘Sphagnum spp.) supporting a number
of characteristic higher plants such as Cotton-grasses (Eriophorum
spp.) and Sedges (Carex spp.), and often ‘ insectivorous ’ associates
such as Butterworts (Pinguicula spp.), Sundews (Drosera spp.), and
Bladderworts (Utricularia spp.). In areas of cool and wet climate
where the drainage is poor, such communities may cover considerable
flattish tracts and be known as ‘ blanket bogs’. Owing to the Bog-
mosses’ remarkable power of holding water in their sponge-like
cushions (see p. 51), these last can grow in height and enormously
in width, extending over marshes and even fens or open water,
depositing peat and raising the surface on which they grow. Such
‘raised bogs’ tend to be strongly acidic in reaction and reddish-
brown in colour.
Although bogs are commonly colonized by Heaths or even larger
374 INTRODUCTION TO PLANT GCEOGRAPHY [CHAP.
woody plants, the Mosses seem to control matters as long as they
remain fully active. However, with draining or natural drying out
of the surface, the succession usually proceeds to the local forest
or other climax—such as highmoors or drier heathlands in exposed
situations. Lowland- or meadow-moors are usually hydroseral
communities occurring on circumneutral peat accumulations result-
ing from the filling up of lakes and preceding the timbered stages
of succession. Here the upper layers above the level of ground-
water are often acidic in reaction, and either support heathland
communities dominated by Heaths in drier parts, or bog-like ones
dominated by Cotton-grasses or Moor-grass (Molinia coerulea) in
damper parts. Where acid conditions are developed by such humus
accumulation above basic rocks, ‘ flushes ’ from springs or superficial
drainage may continually bring down fresh supplies of basic salts
and lead to ‘ spring flush > communities very locally. Especially in
regions of marked topographical and geological variation is it common
to find whole series of moorland, dry heathland, pastured grassland,
and scrub or woodland communities existing side by side within a
relatively small area.
Besides the more extensive hinterland communities already men-
tioned, those of sea and other cliffs, and of river and other banks,
may be distinctive, though usually they are very limited in area and
too variable for detailed consideration here. Notable, however, is
the occurrence of cushion and other alpine plants along the dry
margins or beds of watercourses in the lowlands of mountainous
temperate countries on both sides of the Equator. Presumably their
disseminules are washed down from the uplands and their growth
is favoured by the * open’ conditions and general lack of competition
by ranker lowland types, which would prevent their ecesis or rapidly
succeed them in most other habitats.
Finally there are the various weed and allied communities that
follow anthropic disturbance ; for such activities as cultivation or
forest-cutting and their aftermaths are so widespread that in tem-
perate regions there is relatively little truly natural vegetation left.
Indeed of the more favourable areas capable of supporting climax
forest there can scarcely remain any that are wholly undisturbed
by Man or his domestic animals, though there are many which we
think of (and study) as practically natural. Even crops may be
considered to comprise communities of a sort, however artificial and
temporary they may be. Representative crops and some weeds
were dealt with earlier in this work ; especially do weeds form many
12] VEGETATIONAL TYPES OF TEMPERATE LANDS 375
and various if rather ephemeral communities, examples of which
are all too familiar to every farmer, gardener, and estate-owner.
Moreover, owing to almost universal introduction by Man and to
the similar ‘ openness’ of the habitats involved, these colonies of
weeds tend to be remarkably alike in similar situations throughout
the temperate and allied regions, often involving the selfsame
species in both Old and New Worlds, and in both Northern and
Southern Hemispheres. But in spite of the common luxuriance of
such weed communities, the full abandonment of agricultural or
other waste areas usually allows succession to proceed so rapidly
towards the local climax that within a very few years the weeds are
liable to have disappeared entirely. ‘These areas being usually in
tracts that had been cleared of former forests, the weeds are com-
monly superseded in the first few years by scrub or such weedy
trees as Birches or Poplars, before the return of anything like the
climax forest.
SOME PHYSIOGRAPHIC EFFECTS
Many physiographic effects, such as differences in exposure and
water-conditions due to ridges and depressions, have already been
dealt with, and the vegetation of uplands above the limit of arbor-
escent growth is treated in the next chapter. Outstanding, however,
are differences due to aspect and particularly altitude below the
tree-line, which can lead to marked differences in conditions and
local vegetation. ‘This we have already mentioned and illustrated in
Figs. 83, A, and 83, B, but must consider further here. Such differ-
ences are largely due to differences in local climate as explained in
Chapter X. The communities involved usually lie within the orbit
of those described elsewhere and so need not be treated in any
detail, though a few examples of the effects of (1) aspect and (2)
altitude in temperate and allied regions may be given with advantage.
The most widespread and commonly obvious aspect effect is that
due to orientation with regard to the sun’s rays. For example, in
the Mediterranean region some ridges lying east and west may bear
almost entirely different vegetation on their north- and south-facing
slopes, in extreme instances having not a single ecologicallyimportant
species in common. ‘Thus the south-facing slopes, exposed to the
full glare of the midday sun, tend to be occupied by the highly
xerophilous maquis or garigue of more or less sparse shrubs and
herbs. The ower parts of any steep north-facing slope, however,
376 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
will be protected from at least the strongest insolation and may bear
at the same altitude deciduous forest with hygrophilous ground-
vegetation. On the other hand in moist northern regions the most
luxuriant vegetation may be developed on the southern and western
slopes which receive the greatest benefit from the sun. ‘The tendency
is of course reversed in the southern hemisphere. However, north-
and-south-running mountain ranges may show instead another
aspect effect, namely, marked differences in rainfall on their two
sides. ‘Thus of New Zealand the eastern side, sheltered from the
prevailing westerly winds, has in places no more than one-tenth of
the rainfall of the forested western side, and supports only poor
tussocky grassland over considerable areas.
The general tendency towards cooler and damper conditions as
we ascend mountains usually leads to marked attendant changes in
the vegetation. ‘These may run the gamut from arid plains or
lowland forests, dealt with above, all the way to high-alpine regions
of perpetual snow. With the tundra and other communities lying
above (as in latitudes beyond) the tree-limit, we shall be concerned
in the next chapter. ‘Those communities developed between such
extremes and the general run of lowlands in temperate regions
usually involve faciations or extensions of one or other of the forest
types already described. However, in some instances, as most
notably the desert uplands of parts of temperate Asia, there may be
instead almost plantless wastes or expanses of moving sands—with
salt tracts of various extent, and only occasional oases supporting
deciduous trees such as Poplars.
A good example of the usual forested sequence in mountainous
districts is seen in western North America, where, above a basal
zone of ‘ improved ’ plains vegetation, the ‘ montane forest ’ extends
from the arid foothills upwards into the mountains through an
altitudinal range of often some 2,000 metres. ‘The main dominants
are Ponderosa Pine (Pinus ponderosa), White Fir (Abies concolor),
and Douglas Fir (Pseudotsuga taxifolia), though many others occur,
the closest relationship being with the Pacific ‘coast forest’.
Extending above, through an altitudinal belt of commonly some
1,000 metres, comes the ‘subalpine forest’, which is related
primarily to the boreal forest but also to the coast and montane
forests, the main dominants being species of Picea and Abies—
particularly P. engelmannit (Engelmann Spruce) and A. lasiocarpa
(Subalpine Fir), with often some Lodgepole Pine (Pinus contorta
var. /atifolia) and allied species. While the multiplicity of dominants
12] VEGETATIONAL TYPES OF TEMPERATE LANDS 377
makes for varied groupings, there is a tendency towards pure con-
sociations near the timber-line ; at its upper levels the forest also
becomes less luxuriant and the canopy lower, until it passes into
‘elfin wood’ and ultimately “ Krummholz’ of stunted and twisted
trees (see Fig. 108) about where the alpine tundra begins. How-
ever, mountain vegetation has no uniform pattern but varies from
range torange. ‘hus in many mountainous regions of the temperate
Fic. 108.—Pine *‘ Krummbholz’ at timber-line in the Rocky Mountains. (Phot.
W. S. Cooper.)
belt, as for example in central Europe and the White Mountains
of New England, there is a tendency for the lower zone of montane
forest, like the basal tracts, to be dominated by broad-leafed
deciduous trees, and only the higher (subalpine) levels to be pre-
dominantly coniferous. Often a fairly wide belt of mixed deciduous
and evergreen forest intervenes; and although there is a general
tendency for corresponding zones to decrease in altitude towards the
poles, there may be wide variation according to local conditions
even on the selfsame line of latitude.
FURTHER CONSIDERATION
Although this chapter represents in outline the results of considerable
reading and personal experience, often in obscure periodicals or remote
378 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
places, further details about most of the subjects treated may with
advantage be obtained from the standard general works dealing with the
vegetation of the land-masses of the world. ‘These include :
A. F. W. Scuimper. Plant-geography upon a Physiological Basis, transl.
and revised edition (Clarendon Press, Oxford, pp. xxx + 839 and 4
additional maps, 1903). See also the ‘third’ German edition revised
by F. C. von Faber and cited on p. 23.
M. E. Harpy. A Junior Plant Geography (Clarendon Press, Oxford,
pp. 1-192, 1913).
M. E. Harpy. The Geography of Plants (Clarendon Press, Oxford, pp.
xii + 327, 1920).
D. H. CampsBe.ti. An Outline of Plant Geography (Macmillan, London
(and New York), pp. ix + 392, 1926).
A. G. Tanstty & T. F. Cuipp. Aims and Methods in the Study of
Vegetation (Crown Agents for the Colonies, London, pp. xvi + 383,
1926).
M. I. Newsicin. Plant and Animal Geography (Methuen, London, pp.
xv + 298, 1936).
Concerning pertinent ecological principles, and for some local examples,
see the works of Tansley, Leach, Weaver & Clements, and Oosting, cited
at the end of Chapter X.
‘Treatment of the vegetation of different regions is extremely “ patchy ’,
even in the most populous of the temperate and allied lands as they were
considered in the present chapter. ‘Thus whereas on many areas there
is an extensive literature, including numerous accounts prepared by
trained and accomplished observers, on others there is very little. ‘The
total, however, is vast but scattered. Particularly have many valuable
and often well-illustrated accounts appeared in the British Journal of
Ecology, which has been published continuously since 1913. Others have
appeared from time to time in the American journal Ecology, which has
been published regularly since 1920, in the companion Ecological Mono-
graphs, of which a volume has appeared each year since its institution in
1931, and in the German Vegetationsbilder, of which 26 volumes were
published during 1904-44. Further accounts are appearing in the more
recently founded international journal Vegetatio (Junk, Den Haag, 1948-).
Many of the pertinent papers are cited in S. F. Blake & A. C. Atwood’s
Geographical Guide to Floras of the World (see p. 214).
A model of the kind of work which is desirable for each and every
region is Sir A. G. Tansley’s monumental The British Isles and Their
Vegetation (Cambridge University Press, Cambridge, Eng., pp. xxxviii
- 930, 1939), of which a new edition in two handier volumes is now
available. Examples of useful books on other, mainly temperate, lands
include L. S. Berg’s Natural Regions of the U.S.S.R. (Macmillan, New
12] VEGETATIONAL TYPES OF TEMPERATE LANDS 379
York, transl. edition, pp. xxxi + 436, 1950) and those published in Die
Vegetation der Erde—such as J. W. Harshberger’s Phytogeographic Survey
of North America (Engelmann, Leipzig, pp. Ixiii + 790 and map, 1911)
and L. Cockayne’s The Vegetation of New Zealand, second edition
(Engelmann, Leipzig, pp. xxvui + 456 and additional illustrations, 1928).
As examples of what may with advantage be done in elucidating a single
if generalized type of vegetation in one important region, we may cite
E. L. Braun’s Deciduous Forests of Eastern North America (Blakiston,
Philadelphia & Toronto, pp. xiv + 596 and map, 1950) and J. E. Weaver’s
North American Prairie (Johnsen, Lincoln, Nebr., pp. x1 + 348, 1954),
and, as a study of a particular ecological aspect wherever it may crop up,
V. J. Chapman’s Salt Marshes and Salt Deserts of the World (Leonard
Hill, London, pp. xvi + 352 -+ index, in press).
Whereas the world’s /ongest plants are probably some lianes of the
tropical rain forest which are reputed to exceed 655 feet (200 metres, cf.
P- 431) in length (and hence not to be rivalled by the giant Pacific Kelp
Macrocystis pyrifera, at least according to recent accounts—cf. p. 535), it
is in the temperate regions that there grow what appear to be the world’s
tallest plants—see p. 63. For this proud title there is considerable
doubt about the validity of claims of the past and some even about con-
tentions of the present, but the oft-quoted and apparently well authenti-
cated 364 feet cited on p. 63 as that of the tallest living Coastal Redwood
no longer stands, as the tree has lost its top and is now only 346 feet high.
News of this unfortunate loss has arrived as the present volume is in the
press, and, at the same time, from Dr. Lincoln Constance, of the Uni-
versity of California at Berkeley, details of another tree of Sequoia seniper-
virens, growing in the Bull Creek Flat area, that is reported to be 368-7
feet high, though he stresses that this measurement has not been finally
authenticated. Whereas even this height was apparently exceeded by
some Eucalypts growing is southeastern Australia in fairly recent times,
where heights of up to 500 feet are widely cited and one of 375 feet for a
specimen of Eucalyptus regnans seems to be well accepted (cf. A. R.
Penfold and J. L. Willis’s Eucalyptus : Botany, Cultivation, and Utilza-
tion, Leonard Hill, London, in press, and J. L. Willis m /itt.), there do
not appear to be among standing trees any very close rivals of the tallest
Sequoia sempervirens. Moreover, as the tallest known living Eucalypt
(growing in Tasmania, and also of the so-called ‘ Mountain-ash’, £.
regnans) was only 322 feet high in June, 1956, it is likely to be a good many
years before the Australians can again rival their American cousins in
the possession of the world’s tallest living tree.
CHAPTER XIII
VEGETATLIONAT TYPES “OF POLAR uN
EN Ds EUG sri Ares
The Arctic and Antarctic make up the ‘ polar lands’ and are
roughly those lying, respectively, north or south of the limit of
arborescent growth, even as the high altitudes here considered are
those above the timber-line. This ‘ tree-limit’ approximately
coincides with a mean temperature of 10° C. (50° F.) for the warmest
month (normally July in the Arctic). Actually, though use of the
tree-limit is a great improvement on the astronomically determined
but biologically misleading arctic and antarctic circles, satisfactory
delimitation of the Arctic and Antarctic can scarcely rest on such
a simple basis ; nevertheless for our present purpose it will suffice to
tell us approximately what to consider in this chapter, and what
to exclude,
Beyond the stunted ‘ elfin wood’ and twisted *“ Krummbholz ’ that
top the upper timbered slopes of mountains in forested regions, or
the usually more open ‘ taiga’ that terminates the poleward limit of
forests, is normally a zone of relatively luxuriant ‘tundra’. This,
by definition, is treeless, though in its most southerly tracts it may
contain shrubby examples of the forest dominants as well as, in
favourable situations there and elsewhere, bushes of other sorts.
‘The great and extensive examples are afforded by the Arctic, the
Antarctic having relatively little ice-free land apart from scattered
islands, and the high-alpine tracts being also limited. Consequently
most of this chapter is concerned with the Arctic, followed by
briefer mention of some antarctic and high-alpine features which
are often comparable—without, however, being by any means
identical.
[f we recognize some modern refinements! the Arctic may be
generally characterized as treeless, with the winters largely dark and
cold and the mean temperature of the warmest month p/us one-tenth
of the mean of the coldest month over a cycle of years not more
* Auggested, for example, in the Journal of Ecology, vol. 39, pp. 308-315,
1951.
380
VEGETATIONAL TYPES OF POLAR LANDS 381
than g° C., with less than fifty days between spring and fall frosts,
with the subsoil in most places permanently frozen, with an annual
precipitation normally below 50 cm. (commonly and widely below
25 cm.) and largely in the form of snow which drifts and is packed
tightly by the wind, with the soil generally moist in the summer,
and with sheltered salt as well as fresh water frozen over during
much of the winter. Both arctic and high-alpine regions typically
exhibit marked microhabitat effects and consequent variability from
spot to spot. ‘Their slopes may also undergo ‘ solifluction ’, which
is a slow flowing or creeping downwards of the comminuted surface
material over a frozen or other hard substrate, while in flatter areas
the sorting in relation to frost action of the surface soil into various
kinds of ‘ polygons’, most often with the finer material in their
centres, is extremely widespread especially in the Far North.
In spite of its treelessness and generally dwarfed nature, giving,
to the layman, an impression of monotonous sameness, the vegeta-
tion of arctic regions varies very markedly from place to place.
This variation is often extreme in closely contiguous areas of different
habitats or, it sometimes seems, without involving any marked
difference in conditions—even suggesting that repeated readjust-
ments to disturbance outweigh any tendency to equilibrium. Indeed
one of the most striking features of arctic vegetation is its extreme
variability from one small area to the next—in the absence of
sufficient growth to control the physical conditions of the environ-
ment, which conditions themselves often vary rapidly and even
drastically from spot to spot. "Thus whereas in a forest, for example,
the vegetation largely determines habitat conditions (including the
microclimate), in the Arctic the vegetation is relatively impotent.
Here the struggle of plants tends to be with the inimical forces of
a harsh physical environment rather than with hostile competitors
as in more favourable situations, though there is still plentiful
competition between plants in the more favourable arctic habitats.
Such competition is particularly rife where growth is relatively
luxuriant towards the southern limit of the Arctic; similarly in
high-alpine regions it is found mainly towards the lower limits, as
in the Antarctic towards the northern boundary.
In the arctic regions land is ranged practically around the North
Pole, though unlike the situation in the southern hemisphere there
is none at the very highest latitudes. In spite of considerable
differences in flora, especially at the lowest latitudes of what we
recognize as the Arctic, the over-all picture of vegetation is closely
382 DINED RODMC LON ae OM Pe AUNsis G BO} GIRVAVESEINYs [CHAP.
similar in the different sectors into which the Arctic may conveniently
be divided (see Fig. 46). Thus the vegetation developed under
similar habitat conditions in any particular climatic belt ranged
around the top of the globe tends to look much the same 1n whatever
sector it may lie, and there do not seem to be any major subsidiary
regions that can be singled out, such as the Mediterranean or various
semi-deserts in the temperate zone.
Under the prevailing cool conditions, water is very widely
sufficient in the Arctic for such limited growth as the climate, etc.,
allows, and the main vegetational differences in any particular
belt are rather in accordance with the actual habitats (such as were
described in Chapter XI). Thus local edaphic or physiographic
variations can ring the most immediate and fundamental changes
in the local plant life. On the other hand a progressive and almost
regular over-all depauperation of the vegetation is to be observed as
we go farther and farther north; and as this tends to be rather
closely comparable in the various sectors, it is deemed expedient to
separate each sector (and consequently the Arctic as a whole) roughly
into three main belts. ‘These are the /ow-Arctic, in which the vegeta-
tion is continuous over most areas, the middle-Arctic, in which it is
still sufficient to be widely evident from a distance, covering most
lowlands, and the high-Arctic, in which closed vegetation is limited
to the most favourable habitats and is rarely at all extensive.! ‘The
following outline account of the main vegetational types of the
Arctic will accordingly have, under each major heading, some con-
sideration of the expression of this type in each of these three belts,
ranging from south to north. Examples of low-arctic lands are the
southern portions of almost all sectors, of middle-arctic lands Jan
Mayen Island and the vicinity of Point Barrow, Alaska, and of high-
arctic lands the whole of the Spitsbergen Archipelago, and the
Canadian Eastern Arctic north of Lancaster Sound.
Arctic ‘TUNDRAS
‘The term ‘ tundra’, meaning essentially a treeless plain, has been
used in so many and often such vague senses that it seems desirable,
if we are to retain it at all, to limit its use so that it will have a more
precise scientific connotation. In the present work the tundra
proper is understood as the usually ‘ grassy’ formation lying beyond
(or in some extra-arctic places forming patches within) the limit of
' See Frontispiece for a first, tentative attempt to delimit these three arctic belts.
13] VEGETATIONAL TYPES OF POLAR LANDS 383
arborescent growth—except where shrubs or undershrubs pre-
dominate (in scrub and heathlands), or vegetation covers less than
half of the area (in ‘ fell-fields’ and ‘ barrens’). Although this still
includes such special cases as salt-marshes and manured areas; it
is customary to consider these separately, as is done in the present
work. On the other hand, it excludes the taiga and at least the
forested parts of the mixed ‘ forest-tundra’ described in the last
chapter (p. 348). Generally comparable types occur in antarctic
regions where, however, suitable land areas are relatively small.
Alpine tundra bears a similar relationship to the timber-line on
mountains. Instead of true Grasses which, however, are rarely
absent, grass-like plants such as Sedges (Carex spp.), Cotton-grasses
(Eriophorum spp.), Rushes (Juncus spp.), and Wood-rushes (Luzula
spp.) commonly afford most of the ‘ grassiness’ of the tundra,
though various perennial forbs are usually associated, as are often
a sprinkling of dwarf woody plants.
Even in this restricted sense the tundra developed in almost any
arctic region is usually very variable, different areas supporting
widely different types. The variation takes place particularly with
differences in exposure and in water and other soil conditions, and,
at all events in low- and middle-arctic regions, affords faciations
far too numerous even to mention here. We may, however, dis-
tinguish and outline, besides a general central type, the tundras of
damper depressions on the one hand and of drier exposed areas on
the other.
The general run of tundra which covers a large proportion of the
lowland plains and some less extensive upland areas of most low-
arctic regions is commonly a rather thin ‘ grassy ’ sward dominated
by mesophytic Sedges such as the Rigid Sedge (Carex bigelowii agg.)
and Grasses such as the Arctic Meadow-grass (Poa arctica s.1.), with
various associated forbs and under-shrubs including dwarf Willows.
The whole forms a continuous if often poor sward commonly 15—
35 cm. (approximately 6-14 in.) high in which a mixture of various
Bryophytes and Lichens usually forms a rather poorly marked second
layer a very few centimetres high.
Commonly the low-arctic tundra is a mosaic made up of faciations
having each some lesser number of the total association dominants,
and including consociations having only one of these. ‘The areas
of the component communities are often small and the variation
from spot to spot in the tundra is accordingly usually considerable.
In addition there are often local societies dominated by species other
384 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
than the association dominants. ‘The (sometimes unaccountable)
mixing and even intergradation of all these communities is often
intricate and may be suggestive of their relative youth, many having
apparently failed to come to a state approaching equilibrium with
the environment since emergence from glaciation or other extreme
disturbance. Actually, it may be questioned whether, in many areas,
even relative equilibrium can be attained in the face of the persistent
frost-activity, and it has been claimed that the whole system con-
stitutes an ‘open’ one in which the main tendency is repeated
Pe toe B. ee : i.
Fic. 109.—Tundra on Southampton Island, Hudson Bay. ‘The depressions
support low bushy Willows.
readjustment to almost perpetual disturbance. Fig. 109 shows an
area of low-arctic tundra in eastern Canada.
With the generally poor drainage resulting from the soil being
permanently frozen to not far beneath the surface, damper depres-
sions or marshy open tracts tend to be plentiful although often of
quite limited extent ; indeed they are rarely absent except in regions
of porous substrata and low water-table. In the low-Arctic they
are commonly rather luxuriantly vegetated, the sward often being
taller than it is in drier areas. ‘They are usually dominated by
Cotton-grasses and relatively hygrophytic Sedges such as marshland
ecads of the Water Sedge (Carex aquatilis agg.), and by Grasses such
as the Arctagrostis (Arctagrostis latifolia s.l.), with a few hygro-
13] VEGETATIONAL TYPES OF POLAR LANDS 385
philous Willows or other ground-shrubs and many hygrophilous or
ubiquitous forbs. ‘Typical among these last are Viviparous Knot-
weed (Polygonum viviparum) and the bright-flowered Yellow Marsh
Saxifrage (Saxifraga hirculus agg.). ‘The fairly luxuriant cryptogamic
layer is largely composed of Mosses, and helps to consolidate the
whole. An example is seen in Fig. 110. Often these marshy areas
are beset with small hummocks commonly about 25 cm. high, and
introducing drier conditions on their tops, which may then support
heathy plants and Lichens. Such hummocky tracts are known as
Fic. r10.—Marshy tundra near the south shore of Hudson Strait, dominated by
Cotton-grasses, Sedges, and Grasses, with dwarf Willows creeping among the
subdominant Mosses.
‘hillock tundra’. In other instances tundras, especially of the
damper types, are liable to be much interrupted by various of the
geodynamic influences prevalent in cold regions—such as, par-
ticularly, solifluction and ‘patterned soil’ (polygon—see p. 381)
formation of various kinds.
The drier tundras of raised areas or well-drained surface material
in low-arctic regions tend to be much poorer and thinner than the
damper types. ‘Typically they are composed of an extremely
various array of more or less xerophilous Sedges (such as the Rock
Sedge, Carex rupestris), Willows (particularly the Arctic Willow, Salix
arctica s.1.), Grasses (such as Alpine Holy-grass, Hierochloe alpina),
386 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
Northern Wood-rush (Luzula confusa agg.), and various forbs (such
as the same Viviparous Knotweed), in addition to Mountain and
Arctic Avens (Dryas spp.), which are somewhat woody, and which
may dominate considerable areas. But although scattered heathy
plants occur in them, these areas are scarcely heaths, any more than
are the lichen-rich ones dominated by xerophilous Sedges that
characterize dry and exposed situations (see p. 392). Moreover
their vegetation is usually rather poor, often barely covering the
Fic. 111.—Dry tundra on raised area overlooking Hudson Bay, composed princi-
pally of an intricate mixture of xerophilous Lichens, Grasses, Sedges, and other
herbs. Dwarf woody plants also occur, and the surface is interrupted by pro-
jecting lichen-covered boulders.
ground in spite of a plentiful admixture of Lichens and sometimes
also of Bryophytes. Fig. 111 shows such an area in which boulders
project through the thin and somewhat heathy, lichen-rich vegetation.
Especially on limestone or porous sandy substrata is growth often
poor and the vegetation relatively sparse, although the component
flora particularly in calcareous areas may be very various.
It would accordingly seem that the major variations in the precise
type of tundra take place chiefly, but by no means solely, with
local water conditions working through exposure or edaphic factors,
13] VEGETATIONAL TYPES OF POLAR LANDS 387
while very locally the effect of frost action may be paramount.
Thus differences in substratum, as between limestone and acid-
weathering rock, can introduce vegetational differences due to
particular plants’ preferences quite apart from water-relations, while,
as an example of the entry of another factor, heavy pasturing can
lead to increased grassiness as in temperate regions. In addition,
polygon-formation and solifluction may cause persistent disturbance.
It may be noted that, whereas the dominants are usually at least
specifically distinct in different types of low-arctic tundra, some of
the less exacting, more tolerant associates may be present in a wide
range of habitat types. ‘This again is comparable with the situation
in cool-temperate regions and, it often seems, obtains still more
forcibly to the north. ‘Thus in the Far North some of the hardier
plants, such as Viviparous Knotweed and some of the Saxifrages,
grow in an extraordinarily wide variety of habitats, ranging from wet
to dry, exposed to sheltered, and open-soil to vegetationally ‘ closed ’.
The middle-arctic belt is characterized by tundras of a generally
poorer type, both in the matter of flora and luxuriance of develop-
ment, than the low-arctic ones. ‘Thus some of the plants which
were important in low-arctic tundras are absent, though all of the
dominants, etc., mentioned above for low-arctic tundras can, and
frequently do, occupy a similar position also in middle-arctic regions.
Moreover the range of types is much the same, damp, mesophytic,
and drier ones being distinguishable. An example of the second,
dominated by mixed Grasses and Sedges, in northernmost Alaska
overlooking the Arctic Ocean, is shown in Fig. 112, from which it
may be seen that growth tends to be lower and poorer than in low-
arctic regions, though this particular area is only just middle-arctic
in type.
In high-arctic regions still further depauperation is general, and
indeed only limited and relatively few areas are sufficiently vegetated
to be designated as tundra. ‘These areas are chiefly marshy ones
and may be still dominated by Sedges, Cotton-grasses, and Grasses
—often of the same species as in the South, and including similar
associated forbs, though woody plants apart from prostrate Willows
are usually absent. Mosses commonly consolidate the whole, and
in some places appear to dominate. Fig. 113 shows an unusually
extensive area of marshy tundra in Spitsbergen, characterized by
peaty hummocks up to 25 cm. high, and of a type commonly termed
‘hillock tundra’. While the main dominants in such areas are
commonly Sedges and Grasses growing on the sides or tops of the
388 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
Fic. 112.—Mesophytic Sedge-Grass tundra on the north coast of Alaska near
Point Barrow, overlooking the Arctic Ocean. Ice-pups are seen stranded on the
shore, and in the distance the margin of the polar pack-ice is visible. The tundra
in flat areas is usually closed; but on slopes, where disturbed by geodynamic
influences, it is commonly discontinuous.
Fic. 113.—Extensive area of damp ‘ hillock tundra’ on the coast of Spitsbergen.
Note the grassiness of the hummocks but frequent puddles of water between.
13] VEGETATIONAL TYPES OF POLAR LANDS 389
hummocks, the microhabitat effect is extreme, the microhabitats
ranging from depressions occupied by dark boglets or puddles of
‘free’ water (seen in Fig. 113) to dry hillock tops occupied by
Lichens or, in favourably sheltered situations, xeromorphic ground-
shrubs.
Tracts of ‘ grassy’ mesophytic tundra of any substantial extent
Fic. 114.—Discontinuous tundra-like tract of mixed Grasses, Northern Wood-
rush (Lusula confusa agg.), forbs, and Polar Willow (Salix polaris agg.), in inland
valley, West Spitsbergen, grazed by a pair of wild Reindeer. Beyond is the dry
bouldery bed of a melt-water stream that is dry during most of the summer, and,
behind, barren scree and other slopes.
are not common in the high-Arctic, though Fig. 114 shows a dis-
continuous but tundra-like patch of mixed Grasses, Northern Wood-
tush (Luzula confusa agg.), forbs, and Polar Willow (Salix polaris
age.), that is sufficiently developed to attract Reindeer. It is situated
in a sheltered valley well inland in Spitsbergen, by the side of a
bouldery bed of a snow-water stream that is dry during most of
the summer. Still drier types of tundra in these farthest north
lands tend to be dominated largely by Lichens and to be much
interrupted by rocks or bare patches—especially in exposed situations.
390 INTRODUCTION TO PLANT GEOGRAPHY
So far as regular ecological successions are concerned, these are
especially problematical in the Arctic. It has, however, been sug-
gested that the marshy and dry tundras may be subclimax and the
mesophytic ones climax or preclimax, the scrub and _ heathlands,
which are developed in the most favourable situations (see below),
being either postclimax or, perhaps, indicative of a more general
climax to be expected ultimately in sufficiently favourable situations,
though at present a mixed ‘ polyclimax’ is commonly found. ‘The
significance of different ‘stages’ in the hypothetical successions
may, however, vary from place to place. ‘Thus, in the Far North,
heathy plants are apt to be so restricted to the most favourable
situations as to suggest that without major climatic change they
could not become widely dominant in the manner already obtaining
in some places in the low-Arctic. Moreover, frost and other dis-
turbance is so widespread, inter alia impeding or even preventing
the maturation of soils, that it seems as though many areas undergo
a kind of perpetual readjustment rather than exhibit the tendency
to equilibrium which is implicit in a real climax.
ARCTIC SCRUB AND HEATHLANDS
A shaggy scrub of Willows and/or Birches is commonly developed
on the most favourable slopes, in damp depressions, and especially
along watercourses and the margins of lakes in low-arctic regions.
It is commonly around 60 cm. (about 2 feet) high, as in the example
shown in Fig. 115, but tends to become lower and more restricted
northwards until, about the centre of the middle-arctic belt, it
becomes usually very limited in extent and stature. However, in
the most favourable situations in the extreme south the Willows
may be luxuriant: (cf. Fig. 116) and even exceed the height of a
Man, and especially in southwestern Greenland the scrub is quite
extensively developed, in some places including arborescent Birches.
‘These Greenland: Birch ‘ forests’ are of very limited extent, with
the trees scattered and scraggy though sometimes nearly 6 metres
in height and 25 cm. in stem diameter. ‘Their areas have been
termed subarctic but seem too limited to separate on an over-all,
world basis ; they are also too fickle, the development of an arbores-
cent habit being evidently dependent on local shelter, etc. Apart
from these larger Birches, the main dominants in different regions
are most often the Dwarf Birch (Betula nana agg.) or Scrub Birch
(B. glandulosa agg.), or such shrubby Willows as the Glaucous
Fic. 115.—Tangled Willow scrub in the low-arctic belt of the Northwest Ter-
ritories, Canada. ‘The scrub occupies a slight depression whose depth is indicated
by a spade resting on the ground (in centre).
ite %s Noite ; ee 4 i oe ee om
Fic. 116.—Patchy scrub of Glaucous Willow (Salix glauca s.1.) and Scrub Birch
(Betula glandulosa agg.), up to nearly 2 metres high, in southwestern Greenland.
The scrub is interrupted by grassy tracts of fair turf which appear to result from
ancient pasturing (see page 222).
gue
392 INTRODUCTION LO RwAN DL 16 EOIGR AEP Ey. [CHAP.
Willow (Salix glauca s.1.), the Broad-leafed Willow (S. cordifolia s.1.),
the Feltleaf Willow (S. alaxensis agg.), or Richardson’s Willow (S.
richardsonii agg.). Often two or more of these shrubs will dominate
a mixed association. In some places bushes of Green Alder (Alnus
crispa agg.) are present and may be locally dominant.
Such scrub at its best is so thickly tangled and produces so much
litter that few associated plants occur, apart from tall Grasses such
as Bluejoint (Calamagrostis canadensis agg.) and occasional straggling
forbs. But where the dominants are less luxuriant, an extensive
flora is often found, including a considerable variety of herbs and
Mosses, or, in dry situations, subdominant heathy plants such as
Crowberry (Empetrum nigrum s.|.). Also characteristic of dry scrub
are patches of tall Cladonias, Stereocaulons, and other Lichens, with
or without Polytricha or other coarse Mosses. ‘To the north such
scrub thins out gradually, its most northerly expression about the
northern limit of the middle-arctic belt being usually in the form
of single or scarcely confluent bushes that rarely exceed 50 cm. in
height and are usually much lower, though often quite wide.
Heathlands are more widespread and various in the Arctic than
is scrub, though still commonly occupying only a very small propor-
tion of the total area. ‘They are usually characterized by being
dominated by members of the Heath family (Ericaceae) or by heath-
like plants such as, particularly, Crowberry. Sometimes, however,
broad-leafed plants such as Avens (Dryas), or Sedges such as the
Nard Sedge (Carex nardina s.1). or Bellard’s Kobresia (Kobresta
myosuroides), may dominate dry and usually exposed, lichen-rich
areas that are often classed as heathlands rather than among the
drier tundras with which they seem more properly to belong (see
p. 386). Leaving aside such cases it may be said that heathlands
in the Arctic tend to be confined to the more favourable, sheltered
situations that are snow-covered in winter—provided they are not
too moist in summer. In many regions they characterize coarse-
grained rather than clayey soils, as pointed out by Professor ‘Thorvald
Sorensen (in Iitt.).
In the low-arctic belt the heathlands are usually covered by a
continuous thick sward of mixed woody and herbaceous plants, the
main dominants being typically 8-15 cm. high. ‘These commonly
include Crowberry, Arctic Blueberry (Vaccinium uliginosum subsp.
alpinum), Mountain Cranberry (V. vitis-idaea agg.), Arctic Bell-
heather (Cassiope tetragona), Narrow-leafed Labrador-tea (Ledum
palustre agg.), Dwarf Birch, and various diminutive Willows. Often
13] VEGETATIONAL TYPES OF POLAR LANDS 393
the dominants themselves are much mixed, and usually they are
consolidated below by a layer of cryptogams in which Mosses or
Lichens commonly subdominate according to whether the situatioa
is relatively moist or dry, respectively. Fig. 117 shows an area of
dense mixed heath on the south shore of Hudson Strait. In the
drier situations there may occur frequent gaps in the heath which
are actually dominated by Lichens—particularly by ‘ Caribou-moss’
Cladonias that may form a sward 5 or more cm, high. In depres-
sions and behind obstructions where snow drifts deeply in winter,
Fy nao Pe
Fic, 117.—Dense low-arctic heath dominated by Arctic Blueberry (Vaccinium
uliginosum yar. alpinum) in northernmost Quebec, lLight-coloured Lichens and
leaves of dwarf Willows and Sedges are visible, To the left of the sheath-knife
is a flowering bushlet of Lapland Rose-bay (Rhododendron lapponicum),
a characteristic dark (except when flowering) heath dominated by
Arctic Bell-heather usually develops, often with associated Sedges
and Mosses at least where the soil is lastingly moist. Such an area
is shown in Fig, 118 and, apart from a zone of more mixed heath
that may develop outside, usually constitutes the outermost of the
zoned series of subclimaxes developed in late-snow areas as described
on pages 402-5.
In the middle-arctic belt, heathlands are usually somewhat lower
in stature and more restricted in area than to the south, having the
appearance of postclimaxes developed in the most favourable situa-
tions. Of the cited dominants Mountain Cranberry has usually
394 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
Fic. 118.—‘ Snow-patch’ darkened by Arctic Bell-heather (Cassiope tetragona),
southern Baffin Island.
Fic. 119.—Mixed middle-arctic heath with many light-coloured and other Lichens.
Bushlets of Narrow-leafed Labrador-tea (Ledum palustre agg.) are seen to the
right and left of the pipe, which is 15 cm. long and gives the scale. Northern
Baffin Island.
13] VEGETATIONAL TYPES OF POLAR LANDS 395
disappeared, and although the taller ones may still exceed 20 cm.
in height the sward is usually only 5-10 cm. high. Whereas it may
still be fairly dense, more often the ‘ heath’ is of scattered ground-
shrubs with intervening thin patches of Cetrarias, Alectorias, and
other Lichens as seen in Fig. 119.
In the high-arctic belt heathy plants are entirely absent over
considerable areas, the tracts that are popularly spoken of as ‘ heaths ’
being usually dominated by Avens, Sedges, or even Lichens. How-
ever, Crowberry or Arctic Blueberry plants are to be found in some
regions, dominating limited heathy communities in unusually favour-
able situations, while Arctic Bell-heather is quite widespread, char-
acteristically forming a dark tract where the snow accumulates
sufficiently to form a good protective covering in winter (though
disappearing early in the growing-season).
ArcTIc FELL-FIELDS AND BARRENS
These are types in which the evident vegetation occupies less
than half of the area; and whereas the two categories are scarcely
to be rigidly distinguished, it is usually those tracts that bear rela-
tively few and scattered plants that are referred to as ‘ barrens’.
Fell-fields typically have a surface of frost-shattered detrital material
including much finer ‘ soil’ and usually support fairly numerous
different species forming mixed communities, whereas barrens are
apt to be characterized by one prominent size of particle and a
single species of plant, such as Mountain Avens or Purple Saxifrage
(cf. Fig. 128). ‘This is especially the case when they occupy the
most exposed situations.
Where sufficient moisture is present these poorly-vegetated areas,
like some tundras, are commonly disturbed by all manner of frost-
heaving and allied effects—such as solifluction on slopes and polygon-
formation on the flatter terrain. ‘The solifluction is generally
manifest in streaks extending longitudinally downhill, adjacent
streaks being either of different material or accentuated by vegeta-
tion which cannot grow on the more dynamic parts. ‘The‘ polygons ’
are almost endlessly variable but very commonly have the form of
polygonal or circular areas }~2 metres in diameter and composed of
finely comminuted soil that is apt to be too dynamic to support
any plants at all, separated by narrow intervening tracts containing
most of the larger stones and often raised and vegetated or in other
cases barren (Fig. 120). Or the polygons may be separated by
396 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
narrow cracks or troughs that afford shelter for plants which in the
Far North often grow better in such microhabitats than in surround-
ing areas.
On steep slopes and below weathering crags, still more dynamic
and often poorly vegetated ‘screes’ are common. Also often con-
stituting barrens of one sort or another are ‘ raised beaches’ near
sea-shores ; for although some are well vegetated, many others are
the reverse, owing to exposure, recent emergence, or an unfavourable
Fic. 120.—‘ Polygons’ in northernmost Spitsbergen. ‘The stony intervening
tracts are here almost barren, but often in other instances are covered with
vegetation.
substratum. Altogether these poorer types of vegetation—or ter-
rain, for often plants are scarcely at all in evidence—are so numerous
and variable in the Arctic that only a few examples can be mentioned
here.
In low-arctic regions fell-fields, barrens, and the like, are found
chiefly in upland districts and in exposed areas near the coast—
especially where the substratum is of porous material. Here, owing
to lack of stability, to local aridity, or to extreme exposure, such
fell-field or ‘ half-barren’ areas as that shown in Fig. 121 occur,
in which Mountain Avens, Nard Sedge, and various tufted Saxifrages
and other herbs form irregular patches of vegetation. A fair number
of cryptogams are often admixed, though usually they are of poor
13] VEGETATIONAL TYPES OF POLAR LANDS 397
growth. In the most unfavourable situations of all, this type may
thin out to a stony barren supporting little more than diminutive
crustaceous Lichens and very occasional depauperated tussocks of
Avens or Purple Saxifrage.
Although such poorly-vegetated areas tend to be more numerous
and widespread in the middle-arctic zone than farther south, they
still do not normally occupy the general run of lowland terrain but
are chiefly encountered in exposed situations (as for example in the
Fic. 121.—Fell-field on calcareous soil in exposed situation, northernmost
Labrador.
foreground of Fig. 128). An extensive example is shown in Fig.
122, from an altitude of 671 metres (2,200 feet) in central Bafhn
Island, in which Lichens of poor growth cover much of the surface,
vascular plants being virtually absent.
In most high-arctic regions ‘ open’ and often extremely sparse
vegetation is the general rule, and so fell-field and barrens areas are
widespread and plentiful. A relatively well-vegetated and extensive
area, reminiscent of many observed by the author in far northern
Ellesmere Island and Spitsbergen, is seen in Fig. 123, which he
took, however, in the vicinity of the North Magnetic Pole on Prince
of Wales Island. It shows a monotonous expanse of mixed but
scattered and open, diminutive herbs and terricolous (7.e. earth-
inhabiting) Lichens, with occasional small tufts of Avens (in this case
Dryas integrifolia agg.). ‘The general aspect is desolate in the extreme
fo)
398 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
Fic. 122.—Lichen barrens in the uplands of central Baftin Island, looking south.
Note the virtual absence of higher plants and the abundance of persistent snow-
patches on north-facing slopes.
Fic. 123.—Monotonous tract of prairie-like fell-field in the vicinity of the Magnetic
North Pole, Prince of Wales Island, Canadian Arctic Archipelago. ‘The sparse
vegetation consists of scattered diminutive herbs, terricolous Lichens, and occa-
sional tufts of Arctic Avens, all growing in open formation.
13] VEGETATIONAL TYPES OF POLAR LANDS 399
in most of such exposed high-arctic tracts, with a large proportion
of the area typically occupied by barrens supporting little more than
occasional tufts of Arctic Poppy (Papaver radicatum s.\.) or Purple
Saxifrage (Saxifraga oppositifolia agg.), although more mixed fell-
fields may occur in less unfavourable situations. On well-drained
banks there may be Grasses and some hardy but attractive forbs,
while very occasionally under the most favourable conditions a
limited tract of thin heath may be developed, though often one can
trek for days without encountering such a manifestation. ‘The only
at all extensive tracts of closed vegetation are mostly of rather thin
marshy and often hummocky tundra (Fig. 113), and even here the
dominants rarely exceed a height of 30 cm. above the surface from
which they grow. Yet most of these vegetation-types and a fair
range of vascular plants are to be found right up to the highest
latitudes of land, between 83° and 84° N., in marked contrast to the
situation in Antarctica (see pp. 415 ef seq.).
SEASIDE AND OTHER LOcAL ‘TYPES
On sandy and fine-shingle maritime beaches in both low- and
middle-arctic belts there are usually scattered plants of Sea-purslane
(Arenaria peploides agg.) and Sea Lungwort (Mertensia maritima
agg.) on the foreshore and, farther up, stabilizing beds of Lyme-
grass (Elymus arenarius s.1.) which may be fairly tall and luxuriant
(Fig. 124). In the high-arctic belt, Lyme-grass is unknown and
the other two are rare, so exposed sandy and shingly shores are
liable to be barren around high-tide mark.
In sheltered and less well-drained seaside areas, muddy or sandy
‘ salt-marshes ’ are common though usually of very limited extent
in the Arctic. Even more than many other types of vegetation,
they show close similarity of form all around the southern portions
of the Arctic—and also, with natural depauperation, far northwards.
Thus in low-arctic areas they typically consist of a dwarfish grassy
sward dominated by Alkali-grasses (particularly the Creeping Alkali-
grass, Puccinellia phryganodes agg.) and Sedges (particularly the Bear
Sedge, Carex ursina, and phases of the Salt-marsh Sedge, C. salina
s.l.), with associated Low Chickweed (Stellaria humifusa), Scurvy-
grass (Cochlearia officinalis s.1.), Pacific Silverweed (Potentilla egedit
agg.), and other halophytes. Fig. 125 shows such a salt-marsh
on the south coast of Baffin Island. Except for the usual absence
farther north of the Silverweed and the substitution of Salt-marsh
"400 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
Fic. 124.—Shingly beach bound by swarded Lyme-grass (Elymus arenarius s.1.).
Farther down, between tide-marks in this sheltered inlet, the shore is darkened
by algal growth. Hudson Strait, northeastern Canada.
Fic. 125.—Looking down on a salt-marsh dominated by Pacific Silverweed
(Potentilla egedii agg.) (flowering) and Creeping Alkali-grass (Puccinellia phryganodes
agg.) (light-coloured stolons). Pipe gives scale. South coast of Baffin Island.
13] VEGETATIONAL TYPES OF POLAR LANDS 401
Sedge by the doubtfully specifically distinct Hoppner Sedge (Carex
subspathacea), the same plants generally play a similar role in middle-
and high-arctic regions, though with increasing depauperation. It
seems probable that they are unable to alter their habitat markedly,
at least if and when it has reached the approximate level of the
highest tides, and consequently that they represent a subclimax
which will persist indefinitely.
Another type of local climax is engendered by the perennial
manuring that takes place around the ‘ bird-cliffs ’ where countless
Fic. 126.—Luxuriant ‘ patchwork quilt’ of mixed and many-coloured Lichens
and Mosses developed near top of bird-cliff. Northernmost point of Quebec,
overlooking Hudson Strait. Scale indicated by pack on left which is 60 cm. high.
sea-birds nest every summer. Here unoccupied ledges may support
coarse Grasses and rank Scurvy-grass, the rock faces being covered
by Lichens, often of extraordinary size. ‘The tops of the cliffs
typically support near their edges and in damp depressions a rich
grassy sward, and, stretching back for 100 metres or so, a luxuriant
and dense ‘ patchwork quilt’ of mixed and variously coloured Lichens
and Mosses. This is seen in Fig. 126 and is due to manuring
effects engendered apparently largely by scavenging Sea-gulls and
Birds-of-prey, though in some instances Foxes and Polar Bears are
also involved. The situation being usually very exposed, the
adjacent unmanured cliffs and hinterland are apt to be practically
barren and in striking contrast. More local manuring may also
402 INTRODUCTION TO’ PLANT GEOGRAPHY [CHAP.
give remarkable effects, such as the grassy or flower-decked swards
that develop around human habitations, mammalian burrows, or,
particularly, the nesting-grounds of Geese, Eiders, and other
gregarious wildfowl (Fig. 127). An instance of a different kind
is shown in Fig. 128, in which a dense grassy patch is developed
around a boulder in an exposed Purple Saxifrage barren overlooking
the sea ; to such prominences Birds and predators repair, manuring
the ground in the immediate vicinity and doubtless sometimes
Fic. 127.—Looking down on a luxuriant mossy mat on the manured periphery
of a wildfowl nesting-ground in Spitsbergen. ‘The plant flowering on the right,
above the matchbox (giving scale), is Yellow Marsh Saxifrage (Saxifraga hirculus
agg.), the flowers on the left are of Alpine Brook Saxifrage (S. rivularis agg.),
and the small ones below are of Fringed Sandwort (Arenaria ciliata s.1.).
bringing in viable seeds. ‘The result is often a luxuriant if limited
sward in an otherwise seemingly sterile situation.
As very little cultivation or other human disturbance has so far
taken place in most arctic areas, few tracts bear witness to such
change ; on the other hand a common and widespread type of local
climax is that engendered by the drifting and late-melting of snow.
This tends to take place similarly each year and to lead to char-
acteristic vegetational zonation within the area of the drift. The
zones produced are of subclimax nature although the outermost may
be considered postclimax—especially in the Far North where the
13) ; VEGETAL TONALS DYPES) OF POLAR LANDS 403
most widespread and often the sole heathy plant, Arctic Bell-
heather, may be practically confined to such situations. In the low-
and middle-arctic belts the outermost zone of such ‘ snow-patches ’,
which is well protected in winter by snow but does not have its
growing-season markedly reduced by late melting, is commonly
vegetated by a luxuriant mixed heath (Fig. 129), or, in lastingly
damp situations, by a thin Willow scrub. Farther in, where
the snow. drifts sufficiently deeply for the growing-season to be
Fic. 128.—Purple Saxifrage barren on exposed ridge overlooking the sea in
northernmost Baffin Island. The Saxifraga oppositifolia agg. forms only scattered
tufts and small dark matlets that scarcely show in the photograph; nevertheless,
around a prominent boulder that has persistently acted as a perch, there is a
luxuriant grassy sward where the ground has been manured.
appreciably shortened, Arctic Bell-heather characteristically forms
a dark belt, often being the sole dominant as in Fig. 118. In
other instances this belt may be more mixed and only 1-3 metres
wide, as in Fig. 129, which shows in the background the more
barren inner zones. ‘These are apt to vary considerably in number
and vegetation in different places and circumstances. However, they
typically include towards the outside a zone of dwarf Willows
(particularly Herb-like Willow, Salix herbacea, as in Fig. 130),
and, farther in where the growing-season is too short for woody
plants, a sparsely vegetated zone with a considerable variety of
Fic. 129.—‘ Late-snow ’ patch in the highlands of central Baffin Island, showing
in foreground the outer zone of mixed Arctic Blueberry heath, then a narrow belt
of dark Arctic Bell-heather interrupted by light-coloured Lichens, and behind,
a Herb-like Willow zone (see Fig. 130), the centre of the snow-drift area where
the snow melts latest being occupied by a herb ‘ barren.’
Fic. 130.—Looking down on the Herb-like Willow zone of the late-snow area
shown in Fig. 129. The rounded leaves are those of the dominant Willow, the
ground being characteristically encrusted with cryptogams. Pipe 15 cm. long
gives scale,
13] VEGETATIONAL TYPES OF POLAR LANDS 405
Bryophytes and open-soil herbs such as the Mountain Sorrel
(Oxyria digyna).
Many of the smaller snow-patches, of course, disappear well before
the end of summer, especially in the South, and consequently show
only the outermost zones. On the other hand around the centre
of the deeper drifts, which usually form in ravines, depressions, or
behind banks or ridges, the snow may melt only towards the very
end of summer or in a cool season not at all. Here most herbs are
unable to persist and even cryptogams are little in evidence, though
some tufts or limited mats of Bryophytes and investments of Algae
are usually to be found, together with the diminutive grass Phippsia
algida agg. (Frigid Phippsia). ‘Towards such centres various plants,
including attractive Saxifrages and Buttercups (Ranunculus spp.),
are often to be found flowering at the very end of summer, sometimes
being caught still in bud by the frosts and snow of a new winter.
In the Far North the zones are apt to be reduced in number but
extended in area, the fell-fields and barrens often representing inner
zones where the modest snow-covering melts late under the pre-
vailingly cool conditions. Here, as on mountains farther south,
many of the snow-patches are perennial or even eternal, the chief
growths near their centres being of Bryophytes and Algae in the
run-off below.
SERAL ‘TYPES
Whereas the apparent arctic counterparts of many southern seral
types have already been dealt with, being often seemingly incapable
of leading to permanent higher vegetation or best regarded as sub-
climax (in view either of persistent disturbance or of the extreme
slowness of vegetable build-up), other instances of seral stages remain
to be mentioned. Outstanding are the marshy and boggy ones of
hydroseres (the fully aquatic communities comprising the early
stages of which will be dealt with in Chapter XV), various ones of
lithoseres, and the ‘ flower-slopes ’ that probably belong to mesoseres.
The hydrosere of arctic lakes and tarns usually has as its first
aerial stage a ‘reed-swamp’ of aquatic Sedges (particularly the
Water Sedge, Carex aquatilis agg.) and /or Cotton-grasses (particularly
the Tall Cotton-grass, Eriophorum angustifolium agg.), though some-
times Common Mare’s-tail (Hippuris vulgaris s.l.), or such coarse
Grasses as the Tawny Arctophila (Arctophila fulva agg.), may largely
or wholly take their place. Any of these plants may form luxuriant
406 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
beds where the bottom is soft and not more than about 40 cm. deep.
They generally project some 20-40 cm. above the water in the south
but usually less in the Far North, where Mare’s-tail is normally
absent. Commonly such beds are accompanied by aquatic Mosses
and, of course, numerous small Algae. Behind there stretches
typically a marshy sedge-meadow with the same ‘ grassy ’ dominants
and, in addition, Arctagrostis and lowly Willows. ‘This in turn
merges into damp tundra. Fig. 131 shows such a sequence in
the low-Arctic, though it should be noted that in exposed situations,
especially farther north, wave action may prevent reed-swamp
formation, a definite ‘hard line’ then delimiting terrestrial from
Fic. 131.—Luxuriant lakeside marsh dominated by Water Sedge (Carex aquatilis
agg.) and Tall Cotton-grass (Eriophorum angustifolium agg.), which both extend
out into the water. Southampton Island, Hudson Bay.
aquatic communities. On the other hand there may be an ecotone
of Willow scrub that, at least on its lower side, probably represents
a later stage in the hydrosere.
Boggy areas, typically dominated by Bog-mosses (Sphagnum spp.)
and rather strongly acidic in reaction, are chiefly developed in the
southern portions of the Arctic. Baked-apple (Rubus chamaemorus)
is a characteristic inhabitant of them. Often they are well developed
around pools in peaty tracts and are colonized by heathy plants,
with little doubt ultimately turning into heathlands. In the Far
North, however, many tarnside areas remain to this day uncolonized
by higher plants—as in the right background of Fig. 132, although
the foreground, is vegetated by a fine bed of Scheuchzer’s Cotton-
grass (Eriophorum scheuchzert).
13] VEGETATIONAL TYPES OF POLAR LANDS 407
Lithosere stages are abundant in the Arctic, where much of the
terrain is of more or less bare rock that has, in many cases, been
freed from glaciation only in relatively recent times. Nevertheless
there is no doubt that succession is proceeding, however slowly—
at all events in areas that are not too rigorously exposed or lastingly
snow- or ice-covered. ‘Thus rock faces, whether of glacial boulders
or of detrital, cliff-face, or some other nature, are apt to be largely
invested with crustaceous and foliose Lichens, and to occupy con-
siderable areas. On the other hand, rock crevices or interstices
Fic. 132.—Fine bed of Scheuchzer’s Cotton-grass (Eriophorum scheuchzert) beside
tarn in northern Spitsbergen, though the waterside behind (on right) is devoid
of higher plants.
often support higher life-forms, so that in time a moss-mat or mixed
herbaceous community develops, and, ultimately, heathy vegetation
in suitable situations.
Screes, if not too active, may also be bound by hardy plants—
especially in low-arctic regions, where dark strips stabilized by
vegetation often extend downscree slopes. Also commonly stabilized
by vegetation are inland sandy areas, though the psammosere may
advance little beyond the pioneer stage of sand-binding Mosses (such
as Polytrichum spp.) and ground-shrubs (such as Crowberry and
Alpine Bearberry, Arctostaphylos alpina agg.). Consolidation of the
ground-shrubs produces already an advanced type of vegetation. On
408 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
Fic. 133.—Top of flower-slope below weathering crag in southern Baffin Island.
On right is seen flowering Three-toothed Saxifrage (Savxifraga tricuspidata), on
left Arctic Fireweed (Epilobium latifolium) and Alpine Chickweed (Cerastium
alpinum s.1.). In the centre are two species of Fleabane (Erigeron). Knife with
handle 12 cm. long gives scale.
FIG. 134.—Spitsbergen flower-slope dominated by Alpine Arnica (Arnica alpina
s.l.). The background of swarded cryptogams helps to make this high-arctic
community look unusually luxuriant.
13] VEGETATIONAL TYPES OF POLAR LANDS 409
both rock and sand, a dense mat of the ‘ silvery’ Moss Rhacomitrium
lanuginosum may cover substantial areas and apparently persist for
many years, though in the end it is usually colonized by Lichens
and Grasses, etc., to form what is sometimes termed ‘ Rhacomitrium
heath ’.
The mesosere is represented by relatively short-lived communities
in such favourable situations as alluvial deltas and the beds of
receding tarns, and apparently by longer-lived types on the earthy
or gravelly ‘ flower-slopes’ that form such a pleasing feature on
steep south-facing inclines particularly in low-arctic lands. Fig.
133 shows an example in which Fleabanes (Erigeron spp.) and
Saxifrages are prominent, although a large variety of other forbs
occur—typically much-mixed and in such profusion that the usual
dominants are excluded. ‘The writer has even encountered very
limited communities of this type near 78° N. in Spitsbergen and
at high altitudes in southern Greenland, growing under an unusually
favourable combination of conditions of shelter, aspect, water,
aeration, and soil—cf. Fig. 134.
HicH ALTITUDES
In general, temperatures get lower and lower as we ascend moun-
tains, and higher and higher as we go farther and farther south at
a particular altitude (such as sea-level) in the northern hemisphere.
Precipitation, windiness, fogginess, and the intensity of radiation
also tend to increase with altitude on mountains, though precipitation
falls off at the higher altitudes and other conditions often interfere
with radiation. Consequently, particular zones of vegetation in
general get higher and higher in the mountains as we travel towards
the Equator, though something like arctic conditions and attendant
vegetation may still be found at very high altitudes in tropical lands.
But although a general similarity to high-latitude lands prevails in
high mountains even near the Equator, the light and other climatic
regimes are by no means identical, while the floras are not necessarily
even comparable. Nevertheless the very general (but often only
superficial) similarity between polar regions and high altitudes else-
where, which to some degree may extend to the vegetation, makes it
desirable to treat high-altitude plant communities here, though
very briefly.
While in high-arctic lands there is the usual general tendency
towards limitation of flora and depauperation of vegetation as
410 INTRODUCTION TO PLANT GEOGRAPHY
mountains are ascended, fairly luxuriant if limited patches of more
or less closed vegetation are to be seen in some places up to altitudes
of at least 7oo metres. Around such altitudes extensive grassy
tundra can occur in middle-arctic regions, as it can even higher up
in low-arctic regions—where, on the other hand, extraordinary
barrenness may already prevail lower down. Indeed remarkably
drastic variation is apt to be found in arctic uplands, often from
spot to spot in closely contiguous areas. ‘Thus while in one place
the aspect is as of a plantless desert, in another there are plentiful
hardy rosette and other herbs such as Saxifrages and Arctic Poppy
(Papaver radicatum s.|.), while in favourable situations a more or
less continuous tundra may prevail, or even a closed ‘heath’. In
general, something approaching most lowland communities persists
well up into the mountains in the Arctic, while even near glaciers
or perennial snow, and at quite high altitudes and latitudes, the
climber may be agreeably surprised by a considerable show of
vegetation including herb- or moss-mats, and even heaths or flower-
slopes in the South. However, at very high altitudes even towards
the southern boundary of the Arctic, extreme barrenness is usually
encountered, with much snow and névé persisting through the
summer wherever the local topography allows, and little else save
crustaceous and foliose ‘'Tripe de Roche’ and other Lichens on the
exposed rock faces (Fig. 135).
In temperate regions the zone of tundra, etc., just above the
timber-line is in many ways comparable with low-arctic regions
near sea-level, while higher up in both instances a similar sequence
of altitudinal climaxes prevails. ‘Thus tundras of various types
roughly comparable with those of the Arctic are found above the
elfin wood to the south, with often extensive tracts of scrub (Fig.
136, and cf. Fig. 115) where conditions are suitable and pasturing
is not too severe. Above this may stretch increasingly limited tracts
of ‘alpine meadow’ consisting of short and more or less matted
Grasses, Sedges, and forbs or under-shrubs (Fig. 137, A); heathlands
may develop in particularly favourable situations, and fell-fields or
barrens in detrital or exposed ones. Mossy mats are especially
characteristic of run-off areas below snow-banks hereabouts.
Still higher up and nearer the Equator there is not merely a rigorous
climate to contend with but, also, geodynamic influences which are
often powerful, so that conditions and the attendant vegetation
may vary considerably from spot to spot. Almost all the vascular
plants persisting here are chamaephytes or hemicryptophytes, and
Riets Beg as r.
Fic. 135.—High in the mountains near the margin of the ice-sheet in southern
Greenland. Valley glaciers are plentiful and patches of snow and névé (iced firn)
persist through the summer where local topography allows. ‘The macroscopic
vegetation is often limited to Lichens on the exposed rocks.
Fic. 136.—Upland scrub of Dwarf Birch and silky-leafed Willows constituting
an altitudinal climax above tree-limit in northern Norway.
AII
/
in British Columbia, Canada. (Phot. W. S. Cooper.) B, plants flowering at
about 5,944 metres (c. 19,500 feet) on Mohala Bhanjyang, West Nepal: visible
in the gravelly fell-field are Lagotis glauca s.l. (to left of English shilling giving
scale), Potentilla saundersiana var. caespitosa, Pedicularis sp., Arenaria sp., 2
Leguminosae, and one each of Umbelliferae, Cruciferae, and Gramineae—also
several Lichens. (Phot. O. Polunin.)
412
Fic. 137.—High-alpine vegetation and flowering. A, a high-alpine ‘ meadow’
is
VG TATVONALS DYEES OF POLAR LANDS 413
a large proportion are obviously xeromorphic, being reduced in
stature, very hairy, or otherwise modified to conserve water. As
compared with those of lowland plants, the leaves tend to be smaller
and thicker, with a greater development of protective tissue. Indeed
the general aspect is much as in the Arctic, with hardy cushion or
rosette plants including bright-flowered Saxifrages often in evidence,
and Lichens and Mosses plentiful, any woody plants being excessively
dwarfed. However, more and more of the actual species tend to
be different as we travel south, until in the tropics very few arctic
inhabitants are left, although the general aspect may still be some-
what arctic-like at very high altitudes. Moreover, there are fre-
quently anatomical and other differences between arctic and alpine
plants even of very close systematic alliance. Fig. 137, B, shows a
wide range of plants flowering at an altitude of about 5,944 metres
(c. 19,500 feet) on Mohala Bhanjyang, West Nepal, in the Himalayas
—higher than flowering plants are commonly supposed to go in any
number, and perhaps constituting a record in this respect. ‘The
genera are often represented in the Arctic, and at least one arctic
species, Lagotis glauca s.l., is visible. At lower altitudes in the
tropics and especially down near timber-line, the ‘ mountain grass-
land’ is often very luxuriant and inclusive of woody associates
which do not, however, normally reach a greater height than the
herbaceous cover, though in some places a tall scrub may occur
between the Krummbholz and fell-fields.
In some temperate and tropical regions, especially of an arid
nature, and in keeping with the general reduction of precipitation
at very high altitudes, dry conditions may prevail above the timber-
line. Here the effect of aspect is often particularly noticeable.
Thus in parts of central Asia, steppe-like vegetation may persist on
the southern slopes of mountains, while, on the shadier northern
slopes, more luxuriant alpine meadows often occur at similar altitudes.
In these meadows may be tall forbs, and on the slopes below are
often many trees—in marked contrast to the arid steppe-like com-
munities on the south-facing side (cf. Fig. 83, B). Elsewhere, xeric
grasslands in which the tussocks of the dominants fail to coalesce
may be common on slopes around 3,000-4,000 metres, below the
fell-fields, while on plateaux at similar or even higher altitudes, such
as the ‘ punas’ of South America and the ‘ pamirs’ of Tibet, ex-
tremely severe drought and temperature conditions may prevail and
the vegetation consist of sparsely scattered tufts of Grasses and
hardy cushion-plants (Fig. 138). In the high Andes of Peru, etc.,
414 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
the cushions formed by some species may be so large as to resemble
‘recumbent elephants’ (C. A. W. Sandeman im Iitt.).
At the very highest altitudes, for example above 6,000 metres
(19,685 feet), Lichens are the chief or only macroscopic plants to
persist, their growth being usually poor. ‘These very high altitudes
also introduce the so-called ‘ cold deserts’ of temperate and even
tropical regions which, surrounded by tundra and other high-alpine
zones, are developed in the Rocky Mountains of North America
and the Andes of South America, in the Himalayas (O. Polunin
voce), and to some extent in the Norwegian Alps as well as on
Fic. 138.—Alpine puna-like formation near mountain summit in Colombia.
Farther south in South America the cushion-plants are often much larger. (Phot.
R. E. Schultes.)
mountain ranges elsewhere. ‘They are often snow- or ice-bound,
as are of course the ice-sheets of the polar regions, of which the
most extensive are the Greenland ice-cap in the northern hemisphere
and the Antarctic ice-cap in the southern hemisphere. Even around
21,000 feet there may be occasional flowering plants persisting in
the Himalayas, although crustaceous Lichens are more frequent and
go higher (O. Polunin voce, and cf. above).
Whereas in some cases particular zones of vegetation may encircle
a mountain at a fairly uniform level and preserving a fairly uniform
breadth, frequently they are tilted, being often higher on the
13] VEGETATIONAL TYPES OF POLAR LANDS 415
equatorial side; or else they may be narrow on one side, or dis-
continuous, or merely partial. Moreover as they usually merge only
gradually into one another, such vegetational zones are often difficult
to distinguish unless they are in full development.
ANTARCTIC TYPES
The vegetation types of antarctic and adjacent regions have become
sufficiently known in recent years for the broad generalization to
be made that they are reasonably comparable with those of the
Arctic, even if there is a tendency towards more tussock formation
and less woody plants in the South. ‘They also tend to be treeless
to much lower latitudes ; in conformity with our delimitation of
the Arctic, these treeless regions and their vegetation will be con-
sidered here as more or less antarctic, even though many of the
islands lie far from the Antarctic Continent and are commonly
referred to as subantarctic. Although the floristic composition is
largely different, the antarctic flora being in general very limited
and widely peculiar, with often a high degree of endemism, plant
communities in low-antarctic regions often look much like those
developed in the Arctic under comparable circumstances in similar
situations. Furthermore, much the same range of vegetation-types
is found in the Antarctic and Subantarctic as in the Arctic, whose
plant communities we have described and illustrated sufficiently
above. Consequently a very general account of the main antarctic
and subantarctic types should suffice. In this it should be recalled
that antarctic and subantarctic lands are for the most part widely
scattered, often being extremely isolated and having extraordinarily
limited floras, which in the case of the ice-free portions of the
Antarctic Continent consist almost entirely of lowly cryptogams.
The Antarctic is a very bleak and stormy region possessed of two
main climatic areas. The northern one may be described as sub-
antarctic and is maritime, with warmer winters and cooler summeis
than much of the Arctic, and powerful winds throughout the year.
There is heavy precipitation in the form of snow or rain, and little
distinction between the seasons. ‘The subantarctic islands have this
climate. The other, southern and central region is extremely cold
and stormy, having a continental climate without summer warmth.
Thus on the entire Antarctic Continent there is scarcely any place
having the mean of the warmest month of the year above o° C,
In contrast to the Arctic, which apart from ice-caps is largely
416 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
free from snow in summer and widely favourable for plant life, the
Antarctic Continent is permanently ice-covered except for very
limited areas chiefly about its borders. Here the ice and snow may
disappear for a brief period in the most favourable season and allow
some depauperated, almost entirely cryptogamic, vegetation to grow.
A few ‘ oases’ up to a reputed 300 or so square miles in area have
been reported in recent decades to occur at varying distances inland,
though mainly near the coast. ‘They are largely free from snow
Fic. 139.—Crustaceous and foliose Lichens on rocks near shore, Goudier Islet,
Antarctica. From these rock faces, especially where attacked by Lichens, and
from marine and wind-borne debris, there may accumulate in crevices and depres-
sions a fair soil on which fruticose Lichens and Mosses often grow. (Phot.
I. M. Lamb, courtesy of Falkland Islands Scientific Bureau.)
in the favourable season and are reported to have lakes green with
Algae but to be otherwise devoid of evident life, though apparently
they have not been scientifically investigated. Many rock faces
even well away from the ice appear entirely barren, at least from a
distance, though in some places especially near the shore a fair
growth of crustaceous Lichens may be found; here there may be an
accumulation of soil, and, growing on it, fruticose Lichens and Mosses
(Fig. 139). ‘This poverty of the antarctic vegetation especially on
the Continent must be related to the very unfavourable climate—
13] VEGETATIONAL TYPES OF POLAR LANDS 417
particularly to the coolness of the ‘ warm’ season and to the frequency
and persistent strength of the winds—and also to the isolated
situation which makes immigration extremely difficult. Almost the
entire Continent belongs to the zone of ‘ perpetual frost’, only a
few peripheral areas having a ‘tundra climate’, though this is
enjoyed by most of the subantarctic islands.
Only two species of vascular plants, a Grass and a Caryophyll, are
at all well known from the Antarctic Continent (a segregate of the
Grass is claimed by some to constitute a third species), but there
are fairly numerous Lichens, Mosses, and Algae, many of which are
widespread. ‘The microthermic vascular flora of surrounding
islands, however, includes a fair number of more or less circumpolar
species. Characteristic components of this flora are Colobanthus
crassifolius and Lyallia spp. (the former being known from the
Antarctic Continent, and both belonging to the Caryophyllaceae),
Pringlea antiscorbutica (the Kerguelen Cabbage, belonging to the
Cruciferae), Acaena spp. (Rosaceae), Azorella spp. (Umbelliferae),
and the Grass Deschampsia antarctica (also found on the Antarctic
Continent). Mixed with various hardy cryptogams, these and other
vascular plants form a thin tundra which, with increasing luxuriance,
extends northwards over the Antarctic Islands and into much lower
latitudes—for example in southern South America. Nevertheless
this ‘ subantarctic ’ tundra occupies only a tiny area when compared
with its arctic counterpart. Like the latter, it includes some
shrubby plants, mostly of cushion-form, and often tussocky Grasses,
as on South Georgia.
The Antarctic Continent is largely covered by the world’s greatest
ice-cap and consequently is a vast polar waste. Only here and there,
on ice- and snow-free spots of the shore or inland ‘ oases ’, on steep
walls of rock or stony slopes, and on mountain-peaks protruding
from the ice, are found the Lichens, Mosses, and Algae mentioned
above—the vegetation at the best being in general far poorer than
is to be found in all but the most barren of arctic habitats. Even
Bacteria appear to be relatively few in number. Most of them, and
many of the largest known patches of macroscopic vegetation, have
been found in the Graham Land sector of western Antarctica below
the 68th parallel of S. latitude. Here, in the most suitable situations
within areas of favoured climate, may be found the two or three truly
antarctic vascular plant species and patches of cryptogamic vegeta-
tion that are locally more or less closed—particularly with such
Mosses as Brachythecium antarcticum and species of Grimmia and
418 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
Andreaea. ‘The most luxuriant vegetation is often found in areas that
are manured (but not much trampled) by Penguins, etc. Lichens
inhabit the rocks, and include species of Caloplaca that introduce
bright colours much as in the Arctic, while Mosses may form an
almost continuous investment very locally (Fig. 140). But in general
the vegetation even around the periphery of the Continent is sparse
and only encountered in occasional favoured areas, most tracts
being ice-covered and devoid of evident plant growth, though the
Fic. 140.—Luxuriant growth of Mosses, broken chiefly by rocks bearing Lichens,
extending up snow-melt gully in area frequented by Penguins, Deception Island,
Antarctica. Rarely is more luxuriant vegetation to be seen on or near the Antarctic
Continent. Note the small tussocks formed by the chief Moss, and the more
typical barrenness of the ground behind. (Phot. I. M. Lamb, courtesy of Falkland
Islands Scientific Bureau.)
surface snow when tested has usually proved to contain some viable
Bacteria and often also spores of Moulds and Yeasts which had
presumably been carried thither by air currents.
Further south, in the perpetually frozen zone, only a few sheer
walls of rock or peaks or other situations that become bare of snow
in the brief ‘summer’ carry a sparse vegetation consisting entirely
of hardy cryptogams. However, some Mosses and Algae and fairly
numerous Lichens persist to at least 78° S., sometimes practically
covering suitable manured areas near the coastal shelf-ice in deep
13] VEGETATIONAL TYPES OF POLAR LANDS 419
bays, while, inland, three Lichens have been reported from snow-
free rocks of the Queen Maud Mountains within 237 nautical miles
of the Geographical South Pole. Samples of snow collected here-
abouts yielded seven different species of Bacteria; but in general
we can describe the interior of Antarctica as almost devoid of
macroscopic vegetation, and supporting precious little microscopic
growth or even life.
In the wide seas that surround the Antarctic Continent, there are
scattered islands and archipelagos whose low summer temperatures
and vegetational characteristics indicate polar affinities. They
include Kerguelen Island in about lat. 49° S. and long. 70° E., whose
barren appearance is widely attributed to very stormy drying winds
coupled with the low temperature of the ground. ‘The vegetation
is largely dominated by Azorella selago and to a lesser extent by
Acaena adscendens, the former often determining the general appear-
ance of the landscape in sheltered situations in the interior. ‘Thus
in otherwise desert-like areas it may form tussocks up to } a metre
high and 1 metre wide. In some places an almost continuous,
swarded tundra of these and other plants may be developed, and some
slopes may be more or less green, as may be damp depressions. ‘The
prevailing winds being westerly, it is chiefly on the sheltered east-
facing slopes that the most luxuriant vegetation develops. Here the
Azorella tussocks have associated Acaena, Agrostis antarctica, and
Lycopodium saururus, which tend to overgrow them chiefly from
the eastern side, while blocks of rock may be largely covered with
Lichens such as Neuropogon spp.—again most luxuriantly on their
eastern sides.
In suitable situations on Kerguelen the Azorella or other
‘cushions’ tend to coalesce to form a continuous cover which in
the most favoured ‘ oases’ may be replaced by almost pure Acaena.
Species associated with the Azorella are commonly few, though
usually some crustaceous Lichens are to be found on the stones, and
there may occur such dicotyledonous plants as Pringlea antiscorbutica,
Colobanthus kerguelensis, and Lyallia kerguelensis, and the Grasses
Agrostis antarctica and Festuca kerguelensis. Like the dominant
Azorella, most of these plants are of tussocky growth, as are associated
Mosses such as species of Rhacomitrium and Blindia. Acaena forms
a more even, if wavy, meadow-like community which from a distance
may look like a relatively smooth heathland. Commonly associated
with it are the same Pringlea, Galium antarcticum, Ranunculus
biternatus, and various Grasses. Apart from Pringlea, which may
420 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
represent a relic from some past age, the plants in the Azorella
community are all more or less xeromorphic. ‘Those of the Acaena
community, on the other hand, are more or less hygrophytic in
character, lacking any obvious means of protection against the
mechanical and desiccating effects of the wind. ‘Thus from the
rampant main axes of this dominant, which form a thin-meshed
network on the ground, leafy shoots ascend to a height of often
20-50 cm. (about 8-20 in.). In rocky places various Ferns occur,
including a Filmy-fern (Hymenophyllum peltatum) and forms of the
familiar northern Polypody (Polypodium vulgare agg.) and Brittle-
fern (Cystopteris fragilis s.l.). Limited to the salty beaches are
Cotula plumosa and Tillaea moschata.
The vegetation of the other islands of the Kerguelen Group, such
as the Crozet and Prince Edward Islands which extend westwards
in comparable latitudes to 38° E. longitude, and the nearer but more
southerly McDonald and Heard Islands, does not appear to show
any important deviations in its general character from that of
Kerguelen Island itself. However, McDonald and Heard Islands
tend to be particularly barren, presumably owing to their higher
latitude (c. 53° S.). Of the thirty species of flowering plants known
from the Kerguelen Group, no fewer than six (20 per cent.) are
endemic, the Pringlea (Kerguelen Cabbage) being the sole known
representative of an apparently endemic genus.
Of the islands lying south of New Zealand, Macquarie Island
(about 55° 5S.) is antarctic in character, with few and usually dwarf
woody plants (such as Coprosma repens, though the taller Acaena
adscendens and an allied species also occur). Wide stretches of the
hills are taken over by the yellowish tussocky grass Poa foltosa,
between whose tufts occur here and there larger ones of Stilbocarpa
polaris and silvery rosettes of Pleurophyllum hookerit as well as two
species of Acaena. Azorella selago is reported to form large cushions
on the wind-blown summits of some of these hills and to harbour
other plants as on Kerguelen. On the rocks of the shore are tussocks
of Colobanthus muscoides, Tillaea moschata, and a small endemic Grass
besides the more familiar Festuca erecta. In swampy as in some
drier places Poa foliosa is typical, and on the beaches Cotula plumosa.
The nearest land is 650 km. away, and as it is considered unlikely
that any vascular plants survived the severe Pleistocene glaciation
on Macquarie Island, it is thought that migrating sea-birds must
have been the main agents of importation of the thirty-five species
of vascular plants known to grow on the island—cf. page 114.
13] VEGETATIONAL TYPES OF POLAR LANDS 421
Even though they lack arborescent growth and must be men-
tioned here, most other ice-free islands of the South that have been
investigated appear to be scarcely polar in type. ‘Thus the Falkland
Islands near southern South America and the Antipodes Islands
near New Zealand support quite large bushes, besides Grasses up
to 1-5 metres in height which are apt to grow so closely together as
to prevent the entry of other plants. The large island of South
Georgia, however, which lies about 1,200 miles east of Tierra del
Fuego, is within the zone of pack-ice and has a clearly antarctic
character. In spite of persisting glaciation and poverty in species,
the vegetation is relatively luxuriant near the shore and in sheltered
valleys. Its character is chiefly determined by a few plants, such
as the tussocky Grass Poa flabellata and the somewhat shrubby
rosaceous Acaena adscendens, which are often so overwhelmingly
dominant that other plants play only a minor role. The Poa tufts
may be quite tall, the height of a Man being commonly approached
by the long and stiff leaves protruding from tussocks that themselves
often exceed }ametrein height, and that are separated by bare spaces
in which, when the area is sloping, water flows away quickly after
snow-melt or heavy rain. Such vegetation is largely confined to
seaside situations. On rocks near the beach a thick turf and sward
may be formed, especially in manured areas, and sometimes over-
lying deposits of peat. In some places an unbroken grassy tundra
may extend to an altitude of 200 or even 300 metres on sheltered
north-facing slopes (which in the Far South of course tend to be
the most sunny and favourable). Other such slopes, especially
where damp, and the banks of brooks, may be covered by the
Acaena.
Inland areas on South Georgia which have not been so invaded
and are not too swampy, often support a meadow-like community
of such Grasses as Festuca erecta, Deschampsia antarctica, and a
relative of the European Phleum alpinum, often with associated
Acaena adscendens. Mosses and especially Lichens may here play
an important role in the consolidation of the vegetation, ‘ Reindeer-
moss’ Lichens (including a Cladonia allied to C. rangiferina) and
members of the lichen family Stictaceae being often abundant, while
Neuropogon melaxanthus and allied species may practically cover the
rocks in the higher zones. Intermediate situations frequently sup-
port mixed cryptogamous carpets, and exposed ones little save
scattered Lichens. Low-lying swampy areas, on the other hand,
are typically inhabited by a community dominated by Rostkovia
422 INTRODUCTION OSE WANT GE OG RAR THs
magellanica, which gives them a characteristic dark-brownish colour.
Associated are a few other flowering plants and some Liverworts,
and many more Mosses. ‘The freshwater aquatic plants include
a Water-starwort (Callitriche antarctica), a Buttercup (Ranunculus
biternatus), and several Bryophytes besides a greater number of Algae.
Vegetation-types of fresh and salt waters, and of snow and ice,
wherever they may be developed, will be described in Chapters XV
and XVI.
FURTHER CONSIDERATION
The only book devoted to truly arctic vegetation is the present author’s
Botany of the Canadian Eastern Arctic, Part III, Vegetation and Ecology
(Department of Mines and Resources, Ottawa, Canada, National Museum
Bulletin No. 104, pp. vil + 304 and map, 1948), which gives illustrated
descriptions of the vegetation-types recognized in the vast eastern parts
of arctic Canada. Otherwise the above account has resulted from perusal
of numerous published papers as well as from personal experience in a
considerable proportion of the regions involved. Much the same ts true
of the brief consideration given to high-alpine regions, concerning which
the appropriate parts of most of the general as well as regional works
cited at the end of Chapter XII may be found helpful if further details
are desired. As if in tribute to the enterprise of its authors, the Yournal
of Ecology, published regularly since 1913, is remarkably rich in well-
illustrated accounts of the vegetation of various arctic and alpine regions,
including some of the most rigorous and difficult of access.
All truly arctic vascular plant species recognized to date, including
those mentioned in the above chapter, are described and illustrated in
the author’s Circumpolar Arctic Flora (Clarendon Press, Oxford, pp.
XXVill + 514, 1959).
CHAPTER XIV
MEGE i eEtONnAl -RVPES. OF -sLROPLCAL
AND wD PACE NT TAN DIS
In the last two chapters we dealt with the main types of land
vegetation found in temperate and polar and some adjacent or allied
regions. It now remains for us to complete this survey of vegeta-
tional types developed on the land of the world with some con-
sideration of those of tropical and adjacent regions. ‘This considera-
tion will be a mere brief outline that can scarcely do justice to the
range of types that includes the most luxuriant, complicated, and
often changeable vegetation on earth. Nevertheless one hopes it
may help relate some of these types to those of other regions, and
at least assist the inhabitant of the latter in his appreciation of what
is found in the tropics.
‘TRopicaL RAIN FORESTS
These, especially in equatorial regions, constitute the most
luxuriant of all vegetation-types. ‘They occur chiefly where soil
conditions are favourable in moist tropical lowlands and where
there is scarcely a distinct (or at all events no long and severe) dry
season. ‘Their chief development is: (a) in the Amazonian region
of South America, whence they range northwards in the Caribbean
and Gulf of Mexico regions to nearly the ‘Tropic of Cancer, south-
wards past the Tropic of Capricorn in Brazil, and westwards to the
Pacific Ocean coast of Colombia and Ecuador; (6) about the
Equator in central and western Africa, extending southwards past
the ‘Tropic of Capricorn in eastern Africa and Madagascar ; (c) in
western India and Ceylon ; and (d) in the Malayan region whence
they range north to the Himalayas, northeast to Indo-China and
the Philippines, and south and east through much of Indonesia and
New Guinea to Fiji and adjacent archipelagos of the western
Pacific, with an intermittent extension in eastern Australia well
past the Tropic of Capricorn. ‘These are the regions in which
tropical rain forest appears to be the natural climax under present
423
424 INTRODUCTION TO PLANT CEOGRAPHY [CHAP.
conditions, although in most of them it is tending to diminish
rapidly in area owing to the activities of Man, and in some consider-
able tracts has disappeared altogether. Replacement is mainly by
secondary growth on areas of cultivation. In addition, subtropical
rain forests (which seem best treated here) occur widely in central
and southern South America, around the Tropic of Cancer in Central
and North America and in eastern China, farther north than the
tropical rain forest in the Himalayan region and farther south in
East Africa, and also in Hawaii and southeastern Australasia.
The country occupied by tropical rain forests is usually flat or
rolling, though they may extend up the lower slopes of mountains
to an altitude of about 1,000 metres (3,281 feet) or even higher.
In some areas rain falls almost every afternoon and night practically
throughout the year, in others there are one or two dry seasons! of
not more than three months each. Often the rain will pour down
for days or weeks, and everything becomes soaked in a thick grey
mist. ‘lhe temperature is relatively high and uniform, the annual
means being normally around 25—26° C., and the rainfall commonly
totals between 200 and 400 cm. annually, though in places there
may be much more. ‘The relative humidity also tends to be high,
being usually above 80 per cent., though comparatively low values
may obtain for short periods. Some notion of what the climate
is like may be obtained from the tropical palm houses of botanical
gardens. But although the light is dazzling when the sun shines
on the upper canopy from its midday position high in the sky,
beneath the commonly three stories of trees a sombre gloom prevails,
the atmosphere being humid and close. Nevertheless some rays
may penetrate and sun-flecks prevail—and, it seems, be micro-
climatically and physiologically important.
In these tropical rain forests it is chiefly in the tree-canopy that
animal life flourishes—of innumerable and sometimes gaily-coloured?
Insects, Tree-frogs, Lizards and Snakes, Birds, Squirrels, Monkeys,
and so forth, many of which never touch the ground during their lives
(I. V. Polunin voce). ‘The component plants may lose their leaves
individually each year or so ; but there is no regular seasonal change
' Professor Paul W. Richards points out (77 litt.) that although a dry season cannot
be adequately defined in terms of months with less than a certain minimum rain-
fall, dry seasons in these regions may be considered as consisting approximately
of those months having less than 4 inches (about 10 cm.) of rain.
* In general, however, protective coloration is more characteristic of rain-forest
fauna—particularly with the Insects, which tend to be brown or green and to
harmonize with their environment (J. A. R. Anderson and I. V. Polunin in Itt.).
14] VEGETATIONAL TYPES OF TROPICAL LANDS 425
affecting the whole vegetation, flowering and fruiting going on all
the time, though with particular species tending to have their own
definite seasons in these and some other respects. ‘Thus whereas
in some species the different individuals may lose their leaves at
entirely different times, more often there is approximate synchroniza-
tion of this event between them each year—but not between members
of different species to nearly such an extent as in most temperate
forests. [he dormant buds are most often small and unprotected,
but frequently develop after several or many years, so giving rise to
‘ cauliflory ’ (the formation of flowers on old ‘ bare’ wood), which
is particularly common in these regions.
The main plant components of the tropical rain forest are normally
the following seven :
1. The forest trees. 'Vhese form the main structural component,
sometimes referred to loosely as the ‘ roof’ or ‘ canopy’, which is
typically made up of three more or less separate strata characterized
by different types of trees. These ‘stories’ or ‘ layers’, as they
are also called, are usually ill-defined and indeed seldom easy to
recognize by casual observation, owing to the fact that species of
all manner of intermediate heights are commonly present, while
upgrowing young trees may be of almost any height up to the stratum
to which their kind belong, and even different component species
of a stratum often have different heights. In general, however,
there can be distinguished strata consisting of trees whose crowns
vary in height about a mean, and commonly there are three such
strata in tropical rain forests.
The roof of the forest has usually an irregular profile, the trees
of the highest (A) stratum being often more or less widely spaced
and rarely forming a continuous layer to which the term ‘ canopy ’
may be applied. The second (B) stratum, or sometimes even the
third (C), is commonly the highest layer of tree crowns forming a
continuous mass. The crowns of the B stratum typically extend
from about 15 to 30 metres in height, while the still shorter trees
composing the C storey are usually small and slender and have
narrow tapering crowns commonly 5-15 metres high. Fig. 141 is
a profile diagram of typical mixed rain forest in British Guiana in
which the three tree strata are barely recognizable. When, as in
this case, the two upper strata are much broken, the third is usually
dense ; but when the upper ones are dense the third is apt to be
much less well developed—as in Fig. 142. In the former circum-
stances Palms may be prominent, as in the example shown in
Mats 10 5
Fic. 141.—Profile diagram of primary mixed tropical rain forest, Moraballi Creek,
British Guiana, showing all trees over 4:6 metres high on astrip about 45 metres
long and 7:6 metres wide. (After Davis & Richards.)
a 4
VV
i evil
ET. 100 -180. ~“60'> 400 3270p
Ms 30 20 10
Fic. 142.—Profile diagram of climax evergreen forest in Trinidad, British West
Indies. ‘The community is a consociation of Mora (Mora excelsa, marked M) up
to about 45 metres high. The diagram represents a strip about 65 metres long
and 7:6 metres wide. (After Beard.)
426
VEGETADIONAL DYPES OF TROPICAL LANDIS 427
Fig. 143, or there may be many tall shrubs. Mostly, however,
dicotyledons predominate, the large Palms, Bamboos, and some-
times ‘T'ree-ferns being evident chiefly in disturbed areas.
Note the different heights of the trees and
the feathery leaves of Rattans and other Palms.
. 143.—Tropical rain forest in the Philippine Islands.
The trees of each stratum usually represent numerous different
species belonging to various families, a considerable proportion of
the lower ones being young members of species that are dominant
428 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
(or more often co-dominant) above. Indeed one of the striking
characteristics of most tropical rain forests is the extremely mixed
dominance, so that a species commonly occurs only from one to
three times in an acre. Local consociations of single dominance
may, however, be developed (Fig. 142), though in real tropical rain
forests this appears to be rare. ‘The leaves of the trees are com-
monly of medium size, having an area of 2,000-18,000 sq. mm.
They are usually entire and ‘ leathery’, and dark-green with glossy
surfaces. ‘hus they belong to the laurel or large-sclerophyll type,
being mostly oblong-lanceolate to elliptical in outline, often with
extended ‘ drip-tips ’. However, the type of leaf varies considerably
with the stratum, drip-tips, for example, being scarcely ever found
on leaves of mature trees of the higher strata. In many tropical
rain forests, foliage extends almost continuously from the herbs on
the ground to the tops of the dominants, and although many large
trees are present, this foliage of one sort or another mostly hides
their trunks (Fig. 144). In other instances the canopy is exceed-
ingly dense and there is little development of undergrowth and
ground-covering, so that the trunks of the trees stand out in the
gloom as huge columns. Often they show ‘ plank’ buttresses as
in Fig. 145, where an intermediate amount of ground-vegetation
is visible.
Although it is often contended that competition between the trees
finds expression chiefly in the struggle towards the light, actually
the different strata have each their own species. Normally these
individually reach their own particular level before attaining full
development and thereafter make no attempt to pass that level.
The arboreal species thus seem to fall into groups having a particular
height-limit and degree of tolerance to shading by the next stratum
above—or, in the case of the topmost stratum, presumably demand-
ing full exposure—the struggle towards the light being mainly in
the immature stages. Nor does there appear to be any intense
struggle for root-room—a feature that has been confirmed by Dr.
R. E. Schultes (7m Uitt.), with the rider that it is unexpected and
more ought to be made of it.
It should be recalled finally that in these tropical rain forests, not
only flowering and fruiting but also the loss and replacement of
leaves can take place at any time of the year and in fact normally
does take place at all times (considering the vegetation as a whole).
Thus in many species the leaves appear to be renewed annually,
and individual trees devoid of leaves may be observed in the forest
14] VEC EA ONAL SENG ES. OF TROPICAL LANDS 429
Fic. 144.—Another scene of tropical rain forest in the Philippines. Note the
density of the foliage, which often hides the large tree-trunks.
Fic. 145.—Base of tree-trunk showing exaggeratedly buttressed roots in tropical
rain forest.
430 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
at any period, though a new crop soon develops on them. In this,
as was already indicated on pp. 424-5, particular species tend to
have their own special times—for example, members of the genus
Hevea in South America commonly lose their leaves at the end of
the dry season, regularly, each year, just before flowering.
2. Herbs, etc. Where the tree strata are not too dense and
sufficient light penetrates, there may be a fair development of green
ground-vegetation which, like the dominant trees, is independent of
external support. Such lower vegetation in moist situations tends to
be largely herbaceous, Ferns and Selaginellas being often prominent,
whereas on dry ridges it may consist largely of woody plants. In
other cases a shrub stratum (D), consisting mainly of tallish woody
plants, may be reughly distinguishable, with, below, a ground-layer
(E) of herbs and tree-seedlings up to 2 metres in height. ‘The
shrub layer often includes some coarse herbs such as Scitamineae
(Bananas, Gingers, etc.) which may exceed 5 metres in height.. But
in general, in spite of the prevailingly warm and humid conditions,
herbs and other lowly plants are little developed on the ground
owing to the lack of sufficient light. ‘Thus in lowland rain forest
any luxuriant herbaceous ground-vegetation is found chiefly in
clearings and by streams and openings where illumination is above
the average for the level, while in the interior of the forest green
herbs—apart of course from epiphytes—are found chiefly as widely
scattered individuals or scarcely at all. On steep slopes, however,
more light tends to penetrate owing to the angle of the (lateral)
rays, and herbaceous vegetation is generally more abundant, though
still the number of herbaceous species is liable to be far smaller than
that of different trees. Indeed, in contrast to the situation in
temperate regions, the herbaceous vegetation in tropical rain forests
is almost always far less various than the arborescent, and, with
relatively ‘ open ’ conditions, is more apt to form ‘ families ’ of single
species. ‘The herbs belong to various (if usually few) systematic
groups but typically include members of the Madder family
(Rubiaceae) as well as some Grasses and members of the Sedge
family (Cyperaceae) in addition to Ferns and Selaginellas, and,
in the Amazonian region, Marantaceae and Melastomaceae. ‘Their
foliage is usually thin, sometimes variegated, and very variable in
shape, in contrast to that of the dominant trees.
3. Climbers. We now come to the first of the three groups of
plants that, although they are still green, are dependent on external
mechanical support and afford the main ‘forest furnishings’ ; of
14] VEGETATIONAL TYPES OF TROPICAL LANDS 431
them the climbers or ‘vines’ are generally the most important.
Indeed the woody climbers, also called lianes (or lianas), are apt to
be so large and numerous as to afford one of the most impressive
features of the tropical rain forest. ‘They may be thin and wire-
like or rope-like, or as thick as a man’s thigh, vanishing like cables
into the mass of foliage overhead, or here and there hanging down
in gigantic loops. Often they are unbranched up to the profusely
branched crown. Some are said to attain lengths of over 200 metres,
ascending one tree, then descending to the ground before ascending
another, and so on. Often they pass from tree to tree and link
the crowns so firmly that even if a tree is cut through at the base
it will not fall.
Lianes are most abundant where the forest has been disturbed,
or about its margins—as for example along river banks where they
may completely screen the interior of the forest. In addition to
the large woody climbers that reach the crowns particularly of the
B stratum of trees, there are usually some small, mainly herbaceous
ones (including Ferns) that seldom emerge from the shade of the
undergrowth. Among the climbers the large lianes comprise, how-
ever, by far the more numerous synusiae (groups of plants of similar
life-form, each filling much the same ecological niche and playing
a similar role, and contributing to a biocoenosis—cf. p. 321). ‘These
large lianes belong to many different families and genera—chiefly
of dicotyledons, though Climbing Palms or Rattans are often
prominent among them.
The climbers as a whole include twiners which by the revolving
movement of their growing tips become wound around their sup-
ports ; also root-climbers and tendril-climbers which have specialized
sensitive roots and tendrils, respectively, for attachment; and
scramblers which lack such abilities or organs but scramble over
other plants, often being aided passively in their climbing by
recurved spines or wide branching. Many species use more than
one method. As the crowns of tropical trees tend to be less branched
and less leafy than those of temperate trees of similar size, woody
climbers help to close the canopy and decrease the penetration of
light. ‘They may also mis-shape the crowns or constrict the stems
of trees, though such things are more regularly done by stranglers
(see pp. 435-6).
4. Epiphytes. 'Uhese are plants which grow attached to the trunks,
branches, and even living leaves of the trees, shrubs, and lianes,
such situations being the only ones available in closed forests for
432 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
species of small stature but having high light-requirements. A few
of the larger types and many of the small ones grow in the rain-
forest undergrowth, being supposedly species that are intolerant of
root-competition or smothering by fallen leaves. All have to put
up with lack of soil and hence paucity of mineral nutrients, and a
more or less precarious water-supply, though in this last connection
we should recall the persistently heavy and often daily rainfall in
most of their habitats.
Epiphytes do not ordinarily have any ill-effect upon the supporting
‘host’, and, though constituting a very characteristic element in
the structure of the forest, play only a minor role in its economy.
This may even be the case when the epiphytes are so abundant as
to form an almost continuous investment of tree-trunks, as they
commonly do in uplands where the tree-canopy is thin or relatively
simple (cf. Fig. 162). ‘They do, however, play an important part
in the ecosystem as habitats for animals, and they are further inter-
esting in showing many remarkable structural adaptations. ‘Their
number and diversity are great, usually involving a wealth of crypto-
gams of all lower groups as well as Pteridophytes and flowering
plants, including some shrubs. Indeed it is the presence of a wide
range of epiphytes which especially distinguishes the tropical rain
forest from temperate forest communities, though epiphytes are
characteristic also of montane and subtropical rain forests, and may
be even more luxuriantly developed therein. Moreover, different
species of trees frequently show distinctions in their epiphytic
floras, supposedly because of different chemical constituents of rain-
wash as well as for the more obvious reasons of shade or bark-
textlines etc:
A few of the more striking types of adaptations of epiphytes should
be mentioned. Many are constructed so as to collect a substitute
soil, which is mainly derived from the dead remains of other plants,
being often assembled by Ants which inhabit the plant’s own root
system that grows into the so-constituted ‘ vegetable flower-pot ’ of
these ‘ nest-epiphytes’ (Fig. 146). Others have to be able to
absorb water rapidly and for this purpose often have spongy ‘ vela-
men’ roots; they also have to be able to conserve the water they
get, and consequently are often markedly xeromorphic or possessed
of special reservoirs (e.g. Fig. 147) or storage tissues (in ‘ tank-
epiphytes ’).
Three main classes of rain-forest epiphytes may be recognized,
corresponding to different microhabitats: (a) extreme xerophilous
14] VEGETATIONAL TYPES OF TROPICAL LANDS 433
Fic. 145.—An epiphytic Fern (Drynaria sp.) which has small humus-gathering
leaves and larger photosynthetic ones that also produce spores. ( about 3's.)
Fic. 147.—An epiphytic Bromeliad (Billbergia sp.) with a mass of fibrous roots
investing the branch of the ‘ host’ tree. Note that the leaves form urn-shaped
cups for collecting and holding water, which is absorbed by special hairs on their
insides‘ (xeon)
434 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
epiphytes, living on the topmost branches and twigs of the taller
trees, such as some Bromeliads and, remarkably enough, Cacti ;
(b) sun-epiphytes, usually xeromorphic and occurring chiefly in the
centres of the crowns and along the larger branches of the upper
tree-stories, and usually comprising the richest of the epiphytic
synusiae in both species and individuals ; and (c) shade-epiphytes,
mainly found on the trunks and branches of C-stratum trees, or on
the stems of the larger lianes. ‘The shade-epiphyte synusia consists
chiefly of Ferns, and most of its members show no trace of xero-
morphy. ‘The average vertical ranges of the different synusiae
depend on the light factor. ‘Thus they tend to be constant within
any one type of forest but differ in different types, being high in
forests where the top strata are dense, relatively low in more open
types of forest, and still lower on isolated trees or the margins of
clearings or rivers.
Further adaptations (or anyway beneficial specializations) widely
exhibited by epiphytes are wind-borne spores (such as those of
Ferns) or seeds (such as those of Orchids) or fruits, though other
seeds and fruits are commonly dispersed by animals. Indeed it is
difficult to conceive of epiphytes being able to maintain themselves
without some effective means of dispersal of their propagules.
Some types, often termed hemi-epiphytes, develop long aerial roots
which reach the ground and so link epiphytes with the next group
of ‘forest furnishings’, the stranglers.
The epiphytic vegetation of tropical rain forests often includes
abundant Algae, Lichenes, Musci, and Hepaticae ; indeed, with the
usual absence of the mossy layer on the forest floor, all the Lichens
present and almost all of the Algae and Bryophytes are normally
epiphytic except occasionally in spots where fallen leaves have not
collected. There are, however, all manner of ‘ associules ’ on stones,
fallen logs, and so forth, as well as on tree-trunks down to ground-
level. Otherwise, non-vascular plants contribute widely to the
classes of sun- and shade-epiphytes and also grow as ‘ epiphyllae’
on (normally living) leaves—in the last instance mainly in the shady
undergrowth. Actually, the most abundant epiphytes of the shade
community tend to be Bryophytes, which often carpet the branches
of shrubs or hang down in the air, while the epiphytes of the sun
community include also many foliose Lichens, the Bryophytes of this
synusia tending to be more compact and xeromorphic. The
epiphyllae are mainly Algae, Lichens, or leafy Liverworts, and are
found chiefly on the upper surface of rather long-lived evergreen
14] VEGCE PALOMA LY PES OF TRORTCAL LANDS 435
leaves of tough consistency. ‘lhey often have interesting structural
modifications which appear to assist their adherence to the sub-
stratum, though normally they do not have any appreciable ill-effect
on leaves even when they largely cover their surfaces.
5. Stranglers. ‘hese are plants which begin life as epiphytes but
later send down roots to the soil, becoming independent or nearly
so, and often killing the tree which originally supported them.
Fic. 148.—Roots of Strangling Fig on a large tree-trunk.
They thus form a synusia which is biologically intermediate between
dependent and independent plants. Most familiar and plentiful in
both species and individuals are, widely, the Strangling Figs (Ficus
spp.), which may play a considerable part in the economy as well as
physiognomy of the rain forest. ‘The seeds commonly germinate
far up in the forks of tall trees and from the epiphytic bush first
formed are developed long roots which descend to the ground, those
nearest the trunk of the supporting tree branching and anastomosing
until it is encased in a strong network (Fig. 148). After a time
the original tree usually dies and rots away, leaving the strangler,
436 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
whose crown has meanwhile become large and heavy, as a hollow
but independent tree in its place (Fig. 149). Species of Clusia,
forming large crowns but seldom killing their hosts, are often the
most plentiful stranglers in the South American rain forest.
6. Saprophytes. ‘These, the plants obtaining their nutriment from
Fic. 149.—An old specimen of Strangling Fig in which the roots serve as trunks,
the original ‘ host’ having disappeared.
dead organic matter, together with the parasites, comprise the non-
green, heterotrophic components of the tropical rain-forest vegeta-
tion. As in temperate woodlands, the vast majority are Fungi and
Bacteria which aid in organic breakdown—chiefly near the surface
of the soil. ‘There are, however, in addition usually some small
associated flowering plants such as certain Orchids and mem-
bers of the Burmannia family (Burmanniaceae) and the Gentian
family (Gentianaceae), as well as others of the Triuridaceae and
Balanophoraceae, which contain little or no chlorophyll and live
by the same saprophytic means. ‘They are chiefly found in deep
shade on areas of the forest floor where dead leaves tend to accumulate
14| VEGCEDATTONALS DYPES TOF TROPICAL LANDIS 437
to a greater depth than usual—e.g. in slight hollows or the angles
between the buttresses of trees—but they may be absent from
considerable tracts, especially in rain forest with a marked dry
season. For in general there is very little humus accumulation in
the tropics, owing to the rapidity of decay and breakdown of
organic matter. ‘he forest floor normally is barely covered by a
thin litter of leaves, and commonly shows through in frequent
bald patches.
Fic. 150.—Flower and buds of Rafflesia manillana, a true parasite on the roots
of a Cissus vine. Rafflesia has no regular leaves or stem, and no chlorophyll, the
flowers growing directly from the roots of the host; an allied species, R. arnoldit,
has the largest known flowers (about a metre in diameter).
7. Parasites. Of these there are, apart from Fungi and Bacteria,
two main synusiae in the tropical rain forest—the root-parasites
growing on the ground (Fig. 150) and the semi-parasites (often
termed hemiparasites) growing epiphytically on the trees. The
former are few and of little importance, comprising two small but
remarkable families. On the other hand, the epiphytic semi-
parasites all belong to the Mistletoe family (Loranthaceae—Fig.
151) and are numerous in species and often abundant, being met
practically throughout the area of the rain forest. ‘They are woody
shrubs that in forests of fair density occur on the twigs and branches
438 INTRODUCTION TO PLANT CEOGRAPHY [CHAP.
of the taller trees, whereas in open types or areas they may descend
almost to ground-level. ‘Their vertical distribution thus corresponds
with that of the autotrophic sun-epiphytes, and seems to be deter-
mined chiefly by intolerance of shade.
In subtropical regions where the rainfall is abundant and well-
distributed, rain forests occur which are similar to the tropical ones
except for their tendency to be less luxuriant and dominated by
fewer species, and to include temperate elements but a smaller total
Se eee RX
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+
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Fic. 151.—A tropical hemiparasitic Mistletoe, Viscum orientale, the root of which
forms a single haustorium (absorbing organ).
flora. Often there are fewer lianes and epiphytes as well as, especi-
ally, trees; and often the middle (B) tree storey predominates,
although traces of both the others may be developed. Examples
are found bordering on the tropical rain forests and elsewhere as
already indicated above, and include the more luxuriant “ hammocks ’
of southern Florida. Such special features as plank-buttressing and
cauliflory, characteristic of the tropical rain forest, become less
evident or disappear. As temperate regions are approached, these
subtropical rain forests pass into the poorer but still evergreen,
warm-temperate ones described in Chapter XII. It should also be
noted that, in periodically drier areas inhabited by other types of
14] VEGETATIONAL TYPES OF TROPICAL LANDS 439
vegetation, there often occur, along rivers, ‘ fringing forests ’ which
are evergreen and otherwise reminiscent of the rain forest (although
usually less luxuriant).
In spite of the widespread destruction wrought by Man particularly
during the past century, it has been computed that one or other
form of tropical rain forest or ‘ seasonal’ forest (see below) still
occupies about half of the total forested area of the world.
‘TROPICAL FORESTS WITH A SEASONAL RHYTHM
Where marked dry seasons occur in the otherwise humid tropics
and the dominants are dependent upon seasonal rainfall, the vegeta-
tion presents a much more varied appearance. Here conditions
tend to be so ‘ critical’ that slight differences in the climate or soil
may introduce marked changes in the plant formations. Actually,
regions with one or sometimes two pronounced dry seasons of several
months’ duration occupy a much greater area in the tropics than
those with a constantly humid climate and luxuriant evergreen
vegetation. Such ‘ seasonal’ climates of marked extremes are par-
ticularly characteristic of the interiors of continents. When their
areas are timbered and tropical or subtropical, they may be classified
in the following three main groups of descending water-availability :
1. Widespread though variable are the monsoon forests or similar
‘seasonal ’ forests, developed in regions enjoying abundant rainfall
during the wet season, but having this alternating with a pronounced
drought lasting from four to six months or sometimes longer. ‘The
total amount of rainfall is usually less than in tropical rain forests,
being commonly between 100 and 200 cm. per annum if we include
in this category areas dominated by a rather wide range of deciduous
types ; and there are marked daily and seasonal changes in tempera-
ture as well as, commonly, strong winds. Often the character of the
forest has been changed by human interference, to which its main-
tenance may even be due, for the dominants are apt to be fire-
resistant.
Monsoon or allied forests are found in the areas of the true
monsoons in India, Burma, Indo-China, and southwards to northern
Australia, as well as on the margins of the tropical rain forest in
Africa, Madagascar, Indonesia, and Central and South America.
Their vegetation is not as luxuriant as that of the tropical rain forest,
440 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
though varying in appearance from an impoverished form of the
latter down to the savanna-woodland type described below. ‘Thus
in comparison with the rain forest, the monsoon forest tends to be
more open, with the trees farther apart and no such scramble of
all plants for light, while in the dry season (see p. 424) most trees
shed their leaves and the landscape takes on a ‘ wintry ’ appearance.
The degree of defoliation depends, however, on circumstances—
particularly on the severity of the season and the proximity of water-
courses, along which trees tend to retain their foliage throughout
the year. Some evergreen trees persist except when the dry season
is particularly rigorous, and indeed there is apt to be relatively poor
correlation with the dry season, many species tending to produce
young leaves some time before it is over. ‘This dry season is often,
moreover, the period of flowering, so that altogether the monsoon
forests at this time do not present as lifeless an aspect as do temperate
deciduous forests in winter. ‘They do, however, resemble these
forests rather than tropical rain forests in that the trees have thick
bark, exhibit growth-rings in the wood, lack plank-buttresses, and
are usually not more than 12 to, at the most, 35 metres in height.
The trunks of the trees tend to be massive and fairly short, the
crowns being usually round and large, spreading widely from often
no great height above the ground. The branches are commonly
rather stout and gnarled, and the bark is typically fissured or scaly.
‘he community consists of three main tiers—the canopy which is
liable to be much interrupted, the undergrowth which tends to be
dense but to have small and hard leaves, and the ground or ‘ field ’
layer consisting mainly of more or less lowly herbs. ‘The leaves of
the trees are usually thin but may be larger than those of the tropical
rain forest, as in the case of ‘Veak (Tectona grandis). ‘They function
only during the rainy season and so require no particular protection,
being commonly hygrophilous (possessed of features characteristic of
inhabitants of humid situations). ‘The climbers are fewer and smaller
than in the rain forest, being often herbaceous, and vascular
epiphytes are normally found only in the canopy. Consequently
the undergrowth is often luxuriant, consisting of shrubby thickets (or
sometimes of tall tufty Grasses when the forest is immature, though
this is rather a feature of savanna-woodland). Bulbous and other
geophytes are also commonly present in considerable numbers and
flower in the dry season, whereas the shrubs tend to blossom at
the onset of the monsoon rains before the leaves appear, and many
herbs follow suit during the rainy season. Although the Teak
14] VEGETATIONAL TYPES OF TROPICAL LANDS 441
forests may be relatively uniform, a feature of most other types of
monsoon forests is the variety of tree-species involved, which may
number forty or fifty in a single tract. In general, however, both
flora and vegetational luxuriance are markedly poorer than in the
tropical rain forest, though the most striking difference lies in the
seasonal nature of the monsoon forest.
2. Savanna-woodlands or ‘ parklands’ are often found where the
rainless period is more prolonged and the annual rainfall less heavy
than in true closed forest. ‘The trees are mostly widely scattered
save in favourable situations (such as occur near watercourses), show
increasing xerophily and resistance to drought as water-availability
decreases, and are often leafless during the dry season. ‘The
vegetation is open and park-like, being rich in terrestrial herbs and
especially Grasses, but very poor in lianes and epiphytes. Bulbous
and other geophytes are often abundant. ‘The trees are commonly
stunted to little more than tall shrubs, being usually much less than
20 metres high and sometimes overtopped by tall Grasses during
the rainy season. Although normally various, they are most char-
acteristically and widely members of the Pea family (Leguminosae),
which frequently dominate alone. Some of the trees may be quite
lofty ; but usually they are of lowly stature, often with squat stems
and thick fissured bark, the crowns being commonly flattened
or umbrella-shaped—allegedly in relation to wind, though this
presumption has been questioned. Thus Mr. A. C. Hoyle (voce)
believes that the causal factors are complicated and include high
insolation at times of limited water-supply. The leaves of the trees
are usually xeromorphic and their buds well protected, but flowering
frequently takes place late in the dry season.
In some places, as for instance in South Africa, it is contended
that the more open parkland or ‘tree-veld’ is successional, the
scattered Acacias and other trees attracting Birds and Mammals that
drop seeds around. In this manner a considerable variety of other
trees, shrubs, and climbers may be brought in and locally oust the
tall Grasses, so that a patchy type of savanna-woodland develops.
Even this may not be truly climax but due to edaphic or biotic
influences (particularly fire), and indeed it may well be that most
savanna-woodland is a fire climax in which the trees become self-
selected for fire-resistance.
In these well-lighted, open types of woodlands a few small lianes
and epiphytes may occur, though often the latter, particularly, are
442 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
lacking. On the other hand in favourable situations where the
trees grow close together, there may be a fair array of xerophilous
epiphytic Bromeliads, Orchids, and Ferns, that seem more properly
to belong to the rain forest.
Savanna-woodland of one sort or another is found very widely
in tropical and subtropical regions including much of Cuba and
elsewhere in the Caribbean, Brazil and northern Argentina, East and
central Africa both north and south of the Equator, and occupying
much of India and China as well as of northern and eastern Australia.
3. Thorn-woodlands, with similar or allied types called tropical
thorn-forests, thornwoods, thornbush, caatinga, etc., are usually still
more xerophilous, being found in areas of still lower rainfall and
more prolonged drought-period than the often poorly-differentiated
but usually much more grassy savanna-woodlands. Indeed Grasses
are often lacking in the drier thorn-woodlands, or tend to be
segregated into clumps separated by areas of bare soil. ‘The present
group of vegetation-types are found chiefly where the annual rainfall
is between 40 and go cm. but variable, with the temperature high
all the year round, ranging from 15 to 35° C. ‘The terms quoted
are not synonymous in that they are apt to be applied to communities
of different physiognomy growing under different circumstances in
different regions. And just as the major types dealt with previously
in this section grade into one another, as indeed do many vegetation-
types elsewhere, so do the present variants intergrade and inter-
digitate with the savannas and grasslands dealt with below. More-
over, various and even quite different types and sequences may be
involved where local water or other conditions change markedly.
Thus succulents are often most in evidence in areas of particularly
coarse, over-drained soil, and may form distinctive communities
alternating with one or another type of thorn-woodland.
The foliage of the dominants of tropical thorn-woodlands is
deciduous or markedly xerophilous, or often reduced to mere scales,
thorns or prickles being a common feature as the above names
imply. Switch-plants with woody photosynthetic stems are also
characteristic of the community. ‘The roots of the main plants are
much branched, and competition among them for water may be
severe. Often they penetrate very deeply, shallow-rooting types
such as Grasses having little chance of success. Many of the woody
plants store water for the dry season in swollen trunks or roots, as
in the case of the Brazilian Bottletree (Cavanillesta arborea) whose
14| Ver GE AG OINAI inern Ss OF TROPICAL EADS 443
trunk swells to form an almost barrel-shaped structure. There are
also sometimes tracts dominated by arborescent succulents, including
giant Spurges (Euphorbia spp.) in the Old World and characteristic
members of the Cactus family (Cactaceae) in the New World. A
few xeromorphic herbs may occur, such as terrestrial Bromeliads
with sharp-edged leaves. ‘The accompanying shrubs are no less
xeromorphic, being often thorny Acacias or other members of the
Pea family, and sometimes forming a grey bushy jungle 3—5 metres
high, while a few thin woody climbers may also occur. Epiphytes
are usually absent, though Spanish-moss (7i/landsia sp.) may be
plentiful locally. With the onset of the rainy season the leaves and
flowers emerge and vast numbers of geophytic herbs may spring
from the soil, the rhythm of life being even more strongly marked
than in the monsoon forests.
Tropical thorn-woodlands and -parklands, etc., are widely developed
in dry regions: for example, in northeastern Brazil and elsewhere
in tropical South America, in the islands of the Caribbean as well
as in Central America and Mexico, and in and about the Sudan and
regions bordering on the southern Red Sea and the Gulf of Aden.
They also occur in southwestern Africa, in India, and in the west
and centre and northern hinterland of Australia. ‘Thorn-woodlands
and allied types often occupy sandy or limestone soils that are very
permeable to water, and alternate with savanna which tends to
prevail on the stiffer soils that retain rainwater near the surface.
In places where there is a local increase in humidity, as for example
in depressions or ravines, this savanna passes to savanna-woodland.
TROPICAL AND SUBTROPICAL SAVANNAS AND OTHER GRASSLANDS
As in other major regions, grasslands in the tropics and subtropics
constitute one of the main vegetational types and are usually, but
not always, dominated by members of the Grass family (Gramineae).
For sometimes grass-like plants of other affinity (particularly of the
Sedge family, Cyperaceae), may be the dominants over considerable
areas, or plants of quite different types may play a similar role.
Savannas are by some considered synonymous with treeless grass-
lands or steppes but in the present work the term is employed to
denote areas which ecologically speaking appear to be true grass-
lands, in that Grasses or similar herbs seem to be the real dominants,
but in which trees or tall bushes (in ‘ bush savanna ’) occur in open
formation and give a particular character to the landscape. Nor-
444 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
mally these tall woody plants are more or less widely scattered except
in unusually favourable circumstances such as obtain along water-
courses or elsewhere that water is relatively plentiful. ‘The occur-
rence of more hygrophilous ‘ meadows’ is rare in the tropics and
is clearly due to some particular local factor or factors of disturbance.
Although many grasslands in the tropics, as elsewhere, appear to
be ‘ natural’ to the extent that they do not owe their existence to
direct interference by Man, it now seems clear that there is no such
thing as a ‘tropical grassland climate’ and quite possible that
tropical grasslands are not in fact climatic. ‘This may even be the
case with the savannas which in one form or another constitute
their most common expression, for in the tropics taller woody plants
are rarely absent from such areas. Certainly many of these tracts
owe their persistence or very existence to fires or browsing animals,
or are edaphic climaxes due to local soil conditions, while others
appear to be seral. ‘Thus fires often destroy woody and other
dicotyledonous plants and Palms without appreciably damaging the
underground parts of the Grasses. But whatever their ecological
significance may be, these grasslands constitute characteristic types
of considerable economic as well as areal importance.
Savannas or their treeless counterparts are very widespread in
tropical and subtropical regions, where they often cover vast tracts—
though not without considerable local variation within their own
areas, as in the cases of many South American ‘campos’ and
‘llanos’. Examples are to be seen in southwestern North America
and the West Indies, in Central and South America both north
and south of the Amazonian forests, and in very many parts of
Africa such as the Sudan and within as well as around the closed
forests of the Congo, etc. ‘They also occur in central Madagascar,
in disturbed and upland areas of India and elsewhere in Asia, and
to the north of the central desert tracts of Australia. ‘The climate
is hot, with a moderate range of temperature and a fair rainfall
often exceeding 100 cm. annually and well spread over 120 to 190
days, during which ‘ rainy season’ large areas may be constantly
under water. On the other hand there is a prolonged drought
lasting often for six or seven months of the year, and a tendency to
desiccating winds. With such substantial rainfall, more or less
xerophilous woodlands are apt to predominate in the absence of
disturbance—at least elsewhere in areas of very high temperatures
and prolonged rainless periods during the vegetative season. ‘These
woodlands may include the savanna-woodlands, which are dis-
14| VEGELDADLONAL DY PHS OF TROPICAL LANDS 445
tinguished by the fact that in them the trees appear to be dominant,
although such types grade into savannas, even as these last grade
into treeless grasslands. ;
The savanna presents mostly a park- or orchard-like appearance
—a landscape typically of plains of tall Grasses with scattered trees
and shrubs. In hollows or swales the trees frequently grow close
enough together to form woods, whereas on ridges they are sparse
or wholly absent, the vegetation thus constituting a steppe. ‘The
Grasses commonly exceed the height of a Man, but range, in different
instances, from less than 1 to more than 4 metres high, and form
Fic. 152.—Palm-savanna in southern Florida.
a yellowish straw frequently topped by silvery ‘spikes’. ‘They
typically grow clustered in dense tufts which exhibit, in the intervals,
patches of bare soil often of a reddish or yellowish hue. Low
bushes with small and hard evergreen leaves and often prickles or
thorns may occur among the Grasses.
The trees which appear at greater or lesser intervals in the savanna
are usually stunted and gnarled but sometimes lofty. While many
are deciduous, others are evergreen; common heights are 3-6
metres. ‘They belong to characteristic species not usually occurring
in the forest, and commonly include Palms (Fig. 152) or other
plants of peculiar habit (Fig. 153). Often the fast-growing, coarse
and stiff Grasses interpenetrate the lower branches of the trees and
remind the ecologist of their tendency to dominate. Elephant Grass
(Pennisetum purpureum) may exceed 5 metres in height and form
an almost impenetrable ‘ thicket’. ‘The trees typically include some
446 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
Acacias and other members of the Pea family (Leguminosae), and,
in Africa, the Baobab (Adansonia digitata), with its hugely swollen,
water-storing trunk. ‘They often have thick and corky, fissured bark
and in favourable situations may form groves. In uplands the
Grasses tend to be lower and more mixed with forbs, though even
in the lowlands some forbs are to be found—both hardy perennials
and others having tubers or bulbs, which enable them to burst
forth into leaf and flower with the recurrence of the rainy season.
Fic. 153.—Savanna in Australia under rainfall of 25-75 cm. annually.
(Phot. D. A. Herbert.)
Although many present-day savannas and treeless grasslands have
probably resulted from clearance of closed forest, they usually
experience a longer period of drought each year than do forests.
However, they have more frequent rains and usually less permeable
soil than the relatively arid thorn-woodlands. ‘Thus the rainwater
that falls is readily available for the shallow-rooting, tussocky
Grasses whose dead straw also accumulates to form a mulch, and
whose close ‘ felt’ of roots further helps in water-retention. On
the other hand, tree-growth is largely restricted to places where
the ground-water lies at no great distance from the surface, most
of the successful trees having roots penetrating deeply enough to
14| VEGETATIONAL TYPES OF TROPICAL LANDS 447
tap this more lasting source. ‘They also tend to have relatively low
and compact crowns, often of spreading, umbrella-like shape,
allegedly as a result of exposure to winds. But in many areas they
are kept at bay or ousted by burning, or by the intensive grazing
of herds of wild or domesticated Mammals ; of these, many of the
world’s largest are inhabitants of the great grassy plains of tropical
and subtropical regions.
SEMI-DESERT SCRUBS
The arid bushlands characterized by scrubby Acacias and other
xeromorphic shrubs are often included among deserts, though
actually it seems preferable to consider them as transitional between
true deserts and savannas or thorn-woodlands. In tropical and sub-
tropical regions they are found particularly on stony or rocky
hill-sides, in open rolling country, and on sandy or gravelly areas
exposed to the full glare of the sun. Examples may be seen in
the southwestern United States and adjacent Mexico, along the
foot of the Andes, and in Africa especially bordering on the Sahara.
In the East they occur in Arabia and elsewhere near the northern
shores of the Indian Ocean, and also in Australia. ‘The climatic
conditions supporting these warm-region scrubs lie between those
of the desert proper and of thorn-woodlands, the temperatures being
variable but high at all seasons, and the rainfalls occasional though
seasonal and commonly averaging from 20 to 50 or more cm.
annually. Important is the seasonal distribution and _ general
reliability of the rainfall.
Many of the plants are veritable caricatures, such as those that
are grotesquely swollen to store water.
The bushes in these semi-deserts are often of fair size and either
grow separated or aggregated into a more or less continuous scrub,
most often of thorny Acacias. Grasses if present are dwarfed and
wiry, and usually reduced to isolated bunches or ragged patches.
Other herbs tend to be leathery- or fleshy-leafed, or, in numerous
instances, geophytic, with underground storage of food and water.
Cacti in the New World and cactus-like Euphorbias in the Old
World frequently form a characteristic feature of the usually open
vegetation. Like some of the other plants, including the thorny
Acacias, they are often beset with prickly spines. Large-leafed
Agaves, Aloes, and Yuccas are also characteristic inhabitants, as are
smaller succulents. Although some of the shrubs may be fairly
448 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
dense and exceed the height of a Man, they are rarely close enough
together in these semi-desert bush-lands to obscure the entire horizon.
Fig. 154 shows a relatively well-vegetated area in Australia.
In some subtropical regions such as occur in southwestern North
America, a shrubby climax may be developed where the annual
rainfall averages as little as about 8 cm., provided it is reasonably
distributed and reliable. ‘The dominants tend to be many-stemmed
but sparsely open, with widely-spreading roots. Although the
Fic. 154.—Semi-desert ‘ bush-land’ in Australia.
formation is commonly called ‘ desert-scrub’ it seems best men-
tioned here, even as its more temperate counterpart, characterized
by the Creosote-bush, was described under semi-deserts in Chapter
XII. Indeed the southern, subtropical extensions towards Central
America have little of a regular nature to distinguish them from the
northern type, and much the same is true in other regions. ‘Thus
while the dominants tend to be lower, the Creosote-bush being in
the South only about a metre high and others (such as Bur-sages,
Franseria spp.) commonly lower, the transition is normally so
gradual that no further type emerges or account need be given
here.
14] VECGETALIONAL LY PES OF TROPICAL, WANDS 449
‘TROPICAL AND SUBTROPICAL DESERTS
The hot deserts are areas in the tropics and subtropics having
such very slight precipitation that they typically support at best
only a scanty growth of scattered plants. Even if the nights are
cool, with dew and mists occurring especially in central regions,
the days are normally blazing hot. ‘The environment is thus severe
and the general run of habitats so unfavourable that they can only
be colonized by appropriately adapted plants and animals. Bald
expanses of flat or rolling plains predominate, all sunbaked and
windswept, with wide tracts of yellowish sand-dunes or browner
gravels or more rugged rocky floors, and in places broken scarps
of naked hills. Sparingly dotted in the less inimical situations may
be low and dry, strange-looking plants, or, in the very occasional
damper situations, luxuriant but limited oases.
Hot deserts chiefly occur well to the north and south of the
equatorial zone, which imaginative generalizers are apt to say is
“hemmed in’ by them. Although probably more extensive than
they used to be, owing to interference by Man and his domestic
animals, they are often not nearly so vast and invariable as is popularly
supposed, great areas being occupied by other types which are only
relatively speaking ‘ desertic’. ‘The main examples are the Sahara
and Arabian Deserts, occupying, respectively, much of northern
Africa and southwestern Asia, whence there are extensions eastwards
into northwestern India and northwards and then eastwards into
temperate central Asia. Extensive hot desert and near-desert! areas
also occur in central Australia and the southwestern portions of
North America, and smaller ones in southwestern Africa and western
South America.
The primary cause of hot deserts is paucity or even perennial
absence of rain, though an excessively clear and dry atmosphere,
with scorching sun, usually contributes to the general aridity.
Thus the relative humidity in the daytime is commonly less than
50 per cent. and may drop as low as 5 per cent. ‘The rainfall usually
averages less than 20 cm. per annum, often very much less, while
1'To many who have experienced the real deserts of, for example, northern
Africa and southwestern Asia, it seems unreasonable to refer to the more pro-
ductive of the so-called deserts of North America as properly desertic—hence
the use of this designation here. It was one of the present author’s earliest
experiences in Iraq to show pictures of such North American ‘ desert’ areas to
students who asked ‘ But, Professor, how can you call those areas desert, when
the vegetation is so great?’
450 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
in some areas there may be no rain at all for several years on end.
Such areas may be wholly devoid of macroscopic plants over some
tracts. Intense radiation and considerable changes of temperature
are also common, so that where the ground is rocky or clayey it is
liable to be much fissured. Often it is gravelly, sandy, loamy, or
stony—but none the less arid. ‘Thus although the substratum and
even the topography may change greatly in a single desert area,
the abiding influence is the paucity of water. ‘Tracts of different
type may exhibit different forms and degrees of vegetative develop-
ment, or sometimes virtually none, but all have the desert character,
the stamp of aridity. For example in the western Sahara there are
the pebbly-clayey areas with cushion-plants and succulents, the
sandy or gravelly beds of dry watercourses populated with T'amarisks
(Tamarix spp.), the sand-dunes dotted with heath-like bushes and
grass-tussocks, and the rocky plateaux, most desolate of all, consisting
of split stones and broken rocks with an occasional spiny or other
xerophyte anchored in the fissures. In addition there are saline
depressions which at best support relatively sparse colonies of
dwarfed shrubby halophytes.
Similar ranges of type are found in other deserts, or extremes from
absolutely bare moving sand-dunes to fairly dense heathlands char-
acterized by switch-plants. ‘There may even be open miniature
woodlands, with or without large bushy succulents. ‘The American
near-deserts! are remarkable for their giant Cacti such as the Saguaro
or Sahuaro (Carnegiea gigantea) and their small pebble-like Pin-
cushion Cacti (Wammillaria spp.), as well as for their glutinous
Creosote-bushes (Larrea spp.) and characteristic Ocotillo (Fouquiera
splendens) (Fig. 155). ‘The South African desert is famous for the
unique gymnospermous ‘Tumboa (Welwitschia mirabilis) and the
Desert Melon (Acanthosicyos horrida), as is the Australian for other
highly peculiar plant forms. ‘Thus whereas the desert populations
are normally limited to relatively small and scattered plants, many
of which are thorny, each area may have its own particular character
given by the plants themselves.
Desert plants are adapted in various ways to withstand the adverse
conditions under which they have to establish themselves, grow,
and ultimately reproduce. Many, particularly among the shrubs
and more occasional shrubby trees, have long roots that are said to
reach down sometimes to a depth of 10 or more metres (they may
certainly exceed 15 metres in length) to subterranean water or at
! See footnote on preceding page.
14] VEGETATIONAL TYPES OF TROPICAL LANDS 451
least to damp layers deep down in the ground. Regarding the
‘extraordinarily deep-penetrating root systems’ of 'Tamarisks, it is
even reported that they ‘ could be followed during the building of
the Suez Canal in places to a depth of 50 metres’ (transl.).!_ Other
desert etc. plants, especially among cryptogams, endure drought by
drying up almost entirely without harm to themselves. Yet others
are densely tufted or compacted, and often in addition closely
f;
i
Fic. 155.—Arizona near-desert scene showing the giant Saguaro (or Sahuaro)
Cactus (Carnegiea gigantea) and bushy Ocotillo (Fouquiera splendens). (Phot.
F. Shreve.)
invested with hairs and spines, while many, such as Cacti and cactus-
like Euphorbias, store water in their massive stems or other swollen
organs. Usually these ‘ succulents’ have an extensive system of
roots spread out near the surface of the soil and ready to absorb
considerable quantities of water when it comes, for most deserts have
a short rainy season during which conditions are fairly favourable
for plant growth—especially with the aid of the rich nocturnal dew
which may occur. At such times annuals spring up and quickly pass
through their whole cycle of development, while geophytes, with
1K. Rubner, Neudammer forstliches Lehrbuch (Neumann, Berlin, 1 Lieferung,
p. 180, 1948).
2 After the heavy rains in central Iraq in the spring of 1957, the author observed
small Plantains (Plantago spp.) and Grasses (especially of the genus Schismus) and
452 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
underground bulbous or tuberous storage organs, send up aerial
shoots which flower and fruit but die down with the resumption of
drought. Hence the * flowery carpets’ of delicate mesophytes that
some of the more seasonally-varying deserts often exhibit between
their scattered bushes after adequate rainfall. In contrast to these
ephemerals and ‘ deciduous perennials ’, the shrubs commonly have
very small evergreen xerophilous leaves, or sometimes larger
deciduous ones which are lost after the rainy period; others may
have the leaves reduced to scales, photosynthesis being carried on
instead by green twigs or leaf-like or succulent stems.
Most of the desert plants having perennial aerial parts are extremely
xeromorphic, exhibiting such features as excessive development of
fibrous tissues, thickened or otherwise covered epidermis, sunken
and protected stomata, and reduction or ‘ waxing’ of the transpiring
surface. ‘hey also commonly exhibit the xerophytic feature of high
osmotic value of the cell-sap. Frequently several of these char-
acteristics are shown by a single plant, sometimes to an extraordinary
degree. Dispersal of seeds from whole plants detached by the wind
and acting as tumble-weeds is fairly common, and often seeds will
remain dormant for years on end before germinating when sufficient
water becomes available. Even if many xeromorphic desert plants
may transpire fairly rapidly when water is plentiful, they are able
when necessary, by such features as those already mentioned and
by keeping the stomata closed all day, to reduce their water-loss to
a minimum and so often survive prolonged drought. ‘They also
exhibit considerable resistance to wilting and to injury as a result
of water-loss. ‘Thorny shrubs or broom-like switch-plants and
other drought-endurers are particularly characteristic of deserts, as
are, of course, extreme succulents and many ephemerals, but it is
too ‘ convenient ’ to classify desert plants into any such stereotyped
categories. A few root-parasites also occur rather widely in deserts.
Contrary to popular supposition, the larger succulents are unable
to withstand the conditions of the drier deserts.
Where there is a lasting supply of water, as along the banks of
rivers whether permanent or seasonal, or where the ground-water
rises to near or sometimes above the surface, as in oases, the vegeta-
tion is able to demonstrate at once the natural fertility of the soil
other ephemerals in his desert quadrats to grow up, flower, ripen seed, and die
down—all within a period of about five weeks, though this is only in the warm-
temperate belt. It is, however, possible that germination had taken place before
the main rains came, and so closer observations must be made in future. Fig. 156
shows the ‘ before and after’ effect of heavy rain in a desert of central Iraq.
14] VEGETATIONAL TYPES OF TROPICAL LANDS 453
a F
Fic. 156.—Areas of desert in central Iraq. A, metre quadrat showing only 1
small perennial tuft (on right-hand side, about two-thirds of the way back), although
numerous tiny seedlings are appearing after heavy rain. _B, close-up of an adjacent,
less gravelly area a very few weeks later, showing species of Schismus (slender
Grass), Malva (broad dark leaves), and Plantago (thin rosettes) developed to
maturity. The nails projecting from the frame are 10 cm. apart in both A and B.
454 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
—provided, of course, that salts are not present to excess. Date
Palms (Phoenix dactylifera) and attractive gardens can thus be
cultivated in otherwise desert areas, and in large oases a fine variety of
tropical and subtropical agricultural crops are produced. Wherever
there is feed for Mammals, desert forms such as Gazelles are apt
to pasture, while for miles around inhabited oases little save poisonous
or distasteful plant material is normally left undisturbed. Along
dried-up watercourses (wadis) the trees may attain large dimensions,
though usually remaining small-leafed and thorny, while perennial
Grasses, which are otherwise rare, often inhabit the sandy or gravelly
beds. Where desert conditions extend into temperate regions, as
in the Gobi, the oases are characterized (as mentioned in Chapter
XII) by tall Poplars and Willows. Here other vegetation is in
keeping with the temperate situation, the crops being such temperate
ones as Barley, Wheat, and Plums. On their poleward side such
cooler deserts are usually bordered by wide steppes, whereas hot
deserts are typically bordered by semi-desert scrub, at least on the
equatorial side.
MANGROVE AND OTHER SEA-SHORE VEGETATION
By far the most characteristic and important vegetation-type of
tropical and subtropical sea-shores is the ‘ mangrove ’ or ‘ mangrove-
swamp forest’ developed on mud-flats which are exposed at low
tide but otherwise normally covered by salt or brackish water, at
least being reached occasionally during the highest tides. Par-
ticularly favourable conditions for the development of mangroves
are found in creeks and quiet bays ending river estuaries, where
tidewaters cause the deposit of river sediment. On the resulting
flats and deltas, the water-borne seeds or seedlings of the colonizing
plants grow, soon forming the characteristic, rather low and dense
forest (Fig. 157). In other cases the mangrove forest in its interior
may consist of sizeable trees and be quite lofty (Fig. 158). In
Malaya and to a lesser extent in Borneo there are large stretches of
uniformly tall, mature mangroves (I. V. Polunin voce). ‘Thus in
Malaya the ‘climax’ mangrove forest consists of relatively few species
which tend to be gregarious, producing stands of uniform height,
whereas in Borneo the stands are not so pure or uniform (J. A. R.
Anderson and I. V. Polunin am Uitt.). Actually, real climax man-
grove now scarcely exists in Malaya, owing to felling on a rotation
of about 30 years, which leads to a retention of this height-uniformity
14| VEG Bava LON ATE a yYerirsS OF “DRO PUCAT) TAINIDS 455
Fic. 157.—A typical Mangrove plant, Rhizophora candelaria, forming a char-
acteristic marginal ‘mangrove’ and showing prominent prop-roots below.
Fic. 158.—Interior of Philippine mangrove-swamp forest at low tide, showing,
below, the aerating prop-roots and the conical erect aerating roots which project
upwards from the mud,
456 INTRODUCTION TO PLANT GEOGRAPHY [CHAP
(J. Wyatt-Smith voce). Frequently, however, the interior tallness
situation is reversed in that the fringe of the mangrove, at least
where it does not consist of young pioneer plants, is made up of tall
trecs, the interior being of much lower or bush-like plants. ‘This
is due to the loss of true mangrove conditions and an approach to
those of the hinterland forest (J. Wyatt-Smith voce).
The abundant strut-like and often arching prop-roots of the
mangrove trees or lower dominants, among other features, cause
deposition of silt and building up of the surface ; often, new mud-
flats are formed and the forest may extend year by year. ‘These
prop-roots are well seen in the illustrations, while Fig. 158 shows
also numerous slender conical aerating roots growing vertically out
of the mud. Both types of roots have ‘ breathing-pores ’ and con-
tain numerous air-spaces that serve for the conduction of oxygen
to the underground parts of the system. ‘This function is rendered
vitally important by the nature of the substratum and by the
usually frequent inundation, and is performed in some cases by
knee-like or keeled projections of roots above the surface of the mud.
Another function of the ‘ pneumatophores ’, as the aerating roots
are called, appears to be to help keep pace with the tendency of the
surface level to rise through deposition, for their underground parts
frequently bear the fine rootlets on which the tree is dependent
for absorption. Different types of mangrove plants bear these
different kinds of roots ; and the species may be mixed together or, al-
ternatively, segregated in more or less pure stands. In mangroves
in general, and particularly in these of the Indo-Malayan region,
there is often to be found a fairly definite succession. ‘The stages
of this are usually characterized by different species, and range from
the pioneers growing on almost continually submerged surfaces to a
mature mangrove forest of often tall trees whose bases may be
inundated by only the highest spring tides or, in some instances,
scarcely ever reached at all.
Many of the characteristic dominants of mangroves have another
feature in common, namely, the ‘ viviparous’ development of the
seeds—that is, their germination while still within the fruits and
attached to the parent plant. ‘The typical arrangement, exhibited
for example by the Red Mangrove (Rhizophora mangle), is for the
primary root of the seedling to burst through the hanging fruit and,
with adjacent tissues, to grow down as a long and dart-like, slender
but bottom-heavy structure (cf. Fig. 26, B). Later on the seedling
drops—root downwards, so that the tip may be driven into the
14] VEGETATIONAL TYPES OF TROPICAL LANDS 457
mud ! if the tide is out—and forms anchoring lateral roots in a matter
of hours, often continuing to grow im situ. Actually the seedling is
buoyant, so that if the tide is in or for some other reason the root
does not stick sufficiently in the mud when it drops, it may be
transported by water to some other situation, there to resume normal
growth if conditions are favourable. Young seedlings are seen
growing in fair numbers among the roots in the foreground in
Fig. 158.
Mangroves often extend some distance inland in brackish swamps
and lagoons, forming a fairly continuous fringe, or occupying islets
between which run the sluggish tidal streams. At high tide they
appear like a flooded forest or, about their low and tangled margins,
like a mass of green or greyish foliage sitting on the water. Some-
times they are replaced by Palms (such as Nipa fruticans) or other
large monocotyledonous plants, while near their climatic limits they
tend to form dense tall thickets rather than forests. Recession of
the tide even in the forested types reveals an ungainly mass of muddy
roots and often grotesque boles. Even the trees are liable to be
mis-shapen and lowly, while bubbles of stinking gas rise from the
rotten mire, and a teeming population of crawling creatures adds to
the atmosphere of gloomy squalor.
The climate is usually hot and humid, and conditions are kept
monotonous by the tides and salinity, though there may be alternat-
ing heavy rainfall and scorching sun. ‘lhe dominants are evergreen
and halophytic, the foliage being leathery, fleshy, or protected by
a glossy exterior or woolly covering against excessive transpiration.
Their growth is often so manifestly dense as seemingly to prevent
the entrance of herbaceous or other vascular plants, though indeed
there are few of these which are adapted to the very specialized
habitat. A dark coating of Red Algae may occur on the stems and
roots submerged by the tides, and some Lichens are usually to be
found on the stems well away from the water ; but other epiphytes
are rare.
The chief development of mangroves is in southern and eastern
Asia, with extensions to northern Australia and the Pacific. Man-
groves also occur about Central America, and, to the east, with
little variation on both sides of the Atlantic. In areas of low pre-
cipitation and general aridity, mangroves and other maritime wood-
lands are usually lacking or only poorly developed, even as are
1 According to Dr. Frank E. Egler (voce), this happens far less frequently than
the text-books imply.
458 INTRODUCTION TO PLANT GEOGRAPHY
forests inland. Exceptions may be afforded by the mouths of large
rivers, where the salt water is diluted with fresh, and conditions of
physiological drought are thereby relieved—provided cloudiness
reduces insolation and, consequently, transpiration.
Although mangroves are usually seral within themselves, and in the
most favourable of tropical rain-forest areas appear to be succeeded
by freshwater swamp-forest (see pp. 461-3) or perhaps sometimes
by tropical rain forest, in other instances there is no certainty that
they can develop alone, by mere accumulation of silt or humus,
into normal land vegetation or even littoral forest. In such instances
the more advanced and stable ‘inland’ types would seem best
considered as edaphic climaxes, that appear likely to persist in the
absence of disturbance.
The other vegetation-types of tropical sea-shores are more or less
comparable with those of temperate regions. Thus the beach
between tide-marks is usually devoid of vegetation on sandy or
shingly shores and bears only Algae on rocky ones, while even above
high-water mark on exposed coasts the sandy tracts are often poorly
vegetated, as are the outermost dunes. However, these last tend
to be bound by Grasses such as Spinifex littoreus, whose rhizomes
give off tufts at intervals, much as do many of the sand-binding
Grasses of temperate regions. Many other littoral plants of the
tropics adopt a similar trailing habit—including Pes-caprae (Ipomoea
pes-caprae), which forms a particularly characteristic and widespread
vegetation-type. Such plants also have the important faculty of
being able to grow out of the sand when covered by its drifting,
and in dry climates their areas of prevalence may extend far inland.
Some other colonists have prop-roots that grow down and anchor
them in the shifting sand, and almost all have a very deep and
extensive root system. In more sheltered situations, shrubs often
become numerous, as may in time small trees, such as Screw-pines
(Pandanus spp., Fig. 159, A). Farther back still—or in quiet creeks
sometimes near high-tide mark—a closed woodland is typically
formed in areas of sufficient rainfall. In subtropical regions, as for
instance the extreme southeast of Iraq bordering on the Persian
Gulf, there may be salt-marshes reminiscent of those of temperate
estuarine flats. An example is seen in Fig. 159, B, where the planted
groves of Date Palms (Phoenix dactylifera) seen on the horizon afford
a characteristic relief.
Littoral woodlands developing out of reach of the highest tides,
for example on sandy and gravelly shores that still retain an abnormal
7 =
t)
SS a hb:
on ede WO
Fic. 159.—A Screw-pine and a subtropical estuarine salt-marsh. A, a Screw-pine
(Pandanus tectorius) with prop-roots. Such plants are very common along the
strand in the eastern tropics and are widely planted for ornamental purposes.
(< 3s.) B, a subtropical salt-marsh in the estuary of the Shatt al-Arab, near
Fao in the extreme southeast of Iraq bordering on the Persian Gulf, showing,
behind, a characteristic grove of planted Date Palms (Phoenix dactylifera.)
459
460 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
amount of salt, are inclined to be highly characteristic and only
gradually, in space or in time, able to take on the aspect of the local
hinterland climax. Often they develop as a belt just inland of the
mangrove. In other places the forest or scrub even in the tract
lying nearest to the sea may be devoid of halophytic species. Where
salinity prevails and the littoral forest differs from that of the general
hinterland, the sandy or stony soil is often almost bare of dead leaves,
and the trunks of the trees are commonly naked ; or they may be
beset with epiphytes, both thick-leafed and cryptogamic, and support
a mass of thin-stemmed climbers. Where the trees are less close
together, there is often a dense undergrowth of shrubs and small
trees, or patches of coarse Grass. ‘The leaves are usually leathery
or succulent, often hairy when young, or hard and sword-like in
the cases of Screw-pines and the leaf-segments of Coconut (Cocos
nucifera) or other Palms. Particularly characteristic trees in such
situations in the Old World are species of Barringtonia. As the
distance from the coast increases, protective measures become less
necessary and pronounced, and the forest takes on more and more
the appearance and flora of the local climax, containing fewer and
fewer species which are not to be found away from the influence
of the sea. In other instances the littoral forest may be deciduous,
or dominated largely by a single species (such as Ironwood, Casuarina
equisetifolia). ‘The proximity of the sea is also expressed in the
buoyancy of many of the seeds and fruits, which are commonly
found in sea-drift, and which help some at least of the characteristic
species to attain a very wide distribution. ‘This is said to be the
case with the Coconut, plants of which form such a characteristic
feature of many tropical sea-shores (Fig. 160).
FURTHER SERAL OR EDAPHIC COMMUNITIES
Apart from various seral types already mentioned, such as the
dunes and, presumably, littoral woodlands dealt with in the last
section, and biotic plagioclimaxes which in some respects are of a
seral nature, there are yet others to consider in tropical and sub-
tropical regions. Outstanding are various types of forested and
reedy swamps, secondary scrubs and forests, and weedy com-
munities of many kinds.
Swamps occur chiefly around the edges of quiet bodies of fresh
water, in sheltered arms of lakes or sluggish rivers, and in filled or
filling hollows where the ground is at least waterlogged and where
14] VEGETATIONAL TYPES OF TROPICAL LANDS 461
free water accumulates on the surface for some period or periods of
the year. Here, as in temperate regions, there is often a luxuriant
development of largely erect monocotyledonous plants, such as
Papyrus (Cyperus papyrus) or species of Reed (Phragmites) or Cattail
(Typha). ‘These form characteristic reed-swamps, with roots under
water or in saturated soil, and with shoots extending more or less
high into the air. Whether the inundation is permanent or periodic,
and regardless of the water-level being relatively stable or fluctuating,
such swamp-plants typically contain air-passages for the aeration of
their roots and other covered parts.
Fic. 160.—Coconut Palms along a tropical sea shore.
In many cases, particularly in the warmer regions, the shallow
water is colonized by shrubs or trees, which may have special aerating
roots after the manner of mangrove types. Swampy grounds in both
the Old and New World tropics are frequently occupied by almost
pure stands of certain species of Palms, while even where the forest
is mixed it is usually much less rich in species, and particularly in
large tree species, than on drier land. ‘These swamp-forest trees
usually, but not always, belong to species not normally found in
the surrounding forests. ‘They are said in some instances, for
example in Burma, to be bare of leaves at the height of the rainy
season, when they stand in a metre or more of water. Like tropical
Q
~
462 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
rain forest, the swamp-forest may consist of several tiers and may
be plentifully supplied with lianes, that in some instances are
described as having a short stem which reaches up only to the surface
of the water in the rainy season, and from which arise dispropor-
tionately long, slender shoots.' ‘The numbers of terrestrial herbs
depend upon such features as the depth and duration of flooding :
often they are few, being chiefly members of the Sedge family
(Cyperaceae). However, epiphytic Orchids and Ferns can be plenti-
ful, as can Mosses and Liverworts.
Whereas the swamp vegetation just described is evidently hydro-
seral, exemplifying stages in the succession, from open water to
forest, that appears to take place in the same general manner as in
cooler regions, there is one peculiarity in the tropics, where humus
does not normally accumulate to any great degree. ‘This is the fact
that the majority of tropical swamp soils are not peaty, containing
as they do little if any more humus than soils of normal drainage.
However, where the water is poor in dissolved mineral matter
(oligotrophic) some peat formation can occur, leading to the develop-
ment of ‘ moor-forests’, which are commonly called the tropical
equivalent of the ‘ highmoors’ of cool regions. Corresponding to
this on one hand and, on the other, to the normal (non-peaty)
swamp soil with a relatively eutrophic water-supply, there thus
appear, in the tropics, to be two types of hydrosere leading to
different types of climax forest which in both cases are edaphic
rather than climatic. For in eutrophic waters the raising of the soil
level is due mainly to the accumulation of inorganic sediments,
while in oligotrophic waters such raising is chiefly the result of
accumulation of plant remains. In both cases the soil level rises
scarcely if at all above the height of the highest water-level once
this has been reached, as conditions for further substantial accumula-
tion then cease to exist in the tropics where organic breakdown is
rapid. Consequently the hydrosere appears to end with the forma-
tion of ground in which the water-table is near the surface during
at least part of the year. Such ground bears forest more or less
like the climatic climax in structure, but different in floristic composi-
tion owing to the local water conditions. It is thus an edaphic
climax and, like the hydrosere which engenders it, exists in two
forms according to whether the soil consists largely of deposited
silt or peat.
‘A similar phenomenon may be observed in some areas of Amazonian rain
forest that are subject to periodical inundation (R. E. Schultes voce).
14| VEGETATIONAL TYPES OF TROPICAL LANDS 463
Particularly characteristic are the peaty moor-forests originating
in oligotrophic waters, which are widespread in the rain-forest region
of southeastern Asia, where they are evergreen and dominated by
dicotyledonous trees. ‘These last may be as much as 30 metres
high on the edge of such areas where the climax has been reached,
but often diminish gradually towards the centre where the vegeta-
tion typically consists of earlier stages, including dwarf forest and
even pools of water. Kneed and other aerating roots may help to
make the surface of the ground an ‘ impenetrable’ jungle even in
the mature forest. ‘The dominants are often species peculiar to this
type of vegetation ; they are relatively few in number and show a
strong tendency to be gregarious, with a single species sometimes
forming an almost pure community. Often associated are Palms
and Screw-pines, with abundant epiphytes and herbaceous swamp-
plants among the furnishings.
In eutrophic waters the early stages clearly consist of free-floating
‘sudd ’ and communities of submerged aquatics, followed, when the
water becomes sufficiently shallow owing to silting, by rooted
floating-leaf vegetation consisting of Water-lilies, etc. This prepares
the habitat for emergent aquatics that soon constitute the reed-
swamp stage, which in turn is succeeded by scrub or low forest.
Though it may contain more trees per unit area, the (edaphic)
climax canopy tends to be more open than in rain forest, so that
light-loving species are commonly included in the undergrowth.
Moreover, the total number of species per unit area of this climax
tends to be smaller than in the rain forest, but greater than in the
seral stages.
It is of interest to note that not only do tropical hydroseres run
much the same course as temperate ones, the recognizable stages
often having a closely comparable physiognomy, but many of the
genera involved are the same in both these main climatic zones,
and not a few of the species are closely related or, in some instances,
identical. This is especially the case in the early stages in eutrophic
waters, when, as in other extreme habitats, only a few co-dominants
or even a single dominant may prevail. ‘Thus in Panama the
Common Reed (Phragmites communis agg.) and/or Narrow-leafed
Cattail (Typha angustifolia) may largely dominate the reed-swamp
stage, as may identical or similar species over much of temperate
North America and western Eurasia. Characteristic inhabitants of
rocks in rushing water are representatives of the peculiar family
Podostemaceae.
464 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
Primary xeroseres in the tropics may be observed for example on
recently emerged or volcano-devastated areas such as the East
Indian island of Krakatau, where after three years an associes con-
sisting of a lower layer of Blue-green Algae and an upper one of
Ferns and other vascular plants was found to clothe the surface of
the pumice and ash in some inland places. Eleven years later, in
1897, the interior supported a dense growth of Grasses, with isolated
shrubs and fairly numerous forbs as well as Ferns. ‘There were,
however, very few lower cryptogams to be seen apart from ter-
restrial Algae. Nine years later still, the shore supported a belt of
well-developed maritime woodland complete with climbers, etc.,
but the inland savanna persisted. Subsequently it developed into
a mixed woodland of fair luxuriance, and came to show every
indication of progressing ultimately to the local rain forest (apart
from some floristic depauperation and, probably, slowness of succes-
sion due to isolation and the consequent difficulties of colonization).
Thus the usual sequence, such as we have seen elsewhere, of domin-
ance by cryptogams and then by herbs and finally by trees, holds
true in this tropical xerosere, though it should be noted that
in rain-forest areas, such as this, there is a preponderance of
phanerophytes among the flowering plants—often from quite early
stages.
As in the case of the hydrosere, it seems that in the tropical
xerosere the number of species goes on increasing to the end, whereas
in temperate regions the numbers of species in both hydrosere and
xerosere tend to rise to a maximum and then decline as the climax
is approached, the decline usually starting when the community
becomes closed. Other humid tropical regions appear to have
xeroseres of a generally similar nature to that observed on Krakatau,
even if the pioneers in some cases are forbs, Grasses, Sedges, or
even woody plants (especially in secondary successions).
‘The sea-shore and littoral communities outside the normal forest,
already dealt with in the last section, are probably indicative of at
least potential successions. However, with the factors of the
environment as overwhelming as they often are in such situations,
it seems unlikely that the successions will become actual as long
as the shore-line continues as at present. Rather does it appear
that each zone is in equilibrium with its particular environment.
‘his, as we have seen, is apparenily the case with many mangroves
as well as with the latest stages of tropical hydroseres. But where
accumulation can continue, as on some sandy shores, the communities
14] VEGETATIONAL TYPES OF TROPICAL LANDS 465
may undergo rapid change and the zones of vegetation be actual
stages in continuing successions.
Very widespread in the tropics are ‘ secondary ’ scrubs and forests
that form parts of secondary or deflected successions engendered
particularly by Man or his domestic animals. When derived from
tropical rain forest, such communities are always more or less
unstable, whether they consist of weedy herbs, scrub, savanna, forest,
or a ‘chaotic wilderness of trees, shrubs, herbs and climbers’.
When left to themselves and protected from burning, felling, and
grazing, they are gradually invaded by primary forest species and
proceed towards the climatic climax which with little doubt would
ultimately be re-established. But where they are subjected to
recurrent fires or persistent grazing, deflected successions set in and
lead to biotic plagioclimaxes. Such are, probably, many tropical
grasslands and savannas. Even in the forest, according to Professor
Paul W. Richards (7m Itt. binis), ‘too frequent cultivation, 1.e.
shifting cultivation on too short a rotation, is one of the most impor-
tant causes of deflected successions ’, and, ‘ especially if accompanied
by intermittent burning and grazing, leads to the invasion of forest
areas by savanna Grasses and eventually to the establishment of
““ derived’ savanna’.
Shifting cultivation, which is practised by native peoples in nearly
all tropical forested areas, is by far the most important cause of
forest destruction there. In its course the trees are felled and
burned, after which one or more crops are raised before the fertility
of the soil is lost by leaching, erosion, and the exposure of humus
to the sun, whereupon the plot is abandoned and another cleared.
The secondary succession following abandonment usually starts
with series or quick-growing herbs and continues with more lasting
ones. During such re-establishment of vegetation, soil fertility
becomes partially or largely restored, so that after some years the
‘secondary ’ (or tertiary, etc.) forest may be cleared and cultivated
again. ‘The practice is, however, liable to be extremely wasteful
—especially when clearing and cultivation are undertaken at too
frequent intervals. Less drastic in their effects are the abandonment
of plantations and selective exploitation of timber, while strong winds
may also fell trees and start secondary successions. On the other
hand, shifting cultivation that is not too intense and short in rotation
is not necessarily wasteful, and may be preferable to some forms of
what is intended to be permanent cultivation, because it is less
destructive of soil fertility. It may also be less conducive to erosion.
466 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
Secondary forest tends to be lower and to consist of trees of
smaller dimensions than does primary forest, the young growth being
often remarkably regular in including even-aged stands of one or a
few woody species, whereas later on the growth may be extremely
haphazard as already indicated. ‘The early dominants are commonly
light-demanding and ‘ weedy’, as are the associated herbs. Such
forest may also be recognized by its floristic composition, which
usually differs markedly from that of the primary forest, even though
there are probably few if any species entirely restricted to the former.
Many of the components of the secondary forest are unusually wide-
spread, some often being introduced aliens, while its trees are mostly
quick-growing (e.g. 12 metres in three years), short-lived, and pos-
sessed of efficient means of seed-dispersal. ‘Their leaves tend to be
of more various sizes and shapes than those of rain-forest trees.
The most shade-intolerant and quickly growing species are, as might
be expected, most characteristic of the early stages of the secondary
succession.
The secondary forest at least when young is often dominated by
a single or small number of species, and usually has a much smaller
flora than the primary forest ; when very old, however, it may be
indistinguishable from virgin forest. On the other hand with long-
continued grazing, mowing, or recurrent burning, secondary savanna
or grassland is commonly formed, often characterized by species
of Lalang Grass (Imperata); alternatively, as in temperate grass-
lands, there may be still further regression with overgrazing, or
invasion by Bracken (Pteridium). In general, however, secondary
successions appear to reproduce in their later stages the changes
characterizing natural regeneration of primary forest, in which gaps
formed by the death of large old trees are first filled by the easily
dispersed and fast-growing dominants of the early stages of secondary
forest. The earlier stages in larger clearings commonly include
quick-growing Grasses and other weeds which are characteristic of
disturbed tropical areas, though they may be even less lasting than
their counterparts in cooler regions.
While shrubs may form a stage in the successions occurring in
rain-forest areas, often they are omitted, dominance passing directly
from herbaceous plants to trees. However, in drier regions shrubs
are apt to be important in the secondary successions, sometimes
remaining as more or less lasting dominants in what appear to have
been previously forested (though ‘ marginal’) areas. Indeed vegeta-
tional differences, such as frequently arise from differences in the
14] VEGETATIONAL TYPES OF TROPICAL LANDS 467
soil, tend to be much more marked in the dry districts of the tropics
than where rain forest prevails. Outstanding are the laterite soils,
typically reddish in colour owing to ferric compounds, which are
extremely poor in alkalis and nutritive salts as well as in water-
retaining capacity. ‘They are consequently unfavourable to most
plants and support relatively poor vegetation—examples being the
forests in Burma dominated by Eng (Ira), Dipterocarpus tuberculatus,
which often forms almost pure consociations and alone grows up well,
the other trees being stunted and gnarled. Similar poor forests
may also be found on light sands, * bare’ limestone, and dry ridges
of acid-weathering rocks, though these areas often support no more
than thorn-woodland or even scrub. Such vegetational poverty 1s
usually in part engendered by the relatively little humus-accumula-
tion, due to rapid breakdown in the tropics. In other cases porous
siliceous soils may be occupied by forests having a particular char-
acter owing to dominance by particular plants, such as Sal-tree
(Shorea robusta) or various Bamboos. Communities of these last
are often virtually pure, containing no associates apart from small
cryptogams, and in many cases apparently owe their origin to
cultivation.
ALTITUDINAL EFFECTS
The vegetation-types of high altitudes above the tree-limit in
tropical as well as other regions were covered in a general way in
the last chapter, and the communities of fresh and salt waters are
treated in Chapters XV and XVI, respectively. But here we must
consider briefly the upland types occurring below the timber-line
in the tropics and subtropics.
The basal zone of a range of mountains has in general a greater
rainfall than the neighbouring lowlands, being consequently often
occupied by communities resembling the relatively moisture-loving
ones of the lowlands. This is true in tropical regions where,
accordingly, rain forest is very widespread and frequently very
luxuriant on the lower slopes of mountains. Above comes the
montane zone, where the precipitation is often phenomenally high,
and which in the equatorial region is still tropical in its lower levels ;
but at higher levels here, and throughout its altitudinal range to
the north and south, the montane zone is rather temperate in type,
with vegetation corresponding more to temperate rain forest. Thus
the dominants are often of particularly massive growth and rich
branching, but devoid of plank-buttresses ;_ they are evergreen but
468 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
tend to have smaller leaves than those of the tropical rain forest.
The general foliage, too, is less dense, and commonly only two tree
strata are discernible, allowing light to penetrate and _ plentiful
ground-vegetation to develop (Fig. 161). A fair amount of humus
may accumulate but stranglers are absent.
Fic. 161.—Two-storied montane rain forest at an altitude of 740 metres in the
Philippine Islands.
Although more and more temperate species enter as we ascend
in the upper forested zones on tropical mountains, the total flora
tends to decrease. But whereas this decrease is particularly marked
in the case of trees, it is not accompanied, as it is in unfavourable
lowland situations, by any marked tendency to dominance by single
species. Leaves in general become fewer in number and usually
more slender than at the lower levels, and the epiphytes are usually
smaller, almost all herbaceous, and mostly limited to Ferns and
Bryophytes or more lowly cryptogams. Small climbers as well as
epiphytes are, however, often abundant in the two-storied upper
montane forest, as shown in Fig. 162. Epiphytic ‘ mosses’
(mostly leafy Liverworts) tend to be particularly numerous and
luxuriant where mists prevail in these and any still higher forests,
14] VEGETATIONAL TYPES OF TROPICAL LANDS 469
characteristically blanketing the trunks and also hanging from
almost every possible point, so that they create far more of a ‘ show’
than in the three-storied rain forest—hence the name of ‘ mossv
forest’ frequently applied to such types.
Fic. 162.
Epiphytes on trunk of tree near upper limit of montane rain forest
in the Philippine Islands.
Often the subalpine zone is little marked, except by reduction
in the size of the trees and in their foliage which may gradually
acquire a more xeromorphic structure, or, in some cases, be deciduous.
In time, as we ascend, only a single storey is left, corresponding to
the lowest tree one of the tropical rain forest. Often it is exceedingly
mossy. As the trunks of the trees become shorter and relatively
thicker, the branches tend to enlarge, at least in proportion, until
growth becomes irregular as elfin wood is reached. In this the
trees are twisted and stunted, being especially low and grotesque
in the extreme forms known as ‘ Krummbholz ’, though still commonly
festooned with Mosses etc. (Fig. 163). ‘This elfin wood and finally
Krummholz marks the termination of the forest and the beginning
of the treeless alpine zone which is vegetated by scrub, tundra, and
470 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
heathy or herbaceous vegetation, with ultimately, far above, sparse
fell-fields, etc., as described in Chapter XIII. Sometimes, as in
the tall mountains of New Guinea, other zones are interpolated.
In tropical and subtropical regions of dry climate, as in temperate
lands, forest (or intermediate savanna, etc.) may appear only in the
montane zone—or occasionally not at all, as on the western slopes
of the Andes in parts of South America, where scrub, steppes, or
arid ‘ punas’ characterized by large cushion-plants prevail. Near
&
Fic. 163.—Mossy elfin forest near summit of mountain, Philippine Islands.
Trunks, branches, and aerial roots of trees are covered with festoons of Mosses.
the latitudinal limits of the subtropics, something approaching
deciduous summer forest and, above it, forest characterized by
evergreen Conifers, may in some places constitute the upper limits
of arborescent vegetation and simulate the march into higher
latitudes (cf. Fig. 83, B). Details also vary elsewhere in many other
ways, such as the altitudinal limits involved, though these last tend
to become depressed with increasing latitude, proximity to coasts,
or exposure to prevailing winds. However, owing to the ‘ height ’
of the sun and the wide angle of incidence of its downpouring rays,
the influence of aspect tends to be far less marked in tropical than
in temperate and polar regions.
14] VECEDADTONAL SS: YPES (OF TROPICAL LANDS 471
FURTHER CONSIDERATION
Although it is in the tropics that the most luxuriant and complicated
of all vegetation occurs, most of the pertinent literature is again tiresomely
scattered. For such subjects as it covers, however, this is happily
remedied by the first book cited below ; the others are useful for further
details or vivid illustrations :
P. W. Ricuarps. The Tropical Rain Forest: an Ecological Study
(Cambridge University Press, Cambridge, Eng., pp. xviii + 450,
1952). Brings together the available knowledge concerning tropical
rain forests and related topics. See also the more general works of
Schimper, Faber, Hardy, Campbell, Tansley & Chipp, and Newbigin
cited at the end of Chapter XII.
W. A. Cannon. Botanical Features of the Algerian Sahara (Carnegie
Institution, Washington, D.C., Publication No. 178, pp. vi + 81 and
36 plates, 1913).
W. A. Cannon. General and Physiological Features of the Vegetation
of the more Arid Portions of Southern Africa, with Notes on the
Climatic Environment (Carnegie Institution, Washington, D.C.,
Publication No. 354, pp. villi + 159 and 31 plates, 1924).
J. S. Bearp. The Natural Vegetation of Trinidad (Oxford Forestry
Memoirs, No. 20, Clarendon Press, Oxford, map and pp. vi + 7-152,
1946).
G. M. Roseveare. The Grasslands of Latin America (Imperial Bureau
of Pastures and Field Crops, Aberystwyth, Bulletin No. 36, pp.
1-291, 1948).
R. E. Hottrum. Plant Life in Malaya (Longmans, London etc., pp.
viii + 254, 1954).
Concerning special tropical items or regions there are, in addition to
the works already cited, numerous and usually well-illustrated papers in
the Journal of Ecology, which has been published continuously since 1913,
and, almost always in German, in the 26 volumes of Vegetationsbilder
published between 1904 and 1944 and in the long series of monographs
edited by A. Engler & O. Drude and entitled Die Vegetation der Erde
(Engelmann, Leipzig, 1896 onwards).
CHAPTER XV
VEGETATIONAL TYPES (OF Websr
AND UNG SeN DS AL EN ES Waar
Reed-swamp and other semi-aquatic types of vegetation in which
at least half of the plant body is aerial have already been treated
in Chapters XII-XIV, dealing with the terrestrial vegetation of dif-
ferent climatic belts. ‘This leaves the more fully aquatic freshwater
communities, with some others, to be dealt with in the present
chapter, followed by the marine ones in Chapter XVI. Such a
separation of aquatic from terrestrial habitats and attendant vegetation
seems the more proper when we reflect that whereas on land it is
the climatic factors which are of primary importance in determining
the distribution of particular vegetation-types, in aquatic media it
is rather the chemical composition that is fundamental in this respect.
This is particularly the case with salinity, which gives us our primary
division into fresh and salt waters. Yet the ecological differences
between terrestrial and aquatic habitats are largely matters of degree,
chemical and physical conditions in the soil being still extremely
important in the former, for example, and light and temperature in
the latter. Even the distinction between fresh and salt waters is
incomplete, as there are various intermediate ‘ brackish’ waters of
varying degrees of salinity and, in addition, inland salt-lakes ; these,
however, except in such instances as the Caspian Sea which are of
marine origin, seem best dealt with in the present chapter, leaving
only marine types to be considered in the next.
SOME FEATURES OF THE FRESHWATER AQUATIC ENVIRONMENT
‘The temperature and some other conditions vary less in aquatic
than in terrestrial habitats, water acting as an effective ‘ damper ’
on changes concerning heat. Moreover, major bodies of water
exercise an equalizing influence on the temperature of adjacent air.
As a result of the minimization of variation in aquatic environments
and of the fact that, obviously, they offer no problem of water-supply,
aquatic plants and vegetation-types tend to be more widespread than
472
VEGETATIONAL TYPES OF FRESH WATERS 473
terrestrial ones. However, a marked vertical distribution occurs, the
vegetation in sufficiently deep waters being divided into zones ac-
cording to the different depths. In this delimitation light is usually
the main factor, though the temperature of water-masses and the
local chemical composition and especially aeration of the water may
also be important. ‘The zones, at various depths in water, that are
characterized by different forms or abundance of life, largely repre-
sent stages of decreasing intensity of light. ‘They range from a
relatively bright surface ‘ euphotic’ zone in which the light is suf-
ficient for the normal development of large plants, through a dim
‘ dysphotic ’ zone in which photosynthesizing small Algae and even
Mosses may still flourish, to a dark and relatively deep ‘ aphotic ’
zone in which only non-photosynthesizing organisms can exist.
Owing to the varying turbidity of waters due to suspended particles,
and to the different penetration of the sun’s rays at different angles,
the limits of these zones, which are themselves imprecise, lie at very
different depths in different instances.
In summer, lakes in temperate regions experience a marked rise
in temperature, particularly on and near the surface, but this is
followed by a decrease in autumn. Such fluctuations engender
vertical convection currents and eddies which lead ultimately to a
temperature of low value throughout, while at the same time they
are important in aerating the deeper layers (see p. 477, and cf. Fig.
165). ‘These last may vary very little (less than 4° C.) in temperature
between summer maximum and winter minimum. ‘Tropical lakes
may also show a surface temperature fluctuation with changing
weather, but here a reduction in temperature of merely 1-2° C. is
reported to bring about a circulation similar to that which is effected
in cooler regions by a winter cooling of some 20° C.
Limnology is the study of inland waters, including the environ-
ment and all inhabiting organisms and their interrelationships. In
it a distinction is made between the benthos (of organisms growing
on or in the bottom material ; these may be described as ‘benthic’),
the freely floating plankton, and the swimming nekton. ‘The dis-
tinction is, however, somewhat artificial as many organisms are
border-line cases. The plankton and benthos of inland waters are
said to be limnetic, in contradistinction to those of the open sea
which are said to be pelagic. ‘These terms are not much used in
limnology, where the region of open water is commonly distin-
guished as the pelagial. In shallow coastal waters the plankton
is apt to be mixed with forms belonging to the benthos, and
474 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
such waters and their plankton may usefully be described as
neritic.
Halophytes are plants which can tolerate a considerable degree of
salinity. But whereas the land halophytes and those of brackish
waters are usually euryhaline (that is, able to tolerate a wide range of
salt-content in different soils), most aquatic halophytes are more
stenohaline (capable of tolerating only a narrow range of salt-content
in different circumstances, their minimum, optimum, and maximum
being relatively close together). Similarly, ewryphotic and eurythermic
plants are those tolerating a wide range of light and temperature,
respectively, and stenophotic and stenothermic plants are those which
tolerate only a narrow amplitude in such respects.
In general the chief boundaries between different types of aquatic
vegetation are determined by factors comparable with those operating
on land, though the emphasis is often changed. ‘Thus temperature,
salinity, and light are of obvious importance in various ways, as are
movements due to surf and currents. Light-penetration depends
greatly on various factors such as cloudiness of the water (due to
suspended bodies whether living or dead), reflection from the surface,
latitude and the consequent angle of incidence of the sun’s rays,
and content of dissolved substances. Moving water, besides being
better aerated, demands of plants mechanical qualities differing
from those required in still water, and, moreover, stagnant fresh
water tends to have vegetation of very different composition from
running water. Quite apart from this, shelter from currents and
waves can be an important factor and even a major necessity in
aquatic media. Currents, on the other hand, can disperse plants
and improve such conditions as aeration and the potentialities for
nutrition. Rainfall tends to be of significance chiefly in affecting
the salinity of lagoons, particularly in wet tropical regions. Aquatic
plants inhabiting such waters and the mouths of many streams must
be widely euryhaline ; thus certain Diatoms living in waters where
the salt-content changes widely and rapidly are able both to absorb
and let out salt equally quickly, according to the concentration of
the medium.
We have seen that light-penetration in water is a very variable
factor. ‘This is important because different Algae and other aquatic
plants needing light for photosynthesis vary greatly in their actual
light-requirements. As explained in the next chapter, this variation
is in part associated with the predominating colours of the different
groups, but it also exists in fresh waters where Red Algae are usually
15| VEGETATIONAL TYPES OF FRESH WATERS 475
little in evidence and Brown Algae are practically unknown. ‘Thus
different species are differently adjusted as to both wave-length and
intensity of light, the green surface-forms flourishing where the
intensity is high and red rays plentiful, whereas they are hardly able
to utilize the green and blue rays which penetrate deeply into clear
waters. (The still shorter-length ultra-violet radiations, which may
be lethal, penetrate very little.) Photosynthetic activity of higher
plants also diminishes downwards, though the depth at which the
daily assimilation barely compensates for respiration varies greatly
with different species. ‘This “ compensation-point ’ for the Canadian
Water-weed (Elodea canadensis) is about 10 metres, whereas for the
Moss Fontinalis it is about 18 metres under comparable conditions.
Highly important is the physical nature of the substratum, benthic
vegetation assuming a very different character according to whether
the ‘ bottom’ is rocky, gravelly, sandy, or muddy—uin particular,
whether it is hard or soft. Whereas rocky beds are often suitable
for attachment of Algae, soft substrata are favoured by most higher
plants that have to take root. In the intimacy of smaller bodies of
fresh water, the chemical nature of the substratum tends to assume
a greater importance than in large and deep lakes. ‘This is because
the substances in solution exert an influence largely according to
their concentration. ‘Thus, the flora and vegetation often differ
markedly according to whether the water is rich or poor in dissolved
calcium carbonate and some other salts, while the relative abundance
of organic materials or humus may also be important. Ice action
is significant on many polar and other frigid lake-shores as well as
sea-shores, and even in temperate regions can profoundly affect the
nature of the surface and the composition of the marginal flora.
Moreover, extensive freezing of water may significantly alter the
acidity and nutrient and other chemical content of the underlying
medium.
As regards periodic phenomena, temperature in general exhibits
far smaller and less rapid fluctuations in water than in the air, and
is therefore less influential than on land. But whereas perennial
marine Algae even in cold regions may exhibit no period of winter
rest, seasonal differences in both quality and quantity of vegetation
tend to be well marked in small bodies of inland water. ‘This
commonly results from the considerable variation in temperature and
easy formation of ice, while especially in boreal and austral regions
seasonal variations in light may cause a distinct periodicity. More-
over, the amounts of nitrate and phosphate available in bodies of
476 INTRODUCTION TO PLANT GEOGRAPHY | CHAP.
water may also fluctuate seasonally to a marked extent, which again
may be the basis of alterations in the inhabiting population. It is
often contended to be as a result of the prevailingly low temperatures
that cold-loving Diatoms predominate in the lakes of central Europe
in winter and spring, followed by Peridinians in summer and by
Cyanophyceae when the temperature has reached 20° C. However,
as pointed out by Mr. Robert Ross (i /itt.), many other Diatoms are
by no means cold-loving, and it may well be that the control of this
cycle is more chemical than physical, silica depletion for example
being very important. Hotsprings, having a constantly high tempera-
ture, support a largely peculiar and often very limited flora!—as do
snow and ice at the lower end of the scale of temperatures allowing
plant activity (see pp. 489-91).
Although the food requirements of Algae and many other water-
plants are still but poorly understood, it is clear that the presence
or absence of certain mineral salts, derived from the substratum or
inflowing currents, is of outstanding importance in helping to deter-
mine both the composition and the luxuriance of the vegetation
developed. Quite apart from questions of salinity, small freshwater
lakes are particularly dependent on the chemical and physical nature
of their beds. As already explained in Chapter XI, chemical
‘reaction’ (acidity, neutrality, or basicity) and especially nutritive
salt-content are of fundamental significance in determining details
of development of aquatic vegetation—especially plankton—and, of
course, also to the dependent animal populations, in bodies of fresh
water. Hence the classification of the latter largely on the basis of
productivity into ‘ oligotrophic ’ (poor in nutrients, with a hard rocky
bottom and rapidly deepening water), ‘ dystrophic’ (also poor in
nutrients but rich in humus and acidic in reaction), and ‘ eutrophic ’
(poor in humus though commonly silted and shallow, rich in nutrients
including combined nitrogen, phosphorus, and often calcium). Par-
ticularly are the nitrate and phosphate contents of decisive importance
in the matter of biological productivity in fresh waters as well as
salt ones. Also of great significance are the contents of dissolved
oxygen and carbon dioxide, which vary at different depths and in
different seasons—cf. Fig. 165 for oxygen fluctuations.
In oligotrophic lakes, cold-water Fishes such as Trout are often
plentiful ; these lakes commonly show succession towards the eutro-
phic type. Dystrophic lakes, on the other hand, usually lack these
For example in hot springs above 45° C. only Schizophytes appear able to
persist, as indicated on p. 499.
15] VEGETATIONAL TYPES OF FRESH WATERS 477
deep-dwelling cold-water Fishes and sometimes other types too, their
fish-productivity being poor at best, while succession proceeds to
peat bog. Eutrophic lakes usually also lack deep-dwelling cold-
water Fishes, though they are often suitable for Perch, Pike, Bass,
and other warm-water Fishes; succession in them is to swamp or
marsh. Oligotrophic and dystrophic waters are often rich in
Desmids, eutrophic ones in Diatoms and Blue-green Algae.
Among the various ways in which living organisms alter fresh
waters is in the matter of gas-content. In general, green plants
(except in non-photosynthetic periods) remove carbon dioxide and
add oxygen, while animals do the reverse, so that in the upper, well-
lit layers there tends to be a superabundance of oxygen and in the
deeper and darker layers more carbon dioxide than above. Such
considerations lead to the recognition of two types of waters, namely,
those where the gas-content is almost constant in all layers, and those
where it decreases markedly in the depths. ‘The former are chiefly
masses of water in which vertical currents cause almost constant
mixing. In the latter it is common to recognize in summer in tem-
perate regions (1) a wind-stirred and largely homogeneous ‘ epilim-
nion’ or surface layer rich in oxygen (because of contact with the
air as well as photosynthesis), usually extending to a depth of 1ro—-15
metres ; (2) a middle ‘ metalimnion’ or ‘ thermocline’ where the
temperature and oxygen-content decrease rapidly ; and (3) a ‘ hypo-
limnion ’, underneath, where the water is virtually stationary and no
oxygen enters from above. ‘Tropical lakes commonly differ from
those of temperate regions in that the shallower ones ‘ only stratify
for short periods, if at all, while deeper ones may have cyclical
stratification or, if very deep, e.g. Nyassa and ‘Tanganyika, may turn
over very rarely or not at all’ (R. Ross im Itt.).
It is particularly in lakes sheltered from wind-disturbance in
temperate regions that the oxygen stratification tends to follow the
bottom contours, and the surface-waters in periods of quiescence are
often more alkaline than deeper ones. However, in autumn the
surface-waters cool and sink, carrying down dissolved oxygen, and
the deeper masses rise to take their place, so getting aerated by a kind
of reshufling each year. ‘The greater the amount of nutrient material
and accordingly of organic life in a lake, the faster does the oxygen
disappear in the depths during the quiescent period of summer.
Carbon dioxide, being complementary in metabolism, exhibits a
largely reverse trend, disappearing from the well-lit upper layers but
accumulating below. In spite of the relatively small amount in the
478 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
free atmosphere, which averages about 0.03 per cent. of carbon dioxide,
this gas, being easily soluble in water, is widely abundant in lakes.
‘Thus many contain more than 20 c.c. per litre in the depths, although
in oligotrophic lakes the content may be as low as 1 c.c. per litre.
Oligotrophic lakes may also show little variation in the oxygen content
at different depths, in marked contrast to eutrophic ones.
The so-called ‘lime’ content (particularly of calcium carbonate
and bicarbonate) of freshwater lakes and streams varies greatly,
peaty waters being especially lime-poor and ‘ soft’, whereas those
originating in calcareous districts are mostly lime-rich and ‘ hard’.
Variations also occur at different depths and times of the year :
thus in summer periods of relative stagnation, living organisms
may remove much of the ‘ lime’ from the upper layers, while the
deeper ones, which are already rich in carbon dioxide, actually be-
come enriched in lime. Calcium and allied carbonates and bicarbon-
ates have a marked ‘ buffering ’ effect against changes in the reaction
or pH level of a body of water, and as the reaction (or some condition
associated with it) is often important in affecting the flora and conse-
quently the vegetation, so are such ‘ salts’ important. Most favour-
able is a weakly basic to neutral reaction, markedly acidic waters
being biologically unfavourable : hence the limited and peculiar flora
of scarcely buffered bog-waters, the acidity of which is associated
with marked poverty in lime.
With regard to the impress of the environment as a whole, there
is insufhcient data as yet to compare the vegetational productivity
of different climatic zones. ‘Thus although some tropical inland
waters may be more prolific as producers of plant or animal life than
some extra-tropical ones, others are practically barren. Many extra-
tropical lakes naturally rank extremely high in productivity, and for
many which do not so rank a great deal can be done by the addition
of fertilizers or by such manuring as is, for example, practised in
European carp-ponds. For productivity of lakes is related largely
to such factors as chemical content, turbidity, and light. Even in
the Arctic the present writer has collected samples from small lakes
and ponds that have shown a surprising wealth of algal forms: in
one instance of six samples taken in as many small vials from shallow
pools and peaty puddles in Baffin Island in July and August, 1936,
no less than 179 different species or varieties of Algae were determined.
Nearly all of these were microscopic, a large proportion being Des-
mids. Moreover a considerable number of organisms, such as the
familiar planktonic Dinoflagellate Ceratium hirundinella and species
15] VEGETATIONAL TYPES OF FRESH: WATERS 479
of the Entomostracan genera Daphnia and Cyclops, range from the
polar regions to the tropics, so suggesting again that at least those
conditions which are critical for them are relatively uniform over
remarkably wide areas.
PLANKTON
Planktonic organisms are those which float freely on or in a body
of water ; they may be roughly divided into animal and plant types,
constituting, respectively, zooplankton and phytoplankton. ‘The main
categories of freshwater plankton are (7) the ‘limnoplankton’ of
lakes and ponds; (i) the ‘ potamoplankton’ of slow streams and
rivers ; and (7) the ‘ cryoplankton’ of lasting snow, névé, and ice.
The cryoplankton is so distinct in habitat and form that it seems best
treated separately in the next section. In addition we may distin-
guish in lakes and ponds (iv) the ‘ tychoplankton ’ of forms transported
from the littoral or from affluents by currents.
Planktonic organisms must be able to remain suspended in water,
and this they do either by having the power of active locomotion,
particularly by flagella, or, more often, in the case of phytoplankton,
by having suitable ‘form-resistance’. This latter is commonly
expressed in projections from the surface of the usually minute body
and, in addition, for success requires a specific gravity near that of
the surrounding medium. ‘Thin, light, and flat or curved cell-walls,
such as are often found in planktonic Diatoms, help considerably in
this connection; so do gelatinous sheaths possessing almost the
same density as water, and light food-reserves of oil, or, of course,
still lighter bubbles of gas. Very important is smallness of size,
for we all know that large bodies sink more rapidly than small ones
of the same material. Also significant is the specific surface area,
which is the ratio of the total surface area to the volume of the body ;
for the larger the ratio, the greater will be the friction caused during
sinking, and consequently the slower this last will be. Hence the
frequent provision of spines, horns, ridges, and the like on the out-
side of planktonic organisms, such as are illustrated in Figs. 5 (note
especially the forms of Desmids), 6, and 7. Moreover, owing to the
greater viscosity of water as well as protoplasm at low temperatures,
the ability of planktonic organisms to float tends to be greater during
winter thansummer. All such ‘ flotation-adaptations ’ are, however,
unable to prevent fairly rapid sinking of most except the smallest
among the non-motile planktonic organisms if the water is entirely
480 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
static. Rather is it the turbulence of the water caused by the eddy-
diffusion currents that maintains a state of continuous mixing par-
ticularly in the epilimnion, and keeps in this upper layer a sufficiency
at least for survival of the non-motile phytoplankton which chiefly
flourishes here.
Whereas in the sea the differentiation of open-ocean (pelagic) and
near-shore (neritic) planktonic organisms is often marked, so that
the two communities may be very different, in inland lakes the dis-
tinction is relatively poor. Indeed, close relatives of nearly all the
species of phytoplankton here occur also in the littoral, whence the
open waters of the pelagial were evidently colonized. Moreover,
the number of truly planktonic species in lakes is relatively small, a
large proportion of the types found being really ‘ tychoplankton ’
(see above), which have scarcely more claim to membership of the
community than have the particles of inorganic and dead organic
suspended matter (‘tripton’) present. Still, the numbers of actual
‘ plankters ’ (planktonic individuals) may be great, especially when
they exist in highly profuse ‘ blooms ’.
Freshwater plankton communities are composed of (a) producers
and (b) consumers of organic matter, the producers being almost
entirely chlorophyll-containing autotrophic plants. ‘The consumers
or non-producers, including normal Bacteria and Fungi which make
up most of the so-called ‘ saproplankton ’, are dependent upon the
carbohydrates and fats and proteins synthesized by the chlorophyll-
bearing plankters. In general the phytoplankton 1s composed not
only of the relatively large and obvious types which dominate the
so-called ‘ net-plankton ’ and include most of the Diatoms and Dino-
flagellates, but also of extremely minute ‘ nannoplanktonic ’ (micro-
planktonic) species which pass through even very fine nets (of No. 25
silk bolting-cloth) and are usually obtained by centrifuging. In
lakes, from one to a very few species commonly predominate and
make up the vast bulk of the phytoplankton at any one time, and
these may include minute nannoplanktonic types particularly of
Peridinians or other flagellates (e.g. Rhodomonas lacustris). Bacteria
normally contribute only a small proportion of the total ‘biomass’,
and so may even the large species that dominate the net-phyto-
plankton. Inthe zooplankton, on the other hand, such large types
as Daphnia tend to be most prominent.
In general the most plentiful organisms in freshwater phyto-
plankton are unicells or small colonies—whether flagellated or
non-motile—the Bacteria, Cyanophyceae, Chlorophyceae (including
15] VEGETATIONAL TYPES OF FRESH WATERS 481
Desmidiales (Desmids) and unicellular as well as colonial Volvocales),
Bacillariophyceae (Diatoms), Dinophyceae (Dinoflagellates), and
several other groups of flagellates, etc., being commonly represented.
During spring or summer maxima a greenish, yellowish, or brownish
“ soupiness ’ may be evident to the naked eye, the composition and
luxuriance of the community being very variable in time and space.
Thus from an aircraft the colours and general appearance of the
water of even adjacent tarns may be strikingly different, especially
in boreal regions. Except in the tropics where filamentous forms
are sometimes dominant (R. Ross im litt.), macroscopic plants are
commonly lacking in the real plankton, as are Rhodophyceae and
Phaeophyceae, though some Fungi may occur.
Continuous investigation of a lake or pond over a period of years
is likely to reveal striking changes recurring seasonally in much the
same combination and form each year. ‘This variability in type and
abundance at different seasons, or periodicity, as it is called, is shown
by (1) perennial species which occur in different densities at different
times, and (2) ephemeral types which spend the rest of the year
in some resistant stage usually on the shore or lake-bottom. The
fluctuations are due to interaction between the rates of multiplication
and of depletion, the former being dependent on basic biotic in-
fluences and the latter largely on natural mortality, predators, and
mechanical factors such as sedimentation. Nor do spring forms
commonly recur in autumn, owing to the different light and tem-
perature relationships ; for in other than tropical regions these two
leading factors exhibit marked and important differences in lakes at
different seasons.
The circulation of water in winter and early spring brings up
nutrients to the surface layers. ‘This often leads to an early ‘ bloom-
ing’ of Diatoms. But with the onset of summer stratification,
accompanied by a profuse growth of Green and other Algae, the
stock of nutrients cannot be supplemented sufficiently to maintain
abundant new growth and reproduction. ‘This is because most of
the nitrate and phosphate ions, particularly, have already been re-
moved from the upper layers and stored in the bodies of the organisms
that flourish there. Certain Peridinians which can manage with a
minimum of inorganic nutrients are then apt to appear in fair num-
bers. However, in late summer vertical convection gradually ex-
tends deeper, causing a replenishment of the upper layers from the
nutrient-rich water of the depths, so that population expansion can
again take place—cf. Fig. 165. ‘The cycle is perennial and more or
482 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
less perpetual in that the depths are all the time enriched by a‘ rain’
of dead bodies containing the all-important nutrients.
It is chiefly in late summer and autumn, when there has been an
extensive depletion of mineral nutrients but some replenishment
and, meanwhile, a copious increase in organic substances, that
water-blooms of Cyanophyceae occur. ‘Then again in autumn there
may be another Diatom ‘ maximum’. ‘To the extent that each new
population appears only after the requisite conditions have been
provided by its predecessor, there is here a kind of successional
sequence, although actually such phytoplanktonic stages are probably
all proseral in being non-essential to, or at all events not forming
part of, the autogenic main sere.
In connection with the spatial distribution of plant communities
which is the mainstay of our subject, we should recall that each
physiological activity, such as photosynthesis and reproduction, is
greatly affected by various conditions of the environment. Usually
with each such ‘ function’ there is for every pertinent environmental
factor a minimum below which, and a maximum above which, there
is no activity ; somewhere between lies an optimum at which the
function involved is carried on best. ‘These ‘ cardinal points ’, how-
ever, may vary with other environmental conditions, even as they do
of course with different organisms. With such rare exceptions as
perspiration, which generally increases with increasing temperature
until death from overheating occurs, each physiological function
responds in this manner. So does the organism respond as a whole
to change in an external factor—hence the importance of physio-
logical considerations in plant geography. But because several fac-
tors normally change at one time, and indeed go on changing all the
time, the effects are exceedingly complex and usually difficult to
analyse. Moreover the demands and reactions of various types,
species, and even lower entities or different stages of plants are
themselves extremely various.
In water, as we have already seen, some of the environmental
factors which are most variable on land are damped down, but
others retain a strong hold, as it were, on plant activity and distri-
bution. Furthermore, the planktonic population depends not merely
(and obviously) on the systematic groups present and able to grow
and maintain life in the face of often unfavourable conditions, but
also on the rate of reproduction and depletion of the component
forms. Such depletion may be more rapid than in most other
types of plant communities, and numbers may fluctuate greatly
15] VEGETATIONAL TYPES OF FRESH WATERS 483
because of sinking, death, removal by currents, and consumption by
predators.
In lakes there is commonly a marked and steep vertical gradient
of phytoplanktonic distribution, especially when the body of water
is limited in extent. For example, at the surface we may find an
almost continuous investment of often quite large plants such as
Duckweeds (Lemna spp.), Water-hyacinth (Eichhornia crassipes), and
the types shown in Fig. 164. Whereas these macroscopic plants
might be considered as belonging to the littoral, they often occur as
pelagials on ponds and small lakes, especially in warm regions.
Such relatively large floating material comprises the pleuston (hemi-
plankton), which is commonly defined as consisting of macroscopic
plants and as including those floating freely within the body of water
as well as on its surface. Furthermore, there are some microscopic
floating organisms (neuston) which stabilize their position upon the
surface of quiet water by employing surface-tension. For example,
a Green Alga, Nautococcus sp., becomes attached to the upper
surface-film by means of a flotation disk—thus essentially living as
an aerial organism, and forming conspicuous water-blooms of dry
(powdery) appearance. Other neuston organisms hang down in the
water, from the surface-film.
Within the body of the water, most of the phytoplankton is usually
concentrated in the top 10 to 15 metres, its permanent survival being
limited to depths where more food material is made by photosynthesis
than is used in respiration over an average 24-hour period. ‘This
maximum depth is dependent on light-penetration ; even in the
clearest alpine lakes the layer of water that is at all densely populated
by phytoplankton scarcely exceeds 50 metres in thickness. Indeed
in the majority of lakes, at least in the higher latitudes, most of the
phytoplanktonic life is concentrated in the uppermost 5 metres
(according to Professor G. W. Prescott in litt.), while at depths below
about 30 metres the numbers of individuals decline to very small
values. Yet within these limits different species vary greatly in their
preference. Thus more than half of the forms are usually concen-
trated in the uppermost 10 metres or less, while others attain their
maximum concentration at or below a depth of 10 metres. Nota
few types, such as Cyanophyceae provided with gas-vacuoles, have
a specific gravity of less than unity and so are concentrated at the
surface. On the other hand some phytoplanktonic species can have
their maximum density at depths greater than 30 metres, an example
being the Diatom Asterionella formosa in some circumstances in
— SS
= i 5 a | [FHA
7 ans, er S aie (ZN
VEGETATIONAL TYPES OF FRESH WATERS 485
Fic. 164.—Vascular plants floating freely on fresh water. A, a Bladderwort
(Utricularia), showing underwater branches and leaves bearing bladders which
trap minute aquatic organisms. B, another free-floating aquatic plant, Prstia
stratiotes, that has roots and is very widely distributed in the tropics and sub-
tropics. (x 4.) CC, a ‘ Batrachian’ Ranunculus, R. aquatilis s.l., floating in the
surface water of a pond near Babylon, Iraq. ‘The finely dissected leaves are
immersed but the flowers rise slightly above the surface, being photographed
from above.
Europe. Such tendencies result in largely different communities at
different depths—even in the same column of water at a particular
time. This is illustrated by the diagrams A (representing Algae
other than Diatoms) and D (representing Diatoms) in Fig. 165, the
numbers being those of organisms per litre in a Wisconsin lake, and
the 6 component parts of the figure being taken at approximately
monthly intervals during May to October.
The conditions bringing about these varied types of depth-distri-
bution in phytoplankton appear to be those whose cardinal points
limit physiological activity, the depth of greatest population-density
being that of optimum conditions (the resultant of the factors involved).
As we have seen, these optimum conditions vary greatly for different
organisms. ‘The issue is, however, rendered uncertain by the
operation of mechanical factors such as the dynamics of water.
Indeed the most important agent affecting the distribution of plankton
486 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
in general is the movement of the water—particularly the mixing
action of eddy-diffusion currents. Non-motile forms are kept sus-
pended primarily by these, and under conditions of active eddy-
diffusion are distributed more or less throughout the zone of turbu-
lence—apart from a tendency to decrease just below the surface,
MAY 22 JUNE 18
A A
1100
4300,
77—-— — =
I50-—--— -
120g
12-4 _
SEPT.14
A
3700 7000 25 4300 25800,
49 144
14.000 5 57 80 fhe. 45
45
14:3
HOGS
2 ‘ 5:0
22 13:8
5:5
Fic. 165.—Diagrammatic representation of the plankton in a Wisconsin lake during
May to October, the numbers being those of organisms per litre. = Rotifers ;
N Nauplii ; C = Crustacea ; M = depth in metres ; A = Algae other than
Diatoms ; D Diatoms ; O, dissolved oxygen in cubic centimetres per litre;
‘T = temperature in degrees Centigrade ; tr. trace. (After Welch.)
where the latter acts as a brake on eddy-diffusion currents and hence
allows depletion by sinking. ‘There may also be marked differences
on windy and still days, as indicated in Fig. 166 of the representation
of a Blue-green Alga of low specific gravity, which accumulates at
15] VEGETATIONAL TYPES OF FRESH WATERS 487
the surface in calm weather and is then absent below 4 metres (a),
but in windy weather is plentiful down to much greater depths (A).
Yet other Blue-green Algae, such as Aphanizomenon flos-aquae, can
achieve a vertical distribution of 6 metres or more on very calm days
(according to Professor G. W. Prescott in itt.). However, the really
lasting differences in composition occur in and below the thermo-
cline, where the eddy-diffusion currents are curtailed. The latter
may, however, extend to considerable depths during the autumn
a b
10
Fic. 166.—Diagram indicating distribution in a European lake of a cvanophycean
(Glosotrichia echinulata), which is rendered buoyant by included gas-vacuoles :
(a) during calm weather, and (b) during windy weather. (After Ruttner, modified.)
circulation, and at such times lead to a virtually uniform distribution
of plankton (cf. the October section of Fig. 165). Later on, under
the winter ice-cover of cool to frigid regions, stratification again
appears. Another complicating physical factor influencing stratifica-
tion is wave action, and yet another is water renewal, during which
much of the plankton-rich surface water of lakes is liable to be lost
by outflow and commonly replaced by inflowing river or other water
poor in plankton.
The biotic factors affecting phytoplankton are far more numerous
and complicated than the mechanical ones, and no attempt will be
made to analyse them here. They are concerned with such (often
interrelated) processes as reproduction, photosynthesis, secretion of
488 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
external ‘ envelopes,’ parasitism, ‘ grazing’ by animals, and active
movement, as well as their various components. All of these can
be governed in turn by temperature, light, or the chemical composition
of the water as already mentioned. It should be emphasized, how-
ever, that temperature differences are less important than many
others, being often superseded by such factors as light, the effect of
which on photosynthesis in the twilight region (dysphotic zone) is
nevertheless in turn influenced by temperature. ‘The long wave-
length ‘red’ radiation is absorbed in the upper layers so that at a
depth of commonly 15-20 metres in clear water a vivid ‘ green’
coloration predominates where there are objects to reflect the rays.
Absorption and subsequent use for photosynthesis being largely
complementary to the colours of plants, the light of these and of
greater depths is utilized best by phytoplanktonic organisms that
are brownish (such as Diatoms) or reddish (such as the flagellate
Rhodomonas and certain Cyanophyceae). ‘Vhese brownish and reddish
types are often predominant in deep fresh waters, as are Brown and
particularly Red Algae in the sea.
Owing apparently to wind and wave action as well as to the
mysterious avoidance of shallow water by Entomostraca, the character
of the general plankton is often peculiar along lake-shores, where the
abundance of phytoplankton may actually be greater than elsewhere
owing to turbulence and the low incidence of predation. On the
other hand, in deep waters where oxygen tends to be in short supply,
the temperature is often so low that few organisms can exist, and,
therefore, much less of this gas than usual is required for respiration.
Finally, in situations where oxygen is definitely deficient, anaerobic
(that is, living in the absence of oxygen) Bacteria often abound, in-
cluding many having a planktonic habit, while below, about the
surface of the bottom deposits, numbers of Bacteria computed to be
of the order of 100,000 or more per cubic centimetre are frequently
found. As well as by such deep-down productivity, the total biomass
may also be increased as a result of judicious fertilization (addition
of needed materials), an example being the supplying of phosphate
to oligotrophic lakes or ponds in which development is not limited
by poverty in nitrogen.
In contrast to lakes, running waters are characterized by a true
potamoplankton only in those that flow far and gently. Even here
the turbulence is not sufficiently reduced to permit much strati-
fication, conditions being much more homogeneous than in lakes.
Another difference is the transport of water-masses frequently over
15] VEGETATIONAL TYPES OF FRESH WATERS 489
vast distances, often involving great changes in ecological conditions
and even in climate in different parts of the course. The duration
of this transport, depending on speed of current and length of river,
determines whether a true potamoplankton is developed ; for most
of the planktonic organisms that get carried along in many streams,
and particularly in those containing lakes, are merely ‘ tychoplankton ’
washed in from other habitats and doomed to an early death. In
effect, a potamoplankton can be developed only when conditions are
suitable for the growth and reproduction of ‘ selected’ species from
among those which are washed in. ‘True potamoplankton is thus
limited to slowly-flowing rivers of considerable length where the
water takes some weeks to reachthe sea. Onand very near the bottom
of these there is often scarcely any current ; consequently conditions
and communities approach those of standing water. Even where
the flow is rapid it is remarkable how many free-living Algae such as
Diatoms and flagellates are to be found among macroscopic benthic
vegetation which is able to benefit by improved conditions introduced
by the current (see below, especially p. 498).
The planktonic and other aquatic vegetation of ephemeral pools
and puddles is extremely various, depending as it does not only on
local conditions but, very largely, on chance dispersal. Quite often
an extraordinarily rich and seemingly uninhibited development of a
single species is found. In saline lakes, fewer and fewer types of
organisms persist as salinity increases above that of the oceans (3-4
per cent.), but those which do exist may occur in abundance even
at very high concentrations (16-20 per cent.), provided of course no
lethal salts are present and the temperature is not too high.
CRYOPHYTIC COMMUNITIES
The communities developing on snow and ice are in some ways
akin to those found in ephemeral pools, which indeed may result
from the melting of snow or ice and, initially at least, often harbour
the same species. Thus phases of the common arctic and alpine
Red Snow Alga Chlamydomonas (Sphaerella) nivalis are frequently
abundant in pools of snow-water. ‘This ‘ species ’ may represent a
complex of several different ones which undergo a wide range of
intergrading morphological changes through growth under different
ecological conditions.
In general, the ‘ cryovegetation’ of snow and ice is greatly in-
fluenced by the physical and chemical characteristics of the medium,
490 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
though, as it lives on or near the surface, it is primarily affected by
changes occurring there. "Thus variations in salt-content and the pH
level of the surface snow or ice and of any liquid water will influence
the composition of the vegetation, and so will the nature of the
surrounding rock from which inorganic salts are obtained. For
substances from wind-borne dust, including particles resulting from
erosion, dissolve in any surface moisture after these particles alight
on snow or ice and serve as the source of minerals for microorganisms.
Consequently snow-fields and glaciers in the vicinity of acidic rocks,
for example, are apt to support very different vegetation from those
in limestone districts. In general, acidic environments support red
or pink snows and basic ones yield green snows. Red snows,
coloured by various organisms, are found in snow-fields practically
the world over; the much rarer green and yellow snows occur
chiefly in the Arctic and in Europe, although they have been re-
ported also from the United States.
It is perhaps best to refer to the plants growing on snow or
ice as ‘cryophytes’, and the communities they form as cryo-
phytic, for they are scarcely planktonic (that is, free-floating) in
such ‘habitats’. ‘These cryophytes may be usefully classified
according to their preferred environments as growing (1) on ice
—e.g. Mesotaenium berggreni ; (2) on snow and névé (firn)—e.g.
Chlamydomonas nivalis ; (3) on both snow and ice—e.g. Cylindrocystis
brebissoni ; and (4) occuring on snow and ice but only after trans-
portation from their normal habitats—e.g. various Cyanophyceae in-
cluding species of Gloeocapsa.
Although often a single cryophyte predominates in a particular
community, sometimes giving a distinctive colour to the surface of
ice as well as snow, usually others are present, the mixture sometimes
including a dozen or more species, and, in addition, such animals
as Snow-fleas. Dispersal appears to be mainly by wind. Whereas
the colour and texture produced vary with the organism and other
circumstances, * red snow’ commonly appears in spots scattered over
the surface, often involving wide areas, though sometimes the
colonizing is more uniform and extends to a depth of 3 or even
5 cm. ‘The ‘ bloom’ developed by organisms growing on ice may
also extend for miles, as in the purplish-brown form on the largely
snow-free glaciers of southern Alaska and Greenland. ‘This is char-
acterized by filaments of the Alga Ancyclonema nordenskioldu, which
form bunches up to 2 mm. in diameter on the surface of the ice
but chiefly in small hollows formed by its melting. In addition to
15] VEGETATIONAL TYPES OF FRESH WATERS 491
such surface inhabitants, according to Professor G. W. Prescott (in
litt.) “some organisms are embedded in ice’.
Whereas the majority of recognized land cryophytes belong to the
Green Algae—even when they are red, yellow, or brownish in actual
colour—or in several cases to the Cyanophyceae or Bacillariophyceae,
some Fungi and Bacteria may be associated as parasites. A few moss
protonemata have also been found growing on snow or ice, but with-
out developing into leafy plants. ‘The numerous viable but quiescent
spores of airborne Bacteria, Fungi, etc., and tufts or scraps of Mosses
or other ‘land’ plants, that are often present on the surface of old
snow and ice, can scarcely be considered as elements of its vegetation.!
It is on the floating sea-ice of boreal and austral regions, however,
that the most plentiful cryovegetation (of a sort) is commonly de-
veloped. Here Diatoms, particularly, are often abundant in the pools
that result from summer melting, frequently forming considerable
aggregations that render the surface brownish ; they also occur on
the sides and undersurfaces of the floes. Already before the end of
the last century, Nansen (Farthest North, I, pp. 444-5, 1897) reported
from the North Polar Basin :
“one-celled lumps of viscous matter, teeming in thousands and millions,
on nearly every single floe. . . . Whenthesun’s rayshad . . . melted
the snow, so that pools were formed, there was soon to be seen at the
bottom of these pools small yellowish-brown spots. . . . Day by day
they increased in size, and absorbing . . . the heat of the sun’s rays,
they gradually melted the underlying ice and formed round cavities,
often several inches deep. These brown spots were . . . alge and
diatoms. ‘They developed speedily in the summer light, and would
fill the bottoms of the cavities with a thick layer . . . the water also
teemed with swarms of animalcules . . . which subsisted on the plants.
I actually found bacteria... .’
BENTHOS
The vascular plants of semi-aquatic marginal communities have
been dealt with earlier in this work and those that comprise the main
seral stages of shallow waters are discussed later in the present
chapter. Here we must concentrate upon the usually smaller,
attached or loose ‘ bottom’ forms of Algae and other organisms which
comprise the benthos. For, altogether, these make up a substantial
1 Tt is not thought that the living Moss tussock found drifting on an ice-island
near the North Pole (p. 109) actually grew on the ice, but this seems possible.
492 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
proportion of the biomass of most bodies of fresh, brackish, or inland
saline waters. It is chiefly away from the shailowest waters, in
which coarse vascular plants usually predominate provided the bed is
suitable for their rooting, that these smaller benthic and allied forms
are in real evidence, though among the marginal vascular plants there
are usually to be found numerous bottom-attached, epiphytic, and
unattached Algae or other cryptogams. Indeed a definite gradation
can often be traced in the ‘ microflora’ of smaller forms as one passes
from marginal to outer reed-swamps and from the latter to stony
or muddy inorganic beds. For example, there are progressive
changes in the algal flora associated with modifications in the bottom
as it becomes less and less organic in nature, passing from an eutrophic
to an oligotrophic condition. ‘This is regardless of the central lake-
basin usually being covered with sedimentary ‘ ooze’ consisting of
mixed organic and inorganic matter (see pp. 496-7).
The benthic and allied organisms exist about the interface between
free water and the usually heterogeneous bottom or its covering, and
consequently their relationships are apt to be more complicated than
those of plankton. ‘This is furthermore the case because different
lake-basins are formed in various ways and are overlain by different
materials. Also the basins are variously altered after being filled
with water—the shores by wave-action, the bed by deposits of
sedimentary ooze—so that, with time, there is normally a pro-
gressive decrease in depth. Frequently the form indicated dia-
grammatically in Fig. 167 develops, in which, on the outermost
shore, wave action has created a small cliff and an erosional terrace.
To this there is adjoined a usually much wider depositional terrace
that consists of sediments, the surface of which is controlled by
wave action. This latter, sublittoral terrace may extend 100 metres
or more out into the lake before descending more steeply as the in-
fralittoral slope, which, in turn, changes into the central plain of
descending or level deep-water sediments. ‘The water above this
central plain is the main region of pure plankton, for the shore
terrace, the infralittoral slope, and to some extent often the central
plain itself, are all inhabited by benthos.
It is convenient here to distinguish as /ittoral that portion of the
profile inhabited by photosynthesizing plants, and to subdivide it
into the following three zones : (a) ‘The eulittoral, which is character-
ized by fluctuating water-levels and hence conditions, being more-
over a zone in which wave action on shore-lines may have a consider-
able effect. (6) The usually much more extensive sublittoral, which
15] VEGETATIONAL TYPES OF FRESH WATERS 493
is a zone of shallow water, not fluctuating significantly in level, and
bordered on its shoreward side by larger attached plants with leaves
reaching the surface. (c) The infralittoral, which is a deeper region.
Beyond this lies (d) the profundal, in which the light is insufficient
for photosynthesis, the only plants normally present being parasitic,
saprophytic, or chemosynthetic. Such a sequence is indicated in
Fig. 167, though it should be remembered that authorities often
differ as to categorization and terminology.
Limits of the
eo
¢ WaT —] littoral
fe)
hae aed
eee — /
¢
ee /
Redes /
Eulittoral Cliff < /
tof fluctuating”)
water) ——
Erosional Reriaee N
= ie Pelagial —
= —timit of large —}-— _-
Sublittoral tenace a see benthic =
: plants- 7
Infralittoral ‘i
slope
sr
a Limit of photosynthesis —
Profundal
Central plain cE
Fic. 167.—Diagrammatic representation of a typical lake-marginal profile. The
limit of at all large benthic plants is commonly about the bottom of the sublittoral.
In the shore-terrace and other shallow parts of the littoral, where
the bulk and activity of vascular plants and Mosses make seral
advance often quite rapid, there are usually abundant associated
Algae belonging to the benthos. ‘The higher plants are, so to speak,
fugitives from land, the lower ones being ‘ true children of the water ’.
But the higher plants themselves often constitute an important part
of the habitat of the aquatic microflora and microfauna and of any
larger Algae that may be present. Besides firmly attached benthic
types, the Algae include ones that are loose-lying (or in some cases
crawling) on the bottom or, particularly, among dense higher vege-
tation. Unlike the situation with higher plants and parasites, the
organs of attachment of these lower benthic plants do not normally
R
494 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
penetrate the substratum. But these organs are nevertheless effective
and, with the watery medium providing almost unlimited possi-
bilities for development, practically all stone or otherwise suitable
surfaces within the littoral, including dead or living plants, are more
or less thickly populated by attached organisms.
The means of attachment of benthic creatures are many and
various, and often have to afford protection against washing away
by wave action or currents. ‘They include gelatinous stalks of di-
verse structure which are sufficient in quiet water and are found in
many Diatoms and in such animals as Vorticella. Also abundant
in both plants and animals are rigid or gelatinous coverings attached
to the substratum by tiny stalks or by a broader base. In filamentous
Algae the basal cell must bear the entire pull, and so it is often
firmly fastened by a lobed attachment-disk which is closely applied
to the substratum. In agitated water more resistant attachments
are needed and these include thick and shortened gelatinous stalks,
flattened thalli broadly attached to the substratum, and gelatinous
cushions often reinforced with lime.
‘The populations attached to underwater stones and to living plants
—such as parts of various Potamogetonaceae and Pontederiaceae—are
largely different, as is often evident to the naked eye. ‘Thus on stones
and rock surfaces, and often on old pieces of wood, crust-like growths
frequently of considerable thickness predominate, whereas on living
stems and leaves, as well as on such as are rapidly decomposing, the
investment tends to be light and flocculent, often consisting mainly
of filamentous Algae. ‘The relative lightness of the investment on
living and rapidly decomposing substrata appears to be due largely
to their more or less transitory nature, so that they support chiefly
colonists that must be quick-growing and short-lived. It also seems
to be due in part to chemical changes in the immediate environment,
brought about by the living or decomposing ‘hosts’. A_ special
community is afforded by those Algae and Lichens that penetrate
the substratum and hence live partly within the body of stones, etc.—
particularly limestone and snail-shells.
In the eulittoral, owing to the fluctuations in water-level and the
influence of waves and spray, marked changes in conditions often
take place over very small vertical distances, and there may be con-
siderable differences in the composition of the communities from
spot to spot. Most of the species that are resistant enough to thrive
under the rather extreme conditions here obtaining, grow slowly and
can develop only ona firm substratum. In small lakes the eulittoral
15] VEGETATIONAL TYPES OF FRESH WATERS 495
may be restricted to a zone only a few centimetres high, but which
still tends to be well marked and already divisible into belts that are
inundated for varying periods in an average year and consequently
exposed to rapidly changing temperature and other conditions. Thus
in some central European lakes there is often an uppermost ‘ emersion
belt ’ that lies above the water for usually more than half the year
and is coloured brown by the cyanophycean Tolypothrix distorta,
which, when dry, can endure temperatures up to 70° C. without
injury. Below is typically a ‘ surf-belt’ of brownish to reddish-
yellow, pea-like crusts several millimetres thick and extending to
just below the low-water line. ‘This surf-belt is dominated by other
Cyanophyceae—particularly by Rivularia haematites which forms
hemispherical colonies interspersed with stratified layers of lime, and,
in the upper portion, by Calothrix parietina which resembles flat
spots of chocolate. Numerous other forms occur here where mois-
ture is plentiful, including whole hosts of Diatoms and invertebrate
animals. Below, the effect of waves becomes weaker with increasing
depth—often no more than 10 cm. below the lowest water-level—
and the crusts of Rivularia, etc., give way to thick grey-green sedi-
ments including precipitated ‘ lime ’.
We should mention also the psammon, the interesting community
inhabiting the wet capillary spaces of sand-bars and sandy shores.
This forms a zone as much as 2 to 3 metres wide, extending above
high-water mark up to the limit of capillary attraction, and cor-
responds with the edaphon of the soil but consists mainly of Protozoa
and Bacteria, with some Algae near the surface.
The sublittoral, where waves are no longer effective, typically
supports a rich growth of plants and animals of many and various
forms which are, however, killed by even brief emersion. Frequently
the cyanophycean Schizothrix lacustris is dominant, characterizing
a zone that can extend downwards for several metres if the dominant
is not overgrown by filamentous Algae. Here are often freshwater
Sponges and other animals coloured green by symbiotic Algae such
as Zoochlorella. For the water temperature tends to vary little and
photosynthesis to be untrammelled, so an especially rich and varied
life develops—provided the substratum is suitable for attachment and
the composition of the water is not unfavourable.
Deeper down, in the infralittoral, the appearance of the benthos
begins to change markedly at depths usually coinciding with the
thermocline, for here plant life is drastically influenced by the de-
creasing temperature and light-intensity. Noticeable in this twilight
496 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
zone is the marked decline in green types of Algae in favour of Diatoms
which here show especially dark-brown coloration. Nevertheless
the chlorophycean Cladophora profunda and Dichotomosiphon tuberosus
may still be plentiful. Stones at 10-20 metres’ depth are also liable
to have a blackish or reddish to violet covering of particular cyano-
phycean and other forms not found at shallower depths. Here
several types of Rhodophyceae such as species of Hildenbrandia
(‘ Hildbrandtia’, etc.) and Chantransia may occur, the coloration
of which, like the brown of Diatoms, enables them to use for assimil-
ation the short-waved green and allied light-rays that penetrate most
deeply. ‘The occasional green plants that persist here and below are
often strikingly dark in colour, and their assimilation appears to be
favoured either by increases in chlorophyll content or by changes in
the proportions of the component pigments in such a way as to aid
absorption. In some clear lakes the dysphotic region apparently
extends to very considerable depths, fairly frequent plants occurring
(sometimes in fair abundance) to go metres, and a very few Diatoms,
particularly, having been dredged from over 160 metres. It is,
however, by no means certain that these forms were actually living
and reproducing at such depths—that they had not merely sunk from
upper levels.
The depth to which the deeper plant communities extend of course
depends on the transparency of the water; but even in the profundal,
where photosynthesis is no longer possible, or at all events where a
positive assimilation-balance is no longer found, heterotrophic, etc.,
organisms occur on suitable substrata. In addition to animals and
their parasites, the heterotrophs include saprophytic Fungi and
Bacteria living, for example, on rotting wood and leaves. ‘There are
also some autotrophs in the form of chemosynthetic Bacteria liv-
ing under particular and often narrowly circumscribed conditions.
Quite apart from this, Bacteria and Fungi appear to play much the
same role of disease-producing parasites in water as on land, Bacteria
more generally attacking animals and Fungi parasitizing many plants
as well as animals.
Except on outcrops along steep shores, lake-basins are usually
covered by sedimentary ooze which may attain a thickness of many
metres. The ooze normally consists of an intricate mixture of
organic and inorganic matter that is either autochthonous (formed in
the lake itself by vital or physico-chemical processes) or allochthonous
(introduced from outside by inflowing water, falling of dust, etc.).
‘The amount and composition of allochthonous materials will depend
15] VEGETATIONAL TYPES OF FRESH WATERS 497
on numerous factors such as local physiography and the composition
of the rocks whence inflowing waters came; seasonally, pollen grains
may form a particularly impressive form of dust. Autochthonous
materials are the precipitations (such as ‘lime’ and ‘ iron’) that
take place in water, usually as a result of life-processes, and the
sedimentation of plant and animal remains. ‘The so-called ‘lime’
is mainly calcium carbonate, which is precipitated primarily through
the photosynthetic activity of plants that withdraw carbon dioxide
or the HCO3-i0n from dissolved bicarbonates. Much of it is apt
to float as particles in the water and be deposited in shallows as
greyish-white marl; at deeper levels, however, the more abundant
carbon dioxide commonly redissolves any settling particles which
then remain in solution as calcium bicarbonate.
Unlike the situation with lime, the secretion of silicates takes place
directly on living organisms—particularly on Diatoms, whose siliceous
‘shells’, on sinking to the bottom, greatly enrich the ooze with
silica. Such sedimentation takes place chiefly in the pelagial region
of free and deep water and is the origin not only of currently accumu-
lating diatomaceous deposits but also of the ‘ diatomaceous earth ’
in the sediments of long-extinct bodies of fresh and salt waters.
Consequently, and in contradistinction to lime, silica tends to be
far more plentiful in the sediments of the deep central plains than
of the shallow shore-terraces, etc., of lakes—at least when these
latter do not contain a large amount of allochthonous quartz
material.
The organic components of sediments enable lake-bottoms to be
transformed into spheres of often intense vital activity, while even in
shallow waters the carbon dioxide produced by respiring organisms
can lead to extensive re-dissolving of lime. Humic matter in solution
may result in a browning of waters coming in from leached soils
and may be flocculated on encountering calcium or other dissolved
salts when entering lakes—hence the brownish gelatinous sediment
that is commonly found in lakes in boggy regions. Otherwise the
organic component results largely from plankton and dust sedimenta-
tion, from the sinking of pleuston, and from washing in from the
watershed and the littoral zones. Even in temperate regions the
total sediment deposited may amount to several thousands of kilo-
grams of dry weight per hectare annually. Herein thrive ‘ decom-
position’ Bacteria, particularly, forming such ‘end’ substances as
carbon dioxide, water, ammonia, hydrogen sulphide, and methane.
These are thereby returned to circulation, or, in the cases of ammonia
498 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
and hydrogen sulphide, are subsequently oxidized by chemosyn-
thetic Bacteria which utilize, for carbon-assimilation in the dark
(using carbon dioxide as the source), the energy liberated in this
oxidation. However, under acidic conditions, as in bogs, cellulose
is not broken down to methane, and under anaerobic conditions (of
lack of free oxygen, such as occur in the profundal of permanently
stratified eutrophic lakes) decomposition in general stops short at
intermediate organic stages.
In running water, the slower the current the more closely the
benthos approaches in type one or another of the communities of
standing water. ‘This is seen in streams in which rapids alternate
with ‘ lentic ’ stretches having the same substratum ; for in the rapids
the stones are commonly overgrown by bright-green Algae and
Mosses, or in warm waters by thalloid Podostemaceae, but in the
stretches of slow current the growth at least of benthic lower plants
is liable to be markedly less luxuriant. ‘The difference appears to
be due to the fact that whereas in quiet water the organisms are
surrounded by a film of liquid that soon becomes depleted of the
substances they need, in rapid currents the absorbing surfaces of
plants are continually brought into contact with new bodies of water
and hence with new sources of materials.
The benthic plants of the so-called torrential communities have
to be attached sufficiently strongly to resist the mechanical forces
of the current, which may be considerable when it is rapid. Par-
ticularly effective and common as a type is the flat thallus applied
closely to the substratum. This is well exemplified by many
Cyanophyceae and Green Algae as well as by the Red Alga Hilden-
brandia rivularis and many members of the peculiar dicotyledonous
family Podostemaceae. Also prevalent are gelatinous layers and
hemispherical colonies such as those found in the surf-belt of lakes ;
when lime is plentiful they may be held together by it. Attached
floating growths must be particularly strong, with powerful hold-
fasts, as in the cases of tufts or ‘ streamers’ of Mosses or the larger
Algae, while in the more delicate types such as benthic Diatoms
the stalks of inhabitants of swift currents tend to be much shorter
and thicker than those of their lentic relatives. This keeps them
out of the rigours of the main stream. However, since the velocity
of a water-current rapidly decreases as the bottom is approached,
and at a minute distance from the bottom theoretically becomes
zero, the tiniest organisms can easily remain attached to this bottom
or even lie loose and undisturbed. Others can, and often in great
15] VEGETATIONAL TYPES OF FRESH WATERS 499
numbers and variety do, remain protected from the current in tufts
of Mosses or Algae.
The nature of the substratum is, as elsewhere, of importance in
determining the benthic vegetation of streams and rivers. In
general a firm, stony or rocky substratum prevails in rapidly flowing
water, because finer particles are washed away, whereas in lentic
stretches silty deposits are common and usually copious. Here the
rooted vascular plants so important in the hydrosere (as described
in the next section) are often prevalent, and the benthos under
conditions of slower and slower current approximates more and
more closely to that of lake margins.
Temperature may also have a marked influence on the benthos
of running as well as of still waters. ‘Thus, for example, the tempera-
ture often varies markedly in different parts of the same stream,
as well as, of course, in the same place at different seasons—com-
monly with attendant floristic differences. Accordingly, even in
small mountain brooks, the upper part may be dominated (to quote
a European example) by Hildenbrandia and the lower reaches by
another Red Alga, Lemanea, while the vernal period may be char-
acterized by the dominance of Diatoms and the aestival by Green
Algae, followed by a winter reappearance of the Diatoms. In lakes
and ponds, also, there are often three or four seasonal aspects to
be distinguished. Lastingly cold springs from deep rock-strata are
apt to constitute refugia of cold-stenothermal species, while warm
springs lack such types and usually include in their flora specific
megathermal forms. ‘Thus whereas at the lowest so-called ‘ thermal ’
temperature (30 —35° C.) almost all groups of Algae as well as Mosses
and flowering plants are commonly present in favourable alkaline
waters, it is only up to about 38° C. that Green Algae survive, and
up to 45° C. at the highest that the last Diatoms persist. Above
this temperature only Cyanophyceae continue from among these
groups, though they persist in fair numbers up to 55° C. A very
few species have been found growing above 60° C., but none for
certain above 69° C., to which temperature Synechococcus elongatus
appears able to survive. Bacteria, however, can withstand much
higher temperatures, and have been found living in thermal waters
up to at least 775° C. (In a dormant state some Bacteria can
survive one or two boilings at 100° C., and certain Yeasts are capable
of enduring a temperature of 114° C.)
A little needs to be added on the subject of algal epiphytes (that
is, Algae growing attached to other plants). ‘These, in fresh waters,
500 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
usually grow on higher plants—such as aquatic Grasses, Sedges,
Rushes, Horsetails, Water-lilies, Pondweeds, and the like. In
temperate regions they may often be roughly divided into com-
munities living (a) on submerged plants in very shallow waters,
these being mostly Green Algae ; (6) on submerged plants at depths
of between 1 and 3 metres, these being mostly other Green Algae
such as species of Oedogonium and Coleochaete and, in addition,
Diatoms ; and (c) on submerged plants at greater depths.
This last category extends down to about 6 metres and commonly
includes Coleochaete, Diatoms, and Cyanophyceae. Additionally,
subdivision of algal epiphytes is often possible in temperate regions
into (1) winter annuals, (2) summer and autumn annuals, and (3)
perennials. Here the age as well as the nature of the substratum
is of great importance ; but whereas there is a general tendency,
as might be expected, for older leaves, for example, to be better
endowed with epiphytes than younger ones, such a sequence is not
always found. ‘There may also be differences in the epiphytic flora
of the upper and lower surfaces of leaves, and on different parts
of a plant or even organ. Some of these and many other differences
appear to be due to differing light-intensities. Moreover, rapid
growth, as in the case of most of a leaf (other than its tip), tends
to prevent colonization. So far as attachment is concerned, the
nature of the * host’ surface, provided it is large and solid enough,
does not appear to matter except to the motile reproductive bodies
which tend to come to rest most easily in interstices and depressions.
Through germination of such disseminules and subsequent growth,
these sheltered situations often become quickly populated with adults
of the species concerned.
BoGs AND SALINE WATERS
Bogs, which abound particularly in the cooler parts of the northern
hemisphere, form a special habitat in which the substratum is com-
posed of peat. ‘The peat is usually saturated with water and has lying
above it a water-soaked layer of Mosses, particularly of the genus
Sphagnum (Bog-mosses). Domes of such Mosses are often sur-
rounded by swampy moats ; or species of Sphagnum may extend out
into the waters of a lake, often ultimately covering it to the centre.
Such features are typically characterized by attendant zones of
vegetation, the commonly coniferous tree dominants rapidly decreas-
ing in luxuriance and height as they advance into the moat or towards
15| VEGETATIONAL TYPES OF FRESH WATERS 501
the centre of the bog or margin of the lake. Beyond this zone of
often gnarled dwarfs there normally extends one of lowly Heaths,
such as species of Vaccinium (including Blueberries, Bilberries,
Whortleberries, etc.), Chamaedaphne (Leather-leaf), Oxycoccus (Cran-
berries), Andromeda (Marsh Andromeda), and Ledum (Labrador-
tea). The flora is liable to be limited by, among other conditions,
poor aeration not far down.
The surface is typically of slight mounds or hummocks set in a
network of depressions occupied by small shallow puddles. ‘These
last are particularly prevalent towards the centre of the bog or
persisting lake, where they often coalesce to form larger bog-pools.
‘The vascular plants of these very wet depressions are chiefly hygro-
ie ae
s SS SRG RaEMOr fee 7
—= = Lake sediments — —/ \s&
Fic. 168.—Diagram of cross-section through a ‘ highmoor’ bog that has arisen
from a small lake, the contours being exaggerated for clarity. (After Ruttner.)
~
phytic Sedges (Carex spp.) and Cotton-grasses (Eriophorum spp.),
but the zones of recent extension may be of almost pure Mosses.
Quaking bogs are those in which such vegetation, owing to extension
over waters, in part at least floats like a raft, while hanging bogs
are those developed on moist slopes. Fig. 168 shows in diagram-
matic form how a lake may become filled by sediments and ultimately
by Bog-mosses, etc., to form an extensive bog (Sphagnetum).
The bog-waters, including those of puddles, pools, and embedded
lakes, tend to be extremely poor in dissolved salts, strongly acidic
in reaction, and possessed of a high content of humic materials
which often impart a brownish coloration. Such waters are char-
acterized, from the boreal regions to the mountains of the tropics,
by a special microflora of species apparently unable to withstand an
alkaline medium. ‘This microflora is particularly rich in Desmids
but also contains certain Cyanophyceae such as Chroococcus turgidus.
Some Diatoms, Peridinians, and other Green Algae also occur, but
usually in limited variety compared with the predominant Desmids.
502 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
The determining factor appears to be the acidic reaction, coupled
with the low salt-content and the abundance of humic colloids
in solution.
Saline waters occurring inland vary from faintly brackish to
several times the average salinity of the ocean, some being completely
saturated with salts. ‘The plants of very salty inland waters, such
as are found practically throughout the ice-free land of the world
but especially in arid regions, are largely Green Algae, though
Diatoms and Cyanophyceae may also occur, with, in addition, higher
plants around the shallow margins. ‘These higher plants are usually
sea-shore species, so that the communities often resemble some of
those already described. Dunaliella salina is probably the com-
monest of the Green Algae inhabiting brine. It and a species of
Stephanoptera can even form light-green areas on solid salt crusts
where the water has a concentration of 33 per cent. of dissolved solids.
Such waters in salt ponds practically anywhere in the world may be
coloured reddish by the Dunaliella and have their bottoms covered
with a carpet of Cyanophyceae such as Microcoleus chthonoplastes.
Quite large species of Enteromorpha are known among the Green
Algae from salt springs and brine lakes, as well as from fresh waters
and brackish lagoons. Here fluctuating salinities often lead to the
production of peculiar specimens that are difficult to identify :
indeed the characters of several supposed ‘ species ’ can be exhibited
by different parts of a single thallus! In many brackish waters,
such as those near the sea where incoming streams bring fresh water,
a wide range of Algae and other plants of both fresh and salt waters
often flourish : they are normally species tolerant of varying salinity
but, being of different degrees of halophytism, still tend to give
populations that vary with the habitat. ‘Thus under only slightly
brackish conditions the flora is predominantly a freshwater one,
whereas with a close approach to full oceanic salinity it is mainly
marine.
HyYDROSERES
Apart from the bog succession dealt with above, there are the
normal seres occurring in open fresh water—especially around the
shallow margins of eutrophic lakes and ponds. Here the different
stages characteristically form more or less well-marked zones. Such
hydroseres can have their inception in deep waters that gradually
become shallower with the deposition of various materials comprising
15] VEGETATIONAL TYPES OF FRESH WATERS 503
the sedimentary ooze. This last usually includes contributions
from the plankton and pleuston which may accordingly be considered
part of the sere, though their non-essential nature, provided inorganic
sediments are sufficient to build up the bed, makes them rather of
proseral significance. Similarly proseral in nature are many of the
benthic communities of the profundal and lower infralittoral, though
those of the shallower, upper littoral zones commonly form part
of the autogenic main sere. ‘These may include the ‘ Characetum ’,
which often occupies the floors of bodies of water or shoals chiefly
from 8 to 12 metres deep, and is characterized by the curious Green
Algae known as Stoneworts (species of the genera Chara and Nitella),
by Bushy-pondweeds or Naiads (Najas spp.), and by various aquatic
Mosses such as species of Drepanocladus and Fontinalis. ‘These
plants are widely important not only in retaining silt and depositing
humus, but also in ‘ binding’ the surface on which they grow.
The first of what appear to be the normally essential portions of
the hydrosere is the ‘submerged aquatics’ stage. Although this
may to some extent be represented by members of the Characetum,
more familiar in the zone expressing this stage in the north-temperate
regions are such plants as many of the Pondweeds (Potamogeton
spp.), T'ape-grasses (Vallisneria spp.), Water-milfoils (Myriophyllum
spp.), Horn-worts (Ceratophyllum spp.), and Water-weeds (Elodea
spp.). Most of these are normally rooted, but various lower plants
such as the often associated larger Algae and Mosses lack true roots
although they may grow attached to the substratum by means of hold-
fasts, rhizoids, etc. Some even of the higher plants are normally
unattached, floating freely as more or less dense carpets or ‘ beds ’
near the bottom of quiet waters whence they may extend into the
general body of the pond or lake. These submerged plants tend
to cover slopes down to a depth of about 6 metres, where decreasing
light-intensity becomes a limiting factor, though some Mosses may
persist more deeply, Spring-moss (Fontinalis sp.) being said to form
carpets down to depths of about 20 metres in some clear alpine
lakes. Stoneworts (Charales), looking like small vascular plants,
may grow even deeper down, and a single Moss has been found
growing at a depth of 60 metres in Lake Geneva. Fig. 167 showed
the relationship of this stage to other early ones of the hydrosere.
These relatively bulky submerged plants collect silt and organic
matter produced by other living forms and add this material,
together with the results of their own decomposition, to the bed
which accordingly becomes built up more quickly. In time it is
504 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
shallow enough for colonization by other plants which, through long
and flexible upgrowing stems and/or leaf-stalks, enable their leaves
to reach the surface of the water, so constituting the ‘ floating-leaf ’
stage and crowding out most of the submerged aquatics. Examples
of such plants with floating leaves are Potamogeton natans and some
other Pondweeds, some Bur-reeds (Sparganium spp.), and most
members of the Water-lily family (Nymphaeaceae). In a sense also
belonging here is the detached pleuston of Duckweeds (Lemna
Fig. 169.—Leaves of Sacred Lotus (Nelumbo nucifera) projecting out of the water,
and Pistia stratiotes floating on the water, in the Philippine Islands.
spp.), Pistia stratiotes, Water-hyacinth (Eichhornia crassipes), Frog-
bit (Hydrocharis morsus-ranae), Water Buttercups (Ranunculus spp.),
etc., which frequently help to consolidate the surface vegetation of
this stage. Fig. 169 shows a dense community really belonging to
this stage although the leaves of the dominant Sacred Lotus (Nelumbo
nucifera) project out of the water. Floating freely on the surface
are abundant plants of Pistia (cf. Fig. 164, B). Such prolific aquatic
vegetation markedly reduces light-penetration and the turbulence of
the water (cf. also Fig. 164, C). Various Podostemaceae are par-
ticularly characteristic of running warm waters, the vegetation of
which may be separated on this and other grounds.
15] VEGETATIONAL TYPES OF FRESH WATERS 505
The floating-leaf plants tend to be still more prolific than preced-
ing stages and to build up the bed still more rapidly by their col-
lection of silt, etc., and addition of their own material on death.
Consequently in time there can be colonization by swamp plants
and emergent hydrophytes of which at least half the body is aerial.
These rapidly predominate over the floating-leaf plants, many of
which are soon crowded out, and so the reed-swamp stage is attained.
Whereas the water is commonly 4-3 metres deep in the floating-
leaf stage, in the reed-swamp stage it is usually less than 1 metre
deep. Here the main dominants are of such types as the Common
Reed (Phragmites communis agg.), Bulrush (Scirpus lacustris), Reed-
maces or Cattails (Typha spp.), Water Horsetail (Equisetum fluviatile),
and various Sedges (Carex spp.), or Papyrus (Cyperus papyrus) in
tropical rivers, one or other of which often forms a practically pure
stand. Even within this zone there may be some differentiation,
the Bulrushes, for example, occupying the deeper water.
Subsequent events in the hydrosere are outlined and illustrated
in Chapter XI ; but it should here be remarked that many marsh-
plants (helophytes, recognizable within the more general category
of ‘hygrophytes’, or plants of moist habitats) and water-plants
(hydrophytes) are much alike in their morphological and anatomical
characteristics. ‘There are, however, some marked differences, as
the following characterizations will indicate. In underwater parts
the cuticle and other features curtailing transpiration are reduced,
stomata being abolished but aerenchyma (see below) much developed.
Moreover, the vascular tissue even in stems is often arranged centrally
when they develop under water, there being no secondary thickening.
Such a structure gives tensile strength but allows flexibility, as water
affords sufficient support for rigidity to be unnecessary—even
though this same water may exert a dangerous pull. In the stems
of real marsh-plants, however, the supporting and conducting
elements are usually arranged peripherally, as these stems have to
stand more or less erect after the manner of those of land plants
and meanwhile must have sufficient conducting elements for rapid
transpiration. On floating leaves, stomata are usually confined to
the upper surface, while submerged leaves are often finely dissected
to facilitate the exchange of materials. ‘The same characteristic may,
incidentally, save them from being torn by currents.
As it is more difficult to obtain oxygen in water than in air, and
still more difficult in waterlogged mud, most aquatic and marsh
plants have systems of large air-canals or air-filled cells, constituting
506 INTRODUCTION TO PLANT GEOGRAPHY
the so-called ‘ aerenchyma’, which often dominate their anatomy,
extending through their bodies right down to the roots. These
last have a particularly difficult problem of aeration for respiration
in the oxygen-poor ooze.
Finally, although some annual aquatics occur, such as Naiads
(Najas spp.), most higher types are perennial, either living through
any unfavourable season apparently unchanged or else dying down
or, if pleustonic, often descending to the bottom. Even in cool-
temperate regions, many shallow-water types appear to continue
some photosynthetic activity in winter under the ice—at least as
long as this is not covered by a darkening layer of snow.
FURTHER CONSIDERATION
For additional details :
P.S. Wetcu. Limnology, second edition (McGraw-Hill, New York etc.,
pp. Xi + 538, 1952).
F. Rutrner. Fundamentals of Limnology, translated by D. G. Frey
& F. E. J. Fry (University of Toronto Press, Toronto, Ont.,
Pp. Xl + 242, 1953).
F. R. Moutton (ed.). Problems of Lake Biology (American Association
for the Advancement of Science, Washington, D.C., pp. 1-142, 1939).
Useful accounts of particular aspects are to be found in the appropriate
parts of:
A. F. W. Scuimper. Plant-geography upon a Physiological Basis, transl.
and revised edition (Clarendon Press, Oxford, pp. xxx + 839 and
maps, 1903). Especially useful in this connection is the ‘ third’
German edition, Pflanzengeographie auf physiologischer Grundlage,
revised by F. C. von Faber (Fischer, Jena, vol. I, pp. xx + 588,
and vol. II, pp. xvi + 589-1613 and maps, 1935).
V. J. CHapman. An Introduction to the Study of Algae (Cambridge
University Press, Cambridge, Eng., pp. x + 387, 1941).
G. M. Situ (ed.). Manual of Phycology (Chronica Botanica, Waltham,
Mass., pp. xii + 375, 1951).
K. E. Carpenter. Life in Inland Waters : with especial reference to
animals (Sidgwick & Jackson, London, pp. xviii + 267, 1928).
G. E. Hurcurnson. A Treatise on Limnology : vol. I, Geography, Physics,
and Chemistry (Wiley, New York, pp. xiv + 1015, 1957).
CHAPTER XVI
NAB G EAC LO UNA, Shy PES: 1O.F «SEAS
We have now dealt at least cursorily with each of the main vegeta-
tional types of the chief plant habitats, excepting only those of the
oceans and salt-seas which, however, occupy between them slightly
over 70 per cent. of the surface of the globe. Nevertheless they
are relatively uniform over very wide areas and so can be treated
fairly briefly. ‘This is especially true of the open ocean and the
planktonic communities developing in it. On the other hand, on
shallow marine bottoms and particularly around sea-shores, condi-
tions and the attendant plant communities tend to be far more
variable, so that altogether we get a set of categories which are
largely comparable—for example in their planktonic or, alterna-
tively, benthic nature—with those of fresh waters. But before we
deal with each of these in turn, we must consider the sea in general
as a habitat for plants.
SOME FEATURES OF THE MARINE ENVIRONMENT
That the marine environment is widely different from the fresh-
water one is evidenced by the fact that salt-water plants placed in
fresh water, or vice versa, almost invariably perish. ‘The light and
other physical conditions can be, and often are, much the same in
the two media, and temperature fluctuations of the air tend to be
similarly ‘ damped down ’, the differences in salinity being commonly
the decisive factor so far as inhabiting plants are concerned.
The salt-content of the free oceans is around 3°5 per cent., but
in bays and inland seas it may deviate widely from this figure owing
to concentration by evaporation or, alternatively, dilution by fresh-
water streams. ‘Thus preponderance of evaporation causes the Red
Sea to have a salinity of well over 4 per cent. at the surface, while
abundant inflow of fresh water makes the salinity of the Baltic Sea
in many places less than 1 per cent., and in some only o-1-0-2 per
cent. Such reductions in any particular region—apparently regard-
less of climatic or other changes—tend to be accompanied by very
5°7
508 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
marked ones in the numbers of species of Green, Brown, and Red
Algae present, and in the general luxuriance of the vegetation. At
the lower concentrations in middle and high latitudes some of the
persisting Brown Algae, particularly, often take on characteristic
dwarf forms, and the plankton becomes poor in species and more
and more limited to freshwater or brackishwater types. ‘There may
also be superimposed layers of waters of different salinities, and
supporting different Algae—especially when currents of different
origin meet but do not mix.
By far the most abundant solid in solution in sea-water is sodium
chloride (common or table salt), which on the average forms nearly
78 per cent. of the total salts present, contributing over 27 grams
per litre. It is followed in the scale of abundance by magnesium
chloride (nearly 11 per cent.) and magnesium sulphate (nearly 5
per cent.), though salts of calctum and potassium are also fairly
plentiful. Sea-water is a ‘ buffered’ solution, exhibiting resistance
to changes in its degree of basicity (the so-called ‘ reaction’ or
‘ pH level’, due to the concentration of free hydrogen-ions present).
Thus plentiful carbon dioxide is normally available for photosynthesis
without disturbance of the buffered state, and the prevailing slight
alkalinity enables living organisms to extract calctum carbonate, etc.
Especially is this easy in warm seas—hence their numerous large
calcareous shells, coral-reefs, and so forth. Containing, as it does,
all of the chemical elements essential to the growth and maintenance
of protoplasm, sea-water is in general a very appropriate environment
for living cells—provided they are adapted to its concentration of
salts.
Different Algae vary enormously in their tolerance to variations
in the salt-content of the water—from the narrowly stenohaline
species requiring the salinity to remain within a narrow range (these
are represented by most oceanic forms), to the broadly euryhaline
ones that grow in puddles high up on the shore. Here they are
bathed in sea-water when the tide comes in or waves reach them,
the salinity being often further increased through evaporation ; but
after heavy rainfall they may find themselves in almost entirely fresh
water. ‘lhe turgor adjustments involved within the cells in endur-
ing such changes are not fully understood, although it is known that
certain Diatoms living in the mouths of streams where the salinity
varies rapidly are able to take in or let out salt very quickly according
to its concentration in the water.
Owing to the intimacy of aquatic organisms with the medium in
16] VEGETATIONAL TYPES OF SEAS 509
which they live, and also to the general stability of the physical
characteristics of that medium, slight changes in the environment
are apt to be reflected with particular promptness in the plant and
animal population, whose components commonly lack the protection
their terrestrial counterparts have developed. Moreover the
organisms themselves may modify the chemical nature of their
environment by withdrawing or adding substances, so that, for
example, the surface layers of sea-water often become so impoverished
in the essential combined nitrogen and phosphorus as greatly to limit
growth and otherwise change the character of the plankton. Such
impoverishment is due to absorption by organisms whose dead
bodies sink to the depths where they gradually decay, so that
replenishment takes place chiefly by vertical convection currents in
the cooling period of autumn. ‘This replenishment occurs most
actively in temperate and colder seas, and explains the often richer
plankton of these regions than of tropical seas where the lastingly
warm and light surface waters shut off the deep ones and little
vertical convection takes place. Indeed, while tropical seas are
estimated to contain on the average 5 planktonic organisms per
cubic centimetre, the figure for arctic seas varies from 100 to 500.
Nevertheless there is some compensation provided by the greater
depth to which the photosynthetic zone extends in the lower latitudes,
and still more in the short duration of activity each year in the high
latitudes, so that some authorities have recently claimed that there
is little or no significant difference in total productivity between
the seas of high and low latitudes. But for one reason or another,
in spite of the relative uniformity of marine habitats, and the similarity
that prevails over vast areas especially in the open ocean, great
differences do in fact occur in different regions, which are reflected
in their plant as well as animal life.
Among other chemical items it may be presumed that the content
of dissolved oxygen and carbon dioxide, and the variable reaction
(pH), are all important. Where vertical currents are active in seas,
the gas-content and composition of the water will be kept similar
in all layers reached by such currents ; but where no such aeration
etc. takes place, extreme conditions may occur. ‘Thus in the Black
Sea, oxygen is found only to a depth of 183 metres ; below this,
normally respiring plants and animals cannot exist, and much the
same appears to be true in some tropical lakes. Although the situa-
tion with carbon dioxide is substantially reversed through photo-
synthesis, during which it is absorbed and replaced by oxygen,
510 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
actually in the surface layer carbon dioxide can be replenished from
the air, whereas deep down the preponderant tendency is for it to
be given off in respiration and decomposition, oxygen being absorbed.
So here again the two gases are largely complementary. During the
summer and winter periods of relative stagnation, surface waters
tend to be more alkaline than deeper layers, owing to the photo-
synthetic activity of Algae, which removes the carbonate-ion and
leads to a preponderance of hydroxyl-ion.
As regards temperatures, the surface layer of water already shows
much smaller variations than the air lying directly upon it. Con-
versely, water has a regulating effect on the temperature of neigh-
bouring air-masses, and this effect may operate at considerable
distances if the air-masses move over land. ‘The temperatures of
the surface waters of the sea rarely if ever exceed 31° C.; nor do
they fall below the freezing-point of —3-6° C. Deep down in the
ocean the temperatures are commonly rather low and uniform—for
example, 0:6° C. at 2,000 metres’ depth in the Antarctic Sea where
the surface was 1:0° C., and 1-6° C. at 3,000 metres in the equatorial
part of the Pacific Ocean where the surface was 29° C. But in
spite of the relatively small amplitude in this respect, so that perennial
marine Algae even in cold regions may exhibit no period of winter
rest but carry on vegetative activity in summer and reproduce in
winter, while in warm seas the difference in temperature is often
no longer effective, the floristic organization of marine vegetation
depends substantially upon the temperature of the water. ‘Thus the
limits of marine floristic regions tend to coincide with particular
isotherms, and this is especially true of plankton, whose latitudinal
distribution may be related to local temperatures at particular
seasons.
In conjunction with such factors as temperature, the persistence
or importation of disseminules is of obvious importance in deter-
mining the character of the phytoplanktonic population developing
in any one region at various periods of the year. ‘The disseminules
may be transported, as adults or otherwise, considerable distances
and in quantity by ocean currents, and vast distances will be traversed
if a series of generations is involved. Each component type in the
plankton has its metabolic requirements adjusted to particular
temperature ranges and, having withstood unfavourable seasons
elsewhere or in some resistant form, ‘ blooms’ with the return of
suitable conditions, different types succeeding one another largely
according to their specific requirements. Even in the yearly
16] VEGETATIONAL TYPES OF SEAS 511
periodicity in which light plays such an important part, it is clear
that temperature may be decisive in favouring the development,
for example, of northern species very much farther south than usual
in the earlier part of the year, and of southern species northwards
when the water warms up in summer. But although the flora tends
to be more diverse in warm than cold seas, in the matter of total
productivity the reverse may hold—at least of phytoplankton in the
upper layers (see p. 509).
Light is often the dominant factor in determining the local
distribution and extent of aquatic vegetation, being rapidly reduced
with increasing depth. ‘The degree to which water allows light to
penetrate its depths naturally varies greatly with turbidity and other
conditions, but in general the total radiation is reduced to little
more than half its surface intensity at a depth of ro centimetres,
and to a little over one-seventieth at 100 metres. Nevertheless in
some seas there may be a noticeable effect on a photographic plate
as deep down as 1,000 metres. Although photosynthesis doubtless
ceases far above such limits, and probably often in the uppermost
100 metres, it still commonly continues to much more considerable
depths in seas than in fresh waters, owing to the greater transparency
of the former. ‘Thus the euphotic zone in the ocean normally
extends down to 80 or more metres, and the dysphotic zone, of dim
light and consequently very limited plant production, from the base
of the euphotic zone to 200 or more metres. Moreover, because of
the greater thickness of the turbulent layer affected by vertical
diffusion currents, as well as of this deeper photosynthesis allowed
by deeper light-penetration, the habitat of the ‘ surface plankton’
commonly goes far deeper in oceans than in lakes.
The depth to which photosynthesis extends will naturally vary
greatly in different circumstances, being limited markedly by dis-
persion of light due to suspended particles both living and inert.
Moreover, because of the lowering angle of incidence of the sun
and consequently reduced penetration of its rays, this depth usually
gets shallower and shallower at increasingly high latitudes. It is
commonly taken by oceanographers as the dividing-line between the
bottom of the infralittoral (also called by some the sublittoral) and
the top of the deep-sea system, being often placed at around 200
metres as in the accompanying diagram (Fig. 170). ‘This maximum
depth to which photosynthesis extends is also the approximate depth
of water at the outer edge of the continental shelf, and separates
the neritic (shallow water) province from the oceanic province of
512 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
what some students term the pelagic division of the sea (their other
division being the benthic, comprising the ocean floor and shore).
Often the ‘ compensation point’ (at which the daily accumulation
of food as a result of photosynthesis is just balanced by the break-
down during respiration of stored materials) is very much less deep,
being sometimes only a few metres from the surface, though of course
varying with many factors, including the organisms concerned.
It is also convenient to separate an uppermost or ‘ eulittoral ’
zone as extending from the highest to the lowest ‘ normal’ tide-
levels on shores, and a ‘ sublittoral’ extending from the base of
this down to a depth of about 40 to 60 metres, the lower boundary
being set at the lowest limit at which the more abundant attached
plants grow. It should be noted that some authors continue the
eulittoral down to the lower limit of at all abundant attached plants,
their so-called sublittoral beginning here and extending down to
a total depth of about 200 metres, and so corresponding with what
we have here termed the infralittoral. Below the infralittoral is the
deep-sea system, divided into an upper ‘ archibenthic zone ’, extend-
ing to a depth of between 800 and 1,100 metres, and the lower
‘ abyssal-benthic zone ’, in which conditions are practically uniform.
Here the temperatures are always low (— 1° to + 5° C.), solar light
is lacking, and there are no seasons. ‘The various zones, etc., are
shown diagrammatically in Fig. 170.
The component colours of white light are variously absorbed by
sea-water—those of shorter wave-length, such as the very abundant
green, being in general less absorbed than the red, and even the blue,
though the red may be relatively little affected by stains and sus-
pended matter. ‘This differential absorption of different components
of the spectrum seems to be one of the main factors behind the
ecological preferences of different Algae for different depths, though
details are still not clear. ‘Thus in general Green Algae reign in
the uppermost layers where red rays are plentiful, such rays being
apparently essential for healthy growth of many of these plants, while
Red Algae predominate deeper down where green rays still penetrate,
although in each colour-group are species belonging to very different
depths. Brown Algae tend to be plentiful at all depths except the
deepest inhabited by Algae, where Red Algae commonly predominate.
Apparently both the intensity and the wave-length of the light
play a large part in controlling the regional distribution of Algae,
the green surface-forms flourishing under conditions of high light-
intensity and plentiful red rays, though hardly able to utilize the
16] VEGETATIONAL TYPES OF SEAS 513
green rays of the depths. The deep-water Red Algae, on the other
hand, are capable of growing under conditions of much lower light-
intensity, being able to utilize precisely those deeply penetrating
rays of shorter wave-lengths. Only when the light is sufficiently
weakened, do many of these deep-water forms appear able to grow
well in the red-containing light of the surface. Diatoms tend to
flourish best under relatively low light-intensities where the red
component is much reduced, often having their maxima at around
ro metres’ depth and forming rich growths at 15 to 20 metres where
chiefly green and some blue rays prevail.
g tides sa) 7 Limits of
: TING
ema en ee j the littoral
é noe ge
Supralittoral™> pimit FS /
Eulittoral (of tidesyS Low ae
Sublittoral (of large =
vegetation)
ieralicrorale
Rveale: bent” =
(of practically uniform conditions)
Fic. 170.—Diagrammatic representation of typical sea-marginal profile.
The motion of the water is another factor important to marine
vegetation, leading as it does to the occurrence of largely different
forms on surf-pounded and sheltered shores. ‘Thus in exposed
situations, quite apart from the need for strong holdfasts and
‘leathery ’ thalli to prevent detachment and injury by breakers, the
moving water tends to be better aerated than on sheltered shores
and consequently to favour increased biological activity. Moreover
sea currents, whether regular or irregular, commonly carry Algae,
often for considerable distances, and can be one of their chief
agents of dispersal, though for most benthic forms death follows
any prolonged period of detachment.
For plankton, although mixing of waters by upwelling or con-
vection or other currents is widely important, a goodly supply of
nutrients being necessary for rich development, some degree of
514 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
stability is also desirable. ‘Thus any surface and other waters of
excessive turbulence tend to show only moderate phytoplanktonic
populations even when conditions of light and nutrient supply are
very favourable. ‘The chief depauperating influence in such circum-
stances seems to be removal of organisms by descending currents.
Horizontal ocean currents, however, are commonly important in
bringing in types from other climatic belts, as is evidenced by the
widespread distribution of most planktonic species, and even though
living conditions may be similar during only limited periods of
the year. Neritic types carried seawards by outgoing currents often
persist for a while and may even reproduce, though in time they
will perish ; nor can a population resume existence if the water is
persistently devoid of vegetative plants and resting spores of species
capable of taking advantage of suitable conditions. ‘This has been
suggested as the explanation of the poverty of some off-shore com-
munities where the depth of the water, it is thought, may prohibit
the ready ‘return’ of resting stages.
Also important to the local vegetation, except in very deep water,
is the physical nature of the shore or ocean floor—for example,
whether it is hard or soft, fixed and rigid or loose and therefore
likely to be moved by waves or other influences. Normally bare
rock, which on the whole is very inhospitable for land plants, is the
best substratum for the larger Algae that form the vast bulk of
eulittoral and sublittoral marine vegetation. Especially are such
durable rocks as granites suitable for attachment, whereas the softer
schists, shales, and sandstones are insufficiently stable for the attach-
ment of large Algae, carrying at best only rather small species. But
in general the ocean floor is covered by softish sedimentary deposits
that result from weathering and erosion on land or from life in the
sea. ‘here is now some reason for supposing that the chemical
composition of the substratum may yet hold considerable significance
for marine Algae, though this significance is apparently far less than
is commonly the case in fresh waters.
In spite of their rather vague and variable separation, the oceanic
(pelagic) and neritic provinces are very different. ‘The oceanic
province is itself divided into an upper lighted zone and a lower,
dark one: its outstanding features when compared with the neritic
are its great area and range of depth, its transparency owing to the
usual absence of detritus of terrestrial origin, and the consequent
deep penetration of light and resultant blueness. In chemical
composition these off-shore waters are relatively stable, with salinity
16] VEGETATIONAL TYPES OF SEAS 515
almost uniformly high, though the content of plant nutrients may
be relatively low in the upper layer, and these may be only slowly
replaced.
In the neritic province the chemical constitution is more variable,
salinities being usually lower than in the open ocean, and sometimes
markedly so. Moreover they are apt to undergo such seasonal or
sporadic fluctuations that the inhabitants may have to be euryhaline
in nature. However, plant nutrients such as phosphates and nitrates
tend to be more readily available in these shallower inshore waters
than elsewhere—a fact which is of special importance in the produc-
tion of Diatoms, the foremost of ‘ primary sea-foods’. Conse-
quently a unit volume or even unit area of the neritic water is
commonly far more productive than a similar unit of oceanic water,
though the latter as a whole, because of its extent and depth, provides
the bulk of inhabitable space on earth. (The free atmosphere is
scarcely to be considered habitable space except very close to the
surface of the earth, for the organisms that are found free in it
appear to be making only temporary excursions therein, however
protracted these excursions may seem.)
PLANKTON
It seems best, in dealing with the sea, to consider all phytoplankton
together—without separating any macroscopic floating matter as
pleuston. Moreover, barring accidental detachment, for example
of marine vascular plants, when either death or resettling must soon
follow, the ‘ regular’ phytoplanktonic organisms of the sea are all
lowly Thallophytes or Schizophytes. ‘Thus under the heading of
marine phytoplankton are commonly included all of the floating or
drifting forms of plant life of the oceanic and neritic provinces of
the sea—as befits the Greek derivation of the world plankton, which
means ‘ wanderer ’—whether they be microscopic, as in the vast
majority of cases, or quite large, as in the case of Sargasso-weed
(see p. 521). Although the number of different groups of plants
normally represented in plankton is limited, there is no dearth of
variety in form, as Fig. 171 indicates.
The general characteristics of phytoplankton were discussed in
the last chapter, much of which was devoted to the conditions of
life and phytoplanktonic communities of fresh waters. ‘Those of the
ocean are again mainly microscopic and so will not be described in
detail. ‘They are chiefly Diatoms and Peridinians (Dinoflagellates),
516 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
or, In some warm seas and brackish lagoons, Blue-green Algae
(Cyanophyceae). Examples of typical Diatoms and Peridinians are
shown in Figs. 6 and 7, respectively. ‘Thus the brown colora-
Fic. 171.—Photomicrographs of marine phytoplanktonic communities. A, Early
summer © maximum’ of Diatoms off the Atlantic coast of North America. Domin-
ant are Guinardia sp. (short and square) and Rhizosolenia sp. (long and slender),
(x about 35). (Phot. H. B. Bigelow.) B, Late summer ‘ peak’ of Peridinians
(anchor-shaped Ceratium sp.) and animalcules (pencil-shaped Tintinnids) off the
Atlantic coast of North America. ( about 50.) (Phot. H. B. Bigelow.)
16] VEGETATIONAL TYPES OF SEAS iy,
tion of many northern waters at certain times of the year is due
largely to Diatoms, while the Red Sea owes its name to the red
accessory pigment of the planktonic cyanophycean Trichodesmium
erythraeum. Diatoms are present in all seas and indeed almost
everywhere, being usually numerous as regards both forms and
individuals ; Peridinians are also extremely widespread, but tend
to be numerous chiefly in terms of individuals in cold seas and of
different forms in warm ones. Green flagellates and Bacteria may
also be very abundant in marine plankton—the latter especially near
coasts, though there is some doubt as to whether they should be
considered truly planktonic (see p. 521). Various other greenish
brownish, or yellowish types are also prone to occur—such as, respec-
tively, Halosphaera viridis, Phaeocystis, and certain Silicoflagellates.
Marine plankton has to remain suspended in water. Consequently
the component organisms either swim or are very minute or have
some appropriate ‘form-resistance’, meanwhile employing the
seemingly least and lightest possible structural material. This is
towards maintaining the specific gravity near the density of the
surrounding medium—much as in freshwater types. Such needed
buoyancy may be increased by gas-bubbles or, as in the case of
Diatoms, by globules of oil. Adhesion of cells together may also
increase their tendency to float, though, in general, reduction to
small size is highly advantageous. For the rate of sinking of a body
heavier than water, as most phytoplankters (phytoplanktonic
individuals) are, depends upon the ratio of surplus weight to friction,
which is determined mainly by surface area, and the simplest way
to obtain a relatively large surface-to-volume ratio is to reduce the
absolute size. ‘Thus the smaller the body, the less tendency will
it show in general to sink.
Otherwise, if no means of active locomotion or actual flotation
is available, reliance has to be placed on an increase in surface area
and complexity through structural adaptations involving the external
form. ‘This may be as of a bladder, in which much of the relatively
large cell is occupied by light fluid, or of a disk, which sinks in zig-
zag fashion and so covers a greatly increased distance, or of a needle,
which sinks slowly when the long axis lies horizontally—as the
mechanics of sedimentation make it tend to do. Or the elongated
body may be curved or provided with bevelled ends in such a manner
that, if displaced, it is soon brought back to the horizontal position,
sinking being accomplished in wide circles. Or the cells may be
attached in ribbons or chains, or they may be branched or possessed
518 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
of long spines or other projections to resist sinking. One or more
of these features leading to increased form-resistance is commonly
found in planktonic Diatoms, which, it is interesting to note, also
tend to have lighter cell-walls than their benthic counterparts.
Peridinians, in spite of their power of locomotion, may also have
a parachute-like or winged form, or may develop marked asymmetry
that leads to orientation of the sinking body so that the long axis
lies horizontally, accordingly providing the maximum surface and
resistance to passive sinking. It is also interesting to note that, in
keeping with the reduced viscosity of warmer waters, the summer
forms of Diatoms tend to have lighter shells and the southern
Peridinians larger projections than their colder-water counterparts.
The majority of marine phytoplanktonic organisms inhabit the
euphotic zone, and especially its upper layers. ‘The dysphotic
zone 1s commonly very poor in phytoplankton, and the aphotic
zone, except for occasional stray photosynthetic individuals, is
limited to saprophytic, parasitic, or chemosynthetic forms. Seasonal
variations may also be very marked in marine phytoplankton, often
including both spring and autumn maxima when nutrients are
plentiful and lightand temperatures allow rapid development. Quite
apart from this, the seasons may produce a sequence of types favoured
by, or able to withstand, particular conditions.
Owing to the minute size of many of the organisms, only a small
proportion of the total phytoplanktonic population present is satis-
factorily secured by the collecting nets usually employed. Con-
sequently concentration by settling, centrifuging, or filtration is
necessary for proper appraisal. ‘The exceedingly minute material
thus obtained is called ‘ nannoplankton’ and includes the smaller
Diatoms and Peridinians, Bacteria, numerous flagellates such as
Coccolithophores which may be no more than 5 microns in dimen-
sions, and of course the tiniest animalcules.
The composition and density of planktonic vegetation at any
particular place and time naturally depend upon the intensity and
interaction of various factors. Among the more obvious of these
are the rate of reproduction and the rate of removal of individuals
by death or sometimes fusion, by consumption by other organisms,
or by removal through sinking or 1n water-currents. ‘There is also
a dependence on the rate of growth which itself depends upon the
size and type of the parent stock, upon the intensity of the light
falling on the surface and reaching the particular depth, and upon
the concentration and availability of the elements which are essential
16] VEGETATIONAL TYPES OF SEAS 519
for plant growth and photosynthesis—in particular carbon, nitrogen,
and phosphorus. Before all comes the question of available stock.
Sometimes the turbidity of the water is such that the compensation
point (see p. 512) is reached within 5 to 10 metres of the surface, and
at times the water is so depleted of certain elements, in particular
combined nitrogen and phosphorus (which are taken up by the living
organisms), that further development is practically dependent upon
replenishment through death and decay, or importation. Paucity
of certain metallic elements, particularly iron, also seems to be prone
to limit phytoplanktonic growth. ‘Thus fertility may be controlled
by the availability of inorganic nutrients, and marine productivity
in turn limited by the rate of supply of organic foods—as has been
strikingly demonstrated by the extraordinarily increased yield of
“manured’ arms of the sea. Here inorganic nutrients were
added, that led to an increase in organic food in the form of phyto-
plankton, which in turn benefited the Fish.
As in fresh waters, replenishment of nutrients by vertical mixing
currents takes place in the sea most rapidly in autumn and winter
with the cooling and sinking of surface waters, so that in spring
there is a relatively large supply available, and the phytoplanktonic
population can ‘ bloom’ luxuriantly. It seems that in some of the
most actively productive systems of sea-water, almost every atom
of phosphorus and nitrogen must be reassimilated several times each
year to permit the synthesis of the total organic material observed.
The most lastingly productive waters are commonly those where
there is continual upwelling from the depths, such as occurs at the
boundaries of oceanic currents.
Where conditions fluctuate, the phytoplanktonic crop can develop
within a few days of their becoming favourable, whereas a much
longer period is required for the development of zooplankton.
Nevertheless, owing to animal grazing, to sinking below the lowest
level at which they can photosynthesize effectively, to horizontal
transport, and to ‘ indirect factors’ such as changes of temperature
(quite apart from available nutrients), phytoplanktonic populations
tend to vary markedly and often rapidly. Especially is even mild
grazing often extremely effective in reducing the population, so that
it has been calculated that if only one plant out of every ten in a
developing population is eaten, in six divisions 100 plants will pro-
duce but 3,400 individuals instead of 6,400—although a mere 413
have been consumed.
The simple cell division of most phytoplanktonic organisms is a
520 INTRODUCTION LO” PEANT (GEOGRAPHY [CHAP.
very satisfactory and effective method of reproduction that under
favourable conditions probably takes place on the average from once
to twice every twenty-four hours. It has been calculated that in
middle latitudes the increase in total volume of pelagic plants is of the
order of 30 per cent. per diem over the year, so that this proportion
could die or be removed on the average daily without reducing the
plant stock. Whether or not the high latitudes are more prolific
than the low ones in the matter of plankton productivity, as used
to be contended but is now questioned (see p. 509), there seems no
doubt that the total photosynthetic activity taking place in the seas,
which occupy slightly over 70 per cent. of the earth’s surface, is
much greater than that taking place on the land-masses of the
world. Actually, the average productivity of similar land and ocean
areas appear to be roughly comparable over the year, being said
to be of the order of three tons of dry material per acre, though
unit volumes of coastal waters are usually many times more productive
than those of open ocean waters.
In arctic regions there are apt to be phenomenal outbursts of
Diatoms when the sea-ice melts in early summer. Such ‘ blooming’
may be associated with the rapid germination of spores previously
locked in the ice. ‘The population involved tends to be largely
neritic, which is not surprising if we recall that the abundant nutrients
and variable salinity and other conditions simulate coastal ones.
The cryophytic populations developing on the surfaces of melting
sea-ice were described in the last chapter, chiefly on pages 489-91,
the component organisms being termed ‘ cryophytes’. When the
‘spring’ production of Diatoms has come to a low ebb owing
to marked stratification or incipient exhaustion of nutrients, Peri-
dinians (Dinoflagellates) may take over the lead in the matter of
organic production in boreal and austral waters—in particular
species of Ceratium. ‘Their nutrient requirements are lower than
those of Diatoms and their rate of growth slower, and they can
continue to propagate in impoverished waters, besides which they
have the power of locomotion and consequently of adjustment to
the best available conditions. Actually, their daily increase under
summer conditions is only 30 to 50 per cent., as compared with
about ten times as much for some Diatoms. Somewhat similar
sequences may be observed in temperate regions. ‘Thus Fig. 171, A,
shows an early summer maximum of Diatoms off the Atlantic coast
of North America, and Fig. 171, B, shows the later peak of Peridinians
with associated animalcules.
16] VEGETATIONAL TYPES OF SEAS 521
It should be noted that whereas a considerable number of marine
Bacteria may be found in the pelagic zone associated with the
plankton, they are apparently not truly planktonic but attached to
other organisms. Some investigators, however, have claimed that
Bacteria are present in relatively small numbers in the body of free
water, for example at depths of around 5,000 metres, and to that
extent must be considered planktonic. But in any case in the
ocean, the main concentration of Bacteria occurs in the uppermost
few millimetres of bottom-deposit (see below, p. 539).
Of macroscopic marine phytoplanktonic phenomena the most
remarkable is the drift of certain higher Brown Algae belonging to
the genus Sargassum. ‘This familiar genus is predominantly
tropical, fairly large, and well differentiated (Fig. 8, H). Although its
members occur normally as attached littoral plants, they sometimes
become detached and, with the aid of gas-filled bladders, may float
far out to sea with the currents. Most benthic Algae even with
floats will die in time and disintegrate under this kind of treatment,
but two of the species of Sargassum grow exclusively (so far as is
known) in this free-floating manner, though apparently they are
unable to reproduce except by vegetative fragmentation. ‘The
species which thrive in this freely-floating state, Sargassum natans
and JS. fluitans, exhibit marked elongation of the branches as com-
pared with the normally attached species. With some other seaweeds
and animals living attached to or among their branches, they tend
to aggregate in one relatively quiet part of the western Atlantic
which is accordingly known as the Sargasso Sea, whose surface is
largely covered by ‘ floating meadows’ of these so-called ‘ Sargasso-
weeds’ or ‘ Gulf-weeds’. In this manner they constitute the only
such extensive “ drift ’ that is known or likely to occur in the world,
fragments of other drifting Algae, such as detached Fucus, Laminaria,
or Macrocystis, even though sometimes forming extensive aggregates,
being rarely if ever found in a healthy state very far from their
shore or ‘ shallow’ of origin.
BENTHIC ENVIRONMENTS AND LIFE-FORMS
The general features of the marine environment have already been
described in the first part of this chapter, but we must here add
some which are of particular importance to the attached plants
(benthos). ‘These factors are best treated in four groups, namely,
physical, chemical, biological, and dynamic.
522 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
1. Among physical factors we have dealt in some detail with
temperature and illumination, though it should be emphasized that
both these can vary greatly between tide-marks and in pools and over
shallow bottoms. ‘Temperatures may even be elevated sufficiently
to cause death of stenothermic Algae, at least when these are exposed
for long periods by low tides, while Professor V. J. Chapman has
informed the author that he has witnessed the killing of Chondrus
at low spring tides by frost. Moreover, seasonal variations can
induce ‘ migration’ of Algae from one level to another. ‘Thus in
warm regions some of the Algae which in winter occur at high levels
are found only at much lower ones in summer, while in the North,
owing to the unfavourably low winter temperatures of surface
waters, the upward extension of sensitive species occurs instead in
summer. Such ‘ migration’ normally takes place through young
disseminules becoming established at the desired level (cf. p. 524).
In unusually favourable circumstances, light may be sufficient to allow
Algae to live at depths as great as 200 metres, a depth of 180 metres
being reached by quite large numbers off the island of Minorca in
the Mediterranean Sea.
Whereas on land the chemical nature of the substratum is com-
monly important to the rooted plants, in the sea it is its physical
nature which usually matters most. For nutrients are furnished
through the sea-water in which the plants are immersed, the sub-
stratum serving in most instances merely as a place of attachment.
Consequently the degree of hardness, or of smoothness or, alterna-
tively, irregularity of the surface, plays the most important role, each
taxon being apt to evince some (often exclusive) preference for solid
rock, smoothed boulders, gravels, sand, or mud.
Hydraulic pressure, which increases regularly with depth, appears
to have little effect upon benthic Algae except in limiting the exten-
sion into deep waters of types with gas-filled bladders. ‘Thus in
the widespread Brown Alga Ascophyllum nodosum, although the
increased pressure of unusually high tides can cause escape of gas, the
thickness of the bladder-wall is a function of the depth at which
these bladders develop, and an individual transported to a much
lower depth loses the gas in its bladders and dies.
2. As regards chemical factors, we have dealt with the importance
of variations in salinity and noted how a marked lowering thereof,
as for example at the mouths of rivers or in such ‘ continental ’
seas as the Baltic, is commonly accompanied by marked changes
in the algal flora and vegetation. ‘Thus the numbers of species of
16] VEGETATIONAL TYPES OF SEAS 523
benthic Green, Brown, and Red Algae are greatly reduced by the
disappearance of stenohaline types, as is the general luxuriance of
growth ; moreover some of the persisting Brown Algae, particularly,
may take on characteristic dwarf forms. On the other hand the
remarkably euryhaline Green Alga Enteromorpha intestinalis, which
is widely familiar in fresh and brackish waters as well as in normal
sea-water and brine, is reported to undergo optimum development in
diluted sea-water, though it has been suggested that this may be
only where there is an influx of nutrient materials such as those
contained in sewage. Also liable to be subjected to marked varia-
tions in salinity—as well as in temperature, degree of desiccation,
and so forth—are the littoral Algae that are exposed to the air for
varying periods between tide-marks or that grow in small tidal
pools. As might be expected in view of the upward or downward
salinity-trends which these pools have to undergo as a result of
evaporation, or of rains and the influx of freshwater streams, respec-
tively, these littoral Algae have been demonstrated to display greater
tolerance to variations in osmotic pressure than do Algae growing
below low-tide mark. In addition to the Enteromorpha, Fucus
ceranoides and species of Ulva are known to be widely euryhaline.
In reaction, sea-water is somewhat alkaline, owing to most of its
contained carbon dioxide being combined in the form of carbonates
and bicarbonates and to the removal, by plants during photosynthe-
sis, of the (acidic) carbon dioxide formed on dissociation of these
carbonates and bicarbonates. Especially in tidal pools is the alka-
linity apt to be marked, a pH of as much as ro being sometimes
reached after several hours’ exposure by the tide. Although most
Algae will tolerate a pH of up to at least 9, pH 10 appears to be
too high a degree of alkalinity for many ‘ stenoionic’ species (7.e.
those which are narrow in their tolerance of changes in the relative
abundance of free acidic or basic ions), and it has been contended
that this is why such Green Algae as Ulva, having high photo-
synthetic rates, are prone to oust the frailer types, such as many
Red Algae. However, it should be remembered that evaporation
raises the salinity as well as the alkalinity, and so it seems possible
that the reason may be rather the failure of the frailer types to
respond to increases in osmotic pressure of the medium.
It has also been suggested that the low concentrations at which
saturation of oxygen is attained in warm waters may have some
correlation with the less profuse development of Algae in many
tropical as compared with cold waters. However, according to
524 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
Professor V. J. Chapman (zm Uitt.), ‘The evidence suggests that it is a
matter of the relative ratio between photosynthesis and respiration.
In warm waters these are nearly equal and hence there is never a
great development. At lower temperatures the respiration rate
drops more rapidly than photosynthesis, so that there is a greater
excess of photosynthesis over respiration and hence greater growth.’
It is to be expected that the nitrate and phosphate content, which are
known often to control phytoplanktonic development, have some
influence also on benthic Algae ; certainly many of these, such as
Prasiola on land, are greatly favoured, from the high-arctic regions
southwards, by manuring, etc.
3. Among biological factors the successional tendencies are notable,
as exhibited in the repopulation of denuded rock surfaces. Here a
rapid development of Enteromorpha may precede the attachment of
eggs of Bladder Wrack (Fucus vesiculosus) and, apparently, facilitate
the development of young plants of the latter, which later may oust
the Enteromorpha. Again, the building up of silty banks is often
aided by algal growth fostering deposition.
Also important may be the relationships between epiphytic Algae
in the sea and the ‘ hosts’ on which they grow. ‘Thus the epiphyte
is often protected by the host against rough seas or excessive illumin-
ation; or the host may benefit from protection provided by the
epiphyte ; or the load of epiphytes may be too great and cause the
host to be torn away from its point of attachment. In many cases
of widely different affinity the epiphyte is reduced to a disk-like
form completely adnate to the host, while some epiphytes actually
penetrate the host tissues, such penetration usually being accom-
panied by some degree of parasitism. Much the same relationship
exists between many Algae and marine animals, apparently often
resulting in symbiosis. Again, browsing by marine animals such
as Molluscs—and, in some parts of the world, harvesting by Man—
may have a noticeable effect on the algal or other marine vegetation,
and may even cause the disappearance of particular species from
some localities. Finally, diseases can have much the same effect—
as in the case of the ‘ wasting disease’ of Eel-grass (Zostera) off
many northern shores.
The migration of Algae into deeper or shallower water at different
seasons is chiefly manifested by quick-growing species having more
or less continuous reproduction, following which the sporelings only
survive and grow in the most favourable zone for the particular
time of year.
16] VEGETATIONAL TYPES OF SEAS 525
4. What may be termed dynamic factors tend to be most effective
about shores or in shallow waters and consequently to affect the
marine benthos quite markedly. Examples are turbulence, due to
waves and currents, and emersion, due to variations in water-level
particularly as induced by tides. The importance of wave action
has already been implied, and is illustrated by the often very different
algal flora and vegetation developed on exposed capes and in sheltered
bays. ‘The effects are complex but appear to be primarily mechan-
ical, in that the fixation of spores or persistence of fragile Algae is
Fic. 172.—Postelsia palmaeformis. Note the strong but flexible axes which are
resistant to surf. (* about +.) (Courtesy of the National Museum of Canada.)
prevented on heavily battered rocks, whereas an absence of turbulence
in sheltered situations leads to the deposition of sediment which
constitutes an obstacle to the establishment of some Algae. Others,
however, are favoured by this muddy substratum ; and calm con-
ditions may allow a heating of the water which favours many Algae
but leads to the disappearance of some stenothermic species. Some
types, such as the Bladder Wrack, when growing in exposed situations
may show characteristic features enabling them to resist the tearing
action of the waves, while others, such as the surf-loving Postelsia
palmaeformis (Fig. 172) of the Pacific coast of North America,
s
526 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
are favoured by exposure, being at best stunted in sheltered
situations.
Particularly marked is the zonation of Algae living in the intertidal
belt, on shores that are alternately left uncovered by the ebbing tide
and reflooded by the flow. ‘Thus the Algae are regularly emersed
and submersed, their conditions of life being subject to drastic
fluctuations. Owing to variation in the amplitude of the tides them-
selves, there are found, above the low-water mark of the lowest
spring tides, practically all temporal degrees of emersion ; these
take a leading part in determining the localization, at levels where
each finds favourable conditions, of different groups of shore Algae.
Whereas the Algae of permanently submerged zones below low
spring tide-mark are liable to die after short exposure to the air or
to diluted sea-water, those living between tide-marks are more re-
sistant, being often able to withstand emergence for many days on
end, while some (especially among Blue-green Algae) can lose so much
of their water as to become brittle without injury. Again, certain
species die if they are kept constantly submerged ; an example is
Pelvetia canaliculata, which passes most of its life in the air. Yet
others are protected by growing among dense mats of larger Algae
(such as species of Fucus). ‘These Algae exposed to emersion, how-
ever, in general have to withstand considerable degrees of desiccation,
and, in addition, marked changes in salinity and temperature. It
is apparently a combination of these varying factors that produce,
on sea-shores, the characteristic zonations in more or less regular
horizontal bands of different Algae at particular levels which, how-
ever, may vary according to the locality, degree of shelter, and so
on. As pointed out by Mr. F. 'T. Walker (im litt.), quantitative
variation may be not only seasonal and with depth but also cyclic,
over periods of years, while at least for Laminariaceae in Scotland
‘ decrease in seaweed density has been found to be more the result
of a reduction in the number of plants per unit area than of the weight
of individual plants ’.
Various delimitations and systems of nomenclature have been pro-
posed for the main recognizable zones or belts inhabited by benthic
marine Algae. ‘These may be usefully designated, in line with our
earlier subdivision of the ocean’s margin (Fig. 170), as follows :
(1) Supralittoral zone, lying between the upper limit of marine
vegetation and the high-water mark of ordinary spring tides, the
plants being bathed in sea-water only during storms or unusually
high or equinox tides, and in Europe characteristically including
16] VEGETATIONAL TYPES OF SEAS 527
the Lichen Verrucaria maura which may form a continuous black
coating on the rocks. (2) Eulittoral zone, often called simply ‘ the
littoral ’, corresponding to the part of the shore undergoing more or
less regular emersion and submersion by tides or surf. This zone
varies in width from shores with no noticeable tides, where it includes
only the band regularly reached by surf, to those with wide tidal
amplitude, where various ecological conditions develop at different
levels and it is necessary to subdivide it into ‘ horizons ’ or ‘ girdles ’
(see below). (3) Upper sublittoral zone, varying with the location
but characteristically extending approximately 20 metres downwards
from the low-water mark of ordinary spring tides to the lower limit of
abundant major benthic vegetation—z.e. to where the light-intensity
is markedly reduced and where there is little disturbance of the water
or rapid variation in the temperature. In temperate regions almost
all the Laminariaceae live in this zone; indeed below it most of
the light-demanding species thin out and it is chiefly those char-
acteristic of deeper waters which persist. (4) Lower sublittoral zone,
extending from the lower limit of the upper sublittoral zone down
to the lower limit of at all plentiful benthic vegetation (usually 40-60
metres in total depth), and characterized by relatively constant
temperature and other conditions, and weak illumination. (5)
Infralittoral zone, extending from the lower limit of at all plentiful
benthic vegetation right down to the lower limit of photosynthesis.
It may be noted that different belts or girdles of vegetation are
commonly discernible within the above main zones, being often
designated and named separately, and that it is sometimes useful to
recognize, above all the rest, an adlittoral zone of constant emersion
characterized by halophytes living a normal aerial life but able to
endure exceptional waves and sprays during storms. ‘The salt is
concentrated by evaporation and may damage or deform ordinary
land vegetation. ‘The adlittoral zone, and zones 3 and 4, are not
distinguished in the accompanying diagram (Fig. 170).
Gymnosperms, Pteridophytes, and Bryophytes do not live in the
sea, and although some Angiosperms occur there, and a greater
number of Fungi and Schizophytes, the vast bulk of marine plant-
life is made up of Algae of extremely numerous and diverse types.
Much as the Raunkiaer system of life-forms that was outlined with
modifications in Chapter III may be useful to give some idea of the
physiognomic composition of a terrestrial flora when actual floristic
knowledge is lacking, so may a more recently proposed system of
biological types of Algae be of service where marine flora is concerned.
528 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
In this system the first three of the following (main) categories are
annual and the remainder are perennial: ephemerophyceae (e.g.
Enteromorpha), being found throughout the year but often forming
several generations ; eclipsiophyceae (e.g. Nereia), being well developed
during only one part of the year and passing the remainder as a
microscopic vegetative form; hAypnophyceae (e.g. Dudresnaya),
differing from these last in that they pass the unfavourable season
in a resting stage; phanerophyceae (e.g. Fucus vesiculosus), having
the ‘frond’ perennial and erect ; chamaephyceae (e.g. Lithophyllum),
having the ‘frond’ reduced to a crust; hemiphanerophyceae (e.g.
Sargassum), having only a part of the erect ‘frond’ persisting for
several years ; and hemicryptophyceae (e.g. Acetabularia), having only
the basal creeping portion of the ‘ frond’ persisting.
It is also possible, and sometimes useful, to categorize marine
Algae according to their particular ecological needs (such as types
living in pounding surf) or gross morphological characteristics (such
as encrusting types) or the nature of their substratum (such as
epiliths attached to rocks, or pelophiles growing on mud). ‘They may
furthermore be divided according to their temperature requirements
into eurytherms and stenotherms, the latter being composed of
micro-, meso-, and mega-thermic species characteristic of low,
medium, and high temperatures, respectively, while much the same
can be done in connection with the factors of illumination and
salinity. Finally, not only do Algae show marked periodicity,
especially in regions where seasonal changes are pronounced, but
the time and duration of development of a species may be differ-
ent in different parts of the world, while some types which in boreal
regions persist throughout the year develop only in winter and spring
farther south.
It should be noted that the plants composing the marine benthos
are very largely lithophytes, attached to rocks or boulders. ‘The
more massive forms are fixed to the substratum by strong adhesive
disks (in Fucaceae) or crampons (in Laminariaceae), small forms
usually having simpler devices. "The number of species flourishing
on sandy or muddy bottoms is often very limited, such substrata,
at least in agitated water, commonly representing virtual desert so
far as benthic Algae are concerned, though in calm and shallow bays
they may be occupied by Eel-grasses (Zostera spp.), or in warm regions
sometimes by other rooted Angiosperms or attached Green Algae.
The few Algae that flourish in such situations are usually provided
with root-like organs of attachment which penetrate the substratum.
16] VEGETATIONAL TYPES OF SEAS 529
BENTHOS
Although only about 2 per cent. of the sea area of the world is
shallow enough to be occupied by benthic plants, the vegetation-
types of the eulittoral and sublittoral are much the most striking and
studied of all to which the term marine can be properly applied.
Numerous books and papers describe these zones in more or less
detail for different parts of the world, soa brief outline of the vegetation
will suffice for each of the four main regions, viz., tropical, warm-
temperate, cool-temperate, and polar.
1. [he marine vegetation of tropical seas is apt to be less luxuriant
than that of cooler regions, though some groups of Monocotyledons
and Green Algae are largely restricted to warm waters, where reef-
Algae may show periodic correlation with the monsoons. Apart
from such manifestations as the Sargasso Sea which are, rather,
planktonic, and were accordingly treated above as such, Brown Algae
tend to be relatively poorly represented in tropical seas whereas
Red and Green Algae are abundant. Although Red Algae are par-
ticularly characteristic of the sublittoral zone, certain species of this
group often form a thick coating of a dirty violet colour on the roots
and stem-bases of the mangrove-swamp dominants which form such
a characteristic feature of the eulittoral vegetation of tropical shores ;
but mangroves, being mainly aerial, were dealt with in Chapter XIV.
Small Green Algae also with low light requirements may form a
characteristic investment on the soft mud, which is rich in organic
material, between the roots of the dominant mangrove trees. In
a few instances Brown Algae have been reported to form fairly well-
developed littoral communities in the tropics.
Unlike the situation in colder seas where the unstabilized bottoms are
often devoid of macroscopic vegetation, the eulittoral and sublittoral
in some tropical regions may have sand or gravel beds populated by
numerous Green Algae. These may even be found in deep water,
many being strongly calcified and in addition accumulating much
sediment. Also forming extensive communities in some places are
marine Monocotyledons such as species of Thalassia and Cymodocea
which may dominate characteristic consociations or associations
down to a depth of some 30 metres. In many parts of the Pacific
Ocean the so-called ‘ coral-reefs ’ are largely built up by calcareous
Algae—in particular by Porolithon onkodes. Such virtually tide-
less seas as the Caribbean naturally have practically no inter-
tidal vegetation or girdling of Algae, the communities near the
530 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
surface being determined in each place very largely by the type of
substratum.
2. In warm-temperate regions the marine benthic vegetation is
very various in different areas such as the Mediterranean and the
seas of southernmost Africa and Australia, the differences being sup-
posedly due largely to historical causes. ‘The absence or paucity
of Fucus and Laminaria may give the vegetation a very different
aspect from that of most northern seas. In parts of the Mediter-
ranean where the high temperatures of the air and absence of appreci-
able tides result in reduction of the littoral vegetation, Eel-grasses
(Zostera spp.) may grow densely in the upper sublittoral where the
bottom is muddy. ‘They are accompanied by epiphytic and other
Algae, while near-by sandy bottoms may be largely covered with
‘meadows’ of Posidonia oceanica, also a Monocotyledon, as deep
down as 60 metres. At 80-100 metres only isolated plants of the
Posidonia occur, though algal vegetation may still be luxuriant at
a depth of 120-130 metres, and, in exceptionally clear water, fair
numbers of Algae may persist to at least 180 metres, or, in some
cases, tO 200 metres.
In rocky places in the Mediterranean the supralittoral zone is
characterized by a dark girdle of the Lichen Verrucaria maura, or of
various Cyanophyceae where the substratum is calcareous, while
eulittoral rocks may support a fine girdle of Rissoella verruculosa
which, though a Red Alga, somewhat resembles the brown Wracks
of more northerly shores. The lithophytic vegetation of the sub-
littoral zones is rather various and rich in different forms, showing
distinctions occasioned by differences in illumination—chiefly at
different depths but also to some extent according to shadows cast
by irregularities of the coast. ‘Thus the shade-species predominating
in the depths are chiefly Red Algae, such as species of Lithothamnium,
while the Brown Algae, such as species of Cystoseira, prefer brighter
areas, and some of the Green Algae, such as Acetabularia acetabula
(A. mediterranea), favour the very brightest spots. ‘The Red Algae
of well-lit places are usually dull in colour; at the other extreme
are species so sensitive to light that they are restricted to shaded
areas even at considerable depths. In any case the active vegetative
season in the Mediterranean largely coincides, near the surface, with
the winter and spring, whereas in deeper water the chief activity
occurs in the summer and autumn. During early summer, Brown
Algae may even prevail over Red Algae in deep water, whereas at
16] VEC ATTONAL TYPES Ob SEAS 531
other seasons Red Algae predominate in poorly-lighted situations.
Some species even exist in different winter- and summer-forms.
Moreover, exposed habitats that support luxuriant vegetation during
winter may be almost barren in summer.
In South Africa the girdles distinguishable in the culittoral are,
uppermost, of Porphyra capensis which in places extends from the
upper limit to mid-tide level. Below this is often a ‘ bare zone’
that may be largely devoid of macroscopic vegetation, succeeded by
a girdle formed of an association of two species of Chaetangium and,
farther down, by two or three other characteristic girdles. The
sublittoral is often dominated by various Laminariales.
3. The algal vegetation of cool-temperate seas is tolerably well-
known and often markedly diverse even in such closely adjacent
bodies of water as the North Sea and the Baltic, the former of which
is far more productive than the latter. ‘The variation seems to be
in accordance with differences in the tides and salinity, which in
the North Sea are both very considerable, whereas in the Baltic
both are weak, so that its Algae are poor in species and development.
Yet in both instances, as in most cool-temperate seas, Brown Algae
predominate. ‘They are chiefly represented by species of Fucus and
Laminaria or their allies, though many smaller types also abound.
Red Algae are also represented by numerous types, but Green Algae
offer less variety, and the only at all widespread marine Angiosperms
are Eel-grasses. The eulittoral is much wider in the North Sea
than in the Baltic, and in some places bears more abundant vegetation
than does the sublittoral, though this is not the case in Scotland
(according to Mr. F. T. Walker im Jitt.). Actually, in the Baltic
Sea most of the marginal vegetation is commonly in the sublittoral,
as the tides are weak and ice frequently grinds against the upper
shores which are thereby rendered inhospitable for macroscopic
plants.
In most exposed situations away from salt-marshes in cool-
temperate seas, macroscopic vegetation in the eulittoral is virtually
limited to rocky and bouldery substrata, any shingly, sandy, or
muddy beaches being largely barren. ‘This is not, however, neces-
sarily the case low down, or, particularly, in some estuaries and
sheltered creeks, where plentiful between-tides vegetation may de-
velop even on ‘soft’ bottoms. Unlike the situation in warm-
temperate regions, in cool-temperate seas the algal vegetation of winter
tends to be poorer than that of summer, which for most species is
532 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
the main period of vegetative activity in the boreal and austral regions
(though of course involving antithetic times of the year in the nor-
thern and southern hemispheres). Many perennial types, however,
reserve their main reproductive activity for winter, a few, such as
species of Fucus, being in this respect independent of the season,
while again relatively few, such as species of the Rhodophycean genus
Polysiphonia, reproduce mainly in summer. Particularly striking
differences between winter and summer states are exhibited by those
types which shed their assimilating ‘ fronds’ at the beginning of the
cold season, the frondless residue being often covered with repro-
ductive organs. Others, such as many Laminariales, have an inter-
calary zone of growth producing a new ‘frond’, the old one being
cast off as a whole in the spring: this commonly happens at about
the same time to all the representatives of a species in a particular
region.
‘The most characteristic sequence on open shores and, for example,
in the English Channel, shows a supralittoral zone formed for the
most part by Lichens, with girdles of yellow or orange Xanthoria
parietina and Caloplaca marina above, and of blackish Verrucaria
maura below. ‘The eulittoral is characterized by a series of girdles
formed largely by individual Brown Algae—for example, successively
from top to bottom, by Pelvetia canaliculata, Fucus spiralis, F.
vesiculosus, and F.. serratus. Lower down, uncovered at most only
by the lowest spring tides and forming the upper sublittoral, are
girdles of large Kelps (Laminariales). Commonly Laminaria digitata
is dominant near low-water mark and L. cloustoni farther down, or
L. saccharina may replace the latter where the substratum is more or
less sandy. ‘lo these normal girdles which have analogies elsewhere,
as for example on the Atlantic coast of North America, various
facultative ones may be added; nor is there any exact correlation
with tidal level, for, especially on strongly exposed rocks, wave
action induces a displacement upwards of the upper girdles to a
height roughly corresponding with that attained by the waves.
In some circumstances, various Green Algae can largely take the
place of the characteristic tidal girdles of Brown Algae. Ecological
factors can also favour the presence of quite a range of other algal
communities that are not restricted to such narrow bands and
definite levels along the coasts : such are, particularly, the communi-
ties of tidal pools and grottoes, and of local run-off areas. Even
sandy and muddy shores may be well populated in the upper
sublittoral by Eel-grasses accompanied by characteristic epiphytic
16] VEGETATIONAL TYPES OF SEAS 533
Algae. In the lower sublittoral belt, down to some 30 or more metres,
gravelly or coarse sandy bottoms are characterized by an abundance
of branched and unattached Lithothamnium calcareum. Pigi173
depicts a scene near the base of the girdles of Fuci on a rocky part
of the Atlantic coast of temperate North America, showing some
tufted Red Algae and large Kelps (Laminariales) at the lower levels,
and Fig. 174 1s a drawing of a typical Kelp.
FIG. 173.—Scene at low tide on a rocky sea-shore of the eastern United States.
Above are seen luxuriant Wracks (Rockweeds, species of Fucus), and below are
a tufted Red Alga and some large Kelps (Laminariales). (Courtesy of Chicago
Museum of Natural History.)
On muddy tidal flats or shallow bottoms in sheltered situations,
Blue-green Algae often form a delicate investment which binds the
surface, while some forms of Fucus can live merely resting on mud
without being attached. Often they are partly embedded in the
surface and, with some other forms including various Green Algae
and halophytic terrestrial Angiosperms, favour estuaries where salinity
and other conditions vary markedly, such vegetation-types merging
into those of salt-marshes (see Chapter XII). The vegetation of
534 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
inland saline waters ranging from saturated brine to slightly brackish
lagoons, etc., was mentioned in Chapter XV.
The Pacific Ocean supports a widely different benthic algal
vegetation from the Atlantic, so that the Pacific coast of North
America is very different in this respect from the Atlantic coast.
For although in the Pacific the eulittoral may be dominated by
species of Fucus (as well as of Egregia), the lowest portion of this
zone is characterized by the unique laminarian Postelsia palmaeformis
Yl wall ze
wt care i)
wD
Fic. 174.—A characteristic Kelp, Alaria dolichorhachis ( 4). (After Kjellman.)
(Fig. 172) on rocks exposed to heavy surf, while the sublittoral
contains numerous other Lamuinariales, often of peculiar form and
great size. Examples of these are Macrocystis pyrifera (Fig. 175)
and Nereocystis, which are commonly attached on rather deep bottoms
10-30 metres down, and may thus be considered as extending the
upper sublittoral downwards. However, they possess such a very
long ‘ stalk’ that the fronds, borne at the summit and often accom-
panied by air-bladders, can spread out on the surface of the water
even at high tide. Such plants thus project their upper parts into
the ecological sphere which is most favourable especially in the all-
important matter of illumination.
In southern South America and the less frigid of the austral islands,
there may be recognized in the eulittoral an uppermost girdle of
various drought-resistant Algae, a middle-to-lower one of surf-re-
16] VEGETATIONAL TYPES OF SEAS 535
sistant Red Algae or, in less exposed situations, of various Green
and Brown Algae, and a lower surf-girdle of large Brown Algae
such as Durvillea antarctica. Large Laminariales such as Mac-
rocystis pyrifera, and crustose and other Red Algae, together
characterize the sublittoral. On the southernmost Australian and
New Zealand coasts the benthic vegetation corresponds for the most
part to that of southern South America, though Eel-grasses may
abound where the bottoms are sandy or muddy. Thus the same
Fic. 175.—A giant Pacific Kelp, Macrocystis pyrifera. (x about zso.) (After
Skottsberg.)
Durvillea is a characteristic component of the flora, and the same
Macrocystis reaches a reported 60 metres in length (according to
recent accounts ; old reports that it attains much greater lengths
have not been substantiated). It is interesting to note that a Lichen
determined and widely cited as Verrucaria maura, which charac-
terizes the supralittoral belt of so many shores in the northern
hemisphere, plays a similar role in South America and New Zealand.
4. In arctic seas the algal flora is rather limited but the vegetation
is sometimes luxuriant. Indeed, in the Arctic and Antarctic it is
chiefly in the sea that life abounds, the plant and animal communities
on land being of relatively poor development. Particularly robust
and abundant are some of the gregarious Brown Algae, such as species
of Laminaria and Fucus, although many Red and some Green Algae
usually occur in addition. ‘The vegetation is largely limited to rocky
and bouldery substrata, sandy and muddy ones being usually
devoid of major growths except in a few situations towards the south-
ern extremity of the Arctic where the common Eel-grass (Zostera
marina) may form fair ‘ beds ’.
536 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
Even on the most rugged rocky shores in the Arctic and much of
the Subarctic, the eulittoral zone is poorly vegetated in exposed
situations owing to the rigours of the climate and particularly to
the tearing and grinding of ice, which tides and breakers keep in
motion when once it has broken up in early summer. ‘Thus the
eulittoral and immediately adjacent sublittoral tend to be ‘ polished ’
by ice-floes and -pups or splinters of ice which waves and currents
carry against exposed shores through much of the growing-season.
BSscassuces. Saino
Fic. 176.—An arctic foreshore photographed from near low-tide mark, showing
eulittoral boulders with sides covered by Brown Algae (chiefly Bladder Wrack,
Fucus vesiculosus), whereas their exposed upper surfaces are kept bare by ice-
action. Pangnirtung, Cumberland Sound, Baffin Island.
On the other hand in sheltered situations, including bays and the
interior landward shores of nearby islands, and even the sides of
boulders (Fig. 176), where there is protection from the ice, species
of Fucus may form a luxuriant investment—except in the very Far
North where their growth tends to be poor.
It is, however, in the middle and lower sublittoral, out of reach
of floe ice and chiefly at depths of 3-25 metres, that benthic plant life
really flourishes in the Arctic—provided the bottom is hard. Here
giant Laminariales may form extensive ‘ beds ’, species of Alaria and
16] VEGETATIONAL TYPES OF SEAS 537
Agarum as well as of Laminaria abounding, associated with Red
Algae such as species of Lithothamnium and Lithophyllum, and often
beset with epiphytes. Even on desolate Akpatok Island in Ungava
Bay the present writer has measured specimens of Laminaria longi-
cruris up to 47 feet (14:3 metres) in length, and doubtless longer
ones occur in the great off-shore beds. Such luxuriant algal vege-
tation which survives the arctic ice may almost be compared with
the Pacific Macrocystis that ranges nearly to the tropics. Although
the lower limit of the photic region is often placed at only about
36-40 metres in the Arctic, even off far northern Spitsbergen Deles-
seria sinuosa has been dredged ‘ quite fresh’ from as deep down as
155 metres and Ptilota pectinata has been brought up from a re-
ported 274 metres. Presumably these were detached specimens ;
but it may be noted that vegetation of Phycodrys and Pantoneura has
been reported from depths down to 118 metres off the indubitably
arctic Jan Mayen Island.
The periodicity of arctic Algae appears to be much like that of
cool-temperate ones, and indeed many species are common to both
regions. However, in conformity with the virtual absence of annuals
among the higher plants on most areas of arctic land, none at least
of the more massive types of Algae is supposed to be able to complete
its cycle of development in less than a year in the Arctic. Also in
conformity with the situation in cool-temperate seas, it seems that
vegetative activity prevails in summer and reproductive activity in
winter for types growing well below low-tide mark, even though the
temperature is commonly from —1° to —2° C. during the latter
season when darkness largely prevails. Indeed it has been claimed
that the richest local algal vegetation may occur at depths where the
temperature does not rise above o° C. at any time of the year.
Owing in part to differences in such conditions as those of tem-
perature and salinity, and in part to factors not yet understood,
different arctic seas in spite of their connection with one another are
apt to possess distinct algal floras, the dominant Laminariales, for
example, being often different in Spitsbergen, Siberia, and arctic
America. Beyond this, the benthic flora and vegetation tend to be
far poorer in waters of low salinity, such as occur near the heads of
fiords into which large streams flow, in Hudson Bay, and off the
coast of parts of Siberia near the mouths of great rivers, than they
are in waters of more normal marine salinity.
Inthe Antarctic, again, the vegetation of the eulittoral zone is often
very poorly developed because of exposure and ice, which tears plants
538 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
from the rocks or otherwise prevents them from developing properly.
Consequently the types occurring are mostly small, though repre-
sentatives of Green and Red as well as Brown Algae commonly occur
—including, particularly, crustose Corallinaceae. Only these and
other calcareous Algae seem to be unaffected by the almost constant
surf and rubbing ice-floes of open coasts, though in sheltered pools,
lagoons, and coves the vegetation may be quite luxuriant even in the
eulittoral. ‘The sublittoral is again characterized by gregarious
Brown Algae of substantial size, commonly including species of
Desmarestia and Cystosphaera, down to a depth of some 30-50 metres,
with usually an assortment of associated Red Algae. ‘The larger
species among the Brown Algae include Lessonia simulans up to
54 metres long, and the general richness is said to be not inferior to
that of the arctic sublittoral. Although doubtless some Red Algae,
particularly, grow at greater depths, the examples dredged from
hundreds of metres down were evidently not growing there, drifting
Algae being commonly encountered in the Antarctic as elsewhere.
As no large rivers exist in the Antarctic, the discharge being
chiefly in the form of icebergs which float far before melting appreci-
ably, there is little freshening of the water near the coasts in the
manner which apparently impoverishes the algal flora of many
boreal seas. On the other hand in both the Arctic and Antarctic,
wherever the inland-ice or glaciers extend down and calve into the
sea, no eulittoral vegetation can exist and even sublittoral Algae are
liable to be injured. ‘Thus vegetation here, and on shores invested
with shelf-ice, is chiefly found about stretches of beach that are free
from ice in summer. Even on such beaches there is a freezing more
or less ‘* solid’ in winter—which does not, however, preclude the
existence of large Algae.
APHOTIC BOTTOMS
The average depth of the oceans being computed at nearly 4,000
metres, most areas of sea-floor are deep and dark, being reached,
if by any daylight at all, only by an insufhcient amount of it to allow
the normal growth of benthic Algae. Phosphorescence is entirely
inadequate. Such Algae as have been dredged up on occasion from
the aphotic zone have drifted there and, being unable to photo-
synthesize, will not long persist. Nevertheless, these ocean depths
have their flora of saprophytes, parasites, and chemosynthetic
organisms that form vegetation of a sort. ‘This deep-sea-floor
16] VEGETATIONAL TYPES OF SEAS 539
vegetation is mainly composed of Bacteria, which occur in great
abundance in the upper layers of bottom sediments and, although of
course microscopic and scarcely evident in their physical effect, are
believed to play significant roles in determining the character of the
deposits. ‘Thus they form humus and precipitate compounds of
calcium, iron, and manganese. ‘They are mostly motile rods and
comma-shaped forms, and are much more often coloured but less
frequently spore-forming than is the case with terrestrial types.
The greatest numbers of marine Bacteria have been found in
coastal waters where life is most prolific. In the upper waters, and
especially within the uppermost 50 metres, there are often a consider-
able number attached to floating organisms and other particulate
matter; but the main concentration is on the bottom—especially
just below the mud-water interface. Here as many as an estimated
420,000,000 cells per gram of wet mud have been observed. More
or less teeming populations of this nature appear to be practically
world-wide, marine Bacteria being capable of development at un-
usually low temperatures, and ocean depths being relatively stable
and not too low in this respect. ‘Thus even in the Arctic Ocean,
Bacteria in great numbers may be responsible for the colours of
bottom deposits, including those at very considerable depths, and
it has been estimated that at latitude 82° 42’ N. in the North Polar
Basin there are from 34 to 7 tons of bacterial matter per cubic
kilometre of sea water, while farther south, in Barents Sea, the
uppermost 4 cm. of bottom mud are estimated to contain 20 gm.
of Bacteria per square metre.
The presence of numerous saprophytic Bacteria attached to the
planktonic organisms of the uppermost layers of the ocean, whether
or not such Bacteria be considered actually planktonic, results in
prompt decomposition of much of the dead material before it can
sink to great depths, it being claimed that a considerable proportion
of the mineral nutrients are returned to the water within or a little
below the euphotic zone. Nevertheless sufficient elaborated material
reaches even very deep ocean beds to support a teeming population
of, particularly, animals and Bacteria. ‘The Bacteria are chiefly found
in a thin layer of surface ooze in which is concentrated a large pro-
portion of the organic detritus—dead bodies and parts thereof—
which is constantly sinking and supplying food for them as well as
for competing animals. ‘These bottom Bacteria are in general more
numerous in fine than coarse deposits, and occur principally near
the surface of such deposits. ‘Thus although viable cells have been
540 INTRODUCTION TO PLANT GEOGRAPHY
recovered from a depth of more than 3-5 metres below the surface of
marine sediments, their numbers rapidly decrease below the first
few millimetres from the interface.
FURTHER CONSIDERATION
There is still no single work adequately covering the topics discussed
in the above chapter, although further useful details are to be found in
the appropriate parts (almost always obvious from their headings) of such
books as those of V. J. Chapman and of G. M. Smith (ed.) cited at the
end of Chapter XV. Particularly valuable for accounts of general con-
ditions and phenomena in seas is H. U. Sverdrup, M. W. Johnson, &
R. H. Fleming’s The Oceans : their Physics, Chemistry, and General
Biology (Prentice-Hall, New York, pp. x + 1087, 1942), while the ‘ third’
edition of Schimper’s Pflanzengeographie auf physiologischer Grundlage,
revised by F. C. von Faber (Fischer, Jena, vol. II, 1935), gives on pages
1447-98 accounts and illustrations of many of the communities involved.
The individual Algae concerned are usually treated in the volumes of
Fritsch cited at the end of Chapter II, and in F. Oltmanns’s Morphologie
und Biologie der Algen, second edition (Fischer, Jena, especially vol. III,
pp. vil + 558, 1923), while the marine vegetation of different oceans and
stations is described in various works too numerous to mention but often
selectively cited in one or more of the above books.
CHAPTER XVII
EANDS CAPES AND VEGETATION
Man’s handiwork—as, for example, in the maintenance of par-
ticular types of forest or in their clearance to form arable or grass
lands—often reveals the potentialities, or even directly demonstrates
the productive capacity, of particular areas. ‘Thus the Hazel coppice
with Pedunculate Oak ‘standards’ developed so commonly on
heavy soils in England is, when cleared, normally suitable for Wheat,
as is much of the mixed deciduous woodland of New England.
This is well known to local inhabitants who seek out such terrain.
On the other hand the acidic heathlands found in many areas on
both sides of the North Atlantic tend to be too poor for the growth
of other than the most meagre of crops, being generally best given over
to pasturage. ‘These two examples also illustrate two of the main
divisions of landscape—namely, those which are largely forested and
those which are not.
Outside of the polar, high-alpine, and desert regions, there are
few major land areas in the world that are not substantially covered
by plants, or where plants do not ‘ form the chief embellishment’.
Consequently the vegetation largely characterizes the landscape
locally, and, being made up of different plants having different
requirements and ranges, affords a valuable field of study for in-
terpreting landscapes and predicting the best uses to which various
areas may be put.
LANDSCAPES AND COMPONENT LANDFORMS
In spite of the general importance of vegetation almost everywhere
in the world, the stage for its display, so to speak, is set by the local
conformation of the surface of the earth which exhibits various so-
called ‘landforms’. ‘These last may be subdivided into constructional
and destructional categories.
Constructional landforms of the first order are the continents and
ocean basins, and, of the second order, plains (of horizontal structure
and low relief), plateaux (of horizontal structure but high relief),
541
542 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
mountains (of variously disturbed structure), and volcanoes (of
conical structure). ‘The cognate landscapes of these forms of the
second order are familiar to us all, at least from illustrations. ‘The
vegetation which tends to clothe them normally occupies a secondary,
dependent position in the hierarchy of nature. Moreover it is so
general in the case of continental plains and plateaux, and so bound
up with local features and conditions on mountains, etc., that it
would seem in the former instance to be pointless and in the latter
fruitless to attempt to describe it here. For plains and plateaux
generally have their rocks horizontally-bedded, with their deposits
or even igneous extrusions in flat layers, imposing an over-all
similarity of conditions and attendant vegetation ; contrastingly on
mountains the variability is so extreme as to defy brief description.
Far more numerous and liable to be dependent upon vegetational
development are the landforms of the third order—the so-called
destructional ones. ‘These are produced by the agents of erosion
working on the constructional forms, their characteristics being de-
termined in part by the erosional agent and in part by the con-
structional landform involved. Some of these features, such as river
valleys, are produced directly by erosion, the removal of material ;
others, such as river deltas, are the result of the importation
and deposition of sediment; still others, such as natural bridges,
are residual, being left after the surrounding material has been re-
moved. Each and every one of these landforms tends to have its
own characteristic appearance and to contribute to the various types
of landscapes, which indeed are primarily made up of mixed land-
forms of the second and third orders. Secondarily, they are for the
most part veneered with vegetation, which of course varies according
to local climatic and other conditions, but nevetheless is often widely
comparable and sometimes actually characteristic of a particular
landform even throughout regions of very different climate. ‘Thus
sand-dunes tend to be colonized by similar, coarse, binding Grasses
all the way from the Arctic to the tropics.
As landscapes are primarily made up of various and variously
aggregated landforms, each of which indicates much of what has gone
on before and also of what may logically be expected to follow—
whether naturally or as a result of enlightened disturbance by Man—
their study is of both academic interest and applicational value.
‘This interest stems from the manner in which the earth scientist can,
from appropriate investigation of the landforms, often tell us much
regarding the past history of an area, while the value of such studies
17] LANDSCAPES AND VEGETATION 543
is appreciated when, as is often the case, potentialities for future
development emerge. As an example of an easily recognized feature
we may consider an esker, the elongated ridge of gravelly material
that is often left after the disappearance of anice-sheet. ‘The presence
of a well-formed esker indicates not only that the area was glaciated
in the past but also that the glaciation was of the over-all, ‘ continen-
tal’ type and that the ice was locally stagnant for a considerable time.
Knowing that an esker is formed of assorted gravel or sand, we
can tell at a glance that it will afford an abundance of good building
or road-making material as well as good drainage, though there will
probably be plentiful water and fertile outwash plains nearby. An
ecologist could probably go farther, and tell us, for example, even if
the esker is covered with forest, whether it could be converted into
productive pasturage.
LANDFORMS AND PLANT LIFE
It will be appropriate at this point to consider in turn the various
destructional landforms that are most common or important and,
at the same time, their more characteristic forms of plant life. This
consideration may conveniently be given under the headings of the
chief agents of erosion, which are streams, glaciers, ground-water,
winds, and waves and currents. Each resultant structural group may
then be subdivided into (a) erosional, (6) depositional, and (c)
residual, landforms or minor features. As already suggested above,
the vegetational features of constructional landforms such as plains,
plateaux, and mountains tend to be too general (or, in the case of
mountains, comparably variable in different instances) to be distine-
tive and pertinent in this connection, being moreover already con-
sidered in our treatment of the main climatic belts of the world.
Erosional features made by streams include, besides peneplanes
which are too generalized for specific vegetational characterization,
various kinds of valleys and gorges. ‘hese, being more sheltered
and often much lower than the surrounding uplands, tend to support
more luxuriant vegetation. Indeed many plants, for one reason
or another, are restricted in a particular region to valley sides or
eroded bottoms, and much the same may be the case with whole
plant communities—including some of the most important to man-
kind. Thus, near the northern limit of arborescent growth, trees
are largely or entirely restricted to valleys—for example in northern
Alaska, Labrador, and Lapland—as they are also in many arid regions
544 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
where watercourses alone afford sufficient moisture for their growth.
Besides the more open and mature valleys, streams may cut narrower
gullies, ravines, gorges, and canyons, the steep and usually rocky sides
of which often afford habitats for plants that are not to be found
elsewhere in the vicinity. Consequently such plants may be useful
indicators of peculiar conditions—for example of rock crevices or
‘open’ soil and lack of competition, of a humid or shaded situation,
or Sometimes of a particular type of substratum such as is afforded by
calcareous rock. Quite frequently valleys with the long axis lying
east and west have very different conditions on their north- and
south-facing slopes, which may be more clearly indicated by differ-
ences of flora and vegetation than by ordinary meteorological observa-
tions. ‘These last are, moreover, tedious and often costly to make.
And though everybody knows that a south-facing bank in the boreal
regions tends to be sunnier and warmer than a north-facing one, it
may also be drier and so not necessarily preferable for all cultivational
purposes. Here again plants form useful indicators of the im-
mediately local conditions, Ferns for example being characteristic
of damp and shady banks whereas succulents occur on dry and
sunny ones.
Of depositional features made by streams there are also many—
such as alluvial fans and cones, flood-plain deposits, channel bars,
deltas, and natural levees. Each of these has its tendency towards
supporting a characteristic vegetational type which usually varies
markedly in regions of different climate, particularly—although
certain plants, such as some Poplars, are widely characteristic, occur-
ring in similar situations in a considerable range of climates. ‘These
stream depositional forms, being usually of fine material plentifully
supplied with plant nutrients, and commonly situated in sheltered
valleys, include some of the most vegetationally (and hence agri-
culturally) productive terrain. Being low-lying and often damp,
they may also produce fine pasturage.
The residual features left by stream erosion are, from the point
of view of our present study, relatively minor ; though the vegetation
on their area may so bind the surface as to delay their formation, it
is usually meagre. Examples are divides and monadnocks, and, in
part, those flat-topped erosion remnants known as mesas and buttes.
Even the residual forms of this category which are fairly extensive,
are liable to be rocky and exposed and consequently rather poorly
vegetated.
Erosional features made by glaciers somewhat resemble those made
17] LANDSCAPES AND VEGETATION 545
by streams, except that the valleys are often deeper and U-shaped
and their steep sides are usually scoured. The bottoms may be
scoured too, and retain so little soil that the vegetation suffers in
spite of the favourably sheltered situation. Depressions or deeper
troughs are often occupied by water—finger-lakes, paternoster-lakes,
and, especially, tarns being numerous in glaciated territory. These
bodies of water vary from oligotrophic and unproductive where the
surface material has been scoured away, to eutrophic and productive
where comminuted deposits were left and considerable sedimentation
has occurred. Here may be extensive reed-swamp and other seral
stages. Curques, those rocky amphitheatres where mountain glaciers
started in bygone times, are usually so poorly vegetated that they
stand out to this day as barren and forlorn.
Of depositional features left by glaciers there are many, such as
various types of moraines and glacio-fluvial deposits. ‘Terminal
moraines, located at the ends of glaciers whose margins remained
stationary for a long time, usually take the form of hummocky belts
of small rounded hills and basins which are irregularly distributed
and often enclose lakes or swamps. ‘Their vegetation is consequently
very variable from spot to spot, but is usually luxuriant in favourable
situations as the unassorted material includes much that is of fine
texture and nutritional value. Such areas are often valuable for
market-gardening or pasturage. Ground moraine of variable thick-
ness is left by continental glaciers almost everywhere when they
recede. ‘Though again of unassorted material and beset by lakes
and swamps, and sometimes also by smooth elliptical hills called
drumlins, ground moraines tend to be flatter and consequently more
suitable for large-scale agriculture than terminal moraines. ‘The
glacio-fluvial deposits of streams, which carried much of the finer
glacial debris out beyond the terminal moraines, are various but
individually assorted. Thus the coarser sand and gravel was
usually deposited near the terminal moraines but the finer sand and
clay tended to be carried and deposited much farther away, the fans
often coalescing to form a gently sloping ouwtwash plain. Such
plains are usually well vegetated and suitable for agricultural develop-
ment, though they may be beset with eskers (see p. 543), with conical
or other deposits of assorted sand and gravel known as kames and
widely used for building and road-making material, or with pits
known as kettle-holes. These last are often filled with water and
were left where large blocks of ice had been buried in the moraine
or outwash material and subsequently melted, giving an irregular
546 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
‘thaw-sink’ topography. Erratic boulders of various shapes, sizes,
and provenance constitute a minor depositional feature in many
formerly glaciated areas.
The residual features left by glaciers are also various, though
commonly exposed, rocky, and very poorly vegetated. Such are
both arétes (the ‘ knife-edge’ ridges left between glacial troughs)
and matterhorn peaks, which latter are left when several cirques have
so cut into the different flanks of a mountain as to isolate it and re-
duce it to a spectacular horn or needle. ‘The vegetation in such
instances is almost invariably poor, as befits the lofty exposed situa-
tion and rockiness of the substratum; it is often poorer than on
constructional mountain surfaces, owing to the instability of the
residual ones. In high alpine situations it may consist of little more
than Lichens on the rock-faces, crevice plants where they can find
a roothold, and tussock or other herbs where soil accumulates (cf.
Fig. 137, B). In hanging valleys, the other important member of
this category, the vegetation may be much less poor, owing to the
sheltered situation ; it is, however, usually less luxuriant than in the
associated main valley, at least when compared with the floor of the
latter. ‘The roches moutonnées or ‘ sheep rocks’, those asymmetric
rocky hills smoothed by ice-action which characterize many formerly
glaciated areas, are at once erosional and residual. As with large
erratic boulders, their rock surfaces are often to this day devoid of
other than lichen and similarly dwarfed cryptogamic growth. How-
ever, in favourably sheltered situations, soil and higher vegetation
may largely occupy all but their steeper sides, crevices being especially
favoured, while a thick cap of humus, often supporting trees, may
cover their domed tops (cf. Fig. 92).
Erosional features made by ground-water include caverns and
tunnels, the vegetation of which becomes drastically reduced with
decreasing light away from the orifice and, of course, limited to
saprophytes, etc., in the dark. Even near the orifice where the light
seems fairly strong and green plants prevail, these are chiefly vege-
tative forms of shade-plants suchas various Ferns, the types occurring
farthest in being commonly Bryophytes, Algae, and long-drawn-out
seedlings utilizing stored food-reserves. Sinkholes or swallow-holes
are funnel-shaped depressions in the surface of the ground in regions
of soluble rock, formed either by the collapse of a cavern roof or,
directly, by the solvent action of descending surface-water which
enlarges cracks or joints. ‘The bottoms of such sinkholes may be-
come choked with sediment, so that water can no longer drain out
17] LANDSCAPES AND VEGETATION 547
readily and often a pond develops ; or more luxuriant vegetation
may grow in the sheltered depression than in surrounding areas.
This is especially noticeable on pastured calcareous ‘ downs’ where
water percolates away and the general monotony of the short, grassy
vegetation may be relieved by sinkholes containing dense shrubby
growth or even trees.
Of depositional surface features left by ground-water, spring
deposits are the most frequent and important, usually appearing as
mounds or terraces—commonly of calcium carbonate though some-
times of siliceous material. ‘The vegetation varies greatly according
to local circumstances but is usually distinctive—as is true also of
the ‘spring flushes’. ‘These are most noticeable on slopes below
springs where the irrigating water constantly supplies fresh mineral
salts and the ground consequently bears grassy or forbaceous (i.e.
of herbs other than those of grass type) vegetation that is often
bright-green in colour in the midst of brownish acid-tolerant vegeta-
tion. Such flushes may be evident even if the spring or surface
run-off is not; for they still indicate a local abundance of fresh
percolating water.
Residual features left by ground-water are natural bridges and
chimneys, almost invariably of very limited extent. Although they
introduce whole series of microclimatic and often local habitat
effects, each of which may have its characteristic plant inhabitants,
the resulting communities tend to be extremely limited in area and,
moreover, similar to the types to be found in comparable habitats
about cliffs and steep banks elsewhere.
Erosional features made by winds include blow-outs, which are
broad and shallow depressions scooped out of soft rock or sand in
more or less flat regions, wind-caves, due to differential erosion of
soft materials on hill-sides, or rarely blow-holes extending right
through a hill. These last two types occur chiefly in arid regions
and, because also of the dynamic and often eroded nature of the
surface, tend to be devoid of macroscopic vegetation. Blow-outs
may also be practically barren for much the same reasons—for
example in deserts, where they may be some miles in length and
hundreds of feet in depth. In other instances the lower levels may
become occupied by lakes, complete with attendant vegetation ; or
the soft rock may be colonized by plants, particularly in fissures,
and any sand surfaces in time become bound by grassy or other
vegetation which may gradually cover the area.
Features due to deposition by wind include sand-dunes and loess.
548 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
Dunes are accumulations of sand and are formed in much the same
manner as snow-drifts, being started by some obstruction which
causes an eddy in the sand-bearing wind. Once started, each dune
provides the obstacle which causes its further growth. Sand-dunes
are of very various form and size, ranging from a few square feet
up to several square miles in area, and from inches to perhaps 1,000
feet (3048 metres) in height. Commonly the component sand is
well sorted as to size and rounded as to particle-shape. Dunes are
apt to be formed wherever there is a source of sand available—
for example on a sandy beach or river plain, or where sandstone
disintegrates in a dry climate. Unless ‘bound’ and covered by
vegetation so that many surface particles are held while the wind is
slowed down and no longer has full access to the sand, dunes migrate,
though rarely at a rate of more than 25 feet a year. ‘This migration
takes place through the transfer of sand from the windward to the
leeward side of the dune, and in its course may overwhelm roads,
farms, and forests. ‘The most effective way of stopping the migration
of dunes is to plant on their windward side suitably hardy Grasses
and shrubs adapted to sandy soil and capable of binding the surface.
Such are Marram Grass (Ammophila arenaria), Lyme-grass (Elymus
arenarius s.l.), Volga Giant-wildrye (EF. giganteus), and certain mem-
bers of the Pea family, which are used for this purpose of stabilizing
dunes in various parts of the world and may be followed by suitable
coniferous and other trees. Even when a climax forest is attained
the dune origin is commonly evident—and irksome to the foot-
traveller—owing to the intimate topography of ups and downs caused
by the bodies of the stabilized dunes. Frequently sand-dunes
become stabilized naturally by similar means, the common sequence
being that one of the coarse rhizomatous Grasses will pioneer in
colonization (cf. Fig. 28, A, and Fig. 93), followed by Mosses or
Lichens or other ‘ secondary binders’ on the floor, followed in turn
by less xeromorphic higher plants (for the sand is usually damp
beneath the air-dried surface). ‘These last include shrubs and
ultimately trees when the surface has become fully stabilized and
sufficient humus has accumulated to form a reasonably nutrient soil
(Fig. 177).
Loess is wind-deposited dust ; its deposits are usually without
special form but may be of great extent and considerable thickness.
Its fine texture and content of food-salts make loess a fertile
substratum provided sufficient moisture is present and it is
not too compacted ; it gives an easily worked and agriculturally
17 | LANDSCAPES AND VEGETATION 549
= Po HM as
Fic. 177.—Old sand-dune colonized by shrubs and Pitch Pine (Pinus rigida) after
stabilization by Marram Grass (Ammophila arenaria, persisting in foreground).
Prout’s Neck, Maine.
550 INTRODUCTION LO BLAND GEOG RAP Ti) [ CHAP.
desirable soil. However, if unprotected by vegetation it is easily
eroded.
Residual features left by wind include mushroom rocks and some
mesas and buttes: their tops are usually poorly vegetated owing to
exposure and dryness, their sides being often barren owing to
abrasion and erosion.
Erosional features due to waves and currents include wave-cut
benches, cliffed shore-lines, and sea-caves. Of these the benches
extend down to the lower limit of wave erosion and tend to be well
vegetated by fair-sized Brown and other Algae as indicated in Chapter
XVI. The cliffs commonly introduce a host of crevice and other
microhabitats and consequently support very various plant com-
munities in different spots and instances, though their exposed and
unstable nature usually prevents the attainment of anything approach-
ing the local climax. Deep and narrow inlets and sea-caves are
also very variously vegetated according to local conditions, though
owing to their rocky nature and the frequently poor light in them,
there is considerable limitation of the flora and of sturdy growth.
Sometimes, however, where salt water does not reach and a last-
ing supply of fresh water percolates, so that the atmosphere
is continuously damp, a fine growth of Ferns and Bryophytes
is developed in the mouths of caverns and as far in as light
allows.
Depositional features of waves and currents include beaches, tidal
deltas, and various kinds of bars. ‘The beaches are liable to be
barren, especially where the surface is exposed and dynamic, though
in sheltered situations alongside salt as well as fresh water the
surface may be colonized and, where silty, often well vegetated.
This is especially true in tidal creeks and about the off-shore
periphery of deltas, which in the tropics typically support luxuriant
mangrove vegetation of shrubs or trees as described in Chapter
XIV. In extra-tropical regions a swarded salt-marsh often develops
in such situations. Sand and, especially, mud bars or spits may
also be so adorned when formed in sheltered situations such as
deep bays; more often, however, they are devoid of higher plants,
though in cases of intermediate stability the surface may be bound
by a close investment of Schizophyta or lowly Algae.
Residual features formed by waves and currents include stacks
and arches which, being cliffed and exposed, tend to be poorly
vegetated although they introduce an array of microhabitats com-
parable with those found about cliffs. ‘Though distinctive and often
17] LANDSCAPES AND VEGETATION 551
striking, such features are minor in being uncommon and in occupy-
ing very limited areas.
Additional agents of erosion include gravity, giving for example
screes and talus-heaps below cliffs, and animals (including Man),
which are constantly producing new surfaces, either directly or
indirectly, as has repeatedly been indicated in earlier chapters.
INTERPRETATION AND USES
Landscapes are typically made up of the smaller to medium-size
landforms. However, in viewing a landscape in most parts of the
world, it is chiefly the clothing vegetation which is seen. This,
like all other vegetation, as we have already observed, varies greatly
with local conditions. ‘Therefore it affords a ready and evident
means of distinguishing the landforms and other local features which
make up a landscape, in turn being of assistance in their interpretation
and consequently proving of value to the ecologist when he has to
decide on the uses to which particular areas may be put. For it
has been rightly said that the study of vegetation, especially when
it is of established communities and they are mature, affords more
reliable indications of the action and interaction of local factors than
direct measurements, and is often the best basis for agricultural and
allied planning.
To the common question of what is the explanation of particular
plants growing in particular places, there can scarcely be a satis-
factory general answer. ‘To be sure, the presence of, say, a certain
tree in a spot is an expression of something more or less definite,
and indeed, much of this book has been devoted to consideration of
the kind of items that would have to be satisfied before such presence
could come about, and of what, in consequence, it implies. But
these items vary enormously and complicatedly from instance to
instance and even from spot to spot. ‘Thus before we could have
any chance of contemplating our tree where it grows, a viable dis-
seminule would have had to be produced and liberated at a suitable
place and brought by some means to the relevant point at some time
in the past. Moreover, this disseminule would have to be fortunate
enough in its circumstances to find sufficient water for germination
and for the successful establishment of the resulting plant—which
implies also suitable conditions of climate, soil, and so forth for the
growth of the plant in that particular spot. Any deficiency or serious
deviation at a critical stage might easily have proved fatal. And
552 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
then, in order for the species to become established in any numbers,
it would have to reproduce successfully and withstand predators,
diseases, and competitors. Alternatively, Man may have taken a
hand, though, as we have often seen, in the case of introductions
his continued intervention is commonly necessary for persistence.
In any case, a whole complex of circumstances lie behind the arrival
of a particular plant in a particular place, and a further complex
of conditions has to be satisfied before it can become established and
survive there. Furthermore, for anything approaching permanence,
we have the many problems involved in successful reproduction.
And as both circumstances and conditions are complicated, and it
seems safe to conclude that there are very many thousands of different
combinations of them favouring the existence of particular species
among the hundreds of thousands known on earth, it would seem
futile to attempt to treat these matters systematically here. Even
a series of specific examples from a particular place would be of
little help, unless it were to inhabitants of that one place—especially
as in the final analysis a good deal is left to chance.
What the asker of the above question may often have in mind,
however, is what does the presence of a particular plant tell us about
the place in which it grows—especially in terms of practical applica-
tion? It usually means that the plant involved has found suitable
conditions at least for growth and survival, and that the conditions
under which we may know it to flourish elsewhere are approximated
to here—though we should meanwhile recall that preferences may
change (e.g. at different stages of the life-cycle), as may of course
ecological factors themselves. Although many plants have too wide
an ecological amplitude to mean much in this connection, at least
as far as present-day knowledge goes, others are exacting in their
requirements. Others, again, have specific needs such as the
presence (or absence) of particular substances in the soil. Here
we think immediately of certain ‘indicator’ plants or groups of
plants which are so called because they require (and therefore
indicate) the presence of particular conditions, and may consequently
be valuable in demonstrating that these conditions obtain where
they grow. For the existence of a particular plant or community
in a particular place is an expression within certain limits of a
particular combination of actual habitat conditions, among other
things, and, where these limits are narrow, or a specific one is finite,
we have a ready-made demonstration of the situation obtaining
locally.
17] LANDSCAPES AND VEGETATION 553
These ‘ indicator ’ plants, communities, and other signs are prone
to vary from one region to another, sometimes even of comparable
climate, so that considerable local knowledge is necessary for effective
practice of the principles involved. Imparting such local informa-
tion, even for a single limited area, is beyond the scope of this
introductory book: recourse must be had to detailed local floristic
and ecological works such as are now available for many parts of the
world. But even with much book knowledge, field experience is
usually necessary for reliable interpretation and use, as observations
have to be analyzed into usable data and synthesized into applicable
forms. ‘Thus most of us living on either side of the North Atlantic
know that Willows and Poplars like an abundance of water in the
soil, yet there are wide variations in this respect between different
species of Willows and Poplars, some especially of the latter favouring
rather dry habitats. Indeed one species, popularly known as the
Trembling Aspen (Populus tremuloides), in certain circumstances and
places can be a fair indicator of rather arid conditions—which
emphasizes the need for specific observations and local knowledge.
Moreover, members of an individual species often vary in their
needs for water, etc., in different regions, according to other habitat
factors and to their particular state, while it must also be remembered
that species within a genus are sometimes difficult to tell apart—
even to taxonomists, which too many ecologists are not. Still more
liable to be confused in our minds are members of lower taxa which
may yet exhibit marked ecological preferences or needs, and indeed
different habitat preferences may be shown by physiological strains
lacking any evident differences in form.
Especially do plants tend to become more limited to conditions
expressed by certain habitats as they approach the periphery of
their range. This principle should always be borne in mind to
avoid mistakes in interpretation. ‘Thus Beech forests in western-
most Europe are largely limited to calcareous soils, in England being
commonly associated with chalk, whereas to the east this is no longer
the case. The European Beech does, however, seem everywhere to
be indicative of fairly dry, well-drained conditions, and it has been
suggested that its western association with chalk, especially, is due
to the drier conditions usually found thereon as compared with
adjacent clayey and other soils.
In view of the local variability of soil and some other conditions,
it is only to be expected that individual species of plants are in
general unreliable as indicators, being of relatively “low indicator
554 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
value’ as it is called. Nevertheless many dominants, and some
other special instances such as the European Beech or Ash or even
beds of Stinging Nettles, may be useful as individual indicators.
Far more reliable, however, are groups of species or, preferably,
whole communities. ‘These make up vegetation, which, as we have
seen, often provides the most characteristic (though variable) surface
feature not only of landforms but also of entire landscapes.
Apart from the truism that, in the absence of drastic disturbance,
the luxuriance and form of the vegetation developed on an area
will give to everyone some indication of its productivity, consider-
able local knowledge is commonly required to interpret vegetation
and use it as a basis for agricultural or forestral planning. Given
this local knowledge and some practical experience in its employ-
ment, an observant farmer or forester can usually tell more about
the uses to which a particular tract of land can be put, from observa-
tion of its standing vegetation, than can be determined from much
tedious measurement. Nor is this surprising, for it is often not so
much this or that particular factor of the environment which is
likely to affect the natural vegetation, as the result of interaction of
all the factors obtaining locally. Of this composite result the
vegetation is, in a sense, a measuring apparatus, and an extraordinarily
delicate one. For the type, composition, and degree of luxuriance
of the plant communities are apt to be immediately eloquent on
the subject of local conditions, and it is inconceivable that any
battery of instruments, however complicated and delicate, could ever
really emulate natural vegetation in this respect.
Often the vegetation will not only give an indication of minor
and unsuspected habitat differences but also emphasize gross ones
such as landforms in a landscape. An example of the former is
the common persistence of beds of Stinging Nettles (Urtica dioica)
about the sites of old habitations or areas of loosened soil and, of
the latter, the existence of dark clumps of trees in depressions in
many grasslands. Yet in other instances vegetation may obscure
the local conformation of the earth’s surface, including some striking
if minor landforms, by its very luxuriance.
To such further questions as ‘Where, so far as surface and
vegetation features indicate, ought I to build my house and dig my
garden’, there is again no simple answer: it will depend on the
region and various other circumstances. But a close study of the
vegetation of likely situations, coupled, perhaps, with observation
of the plant growth around already established dwellings in similar
17] LANDSCAPES AND VEGETATION 555
habitats, will give many useful pointers, as it will also in such matters
as shelter and water-supply, if these have to be considered. For
vegetation is often the best indicator of conditions not only in the
air but also below the surface of the ground, landscapes being, as
we have repeatedly observed, frequently best interpreted through
their “ green mantle’. Moreover, the occurrence of similar groups
of plants in different spots usually indicates a close similarity of
conditions, while the converse usually holds, at least throughout an
area of fairly uniform climate. So, altogether, careful and suitably
enlightened observation of vegetation can be of the greatest practical
value.
In these considerations, whether the emphasis is practical or
academic, it should be remembered that vegetation may not only
accentuate but also mould or even be responsible for some landforms.
For plant growth promotes deposition, as of sand on dunes and silt
in water, and increases the accumulation on natural levees of material
from flooding rivers. Plants also cause the chemical separation of
calcium carbonate which is largely responsible for ‘ coral’ reefs.
Moreover vegetation impedes erosion, conserving the soil and
underlying mantle of weathered rock or transported material by
reducing or preventing removal by wind or water. With its flow
thus retarded, rainwater or snow-melt water is permitted to sink
into the ground, the vegetation being in turn favoured in what often
becomes a mutually cumulative effect. And as the soil and under-
lying mantle are protected by vegetation from erosion, so is the
bedrock protected from the weathering that precedes erosion
(apart, of course, from the small amount of weathering of exposed
rock surface that may be caused by plant growth). ‘Thus may parts
of landscapes be moulded ; and hence the wide use of vegetation
for wind-breaks, shelter-belts, dune-stabilization, and prevention of
erosion by excessive run-off and other means. Vegetation, whether
natural or planted, is, or should be, not only the main medium but
also a leading tool of the conservationist.
LAND-USE CLASSIFICATION
For the all-important practice of the most effective land utilization
in an area, intimate familiarity is necessary with the vegetation and
other local manifestations. In the words of Dr. Edward H. Graham
(in his work cited at the end of the present chapter), this should be
‘Not the detailed inspection of the micro-biologist or soil analyst, but
a consideration of landscape as an ecological complex prescribing the
556 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
use which man can make of the land. ‘This involves, before all else
Land-use classification [which] should relate to the physical
capacity of the land to produce given crops for an indefinite period
without exhaustion or waste of the land resource. It does not hold
that production of tilled crops is the highest use of the land, but rather
that this is true only for certain classes of land. Some land, for example
swampy areas, is poorly used if it is not devoted to muskrat or other
wildlife production, for efforts to produce more intensive crops only
result in a waste of time, labor, and materials.’
In addition there are the areas—amounting for example in the
United States to about 2 per cent. of the total land—occupied by
roads, towns, and railways, all of which have to be chosen and
maintained with the greatest possible knowledge and care. Land
classification in the modern sense should therefore include also
inventory and over-all planning.
A very general computation has given the following figures for
the occupation of the world’s land surface before Man transformed
so much of it : 30 per cent. forest or arborescent scrub, 19 per cent.
grassland, 17 per cent. desert, and the remaining 34 per cent. poor
mountain or polar areas (cf. Fig. 65). Most of the disturbed tracts
are considered capable of reinstatement so far as vegetation-type is
concerned, and accordingly these figures may also be regarded as
at least potential ones for the future. Curiously enough, although
many forested areas of past or recent times are considered capable
of cultivation, the same proportion of about 30 per cent. of the total
land surface of the world is also believed to be cultivable, while
another 30 per cent., without being too cold or too dry to support
crops, is of poor grazing-land, marsh, waste, or otherwise unsuitable.
Of the many ways of classifying land, perhaps the most practical
and valuable is according to use-capabilities. ‘This is based, as it
should be for permanence, on natural characteristics such as soil
and other biological conditions—ignoring such immediate con-
siderations as financial aspects or the skill of the operating individual.
In this system the following eight classes are widely recognized with
reference to rural use, though of course urban, industrial, transporta-
tional, and recreational areas should also be established with due
reference to a scheme of land classification wherever possible. ‘The
eight classes are placed in three major groups, namely, A, B, and C.
A. Suitable for cultivation involving tillage
I. Without special practices for the prevention of serious erosion
17] LANDSCAPES AND VEGETATION 557
or other lasting deterioration, though fertilizers and simple crop-
rotations are often used. ‘This land is of very good productivity
and, being level or nearly so, is practically free from any possibility
of erosion. ‘The soil is easy to work, deep, and at least fairly well
supplied with plant nutrients.
II. With simple practices—such as contour cultivation, strip
cropping, growing protective cover-crops, or simple water-manage-
ment. ‘This is good land but not quite as good as that comprising
Class I—owing to physical conditions such as the slope, which may
be just steep enough to make water run off at a speed that will
carry away soil. Or the soil may be so wet as to require drainage,
or, alternatively, may be rather dry. In anycase there are deficiencies
which either limit the use of the land to some extent or require
attention year after year.
III. With complex or intensive practices—such as terracing,
though usually a combination of remedies is needed. This is
moderately good land for cultivation, being more limited than that
of Class II by reason of one or more natural features. It can be
regularly used for crops only with intensive treatment and employing
the best of farming methods. Sometimes the slope demands erosion
control, or it may be undesirably dry or, alternatively, so wet as to
require drainage.
IV. With intensive practices and limited use—such as cultivation
in small plots or on long rotations with only occasional crops. ‘This
land is not suitable for regular production of cultivated crops, most
often because of steepness and consequent danger of erosion.
Commonly it can be cultivated safely perhaps one year in every
six ; in the other years it is best used for hay or pasturage. Some
cases are too dry for dependable cropping without irrigation. It
is only fairly good for arable crops but usually affords good grazing
or forest land provided the rainfall or other precipitation is adequate.
B. Suitable for permanent pasture or woodland but not for cultivation
V. Without special practices—land only slightly susceptible to
deterioration when the range is fully grazed or the forest is widely
cut. It is usually nearly level and accordingly not subject to erosion,
but, because of wetness, climate, or such permanent obstructions
as rocky outcrops, is not suitable for cultivation. However, the
soil is deep and the land has few limitations for any kind of grazing
or for forestry use, which should be entirely successful.
VI. With some restriction in use—land moderately susceptible
T
558 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
to deterioration, so grazing must be by rotation and logging only
with due care, and there may have to be special conservation practices
such as contour furrows across slopes to maintain forage and protect
soil. This is quite good land for forestry or grazing but is some-
what limited for these purposes by shallow soil or steep slopes, or
by excessive wetness that cannot be remedied by drainage to permit
use for crops. Alternatively, in arid or semi-arid regions, there
may be limitation through lack of moisture.
<u Ne e®
Fic. 178.—An example of Class VII land. (Phot. U.S.D.A.)
VII. With severe restriction in use—land highly susceptible to
deterioration, so grazing must be only occasional and felling highly
selective (Fig. 178). On this land extreme care must be exercised
to prevent erosion as a result of pasturing or lumbering.
C. Suitable for wildlife but not for cultivation, pasture, or woodland
VIII. With or without special practices—productive of useful wild
plants, fur, game Birds, Mammals, or Fish; generally serving as
range for wild or semi-wild animals (Fig. 179). Such land is usually
very rough or wet or susceptible to severe erosion. Or it may be
17] LANDSCAPES AND VEGETATION 559
so arid or steep as to be unsuitable for grazing and fail to grow trees.
Examples include rocky foothills, rough mountain land, bare rock
outcrops, coastal sand-dunes, and many swamps and marshes.
Sax WAS! Soe AS
ANS S api
a :
foe
Fig. 180 illustrates all eight land-use capability classes. It may
readily be seen that farms are concerned largely with group A
(Classes I to IV), pasturage and forests primarily with group B
(Classes V to VII), and wildlife largely, but by no means exclusively,
with group C (Class VIII). Areas of Class VIII may, moreover,
have important recreational, aesthetic, or watershed-protection values,
as was pointed out by Dr. Edward H. Graham (7m /itt.). ‘They may
furthermore be of great scientific interest. Open waters may also
be looked upon as belonging to Class VIII, as they are best adapted
to the production of undomesticated plants and animals. Actually,
neglect can cause even the best and most productive agricultural
land to deteriorate, through erosion, practically to the bottom of the
scale, as is illustrated in Fig. 181, which should serve as a severe
warning to any who might ignore conservation practices or at least
reasonable caution.
560 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
Within all of the classes except No. I, subclasses may be established
according to the nature of the most important character limiting
use-intensity. Thus, soils that are subject to erosion are placed in
an ‘ erosion subclass ’, indicated by adding an ‘ e’ to the designation,
as, for example, ‘Class [Ve’. Similarly, soils with a problem
primarily of water-control are placed in the ‘w’ subclass of the
appropriate class. Other usual subclasses are of soil-deficiencies
and aridity, while salinity adds yet another.
CLASS.” ¥-LAND
5 Tame pe s “Seale Sets os 6 Sie
Fic. 180.—Illustration of the eight land-use capability classes. (Courtesy of U.S.
Soil Conservation Service.)
The separation of the different land-use capability groups and,
particularly, classes, naturally depends on different manifestations
and criteria in different regions ; yet to a considerable extent it
tends in some circumstances to take account of floristic and vegeta-
tional features or effects, and, consequently, to be phytogeographical
in basis. For, as we have already observed, the plant life gives us
a living expression of the total environment such as nothing else
can replace. It even provides us with ready-made experiments on,
and something of an interpretation of, what happens beneath the
soil surface. Especially where established communities are con-
cerned, suitably enlightened study of the plant life not only affords
a more exact indication of the combined action of all site factors
17] LANDSCAPES AND VEGETATION 561
than any measurement of individual ones, but it commonly provides
the most reliable ecological basis for any agricultural, forestral, or
allied planning.
FURTHER CONSIDERATION
Almost any modern work on general physical geography or geology
will have some treatment of landforms as such, though the approaches
and classification may vary greatly in different works. A useful account,
largely along the lines followed in the present chapter, is to be found in
Henry D. Thompson’s Fundamentals of Earth Science (Appleton-Century-
Crofts, New York, pp. xili + 461, 1947). As for the usually far wider,
constructional landforms that were deemed to be too variable or, alterna-
tively, too general for useful consideration in any detail here, the mountain
ones are well treated in R. Peattie’s Mountain Geography (Harvard
University Press, Cambridge, Mass., pp. xiv + 257, 1936) and the plains
and plateaux in M. D. Haviland’s Forest, Steppe and Tundra (Cambridge
University Press, Cambridge, Eng., pp. [xi] + 218, 1926).
For further details about land utilization, including many botanical
aspects, reference may be made to Edward H. Graham’s Natural Principles
of Land Use (Oxford University Press, London etc., pp. xiii + 274, 1944).
This gives an extensive and valuable annotated bibliography from which
any interested reader may select further works to his taste. A very
readable introductory book is Paul B. Sears’s Life and Environment
(Teachers College, Columbia University, New York, pp. xx + 175, 1939).
CHAPTER XVIII
PLANT ADJUSTMENTS AND APPLEICATIOGRS
Consideration of both (1) the natural ‘ adaptations’ and (2) the
man-made modifications of plants, in each case (a) of individual
kinds and (b) of vegetation, gives us by cross-inference four sets of
topics. ‘These are all of peculiar interest and importance, aud are
exemplified individually by (7) evolution (which depends substantially
on the favourable modifications or adaptations of individual plants) ;
(i) plant-breeding (in which Man manipulates the characteristics
of individual plants) ; (77) successional change (of natural vegetation) ;
and (iv) combating of erosion (for which planted or encouraged
vegetation is the best weapon). ‘To the extent that these and allied
considerations are often areal, for example in leading to extended
ranges, they should be included among the concerns of plant geo-
graphy. Each is a large subject that can be treated here only in
broad outline or with special reference to the example mentioned.
‘ ADAPTATIONS’ OF INDIVIDUALS
However evolution of plants may have come about—and there
appear to be many and various causes and mechanisms of evolu-
tionary change—it is clear that the tendency is generally towards
forms more suitably adapted to the environment than were their
ancestors. Actually it seems that the main single cause of evolution
in plants has been—and continues to be—the ‘ natural selection ’
of heritable characteristics that are beneficial in competition and
hence advantageous in the general ‘ struggle for existence’. Most
species produce, at least potentially in the form of disseminules,
far more individuals than ever survive, and it is the progeny pos-
sessing favourable variations in structure or function that tend to
persist. When these variations are heritable, they may become
‘fixed’ in succeeding generations and benefit the race ; for in the
intense competition that is characteristic of life on earth, individuals
having even slight advantages over their fellows, for example in
exhibiting taller or more rapid growth, will have the best chances
562
PLANT ADJUSTMENTS AND APPLICATIONS 563
of success. Altogether, variation is one of the most universal attri-
butes of living things, and the outcome of natural selection upon
the varying populations of successive generations is persistence of
the forms possessing the most beneficial characters—or, as it is fre-
quently called, ‘ survival of the fittest ’.
Whether or not we accept this explanation of evolution by natural
selection approximately as propounded nearly a century ago by
Charles Darwin—and most biologists nowadays believe that it is
only a partial elucidation of the mechanisms involved—the fact that
evolution takes place is now almost universally accepted. Moreover
there can be few if any who would deny that the common course of
evolution is through forms which, as it proceeds, become more and
more closely aligned with the demands of particular habitats or
groups of habitats, its outcome being then in races or higher taxa
that are more or less closely adapted to the conditions in which they
grow. ‘That is the keynote of the plant geographical aspect of the
subject ; and whereas we could go on explaining and exemplifying
ad nauseam, it seems best to leave matters here. For there can
scarcely be any group of plants that does not afford instances of
adaptations to the particular environmental factors under which one
or more of its members grow, and which thereby affect its distribu-
tion in actuality or potentiality.
On the utilitarian side we should note that, with crops and trees
—including Maize in the mid-western United States and Conifers
in Scandinavia—it has sometimes been observed that seed from
acclimatized local plants gives better results than seed imported from
even a relatively short distance away. ‘This ‘ regional adaptation ’
tends to be especially marked when the testing-ground lies in an
area of unusually diversified climatic or other conditions. ‘The
reason appears to be that the ‘ native’ strains have been rendered
appropriate to the local conditions through the action of natural
selection on a population that was once more mixed genetically.
Nature, as it were, is in this way doing the plant breeder’s work for
him. Yet the basis here is Man’s husbandry and, usually, importa-
tion of seed in the first instance.
MAN-MADE ADJUSTMENTS
When we come to consider Man’s direct and deliberate manipula-
tion of certain characteristics of individual plants or, particularly,
kinds of plants, we are dealing to a considerable extent with processes
564 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
of selection which are, instead, artificial. ‘Thus plant-breeders select,
from among the variants they observe in nature or can produce
by hybridization or other genetical practices, the most desirable
tendencies which, when heritable, persist in subsequent generations.
Often two or more desirable characteristics exhibited by different
strains are combined in one strain by hybridization or other
techniques. By such means, through the ages, most of our domestic
races of vegetable, cereal, pulse, root, and other crops have come
into existence, and by modern methods are being improved all the
time in relation to modern conditions and needs. ‘Thus strains of
many crops have been developed which are able to overcome
obstacles that previously prevented them from being grown success-
fully in whole regions. Examples of such major feats include the
pushing farther and farther north of the wheat belt in Siberia and
Canada, and cultivation of hardy strains of other cereals higher and
higher up in mountainous areas. Certain fruits, too, are now being
produced successfully in increasingly rigorous climes without the
aid of costly contrivances such as glasshouses. In addition there is
the chemical treatment by various ‘ plant growth substances ’ which
is becoming increasingly important in many crop improvement
connections. Resistance to disease or frost or drought, promotion
of early flowering and rapid fruiting, and all manner of other adjust-
ments which can be made in plants by artificial selection and
enlightened breeding or chemical treatment, have greatly affected
the potential and often the actual ranges of the crops involved—
which again strikes the distributional keynote of our subject.
By such means does Man to a considerable degree mould plants
to his needs and extend the area as well as the productivity of his
crops. It should, however, be re-emphasized that these ‘ artificial ’
strains of plants are not only commonly incapable of withstanding
the competition of native vegetation if this is not kept back by Man,
but that Man has also often to modify the habitat even further for
success. ‘This he accomplishes by such agricultural, horticultural,
or forestral practices as ploughing, fertilizing, mulching, irrigating,
draining, and so on. ‘Thus such domesticated plants may, from
Nature’s point of view, be considered doubly artificial—on one hand
in origin and, on the other, in their habitat. But as plant geography
deals in a practical way with plant distributions as we see them in
the world as a whole, even these ‘ artificial’ plants are important
objects of our study. Indeed Chapter VIII is largely concerned
with their modifications and distributions.
18] PLANT ADJUSTMENTS AND APPLICATIONS 565
VEGETATIONAL ADAPTATION
Just as different kinds of plants are adapted in form or function
for life under particular conditions, and this fact largely limits their
distribution on earth, so the communities which plants make up
collectively, and which as a whole we term vegetation, are limited
in area by local conditions. ‘Thus the general principle of adaptation
of plants to particular habitats applies also to vegetation ; and even
as evolution has tended in a very general way to be towards better
and better adaptation of individuals, so it appears to have been with
vegetational change. Most obviously, the dominant species on
which so much depends, commonly have their areas prescribed by
climatic conditions and, more locally within climatically suitable
areas, by edaphic or other immediate considerations. And so it
largely is with the rest of a community, though local conditions are
often controlled to a considerable degree by the dominants them-
selves, or by other plants of similar life-form. Nor is the total
effect of the community necessarily by any means the same as the
sum of the effects of its components considered individually.
Perhaps the most noteworthy and general tendency to change in
natural plant communities is through the process of succession,
which was considered in Chapter XI. ‘The progression from one
community to another of more efficient energy-utilization which is
succession, and which normally involves domination by higher and
higher life-forms as the stages succeed one another in an area, is
essentially a matter of the changing environmental demands and
adaptational attainments of successive colonists. ‘Thus the largely
different plants involved in different stages have usually such different
habitat requirements that the species of one stage are commonly
ousted by those of the next—because they are not suitably adapted
to its attendant conditions. ‘These conditions, admittedly, are
largely introduced or controlled by the plants themselves ; but the
principle nevertheless holds. The immediate plant geographical
implication is that successional changes introduce different conditions
to which usually very different plants are adapted, and whose dis-
tributions are thereby altered, at least potentially. At the same
time succession tends to render areas unsuitable for many previous
colonists and their ecological analogues, correspondingly limiting
the ranges of such organisms. ‘These and allied changes go on con-
tinuously in the world today, as Man or other agencies disturb
vegetation and initiate or alter successions in various ways. We
566 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
have, moreover, repeatedly seen how many plants become established
in an area only when suitable habitats have been prepared for them
by other species; thus plant distributions, and consequently the
local vegetation, are often complicatedly dependent on the succes-
sional stage reached, or at least on the establishment of particular
plant communities. For conditions which preclude one species or
type will commonly favour some other.
Plants vary greatly in the extent to which they can adjust them-
selves to varying environmental conditions, and the same is true
of the communities which they make up. Thus a patch of Fireweed
will bloom gloriously in the early years of a forest clearing but there-
after wane and soon disappear as the conditions are changed by
colonizing shrubs and saplings; on the other hand, these woody
plants may enter at the same time as the Fireweed and persist right
through to the climax stage. All the above and numerous other
considerations lie behind the present-day distributions of plants and
consequently of vegetational types, as should be abundantly clear
from the present work.
MANIPULATION OF VEGETATION
Modern Man is the biotic superdominant of the world, and the
changes which he effects in vegetation are among the most striking
on earth. Such changes are apt to be of a more or less destructive
nature, examples being heavy lumbering and grazing, burning and
clearing, and the cultivation of lowly crops in place of lofty forests.
Yet many are constructive so far as Man’s needs are concerned—
hence the whole vast industry of agricultural and related practices.
Some are even constructive from the points of view of both Nature
and mankind, an example being the stabilization of dunelands by
‘binding’ vegetation—particularly by planting such coarse sand-
binding grasses as Lyme-grass and Marram Grass. ‘These are often
followed by leguminous plants and Heaths, and, in time, by bushes
and trees. Or afforestation may be effected by direct planting of
young trees in already vegetated areas. ‘Thus stabilization can be
effectively carried out by Man’s intervention, and frequently leads
to luxuriant growth and substantial productivity where there was
previously a mere barren waste. Man also in a sense manipulates
the vegetation over wide areas by perpetuating such features as fire
and grazing ‘ subclimaxes ’.
The above and some other aspects of what may be looked on
18] PLANT ADJUSTMENTS AND APPLICATIONS 567
as Man’s manipulation of vegetation have been at least touched
upon elsewhere in this work ; many are dealt with in more worthy
detail in recent books on conservation and allied topics. Agricul-
ture, horticulture, and forestry themselves are to a large extent
dependent upon Man’s manipulation of habitats and cognate plant
communities, whether the latter consist of individual crops or
mixed ones.
The practical value of the concept of plant communities is widely
recognized to be considerable in forest, range, and wildlife manage-
ment—e.g. in North America—and in vegetation mapping for land
use as often practised elsewhere. ‘Thus in the United States, land
utilization and management are happily to a considerable and ever-
increasing extent based on the indications afforded by vegetation
and on the potentialities of its growth—for example, after the manner
outlined in the last chapter. Moreover, many of the most scientific-
ally based and successful practices involve the judicious modification
of existing vegetation rather than the creation of new, ‘ artificial ’
forms. For such practices the careful recording of the species con-
cerned, as well as of the density and composition of plant commun-
ities, is often important. Usually, samples are taken as a basis for
estimating the general conditions or productivity of the plant cover
over the area of which the samples are representative ; comparable
sampling elsewhere may also serve as a standard for comparing the
vegetation of different areas.
The methods employed in sampling vegetation include the use of
quadrats, which are test areas (commonly squares, hence the name)
of designated size in which the kinds and numbers (or areas) of
plants are recorded, and of transects, which are cross-sections of
vegetation studied along a line or belt. A /ine transect is one in
which there are recorded, by names or symbols, the plants touching
or overlapping a string stretched along the ground, while a belt
transect represents a band of vegetation of designated width. In
each case the recording should be done on a diagram drawn to scale.
A belt transect is essentially an elongated quadrat and is usually
far more instructive than a line transect, but it is more tedious to
construct and record. Also involving intensive labours is the bisect,
which is a cross-section of vegetation as it is revealed by a trench
extending down to the deepest roots. For the investigation of suc-
cession, permanent quadrats may be established and studied from
time to time. Clip quadrats, in which can be determined the oven-
dry weight of the total vegetation clipped from the test area, are
568 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
valuable to indicate local plant productivity. Exclusion or enclosure
of animals may usefully be practised to indicate the extent to which
browsers, etc., are modifying the local vegetation. ‘Transplant experi-
ments of plants or whole clumps of vegetation may also be valuable
in indicating climatic, edaphic, or biotic impress. Vegetation maps
of various types and on various scales, based on the closest possible
observation of the composition and boundaries of the communities
Fic. 181.—Part of a maze of gullies which crosses more than an entire county
in the southern United States and has ‘ permanently’ destroyed more than a
hundred thousand acres of excellent land. The original gully had begun about
70 years previously with a drip from a barn roof. (Courtesy of U.S. Soil Con-
servation Service.)
involved, and often supplemented by quadrats, transects, or other
special methods of study of test areas, are also often prerequisite
to successful application of modern agricultural and forestral
techniques over wide stretches of country. Much the same is true
of soil maps, indicating the type and extent of the main soils of
different areas. In these and other ways, the study and under-
standing of vegetation and its habitats are of fundamental importance
to prudent land utilization and, consequently, to humanity.
18] PLANT ADJUSTMENTS AND APPLICATIONS 569
Conservation, which largely depends on wise treatment of vegeta-
tion, is a rapidly developing theme, of the utmost significance to
civilization. Prevention is far better than cure, but very often we
are too late for the former and sometimes even for the latter. Of
this we have already seen instances, a terrible one being indicated
in Fig. 181. In other cases remedy may be simple, as illustrated
ES
Fic. 182.—Delstructive water-erosional gully in heavily overgrazed pasture in
Illinois. (Courtesy of U.S. Soil Conservation Service.)
in Figs. 182 and 183, where bad water-erosion was stopped by
simply excluding livestock for two years and so allowing succession
to proceed. In yet other cases all that is needed may be a planting
of Willows on a stream-bank or of Lyme-grass on the windward
slopes of sand-dunes. Yet altogether an appalling proportion of the
once-productive land areas of many major countries has been lost
to cultivation owing to unwise practices following Man’s desecration
of vegetation. ‘This has led particularly to devastating erosion, and
judicious planting of suitable crops or binders is often the best key
to the reclamation of such tracts where this is possible. In the
continental United States, for example, the area of cropland that
57° INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
is under either cultivation or rotation cropping is computed to stand
at some 478 million acres, or about one-quarter of the total land area.
Perhaps another 238 million acres could be added to Classes I to
IV lands (see Chapter XVII) that are at present neither cultivated
nor in rotation cropping but are yet capable of being cropped,
especially with intensive practices. But it has been estimated that
Fic. 183.—The same as Fig. 182, two years later. Livestock have been excluded
by fencing, whereupon by natural succession a cover of soil-protecting vegetation
soon developed and controlled soil loss. Instead of a wasteful and dangerous
gully (cf. Fig. 181), the area was restored to potential usefulness. (Courtesy of
U.S. Soil Conservation Service.)
already some 25 million acres originally suitable for cultivation have
been lost through water-erosion and soil-blowing, another ten
million acres have been lost through other types of soil deterioration,
and more than half of the remaining cropland has been damaged
(critically as to 121 million acres and seriously as to 128 million
acres) by one or another of these scourges (according to the 1954
figures given out by the U.S. Department of Agriculture’s Soil
Conservation Service). Other computations have indicated that
more than one-third of the productive top-soil of the United States
18] PLANT ADJUSTMENTS AND APPLICATIONS 571
has already gone, and that the water-cycle has been drastically
disturbed. ‘This is a most distressing situation to which Professor
Paul B. Sears’s book Deserts on the March drew eloquent attention
more than two decades ago. Conservation of soil and other natural
resources is above all a way of life, as people have come more and
more widely to realize in recent years, though still there is not nearly
enough appreciation of its significance, let alone application of
effective measures.
PLANT GEOGRAPHICAL STUDY
A plant geographer should have considerable knowledge of ecology
on one hand and of taxonomy on the other. His work must be
ecologically based in its analysis of environmental factors as an
outcome of which he can tell, for example, why such and such a
plant community is restricted to such and such an area ; and above
all it must rest upon a sufficiently precise taxonomy. ‘Those are
the immediate prerequisites of plant geographical study. For sound
ecological knowledge some understanding of a wide range of basic
sciences even outside of biology is necessary; as for precise
taxonomy, even closely allied and superficially similar plants can
have very different reactions to environmental conditions, and so a
knowledge of the local flora in the particular area of interest is
virtually essential.
The methods of the modern plant geographer must accordingly
be as scientifically based and exact as is humanly possible. Admit-
tedly a good deal may be done by merely determining what grows
where. For this the main requisite, apart from intensive field
investigation, is to know precisely what we are dealing with—namely,
the identity of each particular plant in question, coupled with the
ability to determine that it is essentially the same ‘ kind’ throughout
the ascertained range. But for interpretation and application of
such observations we must have far wider knowledge and biological
understanding—or, instead, an empirical system which could scarcely
be worked out in sufficient detail for general use.
The ecological reactions of closely-related species or even races
of plants can vary markedly, as can, accordingly, their ranges both
individually and as components of vegetation, so the plant geographer
must be able to recognize both habitat and taxonomic differences.
For such ecological items his knowledge of soils and climates and
of their component factors is particularly important, while as tools
572 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
for making taxonomic studies he should have available a good
regional ‘ flora’ and a well-arranged herbarium in which are filed
accurately-labelled specimens, dried and mounted on paper sheets,
of as many as possible of the entities involved. Only with such solid
foundations can plant geography really flourish and be of lasting
value to mankind.
Different plant geographers work in different ways, and even the
same investigator may use widely different methods in different
circumstances—according to the current state of knowledge, accord-
ing to when and where he was trained, according to what he has
set out to accomplish, and so on. If, as is often the case, the object
is to determine as far as possible the geographical range of a particular
plant species, the usual method is to have recourse to a major
herbarium. ‘This should be done wherever possible because,
although perusal of appropriate literature may give a fair idea of
where a particular plant is present and where absent, it is nevertheless
desirable to check the range against actual specimens labelled
accurately and precisely with the localities in which they were
collected. Adequate travel for the purpose of determining the areas
of individual plants in the field is usually out of the question—and
unnecessary if a good herbarium is available in which the findings
of sufficient previous collectors are accumulated. Often, however,
it is necessary to visit (or borrow material from) more than one
herbarium for this purpose, bearing in mind the importance of
checking the identity of every specimen on which a fresh * locality ’
is based, for the very best of herbaria will inevitably contain occasional
misidentifications and imprecisions.
Concerning plant ranges it should be recalled that negative
evidence is insecure at best: because a particular plant species has
not been found in (or at least has not been recorded from) a given
region, we must not assume that it does not occur there—unless,
perhaps, a very limited and well-known area is involved.t Nor
may we conclude, without detailed trials, that the plant in question
is incapable of growing in the area under the conditions obtaining
there. ‘Thus, maps indicating the supposed ranges of individual
species can be misleading in suggesting absences which may not
exist. On the other hand, such maps are valuable especially when
showing (by spots) the definitely known stations. For these purposes
various outline maps are available ; but they must be ‘ spotted ’ with
‘Such instances as that illustrated in Fig. 62 should act as a warning in this
connection.
18] PLANT ADJUSTMENTS AND APPLICATIONS 573
the greatest possible care, and preferably from authenticated speci-
mens in view of the frequency with which literature citations, recol-
lections, previous identifications, and so forth are prone to err. In
this, again, all determinations should be checked.
Allied studies of, for example, the dispersal methods of particular
species, or the distribution of the more complicated communities
which they make up collectively, usually involve ad hoc field investiga-
tions. Of such studies more and more are needed. In most major
populated regions there are nowadays herbaria! with staffs who will
aid in determining plant specimens, if necessary by correspondence,
provided a specimen labelled with the place, date, and collector’s
name is submitted. It is when we attempt to study the more
complex communities which make up vegetation, that lasting personal
contact is virtually essential. Yet such study is both instructive and
rewarding, as vegetation affords a fine indication of local conditions.
This was demonstrated in the last chapter.
FURTHER APPLICATIONAL POSSIBILITIES
Apart from the practices of agriculture and forestry which to a
large extent are ecologically based, conservation of natural resources
such as forests and grasslands probably constitutes the most important
application of ecology, which itself is to a considerable degree plant-
geographical in basis. Modern wildlife and fisheries management
should, however, not be forgotten in this connection. Consequently
the preservation of natural areas for ecological and allied study is
important, and also has its significance for plant geography. With
the current disturbance of so many tracts, frequently so drastically
that indigenous plant indicators disappear and there may be scarcely
any native plants left, it is often only through the ecological or
phytogeographical study of ‘ preserved’ areas that the most appro-
priate schemes of land-utilization and conservation can be worked
out and applied. Such areas are, moreover, the ‘controls’ by
which the effects of Man’s modification of surrounding tracts can
be properly judged. For, to quote Sir Arthur G. ‘Tansley (in R. S.
Adamson’s The Vegetation of South Africa, 1938), ‘A knowledge of
what nature produces when she is left to herself is one of the indis-
pensable requisites of wise exploitation.’
1 These are listed, usually with details of staff, etc., for all portions of the world
in the Index Herbariorum published periodically by the International Association
for Plant Taxonomy, Utrecht, Holland, and sent free to all members of that
Association.
574 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
The introduction of exotics—for crops, mere convenience, or
aesthetic or other purposes—is an application of importance. Species
introduced into a new region or at all events a new environment
may vary in their fate from complete failure of growth to becoming
pests and largely ‘ taking over ’ their new locale, after the manner of
the Prickly-pear in Australia, though this is usually only where
(and not much longer than) there is drastic disturbance by Man.
Nevertheless introductions should be effected with care, preferably
after appropriate trials have been made. ‘The result may of course
greatly extend the area inhabited by the species concerned. ‘Thus
many of the finest coniferous forests of Europe are now of Douglas
Fir or other western North American species, and some considerable
plantations in the eastern United States are of Scots Pine. In
making such introductions it is prudent to consider not only the
conditions in the lands concerned but also the ecological economy
of the individual plants. For example, certain American trees thrive
only in mixed stands, and so application in America of the pure-
stand tendency of European forestry is liable to be unsuccessful
where these species are concerned.
In connection with plant introduction, consideration of what are
known as ‘ agroclimatic analogues’ may be valuable. ‘These are
based on the principle that a given variety tends to be very similar
in its phenological behaviour (that is, with regard to such weather-
affected activities as the times of flowering and fruiting) in areas of
similar climatic and latitudinal conditions, the analogues being areas
that are sufficiently alike in these features affecting crop production
to offer a fair chance for success of plant materials transported among
themselves. Elements of comparison in determining these analogues
are the mean monthly and yearly temperatures, absolute minimum
and maximum temperatures, average monthly, seasonal, and yearly
precipitation, precipitation-evaporation ratios, length of frostless
periods, and latitudes. In addition, soil and other features have of
course to be taken into consideration when making trials. Fig.
184 indicates the application of such comparison to the Ukraine
in terms of United States districts, with distinction between year-
round analogues (for winter and spring crops, the appropriate State
being named in slanting type) and April—October analogues (for
spring crops only, the State being named in vertical type). In other
such maps available at the American Institute of Crop Ecology,
Washington, D.C., the winter and spring crop analogues may be
separated. Consideration of latitude and the time of year takes into
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account the photoperiodic demands of some plants for a particular
length of day for such vital activities as flowering, and the special
needs of others for, say, water which in particular regions tends to
be available at particular seasons.
This matching of geographical areas on the basis of purely physical
data should be verified by biological means, such as are provided
by phenological records of similar plants growing in the two or more
areas concerned. ‘The plants used for such verification should if
possible be pure-line varieties or clones (that is, derived vegetatively
from a common ancestor) ; if the phenologies of given varieties of
Wheat, Potatoes, etc., are more or less similar in climatically and
latitudinally analogous areas, we then have our biological check that
these areas are agroclimatic analogues. Such preliminary tests may
conveniently be effected by exchange and co-operation among
agricultural experiment stations, as may the next desirable stage,
namely, establishment of uniform field trials of selected varieties.
Nor need there be limitation to two areas in this connection ; rather
is it desirable to have a world-wide indication of all areas of similar
phenology, meteorology, and latitude. ‘This will give no guarantee
of successful introduction between the analogous areas, plants and
ecological factors being what they are; but it could act as a useful
guide and save much time and effort, seed and money, which other-
wise can easily be wasted. A simpler device that may already save
some disappointment in introduction is the climograph. ‘This is a
diagram constructed by plotting the mean monthly temperatures at
a point against their precipitation or humidity, and connecting the
twelve points for the year by a closed line. If approximately
coinciding climographs are given by two regions, at least for critical
periods of the year in so far as climate is concerned, it may be
considered worth while to try plant introductions from one to the
other.
Many countries and states attempt to prevent the spread of plant
diseases into new areas by imposing rigid restrictions on the importa-
tion of living plant materials, for these often carry diseases. It is
particularly in fresh areas, or with species or strains that are unac-
customed to their ravages, that the greatest devastation by plant
diseases is likely to take place. A striking example is the virtual
extermination of Sweet Chestnut trees from eastern North America
by a blight introduced from China where it has long attacked with
far less effect a different but related species. Much the same
principles obtain, and quarantine or other restrictions are imposed,
18] PLANT ADJUSTMENTS AND APPLICATIONS Laly,
in connection with human and domestic animals’ diseases, so many
of which are caused by Bacteria.
In wildlife management the most effective activity for increasing
numbers is often improvement of the habitat, and in fresh waters
the effect of adding fertilizer or manure (as in European carp-ponds)
may be remarkable. The same can be true in salt waters, as was
demonstrated during World War II in Scottish fiords where the
addition of nutrients led to the extraordinarily rapid growth of Plaice
and other Fishes. Many agricultural, horticultural, and forestral
practices are comparable with this to the extent that the habitat is
commonly prepared for some particular crop or other plant—which
is usually an exotic form and, very often, unable to persist without
the maintenance of some special man-made habitat. With this
principle of habitat preparation and maintenance, with cultivation and
tillage, we are already familiar; but it should be re-emphasized
here as it is largely applicational and, moreover, can and does lead
to vast extensions in the ranges—albeit artificial—of many plants.
As an extreme case Man is able, with the knowledge born of sufficient
experimentation, to create in the laboratory or greenhouse almost
any environment anywhere, so long as he cares to pay the necessary
price.
Particular plants or vegetation-types often give a useful indication
of the geological substrata on which they grow. For example, faults
in various rocks or different strata of sandstone can exhibit marked
floristic or vegetational differences. ‘his may extend to questions
of presence or absence of valuable minerals and, where no other
than floristic or vegetational changes are visible at the surface, may
be of obvious use in prospecting. In general, however, this subject
has been little stressed except in particular instances and places, so
that the field lies wide open for more precise observation and
application.
Of indicator plants—or, better, groups of plants—there are many
that can be valuable for agricultural or other planning. Examples
include the Cacti and ‘ desert’ shrubs that indicate overgrazing in
many parts of the American southwest, or the Sheep Sorrel and
other small weeds that indicate a similar condition in many cool-
temperate areas. Further indicators of similar or other conditions
are mentioned in almost any modern ecological or allied text-book,
and especially in the works of Clements cited at the end of this
chapter. Indeed, most of the important habitat conditions have
their plant indicators, even though we may not think of them
578 INTRODUCTION TO PLANT GEOGRAPHY [CHAP.
in that light, and they may be only local as emphasized in the next
paragraph.
If, as we often do when walking over a meadow, we come upon a
clump of coarse Rushes, this is a strong indication of damp con-
ditions ; if the Rushes are in a temperate region and accompanied
by such hygrophytes as Marsh-marigold (Caltha palustris agg.), this
combination can be taken as a sure sign of lasting percolation or of
a water-table near the surface. Conversely, certain Heaths and
Lichens are indicative of dry conditions, and certain forbs of a
calcareous substratum—at least in many parts of the world. Whereas
a single indicator species may be usefully suggestive, it is far better
to have a group of them, which with suitable experience may be
taken as virtually infallible. But even as the flora and vegetation
change in different areas, so may such indications, which individually
are chiefly of local use; consequently precise local knowledge is
needed for their application.
Accordingly it would seem superfluous in this very general treatise
to multiply the examples. Suffice it to repeat that land-use science,
employing such methods as plant indicators, can be of very great
importance to the modern world.
Of other plant geographical or, often, primarily ecological principles
and findings that have wide applicational value there are very many
—as indicated, for example, in the concluding sections of Professor
H. J. Oosting’s work cited at the end of this chapter, though the
examples he gives are mainly North American. ‘These principles
include many that are widely employed: in horticulture, for
cultivated plants are as subject as any others to ecological laws ; in
agriculture, where so many of the practices, however familiar and
widespread, are basically ecological; in pisciculture, where the
feeding and maintenance of a rich phytoplankton is all-important ;
in silviculture, e.g. towards deciding whether to maintain temporary
forests of a successional nature or to let the stands develop naturally
towards the climax ; in reforestation of suitable areas (after deciding
which areas are suitable) and revegetation of exposed soils, where
special autecological and local synecological knowledge are normally
essential for success ; in range management, which is largely applied
ecology, though it should be recalled that the most effective practices
are in general those which least disturb the natural balance between
the grassland and its environment ; in pasture and various cropland
choice and treatment, e.g. in relation to suitable indicators and
rotations ; in general land-management, which raises the questions
18] PLANT ADJUSTMENTS AND APPLICATIONS 579
of whether our single-cropping and maintenance for it of open-soil
conditions is really sound, involving as it commonly does the
destruction of soil structure by deep ploughing ; in weed and
disease control, where autecological studies suggest the possible
effectiveness of more judicious land management as well as applica-
tion of selective herbicides ; in conservation, where vegetative cover
is the most effective means of checking erosion ; in water-supply
and wildlife management, where the ecological problems are almost
endless ; in landscaping, where knowledge of the habitat require-
ments of the species concerned is fundamental ; and so on—almost
ad infinitum. Indeed we may suggest without serious fear of con-
tradiction that it is largely, and perhaps in the long run only, through
the enlightened application of effective research in the plant sciences
sensu latissimo that Man can continue to keep abreast of population
increases and feed and clothe the world’s teeming millions.
It may be claimed with good reason that this is the age for the
biological scientist : for whereas the physical sciences have already
achieved a high degree of consistency, in the life sciences our know-
ledge is often still no more than meagre and our understanding
rudimentary (yet the importance of learning more and more is
enormous, as indicated above). And so it is that in plant geography
and some of its relatives, our analytical perception is often poor
and many fundamental principles doubtless still await discovery.
Whether, indeed, with the changing drifts of life, this situation will
ever be fully remedied, remains to be seen. But already there is a
vast body of knowledge which is being added to all the time, and
the present book has as its main object the sifting of this knowledge
and introduction to the principles that emerge.
ADDITIONAL READING
It has been customary to conclude previous chapters with lists of
pertinent works or suggestions for further reading. In the present
instance any good modern text-book of genetics and plant breeding, such
as E. W. Sinnott, L. C. Dunn & T. Dobzhansky’s Principles of Genetics,
fifth edition (McGraw-Hill, New York etc., pp. xiv + 459, 1958), or
M. B. Crane & W. J. C. Lawrence’s The Genetics of Garden Plants, fourth
edition (Macmillan, London, pp. xvii + 301, 1952), should suffice to give
the needful student some background to the cognate facts and principles
on which we have (barely) touched.
The ecological items were dealt with in Chapters X and XI, at the ends
of which further reading was suggested. Concerning erosion and its
580 INTRODUCTION TO PLANT GEOGRAPHY
control, such works as H. H. Bennett’s Elements of Sotl Conservation,
second edition (McGraw-Hill, New York etc., pp. x + 358, 1955) and
Paul B. Sears’s Deserts on the March, revised edition (Routledge & Kegan
Paul, London, pp. x1 -+ 181, 1949) are to be recommended as both vivid
and readable, while John D. Black’s more recent Biological Conservation
(Blakiston, New York & Toronto, pp. xiv + 328, 1954) deals with other
practical aspects and has a useful annotated bibliography.
On the subject of indicators, F. E. Clements’s Plant Indicators (Carnegie
Institution, Washington, D.C., Publication No. 290, pp. xvi + 388,
1920) is still the standard work, although an abridged version appears
in his Plant Succession and Indicators (Wilson, New York, pp. xvi + 453,
1928). Chemical control of plant growth, etc., is dealt with in L. J.
Audus’s Plant Growth Substances, second edition (Leonard Hill, London,
pp. Xxli -+ 553, 1959), and applied ecology in H. J. Oosting’s The Study of
Plant Communities, second edition (Freeman, San Francisco, Calif., pp.
Vili + 440, 1956)
Additional reading on particular topics has been suggested at the
end of almost all previous chapters under the heading of ‘ Further
Consideration ’.
INDEX
References in heavy type are to illustrations. For international understand-
ing and accuracy, scientific (ztalicized Latin) plant names are chiefly used, the
English or other equivalents being cross-referenced to them. The full extent
of such vernacular names is indicated in both text and index by Capitalization
of Initial Letters, the name-sequence being retained in the index — usually
without cross-referencing. Thus ‘ Mountain Sorrel’ will be found so listed
(and there referred to its Latin name, Oxyria digyna), and not under ‘ Sorrel,
Mountain’. Other listings are generally treated similarly. ‘To obviate the
need of a glossary, technical terms are listed in the index and explained in the
text—usually on introduction.
‘A’ stratum in tropical forest, 425
“A” zone in soil, 297, 299
Abaca, see Musa
Abies (Fir), 343, 345, 376
A. balsamea (Balsam Fir), 346
A. concolor (White Fir), 376
A. lasiocarpa (Alpine Fir, Subalpine
Fir), 346, 376
A. sibirica (Siberian Fir), 346
A. spectabilis, 295
Absolute relic, 200
Absorption of solutions, 12
Abyssal-benthic zone, 512, 513
Abyssinia, 253
Acacia (Wattle), 247, 274, 275, 318,
358, 441, 443, 446, 447
Acaena, 417, 419, 420
A. adscendens, 419-21
Acanthaceae, 124
Acanthosicyos horrida (Desert Melon),
45°
Acclimatization, 81,
Accretion, 369
— /erosion balance, 369
Acer (Maple), 68, 102, 105, 246, 268,
340, 341, 343
—, fruit of, 105
A. campestre URield Maple), 340, 342
A. saccharum (Sugar Maple), 269,
308, 341
Acetabularia, 528
A. acetabula (A. mediterranea), 530
Acetic acid, 268
Acid (acidic) soils, 303, 318, 359, 373-4
— water, microflora of, 478, 501
Acknowledgements, Xvii—xix
Aconitum (Aconite, Monkshood), 264
Acorn, 258
Acorus calamus Can flag), 275
Adamson, R. S., 57
Adansonia digitata (Baobab), 446
176, 219, 220, 563
Often, too, they are illustrated.
Adaptations, 7, 16, 75, 82, 93, 94, 97,
128, 449-52, 562-3
— structural, 81-91, 432-4
—, vegetational, 81, 565-6
Adaptive response to environment, 16
Adder’s-tongue, see Ophioglossum vul-
gatum
Adiantum caudatum
89, 121
Adjustments, man-made, 7, 563-4
Adlittoral zone, 527
Adverse conditions, survival in, 78
Aeluropus littoralis, 370
Aerating roots (pneumatophores), 455,
456, 461, 463
Aeration, 86, 87, 298, 305, 343
(Walking Fern),
—, features promoting, 86, 87
—, internal, 86, 87, 505-6
— of soil, 302, 305, 501
—, root-growth and, 82
Aerenchyma, 87, 505, 506
Aerial roots, 433, 434, 435, 436, 470
Aeroplankton, 31
Aesculus (Horse-chestnut), 343
Aesthetics, 277
Afforestation, 7, 566
Africa, 148, 156, 187, 196,
235, 239, 260, 446
—, forest area of, 246, 247
—, monsoon forest in, 439
, origin of flora, 171
, Savanna in, 444
—, savanna-woodland in, 442
229, 233
semi-desert scrub in, 447
thorn-woodland in, 443
—, tropical rain forest in, 423
African Mahogany, see Khaya_ sene-
galensis
— Oil Palm, see Elaeis guineensis
Agar, 41, 259, 265
581
582
Agaricus (Psalliota) campestris (Mush-
room, Pasture Mushroom), 100, 259,
Agarum, 36, 37, 537 [282
Agathis australis (Kauri), 353
Agave (Sisal), 235, 270, 355, 366, 447
Age and area hypothesis, 177, 182—3, 209
of plants, 79, 205
— of world, 128, 145
Aggregation of germules, 323
Agnew, A. D. Q., xix
Agriculture, 3, 551, 555-6, 560, 564,
, earliest known, 254 [573-9
—, regions favourable to, 315
Agroclimatic analogues, 574, 576
Agropyron (Couch-grass), 248, 249, 361
A. repens (Couch-grass), 222, 249
Agrostis, 364
A. antarctica, 419
Aids to dispersal, 5, 97-125
Air cavity, 110
— currents, dispersal by, 99, 106, 418
— dispersal in, 99-101, 163, 181
— travel, dispersal by, 119, 120
—, upper, 100, IOI
Airborne pathogens, 251
Akpatok Island, 537
Alabama, 160
Alaria, 36, 37, 536
A. dolichorhachis, 534
Alaska, 382, 387, 388
—, former land-bridge to, 171
—, ‘ice-bloom’ in, 490
—, middle-arctic region in, 382, 387, 388
—, Potato grown in, 231
—, trees in, 543
Alberta, 343
Alcohol, 255, 269, 270, 275
Alcoholic fermentation, 260-78
Alder, see Alnus and Alnus glutinosa
Ale, 260
Alectoria, 395
Alekhin, V. V., 23
Aleppo Oak, 274
— Pine, see Pinus halepensis
Aleurites (Yung, Tung oil), 235, 265
Alexopoulos, C. J., 74
Alfalfa, see Medicago sativa
Algae, 28-41, 145, 146, 278-9, 422, 458,
473,478, 481, 485, 486, 489, 492-503
—, antarctic, 416, 417, 418, 538
, arctic, 39, 478, 490, 520, 535, 536-8
, arctic hydroseres and, 406
, attachment of, 475, 498, 514, 524
—, attachment organs of, 494, 503, 528
belts in eulittoral, 495
-, benthic, 491, 492, 493, 494, 522, 538
biological types of, 527, 528
‘ blooming ’
—, brine, 502
—, browsed by Molluscs, 524
—, calcareous, 495, 529, 538
of, 29, 480-2, 490, 520
INDEX
Algae, Cambrian, 131
—, cavern, 546
—, depth of water inhabited by, 485,
486, 496, 503, 522, 530
—, dispersal of, 106, 117, 513, 514
—, drought-resistant, 534
—, economic use of, 38, 259, 278-9
—, epiphytic, 434, 492, 499-500, 524,
530-3, 537
—, fossil, 131-2
, heat-tolerance of, 495, 499
—, life-forms of, 527-8
, light relations of, 325, 474, 475, 495,
496, 522
—, littoral, 523, 526-7, 531, 550
, living within shells etc., 494
, Manuring and, 524
, marine, attachment of, 514, 528
, —, conditions of life of, 508-29
, —, distribution in depth, 39, 512, 537
, —, dispersal of, 106, 107, 510, 513-
, —, drying and, 526 {14
—, —, dwarfed, 508
,—, ecological grouping of, 528
, —, light requirements of, 512-13
, —, migration levels of, 522, 524
, —, periodicity etc. of, 524, 528, 537
, —, temperature relations of, 510
, —, tropical, 458
, —, zonation of, 526-7, 532
, microhabitat conditions of, 322
, nutrition of, 29, 32, 37, 39, 476
—, parasitic, 524
—, periodicity of, 475-6, 528, 531-2,
—, pH of water and, 510, 523 (537
—, planktonic, 333, 486
—, pre-Cambrian, 129, 130, 146
— as primary producers, 278-9
—, reproductive periods of, 510, 532
, salt-marsh, 368, 369, 370
, — -tolerant, 502, 508-9
, seasonal maxima of, 481-2
—, soil forms of, 304, 333
, snow-patches and, 405
, summer maxima of, 481
, symbiotic, 495, 524
, temperature relations of, 77, 499,
510, 537
—, terrestrial, 29, 32, 34, 365, 464
—, water-blooms of, 29, 480, 481, 482,
490, 520
—, wave action and, 495, 524, 525, 532
Algal limestone, 130, 132, 279
Algaroba, 258
Algin, 38, 265
Alien flora (aliens), 118, 119, 210, 219, 466
— —, strong competitors and, 221
— —, survival of, 120, 220
— —, undisturbed ground and, 220
Alkanna, 274
Alkali-grass, see Puccinellia
INDEX
Alkali-grass, Creeping, see Puccinellia
— pans, 369 [ phryganodes agg.
— soil, 366
Alkaline water, survival in, 499
Alkaloids, 263, 266
Allee, W. C., 17
Allium cepa (Onion), 91, 258, 282
Allochthonous ooze etc., 496, 497
Allogenic effects etc., 304, 370
Allopolyploids, 179
Allopolyploidy, 204
Allspice, 261
Alluvial cones, 544
— fans, 544
Almond, 258
Alnus (Alder), 325, 342, 344, 372, 373
A. crispa agg. (Green Alder), 392
A. glutinosa (Alder), 340
Aloe, 5, 366
Ald, 355, 366, 447
Alopecurus, 364
Alpine area, diurnal fluctuations in, 11
— Arnica, see Arnica alpina s.1.
— Bearberry, see Arctostaphylos alpina
age.
— Brook Saxifrage, see Saxifraga rivu-
laris agg.
— Chickweed, see Cerastium alpinum s.1.
— Fir, see Abies lasiocarpa
— Forget-me-not, see Myosotis alpes-
tris agg.
— Holy-grass, see Hierochloe alpina
— meadow, 410, 412, 413
— plants, 91, 93, 109-14
— —,, in lowlands, 374
— Timothy, see Phleum alpinum s.1.
Seams tundra, 3775 383, 410, 414, 469
Alps, 156
Alternation of generations, 38, 39, 49, 59,
Alternative hosts, 252 [63-4, 69
Altitude, vegetational changes and, 376
Altitudinal area, 188
— effects, 409-15
— —, in tropics, 467-70
— vicariad, 202
Amanita phalloides (Deadly Amanita), 45
Amaranthus retroflexus (Pigweed), 125
Amazonian forest, 247
— region, 423, 430, 444, 462
Amber, 273
Ambrosia (Ragweed), 280
America, 187, 379
—, crop plants original to, 253
American Aspen, see Populus tremuloides
— Beech, see Fagus grandifolia
— Geographical Society, v, xvil
— Western Buckthorn, 264
— Witch-hazel, 264
Ammoniacum, 273
Ammophila arenaria (Marram Grass),
122, 123, 328, 371, 548, 549, 566
583
Amoeba, 42
Amoebic dysentery, treatment of, 264
Anaerobic conditions, 488, 498
Anaesthetic, local, 263-4
Analogue, agroclimatic, 574, 576
, climatic, 574, 575, 576
Ananas comosus (Pineapple), 235, 239,
258, 282
Anastatica hierochuntina
Jericho), 102
Ancyclonema nordenskioldii, 490
Anderson, D. B., 95
ee2 52
Saat De Aza AS.
Andes, 226, 470
, cushion plants of, 413, 414, 470
—., high-altitude vegetation of, 414
—, montane zone in, 470
—, semi-desert scrub of, 447
Andreaea, 418
Andrews, H. N., 153
Andromeda (Marsh Andromeda), 501
Anemometer, 293
Anemone, fruit, wind-dispersed, 102
A. nemorosa (Wood Anemone), 342
Angelica, 262
Angiospermae (Angiosperms), 12, 21
67-73, 143, 144, 145, 149-51
—, aquatic, 483, 484, 485, 494, 503-5
, derivation of, 144
—., fossil, 143, 145, 196
(Rose of
general account of, 67—74
geological history of, 129, 143, 144,
145, 149
—, marine, 527, 528, 529, 530, 531
, Maritime, 107, 533
, mycorrhizal, 72
—, parasitic, 72, 250, 437, 438
, primitive, suggestions of, 139
, saprophytic, 72, 436-7
—, subclasses of, 73
Animal community, 16
— geography, 2, 15-17, 18, 19
Animals, 14-17, 34, 38, 156, 160, 257
—, dispersal by, 27, 111, 112, 113, 114—-
118
—., ecological significance of, 16, 17, 305
—, ecology of, 15
—, plant-like, 2
—, plants influenced by, 15, 304-5
—, response to climate, 16
Anise, 262
Annatto, 274
Annual habit, survival by, 78, 93
— Meadow-grass, see Poa annua
i plants, 78, 93, 217, 327, 355
— —, aquatic, 506
— —, desert, 93,365, 451
— —, grazing favourable to, 364
— —, scarcity in Arctic, 93, 537
— production by photosynthesis, 520
584
Annual rainfall, see Precipitation (annual)
— rings, see Growth-rings
Antagonism, chemical, 79, 80
—, physiological, 80
Antarctic, 148, 166, 380, 415-19, 537-8
Algae, 416, 417, 418, 538
— Bacteria, 417, 418, 419
— circle, 380
— climate, 415
— Continent, 172, 415, 416, 417, 418
— discontinuous range (distribution),
195, 196
—, distribution of plants in, 184
— flora, 72, 415-22
—, forests absent from, 246
— fossil plants, 171, 195
— glaciation, 156, 416
—, ice action on plants in, 537-8
— ice-cap, 414
— land-bridge, supposed, 171-2
— life abundant in sea only, 535
—, poverty of flora of, 416
— Sea, temperatures in, 510
— types of vegetation, 383, 415-22
—, vegetation of eulittoral in, 537-8
Antarctica, 148, 166, 195, 416, 418, 419
Anthrax, 26
Anthropic disturbance, 374
influence, 311
Anthropogenic relic, 200—-1
Antibiotics, 46, 80, 265
Antipodes Islands, 421
Ants, 117, 432
Aphanizomenon flos-aquae, 487
Aphanothece, 28
Aphids, vectors of virus diseases, 117
Aphotic sea-bottoms, 538-40
zone, 473, 518, 538
Apices of stems and roots, 13
Apocynaceae (Periwinkle family), 272
Apomict, 234
Apomictically, 33
Appalachian Mountains, 341
Apple, see Pyrus malus
Applicational possibilities, 542-3, 573-9
Apricot, 258
Aquatic annual plants, 506
— environment, 317, 472-506
— freshwater communities, 472—506
— Fungi, 117, 481
— habitats, 316-20
’ light and, 317, 474-5, 511-13
— organisms, geographical range of,
317, 472, 478-9
— —, sensitive to environmental
change, 509
— vegetation, factors affecting, 317, 474
— —,, periodicity of, 475-6
— —, photosynthesis of, 473, 475, 477
, widespread occurrence of, 472
Arabia, 447
INDEX
Arabian desert, 365, 449
Araceae (Aroids), 73
Arachis hypogaea (Peanut, Groundnut),
234-5, 258, 259, 265-6, 282
— —, world production of, 238
Arceuthobium, 121, 123
Archaeozoic era, 128, 145
Arches, 550
Archibenthic zone, 512, 513
Archil, 274
Arctagrostis, see Arctagrostis latifolia s.1.
Arctagrostis latifolia s.1., 384, 406
Arctic, 100, 174, 224, 323, 380
— Algae, 39, 478, 490, 520, 535, 536-7
—-alpine discontinuous range (dis-
tribution), 189, 192
— America, 537
—, annuals scarce in, 93, 537
— Archipelago, 162
— Avens, see Dryas integrifolia agg.
— barrens, 395-9
— Bell-heather, see Cassiope tetragona
— benthos, 536-7
—, biological spectrum of, 95
— bird-cliffs, 401
—, ‘ blooming’ of Diatoms in, 520
— Blueberry, see Vaccinium uliginosum
subsp. alpinum
— Canada, 382, 384-6, 391-407, 422
— circle, 380
— circumpolar distribution, 184, 185
—, climatic conditions in, 380-1
—, conditions and plants in, 315, 381-2
—, eulittoral zone, ice-action in, 536
— fell-fields, 395-9
—, Ferns in, 62
— Fireweed, see Epilobium latifolium
—, former floras of, 155, 171
—, frost-action in, 177, 323, 395, 396
— heathlands, 392-5, 404
— hydroseres, Algae in, 406
— lithoseres, 405, 407
— Lycopodineae, 57
— marine vegetation, 535-7, 539
— maritime habitats, 399, 400, 401
— Meadow-grass, see Poa arctica s.1.
— Ocean, 127, 388
— —, Bacteria in bottom deposits of, 539
—, open habitats in, 114, 177, 395, 397
—, perennials in, 78
— Phaeophyceae (Brown Algae), 535,
536, 537 é
—, physiographic aspects in, 294, 410
— plants on southern mountains, 164,
210
— Poppy, see Papaver radicatum s.1.
—, precipitation in, 316, 381
—, regional belts in, 382
—, Rhodophyceae in, 39, 535, 537
salt-marshes, 399, 400, 401
— screes, 389, 407
INDEX
Arctic, scrub, 390-2
—, sectors of, 184, 185
— seral types, 405-9
—, short growing-season in, 78
— snow, coloured, 490
—, southern boundary of, 184, 185, 380
— species, in Tropics, 413
— spora in, 99
— - subarctic ecotone, 292, 343-8
— — -arctic sequence, 173
—, succession in, 390, 405-9
—, theory of origin of floras in, 171
—, trees absent from, 380
— tundra, 213, 382-90
— uplands, 410
— vegetation, 62, 381-409, 535-7
—, vegetative propagation in, 79
— Willow, see Salix arctica s.1\.
Arctium (Burdock), 112
Arctophila fulvaagg.(Tawny Arctophila),
Arctostaphylos (Bearberry), 359 [405
A. alpina agg. (Alpine Bearberry), 407
Area (range), 5, 8, 9, 17, 182, 188, 196,
208, 577
— of world under forest, 246, 247, 556
— patterns, 9
—, potential, 9, 175-7
—, types of, 5, 182-212
Arenaria (Sandwort), 116, 412
A. ciliata s.1. (Fringed Sandwort), 402
A. humifusa (Low Sandwort), 198
A. (Honckenya) peploides agg. (Sea-
beach Sandwort, Sea-purslane),
107, 183, 371, 399
Arétes, 546
Argentina, pampas of, 363
—, savanna-woodland of, 442
—, Triassic flora of, 149
Arid climate in past times, 148, 149
— conditions in rain-shadow of moun-
tains, 160
— — in Permian times, 148
— regions, erosional features of, 547
— —, physiographic effects in, 294
Aridity, evidence of, 450
Aril, 113
Aristolochia (Birthwort), 221
Arizona desert and near-desert, 83, 85,
451
Armeria maritima s.\. (Sea-pink), 369
Arnica alpina s.\. (Alpine Arnica), 408
Arnold, C. A., 145, 153
Aroids, see Araceae
Arrowroot, 269
Artemisia (Wormwood), 173, 365
A. tridentata (Sage-brush), 364
Arthrophyta, 135
Artichoke, 258
Artificial changes in vegetation, 215, 566
Ayes) 274:
— selection, 216, 218, 223, 226, 564
585
Artificial strains, 564
Artocarpus altilis (Breadfruit), 242, 258
Asafoetida, 273
Asclepias (Milkweed, Silkweed), 99, 101,
104, 105, 194, 196, 271
Ascophyllum nodosum, 522
Asexual generation, 38, 39, 59
— reproduction, 32, 37, 39, 45, 64, 69
Ash, see Fraxinus, F. excelsior, etc.
Asia, 187, 192, 234, 235, 355
—, areas under forest, 246
—, coniferous forests of, 345
—, deserts of, 366, 376, 449, 452, 453
—, early cultivation in, 253-4
—— oral On 156
— Minor, Rye in, 226
— —, semi-deserts of, 365
—, savanna-type country in 444
—, trees introduced from, 243
Asparagus, 258
Aspect, ecological, 333, 340
, physiographic, 285, 294, 295, 296,
322, 375, 413, 470, 544
— (seasonal) society, 334, 340
Aspen, see Populus
Aspidium, 264
Assam rubber, 272
Association, 333-4, 335, 349, 392, 529
Associes, 334, 349, 464
Associule, 434
Asterionella formosa, 483
Asthma, treatment of, 264
Astringent, 264
Atlantic elements of flora, 211
Ocean, 157, 174, 249, 516, 533
— —, mangroves of, 457
— -—, woody plants common to both
— Period, 173 [sides of, 243
Atmometer, 291, 293
Atmospheric conditions affecting life, 10
Atrichum (Catharinea), 53
Atriplex (Orache), 371
A. canum (Salt-bush), 365
Attachment, algal, 475, 494, 498, 503,
Attar of roses, 275 [514, 524, 528
Atwood, A. C., 214, 378
Audus, L. J., 13, 580
Aureomycin, 265
Auricularia, 45
Austral zone, 186, 520, 532
Australasia, glaciation in, 160
—, Glossopteris flora in, 149
—, grassland in, 363
—, land-bridge to, supposed, 171
Australia, 63, 194, 196, 197, 247, 257,
263, 264, 379, 446, 448
—, alien plants in, 119, 249, 574
—, area of forest in, 246
—, benthos of, 530, 535
—, deserts in, 365, 449, 45°
—, European weeds in, 249
586
Australia, grasslands in, 360, 363
—, Jurassic flora in, 149
—, mangroves of, 457
Olive grown in, 235
peculiar flora and fauna of, 15, 153
polar origin of floras and, 171
Prickly-pear in, 248, 574
savanna in, 363, 444, 446
savanna-woodland in, 442
sclerophyllous woodland in, 354, 357;
semi-desert bush in, 447, 448 [358
similarities of organisms with S.
America, 170
—, thorn-woodland in, 443
—, tropical forest in, 423, 439
—, warm-temperate rain forest 1 In, 351
Autochthonous ooze, 496, 497
Autogenic effects etc., 304, 370
Autopolyploidy, 204
Autumn plankton, 481, 482, 518
Autumnal coloration, 340
Auxospore, 33
Available water in soil, 301
Avena (Oat), 115, 226
—, world production of, 229
A. sativa, etc. (Oat), 226, 257, 282
Avens, see Dryas and Geum
Avicennia (a dominant of mangroves),
Avocado, 258 [107
Awned fruits, 125, 217
zorella, 417, 419, 420
A. selago, 419, 420
“ B? stratum in tropical forest, 425, 431,
“B’ zone in soil, 297 [438
Bacillariophyceae (Diatoms), 33, 34, 35,
127, 481, 485, 489, 491, 516
—, benthic, 498
—, ‘ blooming’ of, 481, 482, 520
—, bog-water forms of, 501
—, deep-water forms of, 483, 496
—, dispersal on ice-floes, 109, 491
— distribution in depth, 485, 486, 513
—, epiphytic, 500
—, eutrophic water and, 477
—, on floating sea-ice, 34, 109, 127, 491
—, form-resistance in, 518
—, freshwater surf-belt, 495
, gelatinous stalks of, 494
—, geological history of, 132
—, general account of, 33-34
—., light and, 488, 496, 513
—, marine, 491, 515, 516, 517, 518, 520
—, nannoplanktonic, 518
—, neritic province, 515
—, nutrient requirements of, 520
, pigmentation and light for, 488, 496
—, planktonic, 479, 480, 486, 516, 518
— as primary sea-food, 515
—, salt relations of, 474, 502, 508
INDEX
Bacillariophyceae, seasonal occurrence
of, 481-2, 499, 520
—, sheltered by Cyanophyceae, 29
—., silica secreted by, 33, 497
—, suspension in water, 486, 518
—, temperature relations of, 476, 499
—, turgor-adjustments of, 508
Back-crossing, 176
Bacteria, 2, 26, 27, 28, 145, 146, 251
265, 488, 491, 521
—., air-dispersed, 99-101, 491
, anaerobic, 488
—, antarctic, 417-19
, bottom deposits and, 320, 497-8,
539
—, chemosynthetic, 27,
—., cold-resistant, 77
—, coloured, 27, 539
—, decomposition by, 303, 497
— as food for Myxomyecetes, 41
—, formation of ‘end’ substances by,
—, fossil, 130 [497
—, freshwater, chemosynthetic, 496, 498
—, —, planktonic, 480-2, 485-6, 488
—, general account of, 26-8
—, geological history of, 129, 130
—, heat tolerance of, 499
, industrial use of, 261, 278
, lithosere, 326
—, marine, 517, 518, 521, 539
496, 498, 518,
[538
—, chemosynthesis by, 518, 538
—, concentrated in bottom de-
posits, 521, 539
—, —, planktonic, 517, 518, 521, 539
—, —, toleration of low temperatures by,
—, nannoplanktonic, 518 [539
—, parasitic, 437, 491, 496
—, pathogenic, 26, 80 279, 577
— as plant pathogens, 250
—, pre-Cambrian, 129, 146
—, psammon, 495
—, Purple, 27
—, saproplanktonic, 480, 539
—, snow, 419, 491
—., soil-inhabiting, 26, 303, 305, 333
—, sulphur-oxidizing, 27
Bad-lands, 366
Baffin Island, 394, 398, 399, 400, 403, 404,
— — Algae, 478, 536 [408, 536
Baiera, 141
Bailey, L. H., 253
Baked-apple, see Rubus chamaemorus
Balanophoraceae, 101, 436
Balata, 272
Bald-cypress, see Taxodium distichum
Balfour, I. B., 20
Balkans, 157
Balm, 262
Balsa, see Ochroma lagopus s.1.
Balsam, see Impatiens and I. balsamina
Fir, see Abies balsamea
INDEX 587
Balsam Poplar, see Populus
Balsams, 273
Baltic Sea, ice and marginal vegetation
of, 531
— —, low salinity of, 507, 522
Bamboo, 351, 354, 427, 467
Banana, see Musa paradisiaca s.1.
Baobab, see Adansonia digitata
Baragwanathia, 145
Barberry, see Berberis
Bark, 14, 360, 440, 441, 446
Barley, see Hordeum vulgare s.1\.
Barrens, 315, 333, 383, 395, 396, 397,
398, 399, 403, 404, 405, 418, 531
Barriers, closed communities as, 126, 161
—, dispersal and, 111, 120, 125—6, 161
— to migration, 105—6, 111, 125-6, 202
Barringtonia, 460
Bars, 550
Barton, J., 17
Barton, L. V., 127
Basic element of distribution, 212
Basil, 262
Bass (Fish), 477
Basswood, see Tilia and T. americana
‘ Batrachian ’ Ranunculus, see Ranun-
culus aquatilis s.1.
Batrachospermum, 40
Bay (plant), 262
— rum, 275
Beaches, 399, 400, 421, 458, 535, 550
Beans, 233, 258
Bear Sedge, see Carex ursina
Bearberry, see Arctostaphylos
Beard, J. S-, 426, 471
Beaufort, F. L. de, 17, 19
Beaver, 305
Beech, see Fagus, F. sylvatica, etc.
— forest, 313, 334, 342, 553
Beechnut, 258
Beer, 260, 261
Beet, see Beta and B. vulgaris
Beggar-ticks, see Bidens
Belladonna, 263
Bellard’s Kobresia, see Kobresia myo-
suroides
Bellis perennis (English Daisy), 78
Belt-forming Algae, eulittoral zone, 495
— transect, 567
Beneficial characters, 562, 563
Bennett, H. H., 579
Bennettitales, 139
Benthic Algae, 492, 493, 498-9, 529, 534
— environment, 492-500, 521-8
— life-forms, 521, 527-8
— province, 512
Benthos, 325, 326, 333, 473, 491, 493,
529-38
—, attached to larger plants, 494, 524
—, attachment of, 493, 494, 498, 514,
—, Australian, 530, 535 528
Benthos, freshwater,
—, gradation in, 492
—, marine, 529-38
—, proseral, 503
— of streams and rivers, 499
—, substratum affecting, 494, 499, 514,
—, temperature relations of, 499 [528
—, water currents and, 498-9, 525
Berberis (Barberry), 248, 349
Berges Ss 378
Bergamot oil, 275
Berries, 258
Beta (Beet), 73, 115, 241, 258
B. vulgaris (Beet), 231
Betelnut Palm, 266
Betula (Birch), 102, 222, 246, 274, 281,
340, 341, 342, 343, 344, 345, 346,
359, 372, 373, 375, 390, 411
B. ermanit, 342
B. glandulosa agg. (Scrub Birch), 390,
391
B. nana agg. (Dwarf Birch), 173, 390,
B. odorata, 348 [392, 411
B. pubescens s.1., 173
B. utilis, 295
Beverages, 260, 261
Bicarbonates, photosynthesis and,
497, 523
Bidens (Beggar-ticks), 112, 113
Big Bluegrass, see Poa ampla
— -tree, see Sequoiadendron giganteum
Bignonia, 101
Bignoniaceae, 102
Bilberry, see Vaccinium
Billbergia, 433
Binding plants, 328, 503, 569
Bindweed, see Convolvulus
Biochore, 321
Biocoenosis, 321, 431
Biogeographer, 166
Biogeography, 2, 21
Biological factors, Algae and, 524
— spectrum, 94-5
— system, 9
— types Bs Algae, 527, 528
Biology, 79
Biomass, age: 288, 492
Biome, 211, 304
—, Spruce-Moose, Z2TET
Biota, 304, 322, 330
Biotic barriers, 126, 161
— climax, 331
— community, 15, 304-5
— conditions, 8, 15-16
— factors, 182, 283, 304-11, 566-8
— plagioclimax, 360, 361, 363, 460, 465
— relics, 201
Biotope, 321
Biotype, 25, 164, 176, 180, 206, 220
Bipartite area, 185
Bipolar area, 188
325, 491-500
130,
588
Bipolar range (distribution), 188, 196
Birch, see Betula
— forest, 343, 344, 390
Bird-cliffs, 401
— -perch, 402, 403
— -seed, 119
Birds, 3, 16, 163, 424, 558
— dispersal by, 109, 112-15, 117, 163,
—, frugivorous, I14, 115 [420
—., habitat preferences of, 116, 163
—,manuring by, 401, 402, 403
—, speed in flight, 114
Birthwort, see Aristolochia
Bisect, 85, 567
Black, J. D., 580
— -browed Albatross, 114
— Grama Grass, see Bouteloua eriopoda
— Oak, see Quercus nigra
— Sea, oxygenation of water, 509
region, sclerophyllous woodland
— Spruce, see Picea mariana [of, 355
— Walnut, see Juglans nigra
Blackberry, see Rubus
Bladderwort, see Utricularia
Bladder Wrack, see Fucus vesiculosus
Blake, S. F., 214, 378
Blanket bog, 373
Blechnum spicant (Hard Fern), 348
Blindia, 419
‘ Blooming ’ in Algae, 29, 480, 481, 482,
490, 510, 519, 520
Blow-holes, 547
— -outs, 315, 547
Blown sand, 293, 548
Blue-green Algae, see Cyanophyceae
— Gum-tree, see Eucalyptus
Bluebell, see Endymion (Scilla) non-
scriptus
Blueberry, see Vaccinium
Bluegrass, see Poa and P. pratensis s.1.
Bluejoint, see Calamagrostis canadensis
Boehmeria (Ramie), 270 [agg.
B. nivea (China-grass, Ramie), 235
Bog (moss), 54, 333, 347) 373, 500-1
— -bean, see Menyanthes trifoliata
— Club-moss, see Lycopodium inun-
datum
— -cypress, see Taxodium distichum
— formation, 174, 477
— moat, 500, 501
— -moss, see Sphagnum
— -pools, 501
—, surface features of, 501
— -water, characteristics of, 501-2
, Diatoms in, 501
Bogs, 54, 151-2, 314, 373, 498, 500-2
Boletus, 45
Bonpland, A., 21
Border-line organisms, 1, 2
Boreal coniferous forest (see also
Northern coniferous forest), 347
INDEX
Boreal forest, 343, 376
— period, 173, 174
— zone, 186, 481, 520, 532
Boring Algae, 494
Borneo, 454
Boron, plant growth and, 79
Botanical particle, 99, 106
Botany, 1, 282
Botrychium (Moonwort), 61
Bottom deposits in water, Bacteria in,
320, 488, 497-8, 539
— conditions in water, Algae and, 492
Bougainvillaea, 88, 89
Bouteloua, 361
B. eriopoda (Black Grama Grass), 85
Bower, F. O., 281
Boyces Jas.) 250.258
Brachythecium antarcticum, 417
Bracken, see Pteridium and P. aquilinum
age.
Brackish habitats, 316, 457, 472, 516
— water, 369, 472, 492, 502, 508, 523
Bract, 105
Bramble, see Rubus
Brandy, 261
Brassica, 73
B. campestris (Swede, Turnip), 90, 91,
231, 258, 259
B. oleracea (Cabbage, Kale), 43, 233,
234, 258, 259
Brassicas, 282
Braun, FE. L., 347, 379
— -Blanquet, J., 15, 92, 94, 96, 311
Brazil, coffee cultivation in, 243
— nut, 258
—, savanna-woodland in, 442
—, tropical rain forest of, 423
—, — thorn-woodland of, 443
Brazilian Bottletree, see Cavanillesia
— palm oil, 266 [arborea
Brazilwood, 274
Breadfruit, see Artocarpus altilis
‘Breathing’ roots (see also
matophore), 353, 456
Brewing, 3, 263
Brine, Algae of, 77, 502, 523
Brissenden, E., 21
British Columbia, high-alpine meadow
in, 412
— —, Pacific coast forest in, 348
— Guiana, mixed tropical rain forest
in, 425, 426
— Isles, separation from Europe, 172
— —, flora etc. of, 169, 172
— West Indies, 426
Brittle-fern, see Cystopteris fragilis s.1.
Broad Bean, see Vicia faba
— -leafed trees, 68, 292, 337, 343, 349
— - — Willow, see Salix cordifolia s.1.
Brome-grasses, 217
Bromeliaceae (Bromeliads), 433, 442, 443
Pneu-
INDEX
Brook Saxifrage, see Saxifraga rivularis
Broomcorn, 270 [agg.
Broomroot, 270
Brown, W. H., xix
—, Mrs. W. H.., xviii
— Algae, see Phaeophyceae
— earth, 298
— Seaweeds, see Phaeophyceae
Browsing (see also Grazing), by Mam-
mals, 305, 306, 307, 444
—, by Molluscs etc., 524
—, woody vegetation and, 307, 308
Bryophyta (Bryophytes), 24, 49-54, 145,
383, 386, 422
— as cave plants, 546, 550
—, Carboniferous, 145, 146
—, epiphytic, 434, 468, 469
—, fossil, 133, 145, 146
—, none marine, 527
— as Nothofagus associates, 342
— in snow-patch vegetation, 405
— in tundra, 386
Bryophytes, see Bryophyta
Buckwheat, see Fagopyrum
Budding, in Yeast, 44, 45
Buddleia, 193
Buds, 13, 92-3, 437
—, dormant, 425
—, perennating, position relative to soil,
Buffalo, 306 [92
Buffering, 478, 508
Bulb, 69, 91, 93, 355, 446
— geophyte, 94, 361, 440, 441
Bulbil, 57, 61, 69, 103, 121
Buller, A. H. R., 125
Bulrush, see Scirpus lacustris
“ Buna’ forest, 342
Bunch Grasses, 360, 365
Bur-reed, see Sparganium
— -sage, see Franseria
Burdock, see Arctium
Burke, A. F., xviii
Burma, monsoon etc. forests in, 439,
—, swamp-forest of, 461 [467
Burmanniaceae, 436
Burmese Rosewood, see Pterocarpus
Burn succession, 309, 310 [indicus
Burn-scars, infection through, 310
Burnett, J. H., xvii, xviii, xix
Burning, effect on seres, 309-10, 330-1
Bush-land, semi-desert, 364-5, 447, 448
— savanna, 443
Bushy-pondweed, see Najas
Butler, E. J., 250, 253
Buttes, 544, 550
Buttercup, see Ranunculus
Butterwort, see Pinguicula
Buttonwood, see Platanus
Buttressed roots, see Plank-buttresses
“C° stratum in tropical forest, 425, 434
U
589
“C° zone in soil, 297
Caatinga, 442
Cabbage, see Brassica oleracea
Cacao, see Theobroma cacao
Cactaceae (Cacti), 82, 83, 85, 355, 362,
365, 366, 443, 447, 450, 451, 577
—, epiphytic, 434
—, overgrazing and, 360, 362
Caffeine, 260
Cain, S. A., 21, 158, 180, 181, 207, 212
Cainozoic (Cenozoic) era, 129, 143, 145,
151, 154
— floras, 150-1
Cakile (Sea-rocket), 371
Calamagrostis canadensis
Calamitales, 135, 145
Calamite, 136, 148
Calamites suckowi, 136
Calamus oil, 275
Calcareous Algae, 495, 529, 538
— soil, endemism etc. and, 163
Calcicole (calcicolous), 79, 302, 373
Calcifuge, 79, 302
Calciphoby, 79
Calciphyte, 302
Calcium bicarbonate,
and, 130, 497
— carbonate (‘ lime ’), 79, 475, 495, 497
— —., algal colonies bound by, 498
— —, circulation in lake water, 478
— —, deposited by Algae, 130, 132, 529
— —, encrustation with, 39
— —, freshwater content of, 478, 497
— — in lake sediments, 495
— —., precipitation of, 497, 539, 555
— —, sea-water content of, 508
— —, soil content and vegetation, 79
— —, surface deposits of, 547
California, 309, 356
—, Pacific coast forest of, 348
—, sclerophyllous woodland of, 355
— Soaproot, 266
Callitriche antarctica (Water-starwort),
422
Calluna_ vulgaris
Caloplaca, 418
C. marina, 532
Calothrix parietina, 495
Caltha palustris agg. (Marsh-marigold),
57
Calvatia, see (Calvatia)
Cambium, 13 [giganteum
Cambrian period, 129, 131, 132, 145
Camelina oil, 265
Camellia sinensis (Tea), 243, 282
Camels, 367
Campbell, D. H., 20, 378, 471
Camphor, 275, 276
tree, 264, 27
Campion, see Silene linicol
Campos, 444
agg. (Blue-
[joint), 392
photosynthesis
(Common Heather,
[Ling), 358
Lycoperdon
Dge
Canada, 211, 347, 349, 350, 391,
balsam, 273
—, former flora of, 151, 161-2
—, National Museum of, xix
—, prairie in, 360
, Spruce-Moose biome of, 211
—, Wheat belt in, 564
—,— Stem-rust, losses from, 251
Canadian Arctic Archipelago, 162, 384,
394, 398, 400, 403, 404, 406, 408
— Fleabane, see Erigeron canadensis
— Water-weed, see Elodea canadensis
Canary Grass, see Phalaris canariensis
Candelilla wax, 266
Candles, 266
Cane sugar (see also Saccharum offici-
narum), 239, 241
Cannabis sativa (Hemp), 115, 234, 235,
Cannon, W. A., 471 [267, 270
Canopy (roof) of forest, 342, 357, 425,
Canyons, 544 [431, 440
Caper, 262
Capsella bursa-pastoris
purse), 78, 120, 249
Capsicum, 261
Capsule, 49 52, 53, IOI, 103, 104, 105,
Caraway, see Carum carvi [110,125
Carbon assimilation in dark, 498
— dioxide, distribution in lake water,
— — in seas, 508-10 [476-8
— — in soil, 298, 302
— -14, 174, 254
Carboniferous forest, reconstruction of,
148
— period, 129, 133, 135, 138, 145, 146,
— —, Bryophyta of, 146 [168
Cardamine (Cress), 123
Cardamon, 261
Cardinal points of physiological func-
tion 482, 485
Cardiospermum, 110
Carex (Sedge), in alpine meadows, 410
— in bog vegetation, 373
—, dominant in reed-swamp, 505
—, dominant in sedge-meadow, 372
— in high-arctic heath, 395
— in high-arctic marsh, 387
— in middle-arctic tundra, 387
— in low-arctic heath, 393
— in low-arctic tundra, 385, 386
— in tundra, 383
in wet bogs, 501
— Bee ay 385
Carex aquatilis age.
384, 405, 406
C. bigelowti agg. (Rigid Sedge), 383
C. nardina s.\. (Nard Sedge), 392, 396
C. rupestris (Rock Sedge), 385
C. salina s.\. (Salt-marsh Sedge), 399
C. subspathacea (Hoppner Sedge), 401
C. ursina (Bear Sedge), 399
400,
[412
(Shepherd’s-
(Water Sedge),
INDEX
Caribbean region, Pinus in, 345
— —, savanna-woodland in, 442
— —, tropical rain forest in, 423
— —, thorn-woodland in, 443
— -—, warm-temperate rain forest in,
351
— Sea, seaweeds controlled by sub-
stratum, 529-30
Caribou, 48
— -moss, see Cladonia
Carica papaya (Papaya), 242, 258
Carnauba wax, 266
Carnegiea gigantea (Saguaro, Sahuaro),
45°, 451
Carnivorous plants (insectivorous
plants), 87, 91, 305, 373. 484, 485
Carob, 258
Carpathians, 158
Carpel, 67, 68
Carpenter, K. E., 506
Carpinus (Hornbeam), 174, 341
Carpospore, 39
Carrageen, see Chondrus crispus
Carrot, see Daucus carota
Carum carvi (Caraway), 119, 262
Carya (Hickory, see also Hickory-nut
and Pecan), 174, 341, 342
Caryophyllaceae, 417 ~
Cascara 264
Cashew nut, 258
Caspian Sea, a marine relic, 472
Cassava, see Manihot esculenta
Cassia, 262
Cassie oil, 275
Cassine, 260
Cassiope tetragona (Arctic Bell-heather)
392, 393, 394, 395, 403, 404
Castanea (Chestnut, Sweet Chestnut),
258, 274, 341, 342
—, blighted in United States, 251, 305,
Castilla rubber, 272 [576
Castor oil, 266
Casuarina, 358
C. equisetifolia (Ironwood), 460
Catarrh, treatment of, 264
Catbriar, see Smilax
Catch-fly, see Lychnis
Caterpillars, 305, 306
Catharinea, 53
Cattail, see Typha
Cauassu wax, 266
Caucasus, 338, 343
Cauliflory, 425, 438
Cavanillesia arborea (Brazilian Bottle-
tree), 442
Caverns (caves), vegetation of, 546, 550
Caytoniales, 138, 144, 145
Ceara rubber, 272
Cedarwood oil, 275, 276
Cedrela odorata (s.l. (Spanish-cedar),
247
INDEX
Ceiba pentandra (Kapok), 235, 271
Celery, 258
Cell, 12-13, 28, 30, 31, 40, 45, 47
— division, 13, 26, 28, 29, 32, 33, 519-
— -sap, osmotic value of, 452 [20
— types, 12
— -wall, 12, 29, 33, 269, 518
Cellulose, 1, 29, 269, 270, 498
Cenozoic, see Cainozoic
Censer mechanism, 103
Central America, 172
— —, Conifers in, 345
— —, crop plants originating in, 253
— —, food crops of, 231, 233
— —, fibre plants of, 235
— —, mangroves of, 457
— —, savanna in, 444
— —, semi-desert scrub in, 448
— —, subtropical rain forest in, 424
— —, tropical forests in, 439
— —, tropical thorn-woodland in, 443
— Asia, desert of, 365
— —, steppes of, 413
— plain, 492, 493
Centre of dispersal, 207
— of origin of plant groups, 178, 208
Cerastium alpinum s.l. (Alpine Chick-
Ceratium, 34, 516, 520 [weed), 408
C. hirundinella, 478
Ceratophyllum (Horn-wort), 503
Cereal Rusts, 251, 280
Cereals, 218, 223-9, 257, 564.
—, conditions unsuitable for, 231
—, history of, 226
—, world production of, 225, 227, 228,
229, 230
Cetraria (Iceland-moss etc.), 41, 49,
348, 395
Ceylon, alien flora rampant in, 118
—, Coffee Rust in, 251
—, flora of, 169, 172
—, tropical rain forest in, 423
Chaetangium, 531
Chamaedaphne (Leather-leaf), 501
Chamaephyceae, 528
Chamaephyte, 93, 94, 95, 359, 364, 410
Chamberlain, C. J., 74
Chamomile, 263
Change in vegetation with altitude, 376,
Channel bars, 544 [409-15
Chantransia, 496
Chaparral, 355, 356, 357
Chapman, V. Jz, xviul 312, 379, 506,
522, 524, 540
Chara (Stonewort), 30, 31, 32, 503
—, silting and, 503
Characetum, 503
Character, beneficial, 562, 563
— manipulation, 91, 562-4
Charales (Stoneworts), 503
—, Devonian, 132
592
Charcoal, 268, 271
Chard, 258
Chaulmoogra oil, 263
Chayote, 258
Chemical antagonism, 79, 80
— factor, marine vegetation and, 522-3
Chemosynthesis, 27, 492, 493
—, bacterial, 27, 496, 497-8, 518, 538
Chenopodiaceae (Goosefoot family),
173, 366, 368, 369
Chenopodium album s.\. (Lamb’s-quar-
ters), 249
Chernozem, 298, 299, 300
Cherry, 258, 282, 343
— -laurel, see Prunus laurocerasus
Chester, K. S., 252
Chestnut, see Castanea
Chestnut Blight, 251, 279, 305, 576
— -brown soils, 298
— Oak, see Quercus montana
Chewing gum, 266, 272, 275
— materials, 266-7
Chicha, 261
Chick Pea, see Cicer arietinum
Chickweed, see Stellaria and Cerastium
Chicle, 272
Chicory, 258
Chile, deciduous summer forest in, 338
—, sclerophyllous woodland in, 354
—, scrub and woodland in, 358
—, temperate rain forest of, 354
Chili, 261
Chimneys, 547
China, 235, 253
—, Chestnut Blight from, 576
—, deciduous summer forest of, 341
—, flora of, 161
— -grass, see Boehmeria nivea
— and origin of cultivated plants, 253
—, savanna-woodland in, 442
—, subtropical rain forest in, 424
—, warm-temperate rain forest in, 351
Chinese Forget-me-not, see Cyno-
glossum amabile
Chinook wind, 293
Chipp, Te Es 301; 3795 472
Chlamydomonas, 30, 31
C. (Sphaerelia) nivalis, 489, 490
Chlorella, 259
Chloromycetin, 265
Chlorophyceae (Green Algae), 29, 30,
31, 32, 39, 495, 496
—, antarctic eulitorral, 538
—-, arctic etc., 535
—, benthic, 496, 498, 529, 530
—, bog-water forms of, 501
—,. brine-inhabiting, 502
—, Cambrian, 132
—, cryophytic, 491
—, epiphytic, 500
—, forms in rapidly moving water, 498
592
Chlorophyceae
480-1
—, general account of, 29-32
—, habitats of, 32
— as Lichen constituents, 47
—, marine, 508, 528, 529, 531
, —, light relations of, 512
, —, replacing Phaeophyceae, 532
, —, on sand and gravel in Tropics,
—, —, surf-resistant, 535 [529
freshwater plankton,
mud-forms of, 533
nutrition of, 32, 481
salinity etc. and, 502, 523, 533
—, temperature relations of, 499
Chlorophyll, 1, 27, 29, 32, 41, 278, 437
—, bacterial, 27
a industrial use of, 274
—, modified in deep water, 496
Chloroxylon swietenia (Satinwood), 247
Chocolate, 243, 260
Cholera, 26, 279
Chondrus, 40, 522
C. crispus (Irish-moss,
| 41, 259
Chorda, 36, 37
Chresard, 301
Christmas-greens, see Lycopodium
Chromosome, 176, 178, 179, 204, 264
— number, 206
Chromosomes, doubling of, 264
Chroococcus turgidus, 501%
Church, G. L., xviui
Chusquea, 354
Cicer arietinum (Chick Pea), 233, 258
Cider, 261
Cimicifuga foetida, 191, 192
Cinnamon, 262
Circulation of substances in fresh water,
> 477-8, 481,497
Circumaustral distribution (range), 184,
Carrageen),
186, 187
Circumboreal distribution (range), 184,
186, 187
Circumpolar distribution (range), 184,
185, 187
Cirques, 545, 546
Cissus, 437
Cistus (Cistus), 355
Citron, 258
Citrous plants, see Citrus
Citrus (citrous fruits, citrous plants),
ZAD 2505 272% 2715
Cladonia (Reindeer-moss,
moss), 348, 392, 393, 421
C. rangiferina (Reindeer-moss), 421
C. verticillata, 48
Cladophora profunda, 496
Caribou-
Clan, 334
ClarkesiGa Ie 312
Class, 2
— I land, 556-7, 559, 560, 570
INDEX
Class II land, 557, 559, 560, 570
— III land, 557, 559, 560, 570
— IV land, 557, 559, 560, 570
— IVe land, 560
— V land, 557, 559, 560
— VI land, 557-8, 559, 560
— VII land, 558, 559, 560
— VIII land, 558, 559, 560
Classification, 4, 5, 24
— by life-forms, 92-5
— and nomenclature, 24—6
Clay, 298, 310, 545
Clayey soil, 301, 303, 366
Claytonia, 124
C. virginiana
beauty), 340
Clematis, 88, 89
Clements; EF. E-, 311, 3355 37915775 50°
Cliff vegetation, 372, 374, 401, 550
Cliffed shore-lines, 550
Clifton; (Gy Es74
Climate, xvi, 8, 9, 10, 16, 329, 446, 571
—, animals and, 16
—, antarctic, 415
—, cooling of, 155
—, general account of, 9-12
— as master factor, 9-12, 565
—, post-Miocene deterioration of, 160
—, types of, g-10
Climatic analogue, 574, 575, 576
— barrier, 126
— belt, 154, 207, 543
— changes and distribution, 155-7, 160
— climax, 329-30, 331-2, 465
— conditions, 8, 380-1
— factors, 9, 283, 284-93, 297, 472
— features, 8
— limits, 182
— regions (zones, areas), 10, 20
— relics, 201
— types of soil, 297
— variation, 294
Climatology, 10
Climax, 6, 323, 324, 329-32, 390, 566
—, biotic, 331
—, chaparral, 355, 356
— forest, 325, 326, 371, 426, 548
— formation, 337
— grasslands, 359-60
— mangrove forest, 454
—, progression to, 324, 464
—, species number and, 464
— units, 333, 334
Climaxes, main, 329-34
Climbers (lianes, ‘ vines’), 87, 88, 89,
IOI, 305, 339, 341, 342, 343, 351,
354; 355, 431, 440, 443, 460, 468
— of tropical rain forest, 430-1
—, woody, 99, 101, 340, 352, 431
Climbing plants, see Climbers
— roots, 89
(Virginian Spring-
INDEX
Climograph, 576
Cline, 183, 202
Clip quadrat, 567
Clone, 576
Closed community, as barrier to migra-
tion, 126, 161, 165
Clothing materials, 270-1
Cloudiness, 10, 288
Clove, 262
Clover, see Trifolium
Club-moss, see Lycopodium
— -mosses, see Lycopodineae
— -root, 43
Clusia, 436
Co-dominant, 329, 331-2, 351, 367, 428,
Coaction, 324 [463
Coal, 59, 136, 146, 271
Coal Age, 146
Coal-forest, 138, 141
Coastal Redwood, see Sequoia semper-
virens
Coca shrub, 263, 267
Cocaine, 263, 266, 267
Coccolithophores, 518
Cochlearia officinalis s.1. (Scurvy-grass),
Cockayne, L., 379 [399, 401
Cockroach plant, 276
Cocoa (see also Theobroma cacao), 243,
— butter, 266 [260, 282
—, world production of, 244
Coconut, see Cocos nucifera
— oil, 235, 266
Cocos nucifera (Coconut), 107, 110, 235,
242, 258, 282, 460, 461
Coenopteridales, 138, 145
Coffea (Coffee), 243, 260, 282
—, cultivation in Brazil, 243
— Rust in Ceylon, 251
—, world production of, 245
Coir, 270
Coke, 271
Cola (Cola), 260
Cola nut, 266
Colchicine, 264
Colchicum, 264
Cold, 8, 77, 78, 284, 365
— deserts, 316, 414
— resistance, 77, 100
— springs, population of, 499
—, survival in, 77, 78, 99, 100
— -water Fishes, 476-7
Colds and hay-fever, treatment of, 264
Coleochaete, 500
Collective species, 222
Collenia undosa, 130, 131
Colobanthus crassifolius, 417
C. kerguelensis, 419
C. muscoides, 420
Colocynth, 263
Colombia, 414, 423
Colonies, algal, 28, 29, 30, 31, 33
593
Colonization, 6, 48, 81, 504, 505, 548
Colony, plant, 324, 334, 375, 43°
Colorado, 295, 361, 362
Coloured snow, 490
Columbus, C., 181, 266
Comatricha, 42
Common Adder’s-tongue, see Ophio-
glossum vulgatum
— Bean, see Phaseolus vulgaris
— Chickweed, see Stellaria media agg.
— Dandelion, see Taraxacum officinale
silk
— Heather, see Calluna vulgaris
— Mare’s-tail, see Hippuris vulgaris s.1.
— Pea, see Pisum sativum
—Polypody, see Polypodium vulgare
agg.
— Reed, see Phragmites communis agg.
— Timothy, see Phleum pratense
Community, animal, 16, 17
— biotic; 15; 304
Ee plant, 6, 7, 14, 17, 23, 283, 331-5
Compensation point, 475, 512, 519
Competition (see also Root-competition)
81, 98, 126, 201, 220, 302, 324, 349
—., alien plants and, 120, 126, 221
—, cultivated plants and, 216, 310, 564
—, general consideration of, 81
—, harsh environment and, 381
—, natural selection and, 562
—, optimum conditions and, 302
— of seedlings, 103
— of smother crops and weeds, 248, 280
— among trees, 428
— between trees and Grasses, 307, 360
—, weak in open habitats, 163, 310, 374
—, weeds and, 248, 322
Component factors, reaction to, 9
— landforms, landscapes and, 541-3
Compositae (Daisy family), 80, 99, 276,
—, toxic root-excretions of, 80 [366
Condiments, 261
Condition, biotic, 8
Conditions of environment, 8, 9
— — —, arctic, 315, 380-2
— — —, marine Algae and, 508-27
— unsuitable for cereals, 231
Cone, 57, 66, 67
— -scale, 63
Congo, closed forests of, 444
Coniferae, Coniferales, see Conifers
Coniferous forest, 246, 328, 33°, 333,
343-50, 470, 548, 574
Conifers, 62, 63, 65, 66, 67, 73, 141, 145,
146, 292, 341, 343-51, 357, 479,
—, austral, 345, 353, 354 [563
—, bogs and, 500
—, boreal, 343-6, 347, 348
— in Central America, 345
— cultivated in Europe, 243, 246
—, distribution of, 345, 346
594
Conifers, fire and, 310
—, fossil, 141, 143, 145, 148-51, 173
—, importance of, 65-6, 246
—, introduced, 243, 246
—, New Zealand, 353
—, winged seeds of, 102
Conservation of vegetation, 309, 569, 573
Conservationist, 555, 567-71, 573, 579
Consociation, 334, 377, 383, 426, 467
Consocies, 334
Constance, L., 379
Constipation, prevention of, 265
Constructional landforms 541-3, 561
Consumer animals, 257, 480
Contejean, C., 22
Continental climate, 10-12, 158, 174
— drift, 165, 166, 167, 168, 169, 207
— elements, 211
— extreme (of climate), 12
— island, 206
shelf, 511
Continuous intercontinental range (dis-
tribution), 183-7
Convergent evolution, 202
Convolvulus (Bindweed), 248, 249
Cook sWrReilen 72
Cool-temperate marine
Cooling of climate, 155
Cooper, W. S., 412
Copaiba, 273
Copal, 273
Coprinus comatus (Shaggy-mane Mush-
room), 100
Coprosma repens, 420
Coral, 2
— islands, 109
——sLeelsn 500s 5205555
Corallinaceae, 538
Corallopsis, 40
Corchorus (Jute), 234, 235, 270, 282
Cordaitales, 141, 142, 145, 146, 148, 149
Cordaiteae, 145
Cordaites, 142
Coriander, 262
Cork, 279
— Oak, see Quercus suber
Corn oil, 265
Cornus (Dogwood), 346
C. sanguinea (Dogwood), 342
Corsica, 356
Corylus (Hazel, see also Hazel-nut), 173,
C. avellana (Hazel), 340 [541
Cosmetic manufacture, 265, 266
Cosmopolitan range (distribution), 184
Cosmopolite, 184
Cosmos, 113
Cotton, see Gossypium
— -grass, see Eriophorum
Cottonseed oil, 235, 265
Cottonwood, see Populus
Cotula plumosa, 420
vegetation,
[5305
INDEX
Cotyledon, 73
Couch-grass, see Agropyron and A.
Cowan, R. S., xviii [repens
Cowpea, 258, 259
Crab Apple, see Pyrus malus
Crab-grass, see Digitaria
Crafts; AV S:, 253
Crampon, 528
Cranberry, see Oxycoccus
Crane; IVI: Br 579
Cranes (Birds), 114
Crataegus monogyna (Hawthorn), 340
Creeping Alkali-grass, see Puccinellia
phryganodes agg.
— chamaephyte, 94
Creosote-bush, see Larrea and L. tri-
Cress, see Cardamine [dentata
Cretaceous flora, 149-50
— period, 129, 132, 145, 149
Crevice plants, 407
Crevices, extension of plants from, 327,
Critical factor, 75-81, 294 ie
Croizateles san
Cronartium ribicola (White Pine Blister-
rust), 80, 248, 252, 306
Crop plants, original sources of, 223-
239, 253-4
Crops, 6, 73, 215, 374, 454, 556, 557, 564
—, habitat requirements of, 219-22
—, world yield of, 229
Cross-fertilization, 118
Crotalaria juncea (Sunn-hemp), 235
Croton oil, 263
Crowberry, see
nigrum s.1.
Crozet Island, 420
Cruciferae, 412, 417
Crumb structure of soil, 298
Crustacea, 117, 486
Crustaceous (crustose) Lichen, 47 48.
326, 327, 407, 410, 414, 416, 419
Crustose Algae, 535, 538
Cryophytes, 490, 491, 520
—, Bacillariophyceae as, 491
—, bacterial parasites on, 491
—, Chlorophyceae as, 491
—, Cyanophyceae as, 491
—, fungal parasites on, 491
—, wind dispersal and, 490
Cryophytic communities, 489-91, 520
Cryoplankton, 316, 479
Cryovegetation, 489, 491
Cryptogams, swarded (see also Sward-
forming Lichens), 351, 401, 408
Cuba, 442
Cucumber, see Cucumis sativa
Cucumis melo (Melon), 235, 239, 258
C. sativa (Cucumber), 73, 234, 258
Cucurbits (Cucumbers, Melons,
Squashes, Pumpkins, Water
Melons, etc.), 282
Empetrum and E.
INDEX 59
Cultivated plants weak in competition,
216-18, 564
— relic, 201
Cultivation, 222, 282, 556-7, 570, 577
—., effects of, 216-19, 465
—, principles of, 310-11
Culture medium for Fungi and Bac-
teria, 265
Cultures of freshwater Algae, 259
Curare, 264
Curing, 3, 278
Currant, see Ribes
Current action, 498
Currents, 106, 474, 498-9, 508, 514, 525
—, erosion by, 543, 550
Cuscuta (Dodder) 88, 89, 249
C. epilinum (Dodder), 219
Cushion plants, 274, 327, 374, 413, 414,
417, 419, 450, 470
Custard-apple, 258
Cutch, 274
Cyanophyceae (Blue-green Algae,
Schizophyceae), 28, 29, 130, 145,
146, 476, 477, 488, 491, 495
—, acid bog-water and, 501
— as algal element of Lichens, 47
—, aquatic, 319, 486, 487, 500
—, benthic in torrents, 498
—, brine and, 502
—, Cambrian, 132, 146
—, cryophytic, 490, 491
—, desert forms of, 365, 368
—, Diatoms shielded by, 29
—, distribution in depth, 487, 488
—, eutrophic water forms of, 477
—, fossil, 130, 131, 132, 146
— in freshwater plankton, 480
—, gas-vacuoles in, 483, 487
—, general account of, 28-9
—, heat resistant, 499
—, marine, mud-binding, 533
—, —, planktonic, 516, 517
—,—, in supralittoral zone, 530
—, —, and tolerance of exposure, 526,
530
—, mud-medallions formed by, 368
—, phytoplanktonic, 480
— as pioneers, 326, 464
—, pre-Cambrian, 130
red, 488, 517
salinity and, 502
stones inhabited by, 496
surf-belt forms of, 495
tolerance of severe conditions, 499,
501, 502
—, warm-water forms of, 476
water-blooms of, 29, 482
Spee (Tree-fern), 60, 61
Cycadales, 145
Cycadeoid, 140, I41
Cycadeoidales, 139, 145
Un
Cycadeoidea, 140, 141
Cycadofilicales, 138
Cycadophyta (Cycadophytes), 139, 145,
148, 149, 150
Cycads, 62, 63, 64, 65, 67, 139, 151, 152
a elaloteenaG
Cycas rumphii, 64, 67
Cyclic changes in vegetation, 330
— quantitative variation, 526
— stratification in lake water, 477-8
Cyclops, 479
Cylindrocystis brebissonii, 490
Cymbidium, fruit and seed of, 104, 105
Cymodocea 529
Cynoglossum amabile (Chinese Forget-
me-not), 113
Cyperaceae (Sedge family, Sedges, see
also Carex), 333, 410, 430, 443, 464
—, algal epiphytes on, 500
—, bipolar distribution of, 196
—, desert forms of, 366
—, dominance of, 333, 385, 406, 443
—, fibre from, 270
— as pioneers, 405, 406, 407, 464
— in tropical rain forest, 430
— in tropical swamp-forest, 462
— in woodland, 342
Cyperus distachyos, 370
C. papyrus (Papy rus), 269, 461, 50
Cypress, 246
Cystopteris fragilis s.1.
Cystoseira, 530
Cystosphaera, 538
(Brittle-fern),
[420
‘D’ stratum in tropical rain forest, 430
DDT, 276
Dactylis glomerata (Orchard-grass), 259
Dahurian Larch, see Larix dahurica
Daisy family, see Compositae
Dakota Cretaceous flora, 149
Dalyia racemata, 131
Damars, 273
Damask Rose, see Rosa damascena
Damp scrub, 372, 373
Dandelion, see Taraxacum
rubber, 272
Danthonia (Poverty-grass,
grass), 186, 187
Daphnia, 479, 480
Darlington, P. J., 17
Darrah, W. C., 181
Darwin, C., 16, 109, 112, 114, 115, 117,
Dasheen, 258 [127 563
Date Palm, see Phoenix dactylifera
Datura stramonium (Stramonium,
Thorn-apple), 264. 267
Daubenmire, R. F., 312
Daubeny, C., 20
Daucus carota (Carrot), 231, 249, 258
Day-length and effect on plants, 79
Dead end, in evolution, 31
Wild Oat-
596
Deadly Amanita, see Amanita phalloides
— Nightshade, 263
Death, 69, 75, 77; 97, 250, 303, 305, 482
DeCandolle, A., 22, 223, 252
Decay etc., 27, 278, 280, 303, 497-8, 539
Deception Island, 418
Deciduous habit, 337, 338, 339, 340-4
— perennials 452
— summer forest, 332, 333, 337, 338,
339, 340-3, 376, 377
— — —, mixed; /338, 339,340-2
— = —,, types of, 340-3
Decomposition, arrest of, 498
—, by Bacteria, 303, 497, 539
Deep-rooting habit, 76, 82, 84, 85, 450,
451, 458
— -sea system, 511. 512, 538-40
— -water forms of Diatoms, 483
Defensive adaptations, 218
Deficiency diseases, 79-80
Deflected succession, 330, 363, 465
Delesseria sinuosa, 537
Deltas, 454, 542, 544
Depletion of algal population, 482-3
Deposition of calcium carbonate, 130,
132, 497, 529, 547
Depositional features (landforms), 542,
543, 544, 545, 547, 550
Depressions, 327, 348, 361, 362, 366,
367, 383, 384, 391, 393, 416, 547
Depth of water, Algae and, 475, 495,
496, 503, 522, 530, 537
— — —, chlorophyll and, 496
— — —, green plants and, 475, 496,
503, 527, 530, 537
— — —, photosynthesis and, 474-5
Derived savanna, 465
Deschampsia antarctica, 417, 421
Desert, 16, 83, 85, 87, 91, 93, 213, 246,
316, 318, 333, 364-8, 376, 449-54
—, ‘alkali’ (see also Salt desert), 366,
369
— and semi-desert, areas of, 213, 556
— annuals, 93, 365, 451
—, cold, 316, 414
— communities, 11, 366-8, 449-54
—, dew in, 451
— Melon, see Acanthosicyos horrida
— mesophytes, 452
—, mud medallions in, 368
— perennials, 366, 452, 453
— plants, adaptations of, 83, 85, 93, 360,
450, 451, 452
— —, deep rooting of, 84, 85, 450, 451
— —, high osmotic value of cell-sap of,
— —,, transpiration of, 452 [452
—, rainfall etc. of, 364-6, 449-50
— -scrub, 364, 448
— soil, 365, 366, 368, 452
—, spread of, 213, 307
— steppe, 365
INDEX
Desert vegetation, 83, 93, 365, 366-8,
449-54
Desiccation, resistance to, 78, 100, 526
Desmarestia, 538
Desmids (Desmidiales), 31, 481
—, arctic, 478
—, general account of, 31-2
—, oligotrophic etc. water and,
477, 501
—, Palaeozoic, 132
—, planktonic, 479, 481
Destructional landforms, 542, 543-51
Deterioration of climate, post-Miocene,
Detritus, colonization of, 372 [160
Devonian period, 129, 130, 132, 133,
135, 138, 145, 146
— — Charales, 132
— — forest landscape, 147
Dew (see also Precipitation), 289, 451
Diaspore, 97
Diatomaceous earth, 34, 279, 497
Diatoms, see Bacillariophyceae
Dichotomosiphon tuberosus, 496
Dicotyledones (Dicotyledons), 68, 71,
73, 276, 355, 365, 427, 431
—., fossil, 143, 144, 145, 150
—, herbicides and, 276
Dictyota, 36, 37
Didymium, 42
Dielsyaey 235107
Diffuse area, 188
Digestion, germination and, 115, 117
—, seeds and, 114, 115
Digitalin, 264
Digitaria (Crab-grass), 249
Dill, 262
Dinoflagellates (Dinophyceae, Peri-
dinians), 34, 35, 478, 501, 516
—, flotation of, 51
— in freshwater plankton, 480, 481
—, general account of, 34-5
—in marine plankton, 515, 516, 517,
—, nutrition of, 35, 501, 520 [518
—, photosynthesis of, 35
—, summer forms of, 476
Dinophyceae, see Dinoflagellates
Dioscorea (Yam), 231, 258
Diospyros ebenum (Ebony), 247
Diphtheria, 26, 279
Diploid, 179, 204
Dipterocarpaceae, 247
Dipterocarpus tuberculatus (Eng., Ira),
Disclimax, 330, 331, 359 [467
Discontinuity, types of, 188-97
Discontinuous range (distribution), 55,
188, 190, 192-7
Disease, control of, 576, 579
—, extermination by, 250-1
—, treatment of, 265
Diseases, deficiency, 79
—, human, 26, 27, 279
318,
INDEX
Disjunct area (range), 188, 206, 210
Dispersal (of plants), 5, 97-127, 207
—, agents of, 97-127
— in air, 99-106, 163
— by air currents, 99, 101
— — — travel, 119-20
Dispersal of Algae, 106, 117, 510, 514
— by animals, I11, 112, 113, 114-18,
3°07
—, barriers to, 103, III, 120, 125, 126
— by birds, 109, 112-15, 117, 163, 420
—, centre of, 207
— by growth, 121, 122, 123
— by human agency. 118-21
—, ice and, 100, 106, 107, 109
—, intercontinental, 21
— of marine Algae, 106, 514
—, means of, 98
—, mechanical, 121, 122, 123, 124, 125
— mechanisms, 99-103, 104, 105, 106—
LOw DIQs IAT, Lat r22y 1235
124-5
— of seeds and fruits, 15, 69, 97-125
— by water and ice, 106-11, 293, 468
— — wind, 98, 99-106, 293, 434, 452
Displacement hypothesis, 165
Disseminules, 97-8, 219, 374, 500, 510,
—, wastage of, 98-125 [551
—, wind dispersal of, g9—103, 104, 105
Distribution (range, see also Area), 4, 14,
154, 157, 158, 182-4, 186, 189
—, basic elements of, 212
— of Conifers, 345-6
— of crops, 6, 215-54
—, disease and, 250-1
—, factors determining, 154, 563
—, geographical, 8, 154-252, 572
—, historical aspects of, 8
—, in depth, of Algae, 39, 485, 486, 487,
488, 512
—, limiting factors of, 75, 565
—, modern, foundations of, 5, 154-81
—, natural, types and areas of, 6, 182-
aD ALLELE 72.77 eas [214
— of plant diseases, 248, 250-2
— of weeds, 6, 248-53
Distributional and ecological aspects, 23
Distributions (ranges), types of, 183-
Divides, 544 [197
Dobzhansky, T., 579
Doctrine of signatures, 262
Dodder, see Cuscuta and C. epilinum
Dogwood, see Cornus and C. sanguinea
Dominance, 28, 329-34, 351, 468, 565
— in deciduous forest, 337, 341
—, general consideration of, 329
—, mixed, 247, 428
Dominants, 65, 325, 329-34, 34°, 347,
348, 353, 354, 357, 376, 383, 385,
393, 400, 409, 428, 448, 463, 495,
516, 554, 565
597
Dominants, arctic scrub, 392, 393
—, Conifers as, 65-6, 343-50
—, Cyperaceae as, 333, 443, 505
—, Gramineae as, 360-4, 443-6
—, hydrosere, 372
—, Leguminosae as, 441
—, Phaeophyceae as, 532, 533, 534-8
—, Populus spp. as, 343
—, progressive change in, 323-4, 464
—, secondary forest, 466
—, semi-desert plants as, 365-6, 448
—, taiga, 347, 348
—, thorn-woodland, 442
—, tundra, 383-6, 387, 389
Dormancy, seed, 452
Dormant bud, 425
Dorstenia (Flat-fig), 124
Doubling of flowers, 218
Douglas Fir, see Pseudotsuga taxifolia
Drabble, H., 311
Drepanocladus, 503
Drift-wood, dispersal by, 1og
Drimys winteri, 342
Drip-tip, 428
Drooping Ladies’-tresses, see
anthes romanzoffiana
Drosera (Sundew), 91, 196, 373
— sect, erythrorhiza, 196, 197
Drought-enduring plants, 78, 450-2
— resistance, 10-12, 78, 82, 450-2, 564
— — in Algae, 526, 534
— —, survival by, 78, 452
Drude, O., 22, 471
Drug, anti-malarial, 264
Drugs, 3, 262-5, 267
—, medicinal value of, 262-3
Drumlins, 545
Drummond, M., 95
Dry tundra, 385, 386, 389, 392
Dry-weight of crops, 229
Dryas (Avens), 386, 392, 395, 397
D. integrifolia agg. (Arctic Avens),
396, 397, 398 ,
D. octopetala s.1. (Mountain Avens),
Drying oils, 235, 265 [173, 396
Drynaria, 433
Dryopteris (Shield-fern), 60, 61
Du Rietz, G. E., xviii, 164, 167
Duckweeds, see Lemna
Dudresnaya, 528
Dulse, 259
Dunaliella salina, 502
Dunes (see also Sand-dunes), stabiliza-
tion of, 328, 366, 458, 548, 555, 566
—, migration rate of, 548
Dunne. C579
Durvillea antarctica, 535
Dust seed, 101, 434
Dutch Elm disease, 280, 306
Dwarf Birch, see Betula nana agg.
— -pine, Siberian, see Pinus pumila
Spir-
598
Dwarfing of Algae, 508, 523
— of arctic and alpine plants, 16, 380-2,
— of Postelsia, 526 [413
Dyeing, 3, 273, 274
Dyes, artificial and natural, 274
Dynamic equilibrium, 323, 330
— factors, marine vegetation and, 525-8
Dysentery, 26
Dysphotic zone (twilight region), 473,
488, 495, 496, 511, 518
Dystrophic lakes (waters), 318, 476, 477
Earth’s beginnings, 128, 145
Earthworm, 303, 305
East Africa, 318
— Asia, Algae used as food in, 38, 259
, deciduous summer forest in, 337,
338, 341-2
—-—and eastern N. America, simi-
larity in floras of, 161, 167
— —, Maize production in, 229
— —, mixed! forest in, 341
— —, plant migration in, 157, 161, 171
— —, Pliocene flora of, 151
— —, use of opium in, 263
— Greenland Currrent, 293
Indies, 262
Eastern Canada, low-arctic tundra in,
384
—N. America, deciduous forests of,
— Red Cedar, 275 [341
Ebony, see Diospyros ebenum
Eca, see Econ
Ecad, 183, 384
Ecesis, 97, 323, 326
Ecballium elaterium (Squirting Cucum-
Echard, 301 [ber), 122, 123
Eclipsiophyceae, 528
Ecological and vegetational aspects, 23,
333, 349, 579
— applications, 573-9
— bases, 23
— elements, 211-12
— endemic, 206
— factors (see also Environmental con-
ditions), 283-312, 313, 321, 336
— gradient, 183
— grouping of marine Algae, 528
— limitation, 80-1
— niche, 321, 431
— significance, animals and, 16, 17, 304
—— terms) Zt 3315
— vicariad, 202, 204
— vicariads, contrasted habitats of, 204
Ecology, 4, 92, 283, 571
—, animal, 15
—, land use and, 7, 551-73
Econ (pl. eca), 182, 333; 334, 335
Economic botany (see also Cereals,
Crops, Edible Algae, etc.), 3, 4, 62,
66,.72, 255-82
INDEX
Economic botany, aesthetic and orna-
mental aspects, 277-8
——, environmental and
aspects, 276-7
— Crops series of Interscience Pub-
lishers, 282
Ecosystem, 304, 329, 432
Ecotone, 335, 361, 363, 406
Ecotype, 183, 202
Ectocarpus, 36, 37
Ectozoic transportation, 111-14, 116-17
Ecuador, 423
Edaphic (soil) barriers, 126
— climax, 330, 444, 458, 462, 463
— conditions, 8, 126, 296-304
— factors, 182, 283, 296-304, 386
Edaphon, 303, 316, 333, 495
Eddy-diffusion currents, 480, 486, 487
Edible Algae, 38, 259
Edwards’s Eutrema, see
edwards
Eel-grass, see Zostera and Z. marina
Effects of cultivation, 216-19, 556
— of climatic changes, 155-61
Egg, 57, 59, 63, 524
Egg-plant, see Solanum melongena
Egler, F. E., xviii, 457
Egregia, 534
Egypt, 102
Eichhornia crassipes (Water-hyacinth),
86, 87, 98, 106—7, 483, 504
Eider, 402
Ekman, S., 17
Elaeis guineensis (African Oil Palm, see
also Palm oil). 235, 282
Elder, see Sambucus and S. nigra
Element of flora, Atlantic, 211
Elements of flora and vegetation, 210-
Elemi, 273 [212
Elephant Grass, see Pennisetum pur-
pureum
Elephantopus, 113
Elevation, ecological effects of, 294, 409
Elfin wood (forest), 377, 380, 410, 469,
470
Ellesmere Island, 397
Elm, see Ulmus
Elm disease, Dutch, 280, 306
Elodea (Water-weed), 503
E. canadensis (Canadian Water-weed),
106, 475
Eluviation, 298
Elymus (Lyme-grass), 361, 566, 569
E. arenarius s.1. (Lyme-grass), 371,
399, 400, 548
E. giganteus (Volga Giant-wildrye),
548
Embryo, 62, 63, 69, 110
Emersion, 525, 526, 527
— belt, 495
— killing effect of, 495, 526
ecological
Eutrema
INDEX
Empetrum (Crowberry), 196, 332, 347;
348, 395, 407
E. nigrum s.\. (Crowberry), 346, 359,
Encelia farinosa, 80 [392
End substances formed by Bacteria,
278, 497, 539
Endemic areas, 205-6
— relic, 200
Endemics, 163, 205, 206, 210
—, types of, 205, 206
Endemism, calcareous soil and, 163
—, island floras etc. and, 163, 172, 420
Endive, 258
Endozoic transportation, III, 114-15,
rit), 30907
Endymion (Scilla) non-scriptus (Blue-
bell), 342
Energy, 3, 13, 27, 32, 37, 43, 498
Eng, see Dipterocarpus tuberculatus
Engelmann Spruce, see Picea engel-
England, Beech in, 306 [mannit
—, deciduous woodland of, 333, 541
—, Dutch Elm disease in, 306
—, former flora of, 151, 211
—, New, deciduous woodland of, 333
Engler, A., 22, 74; 471
English Channel, 532
— Daisy, see Bellis perennis
Enteromorpha, 368, 502, 524, 528, 568
E. intestinalis, 30, 31, 523
Entomostraca, 479, 488
Environment, 5, 283-322
—, aquatic, 472-506
—, arctic, 315
, benthic, 492-500, 521-8
, change in, sensitiveness of plants to,
509
—, harsh, replacing competition, 381
, land, types of, 16, 313-16, 320-2
—, marine, 507-15
, natural, of life, 482
, physical, 15, 16, 283-303, 472-9,
507-15
—, plant communities indicators of
nature of, 17, 552-5, 577
—, response to, 5, 6, 183, 482
Environmental conditions (factors, see
also Ecological factors), 6, 7, 8, 9,
14, 17, 23, 293-312, 313, 5545 563,
Se
Eocene epoch, 129, 145, 151, 168
Epacridaceae, 358
Ephedra, 63, 65, 67, 173, 264
Ephedrine, 264
Ephemerals, 11, 365-6, 453, 481
—., desert etc., 11, 78, 289, 368, 451, 452
— of desert steppes, 365
Ephemerophyceae, 528
Epibiotic (relic endemic), 205
Epidemic plant diseases, 80,
279-80
250-1,
599
Epilimnion, 477, 480
Epilith, 528
Epilobium (Fireweed, Willow-herb),
IOI, 566
E. angustifolium agg. (Fireweed), 99,
2175220
E. latifolium (Arctic Fireweed), 408
Epiphyllae (leaf-epiphytes), 431, 434
Epiphytes, 94, 103, 305, 322, 340, 342,
351, 352, 354, 430, 431, 432, 433,
434, 435, 437, 438, 441, 443, 469
—, algal, 39, 434, 492, 499-500, 524,
530), 5329537
—, —, communities of, 499, 500, 524
—, —, freshwater, 499-500
—, —, seasonal groups of, 500
—, conditions of life, 432
—, cryptogamic, 339, 340, 468, 500
—, Ferns as, 352, 433, 434, 442, 468
—, forest, 352, 431-5, 460, 462, 463
— of mangroves, 457
—., rain-forest, 432-5
—, structural adaptations of, 432, 433
Epiphytic Algae, 434, 492, 494, 524
— Angiosperms, 352, 353, 432, 433) 434
— Bryophyta, 434, 468, 469
— Cactaceae, 434
— Chlorophyceae, 500
— Ferns, 352, 433, 434, 442, 468
— Lichens, 48, 434
Epizoic transportation (dispersal), 111,
Epoch, 128, 129 [ar
Equilibrium, 315, 323, 330, 359, 361,
381, 384
—, dynamic, 323, 330
Equisetales, 135, 145, 146, 149
Equisetineae (Horsetails), 55, 56, 135,
138, 149
—, fossil, 136, 150
Equisetum (Horsetail,
55, 56, 135, 500
E. arvense agg. (Field Horsetail), 55
E. fluviatile (Water Horsetail), 505
Era, 128, 129, 144, 145
Erdtman, G., xvili, 173
Ergot, 265
Erica (Heath), 332, 355, 358, 373
E. mackaiana (Mackay’s Heath), 197
Ericaceae (Heaths), 12, 72, 79, 332, 358,
373, 374, 392, 501, 566, 578
Erigeron (Fleabane), 408, 409
E. canadensis (Canadian Fleabane),
249
Eriophorum (Cotton-grass), 101, 373,
374, 383, 384, 385, 387, 405, 501
E. angustifolium agg. (Tall Cotton-
grass), 405, 406
E. scheuchzeri (Scheuchzer’s Cotton-
grass), 406, 407
Erosion, 289, 293, 307, 369, 542, 557-
60, 569
Scouring-rush),
600
Erosion /accretion balance, 369
—, agents of, 543, 551
—, control of, 7, 557-8, 562, 569, 570
—, currents and, 550
—, habitats formed by, 544-5
— impeded by vegetation, 51, 555, 562
—,, loss from, in United States, 570
—, overgrazing and, 307, 309, 569
—, run-off and, 289, 555
—., surface-binding plants and, 51, 277
— by water, 568, 569, 570
—, wind, 291, 547
Erosional features, 543, 544-5, 546, 547,
— landforms, 543 [550
— terrace, 492, 493
Erratic boulders, 546
Eryngium, 102
Erythrorhiza section, see Drosera
Escapes, 219, 220, 248
Esker, 543, 545
Esparto, 269
Essential oils, 274-5
Establishment, 97-8, 323
Eucalyptus (Gum-tree, Blue Gum-tree,
Eucalypt), 63, 247, 264, 274, 282,
3575 358, 379
E. regnans (‘ Mountain-ash ’), 379
Eulittoral zone, 512, 513, 527, 529, 537
— — in Antarctic, 537-8
— — in Arctic, 536
— —,, belt-forming Algae in, 495, 530-8
— — in English Channel, 532
— —, freshwater, 492, 493, 494-5
—— ——,, 1Ge-actioniin, 530; 53751530
— — in Mediterranean, 530
— — in Pacific N. America, 534
— — in South Africa etc., 531
Euonymus europaeus (Spindle-tree), 342
Euphorbia (Spurge), 82, 365, 443
—, succulent, 355, 447, 451
E. tirucalli, 83, 85
Euphorbiaceae (Spurge family), 121,
Euphotic zone, 473, 518 (272
am OCEANIC STE AMO SSO
Eurasia, arid conditions in, 272
—, Cainozoic flora of, 151
—,, flora of, migration into Africa, 156
—, former climatic changes in, 153
—, glaciation in, 156
Europe, 172, 192, 196, 233, 263, 485
—, Algae used as food in, 259
—-Asian discontinuous range (dis-
tribution), Ig1, 192
—, Beech forests of, 342
—, British Isles separated from, 172
—, Coniferous forest of, 345
—, Conifers cultivated in, 243, 246, 574
—, continental drift and, 166
—, crop plants originating in, 233
—, deciduous summer forest in, 337-8
—, former floras of, 149, 155, 157, 160
INDEX
Europe, glaciation in, 158, 162
—, mixed forest of, 341
—, montane forest of, 377
—, nunatak hypothesis and, 164
—, oakwoods of, 340
—, plant migration in, 161, 171
—, post-glacial changes in, 173-4
—, similarities to N. America, 170
—, Sweet Chestnut forest of, 342-3
European Beech, see Fagus sylvatica
— flora, poverty of, 156-7
— weeds dispersed to other continents,
249
Euryhaline plants, 474, 508, 515, 523
Euryphotic plants, 474
Eurythermic plants, 474, 528
Euspermatopteris textilis, 147
Eutrema edwardsu (Edwards’s Eutrema),
185, 186
Eutrophic lakes (waters), 318, 319, 325,
462-3, 476, 477, 478, 498, 545
— —, succession from, 477, 502-6
Evaporating power, 10, 11, 289, 321
Evaporation, 291, 301, 369
Evaporimeter, 291
Evapotranspiration, xvii
Evergreen habit etc., 332, 343, 345-50,
424-8, 429, 440, 445, 457, 467-8
— leaf, 63, 354
— Magnolia, see Magnolia grandiflora
— warm-temperate forest, 350-4, 438
Evolution, 5, 7, 14, 24, 31, 128, 562, 563
—, acceleration of, 155, 563-4
, continuous, 128
, convergent, 202
, dead ends in, 31
—, plant communities and, 565-6
KS OVE, Ds igh 7/
sStaticu132
sitheer vOr 12
Evolutionary development (and past
history), 128-53
— history, 8
— trends, 15
Ewan, J., xvii
Exotics, introduction of, 243, 574, 576
Expectorants, 264
Experimental ecology, 321, 568
Explosive mechanisms (dispersal),
Exsiccation, 307 [121-5
Extermination by disease, 250-1
Extractives, 268-70
Extraneous occurrence, 210
Extratropical regions, 22
Exudates, 272-3
Faber, F. C. von, 23, 378, 471, 506, 540
Faciation, 334, 345, 346, 351, 376, 383
Facies, 334, 345
Factor, biotic, 182, 283, 304-11, 566-7
—, climatic, 283, 284-93, 472
INDEX
Factor, critical, 294
—, dynamic, 525
—, ecological, 8, 283-312
—, edaphic, 182, 283, 296-304
—, environmental, 8, 283-312, 321
—., physiographic, 283, 293-6
Factors, compensation between, 320
— determining distribution, 154, 155-
81
—, interaction between, 283, 293-4
— of habitat, 182, 283-312
Facultative halophytes, 368, 369, 370
Fagetum, 334, 342
Fagopyrum (Buckwheat), 239, 257
Fagus (Beech), 68, 173, 174, 306, 308,
313, 334, 341, 342, 343, 553
F’. crenata, 342
F’. grandifolia (American Beech), 341
F. sylvatica’ (Beech, European
Beech), 208, 246, 342, 554
— —, Rabbits and regeneration, 306
Fairy ring, 80
Falkland Islands, 421
Fallowing, 311
Family, 24 (systematic), 324, 334 (eco-
Fatty oils, 205-6 [logical)
Favourable situations, vegetation of, 11
Feather-grass, see Stipa
Feature, climatic, 8, 10-12, 173-4
—, erosional, 543, 544, 546, 547, 550
Fell-field, 213, 246, 333, 383, 395, 396,
397, 398, 399, 404, 410, 412, 470
— - —, arctic, 395-9
Feltleaf Willow, see Salix alaxensis agg.
Fen, 314, 326, 333, 373
Fenland, 273
Fennel, 262
Fermented beverages, 260, 261, 278
Fern-allies, 138
Fernald, M. L., xviii, 186, 190, 192
Ferns, see Filicineae
—, epiphytic, 434, 442, 468
Fertility of desert soil, 452
Fertilization, 15, 49, 52, 56, 63, 68, 70,
Fescue, see Festuca [71
Festuca (Fescue), 364, 365
F’. erecta, 420, 421
F’. kerguelensis, 419
F.. rubra s.1. (Red Fescue), 369
Fibre plants, 234-5, 270-1
Fibres, 3, 235, 270-1
Fibrous husk, 110
Ficaria verna (Lesser Celandine), 340
Ficus (Strangling Fig), 435, 436
F. carica (Fig), 242, 258
Field experience, importance of, 553,
— Horsetail, see Equisetum arvense agg.
Field layer, see Ground layer
— Maple, see Acer campestre
Fig, see Ficus carica
601
Fiji Islands, 423
Filaments, 28, 29, 30, 31, 33, 35, 43, 45;
_ 13%, 494, 495
Filicales, 138, 145
Filicineae (Ferns), 59, 60, 61, 62, 138,
145, 146, 342, 348, 354, 420, 430,
442, 462, 464
—>) abctic, 62
—, epiphytic, 352, 433, 434, 442, 468
—, fossil, 138, 145, 146, 149
—, general account of, 59-62
—, habitats of, 62, 434, 468, 544, 546,
550
—, spores of, 59, 100, 125, 434
Filmy-fern, see Hymenophyllum
Finger-lakes, 545
Finmark, 344
Fir, see Abies
Fire, beneficial effects of, 310
— climax, 441
—, competition after, 310
—, Conifers and, 310
—., deflected succession and, 465
—, ecological effect of, 305, 309, 310,
350, 352, 359, 441, 444
—, grassland and, 359, 363, 444, 447
—, importance as ecological factor, 309-
—., persistent grassland and, 444 [310
—, selective action of, 310
—, stimulation following, 310
Fireweed, see Epilobium and E. angusti-
folium agg.
Fish-culture, 3, 477, 577, 578
—, dispersal by, 11
Fisher, W. R., 20
Fishes, age of, 145, 146
Fishing Eagle, 117
Fitzroya, 171
Five-needled Pines, 252
Fixed oils, 265
Flagella (sing. flagellum), 26, 29, 32, 35,
125, 479
Flagellates, 77, 125,
Flat-fig, see Dorstenia
Flatmoor peat, 501
Flats, salt-marsh, 368, 369
Flavouring materials, 260, 261, 262, 264
Flax, see Linum usitatissimum
— -seed, world production of, 236
Fleabane, see Erigeron
Fleming, R. H., 312, 540
Flint, R. F., 159, 173, 181
Flixweed, see Sisymbrium sophia
Floating-leaf community, 372, 493, 504
— .-— stage of hydrosere, 319, 325,
326, 353, 372, 504-5
— sea-ice, Algae on, 109, 491
— seeds, 107—II
Food-plain deposits, 544
—, transport by, 107, 108
Flora,
488, 489,
[517
480-1,
3, 572
602
Flora, alien, 118
—, elements of, 210-12
—, European, poverty of, 156-7
—, post-Miocene impoverishment of,
Floriculture, 278 [160
Florida, 352, 445
Floristic diversity (‘ richness ’), 169, 172
— regions of world, 19
Flotation disk, 483
—, dispersal by, 86, 106-11
— of disseminules, 106-11
—, plankton, 479, 483, 517, 518
Flotsam, 108
Flower, 12, 68, 70, 71, 437, 484, 485
—, doubling in cultivation, 218
— -slopes, 405, 408, 409, 410
Flowering, conditions for, 77~78, 79,
284
—, high-alpine, 412, 413
— plants, 67-73
—, temperature and, 77-8
Flowers, drugs from, 263
Fluctuations, climatic, 10, 13, 284-93
Flush, 374
Fly Agaric, 267
Flying apparatus, 106
Fodder (feedstuffs, stock-feed), 3, 265,
266, 267
Foehn wind, 292-3
Foliage, seasonal, 339-40
Foliose Lichen, 47, 48, 326, 407, 410,
416, 434
Folklore of plants, 262
Fontinalis (Spring-moss), 475, 503
Food. 3, 12, 15.255 25759) 203
— -chain, 17, 257
— crops, 223-39, 253, 257-9
— manufacture, 265
— storage by plants, 29, 32, 33, 35, 39,
78, 87, 90, 91
Yeast, 259, 278, 279
Foods, human, 255, 257-9
Forage plants, 234
Forb, 85, 360, 364, 360, 383, 385, 386,
387, 389, 399, 409, 410, 446, 464
Forest (see also Coniferous forest,
Deciduous summer forest, Tropi-
cal rain forest, Tropical seasonal
forest, Warm-temperate rain forest,
etc), 35 11,2 OSs 2ES5 624350240,
247, 295, 337-54, 423-46, 541
—, absent from Antarctic, 246
—, areas under, 246-7, 439, 556
—, Buna, 342
— canopy, 330, 342, 349, 425, 426, 431
—, climate and, 277
—, climax, 3253 33253725540
, closed, of Congo, 444
—, coniferous, 246, 343-50, 470
—, destruction of, 54, 307, 308, 465
— duff, 299
INDEX
Forest floor, 308, 322, 342, 344, 346,
436-7
— furnishings, 351, 430, 434, 463
—., land area of world covered by, 213,
243, 246, 247, 556
—., light-climate in, 285
—, northernmost, 343-5, 346-8
— products, 73, 239, 243-7
—, regeneration of, 306, 307, 308, 310
— roof, 425
— steppe, 363
—, stratification of vegetation in, 291,
338, 339, 349, 425, 426, 427, 440
— trees, mycorrhizal habit of, 46, 72
— -tundra, 348, 383
Forestry, 3, 242-7, 557-8, 564, 573, 578
Form-resistance to sinking, plankton
and 31, 33, 34, 479, 517, 518
— - — of seeds and fruits, 99, 104, 105
Forma, 25
Formation (vegetation unit), 201, 333-4
—, climax, 211, 337
— relic, 201
Former climates in northern Eurasia
and America, 153, 181
— floras, 149, 151, 154, 155, 157, 160
Fosberg, R. F., xvi
Fossil, 15, 56, 57, 62, 128-54, 166, 171,
173, 196, 199
— Algae, 130, 131, 132
— Angiosperms, 143, 144, 145, 150,
152, 171, 196
— Bacteria, 130, 145
— Bryophyta, 133, 145, 146
— Conifers, 141, 143, 145, 148-51
— Gymnosperms, 138-43, 148, 150,
152
— lower plants, groups of, 129-38
— Pteridophyta, 133, 134, 135, 136,
137, 138, 146, 147, 148, 149, 150
— Seed-plants, 138-44
Foundations of plant distributions 5,
154-81
Fouquiera splendens (Ocotillo), 450, 451
Fox, 306, 401
Foxe Basin, 127
Foxglove, European, 264
Fragaria (Strawberry), 113, 122, 123,
239
F. grandiflora (Strawberry), 235, 258
Fragmentation, vegetative propagation
France, 161 [by, 57, 69
Frankincense, 273
Franseria (Bur-sage), 448
Frraxinus (Ash), 104, 105, 341, 342, 554
F. excelsior (Ash), 246, 340
F’, mandshurica, 342
Free-living Algae, 492
Freezing, death from, 288
—, qualities of water and, 475
— point, activity continuing below, 77
INDEX
Freshwater (fresh water) aquatic com-
munities, 472-506
— aquatic environment, 472-9
— Bacteria, chemosynthesis by, 496,
— —, planktonic, 480 [497-8
—, calcium carbonate (‘ lime ’) in, 478
—, — — precipitated in, by plants, 497
—, circulation of substances in, 497-8
—, eulittoral zone in, 492, 493, 494-5
— habitats, 316-20
—, oxygenation of, 477, 488
— plankton, 318, 320, 479, 480, 481-9
— productivity, 476—
—, surf-belt in, 495
Frey, D. G., 506
Frigid Phippsia, see Phippsia algida agg.
Fringed Sandwort, see Arenaria cilata
Fringing forest, 439 [s.l.
Fritsch, F. E., 74, 540
Frogbit, see Hydrocharis morsus-ranae
Frond, 59, 528, 532, 534
Frost, 11, 381, 405
— action in Arctic, 323, 381, 384, 396
—, disintegration by, 296
— -heaving, 177, 296, 315, 395
—, killing by, 77, 322, 522
—, maturation of soil and, 390
Frugivorous birds, 114, 115
Fruit, 68, 69, 98, 101, 102, 104, 105,
CLO; Liz, £13, 122.9123
— -bat, 115
— development, cultivation and, 217-8
—, dispersal of, 69, 101, 104, 105, I10,
113
—., edible, 235, 239, 242, 258-9
—, explosive, 121, 122, 123, 124-5
— setting, temperature and, 78
Fruiting body (fungal), 43, 44, 45
Fruits, crop, 235, 239, 258-9, 564
—, as source of drugs, 263
—, winged, 101, 102, 104, 105
Fruticose Lichen, 47, 48, 416
Biya he Ee J, 506
Fucaceae, adhesive disks of, 528
Fucus (Rockweed, Wrack), 36, 37, 521,
526, 530, 532, 533, 534-6
F ceranoides, 523
F. serratus, 532
F. spiralis, 532
F. vesiculosus (Bladder Wrack), 35,
524, 525, 528, 532, 536
Fuels, 271-2
Fully naturalized aliens, 220
Fungi (Mushrooms, Toadstools,
Moulds, Rusts, Smuts, Yeasts), 1,
43, 44, 45, 46, 129, 130, 145, 146,
418, 437, 481, 491
Fungi, aquatic, 117, 496
—, dispersal of, 99, 101
—, economic significance of 46, 251,
265, 279
603
Fungi, edible, 259
—, fossil, 146
—, general account of, 43-6
—, industrial uses of, 265, 278
—, low temperature and, 77
—, marine, 527
—., parasitic, 80, 250-1, 305, 491, 496
— as plant pathogens, 80, 250, 251, 265,
280, 305
— pre-Cambrian, 129
—, saproplanktonic, 480
—, scavenging, 132, 278, 280, 303
—, soil forms of, 303, 305, 333
—, spore dispersal of, 117, 491
—, spore-ejection of, 121
—, spore germination of, 117
—, spore-output of, 100, 125
Fur, dispersal on, 116
Fustic, 274
Gagea, 366
Galeobdolon luteum (Yellow Archangel),
340
Galinsoga parviflora (Gallant Soldier),
Galium antarcticum, 419 [249
Gallant Soldier, see Galinsoga parviflora
Galls, 274
Gambier, 274
Gamboge, 274
Gametangia, 45
Gamete, 32, 33, 37, 42, 45, 49, 63, 68, 69
Gametophyte, 49, 51, 52, 53, 57; 59, 62,
63, 69
Garcinia mangostana (Mangosteen), 242,
258
Garden Bean, see Phaseolus vulgaris
— Valerian, 264
Gardens, importance of, 277, 278
Garigue, 355, 356, 375
Gas bladders in Algae, hydraulic pres-
sure and, 522
— -vacuoles, in Cyanophyceae, 483, 487
Gaseous exchange, 76, 302, 509-10
Gaultheria (Spicy-wintergreen), 349
Gaumann, E. A., 74
Gaussen, H., 23
Gazelle, 454
Geiger, R., 335
Gelatinous matrix, 28, 29
— stalks on Diatoms and Vorticella, 494
Geminate species, Jordan’s Law of, 202
Gemma (pl. gemmae), 51, 53, 57
Gene, 117, 118, 176
— -flow, 177
General accounts of plant groups, 24-74
— habitat, 313, 321
Generic name, 25
Genetic element, 211
Genetical changes, cultivation and, 216—
— heritage, 175-8 [218
— recombination, 177
604
Geneva, Lake, 503
Gentianaceae, 436
Genus (pl. genera), 25, 187
Geodynamic factor (influences), 293,
296, 369, 388, 410
Geographer, animal, 17
—., plant, vegetation patterns and, 9
Geographical changes, 155, 156, 165-72
— (areal) patterns, 7-9
—range of aquatic organisms,
478-9
— ranges of plants, 5, 6, 182-252, 572
— distribution of coniferous forest,
246-7, 345
— elements, 210, 211
— gradient, 183
— vicariad, 202
Geography, animal, 2, 15, 17, 18
—, plant, see Plant geography
Geography, purpose of, 4
Geological eras and periods, 128-9, 145
— history of plant groups, 128-53
— time-scale, 128-9, 145
— — -sequence, 128-9
— times, arid climate in, 148-9
— —, plant groups in, 144, 145, 146-53
Geology, indicator plants and, 577
Geomorphological relics, 201
Geophyte, 93. 94, 95, 355, 361, 366, 440,
441, 443, 447, 451
Geotome, 335
Geraniaceae (Geranium family, Stork’s-
bill family), 124
Geranium, 123
— family, see Geraniaceae
Germany, 262
—, Pleistocene and present floras of,
161
Germination of seeds, 69, 76, 110, 452
— — —, delayed, 248
— — —, digestion and, 114, I15, 117
Germules, aggregation of, 323
Geum (Avens), 101
G. rivale (Water Avens), 204
G. urbanum (Wood Avens), 204
Giant Horsetails, 135, 136, 149, 150
— Kelp, 37, 535, 537
— Puffball, see Lycoperdon (Calvatia)
giganteum
— Redwood, see Sequoiadendron gigan-
— Reptiles, 155 [tewm
Gibbs ReaD 73
Gigantopteris, 144
Gilbert-Carter, H., 96
Gill Fungus, 45
Gin, 261
Ginger (Zingiber), 90, 260, 262, 430
317;
Ginkgo biloba (Maidenhair-tree), 140,
Ginkgoales, 141, 145, 149 [141
Ginn and Company, xviii
Ginseng, Chinese, 264
INDEX
Girdles of coastal marine vegetation,
526-7, 530, 531, 532, 533, 534-5
Glacial periods (see also Glaciation),
149, 151-3, 156-61
— relic, 201
— survival, 161-5
— time, 129
Glaciation, European flora and, 156-7
—, Permian, 149, 156 [161
—, plant-migration and, 156~7, 160, 161
—, Pleistocene, 129, 156-8, 159, 161-5
—, Quaternary, 156
—, recurrent, 129, 152, 156, 160-1
— regions affected by, 152-3, 156, 157,
158, 159, 160, 161, 162
Glaciers, erosion by, 538, 543-5
—, residual features left by, 545-6
—, valley, 411
Glacio-fluvial deposits, 545
Glasswort, see Salicornia
Glaucous Bulrush, see Scirpus taber-
naemontanit
— Willow, see Salix glauca s.1.
Glechoma hederacea (Ground-ivy), 121
Gloeocapsa, 28, 490
Glosotrichia echinulata, 487
Glossopteris, 144, 148
— flora, 148
Glycine max (Soybean), 233, 259
Gnetales, 62, 65, 67, 139, 145
Gnetum, 65
Goats, 80
Gobi Desert, 365, 366, 454
Godwin, H., xvili, 174, 181
Goeze, E., 22
Goldenseal, 264
Gondwanaland, 148, 196
Goniaulax, 34
Good, R., 19, 21, 157, 187, 194, 196, 214
Goose, 402 [281
Gooseberry, see Ribes
Goosefoot family, see Chenopodiaceae
Gorges, 543, 544
Gorse, see Ulex
Gossypium (Cotton), 102, 234, 235, 265,
—, world production of, 235, 237 [282
Goudier Islet, Antarctica, 416
Gourd, 258
Gradient, geographical, 183
Graebner, P., 23
Graham, E. H., xviii, 555, 559, 561
— Land, 417
Grain crops, 224, 225, 227, 228, 230
Grains of paradise, 261
Gramineae (Grass family, Grasses), 127,
68, 73, 122)'123; 250, 270; 275, 270;
292, 308, 333, 342, 358-66, 383,
385, 386, 387, 388, 389, 399, 401,
410, 412, 413, 417, 419-21, 430,
440, 441, 442, 443, 445, 446, 447,
451, 453, 454, 460, 464, 466, 500
INDEX 605
Gramineae, dispersal of, 101, 116, 120,
3°97
—, dominance of, 360, 361-4, 387, 443-5
— halophilous (halophytic), 369, 371
—, Sand=dune, 122; 123, 328, 371, 458,
Granadilla, 258 [542, 548, 549
Grape, see Vitis vinifera
Grapefruit, 258
Grass family, see Gramineae
Grasses, see Gramineae
— incompetition with trees, 307-10, 360
Grassland, 85, 213, 246, 289, 306, 314,
333, 359-64, 443-7, 556, 578
—,, fire and, 310, 359, 444, 466
Gravel, 298, 315, 328, 366, 449, 522
Grazing (see also Browsing), effects of,
85, 222, 307, 308, 330, 331, 355,
359, 363, 364, 444,447, 465, 466, 558
—, forest regeneration and, 307, 308
Great Britain, 119
— Lakes (N. America), 338, 349, 371
Green Alder, see Alnus crispa agg.
— Algae, see Chlorophyceae
— manure, 279
— plants, depth of water inhabited by,
475, 485-8, 496, 527, 530, 537
— snow, 490
Greenheart, see Ocotea rodioet
Greenland, 119, 162, 172, 186, 391, 409,
411
—, American plants introduced to, 181
—, caterpillars and sheep in, 306
—, cryophytes in, 490
—, cultivated plants in, 231, 233, 234
—., drifting of, 166
—, flora of, 169, 172
—, fossil flora of, 149, 150
—, glaciation in, 156, 162, 414
—, immigrant weeds in, 165
—, woody vegetation of, 222, 344, 390,
Grey-brown polyolic soil, 300 [391
Grimmia, 417
Grinnellia, 40
Grisebach, A., 22
Groom, P., 20
Ground-ivy, see Glechoma hederacea
— layer (field layer), 340, 342, 344, 440
— -pines, see Lycopodineae
— -shrubs, 327, 344, 347,
410
— -water, 301, 316, 374, 412, 546
— - — as agent of erosion, 543
Groundnut, see Arachis hypogaea
Growing-points, 12
— -season, 78, 315, 339, 403, 536
Growth (general), 13
— -form, 92-4
— -inhibitor, 13
—-ring (annual ring), 13, 146, 149
348, 440
— substances, 13
359; 39275,
Guaiacum (Lignum-vitae), 247, 264
Guarana, 260
Guava, 258
Guayule rubber, 272
Guinardia, 516
Gulf of Aden, 443
— of Mexico, 423
— of St Lawrence, 338
— Stream, 293
— -weed, see Sargassum
Gullies, 418, 544, 568, 569, 570
Gully erosion, 309, 568, 569
Gum, 3, 272
——arabics 272274
— tragacanth, 272
— -tree, see Eucalyptus
Guppy, H. B., 127
Gutta-percha, 272
Gymnodinium, 34
Gymnospermae (Gymnosperms),
66, 67, 129, 145, 146, 450, 527
—, fossil, 138-50
Gypsiferous soil, 366
62-
Haberlandt, G., 95
Habitat, "5 ,,'6,73,19/26, 20)32,.38,) 30,
313-22, 329, 472-9, 553, 563, 564,
— preferences of birds, 116, 163 [577
— requirements, 219, 565
— tolerance, 8
— (ecological) vicariad, 202, 204
Habitats of plant groups, 26, 27-8, 29,
32, 33-4, 35, 38, 39, 41, 45, 47-8,
51, 54, 56, 57, 62, 65, 72, 283, 382,
434, 468, 544, 546, 550
— formed by erosion, 544
— of higher plants, 56, 57, 62, 65, 72,
313
— of lower plants, 26, 27-28, 29, 33-4,
35, 38, 39; 41, 45, 47-8, 51, 54, 313
—, seemingly uniform, 9
—, similar, patterns of, 9, 329
Haden-Guest, J. K., 253
Haematomma puniceum, 48
Haematoxylin, 274
Haemorrhages, treatment of, 265
Hail (see also Precipitation), 289
Haines, R. W., 102
Hairs, 82, 101, 102, 104, 366, 433, 451
Hakea, 83, 85
Half-barren, 285, 396, 397
Halophilous Grasses, 369, 371 ,
Halophyte, 301, 368, 371, 399, 457, 460,
474, 533
—, facultative, 368, 369, 370
—., desert, 365, 367, 368, 450
— of inland waters, 365, 502
— of spray zone, 527
Halosere, 324
Halosphaera viridis, 517
Hamamelis (Witch-hazel), 124, 264
606
Hammock (Florida), 352, 438
Hanging bog, 501
— valleys, 546
Hanson @a933)5
Hard Fern, see Blechnum spicant
— water, 373, 478
Hardwood, 246, 268
Hardy, M. E., 20, 355, 378, 471
Hares, 363
Harley, J. L., 46
Harsh environment replacing competi-
tion, 381
Harshberger, J. W., 282, 379
Hashish, 267
Haustorium (p/. haustoria), 89, 438
Haviland, M. D., 16, 561
Hawai, 424
—, native plants ousted by aliens, 118,
Hawk, 115, 306 [221
Hawthorn, see Crataegus monogyna
Hay-fever, 264, 280
Hayek, A., 23
Hazel, see Corylus and C. avellana
— -nut, 258
Heard Island, 420
Heart ailments, treatment of, 262, 263-4
Heat, resistance to, see High tempera-
ture, tolerance of
Heath, see Erica and Heathland
Heathland, 330, 332, 358-9, 374, 450,
541
—, arctic, 392, 393, 394, 395, 409, 410
—, mountain, 410
Heaths, see Ericaceae and Heathland
Hedera (Ivy), 89
H. helix (Ivy), 340
Hedge, sprouting after clipping, 13
Helbeck, H., 254
Helianthemum, 173
Helophyte, 505
Hemi-epiphyte, 434
Hemicryptophyceae, 528
Hemicryptophyte, 93, 94, 95, 307, 360,
410
Hemiparasite (semi-parasite), 437, 438
Hemiphanerophyceae, 528
Hemiplankton, see Pleuston
Hemitrichia, 42
Hemlock, see Tsuga and T. canadensis
Hemp, see Cannabis sativa
Hempseed oil, 235
Henbane, 267
Henna, 274
Hepaticae (Liverworts), 49, 50, 51, 145,
422, 434, 462, 468
—, fossil, 133, 145
Herb (see also Forb, Cyperaceae, Grami-
neae, etc.), 11, 295, 308, 340, 341,
346, 356, 392, 398, 430
- layer, 340, 341, 346, 430
— -like Willow, see Salix herbacea
INDEX
Herbaceous crops and their areas, 223-
Herbalists, 262 [241
Herbarium (p/. herbaria), 572, 573
Herbicides, 275, 276, 280, 579
Herbivores, see Mammals, etc.
Heredity, 64, 91, 175, 562
Heritable variation, 562, 564
Heritiera littoralis, 110
Herons, 117
Hess, R. W., 243, 253
Hesse, R., 17
Heterogeneous area, 192
Heterospory, 57, 59, 135, 137
Heterotrophic organisms (heterotrophs)
436, 496
Heterotrophy, 436, 496
Hevea (Para Rubber), 124, 272, 430
H. brasiliensis (Para Rubber), 121
Hickory, see Carya
— -nut, 258
Hierarchy of classification, 24, 91
Hierochloe alpina (Alpine Holy-grass),
385
High-alpine meadow, British Columbia,
412
— - — vegetation, 409, 410, 411, 412,
413, 414
— altitudes, 10, 409-15
— —, comparison with high latitudes,
409
— -arctic belt or region, 171, 316, 382,
387, 389, 395-9, 401, 409
— --— community, unusually luxuri-
ant, 408
— - — heathland, 395
— - — marsh, 387-9
— temperature, tolerance of, 77, 87,
495, 499
Highmoor, 359, 374, 462, 501
Hildbrandtia, see Hildenbrandia
Hildenbrandia, 496, 499
H. rivularis, 498
Hill, A. F., xviii, 252, 281
— Rice, 224
Hillock tundra, 385, 387, 388
Himalayas, 247, 295, 423, 424
—, ‘cold desert ’ of, 414
—, flowering at high altitudes in, 412,
Hind, R. B., 17 [413
Hippuris vulgaris s.1. (Common Mare’s-
tail, Mare’s-tail), 405, 406
Historical elements, 211
History, past, see Evolutionary develop-
ment
Holard, 301
Holdfast, 35, 36, 38, 39, 498, 528
Holly, see Ilex aquifolium
Holm Oak, see Quercus tlex
Holocene period, 173
Holttum, R. E., 471
Homogeneous area, 192
INDEX
Honckenya, see Arenaria (Honckenya)
peploides agg.
Honeysuckle, see Lonicera periclymenum
Hooded Ladies’-tresses, see Spiranthes
romanzoffiana
Hookworm, infections, treatment of,
263
Hop, see Humulus lupulus
Hoppner Sedge, see Carex subspathacea
Hordeum vulgare s.l. (Barley), 226, 257,
260, 282, 370, 454
— —, world production of, 229
Horizons, soil, 297, 299
Horizontal vicariad, 202
Hormonal herbicide, 276
Horn-wort, see Ceratophyllum
Hornbeam, see Carpinus
Horse-chestnut, see Aesculus
— -radish, 262
Horsetail, see Equisetum
Horsetails, see Equisetineae
Horticulture, 3, 233, 277-8, 564, 577-8
Host, 80, 251-2, 432, 433, 435, 436, 437,
Hot desert, 449 [524
— springs, flora of, 476, 499
Hoyle, A. C., 441
Huckleberry, 258
Hudson Bay, 292, 384, 406, 537
— Strait, 385, 386, 393, 400, 401
Hudsonian belt, 346, 349
Hultén, E., xvii, 185, 189-91, 195, 199,
203, 214
Human diseases, 26, 27, 577
— environment conditioned by plants,
255, 277
— population of world, 224, 256
Humboldt, A., 21
Hummocks, 385, 387, 388, 399, 501
Humic matter, soluble, 497, 502
Humulus lupulus (Hop), 239, 261, 263,
282
Humus, 301, 302, 303, 325, 475, 476,
Soe es
—, formation in water, 318, 325, 503
—, marine, 539
—, Taw, 393
—, in soil, 297, 301, 302-3
—, tropical, 437, 458, 462, 467, 468
Hura, 124
H. crepitans (Sand-box Tree), 121
Hutchinson, G. E., xviii, 16, 506
—, J., xviii, 186, 190, 193-4, 281
Hybrid origin of endemics, 163
—, polyploid, 179, 204, 234, 235, 239
Hybridization, 176, 177, 179, 204, 258,
564
Hydra, 2
Hydrarch subsere, 324 :
Hydraulic pressure, algal gas-filled
bladders and, 522
Hydric mean, 328
607
Hydrocharis morsus-ranae (Frogbit), 504.
Hydrogen-ion concentration, see pH
— sulphide, 497-8
Hydrophytes, 86, 87, 93, 289, 372,
593-5
Hydroponics, 278
Hydrosere, 319, 324, 325, 326, 333, 342,
353, 372; 373, 462, 502-6
—, arctic, 405, 406
—, dominants of, 372
—, floating-leaf stage of, 325, 326, 353,
372, 463, 504, 505
—, marginal to lakes, 502
—, reed-swamp stage of, 319, 325, 326,
372, 405, 406, 463, 505
—, sedge-meadow stage of, 325, 326,
372
—, Stages of, 319, 325, 326, 353, 503-4
— of temperate regions, 325, 326, 372
—.,, tropical, 462, 463
Hyeniales, 145
Hygrograph, 291
Hygrohypnum polare, 109
Hygrophilous forest, 350, 440
Hygrophily, 294, 440
Hygrophyte, 289, 326, 505, 578
Hygrophytic scrub (shrubs), 325, 326,
342, 372
—, woodland (forest), 325, 326, 373
Hygroscopic movements, 56, 125
Hylander, C. J., 281
Hymenaea courbaril (Locust), 247
Hymenophyllum peltatum, 420
Hypha (pl. hyphae), 43, 45, 131
Hypnophyceae, 528
Hypnotic, 264
Hypolimnion, 477
Iberian Peninsula, 197
Ice action on marginal vegetation, 475,
531, 536, 537, 538
a Age, 109, 156, 157
— bloom, algal, 490
— -cap (antarctic, etc.), 162, 213, 414-
417
— - —, climate near, 162
—, dispersal and, 100, 106, 107, I09,
491
— -free tracts, plants persisting on, 162,
163, 164, 165
— island, 109, 491
—, marine, seaweeds and, 531, 536,
537-8
— photosynthesis under, 506
— -pups (and ice-floes), 127, 388, 491,
536, 538
—, recession of, 129, 159
Icebergs, dispersal by, 109, 490, 538
Iced firn, 11, 490
Iceland, 184, 222, 344
— -moss, see Cetraria
608
Idaho, 85
Ilex (Holly, Maté, Paraguay-tea), 260
I. aquifolium (Holly), 340, 342
Illinois, 569, 570
Illuminant, 266
Impatiens (Balsam), 124
I. balsamina (Balsam), 122, 123
Imperata (Lalang Grass), 466
Importance of plants, vital, 255-82
Importation, restricted, disease control
Inclusion, 212 [by, 576
India, 196, 233, 263, 272, 444
—, desert in, 449
—, forests etc. of, 423, 439, 442, 443
—, fossil flora of, 148, 149
—, origin of crop plants in, 253
—, South, poor flora of, 172
—, tropical hardwoods in, 247
Indian Lotus, see Nelumbo nucifera
— Ocean, 447
Indiana, 308
Indicator plant communities, 552-5, 577
— Plants, 54459552) 553-5 57308 S775
eas geology and, 577 [578
Indigo, 274
Indo-China, 423, 439
— -Malayan region, 253, 456
Indonesia, 423, 439
Industrial applications, 3, 46, 239, 261,
265, 268-70, 274, 278
Infralittoral slope, 492, 493
— zone, 493, 495-6, 511 512, 513, 527
Ingold (Gy a 127,
Inhibitors, growth, 13
Inland saline areas, 367, 369-70, 502
waters, 369-70, 492, 502, 534
Inorganic nutrients, 12, 301, 432, 519
Insect vectors, 117, 252, 305
Insecticides, 275, 276
Insectivorous plants, see Carnivorous
plants
Insects, 16, 91, 117-18, 250, 424
Insolation, 284, 288, 294, 321, 375, 376
Insular climate, see Oceanic climate
Interaction of factors, 77, 283, 293-4,
554
Intercontinental
183-7
Interglacial periods, 129, 156, 157, 161
— relic, 201
— time, 129
Internal aeration of plants, 86, 87, 505-6
Interpretation of landscapes and ecolo-
gical data, 551-5
Interrelationships of organisms, 9
Interruption of range of distribution,
183, 188-97
Intertidal belt, zonation in, 526-7, 532
— vegetation, 526, 529, 531-6, 537-8
Intervening tracts (of polygons), 395,
396
range (distribution),
INDEX
Intestinal worms, remedy for, 263
Intracontinental discontinuous
(distribution), 196, 197
Intraneous occurrence, 210
Introduction of exotics, 215, 243, 574,
576
Introgression, 176, 177
Invasion, 324
Invertebrate animals
117, 145, 146, 495
Iodine, algal source of, 265
Ipecac, 264
Ipomoea batatas (Sweet Potato), 231
258
I. pes-caprae, 458
Ira, see Dipterocarpus tuberculatus
Iraq, 254, 355, 365, 367, 368, 370, 451
range
(Invertebrates),
459, 495
—, desert of, 366, 367, 451-2, 453
Ireland, 197, 251
Tris sisyrinchium, 366
Irish-moss, see Chondrus crispus
— Potato, see Solanum tuberosum
Ironwood, see Casuarina equisetifolia
Irrigation, 307, 309, 311, 379, 557
Island floras, 167, 169, 172, 205, 206
222
— —, endemism in, 172, 205
Tsoetes (Quillwort), 56, 57, 58
Isoflor, 208
Isolation, 176, 205-6
Ithyphallus (Stinkhorn), 44, 45
Ivy, see Hedera and H. helix
Jack-fruit, 258
Pine, see Pinus banksiana
Jackdaw, 114
Jactitation, 103, 105
Jamaica, flora of, 169, 172
Jamaican Lace-bark, 271
James, W. O., xviii, 95
Jan Mayen Island, 382 537
Japan, 211
—, forests of, 338, 341, 351
—, fossil flora of, 149
—, marine Algae as food in, 259
Java, 221
Jericho, ancient cultivation in, 254
Jerusalem Artichoke, 258
Johnson, M. W., 312, 540
— -grass, 259
Johnston, M., 20
Jojoba wax, 266
Jones, S. G., 250, 253
Jordan’s Law of Geminate Species, 202
JFovellana, 194
Juglans (Walnut), 258, 341
J. nigra (Black Walnut), 80
Juglone, 80
Jujube, 258
Juncaceae, 500
INDEX
Juncus (Rush), 270, 383, 578
Juniper, see Juniperus and F. communis
Juniperus (Juniper), 295 [s.l.
F. communis s.1. (Juniper), 261, 359
Jurassic flora, 145, 149
— period, 129, 132, 138, 143, 149
Jussiaea repens, 86, 87
Jute, see Corchorus
Kale, see Brassica oleracea
Kalmia (Pale-laurel), 346
Kames, 545
Kapok, see Ceiba pentandra
Karaya, 272
Kavakava, 267
Kauri, see Agathis australis
Kelp (see also Laminaria), 37, 38, 532,
533, 534, 535
Kelps, see Laminariales
Kendrew, W. G., 10
Kenyon, K. M., 254
Kerguelen Cabbage, see Pringlea anti-
scorbutica
— Island, 419
Kerner (von Marilaun),
Kettle-holes, 545
Key factor, 329
Khat, 260
Khaya senegalensis (African Mahogany),
247
Kidney Bean, see Phaseolus vulgaris
Killing, by frost, 77, 322, 522
Kimble, G. H. T., xvii
King, L. J., 253
Klages, K. H. W., 252, 281
Kobresia myosuroides (Bellard’s Kob-
resia), 392
Korea, 351
Krakatau, 464
Kramer, P. J., 335
Krummholz, 377, 380, 413, 469
Kichler, A. W., Frontispiece
Kudzu, 259
Kumquat, 258
A., 114, 127
Lablab, 258
Labrador, 397, 543
— -tea, see Ledum
Lacquer, 273
Lactuca scariola (Prickly Lettuce), 249
Lagotis glauca s.1., 412, 413
Lake-basins, variation in, 492
— -forest, 349, 350
— -marginal profile, 493
— sediments, 492, 496-7, 501
, ‘lime’ in, 495
— water, calcium carbonate in, 477-8
— —, seasonal differences in oxygena-
tion, 477, 486
Lakes, circulation of water in, 473,
481-2, 486
477
609
Lakes, filling up of, 374, 50%
—, glacial origin of, 545
—, productivity types of, 318, 319,
476-8
—, temperature changes in, 473, 475,
—, turn-over of water in, 477 [486
Lalang Grass, see Imperata
Lamb, I. M., 418
Lamb’s-quarters, see
album s.1.
Laminaria, drifted, 521
—, geographical distribution of, 530,
531, 535, 537
L. cloustonit, 532
L. digitata, 532
L. longicruris, 537
L. saccharina, 532
Laminariaceae, 526-8
Laminariales (Kelps), 379, 531, 532
533, 534, 535, 536, 537
Lamium, 249
Land- bridges, 15, 170-2, 207
— flora, climatic regions and, 20
— plants, fossil history of, 133-5, 146
— -masses forming patterns, 8
— - — main, vegetation and, 20
— - —, proximity of, 169, 172
— -use classification (capability classes),
555-61
— - —, ecology and, 7, 551, 567-8
Landforms, 7, 293, 541-51
—, plants and, 543-51, 554, 555
Landscapes and component landforms,
541-3
—, reconstructions of, 147, 148, 150,
152
— and vegetation, 277, 541-61
Lapland, 543
— Rose-bay, see Rhododendron lapponi-
cum
Larch, see Larix and L. decidua
Larix (Larch), 268, 274, 343, 345
L. dahurica (Dahurian Larch), 343,
346, 348
L. decidua (Larch), 246
L. laricina (Tamarack), 346, 347, 348
L. sibirica s.1. (Siberian Larch), 346
Larrea (Creosote-bush), 450
L. tridentata (Creosote-bush),
448
Late-blight of Potato and Tomato, see
Phytophthora infestans
— -snow, see Snow-patches
Laterite soil, 298, 300, 467
Latexes, 272-3
Lathyrus maritimus agg. (Sea-pea), 371
Latitude as limiting factor, 20, 574, 576
Lauraceae (Laurel family), 352
Laurel family, see Lauraceae
Lavandula latifolia (Lavender), 275, 355
Lavender, see Lavandula latifolia
Chenopodium
364,
610
Laver, 259
Lawrence, G. H. M., 74
—, W. J. C., 579
Laxative, 263, 264
Layer (stratal) society, 334
Layering, 64-5, 98
Leach, W., 311, 378
Leaching of soil, 298, 331, 465, 497
Leaf, 12, 59, 63, 91, 137, 143, 148, 393,
404, 427, 429, 433, 437, 440, 467,
468, 484, 485, 504
Leaf-epiphytes (‘epiphyllae’), 431,
434
— -fall, 78, 337-43, 424-5, 428, 430,
— modified as spine, 83, 85 [440
— scars, 137
— -tendrils, 88, 89
Leafy Liverworts, 49, 50, 434, 468
Leather, 273
Leather-leaf, see Chamaedaphne
Lebachia (Walchia) frondosa, 143
Ledum (Labrador-tea), 346, 501
L. palustre agg. (Narrow-leafed
Labrador-tea), 392, 394
Legume, 233, 234, 258, 259
Leguminosae (Pea family), 73, 217, 247,
276, 412, 443, 446, 548, 566
—, cultivated, 231, 233, 234, 258, 564
— as dominants, 441
—, seed-ejection by, 124
Lemanea, 499
Lemna (Duckweeds), 106, 483, 504
Lemon, 258
— -grass oil, 275
— Thyme, 262
Lens esculenta (Lentil), 234, 258
Lentic stretches in streams, 498, 499
Lentil, see Lens esculenta
Leontice altaica, 192
Lepidodendrales, 145
Lepidodendron, 137, 148
L. lycopodioides, 137
Leprosy, 26, 263
—, treatment of, 263
Lespedeza, 259
Lesser Celandine, see Ficaria verna
Lessonia simulans, 538
Lethal temperatures, 288, 499
Lettuce, 258
Leucadendron, 355
Leucojum aestivum
flake), 108
Lianes (see also Climbers), 94, 101, 352,
379, 431, 438, 441, 462
Lichen barrens, 398
— sward, 344, 347, 348
Lichenes (Lichens), 11, 45-7, 48, 49,
102, 103, 132, 145, 305, 339, 344,
346, 347, 385, 386, 392, 393, 394,
395, 397, 398, 401, 404, 411, 412,
413, 416, 421-57
(Summer Snow-
INDEX
Lichenes, algal element of, 46-7
—, antarctic, 416, 417, 418, 419
—, arctic, 385, 386, 392, 393, 394, 395,
397, 398, 401, 404, 407-10, 411
—, aquatic, 494
, desert, 365
, dominant, 395
, dye from, 274
—, edible, 259
, epiphytic, 434, 457
, form-groups of, 47
, fungal element of, 46-7
, high-altitude, 410, 411, 412, 414,
546
—, indicator, 578
—, lithosere and, 326, 327, 407, 416,
418
—, sand-binding, 548
—, sand-dune forms of, 371, 548
—, supralittoral zone, 527, 530,
535
—, tundra, 383, 385, 386, 389, 419
Lichens, see Lichenes
Licmophora flabellata, 33
Life, beginning of, 128-30
— duration of, 79, 100, 205
— -form, 92-4, 95, 202, 212, 324, 329-
341, 528, 565
— - — spectrum, 95
—, seat of, 13
Light-absorption by water, 488, 511-13
—, algal pigmentation and, 39, 496,
512-13
—, aquatic plants and, 317, 473, 492,
496, 503, 511-13, 522, 527, 529-31
— -climate, 284, 285
—, flowering and, 79, 284-5
—, growth and, 79, 284-5, 321
—, measurement of, 285
— -penetration into water, 317 474,
475, 504, 511-13
—, photosynthesis and, 39, 496, 512-13
— -relations of Algae, 39, 325, 474, 475,
488, 495, 496, 512-13, 522, 529-31
— requirements, 79, 283, 432, 474, 529
Lignum-vitae, see Guaiacum
Ligule, 57
Liliaceae (Lilies, etc.), 73
Lilium (Lily), 91
Lime, see Calcium carbonate
— fruit, 258
— -tree, see Tilia
Limestone, algal, 130, 132, 279
Limnetic benthos, 473
— plankton, 473
Limnology, 473
Limnoplankton, 479
Linden, see Tilia
Line transect, 567
Ling, see Calluna vulgaris
Linicolous plants, 21g
532,
.
INDEX
Linseed oil, 235, 265
Linum, early cultivation of. 254
L. usitatissimum (Flax), 217, 219, 234,
236, 265, 270, 282
Liquorice, 264
Liriodendron (‘Tulip-tree), 157, 341
L. tulipifera (Tulip-tree), 341
Litchi, 258
Literature (for further consideration,
etc:); 17, 20-3, 73-4, 95-6, 127,
153, 180-1, 212, 214, 252—4, 281-2,
311-12, 335, 378-9, 422, 471, 506,
540, 561, 579-80
Lithophyllum, 528, 537
Lithophyte, 528, 530
Lithosere, 324, 325, 326-8, 372, 405,
407
—, pioneer plants in, 326, 407
Lithothamnium, 53°, 537
L. calcareum, 533
Litmus, 49, 274
Litter, 297, 301, 307, 308, 392, 437
Littoral Algae, 523, 525, 526, 529-38
— zone, 480, 492, 493, 513, 526, 527
Live Oak, see Quercus virginiana
Liverworts, see Hepaticae
Llanos, 444
Loam, 301
Lobaria pulmonaria, 48
Loblolly Pine, see Pinus taeda
Local climate, 283, 284, 294, 320, 375
— distribution, 182, 183
— endemic, 206
— relic, 200
Localized glaciation, 162
Lociation, 334
Locust, see Hymenaea courbaril
Locusts, 80, 117, 305, 306
Lodgepole Pine, see Pinus contorta var.
latifolia
Loess, 162, 366, 547, 548
Loganiaceae, 193
Logwood, 274
Lokao, 274
Lolium, 364
Long-day plants, 284
— -haired disseminules, 101, 102, 104,
105
Longevity of seeds and fruits, 248
Longleaf Pine, see Pinus palustris
Lonicera periclymenum (Honeysuckle),
Loquat, 258 [340
Loranthaceae (Mistletoe family), 437,
Loss of characters in cultivation, 216,
217, 218
Lost continents, 170
Lotus, see Nelumbo nucifera
Low-arctic belt or region, 382, 383-7,
390, 391, 392, 393, 396, 399, 400,
403, 409
611
Low-arctic Chickweed, see Stellaria
humifusa
— Sandwort, see Arenaria humifusa
— temperature effects (see also Cold,
Freezing, Frost), 8, 77, 78, 284, 285
Lower deck, 292, 347
— plants, fossil, 129-38
— sublittoral zone, 527, 533, 536
Lowland-moor, 374
— Rice, 224
Lowlands, alpine plants in, 374
Lucerne, see Medicago sativa
Luffa, 271
Lumbering, 3
Lumpers, 202
Lusitanian element in British flora, 196,
Lutz, H. J., xviii, 348 [197
Luzula (Wood-rush), 383
L. confusa agg. (Northern Wood-
rush), 386, 389
Lyallia, 417
L. kerguelensis, 419
Lychnis (Catch-fly), 103
Lycoperdon (Puffball), 43, 44, 45, 100
L. (Calvatia) giganteum (Giant Puft-
ball), 125
Lycopersicum esculentum (‘Tomato), 73,
78, 80, 120, 231, 234, 258, 282
Lycopodiales, 145, 146, 149
Lycopodineae (Club-mosses, Ground-
pines, Quillworts, Spike-mosses),
56-7, 58, 59, 135, 137
Lycopodium (Club-moss, Christmas-
greens), 57, 58, 59, 121, 264
L. inundatum (Bog Club-moss), 192
L. saururus, 419
Lycopsida (Lycopsid), 138, 145, 146,
Lyginopteris, 148 [147
L. oldhamia, 139
Lyme-grass, see Elymus and FE. arenarius
Lyngbya, 370 [s.l.
Macademia-nut, 258
McDonald Island, 420
McDougall, W. B., 312
Mace, 261
Mackay’s Heath, see Erica mackaiana
McLean, R. C., 73
Macquarie Island, 114, 420
— —, alien seeds on, 114, 163
Macrocystis, 521, 537
M. pyrifera, 379, 534, 535
Macrozanonia, seed of, 104, 105
Macuna gigantea, seeds of, 110
Madagascar, 196, 439, 444
—, aliens on, 222
Madder, 274
— family, see Rubiaceae
Magnolia grandiflora (Evergreen Mag-
nolia), 352
— family, see Magnoliaceae
612
Magnoliaceae, 157, 352
Mahogany, see Swietenia, S.macrophylla,
and S. mahagoni
Maidenhair Fern, 262
— -tree, see Ginkgo biloba
Main climaxes, 329-34
— habitats, 313-20
— sere, autogenic, 331
— successions 322-8
Maine, 319, 326, 328, 371, 549
Maize, see Zea mays
—, world production of, 230
Major regions of distribution, 212
Malaria, treatment of, 264
Malay Peninsula, 239
Malaya, 232, 253, 423, 454
Malaysia, 171
Mallee scrub, 358
Malt, 260
Malva, 249, 453
Mamey, 258
Mammals, 3, 16, 109, 129, 145, 178,
307, 308, 447, 454, 558, 570
—, dispersal by, 109, 115, 116, 123
—, grazing (browsing), effect of, 305-8,
309, 447, 569, 570
—, response to climate, 16
Mamamillaria (Pincushion Cactus), 450
Man, 3, 15, 20, 46, 54, 81, 126, 145, 184,
250, 374, 424, 439, 444, 542) 552,
563
— as biotic factor, 15, 305, 307-11, 566
— as biotic superdominant, 311, 566
—, dependence on plants of, 3, 20, 223,
255-82
—, desert spread by, 213, 307, 449
—, distribution in world, 256
—, effects of cultivation by, 215-52
—, forest destroyed by, 213, 246, 254,
307
—, manipulation of vegetation by, 566—
571
—, plant dispersal by, 118-21, 187, 215,
220-3, 255-7
—, vegetation etc. changed by, 215, 309,
315, 337, 359, 439, 465, 563, 564
—, weeds favoured by, 218-19, 248, 249,
250
Mangel-wurzel, 259
Mangeldorf, P. C., 226
Mangifera indica (Mango), 242, 258, 282
Mango, see Mangifera indica
Mangosteen, see Garcinia mangostana
Mangrove, see Avicennia, Rhizophora,
and R. candelaria
— belt, epiphytes of, 457
—, climax forest, 454, 456
— -swamp forest, 454, 455, 456-8
—, tanning material from, 274
Mangroves, 333, 454, 455, 456-8, 529
550
INDEX
Mangroves, Algae in, 457, 529
—, geographical distribution of, 457
—, root system of, 455, 456
—, seral development of, 456, 458
Manthot esculenta (Cassava), 231, 269
Manila-hemp, see Musa
Manipulation of vegetation by Man,
566-71
Manure as fuel, 271
Manuring, Algae and, 38, 478, 519, 524
—, vegetation and local, 401, 402, 403,
Maple, see Acer [421
forests, mixed, 341
Maps, 4, 22, 572-3
Maquis, 355, 356, 375
Marantaceae, 430
Marattiales, 138, 145
Marchantia, 50
Mare’s-tail, see Hippuris vulgaris s.1.
Margarine, 266
Marginal habitats, 314, 434, 439, 466
— hydroseres, 326, 502-6
— marine vegetation, ice action on, 531,
536, 537-8
Marijuana, 267
Marine Algae, conditions of environ-
ment of, 507-27
— —, dispersal of, 106-7, 109, 510, 513—
14
— —, distribution in depth of, 39, 512-
38
— —, desiccation and, 526
— —, edible, 38, 259
— —, effect on pH of water, 510, 523
— —, migration levels of, 523, 524
— —, periodicity of, 528, 531-2, 537
— —, temperature relations of, 510-11
—-—, tolerance of exposure, 525-6,
530-6
— —, tropical, 458, 529-30
— —, zonation of, 526-7, 529-38
— Angiosperms, 527-309, 531, 532, 535
— Bacteria, 517, 518, 521, 538, 539
— benthos, 521-38
— Chlorophyceae, 508, 512, 523, 528—
35
— Diatoms, 491, 513, 515, 516, 517-20
— environment, features of, 507-15
— Fungi, 527
— humus, 539
— plankton, 480, 509, 510, 513, 515,
516, 517-21
— Schizophytes, 515, 527
— vegetation, 529-38
— —, chemical factors and, 522-4
— —, coastal girdles of, 526-7, 530-2,
533, 534, 535-8
— —, photosynthesis and, 511-12, 513
— —, physical factors affecting, 522
— —, substratum and, 514, 522, 528
— —, survival of, 514, 536-8
INDEX
Maritime climate, see Oceanic climate
— habitats, arctic, 399, 400, 401, 402
— woodlands, tropical, 454-8, 460
Marjoram, 262
Marl, 497
Marpolia aequalis, 131
Marram Grass, see Ammophila arenaria
Marsh Andromeda, see Andromeda
— -marigold, see Caltha palustris agg.
— -plants, anatomy of, 505
Marshland, 372, 373, 406
-Marshy tundra, 385, 387, 390
Marsilea (Water-fern), 61
Maslin, 226
Mass centre, 208, 211
Master factor etc., 9, 320, 331, 373
Maté, see Ilex
Matterhorn peaks, 546
Mature soil, 297—8
Mead, 261
Meadow, 360, 363, 412, 444, 578
—, alpine, 410, 412, 413
— -grass, see Poa and P. pratensis s.1\.
— -moor, 359, 373, 374
— Saffron, 264
Mean, hydrosere—xerosere convergence
to, 328
— temperature, annual, 286, 287
— — of coldest month, 11
— — of warmest month, 11, 287
Means of dispersal, 98
Mechanical constitution of soil, 298,
301
— dispersal, 121-5
Medicago, 249
M. sativa (Alfalfa, Lucerne), 234,
282, 359
Medicinal plants, 262-5, 266-7
Mediterranean Basin, former flora of,
155
— climate, 10, II
— discontinuous range (distribution),
192, 194
— elements of flora, 210, 211
— eulittoral zone, 530
— region, 194, 231, 263, 279, 294, 345,
350, 354, 355, 382
— —, crop plants of, 234, 253
— Sea, 112, 354
— —, depth inhabited by Algae in, 522
— —., vegetation of, 317, 530
Megaphanerophyte, 94
Megaspore, 57, 59, 63, 135
Megatherm, 211, 285
Megathermic Algae, 499, 528
Melastomaceae, 430
Melilotus, 249
Melon, see Cucumis melo
Melville, R., 281
Meningitis, 26
Menyanthes trifoliata (Bog-bean), 117
613
Merismopedia, 28
Meristem, 12, 13
Merrill, E. D., xvii, 252
Mertensia maritima agg. (Oysterleaf, Sea
Lungwort), 107, 399
Mesarch subsere, 324
Mesas, 544, 550
Mescal buttons, 267
Mesembryanthemum spp., 366
Mesic condition, 328, 329
mean, 328
Mesophanerophyte, 94
Mesophyte, 289, 328, 452
Mesopotamia, 311
Mesosere, 324, 405, 409
Mesotaenium berggrenii, 490
Mesotherm, 285, 528
Mesothermic plants, 155, 201, 528
Mesozoicveray- 129, 132,136;-1 30) 14T,
144, 145, 149, 154
Mesquite, 84, 85, 258
Metalimnion, 477
Metamorphic and igneous rocks, 128
Meteorological data, inadequacy of, 284,
288, 320-1, 544
Mexico, crop plants originating in, 253
—, Jurassic flora of, 149
—, Pinus in, 345
—, vegetation of, 354, 360, 443, 447
— City, fossil pollen from below, 226
Mieven) Fe jel. 17, 20
Meyer, B. S., 95
Micro-distribution, 322
— -endemic, 205
Microclimate, 284, 294, 320, 321, 381,
Microcoleus chthonoplastes, 502 [547
Microfauna, 493
Microflora of acidic etc. waters, 492-3,
501
Microhabitat, 313, 320-2, 381, 389, 396,
432, 550
Microhabitats, algal, 322
—, physiographic change and, 320
Microorganisms, economic uses, 278-9
—, soil, 302-4
Microphanerophyte, 94
Microplankton, 480
Microspore, 57, 59, 135
Microtherm, 285, 528
Microthermic Algae, 528
Mid Grasses, 361, 362
Middle-arctic belt or region, 382, 387,
388, 390, 393, 394, 397, 401
— - — heathland, 393, 394 [410
— - — maritime beaches, 399
— -— tundra, 387, 388
Migrant birds, 112, 116, 163, 420
relic, 200
Migration, 3, 5, 8, 15, 21, 97, 98, 112,
156; 157, 103. 205, 323
— along railways, 119
614
Migration, barriers to, 5,97, 103-6, 111,
125-7, 156
—, climatic deterioration and, 153, 155,
160, 161
— elements, 211
—, glaciation and, 153, 155, 156-65
— levels, marine Algae and, 522, 524
— rate, 177
Milkweed, see Asclepias
Millet, 115, 229, 257
Mimosas, 358
Mineral constituents of soil, 298
— nutrients, 301-2, 432, 482, 490, 519
— salts, Algae and, 475, 476
Minorca, 522
Mint, 262
Miocene epoch, 129, 145, 151, 152
— flora, 151
— landscape, reconstruction of, 152
Mires, 314, 373
Mississippi River, 338
Mississippian period, 129
Mistletoe, see Viscum orientale
— family, see Loranthaceae
Mixed forest, 333, 349, 341, 345-6
rain forest, 425, 426
Mixing, in lake water, 477, 481, 486
Modern distributions, foundations of,
154-81
Modifications of crops and weeds, 215-
223
Molinia coerulea (Moor-grass), 374
Mollusca (Molluscs), 117, 524
Monadnocks, 544
Monkeys, fruits and, 116
Monkshood, see Aconitum
Monoboreal theory, 171
Monoclimax hypothesis, 331-2
Monocotyledones (Monocotyledons),
68, 70, 71, 73, 276, 355, 365, 457,
—, fossil, 143, 145, 151 [461
—, marine, 529, 530
—, stem structure of, 70, 71
Monomorph, 223
Monsoon area, 10, II
— forest, 332, 439-41
Monsoons, benthos correlated with, 529
Montane forest, 350, 376, 377, 468, 469
— zone, 377, 467, 468, 470
Montia, 124
Moonwort, see Botrychium
Moor, lowland, 374
-forest, tropical, 462, 463
— -grass, see Molinia coerulea
Mor (raw humus), 303
Mora excelsa (Mora), 426
Moraceae (Mulberry family), 272
Moraines, 545
Morchella (Morel), 44, 45, 259
Morel, see Morchella
Morphological botany, 4
INDEX
Mortality of disseminules, 125
Moss, see Bog and Musci
— Campion, see Silene acaulis agg.
— etc. mat, 325, 402, 405, 407, 409, 503
— protonema, 52, 491
Mosses, see Musci
Mossy forest, see Elfin wood (forest)
Motile Schizophytes and Thallophytes,
or phases of, 26, 29, 30, 31, 32, 33,
34, 37, 41, 42, 45
Mould, see Mucor mucedo
Moulds, see Fungi
Moulton, F. R., 506
Mounds, 547
Mountain-ash, see Sorbus aucuparia and
Eucalyptus regnens
— Avens, see Dryas octopetala s.1\.
— barriers, 105, 125
— climate, 409, 467, 470
— Cranberry, see Vaccinium vitis-idaea
agg.
— grassland, 410, 413
— heathland, 410
— slopes, physiographic aspect and
vegetation on, 294, 295, 296
— Sorrel, see Oxyria digyna
— vegetation, 294-6, 315, 375-7, 409-15
Mountains, endemism on, 205
Mouse, 276, 306
Mucor mucedo (Mould), 44, 45
Mud, marine, seaweeds and, 522, 525,
528, 529, 530, 531, 532, 533
— medallions, desert, 368
— polygons, 370
—, Zostera and, 528, 530, 532, 535
Muddy tidal flats, 454, 456-7, 533
Muenscher, W. C., 253
Mulberry, 258, 269
— family, see Moraceae
Mullein, see Verbascum
Mummy Wheat, 248
Murlins, 259
Musa (Abaca, Manila-hemp), 235, 270
M. paradisiaca s.\. (Banana), 235, 239,
258, 282, 430
Musci (Mosses), 51, 52, 53, 54, 102, 145,
346, 347, 385, 392, 416, 418, 470,
491
—., antarctic, 416, 417, 418, 421-2
—, aquatic, 325, 406, 493, 498, 499, 503
—, —, light requirements of, 473, 475,
503
—, arctic, 102, 385, 387, 401, 407, 409
—, — heath and, 393, 409
—, —, scrub and, 392
— as pioneer plants, 327, 373, 407, 409
—, bog (see also Sphagnum), 373, 374
—, desiccation and, 78
—, dominant, 387, 500
eran] epiphytic, 322, 434; 468, 469, 470
—, fossil, 133, 145
INDEX
Musci (Mosses), grassland, 364
—, lithosere and, 327, 333
—, sand-dune, 371, 548
—, soil accumulation and, 327
—, spore dispersal in, 125
—, torrent-inhabiting, 498
—, tropical swamp-forest forms, 462
—, xeromorphic, 434
Mushroom, see Agaricus
campestris
— rocks, 550
Mustard, 234, 261
Mutants, 206, 207, 220
Mutation, 166, 176, 204
Mycelium, 43, 44, 45
Mycetozoa, see Myxomycetes
Mycorrhiza, 46, 57, 72, 304, 305, 359
Mycorrhizal habit of forest trees, 46, 72
Myosotis alpestris agg. (Alpine Forget-
me-not), 204
M. sylvatica (Wood Forget-me-not),
(Psalliota)
204
Myriophyllum (Water-milfoil), 503
Myristica (Nutmeg), 113, 261
Myrobalan, 274
Myrrh, 273
Myrtle, see Myrtus communis
— wax, 266
Myrtus communis (Myrtle), 355
Myxomatosis, 306
Myxomycetes (Mycetozoa,
moulds), 41, 42, 42
Slime-
Naiad, see Naias
Najas (Bushy-pondweed, Naitad), 503,
506
Naked ovule, 63
Name, scientific, 25
—, vernacular, 25
Nannoplankton, 480, 515
Nanophanerophyte, 94
Nansen, F., 491
Narcotic, 263, 264
Nard Sedge, see Carex nardina s.1.
Narrow-leafed Cattail, see Tvpha an-
gustifolia
— - — Labrador-tea, see Ledum palu-
stre agg.
National Museum of Canada, xix
Native plants, competition with aliens,
221
Natural area (range), 182, 215
— bridges, 542, 547
— distribution, 182
— dyes, 274
— grasslands, 359, 444
— groupings of plants, 6
— levees, 544, 555
— selection, 176, 177, 220, 562, 563
— —, competition and, 562
Naturalization, 219-23
615
Naturalized aliens, 219, 220
Naupli, 486
Nautococcus, 483
Near-desert areas, 364, 449-50, 451
— East, crop plants original to, 253
Nebraska prairie, 361, 362
Nekton, 473
Nelumbium speciosum, see Nelumbo nuci-
fera
Nelumbo nucifera (Nelumbium speciosum,
Indian Lotus, Lotus, Sacred Lotus),
II0, 248, 504
Nematodes, 80, 250
Nematophytales, 132, 133, 145, 146
Neo-endemics, 205, 206
Neolithic culture, 254
Neotropical discontinuity, 194
Nepal, physiographic aspects in, 295
—., plants flowering at high altitudes in,
412, 413
Nereia, 528
Nereocystis, 534
Neritic plankton, 474, 480, 514, 520
— province, 511, 513, 514, 515
Neroli oil, 275
Nervous afflictions, relief of, 264
Nest (nidus), 326, 327
— -epiphyte, 432, 433
Nesting-ground, 402
Net-plankton, 480
Neuropogon, 419
N. melaxanthus, 421
Neuston, 483
Névé, 410, 411, 479, 490
New England, 306, 333, 377, 541
Guinea, 423, 470
— —, forest area of, 246
— Hampshire, 210
— Mexico, 84, 85, 229
— species, formation of, 176
— surfaces, 323, 551
— York, 147
— Zealand, 114, 194, 196, 420, 421
— —, affinities of flora of, 167
— —., aliens in, 119, 221
— —, Conifers of, 353
— —, endemics of, 205
— —. forests of, 247, 351, 352-4
— —, glaciers and vegetation in, 162
— —, grassland of, 360
— —, Jurassic flora of, 149
— —, marine benthos of, 535
— — rainfall distribution in, 376
— —, weeds in, 249
Newbigin, M. I., 20, 214, 378, 471
Newlandia concentrica, 130, 131
Niche, ecological, 321, 431
Nichols, G. E., xviii
Nicotiana tabacum (see also Tobacco),
239, 266, 276
Nidus, 326
616
Niger seed oil, 265
Night temperature, plant distribution
and, 77, 78
Nipa fruticans, 457
Nitella (Stonewort), 503
Nitrate content of water, Algae and, 475,
476, 524
Nitrates, 368, 475, 481, 515, 519
Nitrogen relations, 82, 303, 319
Node, 69
Nomenclature, 24, 25
Non-drying oils, 235, 265-6
— -fruiting, cultivation and, 218
Nordhagen, R., 198
Norsemen, 181, 306
North Africa, 272
— America, 77, 80, 102, 159, 170, 174,
192, 235, 243, 259, 260, 262,
264, 266, 349, 424, 448, 463
516, 567
, ‘ bad-lands ’ of, 366
“cold deserts’ of, 414
— —, coniferous forests of, 343, 345,
346, 348-50
— —, Cretaceous flora of, 150
— -—, deciduous summer forest of,
337-41
— —, Diatoms of, 486, 516, 520
, discovery of, 181
, forest area of, 246
, former climates of, 151
, glaciation in, 157-8, 159, 162
, grasslands of, 360, 361, 362
— —, hot desert of, 449
, Jurassic flora of, 149
, lake-forest of, 349, 350
, mixed forest of, 340-1
, mountain forest of, 376-7
, nunatak hypothesis and, 162-4
, Pacific coast forest of, 348-9
, plant diseases in, 251-2, 305, 306
, — migration in, 153, 157, 161,
171
— —, savanna in, 444
— —, seaweeds of, 525, 532, 533, 534,
536
— —, separation from Europe of, 166
Trapa in, 160, 199
— —, weeds in, 199, 249
— -—and eastern Asia, similarities in
floras of, 167
— Asia, glaciation in, 158
— Atlantic discontinuous range (dis-
tribution), Ig0, 192
— — Ocean, 202, 342, 532
— -facing slopes, 294, 295, 375, 376,
398
— Magnetic Pole, 167, 397, 398
— Pacific discontinuous range (dis-
tribution), 190, 192
— — Ocean, 170
INDEX
North Polar Basin, 127, 491, 539
aan Pole, 99, 127, 154, 166, 170, 184,
381, 491
— Sea, 531
—-South American discontinuous
range (distribution), 1g0, 192
Northern coniferous forest, 332, 343-50
— Wood-rush, see Luzula confusa agg.
Northernmost forests, 343-5, 348
Northwest Territories, 391
Norway, 347, 411
— Pine, see Pinus resinosa
— Spruce, see Picea abies
Norwegian Alps, 414
— Lapland, 327, 344
Nose and throat disorders, treatment of,
264
Nostoc, colonies of, 28, 365
Nothofagus (Southern Beech), 171, 195,
196, 353, 354
N. antarctica (Southern Beech), 342
Nucleus (p/. nuclei), 28, 43, 178
Nudation, 323
Nunatak hypothesis, 162-4, 197
Nuphar lutea (Yellow Water-lily), 108
Nutmeg (see also Myristica), 261
Nutrients, mineral, 301-2, 432, 519
—, soluble, 12, 301-2, 319
Nutrition, angiospermous, 69, 72
—, algal, 32, 35, 39, 475, 476, 481, 520
Nuts, 258
Nux vomica, 263
Nyassa, Lake, 477
Nylon, 270
Nymphaea (Water-lily), 325, 372, 463,
500
N. stellata, 87
Nymphaeaceae (Water-lily family), 504
Oak, see Quercus
— —Beech association, 333, 334
— forests, mixed, 341
Oakwood, 334, 340
Oasis, 3, 365, 376, 416, 449, 452, 454
Oat, Oats, see Avena, A. sativa, etc.
Obelia, 2
Ocean, average depth of, 538
— currents, 107, 2935 510) 5 13-0415 kO
—, euphotic zone of, 511
—, light penetration into, 511
—, temperature at various depths, 510
— water, characteristics of, 507-12, 514
Oceania, 247, 267
Oceanic climate (insular climate, mari-
time climate), 11, 12, 174
— element, 211
— islands, 105, 206
— province (pelagic), 511, 513, 514, 515
Ochroma lagopus s.\. (Balsa), 247
Ocotea rodioei (Greenheart), 247
Ocotillo, see Fouquiera splendens
INDEX
Odum, E. P., 312
Oedogonium, 500
Ogg, W., 223
Oil of citronella, 275
— of Geranium, 275
— of Lavender, 275
— of Rosemary, 275
— seeds, 282
— storage in plants, 33, 35, 479, 517
Oils, 3, 234, 235, 265, 266, 268, 274,
275
Ointments, 266
Okra, 258
Oldest known rocks, age of, 128, 145
Olea (Olive), 235, 258, 265, 354
O. europaea (Olive), 242, 355
Oligocene epoch, 129, 145, 151
Oligotrophic lakes (waters), 318, 476-8
— water, 325, 462-3, 476, 477) 480,
545
— —, Desmids and, 318, 477, 501
Olive, see Olea and O. europaea
Oliver, F. W., 127
Oltmanns, F., 540
Onion, see Allium cepa
Ontario, 130
Ontogenetic change, 217, 322
Oosting, H. J., 312, 332, 378, 578, 580
Ooze, 496, 497, 506, 539
Open Birch forest, 344
— community, 397, 398, 447
— habitat, 81, 114, 120, 126, 163, 165,
218, 296, 374, 387, 395-9, 544
— —, competition weak in, 163, 374
— -soil plants, 108, 544
Ophioglossum — vulgatum
Adder’s-tongue), 61
Opium, 263, 266, 267
— Poppy, see Papaver somniferum
Optimum conditions, competition etc.
and, 302, 482, 485
Opuntia (Prickly-pear), 248, 574
Orache, see Atriplex
Orange, 258
— blossom oil, 275
Orchard-grass, see Dactylis glomerata
Orchidaceae (Orchids), 72, 73, 101, 104,
105, 352, 434, 436, 442, 462
Order, 24
Ordovician period, 129, 132, 145
Organic matter, 302-4
Organisms, oldest known fossil, 130
—, most primitive, 129
Origin of crop plants, 223-39, 253-4
Oryza sativa (Rice), 224, 257, 261, 282
— —, world production of, 225, 229
Osage orange, 274
Oscillatoria, 28, 370°
Osmotic changes, 369, 508, 523
— pressure, variation in, 523
— value of cell-sap, 452
(Common
617
Osmunda regalis agg. (Royal Fern), 202,
203, 342
O. regalis subsp. spectabilis, 202, 203
Otto of roses, 275
Outliers (of forest), 292, 293, 295, 345,
347; 348, 350
Outwash plain, 543, 545
Ovary, 63, 67, 69
Overgrazing, Cactaceae and, 360, 362,
577
—, effects of, 307, 308, 309, 361, 362,
569
—, erosion and, 307, 309, 569
—., plant indicators of, 577
Ovule, 62, 63, 68, 69, 70, 71
Oxalis (Wood-sorrel), 124
O. acetosella (Wood-sorrel), 340
Oxford Botanic Garden, 102, 119
— Ragwort, see Senecio squalidus
Oxycoccus (Cranberry), 258, 501, 560
Oxygen /carbon dioxide balance in sea,
509-10
— concentration, seaweeds and, 523
— content of fresh water, 476, 486, 488
— distribution in lake water, 476-8,
486
— occurrence in sea, 509, 510, 523
—, phytoplankton and, 488
—, role of, in soil, 302
Oxygenation as limiting factor, 319
Oxylophyte, 302
Oxyria digyna (Mountain Sorrel), 108,
405
Oysterleaf, see Mertensia maritima age.
Pacific coast forest, 67, 348, 376
— Ocean, 167, 169, 457, 529, 534
— — benthos, 534, 535
— —, islands of, 109, 167, 194, 259
— —, temperature distribution in water
of, 510
— Silverweed, see
agg.
Pain, relief of, 263, 264
Paint and varnish manufacture, 265, 266
Palaeotropical discontinuity, 194
Potentilla egedii
Palaeozoic era, duration of, 129, 145,
146
— —, flora of, 130, 132-6, 138, 139,
141, 143, 144, 146
Pale-laurel, see Kalmia
Paleocene epoch, 129, 145, 151
Palm-kernel oil, 266
—oil (see also Elaeis guineensis), 235,
266
— -savanna, 445
— wine, 261
Palmae (Palms, Palm family), 73, 187,
211, 351, 352, 355, 425, 427, 444,
445, 457, 460, 461, 463
—, Climbing, 427, 431
618
Palmae (Palms, Palm family), distribu-
—, fibres from, 270 [tion of, 187
—, fossil, 150, 151, 152
Palmer, Es, 17
Palmetto, 274
Palms, see Palmae
Pamir, 413
Pammel, L. H., 282
Pampas, 360, 363
Pan-endemic, 184
Panama, 463
Pandanus (Screw-pine), 458, 459, 460,
P. tectorius (Screw-pine), 459 [463
Pangaea, 165
Pansy, see Viola
Pantoneura, 537
Pantropic distribution (range), 184, 187
— species spread by Man, 187
Pantropical discontinuous range (dis-
tribution), 193, 194
Papaver (Poppy), 103, 105
P. radicatum s.\. (Arctic Poppy), 399,
410
P. somniferum (Opium Poppy), 217,
Papaya, see Carica papaya [263
Paper, 269
— manufacture, 3, 265
Paprika, 261
Papyrus, see Cyperus papyrus
Para Rubber, see Hevea brasiliensis
Parachute fruits, 99, 104, 105
Paraguay-tea, 260
Parasites, 27, 43, 80, 100, 120, 128, 305,
306, 436, 437, 493, 496, 518, 538
—, tropical forest, 437
Parasitic Algae, 524
— Angiosperms, 72, 101, 250, 437
— Bacteria, 26, 27, 250, 437, 491, 496
— Fungi, 45, 80, 250, 251, 265, 305,
306, 437, 491, 496
Parent taxon, 204
Parklands, see Savanna-woodlands
Parsley, 262
Parsnip, 258
Parthenium argentatum, 80
Parthenogenesis, 69
Partial habitat, 313, 320
Partridge, 112
Pasture Mushroom, see Agaricus (Psal-
liota) campestris
—, sprouting after grazing, 13
Pasturing (see also Grazing), 391, 541,
Patagonia, 338 [557
Patchwork quilt effect, arctic cliff vege-
tation, 401
Paternoster-lakes, 545
Pathogenic Bacteria, 26, 80, 279, 496,
Pathogens, airborne, 251 [577
Pattern, 7; $, 9; 320
Patterned soil, 385
Pea family, see Leguminosae
INDEX
Peach, see Prunus persica
Peanut, see Arachis hypogaea
— oil, 235 265-6
Pear, see Pyrus communis
Peat, 54, 151, 271, 326, 373, 421, 46
—, flatmoor, 501 [500-1
—, highmoor, 501
— -moss, see Sphagnum
—, rare in tropics, 462
Peattie, R., 561
Peaty moor, 359
Pecan, 258
Pectins, 272
Pedicularis, 412
Pedunculate Oak, see Quercus robur
Pelagial region, 473, 480, 493, 497
Pelagic benthos, 473
— plankton (region), 473, 480, 513, 514
— plants, rate of increase of, 520
Pelargonium (Pot Geranium), 275
Pelican, 117
Pelophiles, 528
Pelvetia canaliculata, 526, 532
Pendulum theory, 167
Peneplanes, 543
Penetrant, 212
Penetration of light into water, 317
474-5, 504, 511-13
Penfold, A. R., 379
Penguins, 418
Penicillin, 46, 265, 278
Pennisetum purpureum (Elephant Grass),
445
Pennsylvanian period, 129, 146
Peppermint, 262
Peppers (various), 261
Perch (Fish), 117, 477
Peregrine Falcon, 114, 115
Perennation, 92, 93, 217, 343
Perennial habit, 78, 93, 217, 452, 506
Perfumes (scents), 274-5
Peridinians, see Dinoflagellates
Perilla oil, 265
Periodicals, ecological, 378
Periodicity of Algae, 475-6, 481-2, 528,
Sid
Periods, geological, 128, 129, 132, 144,
145, 149-53
—, glacial, 129, 148-9, 152, 156, 161-2
—, interglacial, 156, 157, 161, 162
—, post-glacial, 151-2, 173-5
Periwinkle family, see Apocynaceae
Permanence of continents, 171
— of oceans, 171
Permian period, 129, 145, 148, 154
— —, arid conditions in, 148, 156
— —, flora of, 148-9
— —, glaciation in, 148-9, 156
Perry, 261
Persian berries, 274
— Gulf, 458, 459
INDEX
Persimmon, 258
Persistence of plants on ice-free tracts,
162-5, 197
— — — after introduction by Norse-
men, 181
Perspiration, 482
Peru), 226) 234, 413
Pes-caprae, see [pomoea pes-caprae
Petal, 68, 70, 71
Petroleum, 130, 271
Peyote, 267
pH (hydrogen-ion concentration ‘ re-
action ’), 302, 303, 317, 478, 508,
509
— of aquatic habitats, 319, 476, 510,
523
—, cryovegetation and, 489-90
—, favourable level of, 478
—, lake-water, 319, 478
—., variation in, 490, 523
Phaeocystis, 517
Phaeophyceae (Brown Algae, Brown
Seaweeds), 35, 36, 37, 38, 39, 481,
508, 530, 531, 535, 536, 538, 550
—, Cambrian, 132
—, depth relations of, 317, 522, 530-8
—, dimensions of, 535, 537, 538
—, dominant, 532-8
—, drifting of, 38, 521
—, dwarfed, 369, 508, 526
—, girdles of, 532, 533, 535-8
—., ice-action and, 531, 536, 537, 538
industrial use of, 38, 265
light and, 488, 512, 530
photosynthesis of, 37
rare in fresh water, 475
replaced by Chlorophyceae, 532
—, salinity and, 508, 523, 526
salt-marsh forms of, 369
surf-inhabiting, 525, 532, 534, 535
—, tropical, 529
Phalaris canariensis (Canary Grass), 115
Phanerophyceae, 528
Phanerophyta scandentia, 94
Phanerophyte, 93-5, 464
Pharmacognosy, 262
Pharmacology, 262
Phaseolus vulgaris (Common Bean, Gar-
den Bean, Kidney Bean), 233
Phenology, 574, 576
Phenotypic characters, 183
Philippine Islands (forests etc. of), 423,
427, 429, 455, 468, 469, 470, 504
— Islands, mangrove-swamp forest in,
AD) ve. sae
Phippsia algida agg. (Frigid Phippsia),
Phleum, 364 [405
P. alpinum s.1. (Alpine 'Timothy), 204,
421
P. pratense (Common ‘Timothy,
Timothy Grass), 204, 259
619
Phlox, 124
Phoenix dactylifera (Date Palm), 242,
258, 370, 454, 458, 459
Phosphate content of water, Algae and,
475, 476, 481, 488, 509, 519, 524
Phosphates, 319, 368, 475, 481, 519, 524
Photic region, in Arctic, 537
Photoperiodism, 79, 284, 576
Photosynthesis, 1, 3, 32, 33, 37, 39, 47,
51, 53, 59, 69, 130, 255, 279, 284,
452, 473, 477, 482, 488, 496, 513,
527
—, annual production by, 520
—, aquatic vegetation and, 317, 320,
473, 475, 477, 493, 511, 512, 513,
520
—, bicarbonates and, 130, 497
—, modified forms of, 27, 29, 39
— /respiration balance, 475, 483, 524
—, temperature and, 77
—, terrestrial and aquatic production by,
520
— under ice, 506
Photosynthetic activity in sea, 509, 520
Phragmites communis agg. (Common
Reed, Reed), 98, 461, 463, 505
Phycodrys, 537
Phyllophora, 40
Phylum (p/. phyla), 24, 46
Physiognomy, 14, 96, 463
Physiographic aspect, 294, 295, 296,
322, 375, 413, 470, 544
— barriers, 125-6
— change, microhabitats and, 320
— climax, 331, 411
— condition, 8
— effects, 294-6, 375-7
— factors, 283, 293-6
— vicariad, 202, 204
Physiological activity, environment and,
75-81, 482
— antagonism, 80
— attributes, 5
— changes in cultivation, 216
— drought, 337, 368-70, 458
— functions, cardinal points of, 76-9,
288, 482, 485
— make-up, 6, 9, 75-80
— reactions, 7, 75-81
Physiology, plant, 4, 75
—, plant geography and, 75-81, 482
Phytogeography, see Plant geography
Phytometer, 321
Phytophthora infestans (Late-blight of
Potato etc.), 80, 251, 279
Phytoplankter, 517
Phytoplankton (see also Plankton), 37,
479, 486, 488, 510, 516, 517-21,
575
—, biotic factors affecting, 480-3, 487-8,
—, collection of, 518 [517-2 1
620
Phytoplankton, distribution in depth of,
317, 483, 485, 486, 487, 488
518, 519, 521
—, predators and, 488, 518, 519
—, rate of development of, 519, 520
—, vertical gradient of distribution of,
483, 485, 486, 487, 519
Phytosociology, see Plant sociology
Piassava, 270
Picea (Spruce), 98, 102, 142, 173, 174,
246, 268, 269, 274, 292, 341, 343,
345, 3475 376
P. abies (Norway Spruce), 246, 346
P. engelmannu (Engelmann Spruce),
376
P. glauca agg. (White Spruce), 346,
348
P. mariana (Black Spruce), 292, 346,
347, 348
P. obovata (Siberian Spruce), 346
Pickering, C., 20
Pigeon, 114
— Pea, 258
Pigmentation, 27, 29, 33, 35; 38, 41, 43,
496
—, algal, ecological significance of, 39,
474-5, 495-6, 512-13, 530-1
Pigweed, see Amaranthus retroflexus
Pike (Fish), 477
Pili-nut, 258
Pills, covering of, 264
Pimbina, see Viburnum
Pimento, 261
Pincushion Cactus, see Mammillaria
Pine, see Pinus
— -barrens, 350
— -nut, 258
Pineapple, see Ananas comosus
Pinguicula (Butterwort), 373
Pinus (Pine, see also Yellow Pine), 102,
104, 105, 142, 174, 246, 268, 269,
273, 341, 343, 345, 355, 377
—., five-needled species of, 80, 252
—, geographical data on, 345
—, pollen of, 104, 105
Pinus banksiana (Jack Pine), 346
P. caribaea (Slash Pine), 352
P. contorta var. latifolia (Lodgepole
Pine), 346, 376
P. halepensis (Aleppo Pine), 355
P. insularis, 66, 67
P. palustris (Longleaf Pine), 352
P. pinea (Stone Pine), 355
P. ponderosa (Ponderosa Pine), 295,
376
P. pumila (Siberian Dwarf-pine), 346
P. resinosa (Red Pine, Norway Pine),
349
P. rigida (Pitch Pine), 549
P. sibirica (Siberian Stone-pine), 346
P. strobus (White Pine), 246, 349
>
INDEX
Pinus P. sylvestris (Scots Pine), 173, 181,
200, 246, 327, 346, 574
P. sylvestris, Lapponian form of, 346,
347
P. taeda (Loblolly Pine), 352
P. wallichiana, 295
Pioneer plants, 28, 29, 48, 51, 325, 327,
328, 369, 371, 456, 464, 548
Pistachio-nut, 25
Pistia stratiotes, 484, 485, 504
Pisum sativum (Common Pea), 88, 89,
233
Pitch Pine, see Pinus rigida
Pitcher-plant, see Sarracenia
— - — family, see Sarraceniaceae
Pityeae, 145
Place of origin, of plant group, 178
Plagioclimax, 330, 361, 373
—, biotic, 360, 361, 363, 456
Plan of book, 5-7
Plane-tree, see Platanus
Planets, other, 127
Plank-buttresses, 351, 428, 429, 438, 467
Plankter, 480
Plankton (see also Phytoplankton), 34,
93; 318, 320, 325, 333) 473; 479-89,
51
—, flotation of, 479, 483, 517, 518, 521
—, freshwater, 473, 479-89, 480, 481,
486, 487, 503 Ge
—, freshwater communities of, 480-1
—, limnetic, 473
—, marine, 480, 509, 510, 513, 515, 516,
517-21
—, —, density of, 509
—, —, influence on environment of,
509, 519
—, neritic, 474, 480, 514, 520
—, net-, 480
—, pelagic, 473, 480, 520
Plant adjustments and _ applications,
562-80
— -breeding, 7, 562, 564, 579
— community, 7, 14, 304, 329, 331-4,
554
— —, evolution of, 565
— diseases, 80, 250-2, 305, 306, 341,
576
— —, distribution of, 80,
250-2
— dispersal, see Dispersal
—- distribution patterns, 7, 8, 9
— geographical study, 571-3
ai geography, I, 2, 3,4, 5, 7, 19, 15, 17,
— —, aims of, 3, 4 [19
— —, bases of, 14
— —, earlier works on, 17, 20-3
, economic significance of, 3, 560,
573-9
— —, historical, 9
— —, physiology and, 175-81, 482
>
120, 248
INDEX
Plant geography, techniques of study of,
————, what it 1s, 1-5 [571-3
— groups, 5, 24~74
— —, centres of origin of, 178, 208
— —, geological history of, 128-53, 145
— growth substances, 13, 276, 564
—, ideal, 12-14
— indicators, see Indicator plants
— -like animals, 2
— migration, 5, 97-8,
165, 171, 323
— nuisances, 279-81
— pathogens, 46, 80, 250, 251-2, 265,
305
—- —, methods of control of, 251-2
-— pathology, 250
— response, 16
— Science Monograph, 282
— — Monographs, i
— sociology (phytosociology), 14-15, 92
Plantago (Plantain), 451, 453
P. maritima s.1. (Sea Plantain), 369
Plantain, see Plantago
Plants, animals and, interdependence of,
5 107/
—as conditioners of environment, 3,
255, 277, 565-6
— characteristics of, I
— landforms and, 543-51
—, various groups of, 24—74
Plasmodium (p/. plasmodia), 41, 42
Plastics, 3, 270
Plasticity, morphological, 177
Platanus (Buttonwood, Plane-tree), 192,
194
P. occidentalis, 192
P. orientalis, 192
Pleistocene epoch, 129, 133, 145, 151,
173
— flora etc., 161, 162
— glaciation, 129, 156-8, 159, 161-5,
420
— persistence vs subsequent immigra-
tion, 161-5
Pleodorina, 30, 31
Pleurococcus, 30, 31%
Pleurophyllum hookert, 420
Pleuston (hemiplankton), 483, 484, 485,
497, 503, 504, 506, 515
Pliocene epoch, 129, 145, 151
— flora, 151, 160
Plum, see Prunus domestica
Plumed fruits, ror, 104, 105
— seeds, 99, IOI, 104, 105
Pluvial periods, 160, 197
Pneumatophore (aerating root),
455, 456
Pneumonia, 26
Poa (Bluegrass,
364
P. alpina, 103
xX
U5 3,005 5-00,
Meadow-grass), 361,
621
Poa P. ampla (Big Bluegrass), 77
P. annua (Annual Meadow-grass),
120, 249
P. arctica s.l. (Arctic Meadow-grass),
383
P. flabellata, 421
P. foliosa, 420
P. pratensis s.1. (Bluegrass, Meadow-
grass), 234, 259
Podocarpus, 353
Podostemaceae, 463, 498, 504
Podzol (podsol), 298, 299, 300
Poison, 264, 279, 280
— -ivy, 280
Polar Bear, 401
— climate, 11, 380-1, 415
— lands, vegetational types of, 11, 380—
409, 415-19
— origin of floras, 171-2
— oscillation, 167
— pack-ice, 388
— tree-line, 212, 343-4, 380
— Willow, see Salix polaris agg.
Poles, wandering of, 167, 170
Pollen (pollen grain), 63, 68, 70, 71, 104,
105, 117-18, 141, 181, 280, 497
— sac, 63
—, sub-fossil, 152, 181
—, transport of, 63, 68, 117-18, 181
— tube, 63, 68, 70, 71
—, viability of, 117
Pollination, 15, 16, 68, 252, 305, 307,
340
Polunin, I. V., 424, 454
—, N., 127, 181, 184, 198, 422
—, O., 412, 414
Polychore (‘ wide ’), 210
Polyclimax, 331, 390
Polyendemic species, 205, 206
Polygons, see Soil polygons
Polygonum viviparum (Viviparous Knot-
weed), 385, 386, 387
Polymorphs, 223
Polyphylesis, 207
Polyploid hybrids, 179, 204, 234, 235,
239
Polyploids, 175, 178-80
— and areas, 178-80
—., distribution of, 179
—., efficiency in competition, 180
Polypodium vulgare agg. (Common
Polypody), 420
Polypody, see Polypodium vulgare agg.
Polysiphonia, 40, 532
Polytopic origin, 188, 207
— species etc., 205, 206, 207
Polytopy and incidence of areas, 206-9
Polytrichum, 392, 497
Pome, 258
Pomegranate, 258
Ponderosa Pine, see Pinus ponderosa
622
Pondweed, see Potamogeton
Pontederiaceae, 494
Poplar, see Populus
Poppy, see Papaver
— oil, 265
Populus (Aspen, Balsam Poplar, Cotton-
wood, Poplar), 68, 102, 246, 269,
281, 325, 340, 342-6, 365, 372, 375,
376, 454, 544, 553
—, dominant, 343
P. tremuloides (American
Trembling Aspen), 346, 553
Pore Fungus, 45
Porolithon onkodes, 529
Porphyra capensis, 531%
Porsild, A. E., 198
Posidonia, 317
P. oceanica, 530°
Post-Miocene flora, impoverishment of,
160
— -Pleistocene flora, 165, 173-5
— - — immigration, 165
Postclimax, 331, 361, 390, 393, 402
Postelsia, dwarfed, 526
P. palmaeformis, 525, 534
Postglacial changes, 165, 173-5, 201
— climatic succession, 173-4, 201
—— Peat wl 5 ls
— relic, 197, 201
Pot Geranium (Pelargonium), 275
Potamogeton (Pondweed), 108, 112, 117,
319, 325, 500, 503, 504
P. natans, 504
Potamogetonaceae, 494
Potamoplankton, 479, 488, 489
Potash, algal source of, 265
Potato, see Solanum tuberosum
—, grown in Far North, 231
—, Late Blight of, see Phytophthora
-infestans
Potential area, 176, 209, 215, 224
Potentilla egedii agg. (Pacific Silver-
weed), 399, 400
P. saundersiana var. caespitosa, 412
Poterium sanguisorba (Salad Burnet), 79
Poverty-grass, see Danthonia
Prairie, 85, 343, 360, 361, 362, 363
—, mid-grass, 361, 362
—, overgrazed, 361, 362
—, short-grass, 361, 362
— soil, 298, 299
Prantl, K., 74
Prasiola, 524
Pre-Boreal period, 173, 174
— -Cambrian era, 128, 129, 130, 145,
146
— - — —, Algae of, 129, 130, 146
— - — —, Bacteria of, 146
— -Pleistocene southward
161
— -Tertiary relics, 201
Aspen,
migration,
INDEX
Precipitation (rainfall, snow, hail, dew),
10, 11, 288-9, 290, 294, 338, 345,
349, 35°, 354, 359, 366, 376, 424,
439, 442, 444, 467, 474, 555, 574
—, annual, 288-9, 290, 381, 441
—, arctic, 316, 381
— in arid regions, 364-6, 447-52
—, world map of, 290
—, chemical, 495, 497
Preclimax, 331, 361, 390
Predation, plankton and, 488, 519
Prescott, G. W., xviii, 483, 487, 491
Prevailing climax, 330
Prevernal aspect, 285, 340, 342
Prickly Lettuce, see Lactuca scariola
— -pear, see Opuntia
Primary climatic grouping, 10, II
— producers, Algae as, 278-9, 515
— sere, 324
— xerosere, 325-8, 464
Primitive Angiosperms, suggestions of,
139, 144
— organisms, 129-30
Primofilices, 138
Prince Charles Island, 127
— Edward Island, 420
— of Wales Island, 397, 398
Principles of cultivation, 310-11
Pringlea antiscorbutica (IKerguelen Cab-
bage), 417, 419, 420
Prisere, 324
Producer plants, 257, 480
Productive capacity, 509, 541
Productivity of land and sea compared,
520
— types, fresh waters, 318, 319, 476-8
Profile diagram, 425, 426
—, lake-marginal, 493
—, sea-marginal, 513
—, soil, see Soil profile
Profundal region, 493, 496, 498, 503
Progression to climax, 324
Progressive changes in dominants, 464
Prop-roots, 455, 456, 458, 459
Proseral benthos, 503
— Lichens, 327
— planktonic ete.
502-3
— Schizophytes, 326
Protea, 355
Protective coloration in animals, 424
— covering, loss in cultivation, 217
Proterozoic era, 128, 130, 131, 145
Prothallus, 56, 57, 59, 63
Protolepidodendrales, 145
Protolepidodendron primaevum, 147
Protonema (pl. protonemata), 52, 53,
491
Protoplasm, 13, 321, 508
Protozoa, 305, 495
Prune, 258
deposition, 325
INDEX
Prunus avium (Wild Cherry), 340, 342,
343
P. domestica ete. (Plum), 111, 242,
258, 454
P. laurocerasus (Cherry-laurel), 343
P. persica (Peach), 111, 113, 242;
258
Psalliota, see Agaricus compestris
Psammon, 495
Psammosere, 324, 328, 371, 407
Pseudo-endemic, 206
— -relic (relict), 200
Pseudobornia, 145
Pseudotsuga taxifolia (Douglas Fir), 243,
_ 246, 295, 348, 376, 574
Psilophyta, 135
Psilophytales, 145, 146
Psilophytineae, 133, 134, 135, 138
Psilophytinean complex, 137, 146
Psilophyton princeps, 134
Psilopsida, 135, 145, 146
Psilotales, 145
Psilotum, 135
P. triquetrum, 134
Psychrometer, 289
Psyllium, 263
Ptarmigan, 116
Pteridium (Bracken), 466
P. aquilinum agg. (Bracken), 62, 204,
249, 348
Pteridophyta (Pteridophytes), 24, 55—-
62, 129, 133, 143, 145, 146, 149,
172, 349, 527
—, epiphytic, 432
—, fossil, 133, 134, 135, 136, 137, 138,
146, 148, 149
—, industrial use of, 264
Pteridophytes, see Pteridophyta
Pteridospermae (Pteridosperms, Seed-
ferns), 138, 139, 141, 144, 145, 146,
147, 148, 149
Pteris longifolia, 61
Pterocarpus indicus
wood), 247
Ptilota pectinata, 537
Puccinellia (Alkali-grass), 369, 399
P. phryganodes agg. (Creeping Alkali-
grass), 109, 399, 400
Puffball, see Lycoperdon
Pulp, 3, 239
Pumpkin (see also Cucurbits), 217, 258
Punas, 413, 414, 470
Pure-stand forestry, 574
Purgative, 263
Purple Bacteria, 27
— Saxifrage, see Saxifraga oppositifolia
agg.
— — barren, 402, 403
Pyorrhoea, treatment of, 264
Pyrenees, 156
Pyrethrum, 276
x*
(Burmese Rose-
623
Pyrus communis (Pear), 242, 258, 261
P. malus (Apple, Crab Apple), 80,
112, 242, 258, 261, 272, 282, 340
Quadrat, 366, 452, 453, 567, 568
Quail, 112
Quaking bog, 501
Quarantine, 251, 576
Quassia, 264
Quaternary period, 129, 151, 155, 168
— (see also Pleistocene), glaciation, 156
Quebec, 393, 401
Quebracho, 274
Queen Maud Mountains, 419
Quercetum, 334
Quercitron, 274
Quercus (Oak, see also Aleppo Oak,
Aunkishwi@ake)OS.Oo,k 7s eas
246, 268, 274, 205, 334, 341, 343,
35055
—, evergreen, 351, 352, 354, 355, 357
— fruit (acorn), 258
Quercus alba (White Oak), 341
O. ilex (Holm Oak), 355
QO. montana (Chestnut Oak), 341
O. nigra (Black Oak), 160
O. petraea (Q. sessiliflora, Sessile
Oak), 340
O. robur (Pedunculate Oak), 340, 541
O. rubra s.1. (Red Oak), 341
QO. suber (Cork Oak), 279, 355
O. virginiana (Live Oak), 352
Quillwort, see Jsoetes
Quillworts, see Lycopodineae
Quince, 258
Quinine, 264
Quinoa, 257
Rabbits, 112, 306, 363
—, regeneration of Beech and, 306
Radiation, 10
Radish, 258
Rafflesia arnoldiu, 437
R. manillana, 437
Rafting, 109
Ragweed, see Ambrosia
Railways, migration along, 119
Rain, see Precipitation
— forest (see also Tropical rain forest,
Warm-temperate rain forest), 11,
247, 352, 354, 424, 426, 427, 429,
467, 468, 469
— —, epiphytes of, 351-2, 353, 431-5
— gauge, 289
— -shadow of mountains, 160, 376
Rainfall, see Precipitation
Rainwash, dispersal by, 107, 108, 124
Raised beach, 396
— bog, 373, 501
Ramaley, F., 281
Ramie, see Boehmeria nivea
624
Range, see Area and Distribution
Ranges (distributions), types of, 183-97
Ranunculus (Buttercup, Water Butter-
cup, Water Crowfoot), 106, 405, 504
R. aquatilis s.\. (© Batrachian ’ Ranun-
culus), 485
R. biternatus (Buttercup), 419, 422
Rape oil, 265
Rapids, vegetation of, 463, 498
Raspberry, see Rubus idaeus
Rat, 276
Rate of evolution, 176, 177
— of increase, pelagic plants, 520
Raticide, 276
Rattan (Climbing Palm), 427, 431
Raunkiaer, C., 92, 96, 527
— life-forms, 94
Raup, H. M., xvii, xvili, 323, 343
Ravines, 405, 443, 544
Raw humus, 303
— materials, 3, 239, 268
Raynor, R. N., 253
Rayon, 270
Reaction, physiological, 75—80
—, plant and habitat, 324
Recent period, 129, 145, 151, 152, 173
Receptacle, 110
Recession of ice, 129, 156, 160-2, 165
Rechinger, K. H., xvi
Recombination, genetical, 177
Reconstructions of former landscapes,
146, 147, 148, 150, 152
Record, S. J.; 243, 253
Recurrent glaciation, 129, 156, 160, 161
Red Algae, see Rhodophyceae
— Fescue, see Festuca rubra s.1.
— Gum, 246
— loams, 298
— Mangrove, see Rhizophora mangle
— Oak, see Quercus rubra s.1.
— pepper, 261
— Pine, see Pinus resinosa
a Sea, 443, 507, 517
— Seaweeds, see Rhodophyceae
— snow, 489, 490
— squill, 276
Redtop, 259
Redwoods, fossil, 158, 199, 209
—, giant, see Sequoia and Sequoiaden-
dron
Reed, see Phragmites communis agg.
— -swamp, 314, 319, 325, 326, 333,
372, 373, 405, 461, 463, 505, 545
Reedmace, see Typha
Reef-Algae, 529
Reforestation, 246, 364, 578
Refuges (refugia), 15, 156, 162-5, 499
Regeneration, forest, 306, 307, 308, 310
Regional adaptation, 563
— belts, arctic, 382
— climatic climax, 330-1, 332
INDEX
Regional climax, 330
Reindeer, 48, 259, 389
—-moss, see Cladonia and C. rangi-
ferina
Relative humidity, 11, 288, 289, 294,
424, 449
Relic (relic) area, 157, 158, 197-201
— species, 197, 199, 200
Relics (relicts), 197, 199-201
Relict community, 331
— plant, 163, 197, 199-200
Religious Tract Society of London, 17
Reproduction, 12, 15, 26, 28, 32, 33, 35,
37, 39, 59, 63, 68, 78, 79, 518, 520
—, periodicity in Algae, 532, 537
Reptilia (Reptiles), 117, 129, 145, 155
Requisites of life, animals and, 3, 15, 255
Residual features (landforms), 542, 543,
544, 546, 547, 550
Resiny 360272
Resistance capacity, resting stages and,
Ua
— to adverse conditions, 77, 78, 79, 81—-
7, 100, 365-6, 381, 450-2, 495,
526, 564
Respiration, 3, 13, 77, 254, 302, 510
— /photosynthesis balance, 475, 483,
524
Resting stages, resistance capacity of, 77
Retrogression, 324, 329, 330, 331, 466
Rhacomitrium, 419
— heath, 409
R. lanuginosum, 409
Rheumatism and gout, treatment of, 264
Rhizoclonium, 368
Rhizoid, 49, 51, 53, 56, 59, 327, 503
Rhizome, 55, 69, 90, 91, 93, I21, 122,
123, 134, 371, 458
— geophyte, 94
Rhizophora (a dominant of mangroves),
107
—, germination of, 110, 456
R. candelaria, 455
R. mangle (Red Mangrove), 456
Rhizosolenia, 516
Rhododendron, 349
R. lapponicum (Lapland Rose-bay),
Rhodomonas, 488 [393
R. lacustris, 480
Rhodophyceae (Red Algae, Red Sea-
weeds), 38-9, 40, 41, 132, 265, 457;
481, 488, 508, 523, 529, 531, 532,
533, 535, 538
—, depth relations of, 39, 317, 512, 537
—, freshwater, 39, 496, 498, 499
—, habitats of, 39, 496, 499, 529-38
—, in mangroves, 457
—, light and, 39, 474, 512, 513, 530, 531
—, shade-species of, 530
Rhubarb, 233, 234, 258
Rhynia, 134
INDEX
Ribes (Currant, Gooseberry), 80, 186,
242, 248, 252, 258, 349
—, distribution of, 186, 187
Rice, see Oryza sativa
Richards, P. W., xviii, 424, 465, 471
Richardson’s Willow, see Salix richard-
soni age.
Ridley HaiNe Tors i]s Li4smn5 ene 7
Rigid Sedge, see Carex bigelowii agg.
Rill erosion, 309
Rissoella verruculosa, 530
River banks, 374, 431, 452
Rivers, benthos of, 498-9
—., dispersal by, 107-8, 489
Rivularia, 28
R. haematites, 495
Roach, 117
Robbins, W. W., 253, 281
Roches moutonnées (sheep rocks), 546
Rock, corrosion of, 326, 494
—, oldest known, 128, 145
— Sedge, see Carex rupestris
— surface, colonization of, 325, 326,
327,407, 411, 416, 514, 522, 524, 533
Rockweed, see Fucus
Rocky Mountains, 346, 377, 414
Root, 12, 32, 90, 91, 302-3, 437, 438,
451, 484, 485
—, aerial, 433, 434, 435, 436, 470
—, climbing, 89, 431
— -competition, 81, 365, 428, 432, 442
— crops, 229-32, 257-8
—, drugs from, 264
— growth, 82
— -parasite, 437, 452
— -system, 85, 137, 364, 432, 458
—, toxic excretion from, 80
Rooting, deep, 76, 82, 84, 450, 451, 458
Roots, inflated, 86, 87
Rootstock, 69
Rorippa nasturtium-aquaticum (Water-
cress), 258
Rosa (Rose), 112, 275
R. damascena (Damask Rose), 275
Rosaceae (Rose family), 417
Rose family, see Rosaceae
—of Jericho, see Anastatica hiero-
chuntina
— oil, 275
Rosemary, see Rosmarinus officinalis
Rosette hemicryptophyte, 94
Roseveare, G. M., 471
Rosin, 269, 273
Rosmarinus officinalis (Rosemary), 275,
355
Ross, R., xviii, 476, 477, 481
Rostkovia magellanica, 421-2
Rotation of crops, 279, 311, 557, 57°
Rotenone, 276
Rothamsted Experimental Station, 223
Rotifers, 486
625
Royal Fern, see Osmunda regalis agg.
Rubber, 3, a 272, 282
Rubel, De
Rubiaceae (Nadder family), 430
Rubner, K., 451
Rubus (Blackberry, Bramble), 258, 342
R. chamaemorus (Baked-apple), 406
R. idaeus (Raspberry), 111, 258
Rudolph, L., 22
Rumex acetosella agg.
249, 577
Run-off, erosion by, 289, 555
Runner, 69, 121, 122, 123
Running plants, 87, 89, 121
Rush, see Juncus and Funcaceae
Russell, E. J., 335
ma Bees BRS
Russia, 77, 360, 361, 365
Russian-thistle, see Salsola kali var.
tenuifolia and SS. pestifer
Rusts (see also Fungi), 43, 280
—, cereal, 251
Rutabaga (Swede), 231
Ruttner, F., 487, 501, 506
Rye, see Secale cereale
(Sheep Sorrel),
Saccharomyces (Yeast), 43, 44, 45, 46,
260, 269, 499
Saccharomycetaceae (Yeasts),
418
Saccharum officinarum
239, 241, 269, 282
Sacred Lotus, see Nelumbo nucifera
Safflower, 274
— oil; 265
Saffron, 262, 274
Safrole, 264
Sage, 262
— -brush, see Artemisia tridentata
— - — area, 364
Sago, 269
Saguaro, see Carnegiea gigantea
Sahara Desert, 102, 156, 449, 450
— —, sea in Cretaceous times, 156
— —, semi-desert areas of, 365
— —, semi-desert scrub near, 447
Sahuaro, see Carnegiea gigantea
St. Helena, endemism in, 205
Sake, 261
Sal-tree, see Shorea robusta
Salad Burnet, see Poterium sanguisorba
Salicornia (Glasswort, Saltwort), 368
Saline habitat, 316, 368-70, 399, 454,
502
— soil, 301, 365, 366, 368-70, 450
— waters, inland, 369-70, 489, 492, 502
Salinity, 316, 318, 368, 456, 460, 472,
489, 507, 537, 560
—, fluctuating, 474, 502
—, variation in, 507-8, 522-3, 531, 533,
537
1s Bohs,
(Sugar Cane),
626
Salisbury, E. J., xvii, 127
Salix (W illow), 68 1025007352222 745
325, 342, 346, 384, 385, 391, 392,
393, 406, 411, 454, 553
— of arctic scrub, 390, 391, 392
— in arctic tundra, 383, 384, 385, 387
—, erosion checked by, 569
—, heathland, 359
—, seed of, 102
— in semi-desert oases, 365
—, snow-patches and, 403, 404
S. alaxensis agg. (Feltleaf Willow),
392
S. arctica s.). (Arctic Willow), 385
S. cordifolia s.1. (Broad-leafed
Willow), 392
S. glauca s.\. (Glaucous Willow), 390,
391, 392
S. pees (Herb-like Willow),
403, 404
S. polaris agg. (Polar Willow), 389
S. richardson agg. (Richardson’s
Willow), 392
Salsify, 258
Salsola kali var.
thistle), 249
S. pestifer (Russian-thistle), 102
Salt-bush, see Atriplex canum
tenuifolia (Russian-
— crusts, Algae on, 502
— desert, 366, 367, 368
— -marsh, 6, 296, 333, 367, 368-70,
458, 459, 533, 55°
—, Algae of, 368, 369, 370
— - —, arctic, 399, 400, 401
— - — Sedge, see Carex salina s.1.
— - — vegetation, 368-70, 400, 459
— - —, warm-temperate, 367, 370
— -marshes, location of, 368, 399
— relations of Algae, 474, 490, 502, 508,
509, 523, 533
Saltatory dispersal, 103
Salts needed by plants, 12, 368
Saltwort, see Salicornia
Sambucus (Elder), 349
— nigra (Elder), 342
Sand fraction in soil, 298
— -binding plants, 122, 123, 277, 328,
371, 400, 407, 458, 542, 548, 550
— -box Tree, see Hura crepitans
— -dunes, 296, 316, 371, 458, 547, 548,
555
—--—, climax forest following, 371,
548
— -—, Gramineae of, 371, 458, 542,
— -—, migration of, 548 [548
—, secondary colonization of, 371
—, stabilization of, 328, 371, 458,
548, 549, 566, 569
— - —, succession on, 371, 548, 549
—, wet, organisms inhabiting, 495
INDEX
Sandeman, C. A. W., 414
Sandwort, see Arenaria
Sandy shores, 371, 399, 458, 495, 532-3
— soils, 301, 303, 443
Santonin, 263
Sap green, 274
Sapodilla, 258
— tree, 272
Sappanwood, 274
Saprophyte, 27, 41, 43, 51, 53, 100, 120,
280, 320, 350, 436-7, 493
Saprophytes, angiospermous,
350, 436
—, cave-flora, 546
—, marine planktonic, 518, 539
— of ocean depths, 320, 538
Saprophytic prothallus, 57, 59
Saprophytism, 80
Saproplankton, 480
Sargasso Sea, 38, 319, 521, 529
— -weed, see Sargassum
Sargassum (Gulf-weed, Sargasso-weed),
37> 515, 521, 528
S. fluitans, 521
S. natans, 521
Sarracenia (Pitcher-plant), 91
Sarraceniaceae (Pitcher-plant family),
1g0, 192
Sarsaparilla, 260, 262
Saskatchewan, 343
Sassafras, 262
Satinw ood, see Chloroxylon swietenia
Saturation-deficit, 289, 291
Savanna, 213, 246, 318, 332, 333, 352,
359, 360, 363, 443-7, 464, 465, 466
—, bush, 443, 444
—, derived, 465
—, geographical locations of, 444, 446,
464
—, Gramineae of, 443, 445, 446, 465
—, montane, 470
—, rainfall and, 444
— -woodlands (parklands), 343, 440-3,
72, 342,
Savoury, 262 [445
Sawdust, 271
Saxifraga (Saxifrage), 108, 116, 387,
396, 405, 409, 410, 413
S. aizoides agg. (Yellow Mountain
Saxifrage), 79, 204
S. hirculus agg. (Yellow Marsh Saxi-
frage), 385, 402
S. oppositifoia agg. (Purple Saxi-
frage), 186, 189, 192, 395, 397, 399,
402, 403
S. rivularis agg. (Alpine Brook Saxi-
frage), 204, 402
S. tricuspidata (Three-toothed Saxi-
frage), 408
Scalding, 87
Scale of geological time, 145
Scandinavia, 181, 184, 231, 343, 348, 563
INDEX
Scandinavia, pre-Pleistocene migration
in, 161
—, Trapa natans in, 199
Scavengers, 43, 46, 278, 280, 303, 305
Scents (perfumes), 3, 274-5
Schery, R. W., 253, 281
Scheuchzer’s Cotton-grass, see Erio-
phorum scheuchzeri
Schimper, A. F. W., 20, 23, 378, 471,
506, 540
Schismus, 451, 453
Schizophyceae, see Cyanophyceae
Schizophyta (Schizophytes), 24, 26-9,
46, 145, 146, 515, 527, 550
Schizothrix lacustris, 495
Schmidt, K. P., 17
Schouw, J. F., 21
Schultes, R. E., xviii, 428, 462
Scientific name, 25
Scilla non-scripta, see Endymion (Scilla)
non-scriptus
Scirpus lacustris (Bulrush), 204, 505
S. maritimus, 370
S. tabernaemontanii (Glaucous Bul-
rush), 204
Scitamineae, 430
Sclerophyll, 354
Sclerophyllous forest (see also Sclero-
phyllous woodland), 332, 356, 357
— shrubs, 289, 356
— woodland, 354-8, 356, 357
— —, geographical locations of, 354,
355, 358
Scotland, 292, 526, 531
Scots Pine, see Pinus sylvestris
Scouring-rush, see Equisetum
Scrambling plants, 87, 88, 89, 431
Scree, 372, 389, 396, 407, 551
Screw-pine, see Pandanus
tectorius
Scrub Birch, see Betula glandulosa agg.
community, IT, 314, 354, 355, 356,
357, 358, 372, 391, 392, 411,
447, 463
— —, arctic, 390, 391, 392
—, desert, 364, 447-8
Scurvy-grass, see Cochlearia (fficinalis s.1.
Sea Arrow-grass, see Triglochin mari-
tima
—-beach Sandwort, see
(Honckenya) peploides
— -birds, dispersal by, 114, 420
— -blite, see Suaeda maritima
— -bottoms, aphotic, 538-40
— -caves, 550
— currents, dispersal by, 106, 107, 513-
14
— -drift, disseminules in, 460
—., freezing-point of, 510
— -gull, 401
— -lettuce, see Ulva and U. lactuca
and P.
Arenaria
627
Sea-level, changes in, 170
— Lungwort, see Mertensia maritima
agg.
— -margin profile, 513
—, organic productivity of, 509, 520
— -pea, see Lathyrus maritimus agg.
—, pelagic division of, 512, 513, 514
—, photosynthetic activity in, 509, 520
— -pink, see Armeria maritima s.1.
— Plantain, see Plantago maritima s.1.
— -purslane, see Arenaria (Honckenya)
peploides agg.
— -rocket. see Cakile
—, surface temperature of, 510
—, vegetation of, 507, 512-40
— water, calcium carbonate content of,
508
——, carbon dioxide /oxygen balance
in, 509-10
— —, oxygenation of, 509-10, 523
S ew. pH of, 523
— —, salt-content of, 507-8
— —, seeds killed by, 111
— —, variable salinity of, 317, 507-8
Sears, P. B., xvii, xviii, 561, 571, 580
Seaside and other arctic vegetational
types, 399-405
Seasonal algal maxima, 481, 482, 519,
520
— aspects of freshwater vegetation, 499
— — of woodland vegetation, 285, 334,
338, 339, 342, 439-41, 449, 450
— development of foliage, 339
— forest, 439
— groups of algal epiphytes, 500
— occurrence of Diatoms, 481-2, 499,
520
— recurrent changes in plankton, 481,
520
— (aspect) society, 334, 340, 360
— variation (change), 146, 522, 530, 531
— — of phytoplankton, 481, 518-20
— vicariad, 204
Seaweeds killed by frost, 522
—, marine ice and, 531, 536, 537-8
—, marine mud and, 525, 529, 532, 533
—, temperature groups of, 528
Secale cereale (Rye), 218, 226, 257, 282
— — —, world production of, 228, 229
Secondary colonization, 200, 371, 424
forest, 307, 372, 465, 466
— forest dominants, 344, 372, 466
— scrub, 460, 465
— sere, 324, 464, 465
— succession, 324, 464, 465, 466
Sedative, 263, 264
Sedge, see Carex
— family, see Cyperaceae
— -Grass tundra, 388
— -meadow, 325, 326, 372, 373, 406
Sedges, see Cyperaceae
628
Sediment, 492, 497, 514, 529, 539-40
—, weight of annual production of, 497
Sedimentary ooze, 492, 496, 497, 503
— rocks, 129
Sedimentation, 497, 501
Seed, 12, 62, 63, 68, 69, 78, 98, 104, 105,
I10, I13, 122, 123, 139, 140, ee
— -coat, 62, 101, I10
— dispersal (see also Dispersal), 69, D8.
122, 123, 452
— dormancy, 452
— ejection from fruit, 121-5
— -ferns, see Pteridospermae
— germination, 69, 76, 452
— —, delayed, 248
— —, digestion and, 114, 115, 117
——OuULpUE 125
— -plants, see Spermatophyta
—, size affected by cultivation, 217
Seedless fruits, cultivated and, 218
Seedlings, competition between, 103
— numerous after rain in desert, 368,
_ 451-2, 453
Seeds and fruits, as source of drugs, 263
104,
— — —, longevity of, 248 [105
— killed by sea-water, 111
Selaginella (Spike-moss), 56, 58, 430
Selection, see Artificial selection,
Natural selection
Selective action of fire, 310
Semi-cosmopolite, 184, 249
— -desert bush-land, 448
—--—, plant dominants of,
447-8
— - —, precipitation on, 364, 447-8
— - — scrub, 333, 364-5, 447, 448, 454
— - — —, geographical locations of,
364-5, 447, 448
— -deserts, 213, 246, 364-6, 447-8
— - —, geographical locations of, 364-5,
382, 447-8
— -parasites (hemiparasites), 437, 438
Semi-drying oils, 235, 265
Senecio squalidus (Oxford Ragwort), 119
Senega, 264
Senescence, 200
Sensitiveness of aquatic organisms to
change in environment, 474, 509
Sepal, 68
Separation of Europe and N. America,
166
Sequoia (Redwood), 157, 158, 199, 205,
209
S. sempervirens (Coastal Redwood),
63, 67, 158, 348, 379
Sequoiadendron (Redwood),
199, 205, 209
S. giganteum (Big-tree, Giant or
Sierra Redwood), 158, 348
364-6,
157, 158,
INDEX
Seral communities of temperate lands,
344, 350, 370-5
— (developmental) counterpart, 333-4
— development of mangroves, 456, 458
— types in Arctic, 405-9
Sereclimax, 331
Sere (see also Succession), 323-8, 503
—, arrest of, 330
—, burning and, 330, 331
—, components of, 323-4
—, grazing and, 330, 331
—, primary, 324
—, secondary, 324, 464, 465
Series of books on crops, 282
Serule, 370
Sesame, 235
Sessile Oak, see Quercus petraea
Sewage, 523
Seward, A. C., 153
Sexuality, 27, 32, 37-8, 39, 45,
68-9
Shade Algae, 530
— -epiphyte, 434
— leaf, 89, 217
— tolerance, 428, 430, 434, 530, 546
Shaggy-mane Mushroom, see Coprinus
comatus
Sheep and caterpillars in Greenland,
306
— rocks, 546
— Sorrel, see Rumex acetosella agg.
Sheet erosion, 309
Shelf-ice, 538
Shells, Algae inhabiting, 494
Shellac, 273
Shelter, distribution and, 293, 317-18,
Sheltering materials, 267-8 [322
Shepherd’s-purse, see Capsella bursa-
pastoris
Shield-fern, see Dryopteris
Shifting cultivation, 307, 465
— of poles, 167, 170
Shingle-beach succession, 371-2
— - — vegetation, 371-2, 400, 458
—, soil accumulation in, 371, 372
Ships of wood, 267
Shithatha Oasis, 367, 368, 370
Shore Algae, 400, 522-32, 533, 534-8
—, physical characters and plant growth
on, 475, 514, 522, 525, 530-6
— vegetation, ice action on, 475, 531,
536, 537-8
— -water plankton, 488
Shorea robusta (Sal-tree), 467
Short-day plants, 284
— Grasses, 316, 361, 362
— growing-season in Arctic, 78, 381
Shrubs, 11, 93, 102, 354-66, 391, 447
49-64,
_ 549
Siberia, 171, 343, 348, 537
—, wheat belt in, 564
INDEX
Siberian conifers, 346, 348
— Dwarf-pine, see Pinus pumila
— Fir, see Abies sibirica
— Larch, see Larix sibirica s.1.
— Spruce, see Picea obovata
— Stone-pine, see Pinus sibirica
Sierra Redwood, see Sequoiadendron
giganteum
Sigillaria (Sigillarias), 137, 148
Significance and distribution of plant
diseases, 250-3
— of weeds, 248-50, 253
Silene (Campion), 219
S. acaulis agg. (Moss Campion), 108,
284, 285
S. linicola, 219
Silica, 56, 298, 476, 497
— secreted by Diatoms, 34, 497
Siliceous deposits, 34, 497, 547
Silicoflagellates, 517
Silkweed, see Asclepias
Silting, 293, 325, 369, 456, 476, 505,
524, 555
—, Chara and, 503
Silurian period, 129, 133, 137, 145, 146
Similarity of floras in N. America and
E. Asia, 157, 161, 167
—of organisms in Australia and S.
America, 170
Sinkholes (swallow-holes), 546
Sinnott, E. W., 579
Sisal, see Agave
Sisymbrium sophia (Flixweed), 125
Sizing of paper, 265, 269, 272, 273
Skunk-cabbage, see Symplocarpus foeti-
dus
Slash Pine, see Pinus caribaea
Slerozem, 300
Slime-moulds, see Myxomycetes
Slippery Elm, 264
Slope, ecological significance of, 296, 375
—, infialittoral, 492, 493
Slug, 117, 306
Smilax (Catbriar), 262
Smith, A. L., 74
—, G. M., 74, 506, 540
—, W. Wright, xvii
Smoking materials, 266-7
Smother crops, weeds and, 280
Smuts (see also Fungi), 43, 251, 280
Snail, 117, 305, 306
Snipe, 114
Snow (see also Precipitation), 11, 93,
290, 345, 350, 381, 392, 479, 506
—, Algae on, 333, 476, 489-91
—, Bacteria on, 419, 491
— Bunting, 116
—, coloured, 490
— -drift, 292, 315, 393, 394, 395, 402-5
— -flea, 490
— -melt, dispersal by, 107, 108
629
Snow-patches (late-snow), vegetation
and, 393, 394, 398, 402-3, 404, 405,
411
Soap, 265, 266, 275
— substitutes, lather-forming, 266
Soapbark tree, 266
Soapberry, 266
Soaproot, 266
Soapwort, 266
Socies, 334
Society, layer (stratal), 334
—, plant, 334, 360, 383
Sodium chloride, 368, 508
Soft bottoms, Algae of, 492, 528, 529-33
— water, 373, 478
Softwoods, 246, 268
Soil, accumulation of, 327, 371, 372
—., acid (acidic), 303, 358
—, aeration of, 302, 305, 501
— Algae, 29, 32, 34, 304, 333
— atmosphere, 302
— Bacteria, 26, 303, 305, 333
—, calcareous, 79, 302, 355, 356, 373.
—, calcium carbonate in, 79 [397, 443
— classification, 297, 298
—, climatic types of, 297-8, 300
— colloids, 298
— communities, 303, 316, 333
— conditions and plant growth, 79
— constituents, 298-304
—, crumb-structure of, 298
—, cultivation and stratification of, 297
—, decomposition in, 302-3, 395
=a desert, 365, 366, 368, 449-52, 453
— fractions, 298
— Fungi, 45, 303, 305, 333
— groups, world map of, 300
— horizons, 297, 299
— map, 300, 568
—, maturation of, 298, 390
—, mechanical analysis of, 298
—, — constitution of, 298, 301
— microcosm, 297, 304
—, mineral constituents of, 298
— moisture, 279, 301-2, 381
—, nature of, 297
— organisms, 297, 302, 303, 304, 333
—, patterned, 385
—, permanently frozen, 381, 384
— polygons, 370, 381, 385, 387, 395,
—., prairie, 298, 299 [396
— profile, 297, 299
— reaction (see also pH), 302, 303, 373,
— salinity, persistent, 370 [374
—, salts in, 301-2, 368
—, sandy, 301
—, stratification in, 297, 299
— temperature, 288, 304
— texture, 279, 297
—, toxins in, 304
— water, 298, 301-2, 321
630
Soil, water relations and, 291, 301, 560
—, zonation in, 297, 299
Solanaceae, 80
Solanum melongena (Egg-plant), 258
S. tuberosum (Irish Potato, Potato,
White Potato), 80, 87, 229,
257, 269, 576
— —, hardiness of, 231
— —, viruses of, 117
— —, world production of, 229, 232
Solifluction, 381, 385, 387, 395
Solms-Laubach, H. zu, 22
Soluble humic matter, 497
Sonchus (Sow-thistle), 249
Sorbus aria s.|. (White-beam), 342
SS. aucuparia (Mountain-ash), 340
Soredium (p/. soredia), 47, 103
Sorensen, T., xviii, 392
Sorghum, 229, 257, 269, 282,
South Africa, 15, 235, 351
— —, desert of, 450
— —, fossil flora of, 149
— —, grasslands of, 360, 363
— —, marine Algae of, 530, 531
— —, Permian glaciation in, 156
— —, sclerophyllous woodland of, 354,
355
— —, semi-desert in, 365
— —, tree-veld of, 441
— —, weedy Opuntia in, 248
— America, 114, 156, 166, 194,
22.09 23i1) 1230, 200) 2635 26045
342, 351, 414, 421, 430
— —, bad-lands of southern, 366
— —, cold desert in, 414
— —, crops originating in, 226, 229,
2315)23459235)12399254
— —, desert in, 365, 449
— —, forest area of, 246, 247
— —., glaciation in, 160
— —, grasslands in, 360, 444
— —, high-altitude vegetation of, 413,
414 ,
— —, marine Algae of, 534-5
— —, migration into, 171
— —, Permian flora of, 148
— —, savanna in, 444
— —, semi-desert in, 365
— —, subtropical forest in, 424
— —, supposed land-bridges to, 170,
— —, thorn-woodland in, 443 nga
— —, tropical forest of, 423, 426, 436,
— Asia, mangroves in, 457 [439
—-east Asia, economic botany and,
233, 239, 242, 262, 264, 272
— - — —, rain forest in, 351, 463
— - — —, Triassic flora of, 149
— -west Asia, crops originating in, 234
— Atlantic discontinuous range (dis-
tribution), 194, 196
— -facing slopes, 294, 295, 375, 376
INDEX
South Georgia, 417, 421
— Ontario, 130
— Pacific discontinuous
tribution), 194
— — Ocean, 170
— Pole, 154, 166, 170, 184, 419
Southampton Island, 384, 406
Southern Beech, see Nothofagus and N.
antarctica
— — forest, 342
— hemisphere, glaciation in, 156
— mountains, Arctic plants on, 164
Sow-thistle, see Sonchus
Soybean, see Glycine max
oil, 265
Spanish-cedar, see Cedrela odorata s.1.
— -moss, see Tillandsia and T. usneoides
Sparganium (Bur-reed), 504
Sparhawk, W. N., 243, 253, 281
Spartina, 270
Spearmint, 262
Special creation, 207
Species, 14, 25, 72, 202
—, break-up of, 204
—, collective, 222
— numbers, affected by progress to
climax, 464
Specific epithet (name), 25 [573
Specimens, importance of checking, 571-
Spectrum, biological (life-(from), 94, 95
Speed of flight of birds, 114
Spermatophyta (Seed-plants), 24, 62-
73, 138-44
Spermatozoid, 49, 52, 57, 59, 63, 141
Sphaerella, see Chlamydomonas nivalis
Sphagnetum, 501
Sphagnum (Bog-moss, Peat-moss), 51,
52, 53, 54, 268, 318, 330, 359, 373,
406, 500, 501
Sphenophyll, 136
Sphenophyllales, 135, 145
Sphenophyllum, 148
S. emarginatum, 136
Sphenopsida, 135, 145, 146
Sphenopteris tenuis, 139
Spices, 261, 262, 282
Spicy-wintergreen, see Gaultheria
Spike-moss, see Selaginella
-mosses, see Lycopodineae
Spinach, 258
Spindle-tree, see Euonymus europaeus
Spines on plants, 83, 85, 88, 89, 447, 451
Spinifex littoreus, 458
Spiranthes romanzoffiana (Drooping
Ladies’-tresses, Hooded lLadies’-
tresses), I90, 192
Spirits, 261
Spitsbergen, 181, 284, 285, 382, 387,
388, 389, 396, 397, 402, 407, 408
—., flower-slope in, 408, 409
—., Jurassic flora of, 149
range (dis-
INDEX
Spitsbergen, manuring by birds in, 402
—, potato grown in, 231
—, seaweeds in, 537
—, soil polygons in, 396
Splitters, 202
Sponge, 2, 129
—, freshwater, 495
Spora, 99
Sporangium (pl. sporangia), 41, 42, 44,
45, 49, 56, 57, 59, 134
Spore, 27, 41, 45, 49, 51, 52, 56, 57,
59, 98, 99, 100-1, 131, 327, 434,
490
— dispersal, 99-101, 117, 125, 252
—— EJeECtion, 121, 125
— germination, 52, 56, 117, 520
— liberation, 52
— output of Fungi, 45, 100, 125
Spores washed from air by rain, 106
Sporobolus, 361
Sporophyll, 57, 59
SE OPE Re: 49, 53, 55, 57, 59, 62, 63,
9
Spray, saline, 527
Spring annuals, 355
— deposits, 547
— flowering habit, 340
— flush community, 374, 547
—maxima of Algae, 481, 518,
520
— -moss, see Fontinalis
— plankton, 481, 519, 520
— rainfall, 289, 360
Sprouting, of hedge after clipping, 13
—, of pasture after close grazing, 13
Spruce, see Picea
— gum, 273
— -Moose biome, 211
Spurge, see Euphorbia
— family, see Euphorbiaceae
Squash (see also Cucurbits), 234, 258
Squills, 264
Squirrels, 116, 424
Squirting Cucumber,
elaterium
Stabilization of sand-dunes, 328, 371,
458, 548, 549, 566
— of sere, 324
Stacks, 550
Stage lightning, 59
Stages of hydrosere, 325, 326, 353,
50275
— of xerosere, 325-6, 327, 328, 372
Stamen, 68, 70, 71
Stanford, E. E., 252, 281
Stanley, O. B., 281
Star anise, 262
Starch, 29, 53, 56, 255, 260, 269
Static evolution, 132
Stearn, W. T., xviii, 169, 172
Stebbins, G. L., 179, 181
519,
see Echallium
631
Stellaria (Chickweed), 78
S. humifusa (Low Chickweed), 399
S. media agg. (Common Chickweed),
120, 249
Stem structure, Monocotyledon, 70, 73
a ae Dicotyledonh 7 ne7.3
Stems, drugs from, 264
Stenohaline Algae etc., 474, 508, 523
Stenoionic Algae, 523
Stenophotic plants, 474
Stenothermic (stenothermal), Algae etc.,
474, 499, 522, 525, 528
Stephanoptera, 502
Steppe, 162, 360, 361-3, 365, 413, 445,
454,
—, forest, 363
—, mountain, 413, 470
Stereocaulon, 392
Stereum uffine, 44, 45
Stewart, M. N., 335
Stictaceae, 421
Stigma, 68, 70, 71
Stilbocarpa polar’s, 420
Stimulants, 263, 264
—, chemical, 13
—, heart, 263
Stimulus, directional, 13
Stinging Nettle, see Urtica and U. dioica
Stinkhorn, see I[thyphallus
Stipa (Feather-grass), 361, 365
Stolon, 69, 400
Stoma (p/. stomata), 14, 76, 82, 180, 452,
Stone fruit, 113, 258 [505
— Pine, see Pinus pinea
Stones, Algae living within, 494
Stonewort, see Chara and Nitella
Stoneworts, see Charales
Storage of food by plants, 32, 33, 35, 39,
78, 87, 90, 91
Stored food-reserves, 69, 546
Stories (strata) in forest vegetation, 338,
339, 424-8, 434, 440, 462, 468, 469
Stork’s-bill family, see Geraniaceae
Strains, artificial, 564
Stramonium (see also Datura stramo-
nium), 264
Stranglers, 431, 435, 436
Strangling Fig, see Ficus
Strata, see Stories
Stratal (layer) society, 334
Stratification (cyclical) in lake water,
_ 477, 486, 487
— in soil, 297, 299, 301
— in vegetation, 291, 338-9, 425, 426,
— — —, tropical forest, 425, 426 [468
Strawberry, see Fragaria and F. grandi-
flora
Streams, benthos of, 498-9
—, dispersal by, 107
—, erosion by, 543
—., fertile land deposited by, 544
632
Streams, lentic stretches in, 352, 498
—, plankton of, 479, 488, 489
Streptomycin, 265
Stripping of coastal vegetation by ice,
536, 537, 538
Strobilus (pl. strobili), 56, 140, 141, 142
Strophanthus, 263
Structural ‘ adaptations’, 81-91, 432,
—changes in cultivation, 216-17 [433
— materials, 267-8
Struggle for existence, 126-7, 216, 562
Strut-roots, 455, 456, 458, 459
Strychnine, 263
Stunting of Algae, 508, 523, 526
Sturdiness lost in cultivation, 217
Suaeda maritima (Sea-blite), 369
Sub-Atiantic period, 174
— -Boreal period, 173
— -fossil plants, 174
Subaerial transmigration, 133
Subalpine Fir, see Abzes lasiocarpa
— forest, 350, 376, 469
— Zone, 377; 469
Subantarctic, 415, 417, 419-22
Subarctic forest, 343
— period, 173
— zone, 212
Subclasses of Angiosperms, 73
Subclimax, 330, 331, 335, 350, 363, 390,
— meadows, 363-4 [566
Subdominant, 334, 355, 385
Sublittoral belt (zone), marine, 511, 512,
513, 529, 539, 531, 533, 534, 535-8
— zone, freshwater, 492, 493, 495
Submerged aquatic plants, 326, 493, 503
— benthic stage of hydrosere, 325, 326,
— stones, Cyanophyceae on, 496 [503
Submersion, tidal, 526, 527
Subphylum (p/. subphyle), 25, 46
Subsere, 324, 363, 465
Subsoil, 297, 298, 381
—, permanently frozen, rooting, and,
Subspecies, 25, 201, 202, 203 [346
Substances inhibiting growth, 13
Substratum, aquatic plants and, 475,
494, 499, 503, 529
—, unstable, colonized by tropical sea-
weeds, 529
—, physical characters of, 475, 514, 522
Subtropical deserts, 449-52, 454
— rain forest, 424, 432, 438
— salt-marsh, 458, 459
Subtropics, 212, 443, 458-60, 485
Succession (sere), 6, 81, 313, 322-33,
333» 379, 390, 503, 562, 565, 567,
569, 570
—, deflected, 330, 465
— from lakes, 325, 326, 372, 476, 477
—, mangrove, 456, 458
—, post-glacial climatic, 173-4
— on sand-dunes, 328, 371, 548
INDEX
Succession (sere), telescoping of, 327,
Successions, main, 322-8 [328
—, secondary, 324, 465, 466
Succulent plants, 82, 83, 336, 355, 357,
365, 442-3, 447, 451, 544
— —, bushy, 450
— —, Cactaceae as, 82, 83, 85, 360,
362, 365, 366, 434, 443, 447,
459, 451, 577
— —, desert, 83, 85, 450, 451, 452
— —, Euphorbias as, 83, 85, 355, 443,
Sucker, 69 [447, 451
Suckering, 98
Sudan, 443 444
— -grass, 259
Sudd, 463
Sugar, 1, 269
— Beet, 239, 269
— Cane, see Saccharum officinarum
— Maple, see Acer saccharum
— — -Beech forest, 308
—, world production of, 241, 269
Sulphur Bacteria, 27
Sumac, 274
Summer aspect of vegetation, 339, 340,
341, 349, 351, 499, 532
— (deciduous) forest (see also Deci-
duous summer forest), 332, 337,
338, 339, 340-3
— -forms of seaweeds, 531-2
— maxima of Algae, 476, 481-2, 516,
520
— (season), characters of, 11-12
— Snowflake, see Leucojum aestivum
Sun-epiphyte, 434
— leaf, 89
Sundew, see Drosera
Sunflower (and oil), 265
Sunn-hemp, see Crotalaria juncea
Supralittoral belt (zone), 513, 526, 527,
530, 532, 535
Surf, 474, 525, 527, 528, 534
— -belt, 495, 498
— -sirdle, 525-751535
—, resistance to, 525, 534-5
Surface-binding plants, 51, 277, 533
— deposits of calcium carbonate, 547
— -film of water, 483
— plankton, 483, 511
— reduction, water-conservation and,
14, 82, 452
— temperature of sea, 510
— tension, suspension by, 483
Survival, 75-80 120, 450-2, 499
, annual habit and, 78
—, cold and, 77, 78, 99, 100
—, drought and, 78, 452
—, marine vegetation and, 514, 526
— of the fittest, 563
Suspension in water, Algae and, 486,
— to surface film, 483 [517-18
INDEX
Svenson, H. K., xvii
Sverdrup, H. U., 312, 540
Swale, 445
Swallow, 114
— -holes, 546
Swamp-forest, 353, 461-3
Swamps, tropical 460-3
Sward-forming Lichens, 344, 347, 348,
401
—, grassy (turf), 360, 362, 363-4, 383,
Swedes, 231, 258 [399, 401, 403
Sweet Chestnut, see Castanea
— Chesnut Blight, 251, 341, 576
— — forest, S. European, 342-3
— Corn, 282
— -flag, see Acorus calamus
— Pepper, 261
— Potato, see Ipomoea batatas
Sweets, 266
Swietenia (Mahogany), 247, 268
S. macrophylla (Mahogany), 247
S. mahagoni (Mahogany), 247
Switch-plants, 442, 450, 452
Switzerland, 152
Symbiosis, 46, 305, 524
Symbiotic Algae, 495, 524
Sympatric populations, 177
Symplocarpus foetidus (Skunk-cabbage),
1g0, 192
Synechococcus elongatus, 499
Synthesis, 263
Synusia (pl. synusiae), 431, 434, 435,
437
Systematic vicariad, 204
Systematy, 4
Taiga, 343, 346, 347, 348, 380, 383
—, dominants of, 34
Tall Cotton-grass, see
angustifolium age.
Talus (see also Scree), 296, 315, 328, 366
— -heaps, 551
— sere, colonization and, 372
Tamarack, see Larix laricina
Tamaricaceae (Tamarisk family), 366
Tamarind, 259
Tamarisk, see Tamarix
— family, see Tamaricaceae
Tamarix (Tamarisk), 367, 370, 450, 451
T. pentandra, 370°
Tanganyika, Lake, 318, 477
Tangerine, 258
Tank-epiphyte, 432
Tanner’s Dock, 274
Tanning, 3, 273
— materials, 273-4
Tansley, A. G., xvii, xviii, 311, 368, 378,
471, 573
Tansy, 262
Tapa cloth, 271
‘Tape-grass, see Vallisneria
Eriophorum
633
‘Tapeworm expulsion, 264
Tapioca, 231
Tar, 268
Taraxacum (Dandelion), 99, tor, 248,
249, 258
—, fruit of, 104, 105
T. officinale s.1. (Common Dandelion),
222
'Tarns, 29, 316, 407, 409, 481, 545
Taro, 258, 282
Tarragon, 262
‘Tasmania, 379
Tawny Arctophila, see Arctophila fulva
agg.
Taxodium distichum (Bald-cypress, Bog-
cypress), 160, 352, 353
Taxon (pl. taxa), 24, 177, 182, 202, 553
Taxonomy, 4, 571
Taxus baccata (Yew), 340, 342
Tchihatchef, P. de, 22
Tea, see Camellia sinensis
Teak, see Tectona grandis
Teclatt, E. M., 253
Tectona grandis ('‘Veak), 247, 440
‘Telescoping of succession, 327, 328
‘Temperate area, biological spectrum, 95
— climate, 10
— — heathland, 358-9, 374
— climax grassland, 359-63
— evergreen forest, 352
— lands, seral communities of, 370-5
— —, vegetational types of, 336-79
— rain forest, 350-4
— region, 10, 323, 324, 377, 379
Temperature, 8, 10, 11, 283, 321, 574
—changes in lakes, 473, 475, 486,
495
— -climate, 285-8, 409
— as ecological factor, 285, 288
—, flowering and, 77-8
—, fruit-setting and, 78
— groups of seaweeds, 528
—, lethal, 288, 522
—, ocean, 510
—, photosynthesis and, 77
— relationships, 11, 77-8
— — of Algae, 77, 476, 499, 510, 522
— requirements, 76
—, resistance to high, 77, 87, 499
—, soil, 288, 304
—, world maps showing, 286, 287
— zones, 10
Terraces, 547
Terramycin, 265
Terrestrial Algae, 29, 32, 34, 365, 464
— habitats, 314-16
Terricolous Lichenes, 344, 365, 397,
Tertiary forest, 465 [398
— period, 129, 154, 155, 157
— relic, 201
Tetanus, 26, 279
634
‘Tetraspore, 39
‘Textile industries, 3
Thalassia, 529
‘Thalloid Liverworts, 49, 50
Thallophyta (Thallophytes), 24, 29-49,
145, 146, 515
Thallus, 31, 32, 35; 375 38-9, 498
Thames Valley, 158
Thatching, 268
‘Thaw-sink topography, 546
Theobroma cacao (Cacao, Cocoa), 243,
260, 266
Thermal temperature, 499
Thermocline, 477, 487, 495
Thermograph, 288, 289
Therophyte, 93, 94, 95
‘Thimann, K. V., xviii, 74, 82
Thistle, 248
Thomas, H. Hamshaw, xviii, 144
Thompson, H. D., 561
Thorn-apple, see Datura stramonium
— -woodlands, 442, 443, 446, 467
— —, geographical location of, 443
Thornbush, 442
Thornthwaite, C. W., xvii
‘Thornwood, 442
Thorny scrub, 365
Three-toothed Saxifrage, see Saxifraga
tricuspidata
Thunder-god vine, 276
Thyme, 262
Tibet, 413
‘Tidal action, 369, 525, 526, 536
— deltas, 550
— effects, 319, 526, 527, 531-8
— flats, 533
— girdles of Phaeophyceae etc., 532—8
— pools, 523, 532
Tide-levels, 368, 454, 512, 513
— -mark, 319, 458, 523, 526, 536
Tides, dispersal by, 108
‘Tierra del Fuego, 338, 421
Tilia (Lime-tree, Linden), 105, 173, 341
—, fruit of, 105
T. americana (Basswood), 341
Tillaea moschata, 420
Tillandsia (Spanish-moss), 271, 443
T. usneoides (Spanish-moss), 103, 351,
_ 352, 353 Z
‘Timber, 13, 243, 268
— -line, 377, 380, 410, 413
‘Time-sequence, geological, 128
‘Timothy Grass, see Phleum pratense
‘Tintinnids, 516
‘Tissue, 12, 13, 70, 71, 82, 87, 452
Toad, 117
‘Toadstool (see also Fungi), 43
‘Tobacco (see also Nicotiana tabacum),
world production of, 239, 240
Tolerance of exposure, Cyanophyceae
and, 495, 526, 530
INDEX
Tolerance of high temperatures, 77, 87,
495, 499
— of low temperatures, 77, 539
— of salinity, Algae and, 502, 508, 509,
523, 533
— of shade, 428, 430, 434, 530, 546
Tolypothrix distorta, 495
Tomato, see Lycopersicum esculentum
—, Late-blight of, see Phytophthora
Tonic, 263, 264 [infestans
Tonka bean, 262
Topography, 182, 183, 283, 293-6, 411
— (of taxon), 182, 188
—, thaw-sink, 546
Tornado, 99
Toronto, 161
Torrential communities, 463, 498-9
Torrey Pine, see Torreya
Torreya (Torrey Pine), 192
Torula Yeast, 270
Toxic root excretions, 80
Toxins, soil, 80, 304, 329
Tracheophytes, 54
Transatlantic vicariads, 202, 203
Transcaspian Desert, 366
Transect, 567, 568
Transparency of sea-water, 511, 514
Transpiration, 14, 289, 291, 301, 343
505
—, deciduous habit and, 337, 343
—, desert plants and, 452
—, regulation of, 343, 363, 452, 457
‘Transplant experiments, 56
‘Transportation, ectozoic, 111-14, 116-
17
—, endozoic, IT, 114-15, 116, 117
—, wind, 27, 99-106
Trapa (Water-chestnut), 160, 199
T. natans (Water-chestnut), 199
Tree (see also Forest, etc.), 11, 63, 93,
146, 277, 289, 292, 352, 362, 379,
426, 427, 429, 433, 469, 566
— -ferns, 59, 60, 61, 62, 146, 148, 162,
351, 353, 427
— -limit, 292, 293, 376, 377, 380, 411
— - —, altitudinal, 345, 377
— - —, northern, 292, 345, 348
— of life, 129
— -veld, 441
Trees, absent from arctic regions, 380
—, aged, of slow growth, 348
—, competition between, 428
—, — with Grasses, 307, 360, 361, 363
—, deciduous, 11, 337, 338, 339, 349-4
—, distribution of, 65, 213, 243, 246,
247, 380, 543
—, effect of wind on, 291, 292, 293
—, giant, 63, 67, 348, 379
—., restriction to valleys, 361, 362, 543
Trembling Aspen, see Populus tremu-
loides
INDEX
Triassic flora, 149
— landscape, reconstruction of, 150
— period, 129, 133, 145, 149
Trichamphora, 42
Trichodesmium erythraeum, 517
Trifolium (Clover), 234, 259
Triglochin maritima (Sea Arrow-grass),
369
Trinidad, 426
‘Tripe de Roche, 410
Tripton, 480
Triticum vulgare etc. (Wheat), 115, 223,
224, 226, 255, 257, 282, 454,
541, 564, 576
— —, Stem-rust of, 248, 251
— —, world production of, 227, 229
Triumfetta, 113
‘Triuridaceae, 436
Troms, 347
Tropic of Cancer, 423, 424
— of Capricorn, 423
Tropical America, 243, 247, 261
— Agriculture series (Longmans), 282
(arid area) biological spectrum, 95
— beach vegetation, 458
— cash crops, 282
— climate, 11, 424
— desert, 449-54
— discontinuous range (distribution),
193, 194
— edaphic etc. communities, 460-7
— forest, 350, 423-43, 454-70
— —, disappearance of, 424
— — with seasonal rhythm, 332. 439-43
— —, stratification of vegetation in,
425-31, 434, 438, 440, 462
468-9
— grassland climate, 444
— grasslands, 443-7
— hardwoods, 246-7
— humus, 325, 437, 462, 467
— hydrosere, 462, 463, 464
— littoral woodland, 458, 460
— (moist area) biological spectrum, 95
— ‘ moor-forest ’, 462, 463
—rain forest, 332, 339,
39
— — —, components of, 425-38
— — —, epiphytes of, 431-5
— — —, geographical location of, 423,
424
— — —, herbs in, 430
— — —, precipitation on, 424
— — —, stratification in, 425-31, 434
— reed-swamps, 461
— region, 10, 22, 323, 423, 529
— savannas and other grasslands, 443-7
—seashore vegetation 454-60, 461,
464-5
— seasonal forest, 439-41
— seaweeds, 458, 529-30
379; 423-
635
¥
Tropical seral communities, 460-7
— swamp, 314, 460-2
— thorn-forest (woodland), 442, 443
— — -parkland, 443
— vegetation, 423-71
Tropics, altitudinal effects in, 467-70
—,, arctic species in, 413
—, peat rare in, 462
Trout, 476
True-breeding polyploid hybrids, 235,
239
Truffle, 259, 282
Tsuga (Hemlock), 174, 246, 269, 274
T. canadensis (Hemlock), 341, 349
Tuber, 69, 93, 355, 446
‘Tuberculosis, 26, 279
‘Tufted hemicryptophyte, 94
Tulip-tree, see Liriodendron
tulipifera
Tulipa uniflora, 366
‘Tumble-weeds, 102, 452
‘Tumboa, see Welwitschia mirabilis
Tundra, 174, 213, 246, 292, 300, 315,
332, 333, 348, 380, 384, 385, 386,
388, 389, 410, 417, 419
—, alpine, 377, 383, 410, 414
—, arctic, 382-3, 384, 385, 386-90
—, dominants of, 383-4, 385, 386-9
—, dry, 385, 386, 387, 380, 392
—., grassiness of, 383, 384, 388
—, high-arctic, 387, 388, 389
—, hillock, 385, 387, 388
—, interaction of factors in, 386-7
—, Kerguelen Island, 419
—, low-arctic, 383, 384, 385, 386
—, marshy, 385, 387, 390
—, middle-arctic, 387, 388
—, Pleistocene, 162
— soils, 298
—, subantarctic, 417, 419, 421
— vegetation, 383-9
Tung, see Aleurites
— oil, 265
Tunnels, 546
Turbidity of water, compensation point
and, 519
— — —, light penetration and, 473, 511
Turbulence, sedimentation and, 480,
525
— in water, plankton and, 480, 514
Turf, 360, 362, 363-4, 369, 390, 403,
421
‘Turgor adjustments in Diatoms, 508
—, variable, 508
Turkish Oak, 274
Turmeric, 262, 274
Turn-over in lake water, 473,
481-2
Turnip, see Brassica campestris
Turpentine, 268, 273
Turrill, W. B., 21
anGuelse
477
636
Tussock plants, 284, 285, 336, 361, 413,
415, 417, 418, 419, 420-41, 491, 546
Twilight zone (region, see also Dys-
photic zone), 495-6, 511
Twining plants, 87, 88, 89, 431
Tychoplankton, 479, 480, 489
Types of coniferous forest, 344-50
Typha (Cattail, Reedmace), 319, 372,
461, 505
T. angustifolia (Narrow-leafed Cat-
tail), 463
Typhoid fever, 26, 279
Ukraine, meteorological stations in, 575
Ulex (Gorse), 124, 359
Ulmus (Elm), 68, 121, 173, 246, 280,
306, 340, 341
Ulopteryx, 37
Ulothrix, 30, 31
Ultra-violet radiation, 475
Ulva (Sea-lettuce), 31, 32, 523
U. lactuca (Sea-lettuce), 31
Umbelliferae, 412, 417
Umbrella-shaped trees, 441, 447
Under-shrubs, see Ground-shrubs
Underground stem, 69
Undergrowth of coniferous etc. forests,
344, 349, 350
Undisturbed tracts, aliens rare in, 219-
21
Ungava, Canada, 347, 537
Unicellular organisms, 26, 28, 29, 30,
31-4
Uniformity of flora and vegetation in
early geological ages, 154
United States, 251
— —, algin production in, 38
— —, climatic analogues of, 574, 575,
576
— — Department of Agriculture, 214,
282, 579, 575
— —, endemics of, 205
— —, fossil flora of, 150
—-—, land use and management in,
556, 567, 570
— —, loss from erosion in, 570
— —, plants dispersed by ice in, 109
— —, Scots Pine planted in, 574
— —, soil losses in, 309, 568, 569, 570
— —, use of locally-grown seed in, 563
— —, vegetation of, 337, 338, 339, 341,
343, 345, 348-51, 352, 353, 354,
355, 356, 357, 360-1, 362, 364,
369, 376-7, 445, 447, 451, 485,
486, 533, 568, 569
Units, climax, 333, 334
Upland Rice, 224
Upper air, dispersal in, 100, 101, 106
— sublittoral zone, 527, 530, 532, 534
Upwelling, in ocean water, 513, 519
Urals, 338
INDEX
Urtica (Stinging Nettle), 554
U. dioica (Stinging Nettle), 554
Usnea barbata, 48
Utilization of crops and crop products,
282
Utricularia (Bladderwort), 373, 484, 485
Vaccinium (Bilberry, Blueberry,
Whortleberry), 258, 346, 348, 349,
358-9, 500-1
V. uliginosum subsp. alpinum (Arctic
Blueberry), 392, 393, 395, 404
V. vitis-idaea agg. (Mountain Cran-
berry), 392, 393
Valerian, 264
Valleys, 389, 542, 543, 545
—, trees restricted to, 361, 362, 543-4
Vallisneria ('‘Tape-grass), 503
Valonia, 274
Vanilla, 262
Variation, 25, 562-3
—, climatic, 284-5, 288-9, 291-3, 294
—, cyclic quantitative, of algal vegeta-
tion, 526
— in salinity of sea-water, 317, 507-8,
. 522-3, 531, 533, 537
Variety, 25
Vascular bundles, 54, 70, 71, 73
— plants, see Vasculares
— structure, 54, 68, 70, 71
— system, 54, 505
Vasculares (vascular plants), 54-74,
133-44, 317, 417, 420, 484, 485
— of the Arctic, 422
Vavilov, N. I., 223, 252, 253
Vectors, insect, of diseases, 117, 252
Vegetable fats, 266
— fibres, 270-1, 282
— flower-pot, 432, 433
— ivory, 269
— oils, 265, 271
— sponge, 271
— tallow, 266
Vegetables, fruits used as, 231
Vegetation, 3, 6, 7, 9, 11, 14, 16, 17, 283
314, 315, 336-540, 554-6, 565
—, antarctic, 415-22, 537-8
—, aquatic, factors affecting,
507-15
—, arctic, 62, 380-409, 422, 490, 520,
535, 536, 537
—, artificial changes in, 215, 566-71
5 Clith3725 5550
, conservation of, 309, 569, 573
—, cyclic changes in, 330
472-9,
desert, 93, 364-8, 449-54
elements in, 210-12
erosion checked by, 555, 562, 569,
570
—, freshwater, 479-500, 502-6
— intertidal, 526-38
INDEX
Vegetation, local conditions and, 17,
543-51, 554-5
—, — Variation in, 10
—, localized manuring and, 401, 402,
4093, 421
—, main types of, 332-3
—, Man’s influence on, 15, 215, 309,
315, 465, 563, 564, 566-71
— maps, Frontispiece, 213, 568
—, marine, 507, 513, 515-21, 529-40
— —,, ice-action on, 531, 536, 537-8
— moulding landforms, 555
— of Polar lands, 11, 380-409
—,, primary divisions of, 213
—,, salt-marsh, 367, 368-70
—., sedgy-grassy, 367, 370, 388
—, shingle-beach, 371-2, 400, 458
— of snow-patches, 394, 402-3, 404,
405
—, tropical land, 423-71
—, tundra, 332-3, 383-90, 410
— -types, see Vegetational types
— unit, 201, 323, 333-4
— of United States, see United States,
vegetation of
—., variability in time and space, 4
Vegetational adaptation, 565-6
— belts, 10, 2525 213
— -climatic regions of world, 213
— degradation, fire and, 310
— features, 9, 20
—itypes), 5; 0; LO; 11, 15, 20; 225,23, 76,
283, 332-3, 336-540
— — of bogs and inland saline waters,
500-2
— — of fresh waters, 472-500, 502-6
— — of high altitudes, 409-15
— — of polar lands, 380-409, 415-22
— — of seas, 507—40
— — of temperate and adjacent lands,
336-79
— -— of tropical and adjacent lands,
423-71
Vegetative adaptation, 81-91
— parts of plants, drugs from, 263-4
— propagation (reproduction), 57, 61,
64-5, 69, 79; 98, 107, 177, 179,
521
Velamen, 432
Veld of South Africa, 360, 441
Venetian turpentine, 273
Verbascum (Mullein), 103
Verdoorn, F., 74
Vernacular names, 25
Vernal aspect, 340, 499
Verrucaria maura, 527, 53°, 532, 535
Vertical currents in sea, 509, 511, 519
— distribution of plants in water, 473
— gradient of distribution of plankton,
483, 486, 487
Vetch, see Vicia
637
Viability, 99, 100, 248
Viburnum (Pimbina, etc.), 346
V. lantana (Wayfaring Tree), 342
Vicariad, 201, 202, 203, 204
Vicariads, evolution of, 204
—, types of, 202, 203, 204
Vicarious areas, 201-4
— species, 201, 204
Vicia (Vetch), 234, 259
V. faba (Broad Bean), 231
Viking colonies, 181, 222
— relics, 181
Vinegar, 261
Vines, see Climbers
Viola (Pansy, Violet), 123, 124
Virginia, 341
Virginian Spring-beauty, see Claytonia
virginiana
Virus diseases, 117, 250, 251, 252
Viruses, 80, 117
Viscum orientale (Mistletoe), 438
Vitamins, 257, 278, 279
Vitis vinifera (Grape), 242, 258
Viviparous Knotweed, see Polygonum
viviparum
Vivipary, 456
Vochysiaceae, range of (mainly neo-
tropical), 193
Vodka, 261
Volcanic dust, 100
Vole, 306
Volga Giant-wildrye, see Elymus gigan-
teus
Volvocales, 481
Vorticella, 494
Wadis, trees bordering, 450, 454
Wagtail, 114
Walchia, see Lebachia (Walchia) fron-
dosa
Walker, F. T., xviii, 526, 531
Walker, J. C., 250, 253
Walking Fern, see Adiantum caudatum
Walnut, see Fuglans
— oil, 265
Walton, J., 153
Wang Chi-wu, 341
Warm springs, population of, 476, 499
— -temperate marine Algae, 530-1
— - — rain forest, 332, 350—4, 438
— - — zone, 212
— water, Algae of, 476, 499
— - — fishes, 477
Warming, E., 20, 23
Wasting disease of Eel-grass, 43, 524
Water, I, I1, 12, 14, 86, 106-11, 283,
388, 406, 407, 433, 472-540, 559
— absorption, deep roots aiding 82, 84,
85, 450-1
—, aeration of, 319, 473, 474, 477, 509
— available, 11, 288, 289, 301
638
Water Avens, see Geum rivale
— as barrier to dispersal, 105
— blooms, algal, 29, 480, 481,
520
—, brackish, 472, 492, 502, 508, 523
— Buttercup, see Ranunculus
—, changes in gas content of, 477, 509
— -chestnut, see Trapa and T. natans
— conditions, convergence of, 328
— conservation, epiphytes and, 432, 433
— —, plants and, 14, 82, 83, 85, 450-2
—-cress, see Rorippa nasturtium-
aquaticum
— Crowfoot, see Ranunculus
— currents, benthos and, 498-9, 526
—, depth inhabitable by plants, see
Depth etc.
—, dispersal by, 27, 106-11, 510, 514
—, dystrophic, 318, 476-7
— economy, 14, 76
—., erosion by, 568, 569, 570
—, essential to plants, 76, 301
—, eutrophic, 318-19, 462, 463, 476,
477, 545
— -fern, see Marsilea
— -fowl, 112, 114
—, gas-content of, 477, 509-10
— Horsetail, see Equisetum fluviatile
— -hyacinth, see Eichhornia crassipes
—, light-absorption by, 483, 488, 511—
— -lily, see Nymphaea [13
— - — family, see Nymphaeaceae
— loss, cooling by, 87
— -milfoil, see Myriophyllum
— -movement, plankton and, 486, 514
— - —, plants and, 106-9, 474, 513, 525
—, ocean, 507-12, 514
—, oligotrophic, 318, 462-3, 476-7, 545
—, periodic variation of temperature in,
475, 499, 510, 522
—, phosphate content of, 475, 476, 524
— plants (hydrophytes), 86, 87, 93, 484,
485, 504
— —, anatomy of, 82, 87, 505 [-13
—, penetration of light into, 474-5, 511
— relations, 47, 51, 54, 82-87, ae
— requirements, 76
— retention by grassland, 360, 363, ae
— Sedge, see Carex aquatilis agg.
—, soil, 291, 298, 301-2
— -starwort, see Callitriche antarctica
— storage in plants, 82, 83, 85, 354, 355,
365, 366, 432, 442-3, 446-7, 450-2
— -storage tissue, 82
—, transparency of, 496, 511, 514
—, vertical distribution of plants in, 473
—, viscosity of, 479
— -weed, see Elodea
Watercourses, plants and, 348, 352, 359,
361, 365, 374, 439, 440, 444, 454,
543
483,
INDEX
Waterlogged soil,
559
Watermelon, 258
Wattle, 274
Wave action, Algae and, 495, 513, 525,
288, 296, 302, 460,
532
— —, freshwater lakes and, 406, 492-5
— —., shore erosion by, 492, 493, 543,
Wave-cut benches, 550 [530
Wax, 3, 266
Wayfaring Tree, see Viburnum lantana
Weakness in competition of cultivated
plants, 216-18, 564
Weathering impeded by vegetation, 555
—, soil and, 297, 298, 326-7
Weaver, Jo E-, 311,:335,3 791679
Weed communities, 374, 375
Weeds, 6, 62, 165, 199, 216, 218-19, 248—
50, 280-1, 309-11, 322, 362, 374,
SHY
—, controllof, 280; 310) 3, 57.0
— dependent on human activity, 249,
250
—, derelict areas and, 118 [120, 375
—, disappearance of, on abandonment,
—, dispersal of, 6, 119-20, 248-52
— dispersed from America, 249
— — — Europe, 249
—, injurious effect of, 248
—, open areas and, 120, 165, 375
—, species almost cosmopolitan,
249
—, tolerance of environment, 219
Wegener, A., 165, 169
Welch, P. S., 486, 506
Welwitschia mirabilis (Tumboa), 450
West Indies, 262, 444
Western Eurasia, reed-swamp in, 463
Wet and dry bulb hygrometer, 289, 291
Wheat, see Triticum vulgare etc.
— belt, 564
— Stem-rust, 248, 251
, world production of, 227
Whisky, 261
White-beam, see Sorbus aria s.1.
— Fir, see Abies concolor
— Oak, see Quercus alba
— Pine, see Pinus strobus
— — Blister-rust, see Cronartium ribi-
cola
— Potato, see Solanum tuberosum
— Spruce, see Picea glauca agg.
Whitehead, F. H., xviii
Whortleberry, see Vaccinium
Widespread occurrence’ of
species, 472
Wild Cherry, see Prunus avium
— Oat-grass, see Danthonia
Wildfowl, 402
—, dispersal by, 109
Wildlife management, 556, 558-9, 577
120,
aquatic
INDEX
Willis, J. L., 379
Willow, see Salix
— -herb, see Epilobium
Wilson, J. W., xvii
Wind, no; 11) 2015, 292) 203, 359) 415,
419, 420, 441
— -break, 277, 307, 555
—— —Caves, 5417
— -climate, 293
— as climatic factor, 291-3
—, dispersal by, 27, 98, 99-103, 104,
105, 106, 293, 434, 490
— —, barriers to, 103, 105—6
—, effect on plants, 291, 292, 293, 359
—, erosion by, 291, 543, 547-50
— -transport, 27, 99-106, 434
Wine, 260, 261
Winged fruits, 102, 104, 105
— seed, 101, 102, 104, 105
Winter, 11-12, 288, 361, 380
— aspect, 338, 340, 341, 350, 351 499
— -forms, of seaweeds, 531, 532
—, plants and, 288, 280, 361
Wintergreen, 260, 262
Wisconsin, 341, 485, 486
Witch-hazel, see Hamamelis
Wood (timber), 267-8, 271, 274
— alcohol, 268
— Anemone, see Anemone nemorosa
— Avens, see Geum urbanum
—, destructive distillation of, 268
— Forget-me-not, see Mvyosotis
vatica
— gas, 268
— pulp, 269, 270
— -rush, see Luzula
— -sorrel, see Oxalis and O. acetosella
Woodbury, A. M., 312
Woodland, 16, 332-3, 343, 354-8, 541,
—, hydrophytic, 325 [557
— vegetation, stratification in, 291, 338,
339. 351, 425, 426, 440, 468, 469
Woody climbers (see also Lianes), 101,
— crops, 242-7 [340
— plants of deserts, 366, 450-2
— —, dwarf, 386, 392-5, 413
— — of Greenland, 222, 344, 390, 391
— vegetation, browsing and, 307, 308
World, age of, 128
—, annual precipitation (rainfall etc.)
map of, 290
—, biological spectrum of, 95
— Crops Books series (L. Hill), , 282
—., distribution of Man in, 256
—, floristic regions of, 19
—, forest areas in, 213, 243, 246, 247,
556
— production, Barley, 229
— —, Cane and Beet Sugar, 241
— —, Cocoa Beans, 244
— —. Coffee, 245
syl-
639
World production, Cotton, 235, 237
— —, Flax-seed, 236
—— I Viaizes 2204230
— —, Oats, 229
— —, Peanut, 238
== —5 ltoiinine), AAI}, 242 ¥-
= Sy INIGS, AA 220
— —, Rye, 228, 229
— —, Tobacco, 240
— —, Wheat, 227, 229
—, soil groups of, 300
— temperature maps, 286, 287
—, vegetation-types of, 378, 556
—, zoogeographical realms of, 18, 19
Wormseed, 263
Wormwood, see Artemisia
Wrack, see Fucus
Wright, Miss C., xix
—, J. K., 253
Wilt Eh Vez. 234 rooned2. 2225 cK
Wyatt-Smith, J., 456
Wynd, F. L., 74
Xanthoria parietina, 532
Xerarch subsere, 324
Xeric grassland, 413
— mean, 328
Xeromorphic Pryophytes, 434
— plants, 82, 83, 289, 420, 451, 452
Xeromorphy, 343, 366, 420, 432, 442,
443, 447, 452, 469
Xerophilous epiphyte, 432, 433, 434, 442
— Sedges, 385, 386
Xerophily, 441, 442, 444, 452
Xerophyte, 289, 364, 450, 451
Xerosere, 324, 325, 327, 372, 407, 464
Yam, see Dioscorea
Yautia, 258
Yeast, see Saccharomyces
—, Food, 259, 270, 278
Yeasts, see Saccharomycetaceae
Yellow Archangel, see Galeobdolon
luteum
— Marsh Saxifrage, see Saxifraga hir-
culus agg.
— Mountain Saxifrage, see Sanifraga
aizoides agg.
— Pine, 246
— snow, 490
— Water-lily, see Nuphar lutea
Yew, see Taxus baccata
Ylang-ylang, 275
Yoco, 260
Yucca, 447
Zea mays (Maize), 70, 71, 226, 229, 233,
257, 261, 269, 563
— —, history of, 226, 229
— —, world production of, 229, 230
Zeuner, F. E., 129
640
Zingiber (see also Ginger), 90, 91
TAN, IRo5x BAB, Axi, Dror
Zonation of shore-vegetation, 319, 492,
493, 526-7, 529, 530, 532, 536-8
—, soil, 297, 299
Zone, aphotic, 473, 518, 538-40
—, austral, 186
—, boreal, 186
—, climatic, 10, 20
—, dysphotic, 473, 488, 495-6, 518
—, eulittoral, 492-4, 512, 513, 527, 530,
534
—, euphotic, 473, 511, 518, 539
—, infralittoral, 493, 495-6, 511, 513,
527
—, littoral, 492, 493, 513, 526, 527
—, lower sublittoral, 527, 533, 536
—, montane, 377, 467, 468, 470
INDEX
Zone, sublittoral, 492, 493, 495, 513,
529-38
—, supralittoral, 513, 526, 527, 530,
532, 535
—, twilight, 488, 495-6, 511
Zoned subclimaxes, 370, 393, 402-3,
404, 405
Zones, temperature, 10
Zoochlorella, 495
Zoogeographical realms, 18, 19, 212, 213
Zooplankton, 479, 480, 519
Zoospore, 37, 45
Zostera (Eel-grass), 43, 524, 528, 530,
531, 532, 535
—, mud and, 528, 530
—, ‘ wasting disease ’ of, 43, 524
Zostera marina (Eel-grass), 535
Zygote, 42
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