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AN INTRODUCTION TO THE
STUDY OF ALGAE

CAMBRIDGE

UNIVERSITY PRESS

LONDON; BENTLEY HOUSE

NEW YORK, TORONTO, BOMBAY
CALCUTTA, madras: MACMILLAN

TOKYO: maruzen company ltd

AN INTRODUCTION TO 57
THE STUDY OF ALGAE

V. J. CHAPMAN, M.A., Ph.D., F.L.S.

Fellozv of Gonville and Caius College

University Demonstrator in

Botany at Cambridge

/r* ^* **0/^

I. . , . . t? ^

^\y^ /-

NEW YORK: THE MACMILLAN COMPANY
CAMBRIDGE, ENGLAND: AT THE UNIVERSITY PRESS

1941

PRINTED IN GREAT BRITAIN

CONTENTS

Preface page vii

Chapter I. Classification i

V^II. Cyanophyceae 6

Introduction ^ w iT^'v^ ^

Coccogonales /v^^-^-^-l. ^0\ ^°

Hormogonales /^/p^^^<\<^ ^3

Will. Chlorophyceae '°^p BR'^^^I^I ^^

Introduction ^i^ V .-•^jO'^^ / '*^/ ^^

Volvocales ^^ 22

Chlorococcales 3^

Ulotrichales 44

Oedogoniales 57

IV. Chlorophyceae (^o«^.) 63

Chaetophorales 63

Siphonocladiales 73

Siphonales 84

V. Chlorophyceae (cont.) 98

Conjugales 9^

Char ales 108

Xanthophyceae (Heterokontae) 113

Bacillariophyceae (Diatomaceae) 119

Chrysophiyceae 122

Cryptophyceae 124

Dinophyceae 125

VI. Phaeophyceae 127

General 127
Ectocarpales (Isogeneratae and Heterogeneratae) 132

Cutleriales (Isogeneratae) 154

Sphacelariales (Isogeneratae) 156

Tilopteridales (Isogeneratae) 160

55640

CONTENTS

>. VII.

Phaeophyceae {cont^

Dictyotales (Isogeneratae)
Laminariales (Heterogeneratae)
Fucales (Heterogeneratae)

page 163
163
167
189

VIII.

Rhodophyceae
Introduction

212
212

Proto-florideae (Bangiales)
Eu-florideae

217
220

NemaltonaleSj Gelidiales

220

Cryptonetniales
Ceramiales

224
229

Gigartinales
Rhodymeniales

238
242

IX.

Reproduction
Evolution

245
256

Fossil Forms

266

Physiology
Symbiosis
Soil Algae

279

295
298

XI.

Marine Ecology

306

-lai.

Ecology of Salt Marshes

321

XIII.

Fresh Water Ecology

332

XIV.

Ecological Factors
Geographical Distribution
Life Form

349
360

368

Index yj^

PREFACE

For a long time there has been a great need for a short and re-
latively elementary text-book on Phycology which would be
suitable for University students, and also for those schools which
include visits to marine biological stations as part of their curri-
culum. Such a text-book would not require to be too advanced and
yet should survey the whole field of phycological knowledge, not
only from the systematic but also from the physiological and
ecological viewpoints. The two most recent works on Phycology do
not entirely fulfil this function. Fritsch's Structure and Reproduc-
tion of the Algae must be regarded not only as a classic but also as a
monumental piece of work, but it is somewhat unwieldy in size for
the ordinary student and also it is a compendium of much that he
does not require to know. At the same time it is a book that no
University or research student in Phycology can afford to be
without, whereas this present volume does not pretend to cater for
the research student. The other work, Tilden's Algae and their Life
Relations, is also somewhat bulky, and although it is perhaps more
on the lines of the present volume, nevertheless it is primarily
concerned with systematic phycology. It seems to the present
author, therefore, that there is a place for a relatively short work on
the outlines of Phycology containing the amount of information
that could be conveyed in a course of lectures lasting over a period
of 22-24 weeks at the rate of one lecture per week. No attempt has
been made to produce any work more elaborate, primarily because
Fritsch's volumes will fulfil that need. These, then, are the reasons
for the appearance of this volume.

Relatively few types have been selected from out of each
group; some of these have been described in considerable detail
whilst others are mentioned merely to illustrate the course of
development in either the vegetative or reproductive organs. Every
t}^pe is fully illustrated because the present author firmly believes
in this medium as the best means of teaching. Types that are
regarded as essential for first and second year students are indicated
by an asterisk, and even then it is not intended that they should
necessarily absorb all the details about these species. It may come

viii PREFACE

as a rude shock to some teachers to find that long-established
friends, e.g. Gonium, Vaucheria, have not been asterisked. The
present author believes that such types should have been omitted
from curricula years ago either because they do not convey any-
thing essentially new, or else because recent work has shown them
to be wholly unsuitable types for elementary students. Up to the
present, however, established tradition has kept them firmly
ensconced in their position, but whether they will be able to retain
it remains to be seen. It is suggested that third and fourth year
students should study additional types selected from among the
other species. Certain of the other chapters have also been marked
as suitable for the first and second year students. Several chapters
have been devoted to Ecology because the literature now available
in this branch of the subject ought to be made accessible to the
ordinary student. In these other chapters limits of space have
rendered it necessary to select the material, and it may be felt by
other teachers that some original work has been omitted that
perhaps might have been inserted. In a book of this type such a
feature is inevitable, and the author acknowledges that the choice of
material has been a personal affair and that it is, as such, open to
this criticism. There is a chapter on Physiology, Symbiosis and the
Soil Algae, and also one that is devoted to a surv^ey of reproduction
and evolution. Part of one chapter is devoted to a brief account of
the more important fossil types because it is essential that these
should be studied and compared with their living successors, and
also an acquaintance with these forms materially aids any discus-
sion on evolution.

The algae are now divided into a number of groups, and whilst
it is essential that the student should know that these groups exist,
nevertheless, his attention should be concerned primarily with the
major divisions. For this reason most attention has been given to
the Chlorophyceae, Xanthophyceae, Cyanophyceae, Phaeophyceae,
and Rhodophyceae. This is perhaps somewhat indefensible, but
since the species which are normally encountered by the student
belong principally to these groups, I believe the procedure is
justified.

In order that the student should not be burdened unduly, only
the more important papers have been provided in the references,
but even these are appended only for those who are especially

PREFACE ix

interested in the group. The bibUography therefore does not
pretend to be complete, and the choice of what are regarded as
important papers has lain with the author. It is sincerely hoped
that the majority of University students will find all that they need
to know for a degree course in the volume.

I should like to acknowledge the assistance that I have obtained
from existing volumes and also my indebtedness to the following
publishers for permission to reproduce figures from their works.
Cambridge University Press: The Structure and Reproduction of
the Algae, vol. i, by Fritsch; Algae, vol. i, by West; A Treatise on
the British Fresh-water Algae, by West and Fritsch; Magraw Hill
Book Co. : Fresh Water Algae of the United States, by G. M. Smith;
University of Michigan Press: The Marine Algae of the North
Eastern Coast of the United States, by W. R. Taylor ; University of
Minnesota Press: The Algae and their Life Relations, by Tilden;
The British Museum : Handbook of British Marine Seaweeds, by
L. Newton; University of California Press: Chlorophyceae and
Melanophyceae of the Pacific Coast of North America (2 vols.), by
Setchell and Gardiner; Dulau and Co.: Phycological Memoirs, by
G. Murray.

In addition, reference w^as made to Oltmann's Morphologic und
Biologic der Algen and to Kniep's Die Sexualitdt der Niederen
Pflanzen, but owing to the present conditions it has not proved
possible to get in touch with the publishers.

Acknowledgement for the use of figures is also made to the editors
of the following journals, to whom my thanks are due: Annals of
Botany, Journal of Ecology, New Phytologist, Journal of Gcjietics,
Botanical Gazette, Americanjournal of Botany, American Naturalist,
Annals of the South African Museum, Transactions of the Royal
Society of South Africa, Philosophical Transactions of the Royal
Society, Transactions of the Royal Society of Edinburgh, Proceedings
of the New Zealand Institute, Journal of the Linneafi Society {Botany),
Proceedings of the Linnean Society of New South Wales, Botanical
Magazine {Tokyo), Bidletin of the Torrey Botanical Club, Publica-
tions of the Hartley Botanical Laboratory, Bidletin of the United
States Department of Agricidture, Journal of the College of Agri-
culture, Tohoku {Hokkaido) University, Reports of the Great Barrier
Reef Expedition and Proceedings of the Cambridge Philosophical
Society.

X PREFACE

Use has also been made of figures from the following periodicals
to the editors of which I tender thanks, although conditions have
made it impossible to get into touch with them : Archivfur Protisten-
kunde, Zeitschriftfiir Botanik, Revue gener ale de botanique, Hedwigia,
Berichte der Deiitschen Botanischen Gesellschaft, Jahrbiicher fur
Botanik^ Planta, Flora, Beihefte zum Botanischen Zentralblatt, Le
Botaniste, Osterreichische botanische Zeitschrift, Lunds Universitets
Arsskrifty Botaniska Notiser, Revue algologique, Protoplasma, Arkiv
for Botanik, Svensk botanisk Tidskrift, Nova Acta Regiae Societatis
scientiarum Uppsaliensis, Kongliga Svenska Vetenskapsakademiens
Handlingar, Kongliga Fysiografiska Sdllskapets iLundForhandlingar.

The authors of the various figures are acknowledged in the
legends, and references to the more important papers will be found
at the end of the appropriate section.

Much of this book has been inspired, and indeed used, during
class visits to the marine laboratories at Plymouth, Port Erin,
Lough Ine and Millport, and I can think of no better way of
becoming acquainted with the algae. These visits were initiated
under the tutelage of Mr A. G. Lowndes and to him must go much
of the credit for my interest in this branch of Botany.

I should also like to acknowledge gratefully the encouragement
and help given me by Professor F. T. Brooks, F.R.S., and Professor
F. E. Fritsch, F.R.S., whilst a special debt is due to Dr D. Catcheside
who read and criticized the whole manuscript. I am also indebted
to Dr H. Hamshaw Thomas, F.R.S., who read the section on Fossil
Algae, and to Dr G. C. Evans who read the chapter on Physiology
and the one on Ecological Factors. Dr Godwin also very kindly
read and criticized a portion of this volume whilst it was in
proof. Finally, there has been the help and encouragement given
me by my wife, and it is in no small measure due to her unsparing
help in the drawing of the figures and the preparing of the Index
that this book sees the light of day.

V. J. C.

Gonville and Caius College
May 1 94 1

CHAPTER I

^CLASSIFICATION

In the older classifications the algae proper were simply divided into
four principal groups, Chlorophyceae or green algae, Cyano-
phyceae or blue-green algae, Phaeophyceae or brown algae and
Rhodophyceae or red algae. Now, however, that more is known
about the simpler organisms which used not to be regarded as
algae, it has been realized that there is no real justification for such
a distinction, and so the number of algal groups has been increased.
This is because it has become evident that the Flagellata and other
simple unicellular organisms must properly be regarded as algae,
even though of a very primitive kind. At present it is most con-
venient to divide the algae into ten classes, one of which, the
Nematophyceae, is perhaps somewhat speculative. One of the
principal bases of this classification is the diff"erence in pigmenta-
tion, and a recent study of this problem shows that it is fully
justified.

(i) Cyanophyceae. The plants in this group show very little
evidence of differentiation, containing only a very simple form of
nuclear material, no proper chromatophore and no motile cells with
cilia or flagellae. The products of photosynthesis are sugars and
glycogen. The colour of the cells is commonly blue-green and
hence their name, the colour being due to the varying proportions
of the pigments phycocyanin and phycoerythrin. There is no
known sexual reproduction, propagation taking place by simple
division or else by vegetative means.

(2) Chlorophyceae. This group used to comprise four great
subdivisions, the Isokontae (equal cilia), Stephanokontae (ringed
cilia), Akontae (no cilia) and Heterokontae (unlike cilia). It is now
more in keeping with our present knowledge to place the last
section into a separate class, and this is the procedure adopted in
most recent books. The plants of the Chlorophyceae exhibit a
great range of structure from simple unicells to plants with a
relatively complex organization, whilst the chromatophores also
vary considerably in shape and size. The final product of photo-

CSA I

2 CLASSIFICATION

synthesis is starch together with oil, and a starch sheath can often
be demonstrated around the pyrenoids. In the bulk of the members
of this class the motile cells are very similar and commonly possess
either two or four flagellae, but in the Oedogoniales (Stephano-
kontae) there is a ring of flagellae whilst in the Conjugales (Akontae)
there are no organs of propulsion. Sexual reproduction is of
common occurrence and ranges from isogamy to anisogamy and
oogamy. The colour of the cells is usually a grass green because the
pigments are the same as those present in the higher plants and,
furthermore, they are present in much the same proportions.

(3) Xanthophyceae (Heterokontae). The plants in this group
are usually of a simple nature, but their lines of development
frequently show an interesting parallel or homoplasy with those
observed in the preceding group (cf. p. 264). The chloroplast is
yellow-green owing to an excess of xanthophyll, one of the four
normal constituents of chlorophyll. Oil replaces starch as the
normal storage material, the lack of starch being correlated with the
absence or paucity of pyrenoids. The motile cells possess two
unequal flagellae (occasionally only one) arising from the anterior
end. Sexual reproduction is rare and when present is isogamous.
The cell wall is frequently composed of two equal or unequal halves
overlapping one another.

(4) Chrysophyceae. These form another very primitive group
in which the brown or orange colour of the chloroplasts is de-
termined by the presence of accessory pigments such as phyco-
chrysin. Most of the forms have no cell wall and hence are
*' flagellates" in the old sense of that term, although there are some
members which do possess a cell wall and hence are "algal" in the
old sense of the term. Fat and leucosin (a protein-like substance)
are the usual forms of food storage, whilst another marked feature
is the silicified cysts which generally have a small aperture that is
closed by a special plug. The motile cells possess one, two or,
more rarely, three equal flagellae attached at the front end, but in
one subsection the paired flagellae are unequal in length. The most
advanced habit known is that of a branched filament, e.g. Phaeo-
thamnion (cf. p. 123), whilst the palmelloid types attain to a much
higher state of differentiation, e.g. Hydrurus (cf . p. 1 23), than in either
the Chlorophyceae or the Xanthophyceae. Sexual reproduction is not
certain, and such records as there are point simply to isogamy.

CLASSIFICATION 3

(5) Bacillariophyceae (Diatoms). One of the characteristics of
these plants is their cell walls which are composed partly of
silica and partly of pectic material. The wall is always in two halves
and frequently ornamented with delicate markings, which are so
fine that microscope manufacturers make use of them in order to
determine the resolving power of their lenses. The chromato-
phores are yellow or golden brown containing, in addition to the
usual pigments, accessory brown colouring materials whose nature
is only just being established. One set of forms is radially sym-
metrical, the other bilaterally so. The presence of flagellate stages is
highly probable in the former whilst there is a special type of
sexual fusion in the latter group (cf. p. 122).

(6) Cryptophyceae. There are usually two large parietal chloro-
plasts with diverse colours, though frequently of a brown shade,
whilst the product of photosynthesis is starch or a closely related
compound. The motile cells have two unequal flagellae and often
possess a complex vacuolar system. Nearly all the members have a
"flagellate " organization and there is no example of the filamentous
habit. One type, however, has been described with a tendency
towards the coccoid (non-motile unicell with a cell wall) habit, and
so this must be regarded as the least "algal "-like class. Isogamy
has been recorded for one species.

(7) DiNOPHYCEAE. Most of the members of this class are motile
unicells, but there has been an evolutionary tendency towards a
sedentary existence and the development of short algal filaments,
e.g. Dinothrix (cf. p. 126). Many are surrounded by an elaborate
cellulose wall bearing sculptured plates and inside there are discoid
chromatophores, dark yellow or brown in colour and containing a
number of special pigments. The products of photosynthesis are
starch and fat. The motile cells normally possess two furrows, one
transverse and one longitudinal, although they may be absent in
some of the more primitive members. The transverse flagellum lies
in the former, and the latter is the starting point for the other
flagellum which points backwards. Sexual reproduction, if it
occurs, is isogamous, and it has not been clearly established in the
few cases reported. Characteristic resting cysts are also produced by
many of the forms.

(8) Phaeophyceae. This group comprises the common brown
algae of the seashore and it is worth noting that the majority are

1-2

4 CLASSIFICATION

wholly confined to the sea. The brown colour is due to the presence
of a pigment, fucoxanthin, which masks those other chlorophyll
constituents which are present. The products of photosynthesis are
alcohols, fats, polysaccharides and traces of simple sugars so that
there is evidence of some diversity of metabolism. The simplest
forms are filamentous, and there are all stages of development and
increasing diflferentiation up to the large seaweeds of the Pacific and
Arctic shores with their great size and complex internal and external
diflferentiation. The motile reproductive cells, which possess two
flagellae, one directed forwards and the other backwards, are
commonly produced in special organs or sporangia that are either
uni- or plurilocular. Sexual reproduction ranges from isogamy to
oogamy, but in the latter case the ovum is normally liberated before
fertilization. The life cycles may be extremely diverse and are
perhaps better regarded as race cycles (cf. p. 246).

(9) Rhodophyceae. The members of this class form the red
seaweeds, and although most of them are marine nevertheless a
few are fresh-water. Their colour, red or bluish, is caused by the
presence of the pigments phycoerythrin and phycocyanin, whilst
the product of photosynthesis is a material known as "floridean
starch". Reproductive stages with locomotor appendages are not
known, even the male reproductive body being without any organ
of locomotion. The simplest members are filamentous, and again
all stages of diflFerentiation up to a complex body can be found,
although they do not develop to quite the same degree of complexity
as the Phaeophyceae. Very obvious protoplasmic connexions can be
distinguished between the cells of nearly all forms except in the
small group known as the Proto-florideae (cf. p. 217). Sexual
reproduction is oogamous, the ovum being retained upon the
parent plant, and although the subsequent development of the
zygote is varied to a certain extent, it usually gives rise to filaments
which bear special reproductive bodies or carpospores, and these
latter are responsible for the production of the tetrasporic diploid
plant. Most of the members exhibit a regular alternation of genera-
tions.

(10) Nematophyceae. This is a fossil group of which one genus
has been known for a long time (Nematophyton) whilst the other
has only recently been described (Nematothallus). There is still
considerable doubt as to their true affinities, but it would seem that

CLASSIFICATION 5

a place can best be found for them as a very highly developed type
of alga. Their internal morphology would ally them closely with the
more advanced members of either the Chlorophyceae or the
Phaeophyceae. The only reproduction so far recorded is that of
spores which were developed in tetrads, and therefore may have
been akin to the Rhodophycean or Phaeophycean tetraspores.

REFERENCE

Carter, P. W., Heilbron, I. M. and Lythgoe, B. (1939). Proc. Roy. Soc.
B, 128, 82.

CHAPTER II

CYANOPHYCEAE

^INTRODUCTION

This order used to be known as the Myxophyceae, but as this name
was originally applied to a very heterogeneous group of organisms
it is now customary to employ the name Cyanophyceae. The
members of the group are characterized by a bluish green colour
which varies greatly in shade, depending upon the relative pro-
portions of chlorophyll a, /^-carotin, myxoxanthin, phycocyanin and
sometimes xanthophyll and phycoerythrin. The internal structure
of the cell is extremely simple because a true nucleus and chromato-
phores are absent. Some authors have reported the presence of a
nucleus with rudimentary chromosomes which undergo a form of
mitosis, but these structures cannot be regarded as clearly esta-
blished. The protoplast possesses two regions, a peripheral one
containing the pigment together with oil drops and glycogen, and a
colourless central area which contains granules. Two kinds of
inclusions have been recognized. The metachromatic or a gran-
ules that lie in the colourless central area and which are nucleo-
proteic in nature since they give a Feulgen reaction. These granules
have probably been mistaken by some workers for chromosomes,
especially since it is found that they can divide by simple fission,
although some authorities do not consider that this is even a primi-
tive form of mitosis. The material of the central area is regarded by
such workers as equivalent to the cytoplasm in the cells of higher
plants. The other type of granule is known as the cyanophycin or
jS granule and occurs in the peripheral region. They are in the nature
of a protein reserve, and their presence is probably dependent to a
considerable extent upon the external environment.

The protoplast is normally devoid of vacuoles, and this fact may
explain the great resistance of the plants to desiccation and of the
cells to plasmolysis. In some forms, principally species which are
planktonic, pseudo-vacuoles may be found, and it is supposed that
these contribute towards their buoyancy. The protoplast is sur-
rounded by an inner investment which has been shown to be a

INTRODUCTION 7

modified plasmatic membrane. In addition there is an outer cell
sheath which may surround the whole cell, e.g. Chroococcus, or
form a cylindrical sheath, e.g. Oscillatoria, or an interrupted sheath,
e.g. Anahaena. This is usually composed of a pectic material,
although in the Scytonemataceae it may be made of cellulose. There
is considerable variation in the composition of the different cell
sheaths, and the amount of material laid down frequently depends
upon the external environment. In any case the secretion of pectins
by these plants is regarded as a primitive characteristic. In the
unicellular forms this material is produced at the periphery of the
cell, whilst in a few, e.g. Chroococcus tiirgidus, it accumulates in the
cytoplasm. Protoplasmic connexions between mature cells have
been recorded for one genus, Stigone?na.

The group is characterized by a general absence of well-marked
reproductive organs; there are no sexual organs and no motile
reproductive bodies have ever been observed. It has recently
been suggested that the lack of sexuality can be correlated with the
absence of sterols in the group, an hypothesis that might well repay
further study. The coccoid forms (spherical cells) multiply by cell
division which takes place by means of a progressive constriction,
whilst in other types the cell contents divide up to give a number of
non-motile bodies that are termed gonidia (fig. 5). Crow (1922)
has pointed out that all stages from simple binary fission to gonidia
can be found :

[a) Binary fission, e.g. Chroococcus turgidus.

[b) Quadrants and octants formed, e.g. C. varius.

(c) Numerous small daughter cells are produced in which there
is a retention of individual sheaths, e.g. Gloeocapsa sp. and variants
of Chroococcus macrococcus.

(d) The same without individual sheaths, e.g. C. macrococcus,
Gloeocapsa crepidinum.

{e) x\bstricted gonidia, e.g. Chamaesiphon.

Many of the filamentous forms produce specialized cells known
as heterocysts. These are enlarged cells which possess thickened
walls, and they usually occur singly though occasionally they may
be formed in rows. They develop from an ordinary vegetative cell,
but during development they remain in protoplasmic communica-
tion with neighbouring cells and if they contain contents, as they

8 CYANOPHYCEAE

probably do, these may be expected to differ from those of an
ordinary vegetative cell. Various suggestions have been made as to
their function, and in many cases they probably determine the
breaking up of the trichomes (or threads) into hormogones. These
hormogones are short lengths of thread which are cut off. thus
forming a means of vegetative reproduction among the filamentous
types. The heterocysts may also perhaps act as a food store, or they
may represent archaic reproductive organs which are now function-
less. It has been reported that in Nostoc and Anabaena these cells
may occasionally behave as reproductive bodies. Hormogones,
besides being cut off by the heterocysts, may also be produced by
the development of biconcave separation disks which develop at
intervals along the filament. The hormogones, together with certain
of the filamentous types, exhibit a slow motion, and although ciha
have been described for one species their presence has never been
corroborated. The active and continual secretion of mucilage along
the sides of the filaments is now regarded as the probable mechan-
ism for securing movement. Thick-walled resting spores, or
akinetes, occur in many of the filamentous forms, normally de-
veloping next to a heterocyst. The entire lack of sexuality must be
ascribed to the ancient cell structure and the absence of chromo-
somes together, possibly, with the lack of sterols.

This type of cell structure naturally provides a problem for the
geneticist. There are tw^o possibilities because each cell may con-
tain one single gene or a number of genes (organized self-reprodu-
cing bodies which determine the properties of the cell and of the
organism). The genes must be separated from each other since
there are no chromosomes in which they could be situated, and
they will either be distributed generally throughout the cell or else
in a particular part of it. Since there is no special means of accurate
partition sexuality would be useless because it could not confer the
property of recombination but only of addition.

Many of the forms aggregate into colonies, but in some of the
Chroococcaceae the plant mass is an association of such colonies
and not one large colony or thallus. The form which any colony
may take up depends on (i) planes of cell division, (2) effect of
environment which may determine the consistency of the mucilage,
uneven temperatures, for example, sometimes producing irregular
growth. It has been shown experimentally that the environment

INTRODUCTION 9

may affect the shape of colonies of Microcystis and Chroococcus
turgidus and determines the size of Rivularia haematites (cf. p. 337).
Certain lines of morphological development have been followed by
the group and the various lines may be depicted schematically as
follows (cf. also fig. i):

Plate-like
colonies

Single floating unicell

" /

Regular spherical or^
cubical colonies

'Single attached

unicell

Irregular
spherical
colonies

PLANKTONIC /
/

/ V

/ Attached single Aggregated

filament ^ filaments

Filament with
false branches

True branched
filament

BENTHIC

10/Z

A B C D E F G

Fig. I. Types of trichoma in the Cyanophyceae. A, Hapalosiphon arboreus.
B, Calothrix parietaria. C, Schizothrix purpurascens. D, Oscillatoria margari-
tifera. E, O. proboscidea. F, O. irrigua. G, Arthrospira jenneri. (After Crow.)

As rnay perhaps be expected from a primitive group there is
evidence of homoplastic or parallel development when compared
with plants from other primitive groups, especially the Chloro-
phyceae. Homoplasy can be seen in Gloeothece and Gloeocystis,
Merismopedia elegans and Prasiola (figs. 4, 40), Chamaesiphon and

CYANOPHYCEAE

Characium (figs. 5, 26), Chroococcus and Pleurococcus (figs. 3, 44),
Lynghya and Hormidium.

As a group, the plants are extremely widely distributed over the
face of the earth under all sorts of conditions, frequently occurring
in places where no other vegetation can exist, e.g. hot thermal
springs. Their presence in great abundance in the plankton often
colours the water and is responsible for the phenomenon known as
water bloom, whilst they may also form a large constituent of the
soil algae (cf. Chapter x).

The class is divided into two orders :

CoccoGONALES, which reproduce by means of single cells.
HoRMOGONALES, which reproduce by groups of cells or hormo-
gones.

COCCOGONALES

Curoococcaceae: Microcystis {micro, small; cystis, bladder). Fig. 2.

The thallus, which is free-floating, varies much in shape and
contains a mass of single spherical cells, but the sheaths of the

Fig. 2. Microcystis aeruginosa. A, colony. B, portion of a colony ( x 750).
(A, after Geitler; B, after Tilden.)

individual cells are confluent with the colonial envelope. Repro-
duction of the single cells takes place by means of fission in three
planes, whilst reproduction of the colony is through successive

COCCOGONALES

disintegration, each portion growing into a new colony. The shape
of the colony is primarily determined by the environmental
conditions, and it can be changed by altering the environment
artificially. M. aeruginosa is a very common water bloom alga.

*Chroococcaceae : Chroococciis (chroo, colour ; coccus, berry). Fig. 3 .

The cells are single or else united into spherical or flattened
colonies each containing a small number of cells, the individual

\\Vi A

Fig. 3. Chroococcus. A, C. turgidus, plant ( x 600). B, C. turgidus, protoplasmic
reticulum with accumulations of metachromatin at nodal points. /)^ = plasmatic
granules, mf = microsomes. C, C. macrococcus, normal daughter cell formation.
D, C macrococcus, daughter cell formation with retention of the parent envelopes.
(A, after Smith; B, after Acton; C, D, after Crow.)

sheaths being homogeneous or, more frequently, lamellated.
Plants grown in water produce a concentric envelope but when
grown on damp soil the sheath is often asymmetrical. The outer
integument is not very gelatinous and indeed is quite thin in some
species. The colonies are either free-floating or else they form a
layer on the soil. A study of the cytology of this genus has shown
that C. turgidus represents the simplest condition with the meta-

CYANOPHYCEAE

chromatin granules only just differentiated. In C. macrococcus, a
more complex type, there is a central body which, according to
Acton (1914), contains a fine reticulum with chromatin at the nodal
points, but a reinvestigation of this species is perhaps desirable and
might well lead to a different interpretation (cf. fig. 3). At cell
division this "nucleus" divides by simple constriction, but there is
no evidence of a mitosis. In Gloeocapsa a similar condition is
observed, but in this case with evidence of a rudimentary mitosis.

Chroococcaceae : Merismopedia {merismo, division; pedia, plain).
Fig. 4.

The free-floating colonies form regular plates one cell in thickness
at first, but with increasing age they become irregularly square or

Fig. 4. Merismopedia elegans. A, portion of colony ( x 345). B, portion of
colony (X1125). C, structure in cells about to divide (X1875). (A, after
Geitler; B, C, after Acton.)

rectangular and are often curved or twisted. The cells are spherical
or ellipsoidal and their individual sheaths are confluent with the
colonial envelope. There is every transition from compact (M. aeru-
ginosa) to extremely loose colonies (M. icthyolabe), the number
of cells enclosed in one envelope depending on the rate of division
which only takes place in two planes. In M. elegans, prior to cell
division, an accumulation of chromatin occurs in the centre of the
cells to form a central body or so-called "nucleus" which divides
by constriction immediately preceding cell division. The "nucleus"
then disappears until the next division.

COCCOGONALES

Chamaesiphonaceae : Chamaesiphon (chamae, on the earth;
siphon, a small tube). Fig. 5.

The cells are epiphytic, solitary, or arranged in dense clusters;
they stand erect, are more or less rigid, vary much in shape and are
attached at the base. The sheath is thin, hyaline and ultimately
opens at the apex. Reproduction is by means of gonidia which are
abstricted successively by transverse division from the apex, and as
these gonidia have been regarded as one-celled hormogones the genus
thus forms a link between the Coccogonales and Hormogonales.

1.^^

Fig. 5-

f»T

Fig. 5.
Fig. 6.
Fig. 7-

A B

Fig. 6 A B

Fig. 7.
Chamaesiphon cylindricus with gonidia ( x 1200). (After Geitler.)
Spirulina. A, S. major ( x 1070). B, S. subsalsa ( x 1070). (After Carter.)
Oscillatoria. A, O. formosa (X613). B, O. corallinae (X613). (After
Carter.)

HORMOGONALES (REPRODUCTION BY HORMOGONES)

OscjLLATORiACEAE : SphuUna {spirula, a small coil). Fig. 6.

The trichomes have no proper sheath and are septate, although
the septa are frequently very obscure. The trichomes are simple,
free and coiled into a more or less characteristic spiral.

*OsciLLATORiACEAE : OscUlatoria (oscillare, to swing). Figs, i, 7.

The trichomes are free, smooth or constricted, straight or arcuate
and often form tangled masses, the sheaths being delicate or more

CYANOPHYCEAE

frequently absent. The apical cell is sometimes provided with
a cap or calyptra. There are a number of common species,
O. limosa being frequently found on very damp soils, wet stones
and omer moist places.

OsciLLATORiACEAE : Lytigbya (after H. C. Lyngbye). Fig. 8.

This genus differs from Oscillatoria in the presence of a sheath of
variable thickness and colour, the character of which is largely

.12//.

Pig. 8. Lynghya aestuarii. A, apex. B, C, portions of threads. (After Chapman.)

dependent upon the environment. The plants are either attached or
free-floating. When the hormogones and trichomes escape from the
sheaths it is frequently very difficult to determine whether they
belong to Oscillatoria^ Lynghya^ or some other similar genus.

ScYTONEMATACEAE : Scytonema (scyto, leather ; nema, thread). Fig. 9.

The threads differ from those of the preceding genus in the
presence of heterocysts. The filaments (trichome and sheath) have a
base and apex, and the false branches arise either between two
heterocysts or else adjoining a heterocyst. The intercalary growth
results in strong pressure being applied to the sheath, which finally
ruptures so that the trichome forms a loop outside (fig. 9 A-C).
Further growth causes this loop to break, thus producing twin
branches, one or both of which may subsequently proceed to
additional growth, the branch sheaths extending back into the

HORMOGONALES

parent sheath (fig. 9 E). More commonly, false branching is
initiated by degeneration of a vegetative cell or heterocyst and sub-
sequent growth of the two filaments on either side.

Fig. 9, Cyanophyceae. A-C, geminate branching in Scytonema pseudoguyanense
(A, X 470, B, C, X 340). D, false branching in Calothrix ramosa ( x 570). E, false
branching in Scytonema pseudoguyanense showing branch sheath (bs) terminating
at heterocyst. ^5 = parent sheath, d = dead cell ( x 590). F, hormogones emerging
from parent sheath in S. guyanense ( x 750). (After Bharadwaja.)

*RivuLARiACEAE : Rtvulana (rivuluSy small brook). Fig. 10.

The colonies form spherical, hemispherical, or irregular gela-
tinous masses that are attached to plants or stones, those of R. atra
being especially frequent on salt marshes. They contain numerous
radiating filaments with repeated false branching, each branch
terminating in a colourless hair. The individual sheaths can be seen
near the base of the trichomes, but they are diffluent farther up.
The heterocysts are basal, and in one section of the genus spores are
produced next to them. The genus is also interesting because it
has been shown to contain xanthophyll.

*NosTOCACEAE : Nostoc (used by Paracelsus). Fig. 11.

The gelatinous thallus is solid or hollow, floating or attached,
and varies much in size and shape. There is a dense limiting layer

CYANOPHYCEAE

containing numerous intertwined and contorted filaments with
individual hyaline or coloured sheaths which may be absent,
indistinct or conspicuous. The heterocysts are terminal or inter-
calary and are arranged singly or in series. Reproduction is by

( D

Fig. lo. Rivularia atra. A, plants on stones ( x f). B, transverse section of
thallus ( X 9). C, transverse section of thallus ( x 45). D, single trichome in
sheath ( x 300). (A-C, after Newton; D, original.)

means of hormogones or spores, the latter arising midway between
the heterocysts and developing centrifugally. N. commune forms
gelatinous masses and is fairly common on damp soils.

The closely related genus Anahaena only diifers from Nostoc in
that no firm colony is formed. Some species are often symbiotic
(cf. p. 297), whilst both Anahaena and Nostoc are apparently capable
of fixing nitrogen from the atmosphere (cf. p. 304).

HORMOGONALES

Nostocaceae: Cylindrospermum {cylindro, cylinder; spermum,
seed). Fig. 12.
A characteristic feature of this genus is the large spore which
develops next to the heterocyst at one or both ends of a filament.
The outer wall of the spore is often papillate.

m^<:\^y^

B B A

Fig. II. Fig. 12.

Fig. II. Nostoc. A, portion of colony of N. Linckia ( x 400). B, C, germinating

hormogones of N. punctiforme ( x 900). (After Geitler.)

Fig. 12. Cylindrospermum. A, C. majus ( x 680). B, C. stagnate ( x 340). (After

Geitler.)

REFERENCES

Chroococcus. Acton, E. (1914). An?i. Bot., Loud., 28, 433.

Scytonema. Bharadwaja, Y. (1933). Arch. Protistenk. 81, 243.

Chroococcus, Gloeocapsa, Microcystis. Crow, W. B. (1922). New Phytol.
21, 81.

General. Crow, W. B. (1924). J. Genet. 14, 397.

General. Crow, W. B. (1928). Arch. Protistenk. 61, 379.

Systematic. Geitler, L. (1932). " Cyanophyceae " in Rabenhorst's
Kryptogamen Flora, 14, Leipzig.

General. Poljanski, G. and Petruschewsky, G. (1929)- Arch. Pro-
tistenk. 67, II.

Cytology. Spearing, J. K. (1937). Arch. Protistenk. 89, 209.

CSA

CHAPTER III

CHLOROPHYCEAE

VOLVOCALES, CHLOROCOCCALES, ULOTRICHALES,

OEDOGONIALES

*INTRODUCTION

The older botanists included in the term Chlorophyceae the forms
which are now placed in the Xanthophyceae (cf. p. 113), but in
1897 Bohlin pointed out that some of the green algae possessed
unequal cilia, and in 1899 Luther coined the term " Heterokontae "
for such forms. In 1902 Blackman and Tansley revised the classi-
fication of the green algae using the terms Isokontae, Akontae,
Stephanokontae and Heterokontae. These were adopted by most
workers and remained in use until 1927 when Fritsch included the
Akontae and Stephanokontae in the Isokontae. The term Isokontae
has thus ceased to be of significance and the group is now included
with the Akontae and Stephanokontae in the Chlorophyceae. A
division into two great groups, marine and fresh water, as suggested
by Tilden in 1935, is not at all feasible, because nearly all the
morphologically distinguishable families possess representatives in
both environments.

The cell structure is fairly characteristic, the protoplast often
containing a large central vacuole, which in the simpler forms is
contractile and serves to remove surplus water and waste matter.
The green pigment, which is essentially identical with that of the
higher plants, is contained in plastids : there is usually only one of
these in a cell and its outline may be discoid, star-shaped, spiral,
plate-like or reticulate. There is some evidence to show that these
plastids are capable of movement in response to light stimuli.
Other colouring matter may also be present, e.g. haematochrome in
Sphaerella and phycoporphyrin in some of the Zygnemales, whilst
fucoxanthin (cf. p. 129) is found in Zygnema pectinatum. The cells
are commonly surrounded by a two-layered wall, the inner, which is
often lamellate, being of cellulose, and the outer of pectin, but in
some forms the outer surface of this pectin sheath is dissolved as
fast as it is formed on the inner side. In a few species there is a third

INTRODUCTION 19

layer or cuticle, whilst in others there is an outer mucilage layer,
and in at least three groups (Siphonales, Siphonocladiales and
Charales) lime may be deposited on the walls. The chloroplasts
normally contain rounded bodies, or pyrenoids, which are composed
of a viscous mass of protein. The pyrenoids are usually surrounded
by a starch sheath, starch being the principal product of photo-
synthesis, and it is said that parts of the pyrenoid are successively
cut off to form starch grains, but the evidence for this is not entirely
satisfactory. The pyrenoids are perpetuated by simple division but
they may also arise de novo.

Each cell usually contains one nucleus, but in certain groups a
multinucleate condition is to be found. Each nucleus possesses a
deeply staining body, the nucleolus, together with chromosomes
which are usually short and few in number, although these latter
may be masked during the interphases between nuclear division.
At cell division the pyrenoids and chloroplasts may also undergo
division. The flagellae of the motile bodies are composed of an
axial cytoplasmic filament surrounded, except at the very apex, by a
sheath which probably has the power of contraction, whilst in the
Volvocales the flagellae normally disappear at the commencement
of cell division. The motile cells also possess a red eye-spot, the
detailed structure of which is not yet elucidated in all the groups,
though it appears to contain a primitive lens in the Volvocales. The
red colouring matter is due in part to the chromolipoid pigment
known as haematochrome (cf. fig. 13).

Vegetative reproduction takes place through fragmentation and
ordinary cell division, whilst asexual reproduction is by means of
bi- or quadriflagellate zoospores which are commonly produced in
normal cells because special sporangial structures are rare. These
zoospores are often formed during the night and are then liberated
in the morning : after liberation they may remain motile for as much
as 3 days or for as short a time as 3 min. Their production can some-
times be artificially induced by altering the environmental condi-
tions, e.g. removing the plant from flowing to still water {Ulothrix,
Oedogonium), changing the illumination, transferring to water from
air (terrestrial Vaucheria spp.), or removing from water for 24 hours
(Ulva, Enter omorpha). Each individual cell may produce one or
more zoospores, the number varying with the different species.
Liberation is secured by means of (a) lateral pores, {h) terminal

2-2

CHLOROPHYCEAE

pores, (c) gelatinization of the entire wall, (d) the wall dividing into
two equal or unequal halves. In some species non-motile zoospores
are formed which are called aplanospores, but if these should then
secrete a thick wall they become known as hypnospores. Aplano-
spores which have the same shape as the parent cell are termed
autospores. All these spores develop a new membrane when they
are formed and hence differ from a purely resting vegetative cell or
akinete (cf. fig. 13).

B A

Fig. 13. A, diagram of eye-spot of C/zZaw3'^owonas. _^ = pigment cup, 5 = photo-
synthetic substance. B, diagram of cross-section of eye-spot of Volvox. L = lens,
/)'= pigment cup, 5 = photosynthetic substance. C, aplanospores of Microspora
Willeana ( x 600). D, akinete of Pithophora oedogonia ( x 225). (After Smith.)

Sexual reproduction is represented in all the orders and often
there is a complete range from isogamy to oogamy, the ova usually
being retained on the parent thallus in the oogamous forms (e.g.
Vaucheria, Coleochaete). The isogamous forms are normally di-
oecious, the two strains being termed + and - , and as they are
usually alike morphologically they can only be distinguished by the
behaviour of the gametes. In some cases ( Ulva) relative sexuaUty is
known to occur, weak + or - strains fusing with strong + or -
strains respectively. Indeed, Hartmann (1924) has declared that
all gametes are potentially bisexual, and there would seem to be
considerable grounds for supporting this view. Segregation into +
and - strains occurs during meiosis, a phenomenon which in
many species takes place at the first or second division of the zygote.

INTRODUCTION

The occurrence of sexual reproduction in Nature often marks the
phase of maximum abundance when the cHmax of vegetative activity
has just been passed. It can also be brought on in culture by an
abundance or deficiency of food material or by intense insolation.
Interspecific hybrids have been recorded from Spirogyra, Ulothrix,
Stigeoclonium, Draparnaldia and Chlamydomonas (fig. 14). Another
striking fact is that characters which may develop in some species
under the influence of the external environment are normally found
"fixed" in others. This not only indicates the plasticity of many
members in the class, but the phenomenon might also be of im-
portance in considerations of phylogenetic relationships.

Vegetative evolution would appear to have taken place along
several lines and may be represented schematically thus :

Motile colourless

unicell

(Phacotus)

Motile holophvtic unicell

Palmelloid

colony
(Palmella)

Small motile

colony
{Pandorina)

Non-motile unicell

with motile spores

(Chlorococcuni)

>.
Large motile

colony

( VoJvox)

Net-like

colony
{Hydrodictyon)

Attached

unicell

(Characium)

Specialized simple
filament (Oedogoniales)

Foliose parenchymatous
thallus (Ulvales)

Simple filament
(Ulothrix)

Dendroid

colony

{Prasinocladus)

Small siphonaceous
thallus {Protosiphon)

Branched

filament

(uninucleate)

Branched filament

(multinucleate)
(Siphonocladiales)

Heterotrichous filament

(aerial and basal portion)

(Stigeoclonium)

r Basal cushion
Reduced types -j only

I (Coleochaete)

Aerial filament

only
(Draparnaldia)

Siphonaceous
thallus (Siphonales)

Basal disk

only

(Protoderma)

CHLOROPHYCEAE

The names of the species and genera do not imply that they
provided the actual links in the process of evolution, but that
ancestral forms having an appearance similar to that of the examples
quoted formed the intermediate stages. The examples are given in
order that the student may have something concrete upon which to
visualize the scheme.

As a group the Chlorophyceae are very widespread, occurring in
all types of habitat. A few species, e.g. Endoderma, Chlorochytrium,
Rhodochytrium, are parasitic, whilst several other species participate
in symbiotic associations, e.g. Carteria, Zooxanthella, Chlorococcum
(cf. p. 296).

VOLVOCALES
*Chlamydomonadaceae : Chlamydomonas (chlamydo, cloak; monas,
single). Fig. 14.

The ''chlamydomonad" type of cell characteristically possesses
a single basin-shaped chloroplast, a red eye, one pyrenoid and two

Fig. 14. Chlamydomonas. A, B, vegetative individuals of two parents. Az, Bz,
zygotes of parents. A x B, fusion between gametes of A and B. ABz, zygote of
hybrid. Fi, four hybrid individuals obtained from germination of one hetero-
zygote. C, Chlorogonium oogamum, female showing formation of ovum.
D, Chlorogonium oogamum, male showing formation of antherozoids. E-G, stages
in fusion of C. media ( x 400). H, vegetative division in C. angulosa. I, zygote of
C. coccifera. J, conjugation in C. longistigma ( x 400). K, fusion of naked gametes
of C. pisiformis ( x 400). L-N, stages in fusion of gametes of C. Braunii.
O, fusion of gametes in C. coccifera. P, C. Braunii, palmelloid stage. (A-D,
after Fritsch; E-K, after Scott; L-P, after Oltmanns.)

VOLVOCALES 23

flagellae, and is often strongly phototactic. Variations in the
structure of the cell occur throughout the genus, which contains
about 150 species. There may be more than one pyrenoid present
(C sphagnicola) or they may be completely absent (Chloromonas),
whilst the chloroplast may be reticulate {Chlamydomonas reticulata),
or axile and stellate (C. eradians), or it may be situated laterally
(C.parietaria). It has been said that under cultural conditions many
of the characteristic features can be modified, and that therefore
some of the forms are not true species but are simply phases in
the life cycles of other species.

The motile cells are spherical, ellipsoid, or pyriform in shape with
a thin wall which occasionally possesses an outer mucilage layer.
The two flagellae are situated anteriorly and either project through
one aperture in the wall or else through two separate canals, but in
either case at the point of origin of the flagellae there are two basal
granules whose function is not yet clearly established. Each cell
typically possesses two contractile vacuoles which have an excretory
function. At asexual reproduction the motile bodies come to rest
and divide up into four, more rarely eight or sixteen, daughter cells.
The first division at zoospore formation is normally transverse, and
in those cases where it is longitudinal a subsequent twisting of the
protoplast makes it appear to be transverse. The zoospores escape
through gelatinization of the cell wall, but if this does not occur the
colony then passes into the palmelloid state, which is usually of
brief duration, though in C. Kleinii it forms the dominant phase in
the life history of the species. C. Kleinii may thus be regarded as
forming a transition to the condition found in Tetraspora (cf. p. 34).

In sexual reproduction eight, sixteen or thirty-two gametes are
formed in each cell. In Chlamydomonas longistigma the gametes are
bare (gymnogametes) ; in C. media they are enclosed in a cell wall
from which they emerge in order to fuse (calyptogametes) ; in
C. monoica there is anisogamy as the naked contents of one gamete
pass into the envelope of the other ; in C. Braunii there is marked
anisogamy, the female cell producing four macrogametes and the
male cell eight microgametes ; in C. coccifera there is oogamy, with
the female cell producing one macrogamete enclosed in a wall whilst
the male cell produces sixteen spherical microgametes. In a related
genus, Chlorogonium oogammn, one naked ovum is produced and
numerous elongate antherozoids, whilst cases of relative sexuality

CHLOROPHYCEAE

have been recorded for Chlamydomonas eiigametos. The zygote on.
germination frequently gives rise to four swarmers, and it is
probable that meiosis occurs during this segmentation, the normal
vegetative cells thus being haploid. In C. pertusa and C. hotryoides,
however, the zygote may remain motile for as long as lo days, and
hence it may be considered that these two species exhibit a definite
alternation of generations. In C. variabilis the persistent quadri-
flagellate zygote has for long been known as Carteria ovata (cf. also
p. 297), but it has now been shown that the latter is the diploid
generation of the Chlamydomonas.

The genus is widespread, the various species occurring prin-
cipally in small bodies of water or on the soil.

Chlamydomonadaceae : Goniiim {gonium, angle). Fig. 15.

The colony in the different species is composed of four, eight or
sixteen cells all lying in one plane and forming a flat quadrangular
plate, but it has been suggested that the four- and eight-celled

Fig. 15. Gonium pectorale. A-D, stages in the formation of a coenobium.
E, colony ( x 520). F, zygote. G, H, stages in germination of zygote. J, four-
celled colony. (A-D, after Fritsch; E, after Smith; G-J, after Kniep.)

VOLVOCALES

colonies are merely degenerate forms of the principal species,
G. pectorale. In the sixteen-celled colonies (G. pectorale) there are
four cells in the centre and twelve in the periphery, each cell being
surrounded by a gelatinous wall and fused to the neighbouring
cells by means of protrusions, whilst the protoplasts of the indi-
vidual cells are also united by fine protoplasmic threads. The ovoid
or pyriform cells contain contractile vacuoles and are provided with
a pair of flagellae. The centre of the colony is composed of mucus
and there is also a firm outer gelatinous layer. The shape of the
colony accounts for its mode of progression which is by means of
a series of somersaults around the horizontal axis. At asexual
reproduction all the cells divide simultaneously to form daughter
colonies. If single cells should become isolated then after a time
they will give rise to {a) daughter colonies, {h) akinetes, or (c) a
palmelloid state. Sexual reproduction is by means of naked iso-
gametes, fusion occurring between gametes from separate colonies
as the various species occur in + and - strains. The resulting
quadriflagellate zygote soon comes to rest and subsequently germi-
nates, when it gives rise to four biflagellate haploid cells which are
liberated together as a small colony. When the later development of
these cells is followed it is found that two of them give rise to +
and two to - colonies, suggesting that meiosis must take place at
germination of the zygote.

*CHLAMYDOMONADACEAE:P^wJorma (after Pandora's box). Fig. 16.

The colonies are oblong or spherical and are composed of four,
eight, sixteen or thirty-two cells, sixteen being the normal number
in the common species P. morum. The cells, which are arranged

A ' B

Fig. 16. Pandorina morum. A, vegetative colony ( x 975). B, colony with female
gametes ( x 975). (After Smith.)

CHLOROPHYCEAE

compactly in the centre and are frequently flattened from mutual
pressure, are connected to each other by protoplasmic threads that
are withdrawn during reproduction. Each colony is enclosed in a
gelatinous matrix with an outer watery sheath, and, together with
the next two genera, exhibits some degree of polarity in its pro-
gression. When reproducing asexually the cells first lose their
flagellae and then each one gives rise by several divisions to a
daughter colony. In sexual reproduction signs of anisogamy are to
be found, and the zygote germinates giving one to three biflagellate
spores which then develop into new colonies.

*Chlamydomonadaceae :£'M(fonw« {eu, well ; dorina, meaningless !).
Fig. 17.

The colonies are spherical or ellipsoid, the posterior end often
being marked by mamillate projections. They contain sixteen,

Fig. 17. Eudorina elegans. A, vegetative colony. B, transverse section showing
structure and protoplasmic connections, a = outer layer, b = inner layer of muci-
lage. C, formation of daughter coenobia. D, E. illinoiensis, showing somatic
cells, V. (After Fritsch.)

thirty-two (commonly) or sixty-four biflagellate cells, which are not
closely packed and are frequently arranged in transverse rows, the
flagellae of the individual cells emerging through funnel-shaped
apertures. In most species all the cells give rise to daughter colonies,
but in E. illinoiensis and E. indica the four anterior cells are much
smaller and cannot produce gametes or daughter colonies. This

VOLVOCALES

marks a first differentiation into a plant soma within the group, and
furthermore these somatic cells die once the colony has reproduced.
It would be of great importance if the nature of the stimulus that
induced some of the cells to lose their reproductive capacity could
be determined. It might be possible to investigate such a problem
experimentally on some of the undifferentiated species of Eudorina.
Sexual reproduction is oogamous, the colonies being either
monoecious or dioecious : in the former case the anterior cells give
rise to the antherozoids, whilst in the latter case the antheridial
plates are liberated intact and only break up after swimming to the
female colony where the surrounding walls have already become
gelatinous. The zygote on germination gives rise to one motile
zoospore and two or three degenerate zoospores.^

Chlamydomonadaceae : Pleodorina (pleo, more ; dorina, meaning-
less!). Fig. 18.

This genus is very similar to the preceding one, but the somatic
area is more highly differentiated as it occupies one-third to one-

Fig. 18. Pleodorina Californica. Colony of 120 cells ( x 178). (After Shaw.)

half of the colony, and the total number of cells is greater, thirty-
two, sixty-four or 128. The somatic cells are all situated either in an
anterior or posterior position and they die when the colony has
reproduced. Reproduction follows the same lines as in Eudorina.

^ Inversion of the daughter colonies and of the antheridia takes place during
development (cf. Volvox).

28 CHLOROPHYCEAE

*Chlamydomonadaceae: Volvox (volvere, to roll). Figs. 19-22.

This genus represents the ultimate development that has been
reached along this particular line, each colony forming a hollow
sphere with 500-20,000 biflagellate cells set around the peri-
phery, the flagellae emerging through canals. The interior of the
colony is mucilaginous or else merely contains water, whilst the
whole collection of cells is bounded by a firm mucilage wall. The

Fig. 19. Volvox. A, V. aureus with daughter colonies. B, structure of V. aureus
as seen in section. C, surface view of single cell of V. Rousseletii ( x 2000).
D, the same in side view ( x 2000). (A-B, after Fritsch; C, D, after Pocock.)

individual cells, each containing two to six contractile vacuoles,
are surrounded by gelatinous sheaths, the middle lamellae of which
form a polygonal pattern when stained with methylene blue. The
cells are usually united by two or more delicate cytoplasmic threads,
or plasmodesmae, though these are absent in some species {V. tertiiis).
In V. glohator the cells are sphaerelloid in nature, whilst in
V. aureus they are chlamydomonad in appearance, several individual
chloroplasts being enclosed in wedge-shaped prisms which are

VOLVOCALES

probably morphologically equivalent to cells. For this reason it has
been suggested that the volvocine colony has arisen at least twice in

1 p.m.

1 0.50 a.m.

Q 12.1 0 p.m.

12.50

Fig. 20. Volvox capensis and V. Rousseletii. A-J, stages in the inversion of a
daughter colony. A, denting begins. C, dents smooth out. D, colony round
again. E, 'hour-glass' stage. F, posterior half contracts. G, infolding begins.
H, infolding complete. I, posterior half emerges through phialopore, J, flask
stage begins. K, flask stage ends. L, inversion complete. (All x 150 approx.)
(After Pocock.)

the course of evolution, once from a Sphaerella and once from a
Chlamydomonas ancestry. On the other hand, the great uniformity
of their sexual reproduction can be employed as an argument
against such a diphyletic origin.

CHLOROPHYCEAE

2.43

3.1 7a.m.

2.55a.m,

2.40 a.m.

Fig. 21. Volvox. A-I, stages in the development of the zoospore of V. Roiis-
seletii. A, zoospore just after escape. B, first division. F, preparation for
inversion. G— I, inversion. (All x 375.) J-O, stages in the inversion of a sperm
bundle of V. capensis. (All x 750.) (After Pocock.)

VOLVOCALES

I
ft

Pig. 22. Volvox. A-C, development of oospore of V. Rousseletii ( x 750).
OMi = outer wall. A, flagellar stage. B, mature. C, exospore formation. D— L,
development of daughter colony (gonidium). F, two-celled stage ( x 750).
G, four-celled stage ( x 750). H, eight-celled stage ( x 750). I, sixteen-celled
stage ( X 750). J-L, formation of phialopore ( x 225). (After Pocock.)

32 CHLOROPHYCEAE

The majority of the cells, including all those in the anterior
quarter, are wholly somatic, and only a few are able to give rise to
daughter colonies. When this occurs a cell increases in size and
divides many times to produce a small hollow sphere with a pore
(phialopore) towards the outer edge. These plants {gonidia), which
hang down into the cavity of the parent, then invert, the process
commencing opposite their phialopore, and later they are liberated
into the parental cavity (cf. fig. 20). They remain in the cavity until
the parent tears open, in Volvox aureus at the phialopore of the adult,
in V. glohator at any place. In V. africana it is possible to see as
many as four generations in the one original parent colony.

In sexual reproduction the plants are either monoecious
(V. aureus) or dioecious (F. glohator), and, furthermore, plants re-
producing sexually are usually devoid of asexual daughter spheres.
Cells giving rise to eggs {egg cells) enlarge considerably, but do
not undergo division, and the flagellae disappear, whilst cells
giving rise to the antherozoids [antheridia) divide up into
sixteen, thirty-two, sixty-four or 128 small elongated cells which
form a plate or globoid colony which may invert in the same way as
the asexual gonidia (cf. fig. 21). The fertilization mechanism is not
known for certain, but in the dioecious species the antherozoids are
said to penetrate the female colony and then enter the ovum from
the inner side. The first divisions of the zygote involve meiosis, and
the oospore then develops into a single swarmer that grows into a
"juvenile" plant of about 500 cells which finally inverts before
developing into the adult (cf. fig. 21, also p. 43 for a comparison
with Hydrodictyon and a possible interpretation). There is evidence
that in some species the "juvenile" stage is omitted. One of the
characteristic features of the genus are the inversions that occur at
diflferent stages of the life cycle, and it is difficult to see why they
occur or what the conditions were under which they first developed.
It may be associated with the fact that the cells are formed with the
eye-spot facing the interior, but even then the problem arises as to
how the individual cells came to be arranged thus.

*Sphaerellaceae: Sphaerella {sphaer, ball; ella, diminutive of
affection) (Haematococcus). Fig. 23.

A characteristic of this genus is the area between the protoplast
and the cell wall ; this is filled by a watery jelly and is traversed

VOLVOCALES

by cytoplasmic threads passing from the central protoplast to the
cell wall. The protoplast contains several contractile vacuoles and
one or more pyrenoids, although two is the usual number. Asexual
reproduction is by means of two to four macrozoospores, whilst
sexual reproduction is isogamous or anisogamous. The young
Sphaerella cell is very akin to a Chlamydomonas, and for this reason
some authors would unite the two genera. Large akinetes are

Fig. 23. Sphaerella lacustrh {Haematococcus pluvialis). A, diagram of single
macrozoid. fe = blepharoplast, c = chloroplast, cf = flagellum tube, cw = c&W wall,
n = nucleus, nM = nucleolus, ^ = pyrenoid, ^5 = protoplasmic strand, r = rhizoplast,
5 = stigma. B, encysted plant with haematochrome in centre. C, eight-celled
palmelloid stage. D, diagram illustrating life cycle in bacteria-free cultures.
(After Elliott.)

known which on germination give rise to zoospores, hypnospores,
or gametes. One species forms one of the components of "Red
Snow" because under nival conditions it develops haematochrome
as a result of nitrogen deficiency brought about by the presence of
the snow. Periodic drying also appears to be an essential factor if
the life history of the common species, Sphaerella lacustris, is to be
maintained. Eight-celled colonies (coenobia), which behave just
like Pandorina, are known in the related genus Stephanosphaera.

Tetrasporaceae : Tetraspora {tetra, four; spora, spores). Fig. 24.
The members of this genus form expanded or tubular, convoluted,

CSA

CHLOROPHYCEAE

light green macroscopic colonies. These are most abundant in the
spring when they are attached at first, although later they become
free-floating. The cells are embedded in the mucilage in groups of
four, each group often being enclosed in a separate envelope. Two
or four pseudocilia proceed from each cell to the surface of the main
colonial envelope, each thread being surrounded by a sheath of

&-e^

:o-^®}^e^%r^^- ®>

©■.- "

-A

^:--

(^'■■^■,

Fig. 24. Tetraspora. A, T. cylindrica ( x |). B, portion of colony of T. cylindrica
showing outer envelope ( x 155). C, T. lubrica ( x ^). D, portion of colony of
T. lubrica ( x 500). (After Smith.)

denser mucilage. These structures cannot be organs of locomotion
because they possess no power of movement, but they may repre-
sent such organs which have lost their function or they may be their
precursors. Reproduction is either by fragmentation of the parent
colony or else by means of biflagellate swarmers which may develop
into {a) a new colony, {b) the palmelloid state or (c) a thick- walled
resting spore. The resting spore gives rise to an amoeboid cell on
germination. Sexual reproduction is secured by means of bi-
flagellate isogametes, the colonies being either monoecious or

VOLVOCALES

dioecious, and after fusion has taken place the zygote divides into
four to eight aplanospores which later grow into new colonies. The
place of meiosis in the life cycle is not yet known.

Chlorodendraceae : Prasinocladiis {prasino, leek-like ; cladus, shoot)
(Chlorodendron). Fig. 25.

This genus is to be found principally in marine aquaria where it
starts life as a quadriflagellate swarmer of the chlamydomonad

I
I
I
I
I

I
I
I
I

I
I

Fig. 25. Prasinocladus. A, B, portion of plant showing cell structure, 1-3 = cells.
C, portion of plant showing arrangement of cells at branching. D, portion of
plant with branches and living cell. (All x 1600.) ^^^ = pyrenoid, n = nucleus,
5 = stigma, c = chloroplast, m = basal margin of terminal protoplast, 6r = first
branch, Z>ri = second branch, c/ii-c/z4 = short chambers behind terminal cell at
times of division, ch^ being the earliest, /= minute remnant of flagellae, c^, c^.
= bases of t^vo cells, ^ = papilla, 0 = overlap of lateral wall, Z = entire lateral
extent of one chamber, e^ = papilla pointing upwards, 6c = basal cross wall,
ti, i2 = tops of two cells. (After Lambert.)

type. The swarmer comes to rest and a new wall is formed with
papillae at the base. Then the apex of the old wall ruptures, and
when the contents have developed flagellae they move up, together
with the new wall, so that the new cell becomes enclosed in the neck
of the old one. The flagellae are lost for a time and then the process
is repeated, and in this manner a filament of dead cells is built up

^-2

36 CHLOROPHYCEAE

with a living cell at the apex. An oblique division of the living cell
results in a branch being formed and sometimes one half may cease
to divide, thus leaving a living cell in the middle of the dead cells.
It is evident from a consideration of this process that at each divi-
sion a potential swarmer is formed which is not normally liberated.
On the few occasions when it is freed then the species is perpetu-
ated, but at present the particular conditions under which a swarmer
may be liberated are not known.

REFERENCES

Eudorina. Akehurst, S. C. (i934)- J- ^oy. Micr. Soc. 54, 99.

Cytology. Chaudefaud, M. (1936). Rev. Alg. 8, 5.

Sphaerella. Elliott, A. M. (1934). Arch. Protistenk. 82, 250.

Tetraspora. Geitler, L. (1931)- Biol. Zbl. 51, 173.

Gonium. Harper, R. A. (191 2). Trans. Amer. Micr. Soc. 31, 65.

Eudorina, Gonium. Hartmann, M. (1924). Arch. Protistenk. 49, 375.

Prasinocladus. Lambert, F. D. (1930)- ^' Bot. 23, 227.

Volvox. Lander, C. A. (1929). Bot. Gaz. 87, 431.

Flagellae. Petersen, J. B. (1929). Bot. Tidsskr. 40, 373.

Volvox. PococK, M. A. (1933)- Ann. S. Afr. Mus. 16, 523.

Volvox. PococK, M. A. (1938). jf. Quekett Micr. Club, ser. 4, i, i.

Pleodorina. Shaw, W. R. (1894). Bot. Gaz. 19, 279.

Eudorina, Gonium. Smith, G. M. (1930-1). Bull. Torrey Bot. Club, 57, 359.

Volvox. Zimmerman, W. (1921). Jb. wiss. Bot. 60, 256.

Chlamydomonas. Behlau, J. (1939). Bei. Biol. Pfianzen, 27, 221.

Eudorina. Doraiswami, S. (1940). jfourn. Ind. Bot. Soc. 19, 113.

CHLOROCOCCALES

This is an order which is probably of polyphyletic origin but it is
not proposed to elaborate this problem here.

*=Chlorococcaceae: Characium (a slip or cutting). Fig. 26.

Each plant is a solitary unicell and only possesses a motile re-
productive phase, and it may be supposed that in some previous era
the vegetative phase ceased to be motile and became attached. The
ellipsoidal cells occur singly or in aggregates on submerged plants
or living aquatic larvae, being borne on a short stalk which emerges
from a small basal disk. Asexual reproduction is brought about by
means of biflagellate zoospores which are liberated through a
terminal or lateral aperture. Certain species exhibit anisogamy,
whilst in C. saccatum the sexual and asexual generations are distinct
so that there may therefore be two different cytological generations.

CHLOROCOCCALES

Chlorococcaceae : Chlorochytrium (chloro, green; chytrium,
vessel). Fig. 27.
The swarmers, which may either be zoospores or motile zygotes,
settle on the leaves of aquatics, principally species of Lemna
{Chlorochytrium lemnae), whilst another species is also known which
penetrates the leaves of Polygonum lapathifolium. Tubular pro-
longations grow out from these attached bodies and enter the host,
either by way of the stomata or else between two epidermal cells.

e<:

Fig. 27.

Fig. 26. Characium angustatum. A, vegetative cells ( x 650). B, cell commencing
zoospore formation ( x 650). C, liberation of zoospores: the cell is probably-
broken accidentally ( x 650). (After Smith.)

Fig. 27. Chlorochytrium lemnae. A, entrance of zygote into host. B, resting
cells in leaf of Lemna. C, resting cell. (After Fritsch.)

Subsequently the end of the tube swells out into an ellipsoidal or
lobed structure into which the contents of the swarmer pass. These
swellings, which are to be found in the intercellular spaces of the
host's tissues, become rounded off, and in the autumn sink down
with the Lemna fronds to remain dormant until the next spring. In
Polygonum the swollen filaments even crush the host cells which
may become partially dissolved. In the spring the cell contents
divide up into biflagellate swarmers, which are probably haploid,
and these are liberated all together in a mucilaginous vesicle.

38 CHLOROPHYCEAE

fusion taking place whilst still enclosed or else after they have
escaped. The resulting zygote is motile and quadriflagellate.
Swarmers are also known which do not fuse, and it has been
suggested that these develop from haploid races which have arisen
apogamously, but a simpler explanation would be to regard them
as zoospores. In any case the principal phase in the life cycle would
seem to be diploid. Resting cells are also known in which the walls
are thick and stratified. Species have been reported from mosses
and algae as well as angiosperms, and as many of them have a
decided pathogenic action they must at least be facultative parasites.

*Chlorococcaceae : Chlorococcum {chloro, green; coccum^ berry)
(Cystococcus). Fig. 28.

Much confusion has existed over this genus, as many of the
species formerly described are now known to be phases in the life

Fig. 28. Chlorococcum humicolum.
(After Smith.)

A-F, various stages in the hfe histor>^ ( x 800).

cycles of species from other genera. Some of the species have been
segregated into the genus Trebouxia, the cells of which form the
algal component of several lichens (cf. p. 296). The plants are non-
motile spherical cells which vary much in size, occurring singly or
else forming a stratum on the soil. There is no eye-spot or con-
tractile vacuole ; the chloroplast is parietal, and there may be one or
more pyrenoids. The cell walls are two-layered with a thin inner
layer and an outer gelatinous one which is sometimes lamellose.
The young cells are uninucleate but the adult ones are commonly

CHLOROCOCCALES

multinucleate, and it is in this older condition that the protoplast
divides and gives rise to numerous biflagellate zoospores which are
Hberated all together in a vesicle, usually in the early hours of the
morning. After a short motile phase the flagellae are withdrav^n
and a new vegetative phase commences.

Isogamy and anisogamy are known, but there is no recorded
example of even primitive oogamy comparable to that found in
Chlamydomonas. Under certain conditions aplanospores are formed :
when this happens the parent gelatinizes and a "palmella" stage
results, the cells of which subsequently give rise to two to four
biflagellate gametes. It seems clear that the suppression of motility
has occurred several times in the Chlorococcaceae, a feature which
supports the idea of their polyphyletic origin. The aplanospore stage
also suggests how the genus Chlorella may have arisen. Under
normal conditions Chlorococciim reproduces by means of motile
zoospores, but when subjected to drought these bodies are non-
motile. In nutrient culture solutions of low concentration repro-
duction takes place by zoospores, whilst in highly concentrated
solutions the zoospores are replaced by aplanospores, so that it can
be concluded that the environment may affect the reproductive
mechanism to a considerable extent. C. humicolum is a very common
soil form (cf. p. 299).

Chlorellaceae : Chlorella {chlor, green; ella, diminutive of
affection). Fig. 29.
The globular cells are non-motile, solitary or aggregated into
groups, and usually lack pyrenoids. They reproduce by division

F E D

A B

Fig. 29. Chlorella vulgaris. A, single cell. B, division into four. C, final stage
of division into four daughter cells. D, first stage of division into eight. E, F,
second and third stages of division into eight daughter cells. (After Grintzesco.)

CHLOROPHYCEAE

into two, four, eight or sixteen autospores. Several species often
form a symbiotic association with lower animals when they are
known as Zoochlorella or Zooxanthella (cf. p. 296). The species are
frequently indeterminate systematically and are chiefly studied by
means of laboratory cultures, but in spite of these systematic
difficulties they are common objects for physiological experiments.

Hydrodictyaceae : Pediastnun {pedia, plain ; astrum, star). Fig. 30.

The species of this genus are common components of fresh-
water plankton. The cells form disk-like coenobia, the plane-faced
or lobed cells being arranged in one layer, experiments suggesting

Fig. 30. Pediastrum. A, P. Boryanum ( x 333). B, P. simplex var. duodeniarum
( >< 333)- C, P. Boryanum var. granulatum showing liberation of zoospores.
D, P. duplex with hypnospores. E, P. Boryanum, germination of tetrahedron.
F, P. Boryanum var. granulatum, formation of new plate. (A, B, after Smith;
C-F, after Fritsch.)

that the shape of the cells is determined by heredity and mutual
pressure. At certain stages in the life cycle they bear tufts of
gelatinous bristles which are probably a modification for their
floating existence. There are 2-128 cells in each coenobium,
varying with the species, and whereas the young cells are uni-

CHLOROCOCCALES 41

nucleate the mature ones may possess as many as eight nuclei.
Biflagellate zoospores are formed, the number depending upon the
external physical conditions, and they are usually liberated at day-
break from the parent cell into an external vesicle in which they
swarm for a time, but they soon become arranged into a new
coenobium before the vesicle ruptures. The flagellae are some-
times absent. Isogametes are also formed and liberated singly, and
after fusion the zygote divides up into a number of swarmers ; each
of these subsequently turns into a thick-walled polyhedral cell in
which a new coenobium is formed. There would seem to be very
little justification for placing this and the next genus into the
Siphonales, as some authors have suggested, because their mode of
reproduction is essentially much more akin to that of the Chloro-
coccales.

*Hydrodictyaceae : Hydrodictyon (hydro, water; dictyon, net).

Fig. 31-

•

The number of species are few, the commonest, H. reticulatum,
having a world-wide distribution though it occurs but rarely in
each locality. It is a hollow, free-floating, cylindrical network
closed at either end and up to 20 cm. in length. The individual
coenocytic cells are multinucleate and are arranged in hexagons or
pentagons to form the net. The chloroplast is reticulate with
numerous pyrenoids, though in the young uninucleate cells there
is but a simple parietal chloroplast which later becomes spiral and
then reticulate. H. africanum and H. patenaeforme develop into
saucer-shaped nets, the former with spherical cells up to i cm.
diameter which may become detached and lie on the substratum
looking like pearls. The other species is composed of cells which
may grow up to 4 cm. long by 2 mm. in diameter. Asexual
reproduction in H. reticulatum is by means of numerous uninucleate
zoospores which swarm in the parent cell about daybreak and then
come together to form a new coenobium which is subsequently
liberated, further growth being brought about by elongation of the
coenocytic cells. It is interesting to note that the arrangement of
the daughter cells in the parent coenocyte agrees with the me-
chanical laws for obtaining the greatest rigidity with the maximum
economy of space.

Asexual reproduction is unknown in H. africanum and H.patenae-

CHLOROPHYCEAE

forme. Sexual reproduction in all three species is isogamous in
character and the plants are monoecious. In H. patenaeforme the
zygote is motile for a short time, but in the other two species it
is always non-motile. At germination the zygote enlarges and
divides by meiosis into four biflagellate swarmers which first come

Fig. 31. Hydrodictyon. A-F, development of young net of H. patenaeforme
from the zygote. A, young polyhedron. B, older polyhedron with four nuclei.
C, protoplasm granular just before zoospore formation. D, "pavement" stage.

E, zoospores rounding off and wall of polyhedron expanding to form vesicle.

F, fully formed net still enclosed in vesicle. (A-E x 250, F x 175,) G, portion of
mature net of H. reticulatum. H, polyhedron and young net of H. reticiilatum.
J, H. reticulatum, formation of net in parent cell from zoospores. (A-F, after
Pocock; G, H, after Oltmanns; J, after Fritsch.)

to rest and then develop into polyhedral cells. After resting for a
period these divide to produce zoospores ; the food material in the
angular thickenings of the polyhedrons is used up and all the
swarmers are finally liberated in a vesicle, in which, after a period
of motility, they come together to form a new coenobium. The
vegetative plant is therefore haploid and its development is probably

CHLOROCOCCALES

one of the most remarkable that is to be found among the fresh-
water algae. Pediastrum is a very poor indication of what the
ultimate development of this type of thallus construction could be,
and this provides a problem at present unsolved, namely, the absence
of any intermediate morphological stage between Pediastrum and
Hydrodictyon. Further increase in the size of colony is probably
impossible for purely mechanical reasons. The fact that the cells are
coenocytic also indicates that the siphonaceous habit must have
arisen several times in the course of evolution. Hydrodictyon is
essentially a collection of a number of individual coenocytic plants
because it has arisen as a result of the fusion of a number of
swarmers. Volvox, on the other hand, must be regarded as a single
plant composed of a number of cells connected by strands because
it arises from a single zygote or asexual cell. Gamete and zoospore
production respectively can be obtained in Hydrodictyon by varying
the external conditions artificially. For example, if plants are grown
in weak maltose solutions in bright light or in the dark and are then
transferred to distilled water, zoospores will develop under the first
set of conditions and gametes under the second.

CoELASTRACEAE : Sccnedesmus (scene, rope; desmus, fetter). Fig. 32.

The planktonic colonies are composed of four, eight or, more
rarely, sixteen cells. The two end cells of the chain may differ in

G F

Fig. 32. Scenedesmus. A, 5. acuminatus. B, S. acuminatus with mucilage
bristles. C, S. quadricauda. D, S. quadricauda reproducing. E-I, stages in the
formation of daughter coenobia in S. quadricauda. (After Fritsch.)

CHLOROPHYCEAE

shape from the others and often have processes which are elabora-
tions of the mucilaginous cell envelope : these processes are prob-
ably to be correlated with the planktonic mode of life, whilst tufts of
bristles performing the same function and similar to those of
Pediastrurn are also recorded.

It should be evident from the preceding descriptions that the
Chlorococcales represent a number of very diverse types, some of
which may have indications of distant relationships whilst there
are others whose relationships are extremely vague : a recent paper
even describes some oogamous members.

REFERENCES

Chlorococcum. Bold, H. C. (i 930-1). Bull. Torrey Bot. Club, 57, 577-
Chlorochytrium. Bristol, B. M. (iqiq)- J- Linn. Soc. (Bot.) 45, i.
Chlorella. Grintzesco, J. (1903). Rev. Gen. Bot. 15, 5.
Pediastrurn. Harper, R. A. (1918). Proc. Amer. Phil. Soc. 57, 375.
Hydrodictyon. Mainx, F. (193 i). Arch. Protistenk. 75, 502.
Chlorochytrium. Palm, B. T. (1932). Rev. Alg. 6, 337.
Hydrodictyon. PococK, M. A. (i937)- Trajis. Roy. Soc. S. Afr. 24, 263.
Chlorococcum. Puymaly, A. de (1924). Rev. Alg. i, 107.
Scenedesmus. Smith, G. M. (19 14). Arch. Protistenk. 32, 278.

ULOTRICHALES
*UxoTRiCHACEAE : Ulothrix {iilo, shaggy; thrix, hair). Fig. 33.

The unbranched filaments are attached to the substrate by means
of a modified basal cell which frequently lacks chlorophyll, but even
though attached at first the plants sometimes become free-floating.
Under unfavourable conditions, e.g. nutrient deficiency, rhizoids
may grow out from the cells or else the filaments become branched.
This behaviour suggests one way at least in which the branched
habit may have evolved from the simple filament, in this case
probably representing an attempt to increase the absorbing surface
in order to counteract the deficiency of salts. The cells vary con-
siderably in size and shape and the walls may be thick or thin ; if the
former, then they are usually lamellate. There is a single chloro-
plast which forms a characteristic circular band around the whole or
most of the cell circumference. Vegetative reproduction can take
place through fragmentation, especially when conditions are un-
favourable, the various fragments developing conspicuous rhizoids.
Swarmers are formed from all the cells of the filament except the
attachment cell, but they usually appear first at the apex of the

ULOTRICHALES

filament and then successively in the other cells. They are liberated
through a hole in the side of the cell into a delicate vesicle, and the
subsequent bursting of this vesicle frees the swarmers, all of which

Fig. 33. Ulothrix zonata. A, B, rhizoid formation. C, liberation of swarmers
into vesicle ( x 375). D, germination of aplanospores in the cell ( x 250). E, libera-
tion of gametes ( x 375). F, escape of zoospores ( x 375). G, akinetes of U. idio-
spora. H, palmelloid condition. I, schema to illustrate the different types of
filaments and swarmers. J, K, aplanospores ( x 400). L, zoospore formation
( X 400). M, banded chloroplasts in a portion of the vegetative filament ( x 400).
(A, B, I, after Gross; C-F, J-M, after West; G, H, after Fritsch.)

are positively phototactic. Three types of swarmer are to be found.
(a) Quadriflagellate macrozoospores, of which two, four or eight
are produced per cell. After a motile period these become broader
than they are long, attach themselves to a suitable substrate, and

46 CHLOROPHYCEAE

then a rhizoid and filament grow out opposite each other in a plane
at right angles to the longitudinal axis of the original zoospore.
(b) Each cell produces four, eight, sixteen or thirty-two bi- or quadri-
flagellate microzoospores. These swarmers will only germinate at
low^ temperatures and then more slowly than the macrozoospores,
producing a somewhat narrow filament or else forming resting
spores, (c) Eight, sixteen, thirty-two or sixty-four biflagellate
gametes are produced in each cell and are usually liberated soon
after daybreak. The adult plants are usually dioecious, and after
fusion of the gametes has taken place (parthenogenesis is said not to
occur) the quadriflagellate zygote forms a resting zygospore. This
germinates after 5-9 months giving rise to four or sixteen aplano-
spores, and as meiosis occurs during their production the adult
plants are haploid.

It is said that there are six types of adult filament: + and —
strains producing + or — gametes only, + and — strains producing
both + or — gametes and zoospores and + or — strains producing
zoospores only. Aplanospores, when they are formed, may either
develop into new plants or else they form a temporary "palmella"
state. Akinetes are also recorded. The genus appears primarily in
winter or spring, and the optimum conditions would seem to include
either cold weather or cold water because the plants die down in
summer. The genus is well represented in both fresh and salt waters,
U. zonata and U. flacca being common species respectively of the
two habitats. The nearly related genus Schizomeris, in which the
filaments have some longitudinal divisions, may be considered as
representing an intermediate stage in the evolution of the more
foliaceous forms, e.g. Ulva.

*MiCROSPORACEAE : MicrospoYU {micro, small ; spora, seed). Fig. 34.

This genus is sufficiently distinct from the preceding one to
warrant its inclusion in a separate family. The plants are free-
floating when mature and consist of unbranched threads, the cells
of which have walls of varying thickness, the thicker walls showing
some stratification. In many species the cell wall is in two over-
lapping halves held in place by a delicate inner or outer membrane,
and it is because of this type of structure that the threads readily
fragment into H pieces. In ordinary cell division growth is brought
about by the introduction of new H pieces. The parietal chloroplast

ULOTRICHALES

is often reticulate or else forms an irregularly thickened band, and,
although there are no pyrenoids, the genus is characterized by an
abundance of starch in the threads. One to sixteen biflagellate
(quadriflagellate in one species) zoospores are formed in each
mother cell and are liberated by the thread fragmenting into H

D I C

Fig. 34. Microspora amoena. A, portion of thread. B, early cleavage in swarmer
formation. C, two young cells ( x 745). D, akinete formation ( x 550). E, forma-
tion of aplanospores. F, G, stages in germination of aplanospores. H, liberation
of zoospores. I, zoospores ( x 745). (C, D, I, after Meyer; rest after Fritsch.)

pieces or else by gelatinization of the cell walls. There may perhaps
be biflagellate gametes, but fusion between swarmers has only been
seen in one species, whilst in another species gametes possessing
somewhat unequal flagellae have been recorded. This fact is
extremely interesting and, if true, would make a reorientation of
ideas about this genus essential. Microspora exhibits considerable
variation, particularly in a xanthophycean direction, and in many

CHLOROPHYCEAE

characters it overlaps Tribonema (cf. p. 117). For this reason it is
not impossible that the filamentous Xanthophyceae may be derived
via a form such as Microspora from an ulotrichaceous filament. On
this view Ulothrix and Tribonema cannot be regarded as end-phases
in separate lines of evolutionary development. Any cell of Micro-
spora can produce aplanospores instead of motile bodies, and
akinetes are also frequently formed, either singly or in long chains.
On germination these divide into four bodies, each of which gives
rise to a new filament. This genus, like Ulothrix, is also confined
to the winter or spring months.

Cylindrocapsaceae : Cylindrocapsa (cylindro, cylinder; capsa, box).
Fig. 35-
The filaments, which are unbranched, are attached at the base by
means of a gelatinous holdfast, and when young each thread is com-

Fig. 35. Cylindrocapsa. A, vegetative filament. B, thread with young antheridia
(n) and young oogonium (o). C, fusion of gametes, a = antherozoid, o = ovum.
D, old mature filament. (After Fritsch.)

posed of a single row of elliptical cells with thick stratified walls, the
whole being enclosed in a tubular sheath. In older filaments,
however, the cells divide longitudinally, usually into pairs, and the
gelatinous nature of the threads suggests how the genus Mono-
stroma may have evolved, although Cylindrocapsa itself has de-

ULOTRICHALES

veloped much farther, especially in its mode of sexual reproduction,
because it is anisogamous with elongate antherozoids and large
round ova.

MoNOSTROMACEAE : Monostroma {mono, single; stroma, layer).
Figs. 36, 37.

The thallus develops as a small sac, which in most species
ruptures very early to give a torn plate of cells one layer in thick-

O^x

R=-^D

ULOTHRDC Etc. nONOSTROHA

Fig. 36. Diagram to illustrate the three different types of life cycle found in the
Ulotrichales. i^D = place of reduction division in life cycle.

Fig. 37. Monostroma crepidinum. A, plant ( x |), B, cells of thallus ( x 200).
C, transverse section of thallus ( x 200). D, M. Lindaueri, plants ( x f ). (After
Chapman.)

ness, the cells often being arranged in groups of two or four. In
M. Grevillei the thallus only ruptures in the adult stage, and traces
of the original tubular form can also be seen in adult plants of
M. Blytii, whilst in M. Lindaueri the sac appears to remain entire.

CSA

CHLOROPHYCEAE

The plants are dioecious in respect of sexual reproduction and several
of the species exhibit anisogamy. Biflagellate gametes from separate
plants fuse to give a non-motile zygote which then increases in size
and after some months undergoes meiosis and forms zoospores.
The macroscopic plants are thus all haploid and the diploid
generation is only represented by the enlarged zygote. In this
respect it is sharply differentiated from the genera Ulva and
Enter omorpha, and it possibly has only a distant relationship with
them. Each zoospore from the zygote divides to give eight peri-
pherally arranged cells with a central cavity and this then develops
slowly into a sac. The genus is more widespread than is perhaps
suspected from the literature, frequenting both saline and fresh
waters.

*Ulvaceae: Ulva (Latin for a marsh plant). Figs. 36, 38.

The thallus, which is composed of two layers and is therefore
distromatic, develops from a single uniseriate filament that sub-

Fig. 38. Ulva lactuca.
Oltmanns.)

A, plant. B, transverse section of thallus. (After

sequently expands by lateral divisions, but there is usually no
hollow sac, though exceptions to this are found in U. Linza and
U. rhacodes. The plant is attached at first by a single cell, but later

ULOTRICHALES 51

multinucleate rhizoids grow down from the lower cells and a basal
attachment disk is formed which may persist throughout the
winter, new plants arising from it in the spring. Detached frag-
ments are another frequent means of forming new thalli, whilst
normal asexual reproduction is by means of quadriflagellate zoo-
spores. In sexual reproduction, which occurs in plants other than
those producing zoospores, fusion takes place between isogametes
from separate plants which have been described as + and - . The
gametes may fuse in pairs or they may fuse into "clumps", and
whilst they are positively phototactic before fusion, the zygote is
negatively phototactic, and this change in behaviour causes it to
descend on to a suitable substrate. Hartmann (1929) has shown
that in certain cases there may be relative sexuality among gametes
from different plants, the sex of the older and weaker gametes
becoming changed. Meiosis takes place at zoospore formation and
there is a regular alternation of diploid and haploid generations,
both indistinguishable morphologically, and when this life history
is compared with that of Monostroma the essential differences are
immediately apparent (cf. fig. 36). The plants occur in saline or
fresh water and become particularly abundant when the waters are
polluted by organic matter or sewage.

*Ulvaceae: Enteromorpha {entero, entrail; morpha, form). Figs. 36,
39-

The plants of this genus also commence hfe as uniseriate filaments
which soon become multiseriate and tubular. Like Uha, many of
the species are attached by means of rhizoids, but there are also a
number of forms, especially on salt marshes (cf. p. 330), which are
free-floating for the whole or part of their life cycle. Growth of the
thallus is either intercalary or else through the divisions of an
apical cell. Asexual reproduction is by means of zoospores, and as
meiosis takes place at their formation the life cycle is identical with
that of Ulva because morphologically similar haploid plants are
known. The first division of the germinating zoospore is transverse,
the lower segment forming an embryonic rhizoid. The sexual
haploid plants are dioecious, usually with isogamous reproduction,
the gametes commonly being liberated around daybreak. Aniso-
gamy has been found by KyHn (1930) in E. intestinalis where the
male gamete is small with but a rudimentary pyrenoid. The motile

CHLOROPHYCEAE

phase of the gametes is short, lasting about 24 hours, whilst the
zygote may also remain motile for i hour, although the first division
of the zygote usually takes place after several days' dormancy.

■\

Fig. 39. Enter omorpha intestinalis i.flagelliformis. A, portion of plant. B, origin
of branch of same showing basal constriction. C, D, E, transverse sections from
near base, middle and apex of thallus. 0 = outside, z = inside of tube. F, G, cells
of thallus. (Original.)

Parthenogenetic development of gametes has been recorded for
E. clathrata, and this presumably results in new sexual plants. As
yet no evidence of relative sexuality has been found among the
gametes of this genus.

ULOTRICHALES 53

*Prasiolaceae : Prasiola {prasio, green). Fig. 40.

The young unbranched filament, which is known as the '' Hor-
midium'' stage, consists of a single row of flat cylindrical cells with
thick walls which frequently possess striations. Later on the cells
divide longitudinally and produce a thin expanded thallus, known
as the " Schizogonmm'' stage, which tapers to the base. The cells of

Fig. 40. Prasiola. A, plant of P. crispa. B, ^' Schizogonium" stage of P. crispa
forma muralis. C, D, ''Hormidium" stage of P. crispa f. muralis with akinetes.
E, development of macrogametes in P. japonica ( x 665). F, development of
microgametes in P. japonica ( x 665). G, P. crispa, membrane striations in
'' Schizogonium" stage ( x 650). H (a-d), formation of aplanospores in akinetes
and young plants. (A, B, after Fritsch; C, D, H, after Oltmanns; E-G, after
Knebel.)

the mature thallus are often arranged in fours and possess axile
stellate chloroplasts, whilst another feature is the presence of short
rhizoids that may occur in the stalk-like portion or else growing out
from the marginal cells. In the juvenile filament reproduction
takes place by means of fragmentation as a result of the death of
isolated cells, whilst in the older, more leafy thallus, "buds" can
arise from the margins. Sometimes the cells produce large, thick-

54 CHLOROPHYCEAE

walled akinetes that germinate to form aplanospores from which new
plants arise. In P. japonica sexual reproduction is brought about
by macro- (sixteen per cell) and microgametes (sixty-four per cell)
that are both produced on the same plant so that this species, at
least, is anisogamous. The shape of P. crispa has been shown to
vary considerably with the habitat, the optimum conditions being
those where there is an abundant supply of nitrogen, such as may
be found in areas occupied by bird colonies. The genus, which is
generally absent from the tropics and subtropics, is represented by
saline, fresh-water or subaerial species, the latter being tolerant
towards considerable desiccation and temperature changes. This
resistance is attributed to the lack of vacuoles in the cells and also
to the high viscosity of the protoplasm. Water supply appears to be
the principal factor limiting successful development, especially in
the subaerial species. Some authors consider that the genus is
characterized sufficiently to warrant removal from the Ulotrichales,
but such a change does not really seem to be justified.

Sphaeropleaceae : Sphaeroplea {sphaero, sphere; plea, full).
Fig. 41.

This genus is widely distributed, being most abundant on ground
that is periodically flooded by fresh w^ater. The long, free, un- |
branched filaments consist of elongated coenocytic cells containing
one to seventy annular parietal chloroplasts. These latter have ^
denticulate margins and occupy the periphery of disks of cyto-
plasm, the disks being separated from each other by vacuoles,
although occasionally they may come together to form a diffuse
network. Each disk normally possesses one or two nuclei in its
cytoplasm. In most of the species the septa develop as ingrowths,
though in S. Africana these are replaced by a series of processes
which appear to be comparable to the strands of a Caulerpa (cf. p.
91), but as they sometimes fail to meet at the centre the coeno-
cytes may be continuous.

Vegetative reproduction is secured by means of fragmentation
and there is apparently no asexual reproduction. In sexual repro-
duction although the cells do not change in shape, nevertheless both
oogonia and antheridia are formed singly or in series, the plants
being either monoecious or dioecious. In the formation of oogonia
the annular chloroplasts first become reticulate and then the ova

ULOTRICHALES

are formed without any nuclear divisions being involved. In
Sphaeroplea annulina the ova are non-motile but in S. cambrica
they are biflagellate, and so it may be argued that the motionless egg
has been evolved from the motile one by loss of flagellae. In the
antheridia, on the other hand, the nuclei undergo division and

Fig. 41. Sphaeroplea. A, S. annulina, portion of thallus. B, S. annulina,
chloroplast. C, structure of septum in 5. Africana ( x 375). D, female plant with
ova and antherozoids. E, male plant. F, young zygote. G, zygote with thickened
wall. H, I, young gametophytes. J, spores emerging from zygote. K, L,
S. Africana, transverse sections across the septa ( x 375). (A-C, K, L, after
Fritsch; D-J, after Oltmanns.)

numerous elongated narrow antherozoids are formed which are
liberated through small holes, subsequently penetrating the oogonia
through similar perforations. The fertilized ovum (oospore) becomes
surrounded by a hyaline membrane, and then inside this two new
membranes are laid down, after which the first one disappears. The
new external membrane is ornamented and the contents of the
oospore are now a brick red. Germination stages are only known

56 CHLOROPHYCEAE

for a few species, and in such cases the oospores may remain dormant
for several years before they produce one to four biflagellate swarmers
which very soon come to rest and then grow into new plants. On
germination the zoospores do not always separate and so one gets
a four- or eight-flagellate synzoospore depending on whether it is
composed of two or four zoids. These germinate to a fourfold
seedling or to a seedling with four claws. In some cases the swarmers
in the oospore are completely suppressed and a new filament develops
directly, this type of reproduction being known as azoosporic. The
adult plants are haploid because meiosis is known to take place at
the segmentation of the oospore.

Primitive characters, which seem to be a feature of the genus,
are the numerous ova, the entire lack of specialized organs for pro-
ducing the sexual reproductive bodies and a simple form of zygote
germination, whilst in S. tenuis the reproduction is even more
primitive as there is strong evidence to show that both kinds of
gametes are motile. The plant must probably be regarded as an
Ulotrichacean filament, which, whilst becoming non-septate, has
still retained, many primitive features, and in *S. annulina cells are
frequently found with only one or two chloroplasts thus showing
a gradation towards Ulothrix. There would seem to be very little
justification for following some authors and placing it in either the
Siphonales or Siphonocladiales, though it must be admitted that
S. Africana does have some features in common with those of the
Siphonocladiales.

REFERENCES

Enter omorpha. Eliding, C. (1933). Svensk hot. Tidskr. 27, 233.
Ulvaceae. Carter, N. (1926). Ann. Bot., Lond., 40, 665.
Sphaeroplea. Fritsch, F. E. (1929). Ann. Bot., Lond., 43, i.
Ulothrix. Gross, I. (193 1). Arch. Protistenk. 73, 206.
Enter omorpha. Hartmann, M. (1929). Ber. dtsch. hot. Ges. 47, 485.
Prasiola. Knebel, G. (1935-6). Hedwigia, 75, i.
Monostroma. Kunieda, H. (1934). Proc. Imp. Acad. Tokyo, 10, 103.
Enteromorpha Kylin, H. (1930). Ber. dtsch. bot. Ges. 48, 458.
Ulothrix. LiND, E. M. (1932). Ann. Bot., Lond., 46, 711.
Microspora. Meyer, K. (1913). Ber. dtsch. bot. Ges. 31, 441.
Sphaeroplea. Pascher, A. (1939). Beih. hot. Zhl. 59, Abt. A., 188.
Microspora. Steinecke, F. von (1932). Bot. Arch. 34, 216.
Prasiola. Yabe, Y. (1932). Sci. Rep. Tokyo Bunrika Daig. i, 39.

OEDOGONIALES 57

OEDOGONIALES

^Oedogonium (oedo, swelling; gonium, vessel). Figs. 42, 43.

The three genera, Oedogoniiim, Oedocladium and Bulbochaete,
which comprise this order were at one time classed as a separate
group, the Stephanokontae. Under the new scheme of classification,
however, they must be regarded, together with the other members
of the old Isokontae, as forming the Chlorophyceae.

In Oedogonium the thallus consists of long unbranched threads
which are attached when young, though later they become free-
floating, whilst in the other two genera the filaments are commonly
branched. Each cell possesses a single nucleus together with an
elaborate reticulate chloroplast containing numerous pyrenoids.
The cell wall contains, according to some workers, an outer layer of
chitin, and if they are correct this is of great interest because chitin
is essentially an animal substance. The chromosomes of Oedogo-
nium are especially interesting among those of the algae in that they
have thickened dark segments at intervals along their length.
Vegetative cell division is so peculiar and characteristic that many
accounts of the process have appeared. A thickened transverse
ring, which develops near the upper end of the cell, first enlarges
and then invaginates, the much thickened wall being pushed into
the interior of the cell. Nuclear division now takes place near this
end of the cell and a septum is laid down between the two daughter
nuclei. Next, the outer parent cell wall breaks across at the ring and
the newly formed membrane stretches rapidly now that the
pressure is released — a matter of about 15 min. — so that a new cell
is interposed between the two old portions. The new transverse
septum becomes displaced by differential growth of the two
daughter cells so that it finally comes to rest just below the fractured
parent wall, and it is also evident that the new longitudinal wall of
the upper cell is almost entirely composed of the stretched mem-
branous ring. The old walls form a cap at one end and a bottom
sheath at the other, and as successive divisions always occur at the
upper end of the same cells, a number of caps develop there and
give the characteristic striated appearance to some of the cells. This
method of growth in Oedogonium may be either terminal or inter-
calary, but in the other two genera, as each cell can only divide
once, there is usually only a single cap. This peculiar mode of

CHLOROPHYCEAE

fey,, -r.' r t. 5 •> fiSl
** — -t — 7 ■■'■ •^y;

J .-' ?- •? i

^*'^

i> '-v * •. ', r'. v.?

t^^

Fig. 42. Oedogonium. A-G, stages in cell division in Oe. grande ( x 526).
B, C, formation of ring. F, G, expansion of ring to form new cell. H, formation
of aplanospore in Oe. Nebraskense. I, Oe. ciliatum, position of antherozoid
2 hours after entering egg. J-M, stages in fertilization of ovum of Oe. Ameri-
canum. K, entrance of sperm. L, fusion of gamete nuclei. M, zygote. N, Oe.
Kurzii, dwarf male ( x 175). (A-M, after Ohashi; N, after Pringsheim.)

OEDOGONIALES

division is unique, and although there is no trace of its ancestry
its constancy suggests that the group terminates a line of
evolutionary development. Vegetative reproduction commonly
occurs by means of fragmentation, whilst asexual reproduction is

Fig. 43. Oedogonium. A, idioandrosporous nannandrous filament. B, g>'nandro-
sporous nannandrous filament. C, dioecious macrandrous filament. D, monoe-
cious filament. E-H, stages in development of dwarf male plant ( x 400).
I, antherozoid ( x 480). J, escape of zoospore (X138). (A-D, after Mainx;
E-I, after Ohashi; J, after West.)

secured through akinetes or multiflagellate zoospores, the forma-
tion of the latter being said to depend on the presence of free
carbon dioxide in the water. The flagellae, which may have one or
two rings of granular blepharoplasts at their base, form a circular
ring around an anteriorly situated beak-like structure. This is the
typical oedogonian swarmer, one of which is produced by each cell,

6o CHLOROPHYCEAE

and there are two theories that have been put forward to explain its

origin :

(a) The group arose independently from flagellate organisms
which possessed a ring of flagellae. If this is true then there could be
no real connexion with the other members of the Chlorophyceae.

(b) Several divisions of the two original blepharoplasts and
flagellae took place, thus resulting in the ring structure. If this is
correct then development might well have occurred from a
Ulotrichalean type of swarmer.

When the zoospore is ripe the cell wall ruptures near the upper
end and the swarmer is liberated into a delicate mucilaginous
vesicle, but this soon disappears, thus allowing the zoospore to escape.
After remaining motile for about an hour the anterior end becomes
attached to some substrate and develops into a holdfast, or else the
zoospore flattens to form an almost hemispherical basal cell. The
type of holdfast depends on the species and the nature of the sub-
strate, a smooth surface inducing a simple holdfast and a rough
surface inducing the development of a branched holdfast. De-
velopment of the one-celled germling can proceed along one of two
lines, depending on the species :

(a) The single cell divides near the apex by the normal method
described above, in which case the basal daughter cell persists as
the attachment organ and the upper cell goes on to form the new
filament.

(b) The apex of the cell first develops a cap and then a cylinder of
protoplast grows out pushing it aside, and when the protoplast has
reached a certain length a cross-wall is formed at the junction of the
cylinder and the basal cell. The upper cell subsequently develops
along the normal lines.

Sexual reproduction is by means of an advanced type of oogamy,
the development of sex organs being assisted by an alkaline ^H and
some nitrogen deficiency. In some of the species the oogonia and
antheridia are produced on the same plant {ynonoecioiis forms) : in
other species the oogonia and antheridia appear on different
filaments which are morphologically alike {dioecious hgmothallic
forms). The species belonging to both these groups are termed
macrandrous because the male filament is normal in size. There is a
third group of species in which the male filament is much reduced
and forms dwarf male plants. Such species are dioecious and hetero-

OEDOGONIALES 6i

thallic and they form the nannandrous group. The dwarf males
arise from motile androspores which are formed singly in flat
discoid cells, the androsporangta, produced by repeated divisions of
ordinary vegetative cells. The androspores may be formed either
in the oogonial filament — gynandrosporous species — or on other
filaments that do not bear oogonia — idioandrosporous species (fig.
43). In shape and structure the androspores are small editions of
the zoospores, and after swimming about they settle on the wall of
the oogonium or on an adjacent cell and germinate into a small male
plant which is composed of a rhizoidal holdfast with one or two flat
antheridia above, though in some cases only one antheridial cell
without any rhizoidal portion is formed. Usually two antherozoids
are freed from each antheridium into a delicate vesicle which later
dissolves. The antherozoids are also hke small zoospores, and if
they fail to enter an ovum immediately they may remain motile for as
long as 13 hours. In the macrandrous monoecious species the
antheridia are usually to be found immediately below the oogonia
where they arise by an ordinary vegetative division in which the
upper cell subsequently continues to divide rapidly, thus producing
a series of from two to forty antheridia. The antheridia frequently
develop one day later than the oogonia, thus ensuring cross-
fertilization.

The oogonia are enlarged spherical or ellipsoidal cells arising by
one division in which the upper segment forms the oogonium and
the lower a support cell, or else the latter subsequently divides to
give antheridia. In some species the lower cell may also become an
oogonium so that one can find a series of oogonia on one filament.
Each oogonium contains one ovum with a colourless receptive
spot situated opposite to the opening in the oogonium wall from
which a small quantity of mucilage is extruded. The opening is
either a very small pore, formed by gelatinization of a tiny papilla,
or else a slit, but in either case there is an internal membrane
forming a sort of conduit to the ovum. After fertilization the
oospore often becomes reddish in colour and develops a thick
membrane which is usually composed of three layers. At germina-
tion the protoplast divides into four segments, which may each
develop flagellae and escape as zoospores, or else they function as
aplanospores that later give rise to zoospores. Meiosis takes place
at the germination of the zygote so that the adult filaments are

62 CHLOROPHYCEAE

haploid. In one species it has been definitely established that two
of the zygote segments ultimately develop into male plants and two
into female plants. Zygote germination without meiosis is not
uncommon, in which case it gives rise to what are presumably
large diploid swarmers, and these develop into abnormally large
threads that are always female. Oogonia appear on these diploid
filaments and can be fertilized, but the fate of the zygote is un-
known.

It remains to discuss the possible origin of the androspores, and
there are two hypotheses that may be considered :

(a) The androspore is equivalent to the second and smaller type
of asexual zoospore, such as those found in Ulothrix, but in the
Oedogoniales tliey can no longer give rise to normal filaments. On
this view the nannandrous forms are the more primitive, the
macrandrous having been derived by the androsporangium ac-
quiring the capacity to produce antheridia immediately and hence
never appearing, (b) The androspore is equivalent to a prematurely
liberated antheridial mother cell which subsequently undergoes
further development. On this view the macrandrous species are
the more primitive. West (191 2) considered that the dwarf males
were to be regarded as reduced from normal male filaments, for in
one species the male plants are intermediate in size. At present
there does not appear to be any ver>^ convincing evidence in support
of either theory,

REFERENCES

GussEWA, K. (1931)- Planta, 12, 293.
Mainx, F. (1931)- Z' Bot- 24, 481.
OHAsm, H. (1930)- Bot. Gaz. 90, 177.
Spessard, E. a. (1930). Bot. Gaz. 89, 385.
West, G. S. (1912). jf. Bot, 50, 321.

CHAPTER IV

CHLOROPHYCEAE (cont.)

CHAETOPHORALES, SIPHONOCLADIALES, SIPHONALES

CHAETOPHORALES

A family in which the fundamental structure is the possession of
both a basal and erect system, this type of thallus being known as
heterotrichous (cf. p. 263). In some of the genera, however,
reduction has taken place and only the basal or erect system is now
represented.

*Pleurococcaceae : Pleiirococcus {pleuro^ box; coccus, berry).
Fig. 44.

The systematic position of this alga has varied considerably. By
some authors it has been placed in the Chlorococcales whilst others

Fig. 44. Pleurococcus Naegelii. A, single cell. B, single-celled colony. C, normal
colony. D-F, thread formation. (After Fritsch.)

have placed it in a special group, the Pleurococcales, but as the alga
can occasionally develop branched threads there would seem to be
evidence for regarding it as a much reduced member of the
Chaetophorales. There are, it is true, almost equally sound argu-
ments for the other systematic treatments of the genus, and its place
at present must be largely a matter of opinion. Pleurococcus is
terrestrial and forms a green coat on trees, rocks and soil, growing in
situations where it may have to tolerate prolonged desiccation. The
cells, which are globose in shape and occasionally branched, are
single, or else as many as four may be united into a group. Under
certain cultural conditions branching may be copious. Each cell

64 CHLOROPHYCEAE

contains one chloroplast and there are no pyrenoids. The sole
method of reproduction is through vegetative division in three
planes when one may find up to fifty cells in a group. There is
probably only one species, P. Naegelii, all the other so-called
species being reduced or modified forms of other algae. The
resistance of the cells to desiccation is aided by a highly concen-
trated cell sap and a capacity to imbibe water directly from the air.

*Chaetophoraceae : Draparnaldia (after J. P. R. Draparnaud).
Fig. 45-
The plants, which are confined wholly to fresh water, are repre-
sented principally by the aerial system, the prostrate system being

Fig. 45. Draparnaldia. A, portion of plant ( x f). B, same enlarged. C, rhizoids
in D. plumosa. D, aplanospores of D. glomerata. (A, B, D, after Oltmanns;
C, after Fritsch.)

entirely absent or else greatly reduced. The young plant is originally
attached by means of a much reduced prostrate system together

CHAETOPHORALES 65

with rhizoids from one or two basal cells. The thallus, which is
often invested by a gelatinous matrix of pectins, possesses a main
axis composed of large barrel-shaped cells, each containing a small
entire or reticulate chloroplast and several pyrenoids. This axis is
primarily for support, and it bears much branched laterals that
normally grow out in tufts, the short cells composing the laterals
being almost wholly filled by one entire chloroplast containing a
single pyrenoid. The apices of these branches, which perform the
functions of assimilation and reproduction, are often prolonged into
a hair. In some species rhizoids develop at the base of the branches
and grow downwards thus clothing the main axis with a pseudo-
cortex, but normal growth is generally restricted to a few cells of
the thallus. When grown in culture with increased carbon dioxide
or additional nitrate the plants take on a form very like that of
Stigeoclonium (cf. below). Asexual reproduction is by means of
quadriflagellate macrozoospores, one to four being produced in each
cell. These, after swarming for a few minutes, settle, and germinate
into a short filament which already possesses a hair at the four- or
five-celled stage when it commences to put out rhizoids. Sexual
reproduction is secured by means of quadriflagellate microswarmers
or isogametes which fuse whilst in an amoeboid state, though these
gametes may also develop parthenogenetically. The behaviour of
the microswarmers demands further investigation as it does not
seem to be clearly understood, nor has it been determined whether
the plants are haploid or diploid. In Draparnaldia glomerata the
nature of the swarmer is controlled by the pYi of the medium,
microswarmers being formed under alkaline conditions and
macrozoospores under neutral or acid conditions.

*Chaetophoraceae : Stigeoclonium (stigeo, sharp pointed; clonium^
branch) (Myxonema). Fig. 46.

Many species are heterotrichous and the plants are frequently
enclosed in a broad watery gelatinous sheath. The chloroplast is
band-like and often does not fill the entire cell, especially in the
older parts of the thallus. The aerial part bears branches that
terminate in a colourless hair, the degree and nature of the branch-
ing depending upon illumination, nutrition and the rate of water
flow. There is no localized area for cell division in the aerial
portion, but in the creeping system only the apical cells are

CSA 5

CHLOROPHYCEAE

meristematic. The prostrate system may be (a) loosely branched,
(b) richly and compactly branched or {c) a compact disk, but the
more developed the basal portion the less elaborate is the aerial
and vice versa. Vegetative reproduction is by means of fragmenta-
tion, whilst sexual and asexual reproduction are the same as in
DraparnaldiUy except that there is only one macrozoospore

Fig. 46. Stigeoclonium. A, plant of S. teniie. B, basal portion of S. luhricum.
C, aerial position of ^S. protensum. D, rhizoids in S. aestivale. E, palmelloid state.
(A-C, E, after Oltmanns; D, after Fritsch.)

produced per cell. In two species, however, a third type of bi-
flagellate swarmer is known, and hence reproduction in these
species is comparable to that found in Ulothrix (cf. p. 46). These
extra swarmers, which are probably the true gametes, are few in
number but fusion between them is rare, probably because the
plants are dioecious. In general the microswarmers seem to have
taken over the function of the sexual biflagellate gametes. The
zygote is said to germinate to zoospores, and these then give rise to

CHAETOPHORALES

the germlings in which the erect filament arises first and the
prostrate portion subsequently or vice versa. By increasing the
osmotic pressure or by adding toxic salts to the environment the
thallus passes into a palmelloid state, whilst under other conditions
akinetes can be formed. The plants are confined to well-aerated
fresh water though they have also been found growing on fish living
in stagnant water, but in these cases the movements of the fish
presumably provide adequate aeration.

Trentepohliaceae : Gongrosira {gongro, excrescence; sir a, chain).

Fig. 47-

A genus which lives on stones and the shells of gastropods that

are to be found in fresh and salt water, although there is one

species that is terrestrial. The cushions or plates are frequently

B A

Fig. 47. Gongrosira. A, portion of G. circinnata showing formation of zoospores.
B, dehisced sporangium of G. stagnalis. (After Fritsch.)

lime encrusted and form a tough green stratum with a base that is
composed of one or more layers of cells which give rise to dense,
erect, branched filaments. The sporangia are usually borne
terminally on these erect threads, and in some species they can
even be distinguished morphologically by their greater size,
although generally they do not differ from the vegetative cells. The
sporangia produce biflagellate zoospores, and any of these which are
not able to escape become converted into aplanospores. Biflagellate
isogametes develop from the lower cells of the thallus, whilst the
prostrate portion can also give rise to akinetes.

5-2

CHLOROPHYCEAE

Trentepohliaceae : Cephaleuros (cephal, head; euros ^ broad).
Fig. 48.

These grow as epiphytes and parasites on and in the leaves of
various phanerogams. The plants are composed of one or more
branched interwoven threads from which vertical filaments arise
that bear clusters of stalked sporangia very like those of Trente-
pohlia. Some species bear sterile erect filaments that terminate in

Fig. 48. Cephaleuros. A, leaf of Magnolia infected with C. virescens. B, trans-
verse section of leaf of Michelia fuscata showing filaments and rhizoids (r) of
C virescens. C, transverse section of leaf of Zizyphus with C minimus showing
sporangial branches. D, sporangia of C mycoidea. (A, after Smith; B, C, after
Fritsch; D, after Oltmanns.)

hairs, whilst the parasitic species possess rhizoids which penetrate
the cells of the host, although it has not been clearly established
whether the host cells are killed before or after penetration. Cepha-
leuros virescens forms the red rust of the tea plant which may cause
much economic damage, but the attack is only serious when the
tea tree is growing slowly, because during periods of rapid growth
the alga is continually being shed by exfoliation of the outer tissues.
The disease cannot be controlled by spraying with poisons, but the
bushes can be made less susceptible to attack by treating the soil
with potash. The genus is confined to the tropics.

CHAETOPHORALES

*Trentepohliaceae : Trentepohlia (after J. F. Trentepohl) (Chroo-
lepus). Fig. 49.

The species grow as epiphytes or on stones in damp tropical
and subtropical regions, but they will also grow under temperate

ALOA

GELATINE

COTTON WOOL

Fig. 49. Trentepohlia.
tabulae, cell structure.

A, B, T. montis -tabulae with pectin caps. C, T. montis-
c = cap, i = innermost layer of cell wall. D-F, types of
chloroplast. G, chloroplast in T. lolithus. H, I, T. umbrina, fragmentation of
prostrate system. J, threads of T. aurea bearing sporangia {s). K, T. umbrina,
sporangia. L, M, two stages in the development of the "funnel" sporangium in
T. annulata. N, graph showing decreasing water contents of Trentepohlia,
gelatin and cotton-wool on drying. O, P, T. umbrina, detachment of stalked
sporangium, i.r. = inner, o.r. = outer thickening of sporangial septum. Q, mature
"funnel" sporangium, T. annulata. R, S, gametangia of T. umbrina. (A-G, J,
L, M, O-Q, after Fritsch; H, I, K, R, S, after Oltmanns; N, after Rowland.)

conditions if there is an adequate supply of moisture. The threads
have a characteristic orange-red colouring due to the presence of
y^-carotin which is said to be a food reserve accumulated during
periods of slow growth, but if this is so it would be expected
that it should accumulate under favourable conditions of rapid

70 CHLOROPHYCEAE

growth and disappear under unfavourable conditions when growth
is slow. This is a feature of its metabolism that would seem to
require further investigation. The cells contain chloroplasts that are
discoid or band-shaped and devoid of pyrenoids. Usually both
prostrate and erect threads are present, though the latter are re-
duced in some species. Growth is apical, and the terminal cells
often bear a pectose cap or series of caps which are periodically shed
and replaced by new ones. The origin of the cap is not properly
understood but it is thought to be due to a secretion, whilst its
function may be either to reduce transpiration or else to act as a
means of protection: alternatively, it may simply be a means of
removing waste material. The cellulose walls are frequently
thickened by parallel or divergent stratifications, whilst each
septum between the cells may also have a single large pit which is
penetrated by a protoplasmic strand. The cells are uninucleate
when young and multinucleate when old, but the presence of the
pigment makes the nuclei extremely difficult to distinguish.
Vegetative reproduction is through fragmentation, whilst other
means of reproduction are to be found in three different types of
sporangia :

(a) Sessile sporangia that never become detached. These consist
of enlarged cells which develop in almost any position and they
produce bifiagellate swarmers that may be isogametes.

(b) Stalked terminal or lateral sporangia that are cut off from an
enlarged support cell which may give rise to several such bodies.
The apical portion swells out to form the sporangium and cuts off a
stalk cell underneath that frequently becomes bent. The dividing
septum possesses two ring-shaped cellulose thickenings which
may be connected with the detachment of the sporangium when it
is mature. The detached sporangium is blown away and germinates
under favourable conditions to bi- or quadrifiagellate swarmers.

(c) Funnel-sh^Lped sporangia which are cut off at the apex of a
cylindrical cell, the outer wall splitting later at the septum, thus
liberating the sporangium, the subsequent fate of which is not
definitely known. The sessile and stalked sporangia may occur on the
same plant or else on separate plants. There has been no cytological
work to show whether there is any alternation of generations and
such an investigation would be highly desirable. In one species,
on the other hand, reproduction is wholly by means of aplanospores.

CHAETOPHORALES 71

Rowland (1929) has investigated the physiology of the commonest
species, T. aiirea, in some detail and he found that

(a) drought increases the resistance to plasmolysis ;

(b) if the threads are dried first and then heated together with
cotton-wool and gelatine, the results suggest that the threads hold
water in a manner similar to that of cotton-wool, but that the loss of
water on heating is comparable to that experienced by a colloid
or gel under the same circumstances (cf. fig. 49) ;

(d) the threads can survive desiccation for at least six months;

(e) plasmolysis could only be produced in some of the cells by a
25 % solution of sea salt.

In many respects, e.g. the heterotrichous nature of the thallus,
the diiferent types of sporangia and the orange pigment, this alga
is strongly suggestive of the more primitive brown algae. This
feature, however, is discussed more fully in a later chapter (cf.

P- 255)-

*CoLEOCHAETACEAE : Coleochaete {coleo, sheath; chaete, hair).
Fig. 50-
Most of the species are fresh-water epiphytes attached to the
host by small outgrowths from the basal walls, but there is one
species that is endoph3rtic in Nitella, one of the Charales (p. 108).
Some of the species are truly heterotrichous whilst others only
possess the prostrate basal portion, which is either composed of
loosely branched threads or else is a compact disk. The growth of
the erect filaments is by means of the apical cell whilst the basal
cushion possesses a marginal meristem. Each cell contains one
chloroplast with one or two pyrenoids, and although a character-
istic sheathed bristle arises from each cell nevertheless in the old
plants these may be broken off. These bristles develop above a
pore in the cell wall through which the protoplast extrudes, whilst
at the same time a membrane is secreted over the protruding bare
protoplast. Asexual reproduction takes place in spring and early
summer by means of biflagellate zoospores which have no eye-spot
and are produced singly. After a motile phase the zoospore settles
down and divides either (a) horizontally, when the upper segment
develops into a hair and the lower forms the embryo disk, or

CHLOROPHYCEAE

(b) vertically, when each segment grows out laterally ; in either case it
will be noted that hair formation takes place at a very early stage.

Sexual reproduction is by means of a specialized oogamy, some
of the species being dioecious and the remainder monoecious. The
female organs, or carpogonia, are borne on short lateral branches and
subsequently undergo displacement. Each carpogonium possesses
a short neck or trichogyne (the long neck of Coleochaete scutata
being an exception) the top of which bursts when the carpogonium
is mature. In the disk forms the carpogonia originate as terminal

Fig. 50. Coleochaete. A, C. ^cwZafa, thallus with hairs ( x 150). B, C. pulvinata
with spermocarp ( x 45). C, C. pulvinata with antheridia (a) and young carpo-
gonium (c). D, C. pulvinata, almost mature carpogonium. E, C, pulvinata,
fertilized carpogonium. F, C. pulvinata, formation of envelope around fertilized
carpogonium. G, C. pulvinata, mature spermocarp with carpospores. (A, B,
after Smith; C-G, after Fritsch.)

bodies on the outside of the disk, but as the neighbouring cells
continue gro\^1:h they eventually become surrounded and appear to
be in the older part of the thallus. The antheridia develop in
clusters at the end of branches (C. pulvinata) or from prostrate
cells. They finally appear as small outgrowths cut off from a
mother cell with stages in their development that are strongly
reminiscent of the Rhodophyceae (cf. p. 252). Each antheridium
produces one biflagellate colourless antherozoid which has been
contrasted with the non-motile rhodophycean spermatium.

After fertilization the neck of the carpogonium is cut off and the

CHAETOPHORALES 73

basal part enlarges; branches arise from the underlying cells and
eventually surround the oospore where they form a red or reddish
brown wall, though in the disk forms this wall is only formed on the
side away from the substrate. At the same time the enclosed oospore
develops a thick brown wall and the cells of the outer envelope
then die. The oospore, or spermocarp, hibernates until spring when
it becomes green and divides into sixteen or thirty-two cells, and
these, when the wall bursts, each give rise to a single swarmer which
must be regarded as a zoospore. Meiosis takes place at the segmenta-
tion of the zygote so that there is only the haploid generation. On
the other hand, some observers have recorded the development of
dwarf asexual plants before the reappearance of new sexual ones,
but this is a phase of the life history that demands reinvestigation,
for if it is correct it may mean that there is an alternation of two
unlike generations, an unusual phenomenon in the Chlorophyceae.
Under certain conditions the cells will also produce aplanospores.
The relation of this genus, with its advanced oogamy, to the other
green algae is by no means clear, and although in many of its
features the sexual reproduction is akin to that of the Rhodo-
phyceae, it is commonly regarded as parallel evolution rather than
as indicating a more direct relationship (cf. p. 256).

REFERENCES

Trentepohlia. Brand, F. (1910). Ber. dtsch. hot. Ges. 28, 83.
Trentepohlia. Rowland, L. J. (1929). Ann. Bot., Lotid., 43, 173.
Stigeoclonium. Reich, K. (1926). Arch. Protistenk. 53, 435.
Draparnaldia. Uspenskaja, W. J. (1929-30). Z. Bot. 2,2,, 337.
General. Visher, W. (1933). Beih. hot. Zhl. 51, i.
Coleochaete. Wesley, O. C. (1928). Bot. Gaz. 86, i.

SIPHONOCLADIALES

Until 1935 this represented a well-established order, but in that
year Fritsch placed most of the genera in the Siphonales but re-
tained the Cladophoraceae as a separate order, the Cladophorales,
with affinities to the Ulotrichales. More recently Feldmann (1938),
in a survey of the group, has returned to the earlier idea of a
relationship with the Siphonales via Valonia and Halicystis,
though he also suggests relationships with Chaetophora and Ulo-
thrix. Whatever the relations may be, the present order is clearly

74 CHLOROPHYCEAE

demarcated from the other groups and any affinities would seem to
be somewhat distant.

*Cladophoraceae : Cladophora {dado, branch; phora, bearing).
Figs. 51, 52.
This is a widespread genus that occurs in both fresh and saHne
waters. The sessile forms are attached by means of branched
septate rhizoids, but some of them (e.g. C fracta) may become
free-living later, whilst there is one complete section (Aegagropila)
which is wholly free-living, the species existing as ball-like growths.
The Cladophora thallus is composed of branched septate filaments,
each cell usually being multinucleate, though cells with one nucleus
have been recorded. The elongate reticulate chloroplasts, contain-
ing numerous pyrenoids, are arranged parietally with processes
projecting into the central vacuole, but under some conditions
they break up into fragments. There would not appear to be much
present support for the old view that the chloroplast of each cell
is a complex of numerous disk chloroplasts. The cell walls exhibit
stratification as they are composed of three layers, an inner zone,
a median pectic zone, and an outer zone which is said to be
chitinous. There is very little production of mucilage, and this
probably accounts for the dense epiphytic flora that is frequently
found associated with species of this genus. The branches arise
towards the upper end of a cell and later on are frequently pushed
farther up, a process known as evection, thus giving the appearance
of a dichotomy. All the cells are capable of growth and this is
especially evident in cases of injury, but normally most of the
plant growth is apical and in the section Aegagropila is wholly
confined to the apex. At cell division the new septa arise from the outer
layers and develop inwards, leaving in the process triangular-shaped
spaces which later on may become filled with pectic substances or
folded lamellae. Additional supporting rhizoids usually develop
from the basal and subbasal cells of the lowest branches.

In the Aegagropila group the species can exist as (a) threads, (b)
cushions and (c) balls. The destruction of the old threads in the centre
of the ball results in a cavity which may become filled with water, gas
or mud. In Lac Soro the water in April and May is sufficiently free
of diatoms for light to penetrate to such an extent that photo-
synthesis increases and so much gas collects in the centre of these

SIPHONOCLADIALES

Fig, 51. Cladophora. A, plant with sporangia. B, shoot of Aegagropila holsatica
bearing rhizoids. C, stolon of Ae. holsatica. D, rhizoids of Spongo?norpha
vernalis developing storage cells at the apices. (A, after Oltmanns; B, C, after
Acton; D, after Fritsch.)

CHLOROPHYCEAE

balls that they float to the surface. Their characteristic shape is
brought about by a continual rolling motion over the soil surface
under the influence of wave action, and hence the "ball" forms are
found near the shore whilst the "thread" and "cushion" forms

^^^iiwvw*,^^

Fig. 52. Cladophora. A, ball of Aegagropila holsatica cut through and the dirt
removed ( x |). B, same before cutting ( x |). C, C glomerata, commencement
of septum formation. D, C glomerata, second stage in septum formation.
E, C glomeratum, septum almost complete. F, diagram illustrating evection.
G, H, types of branching. I, C. glomerata, structure of wall at a septum.
J, Spongomorpha coalita with hook branches. K, C. callicoma, structure of chloro-
plast with nuclei and pyrenoids. L, Ae. Sauteri, zoospores in zoosporangium.
M, Ae. Sauteri, zoospores. (A, B, after Acton; C-K, after Fritsch; L, M, after
Nishimura and Kanno.)

are to be found farther out in deeper water where there is less
motion. The harder the floor the more regular is the shape of the
balls, but even so the ball structure would also appear to be inherent
in the alga because "balls" have been kept in a laboratory for eight
years without losing their shape. The following types of branches

SIPHONOCLADIALES 77

have been recognized in the Aegagropila forms: [a) rhizoids,
{h) cirrhoids, both these and the rhizoids being neutral or non-
reproductive branches, (c) stolons or vegetative reproductive
branches. Many of the species of Cladophora are perennial, and in
the section Spongomorpha the rhizoids form a basal expanse from
which new threads may arise each year. In some of the fresh-
water species certain cells may become swollen to form akinetes in
which the walls are thickened and food is stored.

In the section Aegagropila most of the species reproduce vege-
tatively, but biflagellate swarmers have been reported for one
species, Ae. Sauteri, and these are interesting in that they may
germinate whilst still within the sporangium (fig. 52 L, M).
Asexual reproduction in the other species, excluding the section
Aegagropila, is by means of quadriflagellate zoospores (bi-
flagellate in two species) which escape through a small pore in the
cell wall. Biflagellate isogametes are the means of sexual repro-
duction, all the species so far investigated being dioecious. The
zygote develops at once without a resting period. In a number of
species alternation of two morphologically identical haploid and
diploid generations has now been established with meiosis taking
place at zoospore formation. In one or two cases, e.g. Cladophora
fiavescens, the zoospores sometimes fuse, and this irregular be-
haviour is very comparable to similar phenomena found in the
more primitive brown algae (cf. p. 138).

In a few species there is an odd or heterochromosome, and in
a cell the number of zoospores with the odd chromosome are equal
to the number lacking it. Haploid plants of C. Suhriana have six
or seven chromosomes, whilst in C. repens the cells contain either
four or five. In a fresh-water species, C glomerata, a wholly
different type of life cycle is known, and this difference may perhaps
be compared with the various cycles found for Ectocarpus siliculosiis
under different conditions (cf. p. 135). Gametes and zoospores are
both formed on diploid plants and meiosis takes place at gamete
formation so that there is no haploid generation. Whilst zoospore
formation takes place all the year round gametes only appear in
the spring, but the reason for this seasonal restriction is not under-
stood. Parthenogenetic development of gametes has also been
recorded in a number of species. Of the species so far investigated
the chromosomes appear to be present in multiples of 4, and this

78 CHLOROPHYCEAE

probably indicates polyploidy. The following diploid chromosome
numbers have been recorded : C. repens 8 + i, C. Suhriana 12+ i,
C. flavescens 24, C. flaccida 24, C. pellucida 32, C. glomerata 64.

Cladophoraceae : Rhizoclonium (rhizo, root; clonium^ branch).

Fig. 53-

This genus is either marine, brackish or fresh water, several
marine species being found in great quantities on sand or mud
flats. The uniseriate filaments are simple or else possess short
septate or non-septate colourless rhizoidal branches. The threads
are smaller in diameter than those of the preceding genera, and the

Fig. 53. Rhizoclonium. A, part of filaments of R. ripariuyn ( x 90). B, cell of
R. hieroglyphicum to show structure of chloroplast. (A, after Taylor; B, after
Fritsch.)

number of nuclei per cell are also less, usually one to four, although
in the stouter species there may be as many as twenty-four. It has
been found that the number of nuclei contained depends on the
cubical contents of the cell, a feature of size and form that is
analogous to the phenomenon found in the higher plants. The
number of epiphytes may also influence the quantity of the nuclei.
The plants are attached at first but become free-living later, and in
this state some of the larger species are scarcely distinguishable
from small species of the related genus Chaetomorpha. Vegetative
reproduction is by means of biflagellate zoospores which in some
species are said to have unequal flagellae. Anisogamy similar to

SIPHONOCLADIALES

that of the related genus Urospora has been recorded for Rhizo-
clonium lubriciim. Urospora is of interest because the zygote first
produces a Codiolum stage (so called after the alga it resembles),
which is considered to be diploid, and this gives rise to zoospores
from which the normal filaments develop, so that if this interpre-
tation of the life history is correct we have here another rare
example of alternation of morphologically dissimilar generations in
the Chlorophyceae.

*Valoniaceae : Valonia (after the Valoni, an Italian race), "Sea-
Bottle". Fig. 54.

In this genus, which is restricted to warm waters, the young
coenocyte consists of one large vesicle whilst the old one becomes

^(^:^

Fig. 54, Valonia. A, young plant of V. ventricosa. B, young plant of V. utricu-
laris { X i -4). C, adult plant of same, m = marginal cell. D, plant of V. macrophysa
( X 0-8). E, rhizoid of V. utricularis. F, rhizoids from marginal cells at base of
vesicle of V. ventricosa. G, single marginal cell and rhizoid (r) of V. ventricosa.
H, V. utricularis fruiting. I, V. utricularis, germinating swarmer. (B, D, after
Taylor; rest after Fritsch.)

divided up into a number of multinucleate segments. It has been
suggested (cf. p. 280) that it should really be regarded as a coeno-
cytic wall enclosing a fluid, but this interpretation leads to diffi-
culties. In some respects, therefore, the genus provides a link with
the Siphonales. The macroscopic club-shaped vesicle is attached
to the substrate by rhizoids of various types. There is a lobed

8o CHLOROPHYCEAE

chloroplast that congregates with the cytoplasm at certain points in
the older plants and then each group is cut off by a membrane,
thus producing a number of marginal cells. This septation is
regarded as a primitive character that is slowly being lost because in
the more advanced Siphonales it is restricted to the formation of
the reproductive organs. The cells do not necessarily form a con-
tinuous layer and are frequently restricted to the basal region where
they may develop rhizoids, whilst in other species they are nearer to
the apex where they may give rise to proliferations. The lower cells
can form short creeping branches, and as these bear more of the
erect vesicles a tuft of plants is produced. One genus (Siphono-
cladus), classed either in the Valoniaceae or else in a separate group,
resembles Cladophora very closely although the method of seg-
mentation is essentially the same as that of Valonia. Reproduction
in Valonia takes place by means of bi- or quadriflagellate swarmers
which are liberated from the cells through several pores, and al-
though no sexual fusion has been seen as yet, nevertheless meiosis
occurs in V. utricularis at swarmer formation. The plants are
therefore diploid, a condition that is also characteristic of most of
the Siphonales. The reproductive cells may encyst themselves, and
it has been suggested on this evidence that the plant is a colonial
aggregate of coenocytic individuals resulting from the retention of
cysts which have developed in situ. The correctness or otherwise of
this interpretation can only be obtained through a better knowledge
of its phylogenetic history and the reproductive processes of other
members of the group.

*Dasycladaceae : Dasycladus {dasy, hairy; cladus, branch). Fig.

55-
The family Dasycladaceae is very ancient and was formerly
much more widely spread since sixty fossil genera are known
whilst there are only ten living to-day (cf. p. 269). Dasycladus
forms dense growths, up to 5 cm. in height, in shallow waters
where the plants are anchored by means of richly branched non-
septate rhizoids. The central axis bears dense whorls of profusely
branched laterals which are arranged alternately above each other.
The branches arise in whorls of four immediately below the apex
of the parent cell, to which they are united by narrow constrictions,
and although the rest of the main axis is impregnated with lime

SIPHONOCLADIALES

throughout there is none at the constrictions. If the axis or a
branch is decapitated a new apex is regenerated, whilst if a rhizoid
is cut off and inverted it develops a normal apical cell. Short-
stalked spherical gametangia arise at the apices of the major
branches in the upper half of the plant and are cut off by a septum.
The plants are essentially dioecious and produce isogametes that
sometimes exhibit relative sexuality.

Fig. 55. Dasycladus clavaeformis. A, plants. B, assimilatory filaments showing
mode of branching. C, gametangium (g). D, thickenings at base of assimilatory
filaments. 6 = point of origin of branch, / = base of lateral, // = calcified wall,
m = thickened base of wall. (After Fritsch.)

Dasycladaceae : Neomeris {neo, new; meris, part). Fig. 56.

This is a calcareous tropical genus which has been in existence
from the Cretaceous era. The much calcified adult plants have the
appearance of small worm-like masses with an apical tuft of hairs,
whilst very young plants consist of an erect Vauchena-\\k.Q. filament
with a tuft of dichotomously branched filaments at the apex. In the
adult plant the ultimate branches terminate in long deciduous hairs,
whilst the apices of the next lower order of branches dilate and
become pressed together, thus producing a compact surface with
a pseudo-parenchymatous appearance (cf. fig. 56 E). Calcium
carbonate is deposited wherever there is a mucilage layer and an
aggregation of the chloroplasts, but apparently both these con-
ditions must be fulfilled before lime can be laid down. The
principal interest of this form lies in its morphological resemblance
to certain fossil genera (cf. p. 271).

CSA 6

CHLOROPHYCEAE

Fig. 56. Neomeris. A, plants of N. annulata ( x ^). B, young plant of N.
dumetosa ( x j). C, longitudinal section through apex of N. dumetosa ( x J).
D, rhizoid in N, dumetosa ( x J). E, transverse section of thallus of N. dumetosa
in middle of calcified area ( x ^). F, A'', dumetosa, assimilating filaments with
sporangium ( x ^). G, A^. annulata, sporangium ( x 33). H, regeneration of an
injured axis ( x j). (A, G, after Taylor; rest after Church.)

*Dasycladaceae : Acetahularia (acetabular little cup ; aria, derived
from). Fig. 57.

This is a lime-encrusted genus which is confined to warm waters,
extending up as far as the Mediterranean in the northern hemi-
sphere. The plants consist of an erect elongate axis bearing one or
more whorls of branched sterile laterals with a single fertile whorl
at the apex. The sterile whorl or whorls are frequently shed in the
adult plant leaving a mark or annulus on the stem to show where
they were formerly attached. The fertile whorl is composed of a
series of long sac-like sporangia which are commonly fused,
though they are sometimes separate : these are borne on short basal

SIPHONOCLADIALES

segments which are morphologically equivalent to the primary-
branches. The basal segments also bear on their upper surface
small projections, with or without hairs, which form the corona,
whilst in one section of the genus there is also an inferior corona on

Fig. 57. Acetabularia. A, plant of A. crenulata ( x o-8). B, apex of A. medi-
terranea showing corona. C, apex of A. Moebii showing two superposed fertile
whorls. D, A. mediterranea, attachment rhizoid and perennating vesicle {b).
E, A. crenulata, cells near centre of thallus, showing superior corona (c) and leaf
scars (5). F, A. pusilla, vegetative ray segment ( x 44). G, fertile lobes of A.
Schefikii with cysts ( x 44). H, cysts in A. pusilla in a single lobe of the um-
brella ( x 37). I, single cyst of A. mediterranea. J, young plant in first year.
K, L, A. crenulata, apices of ray segments ( x 37). M, A. crenulata, superior
corona ( x 37). N, A. crenulata, inferior corona ( x 37). (A, F-H, K-N, after
Taylor; B-E, I, J, after Fritsch.)

the lower surface. In A. mediterranea two or three years elapse
before the plant attains to maturity. In the first year the branched
holdfast produces an upright umbilical thread, together with a
thin-walled lobed outgrowth that penetrates the substrate in order
to function as the perennating organ. The aerial part dies, and in

6-2

84 CHLOROPHYCEAE

the next year or years a new cylinder arises that bears one or more
sterile whorls of branches, until in the third or even a later year, a
shoot develops which produces one deciduous sterile whorl and a
single fertile whorl or umbrella. Each sac-like sporangium, or
umbrella lobe, gives rise to a number of multinucleate cysts which
are eventually set free through disintegration of the anterior end of
the sporangium. In the spring biflagellate isogametes are liberated
from these cysts and fuse in pairs, or else develop parthenogenetic-
ally. In A. Wettsteinii meiosis occurs at gametogenesis and the
adult plants are therefore diploid. According to Hammerling
the immature plant contains only one nucleus, which is to be
found in one of the rhizoids, and at cyst formation this divides, the
daughter nuclei being carried into the sporangia. The resulting
cysts in the umbrella lobes are uninucleate, but as the single
nuclear condition is in direct contrast to the reports of other
workers it would seem that further cytological study is desirable.

REFERENCES

Cladophora. Acton, E. (1916). New Phytol. 15, i.

Neomeris. Church, A. H. (1895). Ann. Bot., Lond., 9, 581.

General. Feldmann, J. (1938). Rev. Gen. Bot. 50, 571.

Acetabularia. Hammerling, J. (193 1, 1932). Biol. Zbl. 51, 663; 52, 42.

Cladophora. List, H. (1930). Arch. Protistenk. 72, 453.

Neomeris. Svedelius, N. (1923). Svensk bot. Tidskr. 17, 449.

SIPHONALES

This group is characterized primarily by possession of a coeno-
cytic structure in which true septa are rare or absent, the coenocyte
normally having a cytoplasmic lining surrounding a central vacuole
and containing numerous disk-shaped chloroplasts. Cellulose is
largely replaced by callose as the principal component of the walls.
The group may be polyphyletic in origin, and the fact that it
reaches its maximum development in warm waters may be signifi-
cant, not only in respect of the phylogeny of the group itself, but
also in considering the evolution of the Chlorophyceae as a whole.
Most of the genera possess the power of regeneration to a marked
degree, but this can perhaps be regarded as a primitive character
that has persisted throughout the course of time.

SIPHONALES 85

Protosiphonaceae : Protosiphon {proto, first ; siphon, tube). Fig. 58.

The single species common in north Europe grows in damp mud
at the edges of ponds, but a variety is also known from the desert
silt of Egypt which will tolerate temperatures up to 91° C. and salt
concentrations of at least i %. The green aerial portion is more or
less spherical, up to loo^u,. in diameter, grading into a colourless
rhizoidal portion that is occasionally branched. The chloroplast,
which contains numerous pyrenoids and nuclei, is an anomaly in

Fig. 58. Protosiphon hotryoides. A, B, plants, one showing budding. c = chloro-
plast. C, swarmer formation. ^• = vacuole. D, cyst formation. E, zygote ( x 1666).
F, germination of zygote to form zoospores ( x 1026). G, group of plants grown
in a nutrient solution. (A-D, G, after Fritsch; E, F, after Bold.)

the group because of its reticulate character. In very dry places the
rhizoid may be abbreviated to such an extent that the plant looks
like a Chlorococcum, whilst in cultures where nutrient conditions
are favourable one may obtain branched thread-like growths. The
shape of the thallus is determined by the incidence of the light,
unilateral light producing asymmetrical aerial portions, whilst
exposure to bright light and low moisture may also cause an old
thallus to turn brick red. During times of drought resting spores or
cysts are formed w^hich, when conditions become favourable once
more, either germinate directly or else produce zoospores, germina-

CHLOROPHYCEAE

tion in the desert forms occurring at temperatures between 12° and
35° C. Vegetative reproduction takes place by means of lateral
budding, but when submerged the plants also produce naked
biflagellate swarmers which usually act as isogametes, though they
are also capable of parthenogenetic development. Protosiphon
hotryoides is monoecious whilst the desert variety is dioecious, and
this fact alone would seem sufficient justification for regarding the
latter as a distinct species. The zygote either germinates immediately
to give a new plant or else may remain dormant for some time.
The plant is probably haploid, and morphologically is of great
interest in indicating how the more advanced Siphonales may have
arisen.

*Halicystaceae : Halicystis {hali, salt ; cystis, bladder) and Derhesia
(after A. Derbes). Fig. 59.

The gametophytic plants consist of an oval vesicle, up to 3 cm.
in diameter, arising from a slender branched tuberous rhizoid

Fig. 59. Halicystis oralis (and Derbesia marina). A, plant of Halicystis liberating
gametes. B, rooting portion of Halicystis showing old rhizome and line of
abscission (a) and new vesicle (6). C, gathering of protoplasm to form gametes.
/ = lining cytoplasm, ^ = pore of dehiscence. D, male gamete ( x 600). E, female
gamete ( x 600). F, protonemal germling of Halicystis. G, Derbesia plant. H,
Derbesia, with zoosporangia, growing on Cladophora. I, Derbesia, zoosporangium.
J, Derbesia, zoospore. (A-C, F-J, after Fritsch; D, E, after Kuckuck.)

embedded in calcareous Lithothamnia (cf. p. 317) growing at or
below low-tide mark. There are only two species of Halicystis^ one
of which possesses pyrenoids whilst the other does not, though both
contain numerous nuclei in the peripheral cytoplasm. There does

SIPHONALES 87

not appear to be any cellulose in the material composing the cell
wall. S warmers develop in the cytoplasm at the apex of the vesicle
in an area which becomes cut off by a thin cytoplasmic membrane,
the area thus cut off representing a gametangium. Macro- and
microgametes are formed and forcibly discharged in the early
hours of the morning through one or more pores. There are
several crops of these swarmers produced by successive migrations
of cytoplasm into the apical areas at bi-weekly intervals coincident
with the spring tidal cycles. Fertilization occurs in the water, and
the zygote in H. ovalis germinates into a branched protonemal
thread that in 3 months has developed into a typical Derhesia plant
with the erect aerial filaments arising from the basal rhizoidal
portion.

It has been demonstrated only quite recently that both Halicystis
ovalis and Derhesia marina are simply two stages in the life
cycle of one alga, but in addition to the evidence from cultures
the two species have the same geographical distribution. The
mature Derhesia threads produce zoospores that germinate into
prostrate filaments, and these later give rise to slender branched
rhizoids which, after eight months, produce the characteristic
Halicystis bladder. Some weeks after its development the bladder
becomes fertile and so the cycle starts once more. Although the
cytology of the two plants has not yet been worked out the Derhesia
generation is presumably diploid and the Halicystis haploid. It also
remains to be ascertained whether the other species of Halicystis
has a similar life cycle. Growth of the Halicystis vesicles is very
slow and they become shed at the end of the growing season by
abscission, new vesicles arising later from the perennating rhizoid,
and in this manner regeneration may go on for several years. The
genus, formerly regarded as a connecting link between Protosiphon
and members of the Valoniaceae, must now be removed into a
separate family because of this remarkable life history. This new
family must also include Derhesia in the same way that Aglaozonia
is now included in Cutleria (cf. p. 156).

Phyllosiphonaceae : Phyllosiphon {phyllo, leaf; siphon^ tube).
Fig. 60.

This is an endophytic alga that occurs in the leaves and petioles
of the Araceae, most of the species being confined to the tropics,

CHLOROPHYCEAE

although one is found in Europe, including Great Britain. The
thallus is composed of richly branched threads ramifying in the
intercellular spaces of the host. As a result of the presence of the
endophyte the chloroplasts of the host cells do not develop and
yellow-green patches occur on the leaf, whilst at the same time the
adjacent cells may be stimulated to active division resulting in gall
formation, but later on the affected cells die. Reproduction takes
place by means of oval aplanospores.

Fig. 60. Fig. 61.

Fig. 60. Phyllosiphon Arisari. A, leaf of Arisarum vulgar e with whitened patches
due to attack of alga. B, portion of thallus ( x 66). (A, after Fritsch; B, after
Smith.)

Fig. 61. Bryopsis. A, plant of B. plumosa ( x o-6). B, portion of same enlarged
( X 7). C, B. corticulans, rhizoid formation from lower branches. D-F, stages in
septum formation at base of gametangium. ^ = gelatinized material, ;« = mem-
brane, r = ring of thickening initiating septum. G, B. plumosa, female gametangia.
c = chloroplast. (A, B, after Taylor; C-G, after Fritsch.)

*Caulerpaceae: Bryopsis {bryo, moss; opsis, an appearance).
Fig. 61.

Most of the species of this genus are restricted to warmer seas,
though at least two, of which B. plumosa is the commoner, occur
in colder waters. The principal axis, which is often naked in its
lower part, arises from an inconspicuous, filamentous, branched
rhizome that creeps along the substrate and is attached to it by
means of rhizoids. In one species the bases of the lower branches

SIPHONALES 89

develop additional rhizoids that grow down and form a sheathing
pseudo-cortex. The bi- or tripinnate fronds usually have the
branching confined to one plane, the branches being constricted at
the point of origin, whilst the cell membrane is also thickened at
such places. The cytoplasm in the main axis and branches fre-
quently exhibits streaming movements. The function of the
rhizome, especially in warmer waters, is probably that of a peren-
nating organ, although vegetative multiplication can also occur
through abstriction of the pinnae, which then develop rhizoids at
their lower end. The only other known method of reproduction is
sexual. The plants are dioecious and produce anisogametes which
develop in gametangia that are cut off from the parent thallus by
means of septa. Both types of gamete are bifiagellate, but the
microgametes differ from the macrogametes in that they lack
pyrenoids. The gametes are liberated through gelatinization of the
apex of the gametangium, and after fusion has taken place the
zygote germinates at once into a new plant. The plants are diploid
because meiosis takes place at gamete formation ; there is therefore
no haploid generation. The plants can behave like Vaucheria (cf.
p. 95) in their response to certain environmental conditions; thus,
gamete formation is hastened by transference of the plants from
light to dark or by changing the concentration of the nutrient
solution. Inversion of the thallus takes place under conditions of
dull light or when it is planted upside down, and under these
circumstances the apices of the pinnae develop rhizoids. This
exhibition of polarity indicates clearly that the thallus is differ-
entiated internally, but it is still a matter for speculation as to how
such differentiation can occur in an organism which is to all intents
and purposes one unit.

*Caulerpaceae : Caulerpa (caul, stem; erpa, creep). Fig. 62.

Most of the species frequent the quiet shallow waters of the
tropics where they are often rooted in sand or mud, but two have
migrated far enough north to become denizens of the Mediter-
ranean. The prostrate rhizome is attached by means of colourless
rhizoids and gives rise to numerous erect, upright, assimilatory
shoots with apical growth, the form and arrangement of which may
vary very considerably (fig. 62 A-F). Radial branching is regarded
as primitive, whilst the more evolved forms of quieter waters

CHLOROPHYCEAE

possess a bilateral branching system. The genus has been divided
by Borgesen into three groups :

(a) The species of this group, which grow where there is much
mud, possess rhizomes that are vertical or oblique, thus enabling
them to reach the surface even when covered successively by mud
(e.g. C. 'verticillata).

(b) The rhizome in these species first branches at some distance
from its point of origin and it possesses a pointed apex which aids
in boring through sand or mud (e.g. C. cupressoides).

{c) The rhizome is richly branched immediately from its point
of origin and the various species are principally to be found attached
to rocks and coral reefs (e.g. C. racemosa).

Fig. 62. Caulerpa. A, B, C prolifera ( x ^). C, C. racemosa f. macrophysa ( x ^).
D, E, C. sertularioides, side branches ( x ^). F, C. crassifolia f. mexicana ( x ^).
G, structure of wall and two skeletal strands, H, longitudinal section of aerial
portion showing longitudinal (Z) and transverse {i) support strands. I, transverse
section of rhizome with skeletal strands. J, K, L, C. prolifera, reproductive
papillae ( x 5). M, C. prolifera with gametes being liberated. (A-F, after Taylor;
G-I, after Fritsch; J-M, after Dostal.)

It has also been shown that the form of the thallus in some of
the species is largely dependent upon the conditions of the habitat,
a feature particularly well illustrated by the plastic C cupressoides
and C. racemosa:

(i) In exposed situations the plants are small and stoutly
built.

SIPHONALES 91

(ii) In more sheltered habitats the shoots are longer and more
branched.

(iii) In deep water the plants are very large with richly branched
flabellate shoots.

There is no septation, but the coenocyte is traversed instead by
numerous cylindrical skeletal strands, or traheculae, arranged
perpendicularly to the surface and which are most highly de-
veloped in the rhizomes. They arise from rows of structures termed
microsomes, and are at first either free in the interior of the coenocyte
or else connected with the wall, although in the adult state they are
always fused to the walls. The function of the trabeculae, which
increase in thickness at the same time as the walls by successive
deposition of callose, is extremely problematical and may be

{a) mechanical: in this case they would presumably provide
resistance to high turgor pressures, although the presence of high
osmotic pressures in the cells has yet to be proved ;

{h) to enlarge the protoplasmic surface ;

{c) concerned with diffusion, because movement of mineral salts is
more rapid through these strands than through the cytoplasm;

{d) lost or without any function.

In addition to the trabeculae there are also internal peg-like
projections. Vegetative reproduction occurs through the dying
away of portions of the old rhizome thus leaving a number of
separate plants. The swarmers or gametes are formed in the aerial
portions and are liberated through special papillae that develop on
the frond. The sexual reproductive fronds have a variegated
appearance caused by the massing of the biflagellate gametes at the
different points, the swarmers in some species being separable into
micro- and macrogametes. In certain species the whole plant can
produce swarmers, whilst in others the reproductive area is limited,
and in such cases the morphological identity and differentiation of
the frond becomes of great interest. The thallus can be regarded
as composed of a number of individual cells which only become
evident at gametogenesis. Fusion between the swarmers has been
observed in C. racemosa, and it is probable that in all the species the
motile bodies are functional gametes and that the adult plants are
diploid. The genus has been much employed in experiments on
polarity because the structure of the thallus renders it extremely
suitable.

CHLOROPHYCEAE

Codiaceae: Codiutn (fleece). Fig. 63.

This is a widely distributed, non-calcareous genus with several
species living in the colder oceans. The erect and fleecy thallus,
which is anchored either by a basal disk or else by rhizoids, varies
greatly in form and appears as branched worm-Uke threads, flat

B A C F 1 J

Fig. 63. Codium. A, plant of C. tomentosum. B, C. fragile, utricles. C, C. to-
mentosum, single utricle with hairs. D, C. tomentosum, portion of thallus with
medulla and cortical utricles. E-G, stages in formation of constriction at base of
utricle. H, propagule of C isthmocladiim. I, C. toynentosum, female gametangium.
J, C. tomentosum, male gametangium. K, C. toT?ientosum, juvenile thread.
L, C. isthmocladuyn, utricle with propagule. (A, after Taylor; B, C, E-G, J, after
Tilden; D, H, K, L, after Fritsch; I, after Oltmanns.)

cushions, or as large round balls. In C. tomentosum there is a
central medulla of narrow forked threads and a peripheral cortex of
club-shaped vesicles which are the swollen apices of the forked
threads. Deciduous hairs may develop on the vesicles and scars are
to be seen marking their point of attachment, whilst annular
thickenings occur at the base of each vesicle and at the bases of the
lateral branches, although a fine pore is left for intercommunication.

SIPHONALES

The width of these pores in the case of C. Bursa is said to vary with
the season. Detachable propagules develop on the vesicles and
form a method of vegetative reproduction, whilst sexual reproduc-
tion is by means of gametes, which are produced in ovoid game-
tangia that arise from the vesicles as lateral outgrowths, each being
cut off by a septum. The plants are anisogamous, the macrogametes
being formed in green and the microgametes in yellow gametangia.
Some of the species are dioecious whilst others are monoecious,
and in two of the latter the male and female gametangia are borne
on the same utricles. The gametes fuse or else develop partheno-
genetically, but in either case a single thread-like protonema
develops which has a lobed basal portion, and it is from this that the
adult develops through the growth of numerous ramifications of
the one primary filament. Meiosis occurs at gametogenesis and the
plants are therefore wholly diploid and comparable to Fucus (cf.
p. 192). At gametogenesis some of the nuclei in the gametangia
degenerate whilst the remainder divide twice.

CoDiACEAE : Halimeda {Halimeda, daughter of Halimedon, King of
the sea). Fig. 64.
The genus is known from Tertiary times onwards, and it has
played a considerable part in the formation of coral reefs where the

Fig. 64. Halimeda. A, plant oi H. simulans (x 22). B, H. dtscoidea, longitudinal
section showing structure ( x 20). C, central filament: two fuse and subsequently
divide into three ( x 20). D, cuticle of H. opuntia ( x 132-5). E, H. scabra, ter-
mination of filaments ( x 100). F, fruiting plant. G, sporangia. (A, D, E, after
Taylor; B, C, after Howe; F, G, after Oltmanns.)

species are very abundant. The plants are borne on a short basal
stalk that arises from a prostrate system of creeping rhizoids. The

94 CHLOROPHYCEAE

branched aerial thallus is composed of flat, cordate or reniform
segments which are strongly calcified on the outside, the segments
being separated from each other by non-calcified constrictions.
The segments are composed of interwoven threads with lateral
branches that develop perpendicularly and produce a surface of
hexagonal facets through fusion of the swollen ends. Sporangia
develop at the ends of forked threads which vary greatly in their
mode of branching: these threads, which are cut off from the
parent thallus by basal plugs, arise from the surface of the segments
or, more frequently, are confined to the edges. The sporangia
produce biflagellate swarmers whose fate is not known although
they are probably gametes.

Vaucheriaceae : Vaucheria (after J. P. Vaucher). Figs. 65, 66.

This genus differs in many of its characters from the other
members of the Siphonales, and it should perhaps be removed into
the Xanthophyceae. Whereas most of the Siphonales are tropical
genera Vaucheria is essentially temperate, inhabiting well-aerated
streams, soil or saline mud flats, and although some of the species
(e.g. V. Debaryana) may be lime-encrusted it is never to quite the
same extent as in the preceding genera. There is a colourless basal
rhizoidal portion from which arise green, erect aerial filaments with
apical growth and monopodial branching. The cell walls contain
cellulose and pectins whilst the discoid chloroplasts, which lack
pyrenoids, contain more than the normal amount of xanthophyll.
Oil forms the principal food reserve, except that under constant illu-
mination starch is formed, and it is in these biochemical characters
that Vaucheria shows considerable similarity with members of the
Xanthophyceae (cf. p. 113). Septa are only formed in connexion
with the reproductive structures or after wounding. Vegetative
reproduction is secured through fragmentation, whilst asexual
reproduction is brought about by the well-known compound multi-
flagellate zoospores, which are produced singly in club-shaped
sporangia that are cut off from the ends of the erect aerial branches.
The chloroplasts and nuclei congregate in the apex of a filament
before the septum is laid down and the nuclei then arrange them-
selves peripherally. Finally, two equal flagellae develop opposite
each nucleus and then the zoospore is ready for liberation, a process
which is achieved by gelatinization of the sporangium tip. This

SIPHONALES

compound structure must be regarded as representing a group of
biflagellate zoospores which have failed to separate. The zoospore
is motile for about 15 min., after which it comes to rest and germi-
nates, the first thread often being more or less colourless. ' ' Zoospore ' '

Fig. 65. Vaucheria. A, V. sessilts, germinating zoospore. B, V. piloboloides,
developing aplanospore. C, V. piloboloides, escape of aplanospore. D, V. ge-
minata, thread with cysts. E, escape of amoeboid protoblast from cyst.
F-I, V. repens, development and escape of compound zoospore. J, regeneration
and formation of septa in injured thalli. K, sex organs of V. sessilis ( x 100).
L, sex organs of V. terrestris (x 100). M, sex organs of V. geminata (x 100).
N, V. geminata, germinating aplanospore. O, germinating zygote. P, zygote
with four haploid nuclei. Q, portion of compound zoospore, much magnified.
(A, D, E, N, O, after Oltmanns; B, C, F-J, Q, after Fritsch; K-M, after Hop-
paugh; P, after Hanatschek.)

formation can often be induced by transferring the plants from light
to darkness, or from a nutrient solution to distilled water.

Under dry conditions aplanospores may be formed at the
ends of short laterals or terminal branches, whilst if exposed to
greater desiccation the threads of the terrestrial forms become
septate and rows of cysts are formed, thus giving the '' Gongrosira'^
stage. When conditions become more favourable these cysts
germinate either into new filaments or else into small amoeboid

CHLOROPHYCEAE

masses which grow into new filaments. Sexual reproduction is
distinctly oogamous, the different species being either monoecious
or dioecious. The oogonia, which are sessile or stalked, are cut off
by a septum at a stage when there is only one nucleus left in the
oogonium. Some authors maintain that the extra nuclei, which are

Fig. 66. Vaucheria sessilis. Stages in development and fertilization of oogonium.
April 1-6, 1930. ( X 195.) A, young antheridium and " wanderplasm " in place
from which oogonium will arise. B, young oogonium. C, oogonial beak formed;
"wanderplasm" retreating into thread; oil globules passing into oogonium;
antheridial wall forming. D, "wanderplasm" out of oogonium. E, basal wall of
oogonium forming. F, antherozoids emerging. G, oogonial membrane forming
at tip, some antherozoids in egg. H, cytoplasm extruded and rounded off;
fertilization occurring. I, ripe egg. zc^ = wanderplasm. (After Couch.)

potential gametes, degenerate, whilst others consider that the
surplus nuclei, enclosed in a mass of cytoplasm or ''wanderplasm",
travel back into the main thread before the septum is laid down. It
is probable that in some species all the surplus nuclei pass out with
the "wanderplasm", whilst in other species some nuclei may be
left behind and degenerate later after the septum has been laid

SIPHONALES 97

down. The factors that determine the setection of the functional
nucleus from among the number available offer a problem for
future research. In the mature oogonium there is either a beak, the
apex of which gelatinizes, or else several pores through which the
antherozoids can enter the oogonium, fertilization taking place in
situ.

The antheridia, which are usually stalked, commonly arise
close to the oogonia, though in V. sessilis they develop just prior to
oogonial formation. When the septum cutting off the antheridium is
laid down the nuclei divide, and cytoplasm gathers around each
daughter nucleus. The mature antheridium may be colourless or
green, and it opens by one or more apertures near the apex, thus
providing a means of escape for the pear-shaped antherozoids
which bear two flagellae pointing in opposite directions. After
fertilization the zygote develops a thick wall and remains dormant
for some time before it germinates to give rise to a new filament.
The latest evidence shows that reduction of the chromosome
number takes place when the zygote germinates, thus indicating
that the adult plant is haploid. This character is somewhat ano-
malous when contrasted with the diploid status of almost all the
other Siphonales, with the exception of the primitive Protosiphon.
This is yet another reason for suggesting that the true affinities of
Vaucheria are to be found with the Xanthophyceae.

REFERENCES

Caulerpa. ARwmssoN, T. (1930). Svensk hot. Tidskr. 24, 263.

Caulerpa. Borgesen, F. (1907). K. danske vidensk. Selsk. Skr. 7, 340.

Vaucheria. Couch, J. N. (1932-3). Bot. Gaz. 94, 272.

Caulerpa. Dostal, R. (1929). Planta, 8, 84.

Halimeda. Howe, M. A. (1907). Bull. Torrey Bot. Club, 34, 491.

Neomeris. Howe, M. A. (1909). Bull. Torrey Bot. Club, 36, 75.

Halicystis. Kornmann, P. (1938). Planta, 28, 464.

C odium. Schmidt, O. C. (1923). Bibl. bot., Stuttgart, 91, i.

Caulerpa. Schussnig, B. (1929). Ost. bot. Z. 78, i,

Caulerpa. Schussnig, B. (1939). Bot. Notiser, p. 75.

Phyllosiphon. Tobler, F. (1919). Jb. wiss. Bot. 58, i.

Codium. Williams, M. (1925). Proc. Linn. Soc. N.S. Wales, 50, 98.

CSA

CHAPTER V

CHLOROPHYCEAE (cont.) (CONJUGALES, CHAR-
ALES), XANTH OP HYCEAE(HETE ROKO NT AE),
BACILLARIOPHYCEAE, CHRYSOPHYCEAE,
CRYPTOPHYCEAE, DINOPHYCEAE

CHLOROPHYCEAE

*CONyUGALES

The members of this group are somewhat distinct from the other
groups of the Chlorophyceae that have already been described and
at one time they were classed in a separate division, the Akontae.
As their pigmentation and metabolism are fundamentally the same,
however, it would seem desirable to abandon this arrangement.
Their peculiar reproduction suggests that they were evolved at a
very early stage from one of the simpler orders of the Chlorophyceae.
The order is subdivided into two distinct divisions, the Zygne-
maceae which are filamentous and the Desmidiaceae most of which
are not, although recently some desmids have been classed with
the Zygnemaceae.

*Zygnemaceae : Spirogyra {spiro, co\\\ gyra, curved). Figs. 67, 68.

The unbranched filaments are normally free-living although
attached forms are known, e.g. S. adnata, and they form slimy
threads which are known as ''Water-silk" or "Mermaid's tresses".
These grow in stagnant water and are most abundant in either the
spring or autumn, the latter phase being due to the germination of
a percentage of the spring zygospores. Each cell contains one or
more chloroplasts possessing either a smooth or serrate margin and
arranged in a characteristic parietal spiral band. The single nucleus
is suspended in the middle of the large central vacuole by means of
protoplasmic threads that radiate out to the parietal protoplasm.
The chloroplasts, which may occasionally be branched, are T- or
U-shaped in cross-section and contain numerous pyrenoids which
project into the vacuole on the inner side, the majority of the
pyrenoids arising de novo at cell division. The cell wall is thin and
composed, according to some investigators, of two cellulose layers.

CONJUGALES

whilst others maintain that there is only an inner cellulose layer
with an outer cuticle. The whole filament is enclosed in a mucilage
sheath of pectose. Any cell is capable of division, and vegetative

I H G F

Fig. 67. Spirogyra. A, B, cell disjunction (diagrammatic). C-E, cell dis-
junction in S. colligata. F-H, S. Weberi, cell disjunction by replicate fragmenta-
tion, r = replication of septum. I, vegetative structure and cell division,
S. nitida ( x 266). J, K, cell disjunction and development of replicate septa.
(A-H, J, K, after Fritsch; I, after Scott.)

reproduction by fragmentation is exceedingly common, three
methods having been described :

{a) The septum between two cells splits and a mucilaginous
jelly develops in between, so that when one cell subsequently de-
velops a high turgor pressure the cells become forced apart.

{h) Ring-like projections develop on both sides of a septum and
the middle lamella dissolves. Then the rings of one cell evaginate

7-2

100

CHLOROPHYCEAE

Fie 68 Spirogyra. A, B, rhizoid formation in S. fluviatilis. C ^izoids and
h 'ptophort ofi adnata. D-G, stages m conjugation^ 5. vanan.H gerrmnatK>n
of zygospore in S. neglecta. (A, B, after Czurda; C, after Delf, D G,
Saunders; H, after Fritsch.)

CONJUGALES loi

and force the cells apart whilst the rings of the other cell evaginate
after separation {replicate fragmentation) (cf. fig. 67 F-H).

(c) The septum develops an I piece and then when the wall
inverts, due to increased turgor, the I piece is slipped off and the
two cells come apart (cf. fig. 67 C-E).

When two filaments touch they may form joints or genicula-
tions, adhesion being brought about by a mucilaginous secretion
produced by the stimulation of the contact. The formation of such
geniculations, however, has no connexion with reproduction.

Sexual reproduction is secured by the process of conjugation, the
onset of which is brought about by a combination of certain
internal physiological factors combined with the ^H of the external
medium. It commonly takes place during the spring phase and
then the threads come together in pairs, but either one or more
than two filaments may also be involved. The threads first come
together by slow movements, the mechanism of which may be
connected with the secretion of mucilage; then they become
glued together by their mucilage and later young and recently
formed cells in both filaments put out papillae. These papillae
meet almost immediately, elongate, and push the threads apart.
Normally one of the threads produces male gametes and the other
female, but occasionally the filaments may contain mixed cells.
The papillae from male cells are usually longer and thinner than
those from the female cells and so they can fit inside the latter. The
conjugating cells accumulate much starch, the nuclei decrease in
size and the wall separating the papillae breaks, thus forming a
conjugation tube. The whole process so far described forms the
maturation phase which is now followed by the phase of gametic
union. Contractile vacuoles, which make their appearance in the
cytoplasmic lining, remove water from the central vacuole and so
cause the protoplasm of the male cells to contract from the walls.
The male cytoplasmic mass then migrates through the conjugation
tube into the female cell where fusion of the two masses takes place
and this is then followed by contraction of the female cytoplasm,
though in the larger species it may contract before fusion. Fusion
of the two nuclei may be delayed for some time, but in any case the
male chloroplasts degenerate. The process described above is
known as scalariform conjugation, and it includes certain abnormal
cases where cells produce more than one papilla or where the

102 CHLOROPHYCEAE

papillae are crossed. In some monoecious species, however,
lateral conjugation occurs, the processes being put out from ad-
joining cells on the same filament.

The last phase to be described is that of zygotic contraction which
is brought on by further action of the contractile vacuoles, after
which a thick three-layered wall develops around the zygote, the
middle layer or mesospore frequently being highly sculptured.
The zygospore occasionally germinates almost at once, thus pro-
ducing plants that account for the autumn maximum, but it is
usually dormant until the following spring. Meiosis takes place
when the zygote germinates and four nuclei are formed of which
three abort, the plants thus only exhibiting the haploid generation.
A two-celled germling is formed, the lower cell being relatively
colourless and rhizoidal in character. Filaments of two different
species have been known to fuse, the form of the hybrid zygospore
being determined by the characters of the female thread. Azy go-
spores, which have arisen parthenogenetically, and akinetes also
form other means of reproduction.

Zygnemaceae : Zygogonium {zygo, yoked ; gonium, angle). Fig. 69.

The commonest species of this genus, which is sometimes re-
garded as a subsection of the genus Zygnema, is the terrestrial
Z. ericetorum. The cells of this species each contain a single axile
chloroplast, whilst in Zygnema, of course, there is a pair of very
characteristic stellate chloroplasts (fig. 69 A). At low temperatures
the walls develop a very thick cellulose layer, whilst the sap is
coloured violet by phycoporphyrin, especially when the threads are
subjected to strong light. Sexual reproduction is rare but when it
does occur the gametes are formed from only a part of the proto-
plasm. In an Indian species azygospores are apparently the only
means of reproduction and even these are scarce. Aplanospores
and akinetes are commonly formed, and there is one abnormal form
growing on Hindhead heath which only exists in the akinete stage.

Zygnemaceae: Mougeotia (after J. B. Mougeot, a French botanist).
Fig. 70.

The filaments of the different species are commonly unbranched,
although they may occasionally possess short laterals. The chloro-
plast is a flat axile plate lying in the centre of the cell and orientated

CONJUGALES

103

according to the light intensity, whilst the nucleus is to be found in
the centre of the cell on one side of the chloroplast. Fragmentation
takes place by method (a) as described for Spirogyra (cf. above),
and knee joints or geniculations are also common. At conjugation
the gametes are formed from only part of the cell protoplast as in

Fig. 69. Zygogonium ertcetorum. A, Zygnema stellinum, cell and nucleus before
division ( x 500). B, the same, after division ( x 500). C, Zygogonium, stages in
conjugation. m = male nucleus, /) = conjugation process. D, terrestrial form
( X 1065). E, aplanospores formed from drying up of filament ( x 542). (A, B,
after Cholnoky; C-E, after Fritsch.)

Zygogonium^ fusion taking place either by way of papillae or
through a geniculation. The zygote is cut off by new walls and so
becomes surrounded by two or four sterile cells depending on
where the zygospore has been formed. Most of the species are
isogamous but anisogamy is known in Moiigeotia tenuis. Repro-
duction by means of thick-walled akinetes and parthenospores
occurs commonly, at least five species having only the latter mode
of propagation.

104

CHLOROPHYCEAE

D
C

Fig. 70. Mougeotia. A-E, M. ynirabilis, stages in conjugation through loss of
cell wall. F, normal conjugation in M. mirabilis. n = new walls cutting off zygote.
Gr-I, stages in lateral conjugation of M. oedogonioides. J, two azygotes in
M. mirabilis. (A-F, after Czurda; G-I, after Fritsch; J, after Kniep.)

pM^ms^f^

Fig. 71. Mesotaenium. A, plant of M. De Greyi. B-E, conjugation of Cylindro-
cystis Brebissonii and germination of zygospore. (After Fritsch.)

CONJUGALES 105

Desmidiaceae : Mesotaenium (meso, middle; taenium, band). Fig.

71-
This is an example of one of the saccoderm desmids, which as a
group are characterized by a smooth wall in one complete piece
and without any pores. The rod-shaped cells of Mesotaenium are
single, have no median constriction, and are circular in transverse
section. The chloroplast is a flat axile plate containing several
pyrenoids, whilst in some species the presence of phycoporphyrin
imparts a violet colour. The inner cell wall is composed of cellulose
and the outer of pectose. Multiplication takes place by cell division,
the daughter cells being liberated by dissolution of the middle
lamella after a constriction has been formed, though in some cases
this may not occur until a number of cells have been enclosed in a
common mucilaginous envelope. Sexual reproduction is by means
of conjugation, two processes being put out just as in the fila-
mentous forms: these unite and then the middle septum breaks
down so that the two protoplasts can meet in the centre, after
which the conjugation tube may widen. The thick-walled zygote
divides twice, the first division being heterotype, whilst in one
species the divisions result in two macro- and two micronuclei. It
is from these divisions thai either two or four new individuals
arise. The species are to be found in upland pools, peat bogs or on
the soil.

*Desmidiaceae : Closterium (enclosed space). Figs. 72, 73.

This genus is an example of one of the placoderm desmids, a
group that is commonly characterized by the highly perforated
cell wall composed of two parts.

The curved cells have attenuated apices with a vacuole in each
apex which contains crystals of gypsum that appear to have no
physiological function and are probably purely excretory. The
pores are arranged in rows in narrow grooves, cell movement being
secured by the exudation of mucilage through large pores near the
apices. Each semi-cell has one axile chloroplast which is in the
form of a curved cone with ridges on it, whilst in transverse section
it either has the appearance of a hub with radiating spokes or else
looks Hke a coarsely cogged wheel. Cell division is pecuHar and
takes place by one of two methods producing either (a) connecting
bands which appear as striae in the older semi-cells or (b) girdle

io6

CHLOROPHYCEAE

bands (cf. fig. 73). At conjugation, papillae from the two cells meet
or else the naked amoeboid gametes fuse immediately outside the
cells, whilst in C. parvulum there is some evidence of sexual
differentiation. After the gametes have fused two of the chloro-
plasts degenerate and the zygospore on germination divides twice,

Fig. 72. Closterium. A— D, C. Ehrenbergii, stages in cell division. w = nucleus,
5 = septum, z; = vacuole. E, C. lanceolatum, chloroplast structure. F, Closterium
sp. showing structure. w = nucleus. G, C. lineatum, first stage in conjugation.
H, C. rostratum var. brevirostratum, zygospore formation, second stage.
I, C. calosporum, mature z\'gospore. (A-H, after Fritsch; I, after Smith.)

during which meiosis takes place. Two daughter cells are then
formed, each containing one chloroplast and two nuclei, but one of
the latter subsequently degenerates. The genus is wholly fresh
water.

Many of the desmids are planktonic and possess modifications,
e.g. spines, which may be regarded as adaptations to this mode of
existence. The group is extremely widespread, though it is absent

CONJUGALES

107

from the Antarctic and is scarce in waters containing much lime, the
individual species thriving best in soft or peaty waters. The most
favourable seasons for their development are the late spring and

g cl d d I b

Fig. 73. Closterium. Diagrams to explain cell division in species of Closterium
with (B) and without (A) girdle bands. The different segments of the wall are
indicated by shading, i, 2, 3, and A, b, c = the successive generations. The
individuals in i and A have each arisen from a zygote and have not undergone
division, a, b, c/= semi-cells of various ages; c = the connecting band demarcated
by the two sutures s, of the previous generation, and t, of the present ;5', /-girdle
bands developed before (g) and after division ; s = suture bet^veen young and older
semi-cells; r = the line of the next division, (After Fritsch.)

early summer and their resistance in the vegetative state to adverse
conditions would seem to be very great. The evidence suggests
that, as a group, they have been evolved from filamentous ancestors,
possibly by over-specialization of the process of fragmentation.

REFERENCES

Zygnemaceae. Czurda, V. (193 1). Beth. hot. Zbl. 48/2, 238.
Spirogyra, Zygnema. Czurda, V. (i933)- Beih. hot. Zbl. 50/1, 196.
Zygogonium. Hodgetts, W. J. (19 18). New Phytol. 17, 238.
General. Lefevre, M. and Manguin, E. (1938). Rev. gen. Bot. 50, 501.
Spirogyra. Lloyd, F. E. (1926). Trajis. Roy. Can. Inst. 15, 151.

io8 CHLOROPHYCEAE

Spirogyra. Lloyd, F. E. (1926). Trans. Roy. Soc. Can. 3rd series, 20, 75.

Spirogyra. Lloyd, F. E. (1928). Protoplasma, 4, 45.

General. West, G. S. (1915). jf. Bot. 53, 73.

Zygogonium. West, G. S. and Starkey, C. 6.(1915). New PhytoL 14, 194.

*CHARALES

The plants forming this small order represent a very highly
specialized group that must have diverged very early in the course
of evolution from the rest of the green algae, the intermediate forms
subsequently being lost. They are characterized in that they lack
asexual reproduction and possess very complex sexual reproductive
organs. The young plants develop from a protonemal stage, the
erect plants having a structure which is more elaborate than any
type so far described, whilst the thallus is also frequently lime
encrusted. The group is very ancient because fossil members are
found from almost the earliest strata. The living forms are widely
distributed in quiet waters, fresh or saline, where they may
descend to considerable depths so long as the bottom is either
sandy or muddy.

*Nitella {nitella, a little star). Figs. J 4.-77.

The plants have the appearance of miniature horsetails (Equise-
tum) because they bear whorls of lateral branches arising from the
nodes. The nodes are formed by a transverse layer of cells in
contradistinction to the internodes, which consist of one large cell
whose individual length may extend up to 25 cm. in Nitella cernua.
The height of the different species varies up to i m., growth being
brought about by an apical cell which cuts off successive segments
parallel to the base. Each new segment divides transversely into
two halves, the upper developing into a node and the lower into an
internode (fig. 75 B). Branches, both primary and secondary, are
formed by the peripheral cells of the nodes protruding to form new
apical cells, but these soon cease to grow after the branch has
reached a short length. At the basal node of the main plant
branches of unlimited growth are produced: these arise on the
inner side of the oldest lateral in the whorl, thus producing a
fictitious appearance of axillary branching. Multicellular branched
rhizoids with oblique septa function as absorption organs and also
serve for anchorage. The rhizoids develop from the lowest node of
the main axis, but every node is potentially capable of producing

CHARALES

109

them though normally the presence of the stem apex inhibits their
appearance but if this is cut off they will then develop. This
behaviour is very suggestive of an auxin control similar to that

Fig. 74. Charales. A, Nitella batrachosperma, B, Chora hispida. C, underground
bulbil of C. aspera. D, germinating oospore. E, protonema of C.fragilis. F, young
plant of C crinita. a/) = accessory protonema, z = internode, ^ = protonema,
r = rhizoids, rn = rhizoid node, 5 = shoot, sn = stem node, v = initial of young plant.
(After Fritsch.)

found in the higher plants. The cells, which have a cellulose
membrane, contain discoid chloroplasts without any pyrenoids
together with one nucleus. Cytoplasmic streaming is very readily
observed, especially in the internodal cells. Sexual reproduction is
by means of a characteristic oogamy where light intensity plays a

CHLOROPHYCEAE

part in determining the production of the sex organs. The species
are either dioecious or monoecious, in which latter case the
oogonia and antheridia are juxtaposed, the oogonia being directed
upwards and the antheridia downwards, both organs usually
appearing on secondary lateral branches of limited growth.

Fig. 75- Charales. A, i-6, successive stages in development of root node of
Char a aspera. i, double foot joint. 2, dilation of toe of upper foot. 3, toe portion
cut off. 4, 5, subdivision of toe cell. 6, rhizoids growing out. B, 1-3, successive
growth stages of apex of Nitella. In i apical cell is undivided, in 2 it has divided,
in 3 the lower cell has divided into an upper node and a lower intemode.
C, C. hispida, node with stipules. D, A'', gracilis, longitudinal section of node.
E, C. fragilis, branch at node with axillary bud. a = antheridium, ac = ascending
corticating cells, as = apex of side branch, 6«/ = basal node of branch (I), c and
CO = cortical cells, J= descending cortical cells, f = internodal cell, w = nodal cell,
0 = oogonium initial, 5 = stipule. (A, B, after Grove; C-E, after Fritsch.)

Antheridia. Fig. 76.

The apical cell of the lateral branch cuts off one or two discoid
cells at the base and then becomes spherical. The upper spherical
cell divides into octants and this is followed by two periclinal
divisions after which the whole enlarges and the eight peripheral
cells develop carved plates {shields), thus giving the wall a pseudo-
cellular appearance. At maturity these peripheral cells acquire

CHARALES

III

brilliant orange contents. The uppermost discoid basal cell pro-
trudes somewhat into the hollow structure formed as described
above. The middle segment of each primary diagonal cell now
develops into a rod-shaped structure, the manubrium, which bears
at its distal end one or more small cells, the capitula ; every one of

Fig. 76. Charales. A, B, stages in development of antheridium of Chara.
1-3, segments and cells to which they give rise. C, section of almost mature
antheridium of Nitella flexilis. 6 = flask cell, c = extra basal cell. D, C. tomentosa,
single plate with manubrium and spermatogenous threads. E, C tomentosa, apex
of manubrium with spermatogenous threads. a = priman,- head cell, 6 = secon-
dary- head cell. F-I, C. foetida, stages in formation of antherozoids in sperma-
togenous threads. J, mature antherozoid. (A, B, after Goebel; C-E, J, after
Grove; F-I, after Fritsch.)

these produces six secondary capitula from each of which arises a
forked spermatogenous thread containing 100-200 cells. These
antheridial cells each produce one antherozoid, an elongate body
with two flagellae situated just behind the apex. The complete
structure has been regarded as one antheridium, whilst another
view regards the octants as laterals, the manubrium as an internode,

112

CHLOROPHYCEAE

the capitula as a node and the spermatogenous threads as modified
laterals, so that on this basis the antheridia are one-celled and
conform to the normal structure of the majority of the antheridia in
the green algae. This second interpretation, if it is correct, helps
considerably in understanding this peculiar group.

Oogonia. Fig. 77.

The apical cell of the lateral branch divides twice giving rise to a
row of three cells, the uppermost cell developing into the oogonium

Fig. 77. Charales. A-F, Chara vulgaris, stages in formation of oogonium.
A, first division. B, C, division of periphery to form envelope cells. D, coronal
cells cut off. F, mature oogonium. G— I, Nitella flexilis, stages in formation of
oogonium. J, fertile branch of C. fragilis. a = oogonium, 6c = bract cell, hn =
branch nodal cell, ff= coronal cells, e = envelope cells, /= flask cell, z = internode,
w = nodal cell, f = turning cell, 0^ = oogonium stalk cell. (After Grove.)

whilst the lowest forms a short stalk. The middle cell cuts off five
peripheral cells which grow up in a spiral fashion and invest the
oogonium, each one finally cutting off two small coronal cells at the
apex. The oogonial cell cuts off three cells at its base and it is
maintained that these, together with the oogonium, represent four

CHARALES 113

octants, only one of which develops to maturity. When mature, the
investing threads part somewhat to form a neck, and the apex of the
oogonium gelatinizes in order to permit the antherozoids to enter.
After fertilization the zygote nucleus travels to the apex of the
oospore and a coloured cellulose membrane is excreted around it,
whilst the oogonium wall, together with inner walls of the investing
threads, thicken and silicify. Four nuclei are formed by two
successive divisions of the zygote nucleus, meiosis taking place
during this process. One of these nuclei becomes cut off by a cell
wall whilst the other three degenerate. The small cell so formed then
divides and two threads grow out in opposite directions, one a
rhizoid, the other a protonema. The cell next to the basal cell of the
protonema divides into three cells, the upper and lower forming
nodes which become separated by elongation of the middle cell
(fig. 74 D-F). The lower node develops rhizoids whilst the upper
produces a whorl of laterals from all the peripheral cells except the
oldest, which instead forms the apex of the new plant. The mature
plant is therefore morphologically a branch of the protonema.
Vegetative reproduction can take place from secondary protonemata
which develop from the primary rhizoid ring or else from dormant
apices.

Chora (of a mountain stream). Figs. 74-77.

This genus is very similar to Nitella in its method of reproduction,
but the plants are usually larger and coarser as a result of lime
encrustation, whilst the stem is corticated, the corticating cells
arising from the basal nodes of the short laterals, one thread
growing up and another down.

*XANTHOPHYCEAE

As a group the Xanthophyceae exhibit considerably less differ-
entiation than the Chlorophyceae. Two of the most characteristic
features are the replacement of starch as a food reserve by oil and a
greater quantity of xanthophyll in the plastids, although the actual
amount of the latter is partially dependent upon the external
conditions. The pigment turns blue-green when the cells are
heated in concentrated hydrochloric acid and this forms a con-
venient test for distinguishing them from the Chlorophyceae.
The walls are frequently in two equal or unequal portions which

CSA 8

114 XANTHOPHYCEAE

overlap, their composition being principally of a pectic substance
although some cellulose may occasionally be present. The motile
bodies contain more than one chloroplast and are further character-
ized by two unequal flagellae, the longer one often possessing
delicate cilia. The Xanthophyceae exhibit very little regularity in
the formation of reproductive bodies. Sexual reproduction is rare
and in the few known examples is always isogamous, the principal
mode of reproduction being by means of zoospores and aplano-
spores. The majority of the species are confined to fresh water. It
would seem that they have a motile unicell ancestry, the chief
interest of the group being the manner in which evolution has taken
place along lines parallel to those found in the Chlorophyceae.
As a result there exists a set of analogues which, so far as general
morphology is concerned, bear so much resemblance to chloro-
phycean groups that these forms are classed as Heterochloridales,
Heterococcales, Heterosiphonales and Heterotrichales.

Heterochloridaceae : Chloramoeba (chlor, green ; amoeba, chang-
ing). Fig. 78.

This is a naked unicell which is analogous to certain members of
the Volvocales, e.g. Diinaliella. The cells multiply by longitudinal
division, but under adverse conditions ellipsoidal cysts with large
oil globules are developed and these form a resting stage.

Heterocapsaceae : Botryococciis (botryo, cluster; coccus, berry).
Fig. 79.

This fresh-water genus represents one of the palmelloid ana-
logues of the Chlorophyceae, the principal species, B. Braunii,
forming an oily scum on ponds and lakes in spring and autumn,
whilst in late summer the cells are often coloured red by haemato-
chrome. The colonies vary greatly in shape, the cells being radially
arranged into spherical aggregates that are connected in a reticular
fashion by tough, hyaline or orange-coloured strands belonging to
the lamellated mucous envelope. The individual cells are surrounded
by a thin membrane that becomes evident when they are squeezed
out of their envelopes as sometimes happens. Each cell is enclosed
in a funnel-shaped mucilage cup composed of several layers and
prolonged at the base into a thick stalk. In old colonies the mucilage
envelope swells up so that the cup structure is obscured, but al-

XANTHOPHYCEAE

115

though the sheath is so predominant nevertheless its origin is not
clearly known. The cells multiply by longitudinal division, whilst
asexual reproduction by means of zoospores has also been recorded
though it requires confirmation. Normally reproduction is
secured by means of aplanospores, of which two to four are

Fig. 78. Fig. 79.

Fig. 78. A, Chloramoeba heteromorpha, cyst. B, the same, motile phase. c =
chloroplast, « = nucleus, w = vacuole. C, flagellum structure in Monocilia.
(After Fritsch.)

Fig, 79. Botryococciis Braunii. A, colony ( x 300). B, portion of colony showing
cells in their mucilage envelope. C, two cells enclosed in the parent cup. D, por-
tion of colony enlarged ( x 780). E, two cells arranged diagrammatically to show
structure. c = cup, cc = cell cap, cic = ceW wall, /)c = parent cell, pm = pectic
mucilage, ^? = parent thimble, f = thimble. (A, after Smith; B, C, after Fritsch;
D, E, after Blackburn.)

produced in each cell. The colonies decay very slowly, and one of
the principal interests of the genus is the recent discovery that
boghead coal is composed very largely of this organism, whilst the
fossil genera Pila and Reinschia hardly differ from the living
Botryococcus Braunii.

Halosphaeraceae : Halosphaera (halo^ salt; sphaera, sphere). Fig.
80.

The large, free-floating spherical cells possess one nucleus which
is suspended either in the central vacuole or else in the parietal

8-2

ii6

XANTHOPHYCEAE

cytoplasm where it is associated with numerous discoid chloro-
plasts. A new membrane is formed internally and then the old one
ruptures, but as the latter may still persist outside one can often see
what appears to be a multi-layered sheath. Reproduction can take
place by means of zoosporic swarmers but these may be replaced
by aplanospores, whilst resting cysts are also recorded. Although
most abundant in the warmer oceans, especially during the winter
months, its life history is as yet only imperfectly known.

A X B

Fig. 80. Fig. 81.

Fig. 80. Halosphaera viridis. A, mature cell. B, young cell in optical section.
C, mature cell with aplanospores. D, swarmer. (A-C, after Fritsch; D, after
Dangeard.)

Fig. 81. Characiopsis saccata. A, plant. B, probable swarmer formation.
(After Fritsch.)

Chlorotheciaceae : Characiopsis (like Characium). Fig. 81.

The very name of this genus indicates that it is an analogue to the
genus of similar name in the Chlorophyceae. The plants, which are
epiphytic, solitary or gregarious, vary much in shape, even in
pure culture, and they develop from a short stalk with a basal
mucilaginous cushion. The wall, composed of cellulose and pectins,
is in two unequal portions, the smaller upper part forming a lid
which is detached at swarmer formation whilst in one species the
lower part bears internal processes. Although the young cells are
uninucleate and contain one or more chloroplasts the adult cells
are multinucleate containing eight to sixty-four nuclei. Reproduc-
tion is either by means of zoospores (eight to sixty-four per cell) or
else by means of thick-walled aplanospores, which in one species

XANTHOPHYCEAE

117

are said to give rise to motile gametes, although this is a feature that
requires further investigation.

*Tribonemaceae : Tribonema (tribo, thin; nema, thread). Fig. 82.

This is a filamentous analogue to a form such as Microspora
(cf. p. 46) with which it is frequently confused. T. bombycina
sometimes appears in sheets covering ponds and pools and if these
dry up they form an algal ''paper". The unbranched threads are
composed of cells possessing walls of two equal overlapping halves,
with the result that the filaments are open-ended and tend to
dissociate into H pieces. At cell division a new H piece arises in the

■■■■n^'I^'

Fig. 82. Tribonema. A, T. bombycina (X450). B, T. minus, hypnospores.
C, D, construction of H piece in T. bombycina as shown after treatment with
KOH (X675). (A, C, D, after Smith; B, after Fritsch.)

centre and the two halves of the parent cell separate, somewhat as
in the Desmidiaceae. Each cell contains one nucleus, although
Tribonema bombycina may have two together with two or more
parietal chloroplasts. Asexual reproduction is by means of zoo-
spores (two to four per cell) which are liberated by separation of the
two halves of the cell. On coming to rest the zoospore elongates
and puts out an attachment process, and in this state it much re-
sembles Characiopsis. Aplanospores (one to two per cell) and
akinetes, which are formed in chains, also act as additional means of
propagation, whilst sexual reproduction has been seen only once
when some of the motile bodies came to rest first and were sur-
rounded by other motile gametes. Iron bacteria sometimes live

ii8

XANTHOPHYCEAE

symbiotically with this alga and colour it yellow or brown from
ferric carbonate. This substance controls the pU of the water and
thus acts as a local buiTer for the alga whilst the bacteria obtain
their oxygen requirements from the Trihonema.

BoTRYDiACEAE : Botrydiutti (a small cluster). Fig. 83.

This genus belongs to the Heterosiphonales and is analogous to
a form such as Protosiphofi, the commonest species, Botrydium
gramilatiwiy being frequently confused with it, especially as these
two plants are often associated on areas of drying mud. B. granu-
latum makes its appearance during the warmer part of the year
when it is seen that the green, pear-shaped vesicles are rooted
by means of colourless dichotomously branched rhizoids. The

GAMETES

ZYGOTE

Fig. 83. Botrydium granulatum. A, plant. B, swarmer. C, cyst formation.
D, diagram of life cycle. (A-C, after Fritsch; D, after Miller.)

membrane is composed of cellulose and the lining cytoplasm con-
tains numerous nuclei scattered throughout it, whilst the chloro-
plasts, containing pyrenoid-like bodies, are confined to the aerial
part. The shape of the vesicle is influenced by the environment, the
shade forms being elongate or club-shaped. In B. Wallrothii the
unbranched vesicle is covered with lime whilst in B. divisum it is
branched but without lime. When the plants are submerged re-
production takes place by means of numerous zoospores which are
set free by gelatinization of the vesicle apex, but when the plants are
only wet but not submerged aplanospores are formed instead.
Under dry conditions each vesicle develops into a single cyst

XANTHOPHYCEAE 119

(macrocyst) or into several multinucleate spores (sporocysts), or
else the contents migrate to the rhizoids and there form several
cysts (rhizocysts) which, when conditions are again favourable,
either germinate directly to a new plant or else give rise to zoo-
spores. In B. granulatum it is estimated that about 40,000 iso-
gametes are formed in each vesicle, but as the plant is monoecious
many fuse either in pairs or threes, rarely fours, before they are
liberated. Those that do not fuse develop parthenogenetically,
although the stage at which meiosis occurs is not yet known. The
life cycle can be tentatively represented as in fig. 83 D.

*BACILLARIOPHYCEAE (DIATOMACEAE)

Figs 84, 85

These unicellular algae are abundant as isolated or colonial forms
in marine or fresh- water plankton and also as epiphytes on other
algae and plants. They form a large proportion of the bottom flora
of lakes and ponds and occur widely on salt marshes, although
certain diatoms are said to be very sensitive to the degree of
salinity in the medium. In the colonial forms the cells are attached
to each other by mucilage or else they are enclosed in a common
mucilaginous envelope. The plants have characteristic silicified
cell walls which are built up on a pectin foundation and are highly
sculptured. Each shell (frustule) is composed of two halves varying
much in shape, the older (epitheca) fitting closely over the younger
(hypotheca), each half being composed of a valve together with a
connecting band, the latter forming the overlapping portion. The
Diatomaceae are divided into two groups, the Pennatae and Centri-
cae, the former having intercalary bands as well as the connecting
bands. A simple way of distinguishing between these two groups is
that the Pennatae have the shape of date boxes and the Centricae
that of pill boxes. The marks or striae on the frustules are
composed of rows of dots which represent small cavities, and these
are so fine that they are employed in testing the resolving power of
microscopes. The Pennatae have the striae arranged in series with
either a plain area in between (pseiidoraphe) or else a slit that varies
in form and structure (raphe). In the Centricae these structures are
absent and the striae are arranged radially. The raphe is connected
with movement, as only those forms possessing one have the power

120 BACILLARIOPHYCEAE (DIATOMACEAE)

of locomotion, and although the mechanism is not completely
understood it would seem to be connected with friction caused by

Fig. 84. Bacillariophyceae. A, Melosira granulata (Centricae) ( x 624). B,
Pinnularia viridis (Pennatae), girdle view. C, same, valve view. D, P. viridis,
union of valve and parts of adjacent girdle bands. E, P. viridis, termination of the
two parts of the raphe in the polar nodule. F, P. viridis, diagrammatic view
showing the two raphes. G, movement of P. viridis as shown by sepia particles.
I, in valve view; 2, in girdle view. H, diagram to illustrate successive diminution
in size of plant. The half-walls of the different generations are shaded appro-
priately. cw = central nodule, /= foramen, ^ = girdle, /? = hypotheca, ^n = polar
nodule, ?- = raphe, z; = valve, z:; = wall of valve. (A, H, after Smith; B-G, after
Fritsch.)

the streaming of protoplasm. Streams of mucilage pass from the
anterior polar nodule down to the centre of the plant body where it
masses and then spreads out posteriorly in the form of a fine thread
(fig. 84 G). Each cell is surrounded by a cytoplasmic fining with a

BACILLARIOPHYCEAE (DIATOMACEAE) 121

bridge between the two halves of the shell in which the nucleus is
commonly to be found. The chloroplasts are parietal, olive green
to brown, the principal colouring matter being isofucoxanthin,
whilst pyrenoids may be present or absent. The product of photo-
synthesis is a fatty oil. The pelagic forms frequently possess out-
growths which must be regarded as adaptations to their mode of
existence. Cell division normally occurs at night time, and when
the nucleus and protoplast have divided new valves are formed

Fig. 85. Bacillariophyceae. A-G, auxospore formation by two cells in the
pennate diatom, Cymhella lanceolata. A, synaptic contraction. B, after first
mitosis. C, second mitosis with functional and degenerating pairs of nuclei.
D, division of each protoplast into two uninucleate gametes. E, young zygotes.
G, zygotes elongated to form auxospores. H, microspore formation in Melosira
varians ( x 600), I, J, auxospore formation in Rhabdonema arcuatum. K, asexual
auxospores in M. varians. (A-H, K, after Smith; I, J, after Fritsch.)

inside and then the parent connecting bands separate. One indi-
vidual thus becomes smaller and smaller because the size of the
new valve is fixed by the silica contained in the wall of the old
valve and in five months there may be a decrease of three-fifths to
two-thirds of the length until finally the shrinkage is compensated
for by auxospore formation (fig. 84 H). However, a long time elapses
before this rejuvenation is necessary and so auxospore formation is
relatively rare.

At auxospore formation in the Centricae the two halves of the

122 BACILLARIOPHYCEAE (DIATOMACEAE)

shell are thrust apart by enlargement of the protoplast, which
becomes enveloped in a slightly silicified pectic membrane, the
perizonium. No nuclear division takes place, but fresh valves and
connecting bands are formed inside this membrane so that a new
and larger individual results. In the Pennatae a union takes place
between two naked amoeboid protoplasts that have arisen from two
distinct individuals which come together in a common muci-
laginous envelope. These are the gametes, and as meiosis occurs
during their formation the normal diatom cell must be regarded as
diploid (fig. 85). The zygote remains dormant for a time and then
elongates in order to form auxospores, the perizonium either being
the remains of the zygotic membrane or else formed de novo.
Isogamy is the normal condition but a few cases of physiological
anisogamy are known and also apogamy. In addition to auxo-
spores the Centricae also produce microspores, small rounded
bodies with flagellae, but whether these are true gametes has yet to
be established because their fate has not been fully studied. Some
diatoms are also known to produce resting spores but very little is
recorded about these bodies.

CHRYSOPHYCEAE

Fig. 86

This assemblage is principally composed of uninucleate flagellate
forms although certain members do exhibit some algal character-
istics. Like the Xanthophyceae there is considerable morphological
parallelism with the Chlorophyceae indicating that evolution has
taken place along the same lines. Sexual reproduction is rare and
when it does occur is isogamous, the plants probably all being
haploid. They occur most commonly in both fresh or salt water
during cold weather. The colour is golden yellow or brown due to
the presence of the pigment phycochrysin which is contained in a
small number of parietal chromatophores that may also contain
pyrenoid-Hke bodies, although starch as a product of photo-
synthesis is replaced by oil or leucosin. The motile cells are uni- or
biflagellate, and in the latter event one flagellum is beset with fine
cilia ; one of the flagellae is said to provide forward movement and
the other rotation. When an individual has entered the amoeboid
state cysts may be produced endogenously and these have silicified

CHRYSOPHYCEAE

123

walls composed of two equal or unequal parts. The group possesses
the following morphological categories :

(a) Unicellular motile types, e.g. Chromulina.

(b) Encapsuled types, either free or epiphytic, e.g. Dinohryon spp.
{c) Colonial types, e.g. Syniira.

Fig. 86. Chry'sophyceae. A, Phaeothamnion confervicolum. B, Hydrurus
foetidus. C, H. foetidus, apex showing branching. D, Phaeocystis pouchetii.
E, the same, portion of plant. / = leucosin. F, Ochromonas mutabilh. c = chloro-
plast, / = leucosin, 5 = stigma, t; = vacuole. G, Synura ulvella. H, Dinohryon
sertularia, colony. I, D. marchicuni. J, Epichrysis paludosa on Tribonema.
u = vacuole. (After Fritsch.)

{d) Dendroid colonies, e.g. Dinohryon spp.
{e) Rhizopodial or amoeboid types, e.g. Rhizochrysis.
If) Palmelloid types, e.g. Phaeocystis and Hydnirtis, the latter
being a highly differentiated branched type.

(g) Simple filamentous types, e.g. Phaeothamnion,

124

CRYPTOPHYCEAE

It is suggested that the group is still actively evolving, and that
some of the brown types with algal characters may have a relation-
ship with the simpler Phaeophyceae.

CRYPTOPHYCEAE •

Fig. 87

Very little is known about this group. They are mostly specialized
flagellates with two fiagellae but there are a few algal forms, although
none of them is filamentous. The morphological types are :

(a) Naked motile unicells.

(b) Colourless unicells.

Fig. 87. Cryptophyceae. A, Cryptomonas anomala, side view. B-D, Tetra-
gonidiuni verrucatum, D, being the swarmer. /= furrow, n = nucleus, p = pyrenoid,
5 = starch. (After Fritsch.)

(c) Symbiotic unicells with cellulose walls, e.g. some of the
Zooxanthellae which are found associated with Coelenterata and
Porifera.

{d) Palmelloid type, e.g. Phaeococcus, which is found on salt marsh
muds in England.

(e) A single coccoid type, Tetragonidium.

The number of chloroplasts varies, pyrenoids are present, and
there is one nucleus in each cell. Reproduction is by means of
longitudinal fission but some species also form thick-walled cysts.

DINOPHYCEAE 125

DINOPHYCEAE

Fig. 88

This group is predominantly planktonic, naked forms being
most abundant in the sea, whilst in fresh waters one commonly
finds armoured forms which often have spiny processes that can be
regarded as adaptations to their pelagic existence. The majority of
the species are motile and characteristically possess two flagellae,
one directed forward and one transversely, both commonly lying in
grooves and emerging through pores. In one or two cases, however,

Fig. 88. Dinophyceae. A, Peredinium anglicum, dorsal view. B, P. anglicum,
ventral view. C, Cystodinium lunare. D, Gymnodinium aeruginosum. E, Dino-
clonium Conradi. F, Gloeodinium montanum. a = apical plate, ac/) = accessory
plates, «2/) = antapical plates, ^ = girdle, w = nucleus, ^ = precingular plates,
^c = postcingular plates, r = rhomboidal plate. (After Fritsch.)

the flagellae may be situated anteriorly. In some forms ocelli^
which are composed of a spherical lens and a pigment, can be ob-
served ; these are presumably connected with the perception of light
and they must be regarded as an elaborate development of the
ordinary red eye-spot. Two genera also possess nematocysts com-
parable to those found in hydroids. The numerous disk-like
chromatophores are dark yellow or brown in colour and sometimes
contain pyrenoids. There is one nucleus and the food reserve is
starch and fat, whilst the marine Dinoflagellates are noted for
possessing large vacuoles. Multiplication is by means of cell

126 DINOPHYCEAE

division which takes place either during the motile phase or else
during a resting phase. Spherical swarmers of the naked unicell
t}'pe are also known together with cysts and autospores. The
following represent the different morphological types that have
been evolved in the course of evolution :

(a) Motile unicells which are either naked or else enclosed in a
delicate membrane, e.g. Desmokontae and the unarmoured Dino-
flagellates.

(b) Motile unicells with a conspicuous cellulose envelope of
sculptured plates and with the flagellae furrows well marked, e.g.
armoured Dinoflagellates — Peredinium, Ceratium.

{c) Parasitic marine forms which are either ecto- or endo-
parasites.

[d) One palmelloid genus, Gloeodinium.

{e) Colourless and rhizopodial forms.

(/) Coccoid forms, e.g. Dinococcales.

[g) Filamentous forms, e.g. Dinothrix^ Dinoclonium.

Recent work has tended to show that there is no real evidence for
believing that this group is closely related to the Diatomaceae as
was formerly supposed.

REFERENCES

Botryococcus. Blackburn, K. and Temperley, B. N. (1936). Trans. Roy.

Soc. Edinb. 58, 841.
Halosphaera. Dangeard, P. (1932-3). Botaniste, 24, 261.
Diatoms. Geitler, L. (1930). Arch. Bot. 3, 105.
Diatoms. Geitler, L. (1932). Arch. Protistenk. 78, i. .
Charales. Goebel, K. (1930). Flora, 124, 491.
Diatoms. Gross, F. (1938). Philos. Trans. B, 228, i.
Charales. Groves, J. and Bullock- Webster, G. R. (1920-4). The

British Charophyta, i, 2. Ray Society.
General. Kolbe, R. W. (1927). Pflanzenforschwig, 7.
Botrydium. AIiller, V. (1927). Ber. dtsch. bot. Ges. 45, 151.
Dinophyceae. Pascher, A. (1927). Arch. Protistenk. 58, i.
Xanthophyceae. Poulton, E. M. (1926). New Phytol. 25, 309.
Xanthophyceae. Poulton, E. M. (1930). New Phytol. 29, i.

CHAPTER VI

PHAEOPHYCEAE

ISOGENERATAE AND HETEROGENERATAE (EXCLUDING
DICTYOTALES, LAMINARIALES AND FUCALES)

*GENERAL

The algae composing this group range from minute disks to lOO m.
or more in length and are characterized by the presence of a brown
pigment, fucoxanthin; the function of this substance is still not
clearly understood but it is probably connected with the absorption
of light, though not with its utilization (cf. p. 293). In the older
classification the group was customarily divided into three orders :
the Phaeosporeae (including the Laminariales) with motile gametes,
Cyclosporeae with non-motile asexual bodies and non-motile ova
and the Acinetosporeae or Tilopteridales with non-motile asexual
bodies, the sexual reproductive organs being either absent or
imperfect. In 1917 Kylin suggested the following classification:
Tilopteridales (Acinetosporeae), Dictyotales, Laminariales, Fucales
and Phaeosporeae, this last group really being a polyglot assembly
of distantly related forms. Later Taylor (1922) transferred the
Laminariales to the Cyclosporeae and placed the Acinetosporeae in
the Phaeosporeae thus leaving only two orders. In 1933 Kylin
suggested yet another rearrangement with only three groups based
upon the type of alternation of generations, but it is doubtful
whether this new classification has any more real significance in so
far as phylogenetic relationships are concerned.

(a) Isogeneratae. Plants with two morphologically similar but
cytologically different generations in the life cycle, e.g. Ecto-
carpaceae, Sphacelariaceae, Dictyotaceae, Tilopteridaceae, Cut-
leriaceae.

(b) Heterogeneratae. Plants with two morphologically and
cytologically dissimilar generations in the life cycle:

I. Haplostichineae. Plants with branched threads, which are
often interwoven, and without intercalary growth, e.g.
Chordariaceae, Mesogloiaceae, Elachistaceae, Spermato-
chnaceae, Sporochnaceae, Desmarestiaceae.

128 PHAEOPHYCEAE

II. Polystichineae. Plants built up by intercalary growth into

a parenchymatous thallus, e.g. Punctariaceae, Dictyo-

siphonaceae, Laminariales.

e.g. Fucales. In view, however, of the most recent interpretation

of the life history of the Fucales (p. 189) the Cyclosporeae should

now be classed with the Heterogeneratae, division Polystichineae.

It would seem impossible to construct a classification of the
Phaeophyceae on a satisfactory phylogenetic basis because they
would appear to have diverged and converged greatly during the
course of evolution. As a group they are very widespread and are
confined almost entirely to salt water although Pylaiella is some-
times found in brackish water and Lithoderma in fresh water.
Some of the species commonly exhibit morphological variations
and it has been shown that these may depend on [a) season of the
year, and {b) nature of the locality. Church (1920) has given us an
elaborate account of the morphology of the Phaeophyceae, and he
suggested that if a brown flagellate came to rest it could develop in
one of three directions to give :

{a) Uniseriate filaments which occupy a minimum area and
obtain maximum light energy per unit of area, growth being either
distal or intercalary.

(6) A mono- or polystromatic thallus which occupies a maximum
area and obtains a minimum light energy per unit of area.

{c) Mass aggregation.

A morphological examination of the brown algae will show that
development has taken place along each of these directions, often
resulting in plant bodies of a complex construction, and the
following types can be recognized among the various species :

{a) Simple filaments (e.g. Acinetospora).

(b) Branched filaments (e.g. Pylaiella).

(d) Interwoven central filaments (cable type, e.g. Mesogloia).
{e) Basal portion only {reduced filamentous or cable type, e.g.

Phaeostroma).

(/) Filaments uniting to form a sphere {hollow parenchymatous
or modified cable type, e.g. Leathesid).

{g) Multiseptation of primary cable type (e.g. Chorda).

GENERAL 129

{h) Erect filaments with cortication {corticated type, e.g. Spha-
celaria).

(i) Simple or laminate parenchymatous thallus (e.g. Punctaria).

(j) Improved parenchymatous structure with internal differentia-
tion of the tissues (e.g. Laminariales).

(a) to {c) are generally of an ectocarpoid type (like Ectocarpus;
cf. p. 132) with a single central filament, or else of a mechanically
produced cable type when the central filaments are twisted to-
gether by wave action to form a rope or cable. Many of these,
whether reduced or not, exhibit the condition of heterotrichy
similar to that found in the Chaetophorales, but this is a feature
that will be discussed elsewhere (cf. p. 263). The thalli may also
reach a relatively large size and under these circumstances addi-
tional support is obtained as follows :

(i) Increase in wall thickness (Stypocaulon) or the production of
a firmer cellulose material (Sphacelaria).

(2) Twisting and rolling of the threads together.

(3) Development of root branches or haptera.

(4) The appearance of descending and ascending corticating
filaments.

(5) Multiseptation takes place in a longitudinal direction.

Branching may proceed from any cell, and it frequently takes the
form of a regular or irregular dichotomy, although sometimes a
spiral phyllotaxis may be found.

The cells vary greatly in size but they always have distinct walls,
which are usually composed of cellulose, and although they are
uninucleate occasionally they become multinucleate. Plastids are
also present, but the green colour is masked by the brown pigment
fucoxanthin. This, how^ever, can be removed by boiling and the
thallus then takes on a green colour from the chlorophyll, the com-
position of which is not quite the same as that of the higher plants
because chlorophyll h is absent (cf. p. 290) whilst xanthophyll is
also missing in the higher members, e.g. Fucales. The products of
assimilation are alcohols, carbohydrates and oils but no true starch
is formed. Hyaline hairs occur in many forms and their function
has been variously ascribed as

(i) shock absorbers,

(2) respiratory and absorptive organs,

CSA

130 PHAEOPHYCEAE

(3) protection against intense illumination,

(4) protection against epiphytes,

(5) protection against covering by sand or silt,

(6) mucilage organs.

None of the evidence for any of these suggestions is entirely
satisfactory, and the whole problem demands further investi-
gation.

Vegetative reproduction may take place by splitting of the thallus
or else by the development of special propagules (Sphacelaria).
Asexual reproduction is commonly secured by means of uni- or
biflagellate zoospores which are normally produced in speciaUzed
cells or sporangia. In one group (Dictyotales) tetraspores replace
the zoospores, these bodies being produced in groups of four in
each sporangium on plants which do not bear sexual organs. In
yet another group (Tilopteridales) asexual reproduction is by means
of uni- to quadrinucleate monospores. The homologies of these
monospores have been subject to much speculation and they have
been variously regarded as equivalent to

(a) propagules of Sphacelaria,

(b) simple forerunners of tetraspores,

(d) parthenogenetic ova.

The second suggestion is perhaps the most satisfactory in our
present state of knowledge, especially when considered in relation
to the vegetative characters. Sexual reproduction ranges from
isogamy, with both gametes motile and characteristically bearing
two flagellae inserted laterally, through a series in which differentia-
tion first to anisogamy and finally to oogamy can be traced. Only
one species (cf. p. 184) is known in which the ova are retained on
the parent plant, so that, apart from this exception, fertiUzation
always takes place in the water. The change from isogamy to
anisogamy is also accompanied by a corresponding differentiation
of the gametangia.

Both unilocular and plurilocular sporangia are commonly found,
but the fate of their products varies considerably (cf. p. 247). Most
species show an alternation of generations, but this is by no means
regular as there may be considerable modifications. Indeed, the
alternation in the Ectocarpales is so irregular that it has been

GENERAL 131

proposed to term the phenomenon a race cycle rather than an
ahernation of generations, and this would appear to be the better
terminology. Furthermore, the two generations are often not the
same in size, and commencing from species with equal morpho-
logical generations one may have those in which either the sporo-
phyte or gametophyte is dominant down to plants where only the
gametophyte or sporophyte is known. A progression in anatomical
development can be traced, but it seems almost impossible to do the
same when the life histories or reproductive organs are considered.
Three principal types of Hfe cycle have, however, been recognized
by Kylin:

(a) Fucus type. Only the diploid sporophytic plant is known,
with meiosis taking place at gametogenesis, e.g. Fucales. This
type, however, is more apparent than real (cf. p. 189).

{h) Dictyota type. Meiosis is delayed and two similar morpho-
logical generations exist, e.g. Dictyotales, Nemoderma, Lithoderma,
Ectocarpus siliculosiis (certain areas only), Pylaiella littoralis. A modi-
fication of this type is found in Ciitleria where the two generations
are of equal significance but morphologically dissimilar, the gameto-
phyte being the larger.

{c) Laminaria type. Meiosis is delayed but the two generations
are wholly dissimilar, the sporophyte being dominant whilst the
gametophyte is much reduced.

Quite a number of species must now be included in this last
category, although the regular alternation may be masked by com-
plications produced by such phenomena as parthenogenetic
development of the ova. Those members of the Heterogeneratae
(excluding the Laminariales) which exhibit this type of alternation
have a fully developed diploid or delophycee form which is common
in summer, and a much reduced haploid or diploid adelophycee
stage which usually appears during the winter months in one of the
following forms :

(a) In a protonemal stage which reproduces the large form by
means of buds.

{b) In a gametophytic prothallial stage which reproduces the
large form by means of gametes from plurilocular sporangia.

{c) In a plethysmothallial stage which reproduces the large form
by means of swarmers from either unilocular or plurilocular

9-2

132 PHAEOPHYCEAE

sporangia. Until recently these were regarded as arrested sporo-
phytes in a juvenile condition.

Fritsch, however, has suggested (1939) that some of these plethys-
mothalli are really potential gametophytes (prothalli), especially
those dwarf plants which perpetuate themselves by means of
plurilocular sporangia. The term '^plethysmothallus" should be
reserved for plants that are diploid and which have arisen from
diploid swarmers produced in plurilocular sporangia on the
macroscopic plants. It has been suggested that this type of alterna-
tion should be termed an alternation of vegetation growths rather
than an alternation of generations, but it is also equally satisfactory
to regard it as heteromorphic alternation.

REFERENCES

Church, A. H. (1920). Somatic Organisation of the Phaeophyceae. Oxf.

Bot. Mem. no. 10.
Fritsch, F. E. (1939). Bot. Notiser, p. 125.
Kylin, H. (1933). Lunds Univ. Arsskr. N.F., Avd. 2, 29, no. 7.
Taylor, W. R. (1922). Bot. Gaz. 74, 431.
Williams, J. Lloyd (1925). Rep. Brit. Ass. Pres. Address, Sect. K, p. 182.

ECTOCARPALES
(ISOGENERATAE AND HETEROGENERATAE)

*EcTOCARPACEAE : Ectocavpus {ecto, external; carpus, fruit). Figs.
89, 90.

The plants are composed of uniseriate filaments which are
sparsely or profusely branched. The erect portion is sometimes
decumbent and arises from a rhizoidal base, which in some of the
epiphytic species occasionally penetrates the host, and it is also
possible that there may be one or two examples of mild parasitism.
E. fasciculatiis even grows on the fins of certain fish in Sweden, but
the nature of the relationship in this case is not clear. The branches
of some species terminate in a colourless mucilage hair : in young
plants of E. siliculosus these hairs are quite long, but later, with
increasing age, they become much shorter through truncation. The
erect filaments have an intercalary growing region, but the
rhizoids increase in length by means of apical growth. Each cell,
which contains one nucleus together with brown disk or band-
shaped chromatophores, possesses a wall that is composed of three
pectic-cellulose layers.

ECTOCARPALES

133

Generally two kinds of reproductive structures are present, the
plurilocular and unilocular sporangia, but some species possess a
third type, the meiosporangia. The unilocular sporangia always
occur on diploid plants and they give rise, after meiosis, to numerous
haploid zooids which may either function as gametes or else develop
without undergoing a fusion. The sporangia are sessile or stalked
and vary in shape from globose to ellipsoid, the mature ones
dehiscing through the swelling up of the centre layer in the wall.
The plurilocular sporangia, which are either sessile or stalked,
range from ovate to siliquose in shape and are to be found on
haploid or diploid thalli. In E. siliculosus they represent modified

C B D ^ E A

Fig. 89. Ectocarpus. A, E. confervoides, plant ( x 0-44). B, E. tomentosus, unilocular
and plurilocular sporangia ( x 100). C, meiosporangium, E. virescens. D, mega-
sporangium, E. virescens. E, microsporangium, E. virescens. (A, B, original;
C-E, after Kniep.)

lateral branches and arise as side papillae from a vegetative cell in
the filament. The plurilocular sporangia are divided up into a
number of small cells, each one of which gives rise to a zooid and,
when ripe, dehiscence takes place by means of an apical pore, the
contents either germinating directly or else behaving as gametes.
The gametes are usually aHke in size but the sex function becomes
weaker with age so that relative sexuality is induced, the older and
weaker gamete behaving as the opposite sex towards the younger
and stronger gamete.

In one species, E. secundus (Gifjordia secundus), there is well-
marked anisogamy because there are two types of plurilocular
sporangia with large or small loculi that produce zooids which diflfer

134 PHAEOPHYCEAE

in size, the smallest being the antheridia and the largest, or mega-
sporangia, the oogonia. The contents of the larger megasporangia
are sometimes capable of parthenogenetic development, when they
must be regarded as incipient or degenerate ova. In E. Padinae
the unilocular sporangia are absent and there are three kinds of
plurilocular sporangia. One type, which has very small loculi,
represents the antheridia, whilst there are also medium-sized or
meiosporangia, and large or megasporangia. The latter probably
represent the female reproductive organs, but there is, at present,
no definite proof for this hypothesis. It has been suggested that the
meiosporangia may be haploid and the megasporangia diploid in
character, but no cytological data appear to be available. In
E. vtrescens unilocular sporangia are absent and there are only
meio- and megasporangia, both of which always occur on separate
individuals. No fusion between zooids from the two types of
sporangia has been observed, but the zooids of the megasporangia
are not very mobile and frequently germinate inside the sporangium.
This may represent a case of apogamy in which sex has been lost,
or it may represent parthenogenetic development of ova because
the male organs (the meiosporangia) have ceased to function. In
any case it must be regarded as a type in which some degeneration
has occurred.

The life cycles of the species are full of interest, especially in view
of what has been discovered for E. siliculosus. Knight (1929) found
that the plants in the Isle of Man occurred in early spring and late
autumn and were all diploid, the haploid generation being unknown.
They bore unilocular and plurilocular sporangia, the former
producing gametes after a reduction division whilst the latter gave
rise to zoospores. In the Bay of Naples, on the other hand, the
large plants were all haploid and only bore plurilocular sporangia.
The zooids from these behaved as gametes, and after fusion meiosis
commonly took place when the zygote commenced to germinate
because it normally developed directly into a new haploid plant.
Berthold recorded a microscopic form which has since been re-
garded as diploid because unilocular sporangia were found on it,
but Knight was unable to find any such dwarf plants.

A schema illustrating these features is seen in fig. 90. It has been
suggested that the differences between the plants from the two
localities are due to differences in the tides, light conditions or

ECTOCARPALES

135

Q-
O

y3iNi/^

rv.

o..

o o

u
o

-a

T3
Ui
O
O
lU
u

<-<

)J3_LNm

136 PHAEOPHYCEAE

temperature, with perhaps most emphasis on the last. A further
study of the NeapoUtan form by Schussnig and Kothbauer (1934)
has subsequently revealed the existence of unilocular sporangia,
although the products from these did not undergo fusion. The
results of this study do not fit in at all satisfactorily with those of
Knight because it will be seen that there is a considerable seasonal
variation (cf. fig. 90). Yet another study of this species has also been
carried out in America by Papenfuss (1935), and his conclusions fit
in fairly well with those of Schussnig and Kothbauer. It would
seem, therefore, that the somewhat more complex schema of these
later workers is probably the more correct, at any rate so far as the
Neapolitan form is concerned. In America the diploid plants were
found growing epiphytically on Chorda or Spartina and these either
bore pluri- or unilocular sporangia independently, or else both
could be found on the same thallus. The unilocular sporangia
occurred only in summer, whilst the plurilocular were present
throughout the whole year. Although the zooids from both types of
sporangia acted as zoospores and germinated directly, nevertheless
meiosis always took place in the unilocular sporangia, the zooids of
which developed into the sexual plants that were found growing as
obhgate parasites on Chordaria, in some cases the nearest asexual
plants being 20 miles distant. It is suggested, therefore, that
dependence of the sexual generation upon a particular host may be
rather more common than is perhaps suspected. The plants growing
on Chordaria were dioecious and only bore plurilocular game-
tangia. It must also be borne in mind that the variations in the life
cycles of the plants from these three localities may be due to genetic
differences because, although the chromosome numbers may be
identical, this would not exclude such a possibility. This extremely
large genus is now subdivided, and recently a number of new
genera have been established (Hamel, 1939).

*EcTOCARPACEAE : Pylaiella (after de la Pylaie, a French botanist).
Figs. 91, 92.

There is only one species, P. littoralis, and although it is said to
possess a number of varieties yet it is by no means certain that they
may not be ecological or seasonal forms because it has been shown
that the movement of the water can even affect the nature of the
branches. In general appearance the plants are very like Ecto-

ECTOCARPALES

137

carpus, and for many years the species was included in that genus.
The branching is opposite or akernate, but the branches do not
end in a mucilage hair as they do in Ectocarpus. Attachment to the
host plants or to the substrate is by means of rhizoidal filaments,
and near the base the main filaments of the erect thallus are
frequently coalesced into a rope-like structure as a result of wave
action. In some places the plants appear to be confined principally

Fig. 91. Pylaiella littoralis. Portion of plant with plurilocular and unilocular
sporangia ( x 200). (Original.)

to certain host plants whilst in other areas there may be no special
hosts. In the Isle of Man Knight (1923) has shown that in the
spring the plants occur on Ascophyllim nodosum, in early summer
they are to be found on Fucus vesiculosus and in late summer on
F. serratus, yet in north Norfolk the species frequently grows on the
stable mud banks of salt marsh creeks or else on F. vesiculosus. On
the Swedish coast three forms have been noted, two of which are
found on Ascophyllum nodosum, whilst the third, which is a
vernal form that dies off at the end of June, occurs attached to
stones. Of the two forms observed on Ascophyllum it is found that
those directly attached to the host are the more numerous, and

138 PHAEOPHYCEAE

although they persist for the whole year they are most fertile in
winter when they produce unilocular sporangia. The other plants
are really epizoic because they grow on the colonies of Sertularia
(a hydroid) that are to be found on the Ascophyllum. These plants,
which only bear plurilocular sporangia, are most vigorous during
spring and early summer and are dead by the end of July.

This species is readily distinguished from Ectocarpus by the
position of the sporangia because these bodies are nearly always
intercalary, very rarely terminal, and when this latter is the case it is
frequently due to the loss of the terminal vegetative portion. The
unilocular sporangia are cask-shaped and open laterally, dehiscence
of the sporangium being brought about by the swelling up of the
middle layer of the wall, but this process is dependent on the

SEASON HOST CHROM. NO. SPORANGIUM SWARMER

Spring

Early
Summer

Late
Summer

>- cx-^^

Ascophyllum or ^ Diploid ^m ^ 0<-^,

F. vesiculosus 2x n ?^ '

Short ^ ^1 1 - J

circuit I ^L R

" ^ -- " ^ 05---,

» V li
Fucus serratus •> Diploid 2x [j >-Q "^" ^^^^lC-^

Seasonal drift ->

Repetition >

Fig. 92. Pylaiella littoralis. The life cycle according to Knight.

temperature of the water when the plant is flooded by the incoming
tide, high temperatures acting in an inhibitory manner. Meiosis
takes place in the unilocular sporangia, and each zoospore when it
finally emerges possesses one nucleus, two plastids and flagellae
and one eye-spot. After emergence the zoospores usually germinate
singly but they have been known to fuse and thus restore the
diploid condition. The plurilocular sporangia, which are produced
on haploid or diploid plants, are oblong or irregularly cylindrical
and also dehisce laterally, each cell producing one zooid which
emerges singly. The zooids from these sporangia either fuse or else
develop at once, the parthenogenetic zooids arising from diploid
sporangia, principally during the summer in England and through-
out the winter in Sweden, although isolated cases may occur at any
time in the year. The other zooids, which function as gametes or
which may occasionally develop parthenogenetically, arise from

ECTOCARPALES

139

haploid sporangia and are most abundant in spring and early
summer. Fig. 92 is a schema taken from Knight (1923) to illustrate
the life cycle as found in English plants during the course of one
year.

EcTOCARPACEAE : Phaeostromu {phaeo, brown; stroma, mattress).

Fig. 93-
This is cited as an example of a much reduced ectocarpoid form
which occurs as an epiphyte or partial parasite upon marine grasses,
such as Zostera, or else upon other brown algae.

Fig. 93. Phaeostroma Bertholdi. Thallus ramifying in Scytosiphon showing
sporangia {s) and a hair {h). (After Oltmanns.)

Mesogloiaceae : Mesogloia {meso, middle; gloia, shme). Fig. 94.

In this and related genera {Castagnea, Eudesme, Chordaria and
Acrothrix) the construction is of the "cable" or consolidated type
described by Church (1920) in which there are one or more erect
parallel strands enclosed in a mucous matrix, the whole being
interwoven with lateral branches. There are three principal zones
that can be recognized in the plant thallus :

(a) a medulla composed either of one long thread accompanied
by offshoots of the first order or else of a group of long threads ;

{h) a cortex of peripheral assimilatory filaments and colourless
hairs ;

{c) a subcortex composed of offshoots from the medulla.

140

PHAEOPHYCEAE

There is also a certain amount of secondary tissue which in some
parts may be rhizoidal in character.

In Mesogloia there is a single central strand terminating in a hair
and having a distinct intercalary meristem just below the apex.

L.T.4^

P.A.

C.HA

CORTEX-

Fig. 94. Mesogloia veryniculata. A, plant (sporophyte). B, apex of filament with
branches and beginning of cortication ( x 135). C, unilocular sporangia. D, pluri-
locular gametangia on gametophyte. E, diagram to illustrate construction of
thallus (central thread type). C.i/. = colourless hair, L.T.= leading thread with
intercalary growth zone, P.y3.= primary assimilator, 5".^. = secondary assimi-
lator, »S.c. = secondar>^ cortex, 6'.5C. «= secondary' sub-cortex. F, diagram to
illustrate life cycle. (A, C, D, after Tilden; B, E, F, after Parke.)

The cortex is formed of short horizontal filaments with somewhat
globose terminal cells that are packed in a gelatinous material.
The hairs, which are frequently worn away in the older parts of the
thallus, occupy a lateral position, but owing to inequahties of

ECTOCARPALES 141

growth they may appear to be terminal. The unilocular sporangia
are ovoid and are borne at the base of the cortical filaments, but the
elongate plurilocular sporangia, which incidentally are only known
for M. Levillei, replace the terminal portion of the assimilatory
hairs and hence are always stalked. Meiosis takes place in the
unilocular sporangia during zoospore formation, and culture
experiments on M. vermiculata carried out by Parke (1933) have
demonstrated conclusively that the adult macroscopic plant of
summer and autumn is diploid, the zooids from the unilocular
sporangia germinating into a minute winter gametophyte (haploid
adelophycee form) that bears plurilocular sporangia of an ecto-
carpoid type. The zooids from these sporangia fuse and the zygote
develops into the characteristic basal disk from which the central
erect filament of the macroscopic plant arises. There is thus
an alternation of morphologically distinct generations in this
species. The fate of the zooids from the plurilocular sporangia
of M. Levillei is not known.

Mesogloiaceae : Eudesme (well-binding). Fig. 95.

E. virescens, which is the type species of this genus, has recently
been removed from the genus Castagnea to which it is very closely
allied in structure. The branched mucilaginous plants differ from
Mesogloia fundamentally in the presence of more than one central
strand in the medulla. The primary filaments in the medulla, which
originate from a basal disk, have an intercalary growing zone and
terminate in a colourless hair, and as branching takes place from
these primary filaments laterals may develop in such a manner as to
make it difficult to distinguish them from the primaries. The cortex
is composed of club-shaped primary and secondary assimilatory
hairs arranged either singly or in falcate tufts. The unilocular
sporangia develop as outgrowths from the basal cells of the primary
assimilatory filaments, whilst the plurilocular sporangia appear in
secund rows on the outermost cells of the same type of filament.
The zooids from the unilocular sporangia germinate immediately,
or else some considerable time may elapse, perhaps as much as
3 years according to some observers, before any development takes
place. They give rise to a microscopic plethysmothallus on which
plurilocular gametangia similar to those of Mesogloia are to be
found. After zooids have been liberated from the plurilocular

142

PHAEOPHYCEAE

sporangia of the plethysmothallus young macroscopic Eudesme
plants appear, so that it may be assumed that there is a definite
akernation of generations in which the small gametophyte forms the
winter phase.

"NEDULLft-

Fig. 95, Eudesme virescens. A, plant. B, diagram to illustrate thallus construc-
tion (multiple strand type). C.i7. = colourless hair, P.^. = primary assimilator,
»S.^. = secondary assimilator, 5'.C. = secondar>' cortex, 5'.-S'c. = secondary sub-
cortex, S.M. = secondary medulla. C, apex ( x 160). D, thallus with branch and
corticating filament ( x 75). E, unilocular sporangia. F, plurilocular sporangia.
(A, E, F, after Oltmanns; B-D, after Parke.)

Mesogloiaceae : Chordaria (a small cord). Fig. 96.

In Mesogloia there is a single central filament whilst in Eudesme
there are several, but in Chordaria development has proceeded a
stage farther and the branched cartilaginous fronds possess a firm,

ECTOCARPALES

143

pseudo-parenchymatous medulla of closely packed cells that have
become elongated in a longitudinal direction. The cortex is com-
posed of crowded, radiating, assimilatory filaments, which are either

Fig. 96. Chordaria divaricata. A, plant ( x f ). B, apex of young plants showing
commencement of cortication. C, apex of older plant of C. flagelliformis showing
structure of thallus. a = assimilator. D, unilocular sporangia ( x 300). (A, D,
after Newton; B, C, after Oltmanns.)

simple or branched, the whole being embedded in a thick layer of
jelly, thus giving the plant a slimy touch. This type of structure,
even though the growth is still confined to the apex, marks the
highest development of Church's consolidated or cable type of

144 PHAEOPHYCEAE

construction. The oblong unilocular sporangia are borne at the
base of the assimilatory filaments, but plurilocular sporangia are
unknown. When this genus comes to be investigated it will prob-
ably be found to have a life history similar to that of the other
Mesogloiaceae.
CoRYNOPHLOEACEAE : Leathesiu (after G. R. Leathes). Fig. 97.

The present genus provides an example of degeneration in the
cable type of construction. The young plant arises from a small,

Fig. 97. Leathesia difformis. A, plants on Furcellaria fastigiata. B, transverse
section to show thallus construction ( x 24). C, unilocular sporangia (X336).
D, plurilocular sporangia (x 336). (A, after Oltmanns; B-D, after Newton.)

creeping, rhizomatous portion and is composed of a packed mass of
radiating, dichotomously branched filaments which are sufficiently
closely entwined to make the plant mass solid. From these medul-
lary filaments there arises a cortex of densely packed assimilatory
filaments. The young plants are subspherical at first, but with
increasing age the central medullary filaments commence to disin-
tegrate and as a result the mature thallus becomes hollow and
irregularly lobed. Plurilocular and unilocular sporangia are known,
the zoospores from the ovoid unilocular sporangia germinating to
disk-like plantlets 'on which plurilocular gametangia ultimately
appear. These plantlets either give rise to other similar plantlets or

ECTOCARPALES

else to the adult thallus once more. By analogy with other species
the dwarf plantlets with the plurilocular sporangia may be regarded
as haploid gametophytes.

Elachistaceae : Elachista {elachistos, very small). Fig. 98.

Church (1920) regarded this genus as being explicable morpho-
logically on the cable type of construction, although it must be

Fig. 98. Elachista fucicola. A, plants on Fucus vesiculosus ( x 0-36). B, single
plant in section showing penetrating base, crowded sporangia, short paraphyses
and long assimilators. C, unilocular sporangia ( x 120). D, plurilocular sporangia
at base of assimilation thread ( x 220). (A-C, after Taylor; D, after Kylin.)

regarded as a degenerate type in which the true structure is only
seen in the sporeling. This possesses a horizontal portion from
which a number of erect filaments arise, so that in the early stage it
is comparable morphologically to Eudesme. In the older plant the
erect filaments have developed to form a cushion composed of
densely branched filaments matted together and only becoming
free at the surface. The various species are epiphytic on other algae,
Elachista fucicola being especially abundant on species of Fucus.
The pluri- and unilocular sporangia, together with the long hairs,

CSA

146

PHAEOPHYCEAE

arise from the base of the short filaments or paraphyses. The zooids
from the unilocular sporangia germinate in late autumn to give a
branched, thread-like, microscopic gametophyte which persists
throughout the winter. In late winter and spring plurilocular
sporangia develop on the minute gametophytes, and when the
zooids have been liberated they fuse and the zygote germinates into
a new macroscopic Elachista plant.

Myrionemaceae : Myrionema (myrio, numerous; nema, thread).
Fig. 99.

The various species are epiphytic upon other algae, forming thin
expansions or minute flattened cushions or disks that are very

Fig- 99- Myrionema strangulafis. A, young plant ( x 640). B, plurilocular
sporangia ( x 340). C, unilocular sporangia ( x 340). D, 11 -day-old plant from
zoid of unilocular sporangium ( x 336). (After Kylin.)

variable in shape and from which numerous, closely packed, erect
filaments and hairs arise. M. strangulans is especially common on
sheets of Ulva during the summer. The basal portion of the
thallus has a marginal growing region and is composed of crowded
radiating filaments that may, on rare occasions, penetrate the host
plant. The unilocular sporangia, which are not borne on the same

ECTOCARPALES

147

plants with plurilocular sporangia, give rise to haploid zooids, and
these develop into a thread-like gametophytic plant bearing long
filaments, the possession of this type of gametophyte indicating
that the genus is perhaps more closely allied to the Mesogloiaceae
than to the Ectocarpaceae.

Spermatochnaceae : Spermatochnus (sperma, seed; tochniis, fine
down). Fig. 100.

This is essentially one of the corticated types, the filamentous,
cylindrical, branched thallus being derived primarily from a central

E A B C

Fig. 100. Spermatochnus paradoxus. A, plant ( x 0-44). B, apex of young plant
showing origin of cortication. C, portion of old thallus showing structure,
a = assimilator, c = cortical cells, c/= central filament, /i = hair, m = mucilage.
D, portion of thallus showing cortication and pairs. E, paraphyses and unilocular
sporangia ( x 200). (A, E, after Newton; B-D, after Oltmanns.)

axis composed of a single filament with a definite apical cell. Each
individual cell of this filament segments at one end and so definite
nodes are formed. The corticating filaments arise from the nodes,
and grov^h of the cortex is secured by tangential division of the
primary corticating cells, though later more filaments may grow on
top of them. The outermost layer of the cortex bears the assimi-
latory filaments and hairs. As the plants become older mucilage
develops internally and forces the cortex away from the primary
central filament although a connexion is maintained by the threads

10 .2

148

PHAEOPHYCEAE

from each node. Unilocular sporangia, together with clavate
paraphyses, develop in sori, the sporangia arising from the base of
the sterile threads. The life cycle has not yet been worked out, but
if it is at all comparable with the other closely related genera then
the zooids should give rise to a microscopic gametophyte genera-
tion.

Sporochnaceae : Sporochnus {sporo, seed; chnus, wool). Fig. 10 1.

The thallus, which is composed of an inner layer of large cells
with an outer layer of small assimilatory cells, is filamentous with

Fig. loi. Sporochnus pedunculatus. A, plant. B, fertile branch with receptacle.
s = sorus. C, unilocular sporangia (m). e = empty sporangium. (After Oltmanns.)

branches arising alternately and arranged in one plane. On
account of its structure Church considered that it really belonged
to the parenchymatous type of construction, and morphologically it

ECTOCARPALES

149

may represent a transition stage from the corticated to the paren-
chymatous type, although it is usually considered that these two
forms of thallus arose independently. The unilocular sporangia,
which are club-shaped, are borne on branched monosiphonous
filaments crowded together in large oval or elongate receptacles
that bear a cluster of hairs at their apex. This type of reproductive
structure is more or less unique among the simpler forms of the
Phaeophyceae.

ScYTOSiPHONACEAE : Phyllitts (phyllos, leaf). Fig. 102.

The unbranched fronds are expanded, membranous, leaf-like
structures with an internal medulla composed of large, colourless

Fig. 102. Phyllitis Fascia. A, plant ( x f). B, transverse section of thallus with
plurilocular sporangia ( x 375). (A, original; B, after Setchell and Gardner.)

cells and an outer layer of small, superficial, assimilatory cells.
Unilocular sporangia are not known nor are there any paraphyses.
The plurilocular sporangia, which are arranged at right angles to
the surface, arise from the superficial cells and produce zooids that

150 PHAEOPHYCEAE

germinate to give a creeping basal thallus from which a new plant
arises. It is therefore suggested that the plants are wholly diploid
and that the haploid generation has been lost. Yendo (1919),
however, has reported that these zooids can develop after a resting
period into minute protonemal threads bearing antheridia and
oogonia which presumably produced gametes, although no sign of
fertilization was observed. If these observations are correct this
genus must be regarded as anomalous, because normally the
gametophytic generation arises from the products of unilocular
sporangia. It would therefore seem premature to accept this
peculiar life cycle without further evidence, and at present it would
be more in agreement with known life cycles if the plants are simply
regarded as being wholly diploid and without a haploid generation.
In the related genus Scytosiphon it would also seem that only the
diploid generation is present and that the reported protonemata are
not gametophytic as has been suggested by some workers.

Dictyosiphonaceae: Dictyosiphon {dictyo, net; siphon, tube). Figs.

103, 105.

The filamentous plants arise from small lobed disks and have
either a few or many branches, the younger ones commonly being
clothed with delicate hairs. There is a central medulla of large
elongated cells and a cortex of small cells, but in old plants the
medulla is often ruptured and the axis becomes partially hollow.
On the macroscopic plants only unilocular sporangia are found,
each of which is formed from a single subcortical cell. Meiosis
takes place in these sporangia and the zooids germinate to form
microscopic prothalli : these represent the gametophytic generation
and reproduce by means of plurilocular gametangia. The gametes
either develop parthenogenetically into a new protonema or else
two of them, coming from different gametangia, will fuse and the
zygote develops into a small ectocarpoid-like plant. This may either
reproduce itself by means of plurilocular zoosporangia or else it
develops into a plantule from which the adult sporophyte arises.

AsPEROCOCCACEAE : Aspewcoccus (aspero, rough ; coccus, berry). Figs.

104, 105.

The structure of the adult plant is essentially the same as that
of the two preceding genera except that the central filaments

B ' A

Fig. 103. Dictyosiphon. A, plant. B, portion of thallus of the closely related
genus Gobia showing mode of construction with hairs, unilocular sporangia and
assimilators. C, unilocular sporangia of Gohia. (After Oltmanns.)

Fig. 104. Asperococcus bullosus. A, plant. B, unilocular sporangia ( x 225).
C, plunlocular sporangia ( x 225). (A, after Oltmanns; B, C, after Newton.)

152

PHAEOPHYCEAE

degenerate and the centre becomes filled with a gas. The fronds are
simple or branched and bear small superficial cells with sporangia
and mucilage hairs scattered over the surface in sori. The pluri-
locular and unilocular sporangia occur on the same or on different
plants, the sori with unilocular sporangia containing sterile para-
physes in addition. The principal interest of this type is centred
around its life history which has been studied by several workers in

DictuoALbhon
jdanl

t.x

c<.

'l^LiintuLe <-

DICTY05IPH0N

^CK

]

Ect.bUrxt

5motkd.liu£> —

etrel/loTierm. — > Yt — >o<

A5PEf\OCOCCU5

Zx.

Fig. 105. Diagram of life cycles of Dictyosiphon foeniculaceus and Asperococcus
hullo sus.

considerable detail. In A. compressus the life cycle is simple, the
zooids from the unilocular sporangia germinating directly into a
protonemal phase that later turns into small plantules; these can
reproduce themselves successively by means of zoospores from
both pluri- and unilocular sporangia until the advent of favourable
conditions enables the development of the macroscopic phase to
take place once m.ore. There is no evidence of either meiosis or of
gametic fusion. In A.fistulosus it would appear that the life cycle is
dependent upon the behaviour of the zooids from the unilocular

ECTOCARPALES

153

sporangia where meiosis has been shown to take place. If they fuse,
the zygote develops first into a '' strehlonema'' phase, so-called
from the brown alga it resembles, and then into a plantule from
which a new adult plant can arise. In this case there is no evidence
for the existence of a gametophytic generation, nor has any evidence
been obtained to show that such streblonemoid plants can re-
produce themselves by means of sporangia. If no fusion of the
zooids from the unilocular sporangia takes place the '' strehlonema''
phase is again produced parthenogenetically, but under these
circumstances plurilocular sporangia are formed which give rise to

/ADULTS

AUTUnN

^v.

•.Old. bTotonema.

WINTER

-^ADULT
DioecloLLS

SPRING-

/ V

barlKerio^cnsiLi

5unnER
COLPOMENIA 51NU05A

Fig. 106. Life cycle of Colpomenia sinuosa. RD = probable place of reduction
division in life cycle. (Modified from Kunieda and Suto.)

a new '' strehlonema'' generation, nor has any investigator under
such conditions succeeded in obtaining macroscopic plants
again and so it has been suggested, therefore, that sex has been
inhibited in these plants. In A. bullosus the zooids from the
plurilocular sporangia on the macroscopic thallus do not fuse but
germinate directly to give rise to a series of plethysmothalli bearing
plurilocular sporangia : these tide over the winter season, and then in
spring young Asperococciis plants develop in place of the sporangia
on the ectocarpoid plantules. In the unilocular sporangia meiosis
takes place and the zooids develop into minute gametophytic
plants that produce plurilocular sporangia. If the gametes from
these sporangia fuse the zygote develops into a plantule from which
a new macroscopic plant arises, but if there is no fusion then they

154 PHAEOPHYCEAE

merely develop into a new gametophytic generation. Sauvageau
also reported that the zooids from the unilocular sporangia may
give rise to creeping filaments which later produce young plantules
of Asperococciis. This direct reproduction of the macroscopic
plants can only be explained by a premature abnormal fusion of
some of the zooids from the unilocular sporangia. It will be
evident that direct alternation of generations is obscured in this
type through the number of possible independent circuits and
"short cuts". Recently the life cycle in Colpomenia sinuosa, a
member of a closely allied genus, has been described in detail (cf.
fig. 1 06). The adult plants are like Leathesia in appearance, although
they are essentially parenchymatous in structure. It will be seen,
however, that there are two morphologically similar generations,
the dioecious gametophytes appearing in spring and reproducing
by means of anisogametes that are formed in dissimilar gametangia.
The zygote gives rise to new adult asexual plants that reproduce by
means of plurilocular sporangia in autumn.

REFERENCES

Ectocarpus. Hamel, G. (1939). Bot. Notiser, p. 65.
Pylaiella. Knight, M. (1923). Trans. Roy. Soc. Edinb. 53, 343.
Ectocarpus. Knight, M. (1929). Trans. Roy. Soc. Edinb. 56, 307.
Asperococcus. Knight, M., Blackler, M. C. H. and Parke, M. W.

(1935). Trans. Lpool Biol. Soc. p. 79.
Colpomenia. Kunieda, H. and Suto, S. (1938). Bot. Mag., Tokyo, 52,

539.
General. Kylin, H. (1933). Lunds Univ. Arsskr. N.F. Avd. 2, 29, no. 7,

p. I.
Ectocarpus. Papenfuss, G. (1935). Bot. Gaz. 96, 421.
Mesogloia, Castagnea, Acrothrix. Parke, M. W. (1933). Publ. Hart. Bot.

Lab. no. 9.
Ectocarpus. Schussnig, B. and Kothbauer, E. (1934). Ost. Bot. Z. 83, 81.
Phyllitis. Yendo, K. (19 19). Bot. Mag., Tokyo, 33, 171.

CUTLERIALES (ISOGENERATAE)

This order is characterized by trichothallic growth, regular
alternation of generations, and a well-marked anisogamy which in
some respects approaches oogamy. They are generally placed in the
Isogeneratae, even though this leads to a difficulty because in
Cutleria the two generations are not equal morphologically
although they are equal in Zanardinia. On this classification,

CUTLERIALES (ISOGENERATAE) 155

therefore, Cutleria must be regarded as a modified member of the
Isogeneratae or else it must be separated from Zanardinia and put
in a separate family in the Heterogeneratae.

CuTLERiACEAE : Cutleria (after Miss Cutler). Fig. 107.

The gametophyte and sporophyte generations are distinctly
heteromorphic and also differ in their seasonal occurrence, the
former being a summer annual whilst the latter is a perennial
reaching its maximum vegetative phase in October and November

Fig. 107. Cutleria multifida. A, plant ( x i). B, young seedling. C, seedling
slightly older to show branching. D, transverse section of thallus with unilocular
sporangia. E, female gametangia. F, male gametangia. G, ''Aglaozonia" stage.
(A, original; B-D, G, after Oltmanns; E, F, after Yamanouchi.)

with a peak fruiting period in March and April. The gametophyte is
an erect flattened thallus rendered fan-like because of the repeated
dichotomies. The thallus and apices are clothed with tufts of hairs,
each with a basal growing region, whilst the oogonia and antheridia,
which are borne on separate plants, occur in sori on both sides of
the thallus. The antheridia, with which hairs are sometimes
associated, are formed in clusters from superficial cells of the
thallus that divide to produce a stalk cell and an antheridium
initial. The mature antheridium contains about 200 antherozoids,
each of which possesses two chromatophores, and they are much

ir6 PHAEOPHYCEAE

smaller than the mature ova, each of which contains thirty or more
chromatophores.

The oogonia, with which hairs are sometimes associated, are
also formed from superficial cells which divide into a stalk cell
and an oogonium initial. The ripe oogonium contains sixteen to
fifty-six eggs which, after liberation, remain motile for a period of
from 5 min. to 2 hours, whilst the antherozoids can remain active
for about 20 hours. E)ischarge of the gametes takes place at any
time during the day but is at its best about 5 a.m., fertilization
taking place in the water when the diploid number of chromosomes
(48) is restored. Upon germination a small columnar structure is
first formed and then a flat basal expansion grows out from its base
to form the adult sporophyte, which is a prostrate expanded thallus
attached to the substrate by means of rhizoids. It differs so very
much from the gametophyte that w^hen first found it was thought
to be a separate genus and was given the name of Aglaozonia. It
sometimes happens that the ova do not become fertilized, and when
this happens they germinate parthenogenetically to give haploid
Aglaozonia plants, but these do not bear any reproductive organs.
The sporophytic thallus is composed of large cells in the centre with
superficial layers, both top and bottom, of small cells. The sessile
unilocular sporangia, sometimes accompanied by deciduous hairs,
are borne in palisade-like sori or else are scattered irregularly on the
upper surface of the thallus. Each superficial cell first divides into
a stalk cell and sporangium initial, then meiosis occurs and
eventually eight to thirty-two zoospores are formed in each
sporangium. The zoospores on germination give rise to new
Cutleria plants. This life cycle was first worked out by Yamanouchi
(191 2) for the common species Cutleria multifida and its sporophyte
Aglaozonia reptans.

SPHACELARIALES (ISOGENERATAE)

The next three tj^'pes belong to the Sphacelariales, an order
frequently known as the " Brenntalgen " because they possess a
very characteristic large apical cell with dense brown contents, the
detailed classification of the group being based primarily upon the
behaviour of this apical cell at branch formation. The plants have
regular branching and a bilateral symmetry, both of which form
characteristic features. Structurally they can be regarded as

SPHACELARIALES (ISOGENERATAE) 157

strengthened multiseriate filaments, Sphacella perhaps being one of
the more primitive members of the group with a non-corticate
monosiphonous axis.

*Sphacelariaceae : Sphacelaria (gangrene). Fig. 108.

The plants are filamentous with hypacroblastic branching in
which the cell below the apex cuts off two branch initials opposite to

G E

Fig. 108. Sphacelaria. A, plant of S. cirrhosa ( x i). B, apical cell (a) of
S. cirrhosa. C, S. cirrhosa, origin of hair (h). a = apical cell. D, hair at older stage;
<2 = apical cell. E, apex of thallus of S. plumigera showing branches, b; single
segment {s), which later divides into upper {us) and lower {Is) segments ; a = apical
cell. F, origin of branch, b. G, bulbil of S. cirrhosa { x 52-5). H, unilocular
sporangia, S. racemosa. I, zoospore of S. bipinnata { x 1200). J, K, germinating
spore of S. bipinnata (X1200). (A, original; B-G, after Oltmanns; H, after
Taylor; I-K, after Papenfuss.)

each other, although in some cases the initials may remain dormant.
The plants grow attached to stones or other algae by means of
basal disks or rhizoids that have spread down from the lower cells
of the stalk. Mucilage hairs, which arise from the apical cell through
a segment being cut off obliquely, are present in some species
though they may disappear with age. The axis and main branches
form a solid frond due to cortication which commences near the

158 PHAEOPHYCEAE

apex through a series of transverse and longitudinal divisions, until
finally there is an external layer of rectangular cells arranged in a
polysiphonous manner. Unilocular and plurilocular sporangia are
formed on short pedicels, usually on separate plants.

Clint (1927) has studied in some detail the life cycle of S. bipinfiata,
which grows epiphytically on Halidrys (cf. p. 203) in the Irish Sea,
whilst farther south it frequents Cystoseira (cf. p. 205). Although
primarily an epiphyte it is probable that the species is parasitic to
a certain extent. Meiosis occurs in the unilocular sporangia, and
after the zooids have been ejected all together in a gelatinous mass
they fuse in clumps of t^vo to five, the cytology of the clumps being
unknown. Isolated spores may germinate, but under these circum-
stances the sporeUng soon dies. The plurilocular sporangia are
sometimes stalked, and so it is suggested that morphologically they
may be equivalent to branches. The zoospores from these sporangia
are smaller and only contain two plastids as compared with those
from the unilocular sporangia which contain four. Yet another
distinction is that they emerge singly and do not fuse, but germinate
immediately on settling.

Reproduction in this species occurs in early summer and late
autumn, and whilst a certain amount is now known about its life
history it is still a mystery as to how or in what state it survives the
winter. In the north Halidrys breaks off in the winter and no trace
of any Sphacelaria plants can be found on the stumps. It is now
known that the unilocular sporangia are the asexual organs and that
these plurilocular sporangia merely reproduce the diploid generation.
The morphologically similar gametophyte generation has since been
found and it gives rise to isogametes from plurilocular sporangia.
There is thus a regular alternation of generations which agrees with
the facts for other members of the family. Vegetative reproduction
also takes place in this genus by means of modified branches or
propagules which are usually pedicellate and triradiate, the actual
shape varv'ing for the different species. The tropical species, S. tribu-
loides, is said to form the common food of manv Hawaian fishes.

Cladostephaceae : Cladostephus {dado, shoot; stephus, a crown).
Fig. 109.

The plants, which are bushy in appearance, arise from well-
developed holdfasts and are characterized by the ecorticate branches

SPHACELARIALES (ISOGENERATAE) 159

being arranged in whorls with tufts of hairs just below their apices.
Cells just below the apex divide to give a number of branch
segments, this type of branching being known as polyhlastic. The
main axis is corticate and primarily polysiphonous, because the
subterminal cells divide to form cortical cells which then divide
again several times, but as there is also an outer cortex of rhizoids
a pseudo-parenchymatous structure is ultimately formed. Both
unilocular and plurilocular sporangia are formed on stichidia which

Fig. 109. Cladostephus verticillatus. A, plant (x ^). B, apex to show origin of
branch. 6a = branch apex. C, thallus showing cortication. D, unilocular
sporangia ( x 225). E, part of thallus with unilocular sporangia ( x 45). (A, D, E
after Newton; B, C, after Oltmanns.)

arise from the rhizoidal cortex in the internodes between the
whorls of vegetative branches. The two different types of sporangia
occur on separate plants, the unilocular on what must be the diploid
generation as they produce zoospores, and the plurilocular on what
must be the haploid generation because they produce isogametes.

Stypocaulaceae : Stypocaulon (stypo, coarse part of flax; caulon,
stem). Fig. no.

The pinnate frond arises from a well-marked basal system, the
plants in summer having the appearance of shaggy tufts whilst in

i6o

PHAEOPHYCEAE

winter they are more regularly pinnate owing to the shedding of
branches. The inner cortex of the central axis is composed of a
number of cubical cells whilst there is also an outer cortex of
rhizoidal cells, the whole forming a pseudo-parenchyma. Any cell,
whether in the inner or outer cortex, can develop a new apical cell
upon injury, so that there is a great power of regeneration of apical

Fig. no. Stypocaulon scoparium. A, summer form ( x |). B, winter form ( x |).
C, apex, showing branches with cortication. D, unilocular sporangia. (A, B,
original; C, D, after Oltmanns.)

cells. Only unilocular sporangia are known, and these are formed
in groups of up to fifteen on a pad of tissue in the axil of each
branch on the fertile shoot, but as in Cladostephus secondary
sporangia may arise within the empty sheaths of the old ones.
Meiosis takes place in these sporangia, and the zooids on germina-
tion give rise to new Stypocaulon plants. One herbarium plant of
S. scoparium with antheridia and oogonia has been reported, but
these may have been abnormal unilocular sporangia. In view of the
great interest of this observation, however, it would be very
desirable to have a further study made of this alga.

TILOPTERIDALES {ISOGENERATAE)

Haplospora (haplo, simple; spora, seed). Fig. in.

This and Acinetospora belong to a peculiar group of algae, the
life cycles of which are somewhat incompletely known, but it is

TILOPTERIDALES (ISOGENERATAE) i6i

possible that they represent a transition towards the tetrasporic
Dictyotales (cf. p. 163). The chief characteristic of the asexual plant
is reproduction by means of quadrinucleate monospores which may
be equivalent to unsegmented or primitive tetraspores, although
they might equally well be degenerate tetraspores. Sexual repro-
duction is brought about by means of gametes from microgametangia
and larger associated sporangia that may represent oogonia, but

Fig. III. Haplospora globosa. A, portion of plant with uninucleate sporangia,
m (oogonia?), and plurilocular microgametangia, p. B, plurilocular micro-
gametangium. C, monosporangium with quadrinucleate monospore. D, mono-
sporangia. E, F, unilocular sporangia (oogonia?). (A-C, after Oltmanns, D-F,
after Tilden.)

as fertiUzation has not been observed there is an opportunity here
for future research which should also determine whether there
is a regular alternation of generations. The evidence at present
available suggests that there is probably a regular alternation of two
similar generations.

The sexual plants develop intercalary tubular microgametangia
which are produced by the transformation of one or more cells of
the main filament. Besides these organs there are the larger and

CSA

1 1

i62 PHAEOPHYCEAE

spherical uninucleate sporangia (oogonia?) borne on a stalk cell and
partly immersed in the branches. The asexual plant reproduces by
means of quadrinucleate spores formed singly in stalked or sessile,
terminal or intercalary, monosporangia. Meiosis has been reported
as occurring in these sporangia and this would be expected if they
were primitive tetraspores. It would appear, according to some
accounts, that the plants known as Haplospora glohosa and Scapho-
spora speciosa are simply alternate phases of one and the same
species.

In Acinetospora the plant structure is very simple, the slender,
tufted thallus being monosiphonous throughout and frequently
unbranched or else with very occasional branches. No fusion of
zooids from either the uni- or plurilocular sporangia has been
observed, and so this alga must either be regarded as the simplest
member of the Tilopteridales in which sexuality has not yet wholly
developed, or else as a degenerate member in which the sexual
organs have been lost or highly modified. This latter view is prob-
ably the more satisfactory in view of the position of the family as
a whole.

Plants with unilocular sporangia only occur in April and May
and the swarmers give rise to plants bearing plurilocular sporangia.
It has recently been suggested that the monospores are a means
of vegetative reproduction, e.g. morphologically equivalent to
propagules.

REFERENCES

Sphacelaria. Clint, H. B. (1927). Publ. Hart, Bot. Lab. no. 3, p. i.
Stypocaulon. Higgins, E. M. (193 i). Ann. Bot., Lo?td., 45, 345.
Acinetospora. ScHMmx, P. (1940). Ber. dtsch. Gesell. 58, 23.
Cutleria. Yamanouchi, S. (1912). Bot. Gaz. 54, 441.

CHAPTER VII

PHAEOPHYCEAE (cont.)

DICTYOTALES, LAMINARIALES AND FUCALES
DICTYOTALES (ISOGENERATAE)

*Dictyotaceae: Dictyota (like a mat). Fig. 112.

This genus is representative of the Dictyotales, an order character-
ized by a well-marked regular alternation of two identical genera-

Fig. 112. Dictyota dichotoma. A, portion of plant showing regular dichotomy.
B, apical cell. C, apical cell divided. D, group of antheridia surrounded by
sterile cells. E, single antheridial cell and a sterile cell. F, sorus of oogonia.
G, tetrasporangium. (A-D, F, G, after Oltmanns; E, after Williams.)

tions. Asexual reproduction is brought about by means of tetra-
spores produced in superficial tetrasporangia, whilst the sex
organs, which are heteromorphic, are always borne in sori. The
thallus possesses a specialized bilaterality with well-marked apical
growth.

In Dictyota, as represented by the cosmopolitan species D. dicho-
toma, the flattened thallus exhibits what is practically a perfect
dichotomy because there is always a median septation of the apical
cell. Viewed in transverse section the thallus is seen to be composed
of three layers, a central one of large cells and an upper and lower

11-2

i64 PHAEOPHYCEAE

epidermis of small assimilatory cells from which groups of mucilage
hairs arise.

The male and female sex organs are borne in sori on separate
plants, the male sorus being composed of as many as 300 pluri-
locular antheridia surrounded by an outer zone of sterile cells. At
the formation of an antheridium a superficial cell divides into a
stalk cell and an antheridium initial, the final partition of the
antheridium initial into the individual antheridial mother cells
taking place only a few days before the antherozoids are to be
liberated. The mature antherozoid is pear-shaped with only one
cilium, and as each plurilocular antheridium liberates about 1500
antherozoids, a single sorus may generate as many as 450,000. The
number of ova produced are not so numerous, and it has been
estimated that there are about 6000 antherozoids available for each
ovum. The oogonial sorus is very similar to the antheridial sorus,
the large fertile cells, twenty-five to fifty in number, being situated
in the centre and surrounded by sterile cells on the outside. The
oogonia likewise arise from superficial cells that divide into a stalk
cell and oogonium initial, and each oogonium when ripe produces
one ovum. Liberation of both kinds of gamete usually commences
from the centre of a sorus and fertilization takes place in the water,
but during the process the eggs are not caused to revolve by the
activities of the antherozoids as they are in Fucus (cf. p. 197). If
the process is followed under a microscope it can be noted that only
some of the eggs appear capable of attracting antherozoids, whilst
the unfertilized ova develop parthenogenetically ; such plants,
however, always die in culture, though it is possible that in nature
they may persist. The sex organs are produced in regular crops, the
new sori appearing between the scars of the old, and when the
whole of the surface has been used up the plant dies.

After fertilization the zygote develops into a morphologically
similar plant which reproduces by means of tetraspores that are
formed in tetrads in superficial sporangia. At sporangium formation
an epidermal cell swells up in all directions, and after a stalk cell
has been cut ofir the sporangium initial divides twice to give the four
tetraspores, during which the thirty-two diploid chromosomes are
reduced to the haploid number of sixteen. A tetraspore at the time
of liberation is an elongated body and grows at once into a new
sexual plant. In some cases, however, the tetrasporangium fails

DICTYOTALES (ISOGENERATAE) 165

to divide into four spores but germinates as a whole and this
phenomenon probably explains the abundance of sporophytic
plants in certain localities, although the conditions that cause this
abnormality have not yet been discovered. Whilst the sex organs
are produced in rhythmic crops there is no such periodicity in the
case of the tetraspores, and here again there is scope for further
research.

In the related genus Taonia the asexual plant bears tetrasporangia
and hairs in zonate bands across the thallus, and there is some evidence
for a correlation between the tides, or perhaps the Hght conditions
of each intertidal period, and the development of the zones. Each
zone probably corresponds to a single tidal period because a plant
30 days old was found to possess sixty zones of tetrasporangia. The
period between the initiation of each new crop is probably required
in order that the plant may accumulate the necessary food material.
In Taonia also the asexual plants are frequently more abundant than
the sexual, but this is partly accounted for by the persistence of a
sporophytic rhizoidal portion that can give rise to new plants.
More commonly, however, the tetrasporangium fails to divide and
the whole structure germinates before meiosis has taken place.
Plants formed in this way are found to be more resistant and
vigorous than the plants produced from normal tetraspores, and this
may be due to the larger supply of food material available from a
complete sporangium.

Three kinds of rhythmic periodicity for the sex organs of
Dictyota have been described from different localities :

{a) In Wales the sori require 10 to 13 days to develop whilst in
Naples 15 or 16 days are necessary, the gametes being liberated
about once a fortnight in both areas.

{h) In North Carolina liberations occur once a month, at the
alternate spring tidal cycles, although only 8 days are required for
the development of the sex organs. This suggests that the plants
are exhausted after each fruiting and a resting period is necessary
in order to recuperate.

{c) In Jamaica the successive crops take a very long time to
mature, e.g. very little change can be seen even after 22 days. This
results in almost continuous fruiting with two successive crops
overlapping. One very significant feature is that the commonest
species, D, dichotoma, apparently behaves as described above in

i66 PHAEOPHYCEAE

each of the three locahties. It is, however, possible that there is a
genetical distinction between the plants from the different localities
and an investigation along these lines might prove very profitable.
Wherever the plants occur the bulk of the gametes (60-70 %) are
usually liberated in a single hour at about daybreak. On the Welsh
coast the gametes are set free just after each series of high spring
tides during July to October, and it has been suggested that light
plays the part of the determining factor during the intertidal
periods. However, when plants were removed to the laboratory it
was found that the periodicity was maintained, so that it must be
inherited, whilst plants from Carolina likewise retained their
periodicity when transferred to the laboratory, the specimens
fruiting at the same time as those living under natural conditions.
The mean tidal differences vary considerably in the four localities,
ii-i8ft. in England, o-8 ft. at Naples, 3-0 ft. in North Carolina,
and 0-8 ft. in Jamaica. These differences preclude either light or
tidal rise from being the controlling factor because the English and
Neapolitan plants behave similarly even though there is a great
difference in the tides. Regularity of the tidal cycle, however, may
modify the reproductive cycle, because where the tides are some-
what irregular, as in Jamaica, the reproductive rhythm is also
irregular. This rhythmic behaviour is probably not due to any one
factor but has been acquired over a long period of time as a response
to the environment and is now inherited. The phenomenon is not
confined to Dictyota because regular or irregular periodic cropping
has been recorded for species of Sargassum, Halicystis, Cysto-
phyllum, Padina and Nemoderma. Culture experiments are
required in order to determine whether the habit persists in suc-
cessive generations when they are grown under completely non-
tidal conditions, but unfortunately Dictyota has not proved very
amenable to cultural conditions. Finally, it can be argued that
tides and light may have no control over this rhythm and that it
may be associated instead with lunar periodicity, in which case even
cultures will be of no avail. It has been observed that the plants in
North Carolina always fruited at the time of full moon, and it is a
well-known fact that a number of marine animals spawn regularly
at such a period. At present the lunar explanation would appear to
be the most satisfactory, but even that produces difficulties when
the behaviour of the species in Jamaica is considered.

LAMINARIALES (HETEROGENERATAE) 167

REFERENCES

Dictyota. Hoyt, W. D. (1907). Bot. Gaz. 43, 383.

Dictyota. Hoyt, W. D. (1927). Amer. jf. Bot. 14, 592.

Taonia. Robinson, W. (1932). Ann. Bot., Land., 46, 113.

Dictyota. Williams, J. Lloyd (1904). Ann. Bot., Lond., 18, 141, 183.

Dictyota. Williams, J. Lloyd (1905). Ann, Bot., Lond., 19, 531.

*LAMINARIALES {HETEROGENERATAE)

The Laminariales form an order which is principally temperate,
the bulk of the species being confined to the colder waters of the
earth, and there are, in particular, a number of monotypic genera
confined to the Pacific coast of North America. The presence of
such genera suggests that the original centre of distribution was in
the Pacific waters that surround Japan and Alaska. The thallus,
representing the large conspicuous sporophytic generation, is
nearly always bilaterally symmetrical with an intercalary growing
zone, whilst the gametophytes are microscopic. The sporophytes
reproduce by means of unilocular zoosporangia, commonly formed
in sori with paraphyses, whilst the gametophytes reproduce by
means of ova and antherozoids that are borne on separate plants.

* Chord aceae: Chorda (a string). Fig. 113.

The long whip-like thallus, which is clothed in summer with
mucilage hairs, arises from a small basal disk with the growing
region situated just above the holdfast. The hollow fronds are
simple with diaphragms at intervals, the construction of the
thallus being essentially that of a multiseptate cable derived from
the Mesogloia type by further segmentation of descending hyphae to
form a pseudo-parenchyma. The epidermal layer is ultimately
clothed with sporangia, paraphyses and deciduous mucilage hairs,
whilst the central cells become much elongated and support the
filaments that go to form the diaphragm. The zoospores on germina-
tion give rise to small filamentous gametophytes, the male plants
being composed of small cells, each with two to four chloroplasts,
and the female of larger cells with more numerous chloroplasts.
The gametangia are borne laterally or terminally on short branches,
but the plants do not become fertile for at least 3 months after their
formation and they usually require 6 months. After fertilization
the oospore remains attached to the wall of the oogonium. The

1 68

PHAEOPHYCEAE

macroscopic plant is an annual, being abundant in the colder
waters of both hemispheres.

'k^^Slii:j^B

Fig. 113. Chorda Filum. A, plant ( x |). B, transverse section, high-power, with
sporangia. C, female gametophyte ( x 145). D, male gametophyte (X175).
(A, original; B, after Oltmanns; C, D, after Kylin.)

Desmarestiaceae : Desmarestia (after A. G. Desmarest). Fig. 114.

The plants are bushy and usually of some considerable size,
especially the species found on the Pacific coast of North America.
They sometimes bear gall-like swellings which are caused by a
copepod, and similar galls caused by the copepod Harpacticus
chelifer have been recorded from the red alga Rhodymenia palmata.
The erect, cylindrical or compressed thallus arises from a disk-like
holdfast and exhibits regular pinnate branching, the branches either
being elongate or else mere denticulations. The elongate branches
terminate in much-branched uniseriate filaments, which are also to
be found on the denticulations, but as these filaments are deciduous
the plants have a definite winter and summer aspect. Morpho-
logically, the thallus is composed of a single prominent central row

LAMINARIALES (HETEROGENERATAE) 169

of large cells, and these are' surrounded by cortical cells which
become smaller and smaller towards the periphery, the outermost
layer giving rise to the branched hairs.

The unilocular sporangia are on slightly raised portions of the
thallus and develop from cortical cells which undergo scarcely any
modification. Meiosis takes place in the sporangium, and the ripe
zoospores escape in a mass and germinate to give rise to dioecious
filamentous gametophytes which are heterothallic. The smaller
male plants produce terminal antheridia from each of which is

Fig. 114. Desmarestia. A, plant with summer and winter appearance ( x ^).
B, apex showing cortication. C, transverse section stipe. D, female gametophyte.
0 = oogonium. E, male gametophyte. a = antheridium, e = empt>' antheridium.
F-J, stages in seedling germination. (A, after Newton; B, C, after Oltmanns;
D-J, after Schreiber.)

liberated a single antherozoid, whilst the larger female plants
produce the sw^ollen oogonia. Each oogonium gives rise to a single
ovum which escapes, but as fertilization and germination take place
just outside the pore of the oogonium the young sporophyte develops
as far as the monosiphonous stage whilst still possessing a primitive
holdfast in the shape of the empty oogonium. Cortication, which is
best observed near the apex of old plants, commences in the young
plants after a few weeks, and further growth is maintained by an
intercalary growing zone some way behind the apex. It is only just
recently that the real life history of this genus has been established,

lyo PHAEOPHYCEAE

and as a result it has seemed desirable to remove the genus from its
former position in the Ectocarpales to the Laminariales.

*Laminariaceae : Laminaria (a thin plate). Figs. 115-118.

This genus has a very wide distribution in the waters of the north
temperate and Arctic zones, and it is commonly studied because its

Fig. 115. Laminaria. A, L. Cloustoni. B, L. Rodriguez. C-E, normal regenera-
tion ( X ^). C, rupture just commencing. D, E, the new tissues are more
heavily shaded. F, wound regeneration ( x ^). (A, B, after Oltmanns; C-F, after
Setchell.)

morphology is characteristic of the group as a whole with the
exception of Chorda and Desmarestia. Furthermore, it was the
first genus in which the existence of a dwarf gametophyte was
established, thus leading to a new orientation of ideas in the
classification of the Phaeophyceae. The expanded lamina has no
mid-rib and is borne on a stipe that arises from a basal holdfast

LAMINARIALES (HETEROGENERATAE) 171

which can vary greatly in form. The simplest transition area from
stipe to lamina is quite plain, but one may also find folds, ribs or
callosities in that position, which is also the region of intercalary
growth. Laminaria Sinclairii has been studied by Setchell (1905) in
some detail in connexion with regeneration, a common feature
throughout the genus. Three types of growth can be recognized, all
of them confined to the stipe, whilst it is also possible to find all
three processes taking place in one individual :

(i) The ordinary growth and extension of the blade during the
growing season. This hardly merits the description of continuous
physiological regeneration given to it by Setchell unless the concept
of regeneration is to have a wider significance.

(2) Periodic physiological regeneration which represents the
annual process whereby the new blade is formed. The transition
area bulges, due to new growth in the medulla and inner cortex, and
then ruptures from the pressure, thus leaving the frayed ends of the
non-growing outer cortex forming collars, the upper one of which
rapidly wears away. After the rupture the new cells of the medulla
and inner cortex elongate rapidly. The failure of the outer cortex to
grow is probably associated with the proximity of the inner cortical
cells to the medullary hyphae where they can monopolize all the
growing materials, thus cutting oflF any supply to the outer cortex,
but there may, of course, be other factors involved.

(3) Restorative regeneration whereby branches arise from
wounded surfaces, the same tissues being involved as in process

(2)(cf. fig. 115).

Many of the species are used as food by the Russians, Chinese
and Japanese. In Japan, foods derived from about ten different
species of these algae are known as Kombu, kelp gathering from
July to October forming quite a big industry. Goitre is practically
unknown in Japan, and its absence must be largely connected with
the iodine obtained from this algal food. Here we have an ex-
ample of a region where the absence of a disease can be directly
associated with the presence and nature of a particular kind of food.
Apart from food the kelps are generally employed as a source of
iodine and also as fertilizers.

The following brief notes concern a few species that are of more

general interest:

L. Cloustoni. The attachment crampons are arranged in four

172

PHAEOPHYCEAE

lateral rows and there is a long cylindrical stipe which develops
abruptly into the frond.

L. Rodriguezii. The thallus develops annually and splits near the
base, the split gradually extending to the apex. Rhizoids develop on
the crampons of this species.

L. saccharina. The margin is thicker than the central part of the

Fig. ii6. Laminariaceae. A-F, portions of the stipe of Macrocystis passing
successively from the epidermis, A, through the medulla, B— E, to the pith,
F. h = hypha, v — connecting thread, t = " trumpet " hyphae. G, stages in develop-
ment of mucilage canals, L. Cloustoni. H, mucilage canal of L. Cloustoni in
transverse section. c = canal, 5 = secretory cells. I, mucilage canal system in
L. Cloustoni. c = canal, 5 = secretory cells. (After Oltmanns.)

thallus and the wavy lamina is produced by continual growth of the
central portion without any growth in the marginal areas. The stipe
is short and the transition to frond is gradual.

L. digitata. This possesses a digitate frond that arises by gradual
transition from a stipe which tends to be flattened, thus forming a
convenient means of distinguishing it from L. Cloustoni.

Renfrewia. A genus very closely allied to Laminaria but diff"ering

LAMINARIALES (HETEROGENERATAE) 173

from it in that there are no crampons but only a basal attachment

disk.

Morphologically both lamina and stipe in Laminaria can be
divided into three regions (cf. fig. 116), the outer cortex, the
medulla, and the pith or central portion of the medulla. The one-
layered blade first becomes two-layered and then the primary
tubes of the medulla are cut off and separate the two outside layers.

Fig. 117. Laminaria. A-F, stages in development of female gametophyte from
a spore (A-D x 1333, E-F x6oo). G, male gametophyte ( x 533). H, I, first
two stages in development of young sporophyte. J, sporangia {s), paraphyses {p)
and mucilage caps (c). (A-I, after Kylin; J, after Oltmanns.)

Next the cortical cells arise from the cells of the limiting layer by
divisions parallel to the surface. Somewhat later the cells of the
inner cortex elongate, the middle layer of the common wall be-
comes swollen and the cells separate from each other except at a
few points where the connexions become drawn out into short
secondary tubes. Subsequent increase in thickness is due to growth
in the limiting layer and the production of tubes and hyphae
together with a considerable development of mucilage, so that the
central cells become even more separated from each other. Two

174 PHAEOPHYCEAE

types of lateral connecting branches can be recognized, the con-
necting threads and hyphae. The former arise first in the course of
development as outgrowths from the individual cells, but even
when mature they are composed of only a few cells. The hyphae,
which arise later as short branches of small cells cut off from the
original vertical cells, can unite with each other or else they grow by
cell division until finally they contain numerous cells which sub-
sequently elongate very considerably.

One of the most characteristic features of the genus is the presence
in the medulla of "trumpet" hyphae which are modified cells in the
connecting threads and hyphae. At a transverse cell wall the ends of
both cells swell out to form bulbs, the upper bulb always being larger,
but so far no satisfactory explanation of this pecuUarity has been ad-
vanced, though it may be due to purely mechanical requirements.
The transverse wall is perforated to form a sieve plate and a callus
develops on each side, both callus and sieve plate being traversed by
protoplasmic strands. It will be seen that in many respects these
trumpet hyphae resemble the sieve tubes of the flowering plants,
but although the callus is said to be formed in land plants because
of changes in^H, so far no evidence has been published to indicate
whether this is also true for the Laminariaceae. Apart from the
sieve plates the trumpet hyphae also possess spiral thickenings
which appear as striations, and here again there is the problem of
their interpretation (e.g. are they growth zones?), although it is
possible that they have now lost any function they once possessed.
The problem of these trumpet hyphae is still subject to consider-
able speculation : it has been suggested that they may be a storage
or conducting tissue, whilst another suggested function is that of
support, but as the plants are commonly submerged the water would
seem to fulfil this requirement. In some species many of the other
cells also contain pits with a thin membrane across the opening
and these presumably facilitate the diffusion of food materials.

Most of the genera possess systems of anastomosing mucilage
ducts which are normally confined to the stipe, although in
L. saccharina and L. digitata they enter the fronds as well. When
mature there are periodic openings from these ducts to the exterior
and their bases are lined with secretory cells. They arise lysigen-
ously through an internal splitting of the thallus due to cell
disintegration : this is followed by a differential growth so that the

LAMINARIALES (HETEROGENERATAE) 175

canals become more and more submerged in the thallus. The
attachment organs or crampons, which are positively geotropic,
have an apical growth and differ from the rest of the thallus in that
there are no connecting hyphae nor is there any pith. The amount
of conduction necessary in these plants would be expected to be
small, but even so the degree of differentiation is remarkable. So
far as the lamina is concerned the group is usually regarded as

20-1

10-

sterile ^aLt$ sterile

DEC. JAN. FEB.

Fig. 118. Laminaria. A, L. digitata, marked thallus before growth in summer.
B, L. digitata, marked thallus after growth in summer. C, effect of temperature
on fruiting of gametophytes in L. digitata. (After Schreiber.)

primitive because the new portions do not originate separately but
by intercalary growth from an existing portion (cf. fig. 115).

The sporangia and paraphyses are borne in irregular or more or
less regular sori on both sides of the lamina. It is probable that the
zoospores possess an eye-spot, but it must be very small because
in the three species where it has been recorded it was very
difficult to distinguish. The zoospores, which in one or two cases
are reported to be of two sizes, germinate to form minute gameto-
phytes, but on germination they first put out a tube that terminates
in a bulbous enlargement into which the contents of the zoospore
migrate. There the nucleus divides and one daughter nucleus passes

176 PHAEOPHYCEAE

into the tube whilst the other degenerates, but at present the
significance of this phenomenon is obscure : it would hardly seem
to be associated with meiosis because this process takes place in the
zoosporangium. Both kinds of gametophyte show much variation
in shape and size, the male gametophyte being the smaller through-
out as it is built of smaller cells that contain dense chromatophores.

The gametophytes can be cultivated in the laboratory, but for
successful cultivation the water must be sterilized and the cultures
placed close to a north window in winter and 2 or 3 m. distant in
summer. Reproductive organs are only formed at low temperatures,
2-6° C, whilst above 12-16° they are rarely produced, this fact
perhaps accounting for their temperate and arctic distribution (cf.
fig. 118). It is also known that the eggs may develop partheno-
genetically to give a haploid sporophyte which has an irregular
shape, whilst attempts to produce hybrids by artificial fertilization
have so far met with no success. Schreiber (1930) found that the
ratio of male to female gametophytes was always 1:1, and he sub-
sequently showed that of the thirty-two zoospores produced in
each sporangium sixteen gave male and the other sixteen female
gametophytes. The male gametophyte of L. religiosa is reported to
bear unilocular and plurilocular sporangia, but this is so abnormal
and has never been confirmed or reported for any other species that
it can hardly be accepted without further evidence. The ova of
L. saccharina are reported to be capable of producing dwarf fila-
mentous diploid plants which reproduce by means of unilocular
sporangia. If this is confirmed it may be that here we have an
example of a reversion to a primitive filamentous diploid progenitor,
a feature which might help considerably in indicating their ancestry.

The most important characteristics of the gametophytic genera-
tion are :

(i) the male gametophyte always has smaller cells;

(2) the male gametophyte always consists of more than three cells
whereas the female may consist of only one cell, the oogonium.
Under good nutrient conditions both become much branched ;

(3 ) the antheridia are unicellular and produce only one antherozoid ;

(4) any cell of the female gametophyte may function as an
oogonium ;

(5) the male gametophyte degenerates after the gametes are shed
whereas the female gametophyte persists.

LAMINARIALES (HETEROGENERATAE) 177

The young sporophyte first produces numerous rhizoids of
limited growth, but these are later covered by a disk-shaped
expansion from which are produced the haptera or crampons.

Laminariaceae : Saccorhiza {sacco, sack; rhiza, root). Fig. 119.

S. hulbosa used to be known as Laminaria bulhosa, but for some
time it has been removed to a separate genus because it differs
from the other species of Laminaria in several important respects.

Fig. 119. Saccorhiza hulhosa. A, plant ( x ^). B, female gametophyte. C, young
sporophyte. D, E, young plants of -S. dermatodea to show origin of bulb. (A, after
Tilden; B, C, after Kniep; D, E, after Oltmanns.)

The persistent lamina arises from a flat compressed stipe with wavy
edges which is twisted through 180° near the base as a result of
unequal growth, this twisting being regarded as a mechanical
device to facilitate swaying. The young sporophyte is attached at
first by a small cushion-like disk, but later a warty expansion, the
rhizogen^ develops above it and forms a bulbous outgrowth which
bends over and attaches itself to the substrate by means of descend-
ing crampons. As a result of the development of this adult holdfast
the juvenile disk may be lifted completely off the substratum.

CSA

178 PHAEOPHYCEAE

Subsequent growth of the stipe takes place in the outer layer
of the medulla, and in the adult organ five regions can be recog-
nized :

(i) Primary fixing organ.

(2) The bulb.

(3) A flattened twisted area said to provide additional rigidity.

(4) A portion with flounced edges.

(5) A flat straight portion that passes into the lamina.

The existence of these structures is supposed to be correlated
with the large lamina which is cleft into many linear segments.

If, as sometimes happens, the whole of the plant is torn away with
the exception of the bulb, this organ is still capable of reproduction
and assimilation. The advanced external diflFerentiation of the stipe
is not reflected in its histology where the diflFerentiation is poor
because there is no secondary growing region, no mucilage ducts,
and trumpet hyphae are not conspicuous.

Saccorhiza and Alaria are the only two genera in the Lami-
nariales with cryptostomata that are at all comparable to those of
the Fucales (cf. p. 194), the former genus possessing true crypto-
stomata with tufts of hairs. There are three theories concerning
the homologies of the cryptostomata which may be mentioned
briefly here (cf. also p. 196):

(i) They are incomplete sexual fucoid conceptacles which have
failed to develop.

(2) They are forerunners of the sexual fucoid conceptacle.

(3) They are a parallel development with the sexual conceptacles
of the fucoids, but otherwise have no relation to them.

Whilst there is very little evidence for any one of these theories it
may be suggested that the second alternative probably fulfils most
nearly the known facts.

The male gametophyte is filamentous whilst the female fre-
quently consists of only one cell which functions as the oogonium.
After fertilization has taken place the development of the sporo-
phyte to maturity in both species requires only one year so that the
plants are true annuals. Saccorhiza bulbosa is found on the Atlantic
coasts of north and west Europe whereas the other species, S. der-
matodea, is circumpolar and is possibly the parent species from

LAMINARIALES (HETEROGENERATAE) 179

which the other developed, a speculation which is further supported
by the fact that S. dermatodea is more primitive because the stipe is
not twisted nor are the edges so wavy. The young sporophyte first
develops a juvenile blade which does not bear sporangia and then a
new and thicker basal fertile blade is intercalated, but it is only the
juvenile blade that bears the cryptostomata, thus suggesting that
these structures may be juvenile sexual conceptacles.

Laminariaceae : Thalassiophyllum (thalassio, sea; phyllum, leaf).
Fig. 120.

The perennial sporophyte is apparently composed of a spirally
twisted, fan-shaped lamina unrolling from a one-sided scroll

Fig. 120. Thalassiophyllum clathrus. A-F, developmental stages to show the
origin of the single scroll ( x f ). G, adult plant. (After Setchell.)

12-2

i8o

PHAEOPHYCEAE

without any mid-rib. A study of the embryonal stages, however,
shows that the young plant is flat and bilaterally symmetrical. The
two edges then curl up and the plant tears down the centre giving
rise to two lateral scrolls each unrolling from a thickened outer
margin, but as one of the scrolls soon ceases to develop the mature
plant only possesses one scroll borne on a solid bifid stipe with the
vestigial scroll on one of the branches. Slitting is represented by
rows of small holes which commence to develop after the first tear
has taken place.

Lessoniaceae : Lessonia (after R. P. Lesson). Fig. 121.

The plants grow erect and form *' forests" in relatively deep
waters off the shores bounding the southern Pacific, reminding one

Fig. 121. Lessonia. A, adult plants of L. fucescens. B, C, seedling stages in
L. fucescens. (After Oltmanns.)

in appearance of some of the fossil vegetation of the Carboniferous,
although, of course, there is no connexion. The stipe is extremely
stout and rigid, 5-10 ft. long and sometimes as thick as a human
thigh. It appears to be more or less regularly branched in a dicho-
tomous fashion, a feature which is brought about by the lamina
being slit down successively to the intercalary growing region, each

LAMINARIALES (HETEROGENERATA^) i8i

successive segment developing into a new lamina with its own
portion of stipe. Dried parts of the stipe, which can easily be taken
for pieces of driftwood, are used by natives to make knife handles.
This method of causing splitting should be compared with the
other processes found in Nereocystis^ Macrocystis and Postelsia
(cf. below).

Lessoniaceae : Postelsia (after A. Postels). Fig. 122.

This is a monotypic genus, often known as the "sea palm", that
is confined to the Pacific coast of North America where it grows
between Vancouver Island and central California on rocks which

Fig. 122. Postelsia palmaeformis. (After Oltmanns.)

are exposed to heavy surf. The smooth, glossy, cylindrical stipe is
thick but not very long, up to i m. in height. It is erect and hollow
within and bears at its apex a number of short, solid, dichoto-
mously branched structures from each of which hang 100-150

i82 PHAEOPHYCEAE

laminae that bear sporangia in longitudinal folds when they are
mature. Apart from the cryptostomata of Saccorhiza and Alaria it
has also been suggested that the occurrence of these sporangia m
folds may illustrate how the fertile fucalean conceptacle may have
arisen. Such a change would necessitate the development of wedges
of sterile tissue in order to divide up the folds, but whether such a
change could occur in a relatively differentiated thallus is a matter
for speculation.

The numerous laminae are formed by a splitting process in
which a portion of the lamina fails to continue growth whilst the
rest goes on growing, and in this manner a weak area is formed
from which a split commences.

Lessoniaceae : Nereocystis {Nereo, Nereis, daughter of Nereus;
cystis, bladder). Fig. 123.

The plants, which from the recorded observations appear to be
annuals, may attain a maximum length of 90 m. bearing a bladder
up to 2 or 3 m. in length. The long slender stipe is solid and cylin-
drical below but swollen and hollow above, finally contracting just
below the terminal spherical bladder which bears a row of short
dichotomous branches, each giving rise to a number of long thin
laminae. The plant commences with only one blade which divides
twice in a dichotomous fashion, thus producing four blades, and
these form the centre of activity for the remainder through a
process of shtting. The splitting of these four fronds is preceded by
the development of a distinct line along the path of the future slit,
the line representing new tissue, which has in consequence very
little strength, thus forming an area of weakness along which the
slit commences. The plant is found at a depth of from 5 to 25 m.
between Alaska and Los Angeles. Besides being a good source of
potash salts, as the ash contains 27-35 % potassium chloride, the
stalk and vesicle can be treated to yield a candied edible product
called " Seatron". Locally it is called by a number of names, bull
kelp, bladder kelp, ribbon kelp and sea-otter's cabbage.

In the closely related genus Pelagophycus the spores are said to
be non-motile, not even possessing cilia. Further confirmation of
this fact is much to be desired because not only is it an unique state
in the family but it also renders comparison with Nematophyton (cf.

LAMINARIALES (HETEROGENERATAE) 183

p. 275) of great interest. Local names employed for Pelagophycus
are elk-kelp, sea pumpkin and sea orange.

Fig. 123. Nereocystis Luetkeana. A, young plant. B, mature plant. C, branching
from bladder. (After Oltmanns.)

*Lessoniaceae : Macrocystis {macro, large; cystis, bladder). Fig.
124.
The perennial fronds of this giant of the ocean may reach
200 ft. in length, the alga growing at a depth of 20-30 m. in the
North and South Pacific Ocean and near the Cape of Good Hope,
all being regions where the temperature of the water ranges between
o and 20° C. In the juvenile plant the stipe is simple and solid, but
later on it branches one, two or three times in a dichotomous
fashion, although uhimately the branching becomes unilateral and
sympodial, each branch bearing tAvo to eight laminae. The main

1 84

PHAEOPHYCEAE

growing region on each branch is ventrally situated in the terminal
flag or blade, and it is here too that splitting takes place to form the
individual laminae. The splitting is brought about by local gelatin-
ization of the inner and middle cortex together with a cessation of
growth in the epidermal area; this forces the adjacent tissues into
the gelatinized areas until finally the epidermis is ruptured. Two

Fig. 124. Macrocystis pyrifera. A, young plant ( x ^). B, slightly older plant
with primary slit and tw^o secondaries ( x i). C, still older plant. D, young
plant. E, origin of blades at the apex. F, young plant. G, mature plant.
H, sporangial sori ( x yV)- I. transverse section of thallus showing ridges ( x 3-5).
J, surface view of holdfast of old plant showing flattened rhizome. (A, B, after
Brandt; C, H, J, after Setchell and Gardner; D-G, after Oltmanns; I, after
Smith and Whitting.)

kinds of zoospore are recorded, large ones which give rise to the
female gametophytes and smaller ones which give rise to the male.
The appearance of true heterospory in such an advanced alga is a
feature of considerable importance because the phenomenon is
normally associated with the land plants. The eggs are reported to
be fertilized whilst still in the oogonium and if this is so then we
have here the only example among the brown seaweeds of the

LAMINARIALES (HETEROGENERATAE) 185

retention of the ovum on the parent plant. This again may prove to
be a significant feature in a consideration of the origin of a land
flora.

Alariaceae: Alaria (ala, wing). Fig. 125.

This genus is widely distributed throughout the northern
hemisphere, the common species being A. esculenta. There is a

Fig. 125. Alaria esculenta. A, plant oi A. oblonga with sporophylls. B, sporangia
and paraphyses ( x 200). C, germling sporophyte ( x 100). D, female gametophyte
( X 80). (A, after Oltmanns; B-D, after Newton.)

short, soHd, unbranched stipe which is attached to the substrate by
means of small branched rhizoids. It is naked below with an inter-
calary growing zone that allows for continual renewal, whilst above
the growing region the stipe expands into a flattened rachis which
bears each year a fresh crop of marginal rows of sporophylls. The
frond finally terminates in an expanded sterile lamina with a well-
marked mid-rib, which is also an annual production. In addition to
the intercalary growth there is also a marginal growth that imparts

i86 PHAEOPHYCEAE

a wavy appearance to the terminal frond. This bears the so-called
cryptostomata, although these are barely more than tufts of hairs
arising in slight depressions. The sporangia are produced on the
lower blades mixed up with unicellular paraphyses. The gameto-
phytes are protonemal in form, simple or sparingly branched, the
male, as usual, being composed of smaller cells with terminal, inter-
calary, or lateral antheridia, whilst the oogonia on the female
gametophyte are usually terminal. The ovum is fertilized on
emergence from the pore of the oogonium and the young sporo-
phyte develops in situ without the characteristic early appearance of
an holdfast.

Alariaceae: Egregia (outstanding). Fig. 126.

This genus is composed of two species, one having a more
northern distribution than the other, though both are confined to
the waters of the Pacific between Vancouver Island and Lower
California. The whole plant can be regarded as an extension of the
Alaria type in which each branch becomes strap-shaped and bears
three types of outgrowth :

(a) Ligulate sterile outgrowths.

(b) Small fertile outgrowths.

The female gametophyte is composed of one or two large cells
whilst the male plant is composed of numerous smaller ones, both
plants reaching maturity in from 19 days to 4 weeks depending on
the season of the year, e.g. the length of daylight. Maturity is most
rapidly reached at a temperature of 10-16° C, and although at
16-20° C. gamete development takes place nevertheless the
antherozoids are unable to leave the antheridia.

Alariaceae: Eisenia (after G. Eisen). Fig. 127.

The perennial sporophyte arises from a holdfast that is
apparently bifurcate, although the two apparent branches are
actually the lower margins of the primar}^ lamina. The original
elongate stipe, which may be as much as 15 cm. in length, is
persistent and bears a flattened lamina from which pinnules
develop. This primary lamina then disappears leaving two
groups of pinnules or sporophylls attached to the lower and outer
margin of the lamina side of the original transition area, whilst

LAMINARIALES (HETEROGENERATAE) 187

a small partial blade persists at the outer extremity of each false
stipe. New sporangia continually arise at the base of the old ones,
and the genus is interesting because the cuticle is shed when the
sorus is mature (cf. p. 277). This is one of the few Laminariaceae

Fig. 126. Egregia Menzesii. A-C, stages in growth of young sporoph>tes ( x y%).
D, young frond. E, base of mature frond. F, apex of mature frond. G, mature
plant. (A-C, after Griggs; D-G, after Oltmanns.)

in which the number of chromosomes has been counted, the haploid
number being fifteen. Two species are known, one from southern
California and one from Japan.

Alariaceae: Pterygophora (pterygo, wing; phora, bearing).

The perennial sporophyte, which arises from a holdfast of
branched haptera, possesses a simple, solid stipe that is more or

1 88

PHAEOPHYCEAE

less woody, being by far the stoutest known among the algae. The
numerous Hnear laminae, about forty in number, are borne
terminally, and though they have no distinct mid-rib nevertheless
the central portion is much thickened. Long sporophylls are also
produced laterally on both sides of the stipe near the transition area.
These fronds, which possess continual growth, appear first in
February and fruit in the following September or October, the

Fig. 127. Eisenia. A, young sporophyte of £". bicyclis. B-F, stages in develop-
ment of the adult sporophyte of E. arborea. G, base of an adult plant of
E. bicyclis. H, mature sporophyte of E. bicyclis. (After Tilden.)

sporangia and paraphyses being borne in sori on both sides of the
sporophylls and also on the terminal laminae. Pterygophora is a
monotypic genus found from Vancouver Island to Lower Cali-
fornia where it grows characteristically at the bottom of deep
chasms possessing 12-15 ft. of water at low tide. It has been
estimated that individual plants may live for as long as 13 years.

FUCALES (HETEROGENERATAE) 189

REFERENCES

Desmarestia. Abe, K. (1938). Sci. Rep. Tohoku Univ. ivth ser. 12, 475.

Eisenia. Hollenberg, G. J. (1939). Amer. J. Bot. 26, 2^.

Laminaria. Kanda, T. (1936). Sci. Pap. Inst. Alg. Res. Hokkaido Univ.

1, 221.

Laminaria. Kylin, H. (19 16). Svensk hot. Tidskr. 10, 551.

Chorda. Kylin, H. (191 8). Svensk hot. Tidskr. 12, i.

Laminaria. Schreiber, E. (1930). Planta, 12, 331.

Desmarestia. Schreiber, E. (1932). Z. Bot. 25, 561.

Saccorhiza. Setchell, W. A. (1891). Proc. Amer. Acad. Sci. 26, 177.

Eisenia. Setchell, W. A. (1896). Erythrea, 4, 155.

Embryology, Regeneration. Setchell, W. A. (1905). Univ. Cal. Publ. Bot.

2, 115-

General. Williams, J. Lloyd (1925). Rep. Brit. Ass. Pres. Address,
Sect. K, p. 182.

*CYCLOSPOREAE—
FUCALES (HETEROGENERATAE)

The sporophytic plants are even more dominant in the life cycle
than in the Laminariales, but although diploid there is no apparent
asexual reproduction, the plants always reproducing by means of
ova and antherozoids. There is considerable tissue differentiation,
and in their external features the plants exhibit much more variation
than is to be found in the Laminariales. Some workers consider
that the structures called oogonia and antheridia are really macro-
and microsporangia producing mega- and microspores which
germinate before they are liberated from the sporangium, so that
while the reproductive bodies have their origin as spores, neverthe-
less the liberated products are gametes. This view is held by the
present author and is discussed more fully later (cf. p. 258). In the
primitive condition eight ova are produced in each oogonium and
sixty-four antherozoids in each antheridium. Meiosis takes place
during the first two divisions in the formation of microspores, and
as there is often a pause after the second division the first four
nuclei have been regarded as the functional microspores, each of
which subsequently undergoes four mitoses so that they can be
said to germinate to a sixteen-celled gametophyte where each cell
functions as an antherozoid. In the macrosporangium the first four
nuclei formed are regarded as the functional megaspores, and each
of these is considered to germinate subsequently to a two-celled
female gametophyte where each cell functions as an ovum. In

190 PHAEOPHYCEAE

those species where less than eight mature ova are produced it must
be assumed that some of the megaspores undergo abortion.

If the above is to be the correct interpretation, and it would seem
to be more satisfactory than any other theory in comparison with
other members of the Phaeophyceae, then we can say that not only
is there a cytological alternation of generations but there is also a
morphological alternation, although the sexual generation is even
further reduced from the state found in the Laminariales. This
really forms the basis for placing the Fucales in the Heterogeneratae.
The alternative interpretation is that the sexual generation has been
completely suppressed and is solely represented by the gametes, so
that whilst there is a cytological alternation of generations there is
only one morphological generation (cf. also Chapter ix). The
sporangia are borne in flask-shaped depressions of the thallus called
conceptacles, each of which is lined with paraphyses and opens to
the surface by means of an ostiole. The plants of the different
species may be dioecious, monoecious or hermaphrodite. It has
been pointed out that the number of primary rhizoids in the embryo
is proportional to the size of the rhizoidal cell, which in turn bears
a relation first to the size of the egg, and secondly to the com-
plexity of the thallus. On this basis a series of increasing embryonal
complexity may be traced, e.g. Fucus -^Ascophyllum ^Pelvetia ->
Cystoseira ^Sargassum.

Geographically the original centre of distribution was un-
doubtedly the southern Pacific in the waters of Australia and New
Zealand where the greatest number of species are now to be found.
This makes an interesting comparison with the preceding order
whose original centre of distribution was the northern Pacific in the
waters around Japan and Alaska. The Fucales are classified into
five groups, the classification being based primarily upon the
structure of the apical growing cell or cells :

(i) Durvilleaceae. A group comprising two genera, Durvillea
and Sarcophycus, from Australia and Patagonia, both without any
means of apical growth.

(2) Fuco-Ascophyllae. Growth is determined in the adult stage
by one four -sided apical cell.

(3) Loriformes. Growth is due to one three-sided apical cell which
gives rise to a long whip-like thallus.

(4) Cystoseiro-Sargassae. The apical cell is again three-sided but

FUCALES (HETEROGENERATAE) 191

there is copious branching which results in bilateral, radial and
bilaterally radial thalli.

(5) Anomalae, composed of two genera, Hormosira and Notheiuy
both confined to the Antipodes. Growth is brought about by a
group of cells instead of a single cell.

DURVILLEACEAE

Durvillea (after I. D. D'Urville). Fig. 128.

The sporophyte is a dark olive brown or black in colour and
possesses very much the appearance of a Laminaria. The large

Fig. 128. Durvillea antarctica. A, young plant ( x ^). B, adult plant (much
reduced). C, stipe and holdfast ( x ^). (After Herriot.)

solid stipe arises from a scutate holdfast and very soon passes into a
flat, expanded, fan-shaped lamina, which later becomes split into
segments although no definite appendages are produced from this
frond. The ends of the older laminae become frayed and broken off
by wave action, whilst the holdfast may attain a diameter of 2 ft.
through the addition of new tissue annually. If this secondary
growth did not occur the plant would soon be torn from its moor-
ings because the holdfast is continually becoming riddled with
holes through the boring operations of molluscs. The macro- and
microsporangia, which are borne in conceptacles on different plants

192 PHAEOPHYCEAE

as the genus is dioecious, occur over the whole of the lamina, this
condition being regarded as the primitive state for the Fucales. It is
known as the *'bull kelp" and forms submarine forests in deep
waters off New Zealand and the Aucklands down to depths of
30 ft., or else it grows in places continually exposed to surf.

FUCO-ASCOPHYLLAE

*Fucaceae: F«cw5 (a seaweed). Figs. 129-13 1.

This genus contains a number of species that are widely scattered
over the world with the majority in the northern hemisphere,
many of them exhibiting a wide range of form with numerous so-
called varieties. When two or more species occur in the same area
they are generally present in different zones on the shore, probably
dependent upon the degree of desiccation that they can tolerate
(cf. p. 353). The plants are attached by means of a basal disk and
there is usually a short stalk, which continues on to form the
mid-rib of the frond in those regions where the expanded wings or
alae are developed, these latter being of varying width with either
entire or serrate margins. Branching is commonly dichotomous or
subpinnate, and in many species the branches bear expanded
vesicles or pneumatocysts . Sometimes whole portions of the frond
may be inflated in an irregular manner, but the factors causing this
phenomenon are not known, although it is possible that contact with
rock or soil provides the necessary stimulus. With increasing age the
lower portions of the alae may be frayed off by wave action, leaving
only the mid-rib, which then has the appearance of a stipe. The
whole of the expanded thallus is covered with sterile pits or crypto-
stomata similar to those of Saccorhiza, but in fruiting plants it is
only the ends of the branches that become swollen and studded
with the fertile conceptacles. In F. spiralis these conceptacles are
hermaphrodite, containing both mega- and microsporangia ; in
F. vesiculosus and F. serratus the plants are dioecious, the two types
of sporangia occurring on separate plants, whilst in F. ceranoides
either state may be found. A number of very peculiar forms have
been described which commonly occur on salt marshes: these
rarely fruit, reproduction being secured principally by means of
vegetative proliferations (cf. p. 325). The age of Fucus plants has

FUCALES (HETEROGENERATAE) 193

not been studied in much detail but the following figures (Table I)
may be cited from one worker who marked a number of plants :

Table I

Ascophyllum
Species ... F. spiralis F. serratus F. vesicHlosus nodosum

Max. age (yr.) 3^ 4 2^ 2^

Av. age (yr.) i^ 2 i i|

Morphologically the thallus shows considerable differentiation.
The external layer, which is known as the limiting layer, consists of
small cells with abundant plastids and is primarily assimilatory in
function. Below this there is a cortex composed of several layers of
parenchymatous cells which become more and more elongate and
mucilaginous towards the centre, and these probably form the
storage system. In the very centre the cells are extended into
hyphae which are interwoven into a loose tangled web. This central
tissue is called the medulla and probably acts as a conducting
system, because the transverse walls of the hyphae are frequently
perforated with the same type of pit that is to be found in some of
the Laminariaceae. The primary medullary hyphae are relatively
thin- walled, but when secondary growth of the thallus takes place
the new hyphae which result from this process are very thick-
walled and so are probably mainly mechanical in function.
Secondary growth is due to the activity of the limiting layer and the
inner cells of the cortex, the latter tissue being responsible for the
formation of the secondary hyphae (cf. fig. 131) which penetrate
between the primary medullary hyphae and finally outnumber
them. There is a greater development of secondary thickening in
the stipe and mid-rib than there is in the frond, whilst in very old
parts of the thallus the limiting layer may die off and then the
underlying cortical cells take over its function.

Growth in length takes place by means of an apical cell which
lies at the bottom of a slit-like depression that has resulted from the
more rapid growth of the surrounding limiting layer. The apical
cell is three-sided in young plants whilst in the adult thallus it
becomes four-sided, the new segments being cut off successively
from the base and four sides, after which they develop into the
various tissues (fig. 129). Injury, and also the stimulus provided
when the thallus lies on marsh soil, induces new growth in the

CSA 13

194

PHAEOPHYCEAE

neighbouring cells, and in this manner proliferations are formed
which may also serve for vegetative propagation. Both crypto-
stomata and conceptacles arise as depressions in the surface of the

Fig. 129. Fucus. A, adult plant of F. serratus ( x 0-30). B, a marsh form of
F. vesiculosus ( x 0-30). C-E, seedling stages of F. vesiculosus showing origin of
rhizoids and apical tuft of hairs. F, diagram to show method of segmentation of
apical cell, A. 6s = basal segment, 5S = side segments. G, apical cell of young
thallus. H, apical cell of old thallus. (A, B, after Taylor; C-H, after Oltmanns.)

thallus and there are three principal accounts which have been
given of the course of their development :

(i) An early view held by Kiitzing and Sachs in which they were
described as arising as slight depressions in the thallus that later

FUCALES (HETEROGENERATAE) 195

became overgrown by the surrounding tissue. This has since been
abandoned.

(2) According to the second account a linear series of two or more
cells is formed but their horizontal activity then ceases, thus leaving
a terminal initial cell which becomes sunk in a depression as the
surrounding tissues grow up. On this theory the sides of the con-
ceptacle are derived from the limiting layer and underlying cortex,
as Bower (1880) demonstrated for Fucus, whilst in Himanthalia the

o c

Fig. 130. Fucus. A-C, origin of conceptacles in F. serratus. 6 = basal cell,
z = initial. D, juvenile conceptacle of Cystoseira. /i = hair. (After Oltmanns.)

sides are derived from the limiting layer only. Finally, around the
remnants of the one or more initial cells a central mucilaginous
column is formed stretching to the neck of the conceptacle and
connected to the walls by thin strings of mucilage which are later
ruptured. According to this description, therefore, the conceptacles
are the products of one or more initials which may or may not
disintegrate at a later stage (cf. fig. 130).

(3) The third account describes the conceptacle as developed
entirely from a single initial that divides transversely into two un-
equal cells, the upper or tongue cell degenerating whilst the lower

13-2

196

PHAEOPHYCEAE

one gives rise to the walls of the conceptacle. This method of forma-
tion has been successfully demonstrated for Sargassum, Pycno-
phycus and other Fucaceae. It is clear from the investigations that
have been made that both methods (2) and (3) are to be found in the
different species.

Fig. 131. Fucus. A, transverse section "female" conceptacle oi F. platy car pus.
B, transverse section " male " conceptacle of F. vesiculosm. C, portion of thallus of
F. spiralis to show structure ( x 125), D, origin of hyphae at i cm. below apex in
F. spiralis ( x 235). E, microsporangia. F, young, and G, old megasporangium.
H, liberated ova in inner vesicle, e = endochiton, m = mesochiton. I, ova being
liberated, e = endochiton, 7M = mesochiton. J, empty sporangium showing torn
exochiton. K, ovum being fertilized. L, antherozoid. (C, D, after Pennington;
rest after Oltmanns.)

The cryptostomata or hair pits are regarded as a juvenile stage of
the fertile conceptacle (cf. also p. 178) because sporangia are
frequently associated with the hairs or else they occur in the same
cavity after the hairs have been lost. With this interpretation in
view the following morphological series can be arranged :

FUCALES (HETEROGENERATAE) 197

(a) Plants with a continuous patch of hairs and reproductive
bodies, e.g. Laminaria.

(b) Plants with hairs and reproductive bodies in scattered sori,
e.g. Dictyota.

(d) Plants with hairs and reproductive bodies in receptacles
which are confined to apical positions or special side branches, e.g.
Fucus, Ascophyllum.

In the mature fruiting conceptacles there are branched hairs or
paraphyses with the microsporangia borne terminally on the
branches near the base, or else the paraphyses are unbranched and
associated with the megasporangia, which are either sessile or else
borne on a single stalk cell, each megasporangium characteristically
containing eight ova when mature. In those species where the
conceptacles are hermaphrodite all these structures occur together.
The walls of both sporangia are double, and when the gametes are
ripe the sporangia burst, liberating their contents which are still
enclosed in the inner delicate membrane. The expulsion of the
gametes normally takes place whilst the tide is out because the con-
ceptacle is then full of mucilage and the loss of water causes the
thallus to shrink, thus forcing the ripe ova and antherozoids in their
envelopes through the ostiole to the surface. When the tide returns
the inner wall bursts and so liberates the antherozoids, whilst the
inner megasporangium wall inverts and enables the ova to escape.
Fertilization takes place in the sea, the antherozoids clustering
around the ova and causing them to rotate by their activity until
one antherozoid succeeds in entering and fertilizing each ovum.

The fertilized zygote surrounds itself with a wall and very shortly
begins to divide, the direction of the first wall being said to be at
right angles to the incident light. After a few more divisions the
octant stage is reached and then a rhizoid appears on the side away
from the light and grows downward, being followed soon after by
others (cf. p. 289). The upper part of the embryo elongates from a
five-sided apical cell but the end soon becomes flattened, after
which a terminal depression arises that contains the three-sided
juvenile apical cell together with a bunch of hairs. The bunch of
hairs possess trichothallic growth, but they soon fall oflF and the
basal cell of one hair becomes the new four-sided apical cell of the

198

PHAEOPHYCEAE

adult plant. It is perhaps of interest to note that in Fucus vesiculosus
it has been shown that the mature sporophyte contains the diploid
number of sixty-four chromosomes, which appears to be the usual
number in all the Fucales so far examined, with the exception of
Sargassum Horneri in which 2^ = 32.

*Fucaceae: Pelvetia (after the French botanist, Dr Pelvet). Fig.
132.
The fronds in this genus have no mid-rib and are linear, com-
pressed or cylindrical with irregular dichotomous branching. Air

Fig. 132. Pelvetia canaliculata. A, plant ( x |). B, megasporangium (x6o).
C, mature fertilized sporangium ( x 72). we = rejected nuclei. D, germinating
oospores ( x 72). E, microsporangia (x 156). (A, original; B-E, after Scott.)

vesicles may be present in some species but normally they are
absent, especially in the European P. canaliculata, which grows on
rocky shores forming a zone near high-water mark or even above so
long as it is reached by the spray. Modified salt-marsh forms
derived from P. canaliculata are also recorded but these are con-
fined to Great Britain (cf. p. 324); like the marsh forms of Fucus
they are characterized by the general absence of fruiting receptacles,
reproduction being primarily vegetative. The structure of the
thallus is essentiallv similar to that of Fucus, but the Californian
Pelvetia fastigiata also possesses a few cryptostomata which are

FUCALES (HETEROGENERATAE)

199

otherwise absent from the genus. The sporangia are similar to those
of Fucus except that normally only two ova mature, the remaining
six nuclei being extruded from the cytoplasm into the wall, though
in Pelvetia fastigiata one may occasionally find four ripe ova or else
ova that contain two nuclei. In P. canaliculata the two mature eggs
are arranged one above the other, whilst in the Japanese species,
P. Wrightiij they are placed side by side. This difference is probably
dependent upon the relative position of the two megaspores which
germinate.

*Fucaceae: Ascophyllum (asco, wine-skin; phyllum, leaf). Fig. 133.

The plants of this genus are large, often attaining several feet in
length, and are commonly to be found on sheltered coasts at about

Fig. 133. Ascophyllum nodosum. A-C, diagram showing method of branching.
A, apical cell. A^-A^, secondary initials in order of development. AA^ , AA2,
tertiary and quaternary initials. D, plant ( x i). E, microsporangia ( x 225).
F, megasporangium (x2-25). (A-C, after Oltmanns; D-F, after Newton.)

200

PHAEOPHYCEAE

mean sea-level. The thallus of the common species, A. nodosum,
which sometimes bears nodular galls caused by the eel-worm
Tylenchus fiickola, is more or less perennial, and regenerates each
year from a persistent base or from the denuded branches. As in the
two previous genera free-living or embedded forms have evolved in
salt-marsh areas (cf. p. 324), and these differ considerably from the
common parent species, Ascophyllum nodosum, not only vegetatively
but also in the absence of sporangia. The normal fronds have
a serrated margin but no mid-rib and commonly bear vesicles
which are known as pneumatocysts, but when the vesicles are borne
on the little side branches they are termed pneumatophores. The
axis is beset by simple, clavate, compressed branchlets that arise
singly or in groups in the axils of the serrations. These are later
converted into or are replaced by short-stalked, yellow, fertile
branches which fall off after the gametes have been liberated from
their conceptacles. The macrosporangia each give rise to four ova,
the remaining four nuclei degenerating.

The method of branching is perhaps best understood from an
inspection of fig. 133. In spring the main branches divide dicho-
tomously as in Fucus, after which opposite pairs of fertile recep-
tacles or sterile tufts of hairs are produced in notches that are
formed as follows on both sides of the thallus. The apical cell (A)
cuts off another apical cell (^1) that remains dormant for a time,
during which period it is carried up the edge of the groove to the
side of the thallus by the activity of the primary apical cell. The
limiting layer immediately around A^ does not undergo further
growth and so it also comes to lie in a groove. Later on, tertiary
(AA-^) and quaternary (AA^) apical cells are cut off from ^1, the
tertiary cell becoming the apical cell of a sterile or fertile branch.

Fucaceae: Seirococcus {seiro, chain; coccus, berry). Fig. 134.

The mode of branching in this southern-hemisphere genus can
be explained if it is assumed that the lower side of a notch, com-
parable to one of those found in Ascophyllum, develops into a leafy
member (cf. fig. 134). The apical cell cuts off segments on either
side, ^1 and A<^ , which are secondary apicals that become separated
from A through growth of the epidermis. These secondary apicals
divide to give tertiaries, A^ , after which they become separated from
each other by a new leaf organ (/) that develops as a result of the

FUCALES (HETEROGENERATAE) 201

activity of one of the tertiary apicals. Subsequently the secondary
apical, A^, undergoes a series of divisions, thus producing a row of
apical cells each of which develops into a fertile branchlet. The
tertiary apical normally only gives rise to the leaf blade, but it may
divide again sometimes to give a new shoot or a series of fertile

Fig. 134. Seirococciis. A, plant of S. axillaris with fruiting laterals. B, diagram
to show method of branching, b^-hf, blades, b^ being the youngest. C, diagram
showing disposition of apical meristematic cells, A-A3, the former being the
oldest: / = origin of leaf organ. D, E, paraphyses of female conceptacle ( x 135).
F, megasporangium ( x 135). (A-C, after Oltmanns; D-F, after Murray.)

branchlets which will thus appear to grow out from the main
thallus.

LORIFORMES

*Fucaceae: Himanthalia{himant, thong;halia, of the sea). Fig. 135.
The short, perennial frond or button arises from a small disk-
like holdfast, the shape of the button being dependent upon level
because it is short and stumpy when it grows exposed at high

202

PHAEOPHYCEAE

Fig. 135. Himanthalia lorea. A-D, stages in the liberation of the ovum ( x 22).
E, F, abnormal buttons. G, button from bottom of dense zone. H, button from
top of dense zone. I, mature megasporangium. J, plant with fertile fronds. (A-I,
after Gibb; J, after Oltmanns.)

FUCALES (HETEROGENERATAE) 203

levels, whilst it is more elongate at the lower levels where the plants
are submerged for longer periods. From March to July of each
year new receptacles grow out from the centre of the buttons and
form very long strap-shaped and repeatedly forked structures
filled with mucus. Growth curves show that the greatest length is
attained by these annual fronds on plants growing in the lowest
part of the dense zone and that the shortest occur in the highest.
This can be correlated with (a) the greater degree of desiccation at
the higher levels, and (b) the fact that the less frequent flooding
reduces the supply of available salts. Reduction has proceeded so
far in this genus that only one ovum matures in the ripe macro-
sporangium. The liberation of the gametes is controlled by the tides
and exposure and there is a definite periodicity related to these t^vo
factors.

Cystoseiro-Sargassae

Sargassaceae : Halidrys (halt, sea; drys, oak). Fig. 136.

The perennial fronds arise from a conical holdfast and bear
pedicelled air vesicler, but as these are lanceolate and jointed they
probably represent a series of vesicles. There are only two species,
the European i7. siliquosa being hermaphrodite whilst the Californian
one is dioecious. In both, the stalked receptacles form terminal
racemes at the apices of branches, but only one ovum matures in
each macrosporangium. In the Californian H. dioica there are a
number of interesting morphological features :

{a) An unbroken series can often be found which shows every
gradation between a leafy member and the series of vesicles.

(h) Protoplasmic connexions between cells are continuous
throughout the whole of the plant, a feature which should be
compared with the condition commonly found in the Rhodo-
phyceae (cf. p. 212).

[c] The origin of the vesicles appears to be largely dependent

upon the food supply.

{d) The cells in the centre of the mid-rib have definite sieve
plates comparable to those in the trumpet hyphae of Laminaria,
though without the bulbous swellings.

(e) The air chambers and primary hyphae appear to arise in
regions which are losing their vitality, though the significance of
this behaviour is not clear.

204

PHAEOPHYCEAE

Fig. 136. Halidrys siliquosa. A, plant ( x |). B, apex to show branching,
a = primary initial, ai-a4 = secondary initials. (A, original; B, after Oltmanns.)

Sargassaceae : Cystoseira (cysto, bladder; seira, chain). Fig. 137.

The much-branched perennial thallus is either cylindrical or
compressed and arises from a fibrous woody holdfast which has
more or less the structure of a conical cavern. The primary
branches arise from the main stipe towards the base and divide
above into filiform branches and branchlets, but when the latter do
not develop very far one gets what is known as the ''Erica" and
'' Lycopodium" types, so called because of their resemblance to
members of those genera. Seriate rows of small air vesicles may be
inserted in the branches, and when this occurs the row of vesicles
must be regarded as a modified branch. The plants are monoecious

FUCALES (HETEROGENERATAE) 205

or dioecious, the conceptacles being borne in terminal or intercalary
positions on the ramuli, and, as in some of the other genera, only one
ovum develops in each megasporangium, the remaining seven nuclei
degenerating. In the seedling the main shoot is very short and soon
stops growth, and as a consequence it is completely overtopped by

Fig. 137. Cystoseira. A, C. ericoides, plant ( x i). B, portion of same enlarged
( X 4-5). C, germling. D, same, rather older. E, diagram to show nature of
branching in C. ahrotanifolia. (A, B, after Newton; C-E, after Oltmanns.)

the lateral branches. The first two shoots arise opposite each other
but the remainder have a divergence of 2/5. The genus is principally
confined to the warmer subtropical and temperate waters of the
globe.

*Sargassaceae: Sargassum {sargasso^ Spanish for sea- weed). Fig.
138.

- The branching in this genus is radial with a divergence of 2/5.
The primary branch is a sterile phylloclade which bears cr)^pto-
stomata whilst the secondary branch is also sterile and is commonly
reduced to an air-bladder. In some species there may be yet a

2o6

PHAEOPHYCEAE

third sterile branch which is also reduced to an air-bladder, but all
the subsequent branches are fertile and finger-like in appearance.
The plants are attached by means of a more or less irregular, warty.

Fig. 138. Sargassum. A, S. filipendula ( x 0-45). B, base of plant. C, escape of
sporangia each with eight nuclei (X40). D, seedling at rhizoid stage (x 105).
E-G, stages in branching, 6". Thunbergii ( x 0-22). a = main initial, ai = branch
initial, ^2 = secondary branch initial. (A, B, after Taylor; C, after Kunieda;
D, after Tahara; E-G, after Oltmanns.)

solid, parenchymatous base or else numerous stolon-like structures
grow out from the main axis and anchor the plant. The genus,
which is principally confined to tropical waters, is a very large one

FUCALES (HETEROGENERATAE) 207

with about 150 species, some being dioecious whilst others are
monoecious. In the ripe megasporangium only one ovum reaches
maturity under normal conditions, though occasionally eight eggs
may develop. In the former case the single ovum contains all eight
nuclei, but only one of these grows larger and is actually fertilized.
This state of affairs can be interpreted as a failure on the part of
the megaspores and gametophytes to form cell walls, and is a
secondary condition due to still further reduction. In S. filipendula
there is no stalk to the megasporangium and so it is embedded in
the wall of the conceptacle. When ripe the whole megasporangium,
not merely the inner wall and its contents, is discharged and
remains just outside the ostiole attached to the conceptacle wall by
a long mucilaginous stalk.

After fertilization the first divisions take place whilst the zygote
is still attached to the parent plant by this long stalk. In
S. filipendula fertile sporangia or degenerate sporangia are found
in some of the cryptostomata, and this fact has been taken to signify
that these sterile pits are abortive or juvenile conceptacles. The
genus is especially abundant in Australian waters, one species,
S. enerve, being employed in Japan as a decoration for New Year's
Day because, when dried, it turns green. Various species are also
used in the same country for food, but the chief claim to notoriety
in this genus is probably associated with S. nutans, the so-called
Sargasso weed, which from time immemorial has been found as
large floating masses in the Sargasso Sea near the West Indies,
frequent references to it being recorded in the stories of early
travellers to that region. At one time it was thought that plants
of S. natans, together with one or two other species that behave
similarly, were attached in the early stages, but there would now
seem to be good evidence that they remain floating throughout the
whole of their life cycle. Borgesen suggests that these perennial
pelagic species originally arose from attached forms such as
S. vulgare, S. filipendula and S. Hystrix.

S ARGASSACEAE : Turbinaria (like a spinning top). Fig. 139.

The dioecious sporophyte forms a cone-like bush up to 25 cm.
high arising from a branched holdfast. The stiff cylindrical stipe is
crowded with leaves which are triangular or disk-like structures
borne on petioles that represent the primary sterile branch of

208

PHAEOPHYCEAE

Sargassum, these leafy bodies serving not only as assimilatory
organs but also as floats. iVll the subsequent branches, which grow
in corymbose clusters from the base of the phylloclade, are fertile.
The genus is essentially confined to the warm waters of the tropics
and subtropics.

Fig. 139. Turbinaria.
(After Oltmanns.)

Portion of plant with sterile (s) and fertile (/) branches.

Anomalae

*Fucaceae: Hormosira (hormo, necklace; sira, a chain). Fig. 140.

The sporophyte, which has the appearance of a bead necklace,
is composed of a chain of swollen vesicles (internodes) connected
by narrow bridges (nodes). Growth takes place by means of a
group of four apical cells, and these give oflF branches alternately in
a dichotomous manner, the branches usually arising at the inter-
nodes ; but apart from the discoid holdfast, there is no differentia-
tion into appendages comparable to those of the other Fucaceae.
The basal internode is soHd but all the remainder are hollow: the
nodes are also solid because they are composed solely of epidermis
and cortex. The sporophytes are dioecious, the conceptacles being

FUCALES (HETEROGENERATAE)

209

borne on the periphery of the inflated nodes. Although eight ova
are originally formed in the megasporangia only four attain to
maturity, but in this genus, however, it is a case of degeneration of
eggs and not merely of nuclei. Another interesting feature of this
genus is its capacity to form and shed a cuticle that bears the im-

Fig. 140. Honnosira Banksii. A, portion of plant ( x f ). B, longitudinal section
of apex of plant, a = air-filled space. C, transverse section of thallus at internode
( X 1 50). D, longitudinal section of apex. E, transverse section of apex. F, cuticle
being shed (semi-diagrammatic). (A, C, after Getman; B, D, E, after Oltmanns;
F, original.)

pressions of the cell outlines, this feature perhaps being of signi-
ficance v^^hen the problems concerning the Nematophyceae are
considered (cf. p. 277). The genus is monotypic, the single species,
H. Banksii, being confined to Australia and New Zealand where it
grows on rocks and in tide pools of the littoral belt in positions that
are always exposed to the spray.

CSA

210

PHAEOPHYCEAE

Fucaceae: Notheia (a spurious thing). Fig. 141.

The fihform sporophyte grows out parasitically from the base of
conceptacles in Hormosira and Xiphophora, though it is present
most commonly on the microsporangiate hosts. The thallus is soHd
throughout and is composed of epidermal, cortical and medullary
tissues, the epidermis, like that of Hormosira, possessing a cuticle.
The genus differs from Hormosira in that growth is secured by a

Fig. 141. Notheia anomala. A, plant growing out from Hormosira. B, point of
entrance of parasite into host ( x 40). C, conceptacle with megasporangia and
branch shoot, ^ ( x 40). D, mature megasporangia with eight ova ( x 180). (A,
after Oltmanns; B-D, after Williams.)

group of three apical cells instead of four. Branching is irregular,
the new branches arising in the walls of old conceptacles from basal
cells which were dormant during the reproductive phase. There is a
degenerate holdfast which is composed of colourless elongated cells
that penetrate the host and act as absorbing organs, although there
are no actual haustoria. In those portions of Hormosira that are
attacked by the parasite the hollow of the vesicle-like internode
becomes filled up by new tissue formed as a result of the stimula-
tion, but the parasite is apparently unable to attack Hormosira
unless the host is growing in areas where it is continuously sub-

FUCALES (HETEROGENERATAE) 211

merged. The fertile conceptacles, which only contain megaspor-
angia, cover the entire frond, but as microsporangia have never
been recorded the occurrence of meiosis is extremely doubtful,
although at present there is no cytological evidence available. Each
mature megasporangium, which contains eight eggs, is surrounded
by unbranched paraphyses. The genus is monotypic and contains
the one species, Notheia anomala^ which, in view of its habit,
structure and life history, must be regarded as a degenerate type.

REFERENCES

Sargassum. Borgesen, F. (19 14). Mindeskr. Steenstr. p. 3.
Fucus. Bower, F. O. (1880). Quart. J. Micr. Sci. 20, 36.
Hormosira. Getman, M. R. (1914). Bot. Gaz. 58, 264.
Himanthalia. Gibb, D. C. (1937). J. Linn. Soc. {Bot.) 51, 11.
Fucus, Pelvetia. Inoh, S. (1935). J- Fac. Sci. Hokkaido Univ. 4, 9.
Fucus. Nienburg, W. (193 i). Wiss. Meeresuntersuch., Abt. Kiel, 21, 51.
Fucus. Roe, M. L. (1916). Bot. Gaz. 61, 231.
Sargassum. Simons, E. B. (1906). Bot. Gaz. 41, 161.
Notheia. Williams, M. M. (1923). Proc. Linn. Soc. N.S.W. 48, 634.

14-2

CHAPTER VIII

RHODOPHYCEAE

* Systematically these form a large but very uniform group in so
far as their reproductive processes are concerned, although they
may vary widely in the construction of the vegetative thallus. As in
the Chlorophyceae there is one section that is characterized by lime
encrustation, these algae having played a great part during past
geological ages in the building up of rocks and coral reefs (cf. p. 273),
a process which can still be seen going on in the tropical seas to-day.
Morphologically the thallus is built up on one of two plans :

(a) Central filament type in which there is a central corticated or
uncorticated main axis bearing the branches (fig. 144, B).

(b) Fountain type in which there is a mass of central threads all
of which lead out like a spray to the surface, e.g. Corallina.

The cells composing the plants are frequently multi-nucleate,
and contain, in addition to the components of chlorophyll, the red
pigment phycoerythrin together with phycocyanin in some
cases, whilst Polysiphonia is interesting in that it also contains
fucoxanthin. With the exception of the first subdivision, the Proto-
florideae, the cells remain united to one another after segmentation
by means of thin protoplasmic threads or plasmodesmaey which are
very conspicuous in the region of the fusion cell (cf. below), where
their size can be associated with the need for the transmission of
nutritive material. The reproductive bodies are very characteristic,
usually being found on separate plants, but the two sex organs
may occur on the same plant and certain abnormal cases are also
known where sexual and asexual organs are present on the same
thallus (cf. p. 236). The sexual plants are usually all of the same
size, but in Martensia fragilis and Caloglossa Leprieurii the male
plants are smaller than the female.

The male organs, which are probably best termed antheridia
although they have been given other names, each give rise to a non-
motile body, or spermatium, which is carried by the water to the
elongated tip (trichogyne) of the carpogonium or female organ. In this
respect it will be seen that the Rhodophyceae are very distinct from

RHODOPHYCEAE 213

the other algal groups. The carpogonium with its trichogyne is borne
on a special branch {procarp) consisting of a varying number of
cells, whilst the typical auxiliary cell, into which the fertilized
carpogonial nucleus generally passes, is often associated with this
branch, or else forms a part of it. The fertilized zygote commonly
gives rise first to a peculiar diploid generation, the carposporophyte,
w^hich consists of a series of filaments that cut off asexual bodies
or carpospores from their apices. These spores on germination usually
give rise to the asexual or tetrasporic plant which reproduces by means
of tetraspores that are formed in sporangia borne externally or else
sunk into the thallus (cf. fig. 155, F). A common feature in this
group that further emphasizes their uniformity is a tendency for the
2x number of chromosomes to be 40. The Rhodophyceae may be
regarded as the classical example of plants in which meiosis occurs at
different phases in the life cycle, for it may either occur immediately
after fertilization or else at some subsequent period. In the former
case the plants are said to be haplohionts as there is only one kind
of individual or biont, but the individual sexual haploid plants are
termed haplonts. If meiosis is delayed we get an asexual generation
alternating with the sexual and so there are two kinds of indi-
viduals or bionts: this type is therefore known as diplobiontic. It
may be pointed out here that other usages of these terms have been
employed, but the above definitions are those propounded by
Svedelius (1931) who coined the terms, and therefore they are the
correct way in which they really should be employed. The
classification of the Rhodophyceae is based primarily on the
structure of the female reproductive apparatus. After the Proto-
florideae, which lack pit connexions, have been segregated, the
remainder of the red algae, or Eu-florideae, are classified as
follows :

(i) Nemalionales and Gelidiales.

These are regarded as primitive orders which have become more
or less stabilized. In some genera there are no true auxiliary cells,
whilst in others the auxiliary cells are purely nutritive, but never-
theless the beginnings of an evolutionary series can be seen in the
following features :

{a) The development of the hypogenous cells of the carpogonial
branch to form storage organs.

214 RHODOPHYCEAE

(b) The development of special nutritive cells which will
ultimately replace the auxiliary cells in fulfilling the nutritional
requirements.

(c) The development of the carposporic filaments, or gonimo-
hlasts, into creeping threads w^hich may be able to utilize food
contained in neighbouring cells.

(2) Cryptonemiales.

Here there are definite pit connexions to the auxiliary cells,
which serve not only for nutrition but also as starting points for
the gonimoblast filaments. The auxiliary cells develop on special
branches before fertilization and are actively concerned in the post-
fertilization processes.

(3) Gigartinales.

A normal intercalary cell of the mother plant is set aside as an
auxiliary cell before fertilization.

(4) Rhodymeniales .

The auxiliary cells are small, and though cut off before fertiliza-
tion they only develop after that process has taken place.

(5) Ceramiales.

The auxiliary cell is cut oflP from a support cell after fertilization
and as a direct consequence of the process. Series (3)-(5) should
probably be regarded as examples of progressive reduction.

Auxiliary cells absent (Nemalionales)

(Gehdiales) No

Auxiliary cells present before (Cryptonemiales) procarp
fertilization (Gigartinales)

(Rhodymeniales)
Auxiliary cells develop after (Ceramiales) !- Procarp

fertilization J present

In 1926 Sjostedt created tw^o new orders, the Sphaerococcales and
the Nemastomales, but in this volume the genera composing these
two new orders are retained in the orders to which they have
belonged in the past.

The antheridial plants, which are often paler in colour and more
gelatinous, were first mentioned in a letter to Linnaeus in 1767

RHODOPHYCEAE 215

when they were considered to be male by analogy. The antheridia
are either borne over the whole surface (e.g. Dumontia), or else in
localized sori. These sori are reticulate in Rhodymenia, band-like in
Griffithsta, borne on special branches in Polysiphoniay sunk in
conceptacles in Laiirencia and occur on the tips of the thallus in
Chondrus. Very little is known about the seasonal periodicity of the
male plants, which are often less frequent than either the female or
tetrasporic plants, but this may be due purely to lack of observation,
although it is also possible that the male plants are gradually
becoming functionless. The antheridia often appear in an orderly
sequence, being cut off usually as subterminal or lateral outgrowths
from the antheridial mother cell. If they have been borne on a
special part of the thallus (e.g. Delesseria) this may fall off or die
away after fruiting is completed, whilst in other cases the mother
cells simply revert to a normal vegetative state. The different types
of male plant have been classified by Grubb (1925) as follows:

{a) The antheridial mother cell does not differ from the vege-
tative cells either in form or content, nor are the antheridia covered
by a continuous outer envelope, e.g. Nemalion, Batrachospermum.
(b) The antheridial mother cells are differentiated from the
vegetative cells, and the antheridia are surrounded by a common
outer sheath, which is later pierced by holes or else gelatinizes in
order to allow^ the ripe spermatia to escape :

(i) The antheridia develop terminally, e.g. Melobesia, Holmsella.
(2) The antheridia develop subterminally :

{a) Two primary antheridia, e.g. Delesseria sanguinea, Chon-
drus crispus.
(b) Two or three primary antheridia, e.g. Scinaia furcellata,

Lomentaria clavellosa.
{c) Three primary antheridia, e.g. Ceramium rubrum, Grif-
fithsta corallina.
(d) Four primar>^ antheridia, e.g. Polysiphonia violacea,
Callithamnion roseum.
The primary antheridia are commonly succeeded by a second crop
which arises within the sheaths of the first, but a third crop only
occurs in a few genera. In Nemalion, after the spermatium has
become attached to the trichogyne, the nucleus undergoes a
division but only one of the daughter nuclei acts as the fertilizing

2i6 RHODOPHYCEAE

agent: this feature has led to the suggestion that in the more
advanced red algae the contents of the antheridium are equivalent
to a body which formerly did divide.

The tetraspores are either formed in superficial tetrasporangia or
else they are sunk into the thallus, in which case the fertile branch
often becomes swollen and irregular in outline, whilst in the
genus Plocamium there are special lateral fertile branches or
stichidia. Meiosis normally occurs at the formation of the tetra-
spores, but when the spores develop on sexual haploid plants, as
sometimes happens, there is no meiotic division and the products
function as monospores. In Agardhiella tenera apospory is some-
times found and again there is no meiosis so that a succession of
asexual plants can occur. In the Nemalionales reproduction by
means of monospores is quite common though the homologies of
these bodies are somewhat uncertain. In some of the Eu-florideae
(Plumaria, Spermothamnion) polyspores or paraspores develop on
the diploid plants, but it has recently been shown that these are in
some cases morphologically equivalent to tetraspores, whilst in
others, e.g. Plumaria, they form the reproductive organs of a tri-
ploid generation (cf. p. 238). Experimental cultures made on oyster
shells have demonstrated that there are good grounds for believing
that of the four spores in a tetrad two will give rise to female plants
and two to male plants. Observ^ations have been published showing
that monospores, carpospores and tetraspores of some Rhodo-
phyceae appear capable of a small degree of motion, the spores of
the Bangiaceae being the most active among those investigated. The
mechanism of this movement is not understood, and it is doubtful
whether it is sufficient to give it any significance in the reproduc-
tive processes of the plants.

Whilst there are apparently very few truly parasitic species
among the Chlorophyceae and Phaeophyceae, nevertheless in the
present group there are some very definite partial or total parasites.
Ceramium codicola occurs on a Californian species of Codium and is
said to be a partial parasite ; Ricardia Montagnei is probably a total
parasite at some stage of its existence, and the members of the two
genera, Janczewskia and Peysonielliopsis, are probably entirely
parasitic. In European waters Choreonema, Schmitziella, Choreo-
colax, Harvey ella and Holmsella are all to be regarded as partial or
total parasites, and to this list Polysiphonia fastigiata should per-

RHODOPHYCEAE

217

haps be added, since it is always found on one particular host,
Ascophyllum. The order is principally marine, but there are a few
fresh-water representatives, e.g. Batrachospermum, Lemanea and
Hildefibrandtia, which are usually confined to fast-flowing streams
where there is an abundance of aeration.

Proto-florideae

Bangiaceae: Porphyridium cruentum (porphyridium, diminutive of
purple dye; cruentum, blood red). Fig. 142.

This alga has had an extremely varied history, having been
placed at various times in both the Palmellaceae and Schizogonia-
ceae of the Chlorophyceae, near to Aphanocapsa in the Cyano-
phyceae, and among the Bangiaceae in the Rhodophyceae where

ABC D E

•'F

Fig. 142. Porphyridium cruentmn. A-E, stages in nuclear and cell division
( X 1280). F, cells connected by stalks after division ( x 1280). (After Zirkle and
Lew^is.)

it finds a home at present. The single cells are united into a one-
layered, gelatinous colony of a blood red colour which is found on
the soil. Cell divisions take place in all directions, and when a
cell divides the sheath elongates to form a kind of stalk which
eventually ruptures. So far no form of sexual reproduction has
been observed. In each cell there is one large chromatophore with
cyanophycin granules around the periphery and also a central
nuclear-like body, composed largely of anabaenin, which undergoes
a primitive form of mitosis at cell division. Whether this alga
represents a primitive form or else is a much-reduced type cannot
at present be determined.

2i8 RHODOPHYCEAE

*Bangiaceae: Porphyra (purple dye). Fig. 143.

This is a genus which has a very wide range as it extends in the
northern hemisphere from 40° to 71° N. and in the southern from
the Cape of Good Hope to 60° S. It has a variable seasonal
periodicity in English waters where its presence is determined by
the amount of water available, e.g. whether the site is subject to
spray, together with the intensity of light and shade. The plant is

Fig. 143. Porphyra. A, thallus ( x |). B, attachment disk with three primary
blades and four (1-4) secondary. C, formation of carpospores. D-H, formation
of antherozoids in P. tenera. (A, D-H, after Ishikawa; B, C, after Grubb.)

flat and membranous, whilst in the common species, P. umhilicalis,
there are a number of growth forms, the shape, width and length
of the various forms being determined by the age of the plant, the
height above mean sea-level and the type of locality. The plants
are attached by means of a minute adhesive disk which is capable
of producing lateral extensions from which new fronds may be
proliferated. The disk is composed of long slender filaments
together with some short stout ones, those near to or in actual

RHODOPHYCEAE 219

contact with the substrate swelUng up, branching and producing
suckers or haptera which are apparently capable of penetrating
dead wood or the tissue of brown fucoids. In the latter case there
is evidently a capacity for epiphytism once contact is secured, and
there is even some evidence of partial parasitism. In California,
P. naiadiim is an obligate epiphyte on Phyllospadix and Zostera,
two marine phanerogams.

The gelatinous fronds of Porphyra, which are normally mono-
stromatic although they become distromatic during reproduction,
are composed of cells that possess stellate chromatophores with a
pyrenoid, the process of nuclear division being intermediate between
mitosis and amitosis. Reproduction is by means of monospores,
carpogonia, which have rudimentary trichogynes, and antheridia,
the carpogonial areas occupying a marginal position on the thallus.
All the frond, except the basal region, can produce antheridia, but
fertilization has never actually been observed although there is
strong evidence which suggests that it does take place. The male
thalli are paler in colour than the female, and each antheridial
mother cell gives rise to sixty-four or 128 antheridial cells, each of
which produces one spermatium. The fertilized carpogonium divides
into four or eight cells that represent primitive carpospores ; these
are typically diploid whereas here they are haploid because a form
of meiosis occurs when the fertilized carpogonium begins to divide.
The carpospores eventually germinate to form a creeping filament,
and it has recently been shown that spores from these threads
are liberated and when germination has commenced it represents
the commencement of a new Porphyra plant. It is suggested that
the protonemal stage is equivalent to an adelophycean or dwarf
generation in the life cycle, and further work on this part of the life
history might produce interesting results.

The plant, which is called ''lava" in England, "sloke" in Ireland
and "slack" in Scotland, was formerly used as a food when it had
been boiled and seasoned with spices and butter. It is still used as
a food and medicine in Hawaii under the name of Lt?nu Luau. In
Japan, where there are over 2000 acres in cultivation, it is grown on
bamboo bushes planted out between the tide marks where there is
a depth of 10-15 ft. at high water. After collection, the plants are
stirred in fresh water in order to cleanse them, chopped up into
small bits, dried in the air and then pressed into sheets which, after

220

RHODOPHYCEAE

crisping over a fire, can be dropped into culinary dishes in order
to add a savour.

Eu-florideae

NEMALIONALES, GELIDIALES

*Batrachospermaceae : Batrachospermum [batracho^ frog ; spermiim,
seed). Fig. 144.

Two genera commonly found in fresh waters, Batrachospermum
and Lemanea, belong to the Nemalionales. Batrachospermum
moniliforme, which is a very variable species, is found attached to

ilV

his

Fig. 144. Batrachospermum moniliforme. A, plant. B, portion of plant. C, carpo-
gonial branch ( x 480), D, fertilized carpogonium ( x 360). E, mature cystocarp
( X 240). F, antheridia ( x 640). (A, B, after Oltmanns; C-F, after Kylin.)

stones in swift-flowing waters of the tropics and temperate regions.
The thallus is soft, thick and gelatinous, the primary axis, which
grows from an apical cell, being formed of a row of large cells.
Numerous branches arise in whorls from the nodes, the basal
regions of these branches producing corticating cells that grow
downward and invest the main axis. The cells of the thallus are
uninucleate and contain only one pyrenoid. Reproduction takes
place by means of monospores, carpogonia and antheridia, the
latter organs arising as small, round, colourless cells at the apices of
short, clustered, lateral branches. The carpogonia are also terminal

NEMALIONALES 221

and possess a trichogyne which shrivels away after fertihzation.
The nucleus of the fertilized carpogonium divides twice, thus
giving rise to four cells, and from these the short gonimoblast
filaments grow out and finally terminate in a sporangium that
produces a single naked carpospore which soon secretes a cell.
A character of many of the Nemalionales is the occurrence of
meiosis immediately after fertilization so that the carposporophyte
is haploid as in the Proto-florideae.

The life history of the related genus Nemalion is similar to that of
Batrachospermum, except that when the spermatia are liberated
the nuclei are often in prophase, the division being completed when
they have become attached to a trichogyne. This division has
suggested to some workers that the spermatium is really homo-
logous to an antheridium, but it might also be argued that it is a
relic of a time when an antheridium produced more than one
spermatium.

*Chaetangiaceae : Scinaia (after D. Scina). Fig. 145.

This is a widespread genus with its home primarily in the

northern hemisphere, the commonest species, S. furcellata, being

monoecious, although one may find monospores and spermatia on

the same plant. The fronds, which arise from a discoid holdfast,

are subgelatinous, cylindrical or compressed and dichotomously

branched. The centre of the thallus is composed of both coarse and

fine colourless filaments, the former arising from the apical cell and

the latter from the corticating threads. There is also a peripheral

zone of horizontal filaments that terminate in short corymbs of

assimilatory hairs with a large colourless cell in the centre. These

two types of epidermal cell are apparently differentiated near the

apex of the thallus, the small ones giving rise to hairs, monospor-

angia or antheridia. The large colourless cell is said to form a

protection against intense light, but it may also be a relic of a

tissue which formerly had a function that has since been lost. One

or two spores are formed in each monosporangium, whilst the

spermatia arise in sori, forming bunches of cells at the ends of the

small-celled assimilatory branches. The carpogonial branch is

three-celled, the reproductive cell containing two nuclei, one in the

carpogonium proper and one in the trichogyne. The second cell of

the carpogonial branch gives rise to a group of four auxiliary cells

222

RHODOPHYCEAE

which are rich in protoplasm, whilst the sterile envelope of the
cystocarp arises from the third cell.

6 6 6 6 6

Fig. 145. Scinaia furcellata. A, carpogonial branch ( x 700). B, fertilized carpo-
gonium. C, cystocarp ( x 195). D, plant ( x ^). E, antheridia ( x 700). F, young
carpogonial branch ( x 425). G, young cystocarp ( x 232). H, undifferentiated
threads at apex of thallus ( x 425). I, monospores and a hair ( x 340). J, dif-
ferentiated cortex ( X 429). K, life-cycle diagram. (C, after Setchell; D, original;
rest after Svedelius.)

It is now certain that in the related genus Chaetangium, and
probably also in Galaxaura, the wall of the cystocarp arises from
the cell containing the fertilized nucleus, so that it is composed of
fertile gonimoblasts and not sterile tissue. The fertilized nucleus in

NEMALIONALES

223

Scinaia travels to the four auxiliary cells which have fused together
and there meiosis {n=io) occurs, after which one daughter nucleus
passes back into the carpogonium and is concerned with the
development of the gonimoblasts. There are, of course, no diploid
plants because meiosis occurs immediately after fertilization.

Chaetangiaceae : Liagora (after one of the nereids). Fig. 146.

The principal interest of this genus, which is very similar
morphologically to Scinaia, is provided by the species, Liagora

Fig. 146. Liagora. A, carpospores of L. ■ywa'^a ( x 320). B, carpospores in fours
in L. tetrasporifera ( x 320). C, life cycle of L. tetrasporifera. (A, B, after Kylin;
C, after Svedelius.)

tetrasporifera, an inhabitant of the Canary Islands. The carpospores
of this plant divide to give four spores which must probably be
regarded as tetraspores, and although no cytological evidence is
available, nevertheless it is presumed that meiosis is delayed to the
time when the carpospores germinate. In this species, therefore,
the carposporophyte is diploid, but at the same time no independent
tetrasporic diploid generation develops. The remaining species
of the genus behave like the other members of the Nemalionales,
although in L. viscida the carpogonial branch is five-celled instead
of the usual three cells.

Gelidiaceae: Gelidium (congealed). Fig. 147.

In this genus there is no auxiliary cell, but the presence of the
nutrient cells results in the production of a complex structure

224

RHODOPHYCEAE

composed of several carpogonia together with nutrient cells, and
more than one of these carpogonia may be fertilized. The genus is
the principal source of agar-agar, a gelatinous medium much used
in mycology and bacteriology, in the manu-
facture of size and in culinary operations.
Agar-agar is manufactured primarily in -*^ ^^

Japan where it possesses various names,
Kanten, Japanese, Bengal or Oriental isin-
glass, and Ceylon or Chinese moss. The
plants contain about 76 % of the primary V^
gelatinous material, gelose, and are dived ^^^
for between May and October, after which
they are allowed to dry and bleach in the
open, and then they are sold to factories up
in the mountains where the air is pure,
dry and cold. Here the alga is cleaned, Fig. 147. Gelidiumcorneum.
drained and fused into sheets and the (After Oltmanns.)
jelly extracted by boiling. After straining, the jelly is poured into
wooden trays and allowed to cool and then it is cut into bars. In
former times the algae were just simply dried in the sun and the
jelly extracted afterwards by boiling.

CR YPTONEMIALES

*DuMONTiACEAE : Dudresuaya (after Dudresnay de St-Pol-de-Leon).
Fig. 148.
The cylindrical, much-branched thallus is soft and gelatinous,
consisting when young of a simple articulated filamentous axis with
whorls of dichotomously branched ramuli, although in older plants
the central axis becomes polysiphonous and clothed with densely
set whorls of branches. The plants are dioecious, the males being
somewhat smaller, paler and fewer in number than the females.
The carpogonial branches of D, coccinea arise from the lower cells
of short side branches and when fully developed are composed of
seven to nine cells : they are branched once or twice and may have
short sterile side branches arising from the lowest cell. In the middle
of the mature carpogonial branch there are two to three larger cells
which function in a purely nutritive capacity, whilst the auxiliary
cells develop in similar positions on neighbouring branches that

CRYPTONEMIALES

225

are homologous with the carpogonial branches. After fertiHzation
the carpogonium sends down a protuberance containing the
diploid nucleus and this cuts off two cells when it is near to the
nutrient cells of the carpogonial branch. These all fuse together
and sporogenous threads, each carrying a diploid nucleus, then
grow out towards the auxiliary cells on the other branches. When

Fig. 148. Dudresnaya. A-D, stages in development of cystocarp, D. purpurifera.
E, F, stages in development of cystocarp in D. coccinea after fertilization ( x 486).
G, D. coccinea, carpogonial branch ( x 486). H, D. coccinea, antheridia (X510).
(A-D, after Oltmanns; E-G, H, after Kylin.)

these filaments fuse with an auxiliary cell the latter forms a pro-
tuberance into which the diploid nucleus passes, and after this has
divided once the protuberance containing one of the daughter
nuclei is cut off by a wall. The gonimoblast filaments then grow out
as a branched mass from this protuberance of the auxiliary cell.
Each sporogenous thread sent out from the original fusion cell may
unite with more than one auxiliary cell in the course of its wander-
ings through the thallus, so that one fertilization may result in the
production of a number of carposporophyte generations.

CSA

226

RHODOPHYCEAE

SQUAMARiACEAE:Hildefibrandtia{2ifterF. E. Hildenbrandt). Fig. 149.

This genus is characteristic of a small group of red algae all of
which form thin crusts on stones or other
algae, and it is frequently difficult to dis-
tinguish in the field from similar encrusting
brown types such as Ralfsia. The frond is
horizontally expanded into a thin encrusting
layer composed of several layers of cells
arranged in vertical rows, the plants form-
ing indefinite patches that are attached by
a strongly adhering lower surface. The
genus is both marine and fresh water,
Hildenbrandtia rivularis appearing fre-
quently in rivers and streams. The principal

mode of reproduction is by means of ^^g- ^49- Hildenbrandtia

\ . , II- • prototypus. Tetraspores in

tetraspores which are produced m sporangia conceptacles ( x 320). (After

borne in rounded or oval conceptacles that Taylor.)
are sunk in the thallus.

*CoRALLiNACEAE : EpiUthon (epi, above, lithon, stone). Fig. 150.

This and the succeeding type belong to the Corallinaceae, a family
of calcareous red algae which have played much part in the building
up of rocks and coral reefs and which have been known as fossils
from the earliest geological strata. The present type has been
selected because the common species, E. mefnbranaceum, is less
calcified than other members of the Corallinaceae and thus forms
very convenient material for sectioning and demonstration pur-
poses without the trouble of decalcification. The thallus, which
forms a crust on other algae or phanerogams, consists of a single
cell layer composed of large cells, from each of which is cut off a
small upper cell that goes to form the outer lime-encrusted layer.

Further divisions take place internally from the large basal cells
so that one finally obtains rows of erect filaments growing side by
side. The various reproductive organs are borne in conceptacles
on separate plants; in the male plants, for example, there are a
number of two-celled filaments in the centre of every conceptacle.
The basal cells of these threads cut off two antheridial mother cells
which in their turn produce two antheridia, whilst the upper cells
grow out to form the walls of the conceptacle. In the female plant

CRYPTONEMIALES

227

the central threads form three-celled carpogonial branches, whilst
the outer threads develop into two-celled filaments that are
modified auxiliary cells. After fertilization the carpogonium and
the cell below it fuse together and send out a filament to the lower
cell of the auxiliary branch. Finally, all the auxiliary' and nutritive
cells fuse to give one long fusion cell from which very short
gonimoblast filaments grow out. In the tetrasporic plant there are

- -M

c^c5c:scfac5) F

a e
'« «

t UQDOfiQQQQtSlQQQQQ

Fig. 150. Epilithon membranaceum. A, carpogonia ( x 360). B, conceptacle with
ripe carpospores ( x 240). C, young antheridial conceptacle ( x 510). D, mature
antheridial conceptacle ( x 426). E, tetraspores ( x 228). F, G, thallus construction
( X 360). (After Kylin.)

simple filaments which give rise to the tetrasporangia ana branched
sterile filaments that form the roof to the conceptacle by the process
of division and elongation, the original roof being cast off: finally,
a pore develops above each group of tetraspores.

*CoR.\LLiNACEAE : CoralUna (coral). Fig. 151.

Both this and the preceding genus are examples of the "fountain"
type of construction (cf. p. 212). In Epilithon the original
construction has been much modified because of its habit, but
it can be observed extremely well in CoralUna. The erect plants,
which are jointed, cylindrical or compressed, arise from calcified
encrusting basal disks or prostrate interlaced filaments. Branching,
which is frequent, is either pinnate or dichotomous. There is a
central core of dichotomously branched filaments with oblique
filaments growing out at the swollen internodes to form a cortical

15-^

228

RHODOPHYCEAE

layer, the whole being covered by a dense coating of lime, whilst in
C. riibens there may also be epidermal hyaline hairs. The plants are
monoecious or dioecious, the reproductive organs being borne in
terminal or lateral conceptacles. The carpogonia, which are not

Fig. 151. Corallina officinalis. A, portion of plant. B, the same enlarged.
C, carpogonial conceptacle (x2io). D, single carpogonial branch ( x 342).
E, fusion cell, gonimoblasts and carpospores (xiao). F, development of
antheridia ( x 420). G, mature spermatia ( x 648). H, young tetrasporic con-
ceptacle ( X 240). I, mature tetraspore ( x 270). (A, B, after Oltmanns; rest after
Suneson.)

calcified, arise from a kind of prismatic disk formed from the
terminal cells, these cells also functioning later as the auxiliary
cells. As a result of oblique divisions, one to three embryo carpo-
gonial branches are formed on each mother cell, but only one of
these finally develops into the mature two-celled carpogonial
branch with its long trichogyne. After fertilization a long or

CRYPTONEMIALES

229

rounded fusion cell is formed by the auxiliary cells, and this
contains both fertilized and unfertilized carpogonial nuclei. The
antheridia are much elongated, and after liberation the spermatia
round off and remain attached to the antheridial wall by means of a
long thin pedicel in C. officinalis and by a short stalk in C. ruhens.

CERAMIALES
Delesseriaceae : Delesseria (after Baron Delessert). Fig. 152.
The large, thin, leafy fronds, which are bright red in colour,

Fig. 152. Delesseria sanguinea. A, plant. B, apex of thallus to show cell
arrangement ( x 258). C, first stage in formation of carpogonial branch, st-i = first
group of sterile cells ( x 408). D, later stage of same, c/) = carpogonial branch,
5C = support cell, 5^1 = first, and 5?2 = second group of sterile cells ( x 408). E,
mature carpogonial branch, sc = support cell, sti = first sterile branch, st2 = second
sterile branch ( x 720). F, formation of antheridia in related genus, Nitophyllum.
G, transverse section of mature cystocarp in the related genus Nitophyllum.
H, tetraspores ( x 360). (A, F, G, after Tilden; B-D, after Kylin; E, H, after
Svedelius.)

230 RHODOPHYCEAE

possess a very conspicuous mid-rib with both macro- and micro-
scopic veins and they form magnificent plants for pressing as
herbarium specimens. The complex nature of the laciniate or
branched thallus can be seen from the figure. There are three
orders of cells with considerable intercalary division, although the
cortication of the primary cell filaments to form the veins does not
involve intercalary division. The cells of the thallus also become
united by means of secondary protoplasmic threads and they may
also develop thin rhizoids. The cystocarps are small stalked bodies
which are borne on the mid-rib, whilst the tetrasporangia are
produced in special fertile leaflets that arise from the mid-rib, but
as these do not possess the power of intercalary growth they differ
slightly in structure from the vegetative thallus. In the related
genus Martensia each tetraspore mother cell is multinucleate,
containing about fifty nuclei all of which degenerate except for one,
and from this the four nuclei of the tetraspores are produced.

"RnoTiOMELkCEKE: Janczewskia (after E. de Janczewski). Fig. 153.

This is a remarkable hemi- or holo-parasitic genus which is
always to be found on other members (Laurencia, Chondria and
Cladhymenia) of the same family. One of the most interesting
features of this parasitism is that the genus is very closely related to
Laurencia and yet is parasitic upon various species of that genus.
All the species have organs of contact or penetration, the latter
being fungal-like filaments which establish pit connexions with the
cells of the host. Each individual plant is a coalescent tubercular
mass composed of fused branches that grow from an apical cell
buried in a pit as in Laurencia. The sexual plants are dioecious and
the diploid asexual plant also occurs.

*Rhodomelaceae : Polysiphonia {poly, many; siphonia, siphons).
Fig. 154-
The thallus in this genus generally arises from decumbent basal
filaments that are attached to the substrate by means of small
flattened disks. Many species are epiphytic on other algae whilst
P. fastigiata, which is always found on the fronds of the fucoid
Ascophyllum nodosum, is probably a hemi-parasite. The thallus is
laterally or dichotomously branched and bears numerous branches
which are shed annually in the perennial forms before winter and

CERAMIALES

231

are re-developed in the spring. The main axes and branches are
corticate or ecorticate, and possess a polysiphonous appearance due
to the single axial cell series being surrounded by four to twenty-
four pericentral cells or siphons. The corticating cells, when
present, are always shorter and smaller and are often only found
in the basal portions of the stem. The ultimate branches are not

Fig. 153. Janczeivskia. A, J. moriformis on Chondria sp. ( x 6). B, filaments of
J. lappacea in host, Chondria nidifica ( x 180). C, longitudinal section of cystocarp
of y. moriformis (xi8o). D, antheridial conceptacle of J. lappacea (xiSo).
(After Setchell.)

polysiphonous and frequently terminate in delicate multicellular
hairs.

The colourless antheridia, which are formed in clusters, are borne
on a short stalk that morphologically is a rudimentary hair. In
Polysiphonia violacea, where the haploid number of chromosomes
is twenty, the two basal cells of the hair are sterile, the upper one
giving rise to a fertile polysiphonous branch and a sterile hair. One
or more mother cells are formed from all the pericentral cells on
the fertile branch, and each mother cell produces four antheridia

232

RHODOPHYCEAE

in two opposite and decussate pairs, the first and third appearing
before the second and fourth. There is no secondary crop in this
species. The carpogonial branches are also formed from hair

Fig. 154. Polysiphonia violacea. A, plant of P. nigrescens ( x ^). B, life cycle.

C, apex and cells cut off from central cells. D, thallus construction in longitudinal
section. E, transverse section of thallus, P. /a^fz^/a^ww. Z = young tetraspore. F, pro-
toplasmic connections of axial thread. G-J, stages in development of carpospores.
c/) = carpogonium, a = auxiliary cell, g = gonimoblast, 5f = sterile cells ( x 400,
J X 260). K, cystocarp of P. nigrescens with, ripe carpospores ( x 33). L, antheridial
branch (X35). M, a-f, stages in development of antheridia. N, P. nigrescens,
tetraspores ( x 33). (A, K, N, after Newton; B, after Svedelius; C, F, schematic;

D, E, after Oltmanns; G-J, after Kylin; L, after Grubb; M, after Tilden.)

rudiments, the support cell cutting off a small section from which
lateral sterile cells arise. Later on a fertile pericentral cell is cut
off, and this gives rise to the four-celled carpogonial branch, the
carpogonium being of interest because there is also a persistent
nucleus in the trichogyne.

CERAMIALES 233

After fertilization has taken place the auxiliary cell is cut off from
the apex of the fertile pericentral cell and in addition two branch
systems composed of nutrient cells appear. When the zygote nucleus
has divided the two daughter nuclei (only one of the two in
P. nigrescens) pass into the auxiliary cell which has become fused to
the carpogonium in the meantime, and there the two nuclei are
isolated from the carpogonium by a new wall. By this time the
carpogonium and its three lower cells have broken down. The
auxiliary cell then fuses with the pericentral cell and after the two
diploid nuclei have passed into it, it unites with the other support
and axial cells to give a large fusion cell. The diploid nuclei undergo
a number of divisions and the products pass into lobes that are
budded off from the fusion cell. Each lobe then gives rise to a
two-celled gonimoblast filament, the first cell acting as a stalk cell
whilst the end cell produces a carpospore. The wall of the cystocarp
is two-layered, the outer wall being formed from the lateral sterile
cells that are cut off from the support cell, whilst the inner lining is
formed from the axial cell of the fertile segment. The tetrasporangia,
which develop from pericentral cells, are protected by being
embedded in the thallus, a feature which results in the fertile
branch usually being much swollen and distorted.

Ceramiaceae : Griffithsia (after Mrs Griffiths). Fig. 155.

The monosiphonous ecorticate fronds are composed of large
muhinucleate cells connected to each other by a pore, although
this is often closed by a plug. In G. glohulifera the larger cells may
each have as many as 3000-4000 nuclei. Vegetative division is
brought about either by the cutting off of terminal segments from
the end cells or else by the delimitation of a small cell from the
upper edge, but as this grows very rapidly by mere swelling the
appearance of a false dichotomy is produced. In G. corallina
miniature shoots and also delicate colourless branched hairs de-
velop from the large cells of the main thallus. Regeneration can
occur in order to replace an old cell or one that has been wounded,
the process involving the two neighbouring cells which send out
tubes that meet and fuse. The sessile antheridia are borne on the
distal ends of much-branched dwarf shoots which surround the
nodes of the main thallus in tufts or dense whorls, each branch
arising as a protuberance that is cut off from one of the large axial

234

RHODOPHYCEAE

cells. The primary cell of a carpogonial branch, which is cut off
from the apex of a growing cell, becomes pushed down to the side

Fig. 155. Griffithsia corallina. A, portion of plant with short shoots and
branched hairs ( x 18). B, short shoot magnified ( x 312). C, carpogonial branch
( >< 370)- ac = auxiliary' cell, cci, cc^, CC3 = central cells, ^c = pericentral cell,
5C = support cell, s?c = sterile cell. D, antheridial branch ( x 720). E, plant with
antheridia {a) {xyz). F, tetrasporic branch ( x 222). (A-C, F, after Kylin;
D, E, after Grubb.)

where it divides into three cells. The second cell forms the fertile
central cell and gives rise to three pericentral cells, one of which
produces a one-celled branch whilst the others produce two-celled
branches. The basal cell of each of these two-celled branches gives

CERAMIALES . 235

rise to a four-celled carpogonial branch. In the original branch of
three cells the first cell gives rise to a protective branch after fruiting
has occurred, whilst the third cell remains sterile throughout. The
tetraspores, which are borne in whorls, are partly covered by in-
volucral cells. At tetraspore formation, after a small support cell
has been cut off from an ordinary vegetative cell it proceeds to cut
off several side cells, each of which functions as a tetrasporangium.
Finally the support cell cuts off two sterile cells at its apex, the
distal one enlarging to become a protective cell for the whorl of
tetraspores.

*Ceramiaceae: Callithamnion {calli^ beauty; thamnion^ small
bush). Fig. 156.

This is a genus of very beautiful and delicate plants that possess
filamentous branched fronds which are either monosiphonous or
else corticated at the base, the cortication being formed by rhizoidal
filaments. The cells of the vegetative thallus are multinucleate,
and in C. hyssoides there are protoplasmic pseudopodia projecting
internally from the ends of the cells, and although these strands are
apparently capable of some movement their function is obscure.

The antheridia, which form hemispherical or ellipsoidal tufts on
the branches, arise as lateral appendages, the first cell to be cut off
being the stalk cell. This stalk cell gives rise to a group of secondary
cells which later on divide to form branches composed of two to
three cells, each terminating in an antheridial mother cell. In this
genus there may be two or even three crops of antheridia arising
successively in the same place, each mother cell producing about
three antheridia in every crop. The cystocarps, which are usually
present in pairs and enclosed in a gelatinous envelope, arise as
follows. Two cells are cut off from a cell in the middle of a branch
and these function as the auxiliary mother cells. From one of them
the four-celled carpogonial branch is produced, whilst after fertiliza-
tion both auxiliary mother cells divide and cut off a small basal
cell. The fertilized carpogonium also divides into two large cells,
each of which cuts off a small sporogenous cell that fuses with the
adjacent auxiliary cell. As a result of this fusion each auxiliary cell
can receive a diploid nucleus which soon after its entry divides into
two ; one daughter nucleus passes to the apex of the auxiliary cell,
whilst the other, together with the nucleus of the auxiliary cell, is

236

RHODOPHYCEAE

cut off by a wall. It is from the large upper cell that the gonimo-
blast filaments arise and so the mature cystocarp is produced.

The sessile tetrasporangia arise in acropetal succession as lateral
outgrowths of the vegetative cells of young branches. In C. hra-
chiatum mature tetrasporangia and antheridia have been found
on the same plant, whilst other plants have been reported that bear

Fig. 156. Callithamnion. A-I, stages in development of carpospores after
fertilization. J, antheridia. K, the same enlarged. L, secondary spermatium.
M, young tetraspore. N, mature tetraspore. O, amoeboid processes. (A-I, after
Oltmanns; J-L, after Grubb; M, N, schematic after Mathias; O, after Phillips.)

both tetrasporangia and cystocarps. In these cases the nuclei of the
carpospores were found to be haploid whilst those of the vegetative
cells were diploid, so that if fertilization occurred there must have
been two meiotic divisions, one before and one after fertilization.
If only one meiotic division occurs then it must be supposed that
the carpospores arose apogamously. Spermothamnion Turneri is
another plant in which sex organs have also been reported on

CERAMIALES

237

normal tetrasporic plants, but as the procarp bi^anch in this case
develops normally without meiosis the carpogonium is diploid.
Fusion of the nuclei in the carpogonium has been observed so that
the gonimoblast filaments must be tetraploid, but unfortunately
the fate of the carpospores is not known. In S. Snyderae the tetra-
sporangia are replaced by polysporangia which must be regarded as
homologous structures. The mother cells of each polysporangium
contain two to nine nuclei and they give rise to twelve, sixteen,
twenty, twenty-four or twenty-eight spores.

Ceramiaceae: Plumaria {pluma, soft feather). Fig. 157.

The filamentous thallus is much branched, the main axis, which
is monosiphonous throughout, being ecorticate near the apex but

Fig. 157. Plumaria elegans. A, plant (xf). B, antheridial ramuli (xi8o).
C, paraspores (X213). D, tetrasporic ramuli (X126). (A, original; B, after
Drew; C, D, after Suneson.)

corticate below. The antheridia are borne on special branches,
whilst the four-celled carpogonial branch develops from the sub-
terminal cell of an ordinary branch. In northern waters P. elegans
never bears sex organs and only plants with paraspores are to be
found, whilst in southern waters the sexual (« = 3i) and tetrasporic
plants {n = 62) are predominant. Recent investigation has shown
that in this species we are concerned with a triploid race (n = g2) ^^
the northern waters which reproduces by means of paraspores.
There is apparently no relation between the triploid plants and the

238

RHODOPHYCEAE

other two races, and, furthermore, the triploid has the wider distri-
bution because it is able to penetrate into the colder waters of the
north. Tetraspores are to be found on the triploid plants but their
chromosomal complement and fate are not known. Although both
tetra- and parasporangia arise from a single cell it is doubtful if the
two structures are homologous. The reasons for this are first, the
difference in chromosomal complement, secondly, the absence of
any apparent relationship with the haploid and diploid plants, and
thirdly, differences in the mode of development of the para- and
tetrasporangia. This is the first cytological record of triploid plants
in the algae. Paraspores are also known in the related genus
Ceramium but their cytology, and hence homologies, are not known.

GIGARTINALES
Choreocolacaceae : Harveyella (after G. Harvey). Fig. 158.

This and the closely allied genus Holmsella are monotypic
genera each containing a holo-parasitic species, whilst Choreocolax

Fig. 158. Harveyella and Holmsella. A-E, stages in development of gonimo-
blasts after fertilization in Harveyella mirabilis. a = auxiliary cell, 5 = sterile
filaments. F, filaments of parasite, Holmsella pachyderma, in host. G, antheridia
of Harveyella mirabilis. H, tetraspores in Holmsella pachyderma. e = tracks left
after tetraspores have escaped. ^ = sterile cells, i = tetraspores in various stages,
ie = escaping tetraspores. (After Sturch.)

is another parasitic genus very nearly related to them. Harveyella
mirabilis is parasitic on species of Rhodomela whilst Holmsella
pachyderma parasitises Gracilaria confervoides. They have little or no

GIGARTINALES

239

colour of their own as might be suspected from their parasitic
nature, and they send out branched filaments or haustoria into the
tissues of the host. The parasites appear as external cushions lying
on the branches of the host, each cushion, which is surrounded by
an outer gelatinous coat, consisting of a central area that is four to
five cells thick. In Holmsella the carpogonial branch is two-celled
whilst in Harveyella it is four-celled, this feature forming one of the
principal differences between them. The antheridial, carpogonial
and tetrasporic plants are all separate, and the species are said to
pass through the full floridean life cycle twice every year. It is
clear that their much-reduced morphological features are to be
associated with the parasitic habit, and have probably arisen as a
result of the adoption of parasitism.

GiGARTiNACEAE : Choudrus (cartilage). Fig. 159.

This is a widespread genus, many of the species appearing as a
number of varieties, some of which are probably only ecological

Fig. 159. Chondrus crispiis. A, plant ( x f ). B, transverse section of thallus ( x 344).
(A, after Newton; B, after Kylin.)

forms. Chondrus crispus, which is known as '' Irish moss", contains
80 % of water together with salts that control gelatinization. The
plants are often collected and bleached, and then an extract is

240

RHODOPHYCEAE

obtained which can be used in the curing of leather and the
manufacture of size, and also for puddings and medicinal purposes.

*GiGARTiNACEAE : Phyllophora {phyllo, leaf; phora, bear). Figs.
160, 161.
The stipitate fronds expand upwards into a rigid or membranous
flat lamina which is either simple or divided, whilst proHferations
may also arise from the margin or basal disks. Morphologically the
thallus is composed of oblong polygonal cells in the centre bounded
on the outside by cortical layers of minute, vertically seriate
assimilatory cells. In some species secondary tissue has been
observed near the axils of branches or at the base of the frond. The

Fig. 160. Phyllophora Brodiaei. A, plant ( x ^). B, carpogonial branch ( x 250).
C, transverse section of antheridial thallus ( x 450). D, nemathecia with tetraspores
(x 125). (A, original; B-D, after Kylin.)

plants are dioecious and the sex organs are borne in cavities m
small fertile leaflets that are attached to the main thallus, the carpo-
gonial leaflets, which are sessile or shortly stalked, arising laterally
from the stipitate part of the main blade. In P. memhranifolia the
carpogonial branch is three-celled and after fertilization gonimo-
blast filaments are formed which ramify in the tissues, finally
producing pedicellate or sessile cystocarps. In P. Brodiaei the
carpogonium fuses directly with the auxiliary cell and the carpo-
sporic generation is omitted. This method of reproduction must be
regarded as reduction from the ordinary process in so far as the usual
rhodophycean life cycle is concerned. The tetraspores are borne in
moniliform chains packed into wart-like excrescences or nemathecia
which are borne on the female sexual plant. In P. Brodiaei the
absence of carpospores led earlier investigators to regard the

GIGARTINALES

241

nemathecia as belonging to a parasitic plant, which in this case was
given the name of Actinococcus subcutaneus, but it has since been
shown that we are really dealing with a parasitic diploid generation.
In the related genus Ahnfeldtia, although reduction of the life
cycle has gone still further, nevertheless nemathecia still appear and
these also were formerly regarded as a parasite to which the name
Sterrocolax decipiens was given. In this genus, however, there is

— s

Fig. 161. Life cycles. A, Phyllophora viembranifolia. B, P. Brodiaei. C, Ahn-
feldtia plicata. 5 = nionospores. (After Svedelius.)

neither fertilization nor meiosis and only degenerate procarps are
formed; instead the nemathecia contain monospores that develop
as follows. The warts, which arise as small cushions from superficial
cells of the thallus, contain some cells that become flask-shaped
together with other cells possessing denser contents that arise in
groups at the upper ends of the filaments. These latter, which
probably represent degenerate carpogonia, form the generative
cells and they give rise to secondary nemathecial filaments, the
apical cells of which function as the monosporangia. In Ahnfeldtia,
therefore, the sporophytic generation has been completely sup-
pressed, and this modified life cycle should be compared with that

CSA

242

RHODOPHYCEAE

of Lomentaria rosea (cf. below) in European waters where the
gametophytic generation has been secondarily suppressed. The
monospores have been interpreted as morphologically equivalent
to either the carpospores or the tetraspores, the latter interpretation
being the one adopted in this volume.

RHOD YMENIALES

*Rhodymeniaceae: Lomentaria (pod with constricted joints). Fig.
162.

The filamentous fronds are hollow with constrictions at the
nodes, whilst branching is irregular or unilateral. The hollow
central region originates from a branching structure which later on

Fig. 162. Lomentaria clavellosa. A-C, development of carpogonial branch
(x66o). amc = accessory mother cell, ^c = support cell. D, young cystocarp
(X312). ac = accessory cell, 6c = support cell, ^on = gonimoblast. E, mature
cystocarp ( x 90). F, L. rosea, life cycle. G, H, L. clavellosa, antheridia ( x 660).
I, L. clavellosa, plant ( x f ). (A-C, F-H, after Svedelius; D, E, after Kylin;
I, original.)

RHODYMENIALES 243

separates in order to form the outer cell layers, although a few
longitudinal filaments are left in the centre. The plant, which is
enclosed in a thick gelatinous cuticle, may bear unicellular hairs
that have arisen from the epidermal layer. The adult thallus has
developed from a group of eight to twelve apical cells, each of
w^hich produces a longitudinal filament, whilst the corticating
threads develop from lateral cells which are cut off from each
segment just behind the apex. The male plants, which are rare in
nature, bear the antheridial sori on the upper regions where they
form whitish patches. A system of branching threads, which
appears as a preliminary to sorus formation, arises from a single
central cell, and from each of these branching threads two to three
antheridial mother cells grow out and increase in length. De-
pending on the species one, two or three primary antheridia arise
from each mother cell and they may be followed by a crop of
secondary antheridia. The procarp consists of a support cell with a
three-celled carpogonial branch, both these and the antheridial
mother cells being uninucleate, although the mature vegetative
cells are multinucleate. There are one or two auxiliary cells, and
after fertilization one of these receives a process from the carpo-
gonium which carries with it the diploid nucleus. This auxiliary
cell then proceeds to cut off a segment on the outer side, and from this
a group of cells develops that ultimately gives rise to the gonimo-
blasts. The ripe cystocarps are sessile on the thallus and possess a
basal placenta. The tetrasporangia are borne on the diploid plants
in small cavities produced by the infolding of the cortex. In
European waters L. rosea^ which has a diploid chromosome number
of twenty, is only known to produce tetraspores which apparently
arise without undergoing meiosis. Individual spores germinate to
give a new plant or else a whole tetrad may germinate to give a new
plant. In L. rosea, therefore, the gametophytic generation is
wholly suppressed and we have a diplont which behaves as a
haplobiont in respect of its life cycle. In Pacific waters, on the
other hand, the records suggest that the species behaves normally,
whilst the other common species, L. clavellosa, also behaves in the
normal fashion.

16-2

244 RHODOPHYCEAE

REFERENCES

Phimaria. Baker, K. AI. (i939)- Ann. Bot., Lond., N.S. 3, 347.

Xenialion. Cleland, R. E. (1919)- ^^n. Bot., Lond., 33, 323.

Porphyra. Grubb, V. M. (1924)- Rev. Alg. 3, i.

General. Grubb, V. M. (1925). J. Linn. Soc. {Bot.) 47, 177.

Porphyra. Ishikawa, M. (1921). Bot. Mag., Tokyo, 35, 206.

Griffithsia. Kylin, H. (191 6). Z. Bot. 8, 97.

Batrachospermum. Kylin, H. (1917). Ber. dtsch. bot. Ges. 35, 155.

Griffithsia. Lewis, I. F. (1909). Ann. Bot., Lond., 23, 639.

Porphyridiiitn. Lewis, L F. and Zirkle, C. (1920). Amer.J. Bot. 7, 333,

Porphyra. Mangeot, G. (1924). Rev. Alg. i, 376.

Callithamnion. AIathias, W. T. (1928). Puhl. Hart. Bot. Lab. no. 5, p. i.

Harveyella. Sturch, H. H. (1924). Ann. Bot., Lond., 38, 27.

Corallina. Suneson, S. (1937). Lunds Univ. Arsskr. 33, i.

Delesseria. Svedelius, N. (1911, 1912, 1914). Svensk bot. Tidskr. 5, 260;

6, 239; 8, I.
Scinaia. Svedelius, N. (19 15). Nova Acta Soc. Sci. Upsal. 4, i.
General. S\tedelius, N. (193 i). Beih. bot. Zbl. 48, 38.
Lomentaria. S\"EDELIUS, N. (1937). Sym. Bot. Upsal. 2, i.
Polysiphonia. Yamanouchi, S. (1906). Bot. Gaz. 41, 425; 42, 401.
Corallifia. Yaimanouchi, S. (1921). Bot. Gaz. 72, 90.
Porphyra. Rees, T. K. (1940). jf. Ecol. 28, 429.

CHAPTER IX

REPRODUCTION, EVOLUTION AND
FOSSIL FORMS

*REPRODUCTION

In this chapter it is proposed to give a general review of the various
reproduction cycles that are to be found in the three principal algal
groups, Chlorophyceae, Phaeophyceae and Rhodophyceae. It will
also be instructive to ascertain whether such a survey can lead one
to any helpful conclusions in considering evolution among and in
the different groups. A study of fossil forms is of fundamental
importance in any evolutionary or phylogenetic survey, but it
must be clearly understood, however, that as the fossil forms of
algae are largely confined to certain calcareous genera it is very
difficult to draw any decisive conclusions. As a result, hypotheses
must be based almost wholly upon living forms and these may have
advanced far from their primitive ancestors, and furthermore,
evolution may have proceeded at varying rates along the diiferent
lines. For this reason the bulk of the material set out in this
chapter can only he speculative, and students would do well to hear
this in mind. The necessity of basing hypotheses upon living forms
also leads to the further complication that different authors
inevitably propound schemes, and these may differ widely in repre-
senting their views of the lines along which the present living
species have evolved. Here again it cannot be too strongly im-
pressed upon the student that much of what follows must be
attributed to the author's personal opinions, and these are not
necessarily shared by other workers. The student should read the
additional literature critically and then attempt to work out his own
conclusions, and in this connexion it will often be found very
helpful to draw up some form of schematic diagram.

As an essential preliminary it is convenient to recapitulate the
principal life cycles to be found in the three groups, pointing out at
the same time any problems that may arise immediately from such a
survey. The life cycles of representative genera in the Phaeophyceae

246 REPRODUCTION, EVOLUTION, ETC.

are shown in fig. 163, and a study of these enables one to make the
following generalizations :

(i) The life cycle is by no means simple in most of these types
and it frequently has no fixed relation to the nuclear cycle or to the
cycle of reproductive bodies, and so it has been suggested that the
term "Hfe cycle" should be abandoned and replaced by the term
"race cycle" because that indicates more clearly the numerous
possible variations in the life history of any one species. Lying

L An INJURIA

Fucus

NOTHEl/\

Fig. 163. Types of life cycle in the Phaeophyceae and their possible inter-
relationships. i?Z) = position of reduction division in the life cycle.

behind the race cycle is the fundamental nuclear cycle, but this is
often obscured by the frequent repetition of any one generation.
Whether these variations in the life history of any one species, e.g.
Ectocarpus siliculosus, are to be related to differences in environment
or whether they are due to genetical differences is a problem that
still awaits solution.

{2 a) Any thallus in the Ectocarpales, whether it be haploid or
diploid, can produce an unlimited series of the same generation by
means of zooids from plurilocular sporangia. In this connexion it is
extremely instructive to compare and classify the Phaeophyceae in
relation to the two types of sporangia. In Table II it will be seen
that one can distinguish two primary divisions if one regards the

REPRODUCTION 247

antheridia and oogonia as modified plurilocular sporangia. This
concept is inevitably bound up with the phylogeny of the Phaeo-
phyceae because one can either read them as a series commencing
with the undifferentiated plurilocular gametangia of the Ecto-
carpales, or else one can regard these structures as reduced anther-
idia and oogonia in which differentiation has been completely lost.
(zb) The presence of a unilocular sporangium always indicates
the presence of a diploid thallus, and it invariably gives rise to
haploid zooids.

Table II

I. One kind of plurilocular sporangium.

(i) Uni- and plurilocular sporangia on the same individuals, e.g. Ectocarpus

(2) Uni- and plurilocular sporangia on different individuals, e.g. Sphacelaria
hipinnata, Cladostephus.

II. Two kinds of plurilocular sporangia.

(i) Meio- and megasporangia, e.g. E. virescens.

(2) Antheridia and female gametangia ( = plurilocular sporangia).

{a) Unilocular sporangia on separate plants, e.g. Sphacelaria hystrix,
Halopteris filicina.

(b) Unilocular and both gametangia all on separate plants, e.g. Sphace-
laria Harvey ana.

(3) Antheridia and oogonia ( = plurilocular sporangia).
Unilocular sporangia on separate plants, e.g. Dictyota, Laminaria.

(4) Antheridia and oogonia representing modified micro- and megaspor-
angia ( = plurilocular sporangia).

Unilocular sporangia on same plant, e.g. Fucales.

{zc) A haploid zooid, irrespective of the nature of the structure
in which it was produced, can behave either as a gamete or as an
asexual zooid.

(3) In many of the types it cannot be said that there is a regular
alternation of cytological or morphological generations, even
though it is potentially possible. Although by no means entirely
satisfactory, in a good many cases the race cycle can perhaps be
best described as possessing an irregular alternation of generations.

(4) Theoretically it is obvious that there are three possibilities
which can be suggested in order to explain the origin and develop-
ment of the Phaeophyceae :

A. Plants that are haploid throughout their Hfe cycle, except for
the zygote, represent the primitive condition, and the diploid stage

248 REPRODUCTION, EVOLUTION, ETC.

became interpolated by a gradual delay in the occurrence of meiosis.
Against this possibility it may be pointed out that

(i) There are very few Phaeophyceae in which the haploid
generation is wholly dominant. It is possible, of course, that they
were more numerous and have subsequently been displaced by
the more recent types in which the diploid generation plays a more
significant role.

(ii) Ectocarpus siliculosus in its English and Mediterranean forms
would both begin and end the series, and this hardly seems con-
ceivable. This, however, could not be regarded as a fundamental
objection because it might equally well be argued that the species
forms an excellent example of how the process of interpolating the
diploid generation took place.

(iii) The frequency of parthenogenesis in the Ectocarpales
suggests decadence of sexuality rather than the existence of a
primitive condition, but it could also be argued that there is a
decadence of sexuality in the Laminariales and Fucales.

B. Plants with only a diploid generation, e.g. Fucales, are the
most primitive, and the haploid generation has been interpolated
subsequently. If this interpretation is correct the only obvious
source of origin for the group would be from the Siphonales be-
cause a flagellate ancestry would be most unlikely under such
circumstances. The evidence that might be adduced in support of
this hypothesis is tabulated below :

(i) All the Fucaceae are diploid, and these form a large pro-
portion of the Phaeophyceae and also have an extremely wide
distribution.

(ii) In the Laminariales the diploid phase is dominant.

(iii) The haploid phase is frequently omitted in Dictyota (cf.
p. 165) and also in Cutleria.

(iv) The majority of the macroscopic filamentous forms are
diploid, the small ectocarpoid filaments forming the haploid
generation.

One important objection to this view is the concomitant require-
ment that the early Phaeophyceae must have started life with a
highly complex structure, e.g. Fucus, though of course some such
structure can be found in the Siphonales. It must also be re-
membered that the interpolation of a diploid generation into an

REPRODUCTION

249

original haploid phase may have produced plants that were more
successful and which subsequently eliminated their parents in the
struggle for existence.

C. The original ancestors were filamentous with equal haploid
and diploid generations, or perhaps with generations that were
slightly unequal, but that both retained the power of producing a
ciliated zooid which could develop without fusion, e.g. Nemoderma
(fig- 163).

A further consideration of this problem must now be deferred
until the other two groups have been surveyed because a final con-
clusion must incorporate phylogenetic considerations.

SCIMAIA

LjAg-oka Tetr.

Phyllophori

SIPKONIA ETC.

Ahnfeldtia Plicata

LOMENTARIA RO-5EA

Fig. 164. Types of life cycle in the Rhodophyceae and their possible inter-
relationships. i?D = position of reduction division in life cycle.

Fig. 164 is a summary of the principal life cycles that are to be
found among the Rhodophyceae. According to Svedelius (1931)
the primitive cycle is represented by Scinaia, Nemalion and Batra-
chospermum where there is only a haploid generation. Some
postponement of meiosis is seen in Liagora tetr asporif era, but the
maximum delay is reached in Polysiphonia and most other Rhodo-
phyceae where there are two equal generations, the sporophyte
reproducing by means of tetraspores, two of which give rise to

250 REPRODUCTION, EVOLUTION, ETC.

male plants and two to female. Subsequent developments, which
must be interpreted as retrogressive, can be seen in Phyllophora
memhranifolia, where the tetraspores are grouped into nemathecia
on the diploid plant; in P. Brodiaei, where the diploid phase has
disappeared and the nemathecia can be regarded as growing
parasitically in the haploid thallus ; and finally in Ahnfeldtia, where
meiosis no longer takes place and instead the nemathecia contain
monospores. Lomentaria rosea in European waters is another
example of a reduced life cycle, because in this species the gameto-
phyte generation is wholly suppressed, whereas in the other
examples it is the sporophyte generation that has been reduced.

In his studies on the Rhodophyceae Svedelius coined a number
of terms which have subsequently come into common usage :

Haplohiont. A sexual plant with only one kind of individual or
biont, dioecious plants being regarded as representing one kind of
individual.

Haplont. A sexual haploid plant with only the zygote diploid.

Diplohiont. A plant possessing alternation of generations and two
kinds of individuals, and usually with a much greater number of
meiotic divisions since each tetrad of spores involves meiosis. If
Fucus is regarded as possessing sporangia and reduced gameto-
phytes it will belong to this group rather than being treated as a
diploid haplobiont.

Diplont. A sexual diploid plant in which only the gametes are
haploid (e.g. C odium).

The terms ' * haplo- ' ' and ' ' diplobiont ' ' do not necessarily coincide
with the cytological generations, e.g. Codium, and there has been
further confusion from the inaccurate usage of these terms by later
authors, some of whom have introduced completely new interpre-
tations of the w^ords. In the Rhodophyceae the morphological
changes that would be involved make it highly improbable that the
diplobionts were primitive to the haplobionts.

Fig. 1 65 shows a series of typical life cycles that have been found
in the principal members of the Chlorophyceae, and here again it
will be seen that three principal types can be distinguished :

(i) A multicellular haploid generation in which the diploid phase
is present in the unicellular state (e.g. Ulothrtx).

(2) An alternation between multicellular diploid and haploid
generations, both of which are usually morphologically identical.

REPRODUCTION

251

The only definite exception to this morphological equality at
present is seen in Halicystis ovalis where Derbesia marina forms the
diploid generation, although it is possible that a similar state of
affairs may exist in Urospora.

(3) A multicellular diploid generation in which the gametophyte
is reduced to the unicellular state, e.g. Codium.

{i a) A persistent unicellular haploid state alternating with a
persistent or short-lived unicellular diploid state. This can be
regarded as a morphological modification of (i) above or vice versa.

Halicystis CoDiun

Fig. 165. Types of life cycle in the Chlorophyceae and their possible inter-
relationships. i^Z) = position of reduction division in the life cycle.

A Study of these life cycles immediately indicates that as a
series they can be read in either direction, from i->2^3 or from
3^2^ I. On morphological grounds, however, it is more satis-
factory to accept the view that the primitive cycle is that in which
the haploid generation is dominant, and that the sporophyte has
been subsequently intercalated, presumably by a delay in the
occurrence of meiosis as in the Rhodophyceae. Therefore in at
least two of the groups it would seem as if the course of events during
their evolutionary history has been much the same. In the primi-
tive state the haploid filaments would perhaps be monoecious
so that the first development would concern the appearance of
the dioecious condition, e.g. Ulothrix sp. and the Conjugales.

252 REPRODUCTION, EVOLUTION, ETC.

Monostrofna represents another intermediate condition in which the
enlargement of the zygote can be regarded as an incipient delay
before the reduction division takes place.

Summary and Conclusions

In fig. 1 66 are set out some simplified diagrams of the fife cycles
of the principal algal types to be found in all three groups. They have
all been drawn up on the same principle so that comparisons will
be rendered easier. On the hypothesis that the Chlorophyceae are
probablv the original ancestors of most of the algal groups, the
types of life cycle to be found there have been made the basis of the
other diagrams. Chlamydomonas, Ulothrix and Coleochaete can all
be regarded as simple types in so far as their life cycles are con-
cerned, although it is conceivable that the life cycle of Coleochaete
may have been secondarily reduced to the wholly haploid stage, or,
more probably, that morphological evolution took place without
any comparable change in the life history. From a morphological
and reproductive standpoint Coleochaete would appear to be the
only member of the Chlorophyceae from which the Eu-florideae
might be evolved directly, and it is worth noting that the life cycles
of Coleochaete and the primitive Eu-florideae, Scinaia, Nemalion,
Batrachospermum, are identical. It is true that there are differences
in structure between Coleochaete and the primitive red algae, but so
long as there is a complete lack of any intermediate stages it is not
necessarily justifiable to abandon such an origin because there is
an equal lack of intermediate stages for any other source of the
Rhodophyceae (cf. p. 256). It would seem, therefore, that a study
of the life cycles of the Chlorophyceae and Rhodophyceae can lead
one to two conclusions :

(i) Their phylogenetic history follows parallel lines whereby
they commence with a wholly haploid generation and the diploid
generation is subsequently interpolated through a delay in the
occurrence of meiosis. Svedelius (1931) has suggested that the
delay in meiosis came about gradually, but cytologically it is
perhaps easier to imagine one or more sudden delays resulting in
two morphologically similar generations, one of which, the diploid,
subsequently may have undergone modification.

(2) There are grounds for believing that some of the filamentous

REPRODUCTION

253

7v^

Ect. mrestcens

X
NemodiLrrr..dL, Htctyota.

R^D

*^^

/
P

PyUicLU,

_x_ ^

00 o

R^^2x ,)

..r^p

Oi-ctijosihihon.

2x
Punc£a.YL3L,

2 C?

^D 2

PHAEOPHYCEAE

Nothe.ia,

~CdeVch3ete "^H/.OROPHYCe/»e

UtotkriK sp

R^D 2x i
/{a. ti cyst LS

Codium

RHOOOPHYCEAE

7\ i

5cinaia

JLU.§ora.

AtivIeLdtU

.Tifi

Phylloh^ors.
OroduieL

oni^.

Fig. 166. The principal types of life cycle in the Phaeophyceae, Chlorophyceae
and Rhodophyceae and their possible inter-relationships.

254 REPRODUCTION, EVOLUTION, ETC.

Chlorophyceae, in spite of morphological differentiation (hetero-
trichy), nevertheless maintained the simple form of life cycle, and
that those cases where the sporophyte has been interpolated must
be regarded as forming divergent lines of evolution.

Another feature to which attention must be drawn is that in the
Chlorophyceae the interpolation of the sporophyte has proceeded
considerably further, whereby the sporophyte becomes wholly
dominant. In the Rhodophyceae, however, this has only happened
in one case, namely Lomentaria rosea, and even here the gameto-
phyte has only been suppressed abnormally in European waters.
In the red algae there is a reduction series instead, and this leads
back to wholly haploid plants, e.g. Ahnfeldtia, in which the con-
dition has been produced secondarily.

When we turn to the Phaeophyceae the problem is much more
difficult because there are at least two alternatives with very little
evidence to enable one to determine which is likely to be the more
correct :

(i) On the first hypothesis the primitive Phaeophyceae are to be
regarded as wholly haploid, and the series must be read in one
direction in which the sporophyte is again interpolated through a
delay in meiosis, the series terminating with those algae in which
the sporophyte generation is wholly dominant, e.g. Fucus,

(2) On the other hypothesis the primitive Phaeophyceae were
filamentous forms possessing two equal generations, haploid and
diploid, and subsequent development took place along two lines,
one in which the sporophyte and the other in which the gameto-
phyte became increasingly dominant.

There is, of course, the third possibility that the primitive
Phaeophyceae were diploid, having arisen from diploid Chloro-
phyceae such as the Siphonales, but the morphological changes
involved render this possibility extremely unlikely.

Such evidence as may be adduced for either of the first two
hypotheses is summarized below :

(a) Very few members of the Ectocarpaceae are wholly haploid,
and in at least one case, Ectocarpus virescens, the parthenogenetic
development of the eggs suggests a degenerate life cycle rather
than a primitive one.

REPRODUCTION 255

(b) Some of the primitive forms, e.g. Lithoderma and Nemo-
derma, have tv^^o equal generations in the Hfe cycle and a similar
state of affairs is also found among other brown algae, e.g. Dtctyota,
Zanardinia,

{c) The ultimate decision must obviously be largely determined
by the condition of affairs found in the sources from which the
Phaeophyceae arose. On general grounds it is to be supposed that
the Phaeophyceae all arose from one common ancestor, but it must
not be forgotten that the group may have had a polyphyletic
origin, although at present there is hardly any evidence in support of
such a view. Two possible sources of origin for the Phaeophyceae
have been suggested in the past. One is that they arose, as did the
Chlorophyceae, from a flagellate ancestry with intermediate forms
such as Phaeococciis and Phaeothamnion (cf. p. 123). On the basis
of their pigments the Chrysophyceae (cf. p. 122) show a close
resemblance to the more primitive Phaeophyceae and this is not
without significance. If this theory is correct, one must almost
certainly consider that the primitive species, as in the primitive
Chlorophyceae, were wholly haploid and that the diploid state has
been interpolated subsequently. The other hypothesis is that they
arose from some member of the Chlorophyceae, probably among
the Chaetophoraceae. This latter group is characterized by
heterotrichy, a feature which is possessed by some of the primitive
Phaeophyceae, whilst another point in favour of this view is the
lack of any satisfactory existing series between the few known
phaeophycean-like flagellates and the primitive filamentous
Ectocarpales. If we accept an origin of the Phaeophyceae from the
Chlorophyceae, two possible sources may be suggested :

{a) From a member of the Chaetophoraceae which possessed the
heterotrichous habit and two morphologically similar or nearly
similar generations.

{h) From a member of the Siphonocladiales which had a life
cycle with two equal generations, such as is now shown by Chaeto-
morpha or Cladophora Suhriana,

It is tempting to consider whether the Phaeophyceae have not
been derived from a form such as Trentepohlia, and it is much to
be regretted that at present the life cycle of Trentepohlia, so far as
cytological details are concerned, is wholly unknown. Until we

256 REPRODUCTION, EVOLUTION, ETC.

possess a more extensive knowledge of the cytological life cycles
among the Chaetophoraceae it would appear futile to speculate
further on the origin of the Phaeophyceae, and all that can usefully
be done in this chapter is to point out the various possibihties. One
further point remains to be added. In the present volume it has
been suggested that the similarity in life cycles and phylogenetic
histories leads one to the hypothesis that the three groups of algae
are perhaps intimately related. At the same time an attempt has
been made to indicate that there are other w^orkers who believe
that all three groups have had independent origins from different
sources, and that the various types of life cycle have evolved in-
dependently. At present the decision bet^veen these two courses
would seem to be largely a matter of opinion.

*EVOLUTION

Rhodophyceae

It has already been suggested above that the primitive Rhodo-
phyceae, in particular the Eu-florideae, may have arisen from a
member of the Chaetophoraceae such as Coleochaete. It is only
proper, however, to emphasize that this is purely one viewpoint,
and that there are other workers who have sought for an origin of
the group from among the unicellular organisms, but unfortunately
there are very few members of the Protista which can be regarded
as possible sources for the red algae apart from Porphyridium
cnientum. An alternative hypothesis is that which considers the
Rhodophyceae to have been evolved from the Cyanophyceae, the
principal argument in support of this view being the resemblance
between the colouring pigments, primarily in colour because
it has recently been shown that the pigments are not identical
chemically. The principal objection to this theory is the absence
of any form of sexual reproduction among the Cyanophyceae, a
feature which renders the presence of highly specialized sex organs
in even the most primitive Rhodophyceae difficult to explain. Apart
from these theories, however, there is also the possibility that the
Eu-florideae have been evolved from the Proto-florideae, in which
case the origin of the latter group becomes of importance. Two
possible lines of evolution can be suggested, but it does not appear
feasible to discriminate in favour of either one :

EVOLUTION 257

(i) A. Aphanocapsa-^Porphyridium-^Porphyra.
B. Praswla-^Bangta^Eu-f{ondQ2iQ.

Apart from the difference of pigment there is a striking resemblance
in morphological structure and reproduction between Bangia and
Prasiola.

(2) Cyanophyceae -^ Porphyridium -> Porphyra -> Bangia ->
Eu-florideae.

In contrast to this there are those who postulate independent
origins for the Proto- and Eu-florideae on account of the consider-
able diflFerences in structure and reproduction between the two
divisions, but at present it would seem impossible to do more than
point out the different theories that have been put forward. The
later evolutionary changes in the Eu-florideae have already been
mentioned in the introduction to Chapter viii and also in the
earlier part of the present chapter.

Phaeophyceae

Most workers would probably agree that the primitive members
of this group are to be found among those members of the Ecto-
carpales which either possess a single haploid generation or else
two morphologically identical generations. It now remains to
indicate how subsequent evolution may have taken place, but only
a broad outline can be suggested because individual workers have
frequently produced modifications in the lesser details of the
evolutionary sequences. It has been stated that the unilocular
sporangia of the Ectocarpales, in which meiosis occurs, are morpho-
logically equivalent to the tetrasporangia of the Dictyotales. In
this case the plurilocular sporangia which give rise to the iso- or
anisogametes are morphologically equivalent to the gametangia of
the Cutleriales and Dictyotales. One of the outstanding problems
is the origin of the Laminariales and Fucales, and in order to
account for these it would seem necessary to postulate at least two
different lines of evolution, though there were probably even more.
As an example of the simpler type of sequence that has been
suggested the following may be quoted from Svedelius :

Phaeosporeae->Cutleriales->Dictyosiphonales-^
Dictyotales^Laminariales^ Fucales.

CSA 1/

258 REPRODUCTION, EVOLUTION, ETC.

Kylin (1933), on the other hand, whilst agreeing with an origin for
the Dictyotales from the Phaeosporeae suggests that the Fucales
have not arisen from that source. It is possible to imagine a line of
evolution, not only on morphological, e.g. the cable type of con-
struction, but also on reproductive criteria, commencing from
Ectocarpus-^Castagnea^Chordaria-^Chorda^Laminaria, whilst an
alternative source for the Laminariales could also be found in
parenchymatous genera such as Dictyosiphon or Punctaria. It is
extremely tempting to consider whether the Fucales may not have
been evolved from the Laminariales because of the existence of
forms such as Durvillea, and if the oogonia and antheridia of the
Fucales are regarded as modified unilocular sporangia, e.g. micro-
and macrosporangia, then this becomes a possibility. On the other
hand, the oogonia and antheridia might be regarded as more closely
allied to the tetrasporangia of the Dictyotales which must then be
regarded as a possible ancestral source.

Table III

Ectocarpales

Dictyotales

Laminariales

{Macrocystis)

Fucales

Plurilocular

sporangia
Unilocular

= Oogonia

antheridia
= Tetraspor-

= Oogonia

antheridia
= Unilocular

= Oogonia

antheridia
= Unilocular

= Oogonia

antheridia
= Micro- and

sporangia

angia

sporangia

(homo-

sporous)

sporangia

(hetero-

sporous)

mega-
sporangia
(hetero-
sporous?)

From the above table it would seem that the evolution of the
Fucales is associated with the development of heterospory in much
the same way as the evolution of the seed habit is often said to be
associated with the development of heterospory. The origin of such
a habit forms a very distinct problem because there is very little
evidence for such a development among the other phaeophycean
groups. In the Laminariales heterospory but not heterangy is
recorded for Macrocystis, and if it has arisen once it may have been
present in some of the ancestral Laminariales, forms from which
perhaps the Fucales arose. It is also possible that we are pursuing
a false scent in trying to establish heterospory as a feature of the
Fucales, and that if the actual spores are considered, e.g. the
products of the first two divisions in the micro- and megasporangia.

EVOLUTION 259

it would be found that they are really homosporous, in much the
same way as Thomson has suggested that the so-called micro-
and megaspores of the angiosperms are really homosporous. If
this were found to be true, then the problem of the origin of
heterospory in the Fucales would be disposed of and the evolu-
tionary problem much simplified. It is also possible that the
explanation of heterospory and heterangy in the Fucales is to be
found in the retention on the parent thallus of heterothallic game-
tophytes, the stimulus provided by their presence being responsible
for the modification of the original morphologically identical
unilocular sporangia.

One or two authors have recently suggested that an origin for the
Fucales should be sought for among the Mesogloiaceae and
Encoeliaceae (Colpomenia), but the evidence produced cannot be
regarded as wholly convincing. The most recent account by Delf
(1939) considers this problem in some detail. In adult plants of
Fucus there is an apical growing cell which is now known to arise as
follows. In the sporeling a group of apical hairs is formed at the
growing point, each hair possessing basal (trichothallic) growth as
in the Ectocarpales. These hairs die off and the lowest cell of one
hair gives rise to the four-sided apical cell of the adult thallus.
A similar behaviour of the apical hairs is to be seen in Acrothrix
(Mesogloiaceae). New growth from wounded tissue in Fucus also
develops a new apical cell from such trichothallic hairs, whilst the
development of the cryptostomata and conceptacles also appears to
be analogous. It is further suggested that in gross structure, e.g.
primary and secondary medullary filaments and the assimilatory
tissue, the thallus of Fucus shows considerable resemblance to that
of Eudesme as illustrated in fig. 95. Difficulties associated with this
interpretation must be concerned with the differences in size of
the thalli and also the presence of heterospory and heterangy. The
gametophytes of the Mesogloiaceae reproduce by means of pluri-
locular sporangia which do not exhibit either anisogamy or heter-
angy nor do the sporophytic plants exhibit either heterospory or
heterangy. Recent work, however, has shown that the gametophyte
of Colpomenia sinuosa bears organs that must be regarded as
relatively simple antheridia and oogonia, so that there is here an
example of heterangy associated with anisogamy (cf. p. 154).

Whether the Fucales are derived from the Laminariales or

17-2

26o REPRODUCTION, EVOLUTION, ETC.

Dictyotales, it is obvious that their ancestry is not to be found in the
present Hving forms of either group. It would therefore seem best
for the time being to derive the Fucales from the ancestral groups
of either the Laminariales or Dictyotales, recognizing that there is a
definite bridge in both cases, the gap being least perhaps between
the Fucales and Laminariales. The common race cycle found in the
Phaeophyceae with its irregular alternation of generations must have
evolved several times, the course of evolution probably being
determined by the morphological changes, e.g. corticated type,
reduced ectocarpoid type, reduced cable type.

It has been pointed out that the Phaeophyceae can be divided
into two great groups, the Isogeneratae and Heterogeneratae, and
the latest schemes of evolution take these into consideration. In
both Iso- and Heterogeneratae there is a gradual transition from
isogamy to anisogamy, and on these grounds one can perhaps
postulate at least two major lines of evolution. The schema below is
an example of what can be obtained employing this line of approach,
which is probably more satisfactory than one that is purely morpho-
logical :

Fucales

Anisogamy

Dictv^otales?

Tilopteridales

Cutleriales
Sphacelariales

Laminariales
Desmarestiales

Sporochnales
Advanced Ectocarpales (part) \

Chordariales

Isogamy

Reduced
Ectocarpales

Anisogamy
A

Dict>'osiphonales

Punctariales

Isogeneratae

Primitive Ectocarpales

Chlorophycean ancestry Heterogeneratae

Isogamy

The names of the orders given above do not imply that the
present living representatives formed the stages in evolution, but
that types more or less similar to them existed in the evolutionary
sequence.

EVOLUTION

261

Chlorophyceae

Among the primitive forms a schema such as the one below
(modified after Senn) will give an indication of the primary se-
quence of events :

Protomastigineae -^
(colourless)

Primitive
Chrysophyceae

Yellow, vary'ing types of
flagellae and nutrition

-> Chloromonodineae
Yellow, green or
colourless. Leucosin

Mycetozoa Ciliata Fungi / Dinophyceae

Bacillariophyceae

Cryptophyceae Xanthophyceae

brown, /green, blue-green

Phaeophyceae

Euglenineae

Chlorophyceae

Rhodophyceae

The origin of the Phaeophyceae and Rhodophyceae has already
been discussed in some detail and hence will not be considered
further.

With this scheme as a basis it is now possible to consider
evolution among the Chlorophyceae, and here there are two starting
points. One hypothesis commences with the Chlamydomonadaceae
and branches out with a number of lines of evolution, whereas in
the other the Palmellaceae are regarded as the source of the group.
The first of these two theories is perhaps the more satisfactory.
The origin of the Siphonales offers a similar kind of problem to
that of the Fucales because there are very few satisfactory inter-
mediate forms. As a result they are illustrated in the schema as
having been evolved either directly from an unicellular organism or
else from the Siphonocladiales, the latter perhaps forming the more
attractive hypothesis. It has recently been emphasized that the
nearest affinities, even though distant, of the Siphonocladiales, are
with the Siphonales via Valonia and Halicystis on the one hand,
and with Ulothrix via Chaetomorpha on the other.

262 REPRODUCTION, EVOLUTION, ETC.

SCHEME A

CHLAMYDOMONADACEAE

(motile)

y
Spherical / Chlorococcum

colonies / (non-motile, free-living)

Pandorina

Eudorinn

Gelatinous

colonies
Palmellaceae

Dendroid colonies

Characium (non-motile, Enlarged unicell
attached unicell) \^ ? (Protosip/wti)

Ulothrix-

Hydrodictyon ^,

(net'i (simple filament^ ^

Cylindrocapsa ^k^/ isogamy) \ MicTospora

->ConjugaIes

Charales

Siphonocladiales^?

(Dioecious).] Tribonerna
' (Hetero-

trichales)

Pleodorina

Volvox ^g^ -^ ,

Monostroma Schizomeris ^ /
Ulvales-^^ ' Oedogoniales ^

Prastola

Bangia Stigeoclonium

(Heterotrichy, isogamy) Siphonales

V ^ ^

? ? ,Coleochaete Trentepohlia Protoderma Draparnaldia

(oogamy) ? (reduced, (reduced,

., base only) aerial only)
Eu-florideae Phaeophyceae

The names of the genera do not necessarily imply that they
formed the actual intermediate stages, but merely that forms like
them existed in the evolutionary sequence.

SCHEME B

PALMELLACEAE

Charales

Ulvales

Schizogoniales

Hormidium

Ulothrix

Stigeoclonium

Coleochaete

Volvocales

Chlamydomonadaceae
Protosjphon

Siphonales

Pleiirococcus (reduced type)

Siphonocladiales

The Chaetophorales are represented as derived either from the
Ulotrichales (A) or from the Palmellaceae (B), but Vischer has

EVOLUTION 263

suggested that they are derived from the Volvocales on the basis of
a resemblance between them and the simpler forms, e.g. Gongrosira.
It is, however, probably more correct to interpret these simpler
forms as reduced rather than primitive.

From the above two schemas it will be seen that there are a
number of definite morphological tendencies, and it has already
been pointed out that these various lines of morphological de-
velopment are repeated in the different algal groups. Table IV from
Fritsch (1935) provides examples of parallelism in evolution among
the simpler types of algae.

Apart from these examples of evolution among the simpler algae
it is also found that other evolutionary tendencies can be observed
among the more advanced types of algae. Such tendencies are
illustrated in Table V.

The concept of the heterotrichous habit was first advanced by
Fritsch in 1929, and it has become increasingly evident that an
understanding of this habit is of fundamental importance in con-
sidering any phylogenetic or evolutionary problem among the
algae. In the primitive state both the prostrate and erect systems
must be present, but during the course of evolution one of these
has frequently become reduced or lost, e.g. in Endoderma (Chloro-
phyceae), Strehlonema (Phaeophyceae) and Melobesia (Rhodo-
phyceae) only the prostrate system remains. In contrast to this
the thallus in Draparnaldia and most of the Florideae represents the
erect system, the prostrate system having been reduced or lost.
Another fact in connexion with this phenomenon, which needs to be
re-emphasized, is that the most advanced Chlorophyceae exhibit the
heterotrichous habit in its primitive state, whilst this condition is
only found fully developed among the simpler Rhodophyceae and
Phaeophyceae since in the more evolved types one or other of the
systems is reduced. The possible implications of this observation
are immediately obvious.

Finally, a word may "be said about the time when the diflferent
groups first made their appearance. Most authors would consider
that the Cyanophyceae and Chlorophyceae are the most primitive
and therefore appeared first. If, however, the Rhodophyceae and
Phaeophyceae have a flagellate origin then all four groups may be of
almost the same antiquity. There are some workers who believe
that the Cyanophyceae are the most primitive group and that they

264 REPRODUCTION, EVOLUTION, ETC.

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EVOLUTION

265

gave rise later to the Florideae. These, it is then supposed on
fossil evidence together with the similarity in pigmentation, were
followed by the Phaeophyceae and Chrysophyceae, the Chloro-
phyceae being the last group to appear. It must be pointed out,
however, that the absence of fossil remains does not necessarily
mean that a group was absent at any given period : many of the
Chlorophyceae are delicate forms and would not be preserved so
readily as the tougher fronds of the brown and red algae. The
oldest fossil Chlorophyceae to be recognized belong to the Sipho-
nales and Siphonocladiales and they show a high degree of differ-
entiation which suggests that, as a group, they were evolved at a

Table V. Parallelism in evolution among the advanced types of algae

Type of construction

Chlorophyceae

Phaeophyceae

Rhodophyceae

(i) Heterotrichous

*Stigeoc Ionium

*Ectocarpus

* Batrachospermum

filament

(2) Discoid type

*Protoderma

Ascocyclus

Erythrocladia

(3) Crusts or

Pseudopring-

Ralfsia

Hildenbrandtia

cushions

sheimia

(4) Elaborated erect

type

(5) Compact pseudo-

*Draparnaldia

Desmarestia

Callithamnion

Dasycladus

Mesogloia

*Ceramium

parenchymatous

* Char a

type, uniaxial

(6) Ditto, multiaxial

*C odium

Castagnea

Nemalion

(7) Foliose parenchy-

*Ulva

Punctaria

*Porphyra

matous type

(8) Tubular paren-

*Enteromorpha

Asperococcus

Halosaccion

chymatous type

* It is recommended that elementary

students should

only remember these

types together with the fact that the others do exist.

^^ery early stage. With very little evidence to support the view, certain
workers consider that these siphonaceous fossils represent the
primitive green algae, the remainder, principally fresh- water forms,
developing much later. Whilst it is doubtful if many algologists
would subscribe to this interpretation, it is mentioned here because
it is felt that any suggestions, however likely or unlikely, open up
fresh fields of thought and investigation.

On the more orthodox interpretation it would seem as if the
Cyanophyceae and unicellular Chlorophyceae were the most
primitive algae. Some of the Florideae, especially the Proto-
florideae, may have appeared quite early, whilst the Eu-florideae
perhaps developed somewhat later at the same time as the

266 REPRODUCTION, EVOLUTION, ETC.

Phaeophyceae. Whatever the sequence of events, it is quite clear from
the structure of the earHest fossils that considerable evolution had
taken place long before their time.

FOSSIL FORMS

In this section it is merely proposed to give an outline of the
different fossil forms that have been ascribed to the various groups,
but it is not intended to provide a detailed description in every case
so long as the types of structure represented among these fossil
algae have been adequately portrayed. It must be realized that
many of the early forms that have been ascribed to the algae are
relatively unknown because of the poor preservation, and further
examination of new specimens may mean that they will have to be
removed from the algae. For this reason it must be emphasized
that there are a number of doubtful forms from the lowest strata
which can only be tentatively assigned to the algae.

Cyanophyceae

Among the unicellular forms a fossil w^hich has been related
to the Chroococcaceae is recorded from the
Ordovician, It is called Gloeocapsomorpha
and is a colonial form with cells that were
apparently enclosed in a jelly, and whilst it
may have affinities with living colonial forms
it is usually placed in a group called the
Protophyceae. Another plant of Middle
Cambrian age, Marpolia spissa (fig. 167),
which seems to have affinities with the
modern Schizothrix, is also best relegated to
the Protophyceae. Marpolia was represented
by branched filaments which were probably
composed of a trichome enclosed within a
gelatinous or cartilaginous sheath.

Spongiostromata (Precambrianto present day).

Much doubt has been thrown upon the
authenticity of this group, some writers re-
garding them as structures which originated ^-^ ^^^^ Marpolia spissa
as diffusion rings ("liesegang" phenomena) ( x 49-5). (After Walton.)

FOSSIL FORMS 267

in colloidal materials or perhaps in calcareous muds. In the original
description Walcott suggested an affinity to the Cyanophyceae, but
as later workers could only distinguish a purely mineral structure
they suggested the idea of diffusion phenomena. Discoveries of
very comparable algal concretions and laminations in the Bahamas,
however, have made it extremely probable that these structures had
an algal origin. Some examples of these types are shown in fig. 168.
On the basis of Black's discoveries (1933) it may be suggested that
these structures were not necessarily formed by deposition but that
the algae collected and bound the sediment.

•■A",~-.=t>..:

D E f

Fig. 168. Stromatolithi. A, Weedia. B, Collenia. C, D, Cryptozoon. E,
Archaeozoon. F, Gymnosolen. (After Hirmer.)

Porostromata (e.g. Girvanella, Sphaerocodium).

These forms, which are most abundant in the Carboniferous,
have a recognizable microscopical structure, the threads often being
arranged in a radiating fashion: they were probably formed in
much the same way as the algal water-biscuits now found in
South Australia. These range from tiny particles to thick bun-like
forms 20 cm. in diameter, whilst in them are to be found the tube-
like remains of living species of Gloeocapsa and Schizothnx,
Gloeothece and Gloeocapsa are also known to form oolitic granules
in the neighbourhood of Salt Lake City. The presence, however, of
pebbles, or the existence of a granular structure, does not necessarily
involve the presence of algae, and in some cases it is also possible
that the algae were merely included through chance. Pachytheca is
a genus from the Silurian and Devonian which possesses a medulla

268 REPRODUCTION, EVOLUTION, ETC.

of intertwining tubes and a cortex composed of stout, septate,
branched algal filaments that radiate from the medulla to the peri-
phery. Its affinities are extremely uncertain and it may have been a
free-rolling alga of either salt or fresh waters (cf. fig. 169).

There are a few uncertain fossils, ver\^ indistinct and not well
known, ascribed to the Flagellata and Dinophyceae. Recognizable

Fig. 169. Pachytheca. A, transverse section with natural opening through cortex ^
( X 12). B, algal filaments of medulla and inner cortex ( x 240). C, cortex with
algal filaments ( x 60). D, cortex showing degenerate algal threads in tube ( x 150).
(After Lang.)

fossil diatoms are known from the Upper Jurassic, and there was
a very rich fossil diatom flora in the Tertiary, all the specimens
found being closely related to existing families and genera.

Codiaceae

Boueina (cf. fig. 170) is an unbranched form from the Lower
Cretaceous, whilst Palaeoporella (fig. 170), which is composed of
hollow cylinders or funnel-shaped bodies with slender forked
branches, the whole being two to fourteen millimetres long, comes
from the Lower Silurian. Dimorphosiphon, from the Ordovician, is
generally regarded as the oldest known member of the Codiaceae

FOSSIL FORMS

269

and has been tentatively related to Halimeda. It is about ten
millimetres long and is composed of branched tubular cells without
any cross walls, the cells being embedded in a calcareous matrix.
Ovulites, a genus which occurs up to the Eocene, differs consider-
ably from those previously described: the species are little egg or
club-like chalk bodies beset with fine pores and with a large opening
at what was either the base or apex. It has been suggested that

A B

Fig. 170. Codiaceae. A, Palaeoporella
variabilis ( x 12). B, Boueina Hochstetteri.
(After Hirmer.)

Fig. 171. Dasycladaceae.
RJiabdoporella pachyderma
( >< 135)- (After Hirmer.)

perhaps they represent siphonaceous plants in which the apical
tuft of threads has been lost.

Dasycladaceae

This is the best know^n group and contains a very large number
of the fossil algae. It reached its maximum development and
abundance in Carboniferous and Triassic times, and in those days
was far more important than its present living representatives. The
various forms are all based on a type of construction which can be
sufficiently explained by descriptions of a few of the more repre-
sentative types.

RJiabdoporella (fig. 171) seems to be one of the most primitive
genera as it is represented by a purely cylindrical shell that is

270 REPRODUCTION, EVOLUTION, ETC.

studded with pores through which the threads passed. It is known
from the Ordovician and Silurian.

Cyclocrinus (fig. 172) is a genus which grew to about seven
centimetres and looked like a miniature golf ball borne on the end
of a stalk. Narrow branches arose at the apex of the stalk and each
terminated in a flattened hexagonal head, but as the edges of
adjoining heads were fused together to form the outer membrane,
which was only weakly calcified, the cell outlines were clearly

Fig. 172. Dasycladaceae. Cyclocrinus porosus (x 8). (After Hirmer.)

visible. Many species are known from the Ordovician and Silurian,
all somewhat resembling the living genus Bornetella.

Primicorallina (fig. 173), from the Ordovician, had a segmented
stem beset with radially arranged branches, each of which branched
twice into four branchlets.

The type of structure found in Diplopora (fig. 174) was also
shown by many other forms from the Middle Triassic. It was a
few centimetres long and bush-like in appearance, the main stem,
which sometimes had a club-shaped apex, being covered with

FOSSIL FORMS

271

whorls of branches that arose in groups of four, each bearing
secondary branches which terminated in hairs. In the older thalli
the outer part of the branch dropped off leaving a scar on the
calcareous shell. The sporangia are reported to have been modified
branches. Diplopora is a widespread genus from the Triassic rocks
of the eastern Alps, Germany and Siberia.

Fig. 174. Dasycladaceae. Diplopora phanero-
spora ( X 8). (After Hirmer.)

Fig. 173. Dasycladaceae. Primi-
corallina trentonensis ( x 8*25).
(After Hirmer.)

Palaeodasycladus (fig. 175), from the Lower Jurassic, bears a
resemblance to the living species of Dasycladus. Near the base
there were only primary branches, whilst higher up secondary and
tertiary branches were to be found.

Fossil forms, practically identical with living species of Cyjuopolia,
Neomeris and Acicularia have been found in all the recent strata
from the Eocene upwards.

272 REPRODUCTION, EVOLUTION, ETC.

Fig. 175. Dasycladaceae. Palaeodasycladusniediterraneus (x 20). (After Hirmer.)

FOSSIL FORxMS

273

Charophyta

Lagynophora, 3. genus from the Lower Eocene, can be ascribed to
this group, whilst Palaeonitella (fig. 176), from the Middle Devonian,

••. V

i.J.:^'--'

Fig. 176. Charales. Palaeonitella Cranii {x 12^). (After Hirmer.)

may belong here also although its affinities are not so clear.
Gyrogonites and Kosmogyra are names which have been given to
oogonial structures which closely resemble those of Chara, and
which are very abundant in the Lower Tertiary beds of England and
elsewhere.

Phaeophyceae

The principal fossil form ascribed to this group, Nematophyton,
has now been removed to a new group, the Nematophy tales (see

P- 274).

Rhodophyceae

The Melobesiae are represented from the Cretaceous upwards
by species of Archaeolithothamnion, Dermatolithon, Lithothamnion,
Lithophyllum and Goniolithon, some of them only being distin-
guished with difficulty from living forms. The Corallinaceae are also
represented in the Cainozoic by extinct members of present living
genera. There are a large number of forms assigned to an extinct
family, the Solenoporaceae, which existed from the Ordovician up
to the Triassic, but neither their structure nor their systematic
position has been completely established. They formed nodules

CSA

274 REPRODUCTION, EVOLUTION, ETC.

from the size of peas up to several centimetres in diameter in which
the cells were arranged like those of a Lithothamnion although the
cross walls were not well marked.

Nematophyceae: Nematophy tales. Figs. 177, 178.

Two genera are now grouped in this assemblage which has
recently been estabhshed by Lang (1937), and although he regards
these forms as land plants, nevertheless they have so many features

Fig. 177. Nematophy tales. Nematothallus. A, specimens on rock ( x f). B, large
and small tubes, the former with fine annular thickenings (x 150). C, cuticle
(x 150), (After Lang.)

in common with the algae that it is felt proper to include them here.
It is perhaps almost too speculative to suggest that they represent
Church's transmigrant form, but it would appear that they must
either be regarded as highly developed algae which adopted a land
habitat, or else as the most primitive of all true land plants. The two
genera agree closely in their morphological structure, and although
they are both frequently found associated with each other in the
Devonian rocks the two structures have not yet been found in
organic connexion. In spite of this it is very probable that the
leafy Nematothallus was the photosynthetic lamina of the stem-like
Nematophyton and may also have functioned as the reproductive

FOSSIL FORMS

275

organ. In the lowest strata the plants are to be found associated
with remains of marine animals, thus suggesting their power to
grow under marine or brackish conditions, whilst in the higher
strata they occur in beds, which are regarded as fresh- water or
continental, where they are associated with plants that were un-
doubtedly terrestrial. The presence of spores in Nematothallus is
regarded as rendering it unlikely that they were algal in nature,
but the spores may be comparable to the hard-walled cysts such as
are to be found in Acetabularia.

Fig. 178. Nematophytales. Nematophyton, A, longitudinal section (X120).
B, transverse section ( x 120). (After Seward.)

The genus Nematophyton is found in the Silurian and Devonian
rocks where it was first described under the name of Prototaxites
and referred to the Taxaceae, but subsequently it was accepted as
an alga and renamed Nematophyton or Nematophycus. Later the
name Prototaxites was revived and it was placed in the Phaeo-
phyceae, whilst Krausel (1936) recently stated that it must have
had the appearance of a Lessonia (cf. p. 180) and also that it existed
in aquatic habitats which may have been marine, brackish or fresh.
The vahd name is therefore Prototaxites, but as this tends to convey
a false impression of the plant's affinities it would seem more

18-2

276 REPRODUCTION, EVOLUTION, ETC.

satisfactory to retain the better known name of Nematophyton. The
largest specimen is a stem up to two feet in diameter, but what-
ever the size of the stem they are usually composed of two kinds of
tubes, large and small. The large tubes have no cross partitions,
but in some species they are interrupted in places by areas, re-
garded as medullary rays or spots by some authors, which are
wholly occupied by small tubes that in other parts of the thallus
simply take a sinuous course between the large tubes. The wide
tubes, in the latest specimens described by Lang (1937), show no
markings indicative of definite thickening, though striations have
been seen in specimens from other localities. Around the outside
of the central tissue there is a cortex, or outer region, composed of
the same tubes where they bend outwards towards the periphery and
eventually stand at right angles to the surface. The outermost zone
of all is apparently structureless and may well have been a muci-
laginous layer during life.

Nematothallus is a genus composed of thin, flat, expanded in-
crustations of irregular shape and up to 6J cm. long by i cm.
broad, and also constructed of the wide and narrow tubes. The
thallus is surrounded by a cuticular layer that exhibits a pseudo-
cellular pattern, and which includes within the cuticle and among
the peripheral tubes firm-walled spores of various sizes ; in N.pseudo-
vasculosa the spores were definitely cuticularized and so the
suggestion was made that these were land plants or parts of a land
plant. The wide tubes, which have thin pale brown walls, are
translucent in appearance and exhibit distinct characteristic
annular thickenings. The cuticle, which is apparently readily
detached, possesses distinct cell outlines that were probably made
by the ends of the wide tubes from the ordinary tissue where they
became fused together at the periphery, as in the living genera
Udotea and Halimeda. Another species Nematothallus radiata is
more imperfectly known.

From the structure described above it can be seen that the
members of this group are strongly reminiscent of the Laminariales
and Fucales, and it is tempting to suppose that they represent land
migrants from one of these groups. Problems that have to be
solved are : (i) The cuticularized spores ; whilst no such spores with
hard outer walls are known from the brown algae they are recorded
from the Chlorophyceae, e.g. Acetabularia. However, the sug-

FOSSIL FORMS 277

gestion that the spores may have developed in tetrads adds a
further compUcation, at any rate so far as an algal ancestry is con-
cerned, because the Dictyotales and tetrasporic Rhodophyceae do
not show the state of differentiation found in these fossil plants.
(2) The presence of a deciduous cuticle. In this connexion one or
tv^o Laminariales are known to shed cuticles during reproduction,
and the present author has found a deciduous cuticle on some pre-
served plants of Hormosira, a member of the Fucales. It may be
suggested that the plants perhaps had the appearance of a Lessonia
or even of a Durvillea, and a stem diameter of two to three feet does
not preclude them from being algal in character because several of
the large Pacific forms may have stipes of almost this size (cf.
p. 180). It has also been suggested that these forms are related to
the Codiaceae, especially Udotea, and in certain respects it is true
that they have the structure of a siphonaceous plant. Here again
there are several problems that need to be answered: {a) the
presence of two sizes of tubes ; {h) the presence of a cuticle ; (c) the
presence of cuticularized spores; {d) the large size of stem.

The answer to the last problem has already been suggested (see
above) but cuticles in the Codiaceae have not been recorded,
although the present author has been able to detect a structure
something like a cuticle in Halimeda; nor have any species been
reported that possess two distinct sizes of tubes, although grada-
tions in size occur in both Udotea and Halimeda. In this connexion
it may be of interest to refer to Tilden's unsupported suggestion
that the land plants arose from forms such as C odium and C aider pa.
It must be admitted that there are no living members of the
Codiaceae with stems that approach anywhere near the size of
those of Nematophyton. This, however, is not an insuperable ob-
jection as the Nematophytales may bear the same relation to the
livmg Codiaceae that the fossil Lepidodendrons bear to the living
Lycopodiales. For the present, however, the problem must be left
in the hope that further evidence will accumulate.

REFERENCES

Spongiostromata. Black, M. (193 3)- Philos. Trans. B, 222, 165.

Evolution. Delf, M. (1939)- ^^"^ Phytol. 38, 224.

Evolution. Fritsch, F. E. (1935)- Structure and Reproduction of the

Algae, vol. i, p. 12. Camb. Univ. Press.
Heterotrichy. Fritsch, F. E. (1939)- Bot. Notiser, p. 125.

278 REPRODUCTION, EVOLUTION, ETC.

Reproduction. Knight, M. (1931)- Beih. hot. Zhl. 48, 15.
Nematophytales. Krausel, R. and Weyland, H. (i934)- Palaeonto-

graphica, 79, Abt. B, p. 131.
Nematophytales. Krausel, R. (1936). Ber. dtsch. hot. Ges. 54, 379.
Reproduction. Kunieda, H. and SuTO, S. (1938)- Bot. Mag., Tokyo,

52, 539- . ^ ^, ^ , J

Evolution. Kylin, H. (1933). Lunds Univ. Arsskr. N.F. Avd. 2, 29.

Nematophytales. Lang, W. H. (i937)- Philos. Trans. B, 227, 245.

Dasycladaceae. PiA, J. (1920). Abh. zool.-bot. Ges. Wien, 11.

Fossils PiA, J. (1927). In Hirmer's Handb. Palaeobot. Miinchen and

Berlin.
Fossils, General. Seward, A. C. (1931)- Plant Life through the Ages.

Cambridge.
Evolution. Smith, G. M. (1933)- F^esh Water Algae of the United

States, p. 4. New York.
Reproduction. Smith, G. M. (1938). Bot. Rev. 4, 132.
Reproduction. Svedelius, N. (1927)- Bot. Gaz. 83, 362.
Reproduction. Svedelius, N. (1931)- Beih. hot. Zhl. 48, 38.
Evolution. TiLDEN, J. (1935). The Algae and their Life Relations, p. 24.

Univ. Minn. Press.

CHAPTER X

PHYSIOLOGY, SYMBIOSIS, AND SOIL ALGAE

PHYSIOLOGY

It would obviously be impossible to attempt a complete survey in
these pages of all that is known concerning the physiology of the
algae, especially as many species are very suitable objects for the
study of certain branches of physiology and in such cases a
voluminous literature has accumulated. Valonia has been fre-
quently used in experiments on absorption of solutes because of the
large size of the vesicles ; the Charales have been used in studies on
protoplasm because of the large size of their cells and the active
streaming of protoplasm that can be observed in forms such as
Nitella; Ulva, Hormidium and particularly Chlorella have been
repeatedly employed in experiments on assimilation; the eggs of
Fucus have also been objects of study from various points of view,
especially in reference to growth substances. In this chapter
certain recent papers have been selected for a survey because of
their more general interest and bearing on the life of the algae, but
their scope is by no means comprehensive and they have been
chosen in order to provide the student with some idea of the nature
of the knowledge that is being accumulated at present. The chapter
on Ecological Factors (cf. p. 349) will also be found to contain much
that can be regarded as algal physiology and should therefore be
consulted in this connexion.

Chlorophyceae

A recent study by Steward and Martin (1937) of the distribution
and physiology of Valonia at the Dry Tortugas in the West Indies
has brought out some interesting features. There are tsvo species
growing on the reefs that form the Tortugas ; V. macrophysa which
branches freely and V , ventricosa which is unbranched (cf. fig. 54)-
The former only grows in protected places, frequently where there
is no open communication with the sea, whilst the latter grows in
places exposed to the marine currents. The distribution of these
two species is therefore complementary and it is suggested that

28o

PHYSIOLOGY, SYMBIOSIS, ETC.

they are perhaps simply ecological forms. They do, however, differ
from each other biochemically in their K/Na ratio even though the
Cl~ content is about the same in both species, but this is not
necessarily of taxonomic significance. It is pointed out that the
vesicle should not be regarded as an enormous single cell but as a
fluid enclosed within a coenocytic wall composed of living cells.
In contrast to the observations of many workers it was found that
the enclosed fluid or sap of a Valonia plant in contact with sea
water does not have the fixity of composition that has been ascribed
to it. The sap of both species can, for most purposes, be regarded
as a mixed solution of sodium and potassium chlorides, V. macro-

per lit

I I I

12 3 4 5

Size

T5G

rm.

Fig. 179. Size of vesicles of Valonia ventricosa in relation to sap concentration.
(After Steward and Martin.)

physa being poorer in potassium and richer in sodium than V.
ventricosa. The K+ and Na+ content is definitely influenced by
illumination; a bright light, for example, induces a high K+ and
low Na+ content, whilst increased mechanical protection also
raises the potassium and sodium chloride content, the former more
so than the latter. The total salt concentration is also affected by the
size of the vesicle as may be seen by a study of fig. 179.

Another interesting observation was that when the chloride
concentration of sea water is increased, but not otherwise, the sap
will respond to small increments of K+ in the medium. Previously
it had always been thought that changes in concentration of K+ in
the sap were related to the concentration of hydroxyl (0H~) ions
in the medium, whereas it is now evident that the relationship with

PHYSIOLOGY 281

the concentration of CI" in the medium is much the closer. In
nature, Valonia derives its salts from a fluid which is much more
alkahne than its sap, but unlike most other marine plants it does not
appear to be able to accumulate bromide from sea water. The
determining factors for these two species appear to be:

(i) Exposure to surf; a feature of the environment which
operates mechanically and also through variations in pH, oxygen
concentration and temperature. V. macrophysa is tolerant of
considerable variations in the last three factors but it is not tolerant
of the mechanical effects, whereas V. ventricosa responds in the
reverse manner.

(2) Physical character of the substratum.

(3) Composition of the fluid medium, especially in respect of
sodium, potassium and chloride ions.

(4) Illumination.

Rhodophyceae

Some depth studies in relation to photosynthesis by Tshudy
(1934) may usefully be considered here, whilst further references
to this particular problem will be found on pp. 293 and 357.
Nearly all studies of photosynthesis in the algae regard Englemann's
theory that the colour of the alga is complementary to that of the
incident light as the basis for the investigation. For example, in
the green algae the greatest assimilation takes place in the red region
of the spectrum, whilst in the red algae it takes place in the green
region. In 1909 Hanson, considering only those Rhodophyceae
which grow at considerable depths, suggested that the chlorophyll
utilized the energy that was absorbed by the phycoerythrin, in
which case the red colouring matter was simply acting as a passive
colour screen. This theory, however, still left unsolved the problem
of the function of the coloured pigment in those red algae which
always grow in the littoral belt, though it was of course possible
that those algae grew in such situations merely because they could
survive the competition. In 1920, Moore, Whitley and Webster
showed that the Rhodophyceae assimilated less rapidly than the
Chlorophyceae in bright sunlight and more rapidly than the Chloro-
phyceae in diffused light, and so they argued that phycoerythrin
does not act as a passive colour screen but takes an active part in the
process of assimilation. However, their results are partially

282

PHYSIOLOGY, SYMBIOSIS, ETC.

invalidated in that no allowance was apparently made for any
temperature effect.

Tshudy inserted the algae to be investigated into test tubes
which were then placed horizontally in wire baskets that could be
lowered to any required depth. The oxygen was measured in these
tubes before and after each experiment by the Winkler method.
Estimations were also made on blank controls, but as these exhi-
bited some fluctuations the conclusions from the experiments
themselves ought to be accepted with some degree of caution.

Tshudy found that:

(i) At 25 m. depth respiration takes place more rapidly than
photosynthesis, but for the species investigated in that particular
area there is a sHght balance in favour of photosynthesis at 22-5 m.

-^<— Iridaea.
-« — TurnereUa
Voter Contrvl

J L.

15 20 22-5 25

o-sU- — --.^__ ?=^o-^-

1 3 5 7-5 10 15 2022-525 13 5 7-5 10

Depth in metres

Fig. 180. Assimilation of two species of the Rhodophyceae in relation to depth
and weather conditions. A, cloudy day and water slightly choppy. B, clear,
calm day. (After Tshudy.)

(2) Photosynthesis is materially affected by the degree of cloudi-
ness and the state of the w^ater, w^hether it is calm or choppy. The
influence of these tw^o factors can be seen from an examination of
fig. 180.

(3) On clear calm days maximum photosynthesis occurred at a
depth of about 5 m., but on choppy days it occurred at or near the
surface (fig. 180).

From the above results it was concluded that the red phycoery-
thrin acts largely as a colour screen, the plants utilizing the light in

PHYSIOLOGY

283

the same way as aerial shade plants. The colour of the plant would
therefore seem to act in a purely physical fashion and not in any
physiological manner, an interpretation that has also been sup-
ported by Seybold (1934) (cf. p. 293). It must, however, be
remembered that depth studies cannot yield valid conclusions on
the role of pigments unless the spectral composition of the light has
been determined in order that an adequate comparison can be made
with the behaviour of green algae under similar conditions.

Phaeophyceae

A study by Stocker and Holdheide (1937) of the assimilation of
the principal brown fucoids which zone our shores (Fiicus platy-
carpuSy F. vesiculosus, F. serratus) when compared with that of a

rucui bUtycA.r,bu5

14
12

u
J5

-a

•

-1

100 150

12 5 10 50

Light intensity — in thousands of metre-candles

Fig. 181. Assimilation of different algae in relation to hght intensity. (After
Stocker and Holdheide.)

Laminaria, a green and a red alga produced some interesting results.
Assimilation by all these algae decreases under very bright light,
and this agrees with Tshudy's results for clear days when he found
that the maximum assimilation did not take place at the surface
(cf. fig. 181). The optimum Hght intensity for assimilation was
found to be in the same region as that of the cormophytic land
plants, even though these do not usually show a drop beyond
50,000 lux. The optimum temperature for photosynthesis in

284

PHYSIOLOGY, SYMBIOSIS, ETC.

Delesseria, Enteromorpha and Fucus is about 25° C. although there
is a fairly wide range : at low light-intensities, for example, there is
as much assimilation at 5° C. as at 15° C. (cf. fig. 182). In nature
the temperature optimum generally corresponds to the temperature
attained by the thallus in the sun's rays, whilst Ehrke (1931) also

100-

Oi ,' /itg/i U§kt -Sfirin^

'' Lou) U^ht- Su.mTTfcr

I D '

/it|A. L^ki-Stinn^

Surnrncr >

' I 1 L

5 10 15 20 25 30"C 5 10 15 20 25 30 35"C 5 10 15 20 25 30 35°C

I '2 3

Fig. 182. Respiration and assimilation in relation to temperature and light.
A = assimilation. R = respiration. Experimental period = 3 hours, i = Delesseria.
2 = Enteromorpha co?npressa. 3 = Fwcw^ serratus. (After Ehrke.)

found a correlation between the temperature of maximum assimila-
tion and the average temperature of the month of maximum
development.

Table VI

Optimum temperature
for assimilation

17' C.
o"C.

Average temperature in

months of maximum

development

17° C, Aug.-Sept.
0° C, winter and
early spring

Species

Fucus, Enteromorpha
Delesseria

The principal limiting factor for assimilation appears to be the
water content because exposed thalli quickly dry up and cease to
assimilate, whilst respiration also sinks very low (cf. fig. 183). For
the Fucaceae on a normal cloudy day the amount assimilated during
the time they are exposed in 24 hours is only o-7-i-4% of the dry
weight, although the fertile tips of Fucus platy carpus acquire a
slightly higher percentage. On remoistening, Enteromorpha Linza
and Porphyra utnhilicalis take up water at once and very soon
commence to assimilate again, whilst the table below shows that
the Fucaceae behave in a very different fashion.

The influence of rain on the Fuci appears to cause a reversible
depression of the assimilation rate amounting to 19-25 %, and this
factor may assume considerable importance on some coast-lines.

PHYSIOLOGY

285

Table VII. Percentage of normal assimilation reattained

Pelvetia canaliculata

70-80% 8-9 hours after ist tide; exposed for 11 days
previously
Fucus spiralis (and F. 49 % 8-9 hours after ist tide; ^4 hours after

platycarpus)
Fucus vesiculosus

Fucus serratus
Laminaria digital a

3 days' exposure previously 97 %
20% 8-9 hours after ist tide;

3 days' exposure previously 72%
Cannot tolerate 3 days' ex-
posure 42 % ^

Cannot tolerate 1-2 hours' exposure

flooding; ex-
posed 5 hours
previously in
air dryness
10% of max.,
or 90 % R.H.

o
o

u
u

o
o

- fucui 3Crra.tu.i7 - ciou.d
- ' -^unLLSht

—Fucui /bUitLfCArbus-cLoud.

— " '■ -Sunligkt

7 8 g^^o^Rs

12
10
8
6
4
2

~1
-3

A-6&Lm.iLa.tion curoe - Fucm
■Sc rrdfus . /-S, 000 ca.ndU jboocr

Tern/b. -
20

^e.5>bLrd.iion, curue^

30°C

Fig. 183. A, water loss, and B, assimilation in relation to exposure (drying) on
sunny and cloudy days. C, D, effect of temperature on respiration and assimila-
tion of Fucus. Investigational period for assimilation, 5 min.; for respiration,
18 min. (After Stocker and Holdheide.)

286

PHYSIOLOGY, SYMBIOSIS, ETC.

The results so far described would seem to be against a weak light
and cold medium as being the best conditions for the Fucaceae,
but the great development of this group in northern and arctic
seas cannot be overlooked, and the explanation must be the
working together of factors other than merely light and temper-
ature. Hyde (1938), however, explains this development in arctic
waters as due to the indirect effect of lowering the temperature
because this results in an excess of assimilation over respiration.

■M

100
2x40

5°C

30 25 20 15 10
Temperature

Fig. 184. Diagram of paper model to show the combined effects of Hght and
temperature on the rate of apparent assimilation of Fucus serratus. (After Hyde.)

She found that between 15 and 20° C. the assimilation rate could
be increased by raising the light intensity, and that there was a
certain light value (2 x 500) which yielded an optimum in the rate
of assimilation. This effect, however, is not observed at low light
intensities and low temperatures, whilst above 25° C. an increase
of the light intensity causes a marked decrease in the assimilation
rate (cf. fig. 184).

Ehrke (193 1), after carrying out experiments both in the field
and laboratory, found that the respiration of most algae increased
with rise of temperature, and often has not reached its maximum
even when the high temperatures have reduced the assimilation

PHYSIOLOGY

287

rate considerably (of. fig. 182). The assimilation of any alga ex-
hibits a well-marked optimum which depends upon both light and
temperature : at low temperatures the optimum occurs at low light
intensities, and this is obviously significant in the case of those
algae that grow in cold waters. Those algae which behave in the
same way as terrestrial shade plants build up food reserves during
the cold time of the year when conditions are favourable for them,
and then lose the material during the more unfavourable warmer
periods, whilst in summer time those algae which behave like
terrestrial sun plants have a high assimilation rate (cf. p. 359).

/^ssim.

1000

^ Sunlldh.t' , ,

12 5 9 18 26 28 39 47 12 4 8 14 18 26 29 35
Days Days

Fig. 185. Daily drift in assimilation of algae at different temperatures in sunlight
and diffuse light. A, Fiicus serratus, winter plant. B, Porphyra. (After Lampe.)

More recently Lampe (1935) has re-emphasized the fact that the
relation between temperature and distribution is not completely
solved, and furthermore he stresses the fact that it does not appear
to be wholly dependent upon a physiological basis. In winter, the
assimilation rate of Fucus serratus plants is found to rise when it is
measured in sunlight under conditions of increasing temperatures
(cf. also Ehrke above) ; on the other hand, in the case of a red alga
such as Porphyra, when the temperature is raised above 15° C. the
assimilation curve is lowered immediately in difltused light and
after seven days in sunlight (cf. figs. 185, 186). From this it may be
concluded that Fucus is an eurythermal species, i.e. tolerates a wide
range of temperature, whilst Porphyra is a stenothermal species,
i.e. tolerates only a narrow range of temperature (cf. also p. 367).

288

PHYSIOLOGY, SYMBIOSIS, ETC.

At the present time further studies are required in order to ascer-
tain the mechanism involved in the gradual change in value of the
temperature for optimum assimilation as one passes from cold to
warm weather and vice versa. The explanation of this phenomenon
should provide us with the clue to the correlation between algal
distribution and temperature.

We may now turn to other aspects of algal physiology, and here
one may mention some results of Haas and Hill (1933) who found a

Assim

260r

220-

180-

140-

100

Fig. 186. Assimilation of winter plants of Fucus serratus at varying temperatures

and in different light intensities. = vveak diffused light, = strong

diffused light, - • - • = sunlight. (After Lampe.)

correlation of fat content with the vertical distribution, or in other
words with the duration of exposure (cf. Table VIII).

The produc'^s of nitrogen metabolism, however, are not corre-
lated in the same way. The algae are also characterized by an
absence or paucity of free sugars, their place being taken by sugar
alcohols such as mannitol. As these are probably secondary pro-
ducts derived from the free sugar, the latter is not to be found
because conversion to a sugar alcohol removes it as fast as it is
formed. It has recently been discovered, however, that the per-
centage of mannitol together with another substance, laminarin, in
the alga Eisenia hicyclis tends to reach a maximum in the evening

PHYSIOLOGY 289

whilst the mean content for day and night is not appreciably
different. In this alga, therefore, these substances are probably not
direct products of photosynthesis. It has been shown that when
Eisenia is kept in the dark both these substances decrease and so it
is suggested that they should more properly be regarded as food
reserve materials.

Studies on the induction of polarity in Fucus eggs have shown
that unilateral light is normally the strongest determinant but that
polarity can also be induced by an electric current or^H gradients.
There is also the effect of neighbouring groups of eggs or the
nearest large tgg, the first rhizoid developing on the side nearest to

Table VIII

Ether-extracted

fat True fat

Pelvetia canaliculata f. libera (salt marsh) 8-62 8-o

Pelvetia canaliculata (spray zone) 4-88 4*9

Fucus vesiculosus ecad volubilis (salt marsh) 3-76

Ascophyllum nodosum (middle littoral) 2-87

Fucus vesiculosus ~\ 2-60 2*6

Halidrys siliquosa J- (low littoral) ' 2* 18

Himanthalia loreaj
Desmarestia aculeata

Laminaria digitata
Pelagophycus
Macrocystis
Nereocystis
Laminaria Andersonii ^

I-2I

0-65

0-46 0-3

(sublittoral)

0-27

0-34-0-40

I -06

0-65

them. This is probably a growth substance effect since the
presence of growth substances has been demonstrated in eggs,
sperm and fruiting tips oi Fucus. In the case of polarity induced by
the proximity of eggs the growth substances from the neighbouring
ova are not necessary for rhizoid formation but are purely directive.

General

An investigation has been carried out by Biebl (1938) using
hypo- and hypertonic solutions of sea water for determining the
drought resistance and the osmotic relations of algae from different
depths. The algae studied could be placed in three groups according
to their behaviour but irrespective of their geographical locality.

A. Deep growing algae which are never exposed to the air: they
are resistant up to a concentration of 1-4 times sea water.

B. Algae of low water mark and the lower littoral tide-pools

CSA - ^9

290 PHYSIOLOGY, SYMBIOSIS, ETC.

which rarely become completely dry: these are resistant up to
concentrations of 2-2 times sea water.

C. Algae of the littoral belt: these are usually completely
exposed and can resist a concentration of 3-0 times that of sea water.

The behaviour of these algae is summarized in Table IX.

It was found that most of the Rhodophyceae possess a cell sap
which has an osmotic pressure approaching that of the maximum
hypertonic resistance likely to be encountered in their habitat, but
this correlation between cell sap and external medium is not so
evident in the case of the Chlorophyceae and Phaeophyceae. In
their resistance to desiccation the algae fall into the same three
ecological groups as can be seen from Table X.

In group I when the filament is dried up for only a very short time
by means of filter paper the cells die or collapse so quickly that they
do not even recover when put back into sea water. Those of group 2
are less susceptible and those of group 3 hardly susceptible to this
treatment.

A study of the chlorophyll relations in all three algal groups by
Seybold and Egle (1938) revealed the fact that only in the Chloro-
phyceae are both chlorophylls a and h present, their proportions
being the same as those in the submerged flowering plants. In the
Rhodophyceae, Phaeophyceae, Cyanophyceae and Bacillario-
phyceae only chlorophyll a is present, so that if the absence of
chlorophyll h is considered to represent a primitive character, the
Chlorophyceae would have to be regarded as the most recent
group (cf. p. 265). On the other hand it is equally possible that in
these groups the second component has been lost, possibly due to
the introduction of the extra colouring pigment or to some other
factor. In not one of these groups does there appear to be any
relation between depth and quantity of chlorophyll and carotene
present, the actual amount being determined rather by the genetic
constitution. The quantity of pigment per dry or fresh weight is
hss in the Rhodophyceae and Phaeophyceae than it is in the
Chlorophyceae, but the fact that members of the first two groups
assimilate carbon dioxide as rapidly, weight for weight, as those of
the Chlorophyceae indicates that their carbon assimilatory appar-
atus cannot be deficient. It is also evident that the green algae
exhibit a far greater range in the amount of pigment present than
do the red and brown algae (cf. fig. 187).

PHYSIOLOGY

291

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PHYSIOLOGY

293

The light relations in the photosynthetic mechanism of the
algae can be divided into two components :

(a) Physical component, which is the amount of light energy
absorbed by the thallus.

(b) Physiological component, which is the amount of absorbed
light energy that is actually employed in carbon assimilation.

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Fig. 187. Chlorophyll content of the same number of red, green and brown algae
expressed in different terms. (After Seybold and Egle.)

The light energy relations of selected members in the Chloro-
phyceae, Phaeophyceae and Rhodophyceae are illustrated in Table
XI from Seybold (1934) and also in fig. 188.

It is at once obvious that at depths below i m. the Rhodophyceae
and Phaeophyceae are much more efficient as metabolic machines
than the Chlorophyceae, and several species are even more efficient
at the surface. As a resuh of his studies Seybold (1934) concluded
tliat Englemann's theory of complementary colours is only valid
for the physical component of the light relationship, that is, the

294

PHYSIOLOGY, SYMBIOSIS, ETC.

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PHYSIOLOGY

295

pigments only help in the amount of light absorbed and not in its
utilization. We have already seen earlier that other workers, who
have studied members of the individual groups, have arrived at a
similar conclusion since they regard the coloured pigments as
acting in the same way as a colour screen. Apart from the physical
adaptation, in the sense of complementary Hght absorption, there is

700 600

700

600 500/^/i 400
B

50Qjuju,A00

Fig. 188. Absorption curves of A, Monostroma, B, Delesseria, at different
depths. (After Seybold.)

also a physiological adaptation to strong and weak light and to long
and short wave light. The algae can be placed into two groups
depending upon their responses to strong and weak light, or to
long wave and short wave light. This problem, however, is followed
up more closely in the later chapter on algal ecology (cf. p. 359).

^SYMBIOSIS

The most striking and well-known examples of symbiosis in the
algae are provided in those cases where the plants are associated
with animals, especially Coelenterates, or with fungal threads, as in
the common lichens. Apart from these examples, however, there
are other cases which are not so well known, largely because they
are not so common or so conspicuous. Gloeochaete, for example, is
a colourless genus of the Tetrasporaceae which possesses blue
green bodies that look like chromatophores, though they are really
a symbiotic blue-green alga. Gleucocystis is a colourless genus of
the Chlorococcales in which a symbiotic member of the Cyano-
phyceae also forms blue green "chromatophores" that appear as a
number of curved bands grouped in a radiating manner around the
nucleus. In this case the illusion is further enhanced because they
break up into short rods at cell division. It has so far proved

296

PHYSIOLOGY, SYMBIOSIS, ETC.

impossible to grow the blue-green alga separately and it may thus
have lost its power of independent growth. Geosiphon, which is
variously regarded as a Siphonaceous alga or as a Phycomycete,
possesses small colonies of Nostoc enclosed in the colourless pear-
shaped vesicles that arise from an underground weft of rhizoidal
threads. Reproduction by the formation of new vesicles is said to
occur only in the presence of the Nostoc. The presence of chitinous
material in the vesicular wall suggests a fungal nature for Geosiphon^
the vesicles perhaps being galls that are formed on the threads as a
result of the presence of the alga.

«^fcM5^

Fig. 189. Symbiosis. Zooxanthellae in the tissues of a coelenterate, Pocillopora
bulbosa ( X 375). ec = ectoderm, ew = endodemiis, ^2: = dead zooxanthellae,
fg = fat globule, w^ = glands, 5m = structureless lamella, s' = zooxanthellae.
(After Yonge.)

The principal genera taking part in lichen synthesis are Nostoc^
ScytonemUy Cystococcus, Gloeocapsa and Trentepohlia. To what
extent the lichen body is a case of true symbiosis is a problem that is
still not wholly settled: under normal conditions it is probably a
real symbiotic relationship but under abnormal conditions the
fungus may become a parasite and devour the algal component. The
green bodies which are found associated with the cells of Coelenter-
ates and Radiolarians are usually placed in what may be called a
*'form" genus, Zooxanthella (cf. fig. 189). Most of the species
belong to the Cryptophyceae, but in certain of the Coelenterata the
motile phases of some of the algae which have been discovered
suggest an affinity to the Dinophyceae, whilst Chlorella (Chloro-
coccales) is also said to behave as a symbiont of this type. The

SYMBIOSIS 297

non-motile vegetative cells are usually found in the peripheral
layers of the polyp, the larval stages of the host commonly being
devoid of the alga. Most of the algal symbionts are known to have
a motile stage and hence are capable of an independent existence.
The function and relations of these symbiotic algae in the coral
polyps has been discussed at great length by Yonge (1932), and on
the whole there would appear to be evidence for a symbiotic
relationship, the alga obtaining food from the animal, and the
animal oxygen and also perhaps nitrogen from the alga. The prob-
lem of the relationships between algae and animals is by no means
completely worked out, and it is not impossible that in some cases
we really have an animal that is parasitizing the alga. This is
probably especially true in the case of the worm-like creature
Convoluta Roscojfensis and its algal associate Carteria, because the
animal apparently cannot live unless infected with the alga, whilst
under certain conditions it also devours the green cells.

Other examples of symbiosis are provided by Anahaena Cyca-
dearum which lives in the root tubercles of species of Cycas, and
Anahaena Azollae, which is found in the leaves of the water fern,
Azolla filiculoides, though the species oi Nostoc that are to be found
in the thallus of the Liverworts Blasia and Anthoceros are probably
no more than space parasites obtaining shelter.

Epiphytism is extremely common among the algae, whilst there
are also a number of epizoic forms. One may also find endophytic
species, such as Schmitziella mirahilis in Cladophora pellucida, and
endozoic species, such as Rhodochorton endozoicum in the sheaths of
hydroids. The origin of the symbiotic habit among the algae is
probably to be explained as cases of epiphytism in which the
relationship between host and epiphyte became more intimate:
similarly the relatively few cases of parasitism probably arose either
directly from an epiphytic habit or else passed through the
symbiotic phase. Examples of total or partial parasites are Notheia
anomala in the Phaeophyceae, Choreocolax Polysiphoneae in the
Rhodophyceae and Chlorochytrium Lemnae in the Chlorophyceae.

298 PHYSIOLOGY, SYMBIOSIS, ETC.

SOIL ALGAE
Terrestrial algae may be classified conveniently as follows :

(i) Aero-terrestrial species found growing on plants.
(2) Eu-terrestrial,

True soil species :

(a) Epiterranean, or lying in the surface layers of the soil.

(b) Subterranean, or lying in the lower layers of the soil.
So far as is known at present there are no obligate
species of this class.

(d) Casuals.

The study of soil algae, as such, began seriously at the commence-
ment of the nineteenth century with the works of Vaucher,
Dillwyn, Agardh and Lyngbye, whilst towards the end of the
century monographs by Bornet and Flahault, Gomont, Wille and
the Wests, father and son, began to make their appearance. In
1895 Graebner, in a study of the heaths of North Germany, gave
the first account of soil algae as ecological constituents, and sub-
sequently many ecologists have shown that soil algae are pioneers
on bare soil where they prepare the ground for the higher plants
that follow. In such cases the algal flora is generally richest when
the soil is primarily or secondarily naked, e.g. mud flats developing
to salt marsh, or ploughed grassland. A manured soil also has a very
rich flora, whilst the same species are to be found in unmanured
soils, though not in such numbers. The richness of the flora is also
influenced by the moisture conditions, damp soils having a more
varied and extensive collection of algae than dry soils. In recent
years dilution cultures have been widely used in order to give a
quantitative aspect to the work, and the results of such studies have
been to show that there is probably a seasonal variation in numbers,
but that the behaviour depends on the depth and kind of soil.

Subterranean Algae

There are great fluctuations in the numbers of the different
species that compose the flora, but there are no species in the lower
layers of the soil which do not also occur in the surface layers.
Dilution cultures, together with the counting of samples, have

SOIL ALGAE 299

shown that the algal flora is mainly confined to the top 12 in. of
soil with a maximum abundance at about 3-6 in. below the surface.
With increasing depth the number of algae decrease regularly, the
maximum depth at which they have been recorded being two
metres; there is, how^ever, really no conclusive evidence which
shows that algae can grow in the deeper layers where there is no
light, and it is very probable that they are only present in these
layers in a resting phase. > The number of reproductive bodies in the
surface layers reaches a maximum in spring, but in the lower levels
it remains constant throughout the year. In Denmark the quality
of the soil is apparently decisive in determining the luxuriance of
the flora irrespective of whether the ground has been disturbed or
not. In Greenland soil algae have been found down to a depth of
40 cm., and their presence there can only be satisfactorily explained
by the action of water trickling down the cracks because burrowing
animals are absent. A study of soils from all over the world has
emphasized the existence of a widely distributed algal formation in
cultivated soils. This flora consists of about twenty species of
diatoms, twenty-four of Cyanophyceae and twenty species of green
algae, among which Hantzschia amphroxys, Trochiscia aspera,
Chlorococcum humicolum, Bumilleria exilis and Ulothrix suhtilis var.
variahilis are the most frequent.

The growth of these soil algae has been a source of interest and
experiment for a number of years. Roach (1926) has found that
ordinary growth in Scenedesmus costulatus var. chlorelloides is best
in a glucose medium but that xylose is toxic, the factors controlling
the normal growth rate being light, temperature and aeration of
the medium (cf. fig. 190). The same alga has been used for growth
experiments in the dark in order to determine how far such algae
can grow when they are below the soil surface (cf. fig. 191). This
and four other species can be made to grow in the dark provided an
organic medium is present, but they all react differently to the
various conditions and also they vary in the amount of growth that
occurs. At constant temperature, increasing the light intensity
from ^ to J has a far greater effect than increasing the intensity
from J to full sunlight. Under full light the growth curves (cf. fig.
191) rise to an optimum by means of photosynthesis alone, but at
lower intensities the optimum is only approached if additional
nutriment, in the form of glucose, is present as well. There is no

300 PHYSIOLOGY, SYMBIOSIS, ETC.

adequate evidence that such organic media are present in the soil
layers so that it is very doubtful whether growth in the dark can
occur in nature, but it has been shown, however, that Nostoc
punctiforme from the leaves of Gunnera and also a species of
Euglena are capable of growth in the dark.

Cifl

o
3

6-0

^ 5-5

5-0

4-5

4-0

3-5

3-0

iMinerA sslib ^ 1%
1 6Lucose -diffuse

/Minerd sa.lts +

/ / Diffuse d^yli§hi

I /
/ /■

10
Days

Fig. 190. Growth of the soil alga, Scenedesmus, under different conditions of
nutrition and light. (After Roach.)

Even if the algae cannot grow in the lower layers of the soil
because of the darkness, we must still enquire into the process
responsible for their appearance in those layers. The possible
agencies are (i) cultivation, (2) animals, (3) water seepage and (4)
self-motility. Mechanical resistance and lack of light are said to
prevent the Cyanophyceae from moving down under their own
locomotion, and whilst it is possible that algae may move down
through their own motility, further experimental work on this
aspect is much to be desired. The effect of water seepage will

SOIL ALGAE

301

depend on the heaviness of the rainfall, the state of the soil, i.e.
whether dry and cracked, and the nature of the algae, i.e. whether
or not they possess a mucous sheath. Passage through the soil is
facilitated in the filamentous algae either by fragmentation or else
by the formation of zoospores, the factors that are responsible for the
former process appearing to differ for the various species. Many

6-0

5'5

S 5-0

u
a
a

'a
o

to 3-5
o

4-5

4-0

3-0

2 4 6 8 10 DAYS

Fig. 191. Rate of growth of Scenedesmus in a solution of mineral salts under
different light intensities, (After Roach.)

green algae are known to form zoospores when put into water
after a period of dryness, and hence one may presume that a shower
of rain will also induce zoospore formation. Petersen (1935) has
demonstrated experimentally that rain can carry algae down
efficiently to a depth of 20 cm., but that the process is facilitated by
the presence of earth-worms, although these animals probably only
operate indirectly in that they loosen the earth. Farmers in the

302 PHYSIOLOGY, SYMBIOSIS, ETC.

course of their cultivating operations must frequently be respon-
sible for the conveyance of algae down into the soil.

Many of the soil algae, especially the Cyanophyceae, can resist
very protracted spells of dryness as Roach (1920) demonstrated
when soils from Rothamsted that had been kept for many years
were remoistened. Bacteria developed first, then unicellular green
algae with some occasional moss protonemata, and although the
Cyanophyceae appeared last, nevertheless they quickly became
dominant. Nostoc muscomm and Nodularia Harveyana appeared
after the soil had been dried up for 79 years, whilst Nostoc Passer-
inianum and Anabaena oscillarioides var. terrestris appeared after 59
years of dryness. These algae differed in some respects from the
typical forms that are to be found in ordinary soils, but this was
probably only due to the cultural conditions.

Fritsch and Haines (1922) have studied the moisture relations of
some terrestrial algae (cf. fig. 192) and they have shown that:

(i) There is a complete absence or paucity of large vacuoles.

(2) In an open dry atmosphere nearly all the sap is retained.

(3) When the filaments dry up, contraction of the cell is such
that the cell wall either remains completely investing the protoplast
or else in partial contact with it, thus ensuring that all the moisture
which is imbibed will reach the protoplast.

(4) During a drought there is, as time goes on, a decreasing
tendency for the cells to plasmolyse and there are also changes in
the permeability of the cell wall, whilst the access of moisture
normally brings about changes in the reverse direction. The
majority of cells which do plasmolyse lack the characteristic
granules, mainly of fat, that are to be found in most terrestrial

algae.

(5) Those cells which survive after drought do not contain any
vacuoles and possess instead a rigid, highly viscous protoplast which
is in a gel condition. This is the normal state of the vegetative cells
of Pleurococcus and the cells in the '' Hormidium'' stage of Prasiola.

(6) If desiccation is rapid most of the cells will die but some will
plasmolyse and retain their vitality in that state for weeks or months.
In spite of the death of the bulk of the cells no species disappears
from the flora during a rapid onset of drought.

(7) If desiccation continues, the number of living resting cells
will remain constant for several years.

SOIL ALGAE

303

(8) During a very long drought the resting cells of algae below
the surface will still survive.

Apart from the moisture relations there are also other factors
that may be involved. Diatoms can survive very low temperatures,

100

80
60
40
20

c. ...Unaffected A_ ^b^^^S

blcohtlu bLi.sm.oly6<sd.

StTSrvgl-y pU-amolUied

4-1
o

t-t-.

5 days

lOOr

2c/a

lys

3 days

Length of exposure to drought

Fig. 192. Effect of exposure to drought on A, Hormidium\ B, "Hormidium" stage
of Prasiola; C, Zygogoniuyn ericetorum. (After Fritsch and Haines.)

— 80° C. for 8 days or — 192° C. for 13 hours, whilst dry spores of
Nostoc sp. and Oscillatoria brevis can survive —80^ C, though if
they are moist a temperature lower than — 16° C. will kill them. As
the vegetative filaments of Xostoc die after four days at —2 to

— 8° C. this genus must survive severe winters in the form of

304 PHYSIOLOGY, SYMBIOSIS, ETC.

spores. So far as the algae of tropical soils are concerned the dry
spores oi A'ostoc sp. and O. brevis can tolerate 2 min. at 100° C, the
wet spores 20 min. at 60-70° C, and the vegetative filaments
10 min. at 40° C, this latter being a temperature that is fre-
quently reached on open ground in such regions. Acidity and
alkalinity do not appear to be of any great importance, although
members of the Chlorophyceae usually thrive better on basic soils.
It has been demonstrated that Anabaena and Nostoc can fix
nitrogen from the air in the presence of light, but other soil algae
apparently do not possess this power unless they occur in combina-
tion with bacteria, and even then the actual fixation is probably
carried out by the bacteria. It has been found by De (1939) that
Anabaena will only fix nitrogen from the air so long as nitrate is
absent from the soil. The combination of bacteria and algae fix
nitrogen better than the bacteria do alone, so that the algae must
act as a kind of catalytic agent, and it has been suggested that they
{a) provide carbohydrate, and hence energy, for the bacteria, or
{b) remove the waste nitrogen compounds, since it has been shown
that if these accumulate bacterial activity is reduced. In some cases
the algae play a part in aeration because of the oxygen they produce
during photosynthesis, and in this connexion it may be mentioned
that unless certain species are present in the soil of rice fields during
the period they are waterlogged the aeration deteriorates and the
rice becomes much more susceptible to disease. Rice is also capable
of growing in the same field year after year without being manured,
and it has been demonstrated that this is due to the fixation of
nitrogen by the algae present in the soil.

REFERENCES

Physiology. BiEi ., R. (1938). Jb. Wiss. Bot. 86, 350.

Soil Algae. De, P. K. (1939). Proc. Roy. Soc. Ser. B, 127, 121.

Physiology, du Buy, H. G. and Olson, R. A. (1937). Amer.J. Bot. 24,

609.
Physiology. Ehrke, G. (193 i). Planta, 13, 221.
Soil Algae. Fritsch, F. E. (1936). Essays in Geobotany in honor of

W. A. Setchelly p. 195. Univ. California Press.
Soil Algae. Fritsch, F. E. and Haines, F. M. (1922, 1923). Ann. Bot.

36, i; 37, 683.
Physiology. Haas, P. and Hill, T. G. (1933). Ann. Bot., Lond., 47, 55.
Physiology. Hanson, E. K. (1909). New Phytol. 8, 337.
Physiology. Hyde, M. B. (1938). J. Ecol. 26, 118.

SOIL ALGAE 305

Symbiosis. Keeble, F. and Gamble, F. W. (1907). Quart. J. Micr. Set.

51, 167.
Physiology. Lampe, H. (1935). Protoplas?Tia, 23, 543.
Physiology. Moore, B., Whitley, E. and Webster, T. A. (1920). Ann.

Rep. Oceanog. Univ. Liverpool, 36, 32.
Physiology. Nisizawa, N. (1938). Sci. Rep. Tokyo Bunrika Daig. 3, 289.
Soil Algae. Petersen, J. B. (1935). Dansk bot. Ark. 8, i.
Soil Algae. Roach, B. M. (1919). Neiv Phytol. 18, 92.
Soil Algae. Roach, B. M. (1920). Ann. Bot., Lond., 34, 35.
Soil Algae. Roach, B. M. (1926). Ann. Bot., Lond., 40, 149.
Physiology. Seybold, A. (1934). Jb. wiss. Bot. 79, 593.
Physiology. Seybold, A. and Egle, K. (1938). Jb. wiss. Bot. 86, 50.
Valonia. Steward, F. C. and Martin, J. C. (1937). Publ. Carneg.

Instn, no. 475, p. 89.
Physiology. Stocker, O. and Holdheide, W. (1938). Z. Bot. 32, i.
Physiology. Tshudy, H. (1934). Amer. J. Bot. 21, 546.
Symbiosis. Yonge, C. M. and Nicholls, A. G. (1932). Reports of the

Gt Barrier Reef Exp. i, 135. Brit. Mus. Publ.

C S A ■ 20

CHAPTER XI

MARINE ECOLOGY

The algae of the rocky coasts have attracted more investigators
than those of the salt-marsh coast, probably because of the greater
abundance of species, the greater ease in identifying the component
members of the flora, and the v^ell-marked zonation which is so
characteristic of most rocky shores. In spite of these numerous
investigations v^e are still very far from understanding how the
zonation is secured and maintained, nor is there sufficient data
about the environmental factors because most workers have simply
contented themselves with describing zonations in particular areas
and only suggesting possible controlling factors. Furthermore, our
knowledge of recolonization on the sea-shore is very rudimentary,*
and it is highly desirable that more information should be obtained

Table XII. Algal associatiot

Dover

Endoderma
Rividaria- Colo thrix

Schizothrix Fritschii
Enteromorpha- Urospora-

Codiolum
Chrysophyceae-Endoderma-

Lyngbya
Chrysotila stipitata

Fucus spp.

Gelidium-Polysiphonia
Rcdfsia

Enteromorpha-Porphyra
Chalk-boring algae

Rhizoclonium- Vaucheria
Pylaiella littoralis

Enteromorpha intestinalis
Rhizoclonium riparium

Isle of Wight

Peveril Point
(Dorset)

Wembury
(Dorset)

Rivularia-Calothrix —

SUBLITTORAL

Sheltered coasts

Fucus ceranoides Fucus spiralis

Ascophyllum Fucus vesiculosus

Fucus vesiculosus —

Fucus serratus Fucus serratus

— Porphyra

Ulva

Laurencia-
Corallina

Pelvetia
Fucus spiralis

Ascophyllum
Fucus serratus

Lomentaria \
Gigartina J

Halidrys

Laminaria

Chondrus

Lough Ine (Ireland)

Lichina

Hildenbrandtia- Verrucaru

Ralfsia
Upper Chlorophyceae

Fucus spiralis

Fucus vesiculosus var.

evesiculosus
Ascophyllum
Fucus serratus

Porphyra
Bangia- Urospora

Lomentaria
Laurencia

Gigartina

Callithamnion-Ceramium

Nemalion

Himanthalia

Corallina-Lithothamnion

Laminaria

Sublittoral Rhodophyceac

Alaria

Plumaria- Ceramium
Laurencia- Gelidium
Chondrus

Cladophora rupestris
Lower Chlorophyceae.
{Enteromorpha spp.)

* Cf. Rees (1940) upon recolonization.

MARINE ECOLOGY

307

about the factors that cause removal of algae from rocks. Statistical
analyses of drift show that the majority of Laminariaceae are torn
in their entirety from off the rocks, so that removal in their case
cannot be due to epiphytes or to the boring of the stipe by the
mollusc, Patina pelluctda, and as they usually grow beneath low-tide
mark surf action is also removed as a possible destructive factor.
It may be that the continual swell and strong currents finally bring
about their destruction. In the case of smaller algae, however, the
weight and resistance of an excessive epiphytic flora brings about
the uprooting of the host plant. This, and numerous other problems,
await the attention of future investigators.

Table XII contains in a summarized form the principal com-
munities that have been recognized around the coasts of Great
Britain. It is not proposed that any of these should be described in
detail, but it is hoped that the outline provided by this table may be
a guide to students who visit any of these areas. One of the
principal characteristics of any rocky shore is the way in which the
different algae are distributed in zones or belts at the different

f the British Isles

Clare Island
(Ireland)

ichina

lldenbrandtia- Verrucaria

rasiola stipita
nteromorpha intestinalis

eltetia

ucus spiralis

ucus vesiculosus var.

•vesiculosus

ucus serratus

Castletown
(I.O.M.)

Pelvetia

Fucus spiralis
Ascophylltim

Fucus vesiculosus
Fucus serratus

orphyra —

angia- Lrospora- Ulothrix Porphyra- Urospora-

Ulothrix
hodymenia Laurencia-Cladophora-

Rhodochorton
aurencia-Gigartina Laurencia-Lomentaria

allithamnion arbuscula

emalion

imanthalia

orallina

iminaria
icrusting algae
ystoseira

scophyllum

orallina-Lithothamnion
orallina-Cladostephus
hodochorton floriduluvi

Himanthalia

Laminaria

Cromer

(Chapman, J. Linn. Soc.

{Bot.), 1917)

Enteromorpha
Fucus platycarpus

Fucus vesiculosus var.

evesiculosus
Fucus serratus

Fucus-Porphyra-
Enteromorpha

Laurencia pinnatifida

Hildenbrandtia-
Lithothamnion

SUBLITTORAL

Sheltered coasts

Cumbrae
(Scotland)

Enteromorpha intestinalis

Porphyra- Urospora-

Ulothrix
Pelvetia
Fucus spiralis
Ascophyllum
Fucus vesiculosus
Fucus serratus

Laurencia

Gigartina- Cladophora
Enteromorpha Lima

Laminaria

Enteromorpha-Cladophora-
Chordaria

20-2

3o8 MARINE ECOLOGY

heights. On the whole, any one species usually occupies a very
definite vertical range and only occasionally is to be found outside
it, and then there is often some cause, such as the presence of a
rock pool, in which conditions for its existence are favourable. It is
not intended in this chapter to enter into any detailed discussion as
to the causes or factors controlling this zonation, an aspect which is
dealt with more fully in the last chapter (cf. p. 351). It is sufficient
here to point out that these zonations do exist and are characteristic
of a rocky shore. Furthermore, a glance at Table XII will show
that on the whole the zonation is remarkably similar around
most of the British Isles, and the same or very similar communi-
ties can be found at much the same level at the different localities.
The actual number of communities recognized depends upon two
factors :

(a) The locality. It will be observed that the two Irish stations
have a much richer zonation, and this can probably be associated
with their position in relation to the Gulf Stream because this will
tend to produce a mixture of species from both cold and warm
waters.

(b) The personal factor. Each investigator will tend to have a
somewhat different concept of what is represented by an algal
community, whilst the number of communities recognized will
also depend upon the time and thoroughness with which the shore
is examined.

The terminology that has been employed has led to no little
confusion. Algal ecology, as such, commenced later than the
ecology of land vegetation. Some investigators have attempted to
apply the terms used in land ecology to algal ecology, whilst others
have considered that the conditions are sufficiently different to
make this appHcation impossible. Cotton (191 2), for example,
recognized five algal formations at Clare Island :

(i) Rocky shore formation.

(2) Sand and sandy mud formation.

(3) Salt marsh formation.

(4) River mouth formation.

(5) Brackish bay formation.

These were subdivided into associations, the rocky shore forma-
tion containing the associations of the exposed coast and the

MARINE ECOLOGY 309

associations of the sheltered coast. Although the term "associa-
tion" was applied to these communities, it is probable that many of
them are really mere "societies" in strict ecological nomenclature
because they are only transient. At Lough Ine Rees (1935)
classified the formations on a different basis and he recognized
only two, the open and sheltered coast formations. Cotton's
formations were based on substrate or salinity whilst Rees's were
based on shelter. Rees further used the term "association" for
those communities where species that are associated with the
dominants are controlled by the same factors. The difficulty of this
criterion is the time involved in proving experimentally that certain
factors do control the distribution of the species concerned.
Seasonal communities, or those which were locally dominant, were
regarded as societies, whilst the term "zone" was used for those
algal belts which possess horizontal continuity with well-marked
upper and lower limits.

In a study of some New Zealand littoral vegetation Cranwell and
Moore (1938) termed the associations of the successive belts which
follow one another in a regularly recurring sequence as an "associa-
tion-complex". The horizontal belts were commonly continuous
but they could be interrupted occasionally by another community,
e.g. one could have an association fragment of Durvillea in the
Xiphophora belt. It is apparent therefore that there is some
divergence of opinion about nomenclature, and at present, until
a thorough resurvey of the whole problem has been carried out,
it would perhaps be more satisfactory to use a non-committal term
such as "community" which implies no particular status.

THE BASIC ZONATION

Out of the wealth of material available it is apparent that there is
on British coasts what one may term a basic zonation, principally
composed of fucoids, and on this other communities are super-
imposed, the actual number being dependent upon the two factors
already mentioned. This basic zonation is briefly as follows :

(i) An upper Enteroinorpha belt. Such a belt has been recorded
from all the localities except those around Dorset and at Castletown
in the Isle of Man. At Dover there are other species associated
with the Enter omorphay e.g. Urospora and Codiolum. On any coast
there will be a development of an Enteroinorpha community

310 MARINE ECOLOGY

wherever trickles of fresh water run down over the rocks to the sea,
and it is to be supposed that the lowered salinity is responsible for
this development.

(2) A zone of Pelvetia canaliculata can be found on most shores
at about high-water mark and extending up as far as the spray goes.

(3) Immediately below this there is often a zone of Fucus
spiralis or F. platycarpus.

(4, 5) The next two belts vary in position, Ascophyllum nodosum
sometimes being the uppermost and in other places Fucus vesi-
culosus. Where both belts are present there is an intermediate zone
in which the two are mixed.

(6) The lowest fucoid zone is commonly dominated by F.
serratus, but in certain areas it may merge at low-water mark into an

(7) Himanthalia zone.

At the same level as the Fucus serratus belt one may find that it is
partially replaced by communities of red algae, or that there is a
zone of such communities between the Fucus and Himanthalia
belts. There are three communities of this type which may be
frequently encountered in the different localities :

(8) A Porphyra community with which Bangia and Urospora are
often associated.

(9) A Laurencia community, the existence of which is frequently
marked in summer by the development of epiphytic forms such as
Cladophora and Foment aria.

(10) A Gigartina community.

On sheltered coasts Chrondrus crispus may occur at these low
levels. In the sublittoral there is commonly a bed of Laminaria
species in which L. digitata tends to be dominant near low-water
mark and L. Cloustoni farther down. L. saccharina appears in those
areas where the substrate is more or less sandy.

The effect of the height of tidal rise upon the vertical extent of
the zonations is illustrated very well in Table XIII in which the
algal zones from four localities are all reduced to levels based on
mean low-water mark. The small range at Bembridge and Peveril
Point has resulted in a compression and overlapping of the zones,
whereas at Castletown, where the range is large, the zones overlap
but little and occupy a considerable vertical height. The level of
the upper zones in any locaHty is not entirely dependent upon the

THE BASIC ZONATION

311

height of the spring tides. On an exposed coast the shore is open to
considerable wave action and a heavy spray dashes against the rocks
to a height of several feet. As a result of this wave action the upper

Table XIII

Feet
above

M.L.W.

13
12
II
10

Bembridge
I.O.W.

Peveril Point
Dorset

8 Fucus ceranoides Cyanophyceae

7 Fucus ceranoides

Ascophyllum
6 Ascophyllum

5 Ascophyllum
Fucus vesiculosus

4 Ascophyllum
Fucus vesiculosus
Fucus serratus

3 Ascophyllum
Fucus vesiculosus
Fucus serratus

2 Fucus vesiculosus
Fucus serratus

I Fucus serratus

M.L.W. Fucus serratus

— I Fucus serratus

— 2 Halidrys

— 3 Halidrys

Laminaria

Tidal 8-9 ft.
range

Cyanophyceae

Porphyra

Fucus spiralis

Porphyra

Fucus spiralis

Bare

Fucus spiralis

Bare

Fucus serratus

Laurencia

Fucus serratus

Laminaria

Laurencia

Fucus serratus

Laurencia

Laminaria

6-5 ft.

Castletown
I.O.M.

Porphyra

Pelvetia

Ascophyllum

Pelvetia
Fucus spiralis
Ascophyllum
Fucus spiralis
Ascophyllum
Fucus vesiculosus

Fucus vesiculosus
Laurencia
Fucus vesiculosus
Laurencia
Fucus vesiculosus
Laurencia
Fucus serratus
Laurencia
Fucus serratus

Laurencia

Himanthalia

Laminaria
Laminaria

18 ft.

Cumbrae
Firth of Clyde

Enteromorpha
(fresh-water
drainage)

Porphyra

Pelvetia

Pelvetia
Fucus spiralis
Fucus spiralis

Ascophyllum

Laurencia

Ascophyllum

Laurencia

Fucus vesiculosus

Ascophyllum

Fucus vesiculosus

Ascophyllum

Fucus serratus

Fucus serratus
Laminaria
Laminaria
Laminaria

10 ft.

zones are often i or 2 ft. higher than might otherwise have been
expected, and the height by which these zones are elevated is
termed the ''splash zone". At Peveril Point the splash zone is
about I ft., whereas at Wembury in Dorset and Mount Desert
Island in Maine it is computed at 2 ft.

312 MARINE ECOLOGY

There are some features of particular interest from the individual
localities which may suitably be discussed at this stage. Anand
(1937) in his study of the Dover cliffs carried out some experiments
with a view to determining the nature of the controlling factors.
The greatest attention was given to the water relations, and it was
pointed out that the water content of the algal covering depends on

(a) the supply of water, e.g. tides, spray, rainfall, humidity,
moisture of the substrate ;

(b) the water loss due to various causes, e.g. evaporation, drain-
age and capillary attraction of neighbouring belts, the latter being
effective up to a distance of 40 cm. ;

(c) the physical nature of the algal covering, e.g. whether delicate
plants, leathery plants or gelatinous plants, whilst the quantity of
water retained will also depend upon the thickness of the algal
mat.

It was found that the Enteromorpha mat lost 25 % of its moisture
in the first 3 hours of exposure, whilst the Chrysophyceae belt lost
18-4%. The relative loss by evaporation of Enteromorpha and
Chrysophyceae mats is seen in fig. 195 A, whilst the corresponding
loss due to drainage is shown in fig. 195 B, the two sets of measure-
ments being obtained by the simple but ingenious method of
weighing portions of the mat cut out so that they fit into water-
proof paper dishes which could be put back into position on the
shore. The differences in loss for both evaporation and drainage are
due to the gelatinous nature of the Chrysophyceae belt, and this
result is obtained in spite of the fact that the evaporating power of
the air opposite the latter belt is 1-41 as compared to i-i opposite
the Enteromorpha belt, the evaporating power of the Fucus belt
being taken as unity.

Similarly, the concentration of salt in the Chrysophyceae belt,
which shows little variation, can be compared with that of the
Enteromorpha belt which varies considerably with level and length
of time after fall of the tide. The day temperature of the belts only
responds to changes of air temperature in summer, and then it is
always less than that of the air although the seasonal range is greater.
The temperature range is greatest in the Fucus and least in the
Chrysophyceae belt as the latter retains more moisture. If,
however, the period of insolation is at all long, as may well happen

THE BASIC ZONATION

313

in the summer months, then the Chrysophyceae mats frequently
become cracked and fall off.

Light, currents and temperature are the chief factors determin-
ing the incidence of cave vegetation, winding caves showing the
influence of light best. Lack of sunlight stops Fiicus from invading
these areas and when the light intensity is low the Chrysophyceae
also are not able to develop satisfactorily. There is no algal growth

WATEK
LOSS
35

LOSS

90 r

1 2 3

Time in hours

2 3 4

Time in hours

Fig. 193. A, water loss from samples of Enteromorpha and Chrysophyceae belts
when exposed in their original position on the cliff face. B, water loss during
drainage in nature from different levels in the Enteromorpha and Chrysophyceae
belts during successive hours in winter. E1-E3, C1-C3 = successive levels. Water
loss in A and B expressed as % of that originally present. (After Anand.)

in long and relatively straight caves beyond a distance of about
15-5 m. from the entrance where the light intensity has been reduced
to about 1-8% of the Hght outside.

A somewhat different ecological approach was adopted by
Colman (1933) at Wembury. He carried out statistical analyses,
and these showed that so far as the fauna and flora were concerned
there are probably three critical levels :

(a) Between mean and extreme low-water marks of spring tides

314 MARINE ECOLOGY

where the annual exposure is less than 5 %. This marks the lower
limit of several intertidal species.

(b) Between the mean low-water marks of neap and spring tides
where the exposure is about 20 %. This marks the upper limit of
several submarine species.

(c) At the extreme high-water mark of neap tides where there is
about 60 % exposure. This marks the upper limit of several inter-
tidal species.

The least critical level appears to be mean low-water mark of
neap tides where there is about 40 % exposure because the maxi-
mum number of species occurs at this level.

It can be seen therefore that the zonation depends very largely
upon

(a) Extent of tidal rise.

(b) Degree of exposure.

To these two factors may be added yet a third :

This latter feature is very well illustrated in the accounts of the
algal vegetation at Clare Island and Lough Ine in Ireland. Table
XIV sets out the differences to be seen at Clare Island between a
sloping and a flat shore.

Table XIV

Range

Pelvetia zone

Fucus spiralis var. platycarpus zone

Ascophyllum zone

Ascophyllum and F. vesiculosus mixed

Fucus vesiculosus zone

Fucus serratus zone

Sloping shore Flat shore
ft. yd.

2-3 5
5-6 10

10 40
10 30
10 50
10 50

A rather more detailed analysis of the same problem has been
presented by Rees (1935) for Lough Ine.

Apart from the actual control of the zones themselves it has
become increasingly evident that the position of a zone on the
shore is to some extent determined by the temperatures of the
different seasons. Attention was first drawn to this aspect of the
problem by Knight and Parke (1931) in their work on the algal

THE BASIC ZONATION

315

flora of the Isle of Man. They showed, for example, that successive
generations of Cladophora rupestris move vertically up and down
5-8 ft. each year, the movement being rendered possible because

Carrigaclare

Barloge

Creek

Slope of

(

Back

North side

South side

Sheer Rock

75-80°

(sheer)

(75°)

Lichina

Barren to

Lichina

Verrucaria

Fucus

and Ralfsia

spiralis

•\

Ralfsia

Fucus

and dwarf

vesiculosus

Laurencia

var.

evesiculosus

Ralfsia,

Mussels

Corallina and

Laurencia

Lonientaria

Corallina

•

and Laurencia

Porphyra

Corallina

Laurencia

and Laurencia

and large

Laurencia

•

Corallina and

Laurencia and

Porphyra

Gigartina

■>

^ .

Corallina and

Dwarf

Gigartina

Polysiphonia

Chylocladia

Gigartina

Brodiaei

y '

Gigartina and

Corallina and

Laminaria

Core

illina

Corallina

Giga

rtina

digit at a

Himanthalia

and Gigartina

■.

;

Laminaria

Clou

istoni

Clou

istoni

Clou

istoni

Clou

stoni

Clou

stoni

there is a monthly reproduction when the sporelings only survive
in the most favourable zone for the particular time of the year, and
this is not necessarily that in which the parent plants are growing.
Some algae migrate up in winter and down in summer, whilst
others move up in summer and down in winter. It is suggested that
the nature of the response to temperature controls the movements

3i6 MARINE ECOLOGY

of those plants that migrate down in winter, whilst response to
strong insolation determines the behaviour of those that move down
in summer.

MOUNT DESERT ISLAND, MAINE

For purposes of comparison the zonations that have been
described by Johnson and Skutch (1928) from this Western Atlantic
station are of considerable interest. It is a rough coast and the
vegetation is not only much poorer than that of most British
stations but also the zones are less distinct, whilst exposure to
storms is responsible for a splash zone of about 2 ft. The plant
aspect may vary from season to season and from year to year, but
this fact has already become emphasized in describing the British
vegetation. Johnson and Skutch recommended levels based on sea
level as the best means of recording the belts because it is more
convenient for purposes of comparison. This is undoubtedly true,
and those workers who adopt this more troublesome technique
nevertheless vastly increase the value of their investigations. The
littoral communities reported from this area, which has a mean tidal
range of 10*4 ft., are as follows:

(i) A Porphyridiiim cnientum community, which is confined to
the spray zone, has four other species associated with the dominant
alga.

(2) A Calothrix-Vernicaria community which ranges from
+ 9-0 to — 12-0 ft. M.L.W.

(3) A Codiolutn society that appears in summer only with its
lower limit (range +6-0 to —12-0 ft. m.l.w.) determined by the
submergence factor.

(4) There is a Fucus vesiculosus — Ascophyllum community with
the former species predominant in the upper and the latter in the
lower portion of the zone. Range +5-0 to — i2-o ft. m.l.w.

(5) A Bangia-Ulothrix-Urospora community confined to winter
and spring. Range +8-o to —12-0 ft. m.l.w.

(6) An Enteromorpha community which is purely aestival. Range
+ 3-0 to — lo-o ft. M.L.W.

(7) A Porphyra community on the steep slopes from +2-0 to
-6-0 ft. M.L.W.

(8) A Fucus fur catus community in the more shaded parts of the
area with a range of +2-0 to — 6-o ft. m.l.w.

MOUNT DESERT ISLAND, MAINE 317

(9) Rhodymenia community. Range +2-0 to —5-0 ft. m.l.w.

(10) In some years a Spongomorpha arcta society can be found.
Range +2-0 to —5-0 ft. m.l.w.

(11) A Spongomorpha spinescens society appears during the
summer months. Range +2-0 to —7-0 ft. m.l.w.

Only one association is recorded from the subHttoral, but this
ought to be subdivided into three communities if it is to be com-
pared with British coasts.

SubHttoral community

(12) An Alaria-Halosaccion-Lithothamnion community in which
the red alga Halosaccion is most abundant on sloping rocks although
in some parts of the coast it is replaced by Chondrus. Range

-6-0 to +2-0 ft. M.L.W.

Local societies of Saccorhiza may occur between — i -o and
+ 1-0 ft. M.L.W. The kelps and Halosaccion are usually so dense
that they prevent the downward migration of species from the
littoral zones above, although where there is any available space
such a migration will readily occur. This illustrates the effect of
competition in determining zonation.

ZONATION IN WARM WATERS

So far we have only described the vegetation of temperate and
cold waters. It is, however, very instructive to consider briefly the
algal ecology of warm waters and observe how it differs from that
of the colder waters. In the Mediterranean, for example, con-
siderably more attention has to be paid to the sublittoral region,
partly because of the extensive vegetation that persists in such a
place, and partly because the small tidal rise, 20-30 cm., renders
this region much more important. If an ecological survey were to
be carried out in the Caribbean a similar state of affairs would be
encountered. Here the very small tidal rise of less than a foot means
that there is practically no intertidal vegetation, and indeed,
zonation of algal belts is very rare although it may occasionally be
encountered on beach rock. The algal vegetation of the Caribbean
is almost wholly sublittoral, the associations being determined very
largely by the type of substrate. The extent to which the sublittoral
in the Mediterranean is of importance is illustrated in Table XV

3i8 MARINE ECOLOGY

which is a summary of the various schemes that have been proposed
for classifying the vegetation from this region.

In concluding this section a word may be said about the perio-
dicity of the vegetation in the Mediterranean as compared with that
of the English Channel. First of all there is the same pronounced
difference in the floral aspects of the summer and winter months
that has been observed on other coasts. Boreal Atlantic species
such as Ulothrix flacca, U. siihflaccida, U. pseudoflacca, Bangia
fusco-purpurea and Porphyra spp. dominate the flora in winter,
whilst in summer it is the tropical and subtropical species such as
Siphonocladus pusillus, Acetahularia mediterranea, Pseudobryopsis
myura, Liagora visctda, etc., which form the dominant species. In
comparing the behaviour of the Mediterranean vegetation with that
of the Boreal Atlantic one may distinguish several types of algal
periodicity:

(i) Algae with a summer vegetation period in both the English
Channel and the Mediterranean. These algae usually occur at a
considerable depth where there is little or no temperature variation,
e.g. Sporochnus pedunculatuSy Arthrocladia villosa.

(2) Algae with a winter and spring vegetational period in both
the Mediterranean and the English Channel, e.g. species of cold
waters such as Ulothrix flacca.

(3) Algae appearing in the winter and spring in the Mediter-
ranean but during the summer in the English Channel. For these
algae it might be supposed that the temperatures of the winter and
spring in the Mediterranean correspond more or less to the summer
temperatures of the Channel, e.g. Nemalion helminthoides.

(4) Algae found during the summer months in the English
Channel but persisting throughout the year in the Mediterranean,
e.g. Padina pavonia. Their absence in the Channel at other times of
the year may be associated with the low temperatures, and it
ought to prove possible to ascertain the minimum temperature at
which such algae will survive.

(5) Algae of spring and winter in the Mediterranean but per-
sisting throughout the year in the Channel, e.g. Porphyra umbili-
calis, Callithamnion corymbosum. This again is probably related to a
temperature correlation, but in this case the algae concerned will
not tolerate the high temperatures that are reached during the
summer months in the Mediterranean. Comparisons of this

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320 MARINE ECOLOGY

nature are extremely valuable in helping us to understand some-
thing of the biological requirements of the species in question. It is
also evident that they indicate some very profitable hnes of investi-
gation concerning the temperature relations of algae.

REFERENCES

England. Anand, P. (i937)- J- Ecol 25, 153, 344-
England. Baker, S. M. (1909, 1910). New Phytol. 8, 196; 9, 54.
England. Colman, J. S. (193 3)- J- ^ar. Biol. Ass. U.K. 18, 435.
Ireland. Cotton, A. D. (19 12). Clare Island Survey. Part XV. Set.

Proc. R. Duhl. Soc. 31.
New Zealand. Cr\n\\tll, L. M. and AIoore, L. B. (1938). Trans. Roy.

Soc. N.Z. 67, 375.
Mediterranean. Feldmann, J. (i937)- ^^^- -^k- io> i-
England. Gibb, D. (1938). J- Ecol. 26, 96.
Scotland. Gibb, D. (i939)- J- Ecol. 27, 364.
England. Grubb, V. M. (1936). J. Ecol. 24, 392.
North America. Johnson, D. S. and Skutch, A. S. (1928). Ecology, 9,

188.
England. Knight, M. and Parke, M. W. (1931)- Manx Algae, p. 27.

Liverpool.
Ireland. Rees, T. K. (1935)- J- Ecol. 23, 69.
England. Rees, T. K. (1940). J- Ecol. 28, 403.

CHAPTER XII

ECOLOGY OF SALT MARSHES

In comparison with the rocky coast fewer studies have been
carried out on the algal ecology of salt marshes, but those that have
been published can be regarded as having made considerable
advances in our knowledge of these extremely interesting areas.
Their neglect in the past has probably been due to the fact that the
algae are often microscopic and hence not so pleasing aesthetically
even when present in abundance, and also they are more difficult to
determine taxonomically. In practice, however, a detailed study of
any one area often produces the rather unexpected result of a very
extensive flora. For example, the number of species recorded from
the English salt marshes of Norfolk is about two hundred, which
does not compare unfavourably with the number on a rocky coast.

An investigation of any salt-marsh area shows that the algal
communities offer a somewhat different aspect to the algal
communities of a rocky coast. In the latter case it has been seen
that zonation is a characteristic feature together with some super-
imposed seasonal changes and migrations. On the salt marshes it is
not really possible to distinguish any zonation but there may be
well-marked seasonal changes in any one area. Thus on a fairly low
marsh the "Autumn Cyanophyceae " appear in autumn and early
winter, they disappear and are replaced in spring by the Ulothrix
community, which in its turn is replaced during the summer
months by Enterofnorpha and so the cycle proceeds. Furthermore,
as each year the ground level increases in height in relation to the
tide through the continual deposition of silt, the submergences
become fewer and the communities are replaced by others on
account of the modified conditions. As a result there is a definite
dynamic succession of the different communities over a long period
of years. This cannot be seen on a rocky coast where there is no
succession in time and where the succession in space is static.

The phenomenon of dynamic succession in this type of habitat
necessitates a somewhat different approach to the problem of the
status of the community. The continual replacement of one

CS A 21

322 ECOLOGY OF SALT MARSHES

community by another as the marsh increases in height provides
changes that are more akin to those that are found in land habitats.
With this in mind the present author recently attempted a surv^ey of
our present information about the algal communities of salt
marshes. The principal features are set out in Table XVI, and it will
be observed that in the suggested nomenclature the ordinary-
ecological terminations for developing seres has been employed.
Whether this is entirely justified in view of the present somewhat
scanty knowledge may perhaps be questioned, but it is possible that
if the nomenclature can be placed on a proper basis at an early stage
it should facilitate future comparisons.

Table XVI shows that there is not the same ubiquitv* of the com-
munities in the different areas that can be found on a rocky coast.
The reason for this is probably to be associated with the ver\-
different tv'pes of salt marsh that can be found. For example, the
Irish marshes are composed of a form of marine peat, the marshes
on the west coast of England have a large sand component in
the soil, the marshes on the south coast bear a tall vegetation of
Spartina growing in a very soft mud, whilst the east coast marshes
bear a ver}' mixed vegetation growing on a mud that tends to be
clay-hke. In spite of this, however, the Sandy Chlorophyceae,
Muddv Chlorophyceae, Gelatinous Cyanophyceae, Rivularia-
Phaeococcus socies, Catenella-Bostrychia consocies and the Fucus
limicola consocies all have a wide distribution though they may not
necessarily appear at the same relative levels on the different
marshes. On the whole, however, they are very often found in the
same phanerogamic communit^^

A comprehensive tour of the salt marshes of England will show
us that one or more of the communities described above occur in
all the different districts. Where the soil is rather sandy a Vauch-
erietum can be distinguished dominated by V. sphaerospora, but
where the phanerogamic vegetation is ver\' dense or heavily grazed
by animals the algal vegetation is poor, e.g. south and west coast
marshes. The Sandy Chlorophyceae and Vaucheria Thuretii have
a wide distribution, as also the Catenella-Bostrychia community,
whilst the pan flora appears to be richest in East Anglia. Perhaps
the most interesting feature is the distribution of the marsh fucoid
Pehetia canalicidata ecad libera which occurs in north Norfolk,
Lough Ine and Strangford Lough in Ireland and at Aberlady near

ECOLOGY OF SALT MARSHES

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324 ECOLOGY OF SALT MARSHES

the Firth of Forth but with no apparent intermediate stations. The
normal form is present in other areas where there are marshes in the
vicinity, e.g. the west coast marshes, but the marsh form does not
appear to have developed. The evidence at present available would
suggest that it has originated independently in the three areas, and
in that case it can only be concluded that certain conditions must be
fulfilled before the marsh form can develop from the normal
species. This is a problem that is still awaiting solution.

One of the more interesting features of the algal vegetation of
salt marshes is the occurrence of the marsh fucoids. These are
peculiar forms which are either free-living on the marsh or else
embedded in the mud, and they must all at one time have been
derived from the normal attached form. Sometimes they bear a
fairly close resemblance to the attached form but in other cases
they have been very considerably modified, and it is only the
existence of intermediate forms which enables us to indicate the
normal type from which they came. East Anglia is essentially the
home of the marsh fucoids, although Strangford Lough in Ireland is
also extremely rich. In Norfolk, for example, considerable areas
can be found occupied by Pelvetia canaliculata ecad libera, whilst
the three marsh forms of Fucus vesiculosus, ecads volubilis, caespito-
sus and muscoides are also abundant, the last two being embedded in
the soil.

Apart from these forms there are three other loose-lying marsh
forms derived from Fucus vesiculosus but these are confined to the
Baltic, e.g. ecads nanus, suhecostatus and filiformis. A small
crawling marsh form derived from F. ceranoides has been described
from the Irish and Dovey marshes, and another larger free-living
one from Strangford Lough in Ireland; like many others of this
type it is profusely branched, fertile conceptacles are rare and, when
present, are invariably male. F. spiralis vars. nanus and lutarius are
other marsh derivatives, whilst Pelvetia canaliculata not only gives
rise to ecad libera but also to a small embedded form, ecad radicans,
which has been recorded from the Dovey marshes. There is also
another form, ecad coralloides, which has been described from
Blakeney and more recently from the Cumbrae marshes, but until
more is known about this particular ecological form it ought to be
regarded with some degree of caution. Ascophyllum nodosum var.
minor is a dwarf embedded variety, ecad Mackaii of the same species

ECOLOGY OF SALT MARSHES 325

is a free-living form found on American salt marshes, in Scotland
and on the shores of Strangford Lough in Ireland, whilst ecad
scorpioides is a partially embedded form found on the Essex marshes
and on the shores of Strangford Lough. All these forms probably
originated as a result of vegetative budding, although it is also
possible that they have developed from fertilized oogonia that
became attached to phanerogams on the marsh. There is definite
evidence that Ascophyllum nodosum ecad scorpioides arises by
vegetative budding from fragments of the normal plant, whilst it
has been suggested that conditions of darkness may be favourable
for the development of ecad Mackaii. As a group the marsh fucoids
are characterized by

(i) vegetative reproduction as the common means of per-
petuation ;

(2) absence of any definite attachment disk ;

(3) dwarf habit;

(4) curling or spirality of thallus.

In the embedded forms derived from Fucus vesiculosus the three-
sided juvenile condition (cf. p. 197) of the apical cell is retained, the
cryptostomata are marginal and division in the megasporangia is
only partial or else does not occur. It is suggested that these
features are due to

{a) exposure, which results in a dwarfing of the thallus ;

{b) lack of nutrient salts which induces a narrow thallus ;

{c) the procumbent habit and consequent contact with the soil
causes spirality because growth takes place more rapidly on the side
touching the soil.

The cause of sterility may either be a result of the high humidity
(according to Baker, 1912, 1915) or, more probably, because of the
persistence of the juvenile condition as represented by the apical
cell and cryptostomata. The marsh fucoids occur most frequently
either as pioneers on the lowest marshes or else as an undergrowth
to the phanerogams.

One of the more striking physiographical features of salt marshes
is the salt pan. The number, shape and size of these on the diflFerent
salt marshes varies very considerably, but they generally contain a
certain number of algae, especially those pans which occur on the
lower marshes. They are important because they provide a much

326 ECOLOGY OF SALT MARSHES

wetter habitat at levels where normally conditions may be somewhat
dry. Some authors are not prepared to acknowledge the existence
of a pan flora because they maintain that the plants are not per-
sistent. A continual study of pans in one area over a considerable
period of time by the present author showed that a definite pan
flora did exist from year to year, and that many of the species com-
prising it reproduce during the course of their existence. The mere
fact that they can carry out normal reproduction would seem to
validate the recognition of such a flora.

There are several interesting features concerning the pan flora'of
the Norfolk salt marshes which may conveniently be mentioned
here. There are two different types of salt pan, those with soft
floors and those with firm, the algal flora usually being confined to
the latter, although so far there is no explanation of this feature. On
the lower marshes the pan flora is commonly composed of Chloro-
phyceae, whilst with increasing marsh height the Chlorophycean
element decreases and the Cyanophycean element increases. A few
of the constituent members, e.g. MonostromUy are seasonal in
appearance, whilst on some marshes there are pans which contain
algae that are normally associated with a rocky shore, e.g. Col-
pomenia, Polysiphonia, Striaria. These persist from year to year in
spite of the stagnant conditions, and when compared with the
habitats occupied by the same species on a rocky coast it is found
that they are probably growing at an unusually high level. Com-
paring the Norfolk marsh flora with that of a rocky coast the
following two generalizations can be made :

{a) Species that are littoral on a rocky coast are to be found
growing at lower levels, usually sublittoral, on the marsh coast. This
must be ascribed to the lack of a solid substrate at the higher levels
where they would normally grow.

{b) Littoral species of the rocky coast are found growing at
higher levels on the marsh coast. This can be understood in the
case of those species living in pans or in the streams where they are
continually covered by water, but at present it is diflicult to provide
an explanation for the few species which actually grow on the
marshes.

Turning now to the algal vegetation of the marshes proper.
Carter (1932, 1933) has suggested that on the Canvey and Dovey

ECOLOGY OF SALT MARSHES

327

marshes light and space relations, rather than factors relating to
level, influence the distribution of the various species. Whilst this is
undoubtedly true there is no doubt that the increasing height of a
marsh with its consequent greater exposure does nevertheless
effectively determine the upper height to which many plants can go.
The species to be found on the higher marshes in Norfolk are
either fucoids or gelatinous Cyanophyceae, both of which have the
power of retaining moisture. The more delicate Chlorophyceae are
more or less confined to the lower levels. On the other hand a

11 ui IV V VI VII vm IX X

Fig. 194. Distribution in time of the algal communities on the salt marshes at
Canvey and Dovey. I. General Chlorophyceae. II. Marginal diatoms (two com-
ponents, (A) those with a winter maximum ; (B) those with a summer maximum).
III. Marginal Cyanophyceae. IV. Ulothrix community. V. Enteromorpha
minima. VI, Anabaena torulosa. VII. Filamentous diatoms. VIII. Autumn
Cyanophyceae. IX. Phormidium autumnale. X. Rivularia-Phaeococcus . XI. Pel-
vetia canaliculata. XII. Catenella-Bostrychia. (After Carter.)

dense phanerogamic vegetation, such as one finds on the south
coast marshes where the tall Spartina Townsendii must lower the
light intensity considerably, does reduce the quantity of algal
vegetation. A similar state of affairs has been observed on the
grass-covered marshes of New England.

From data available it is possible to compare the distribution in
space (e.g. among the different phanerogamic communities) and
time of the marsh communities recorded from Canvey, Dovey and
Norfolk. Figs. 194 and 195 show the distribution of the Canvey and
Dovey communities and they should be compared with figs. 196

328

ECOLOGY OF SALT MARSHES

and 197 for similar marsh communities of Norfolk. Some of the
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Norfolk. (After Chapman.)

Cyanophyceae, are apt to be overlooked in summer because the
constituent species shrivel up so much or else because the colonies

ECOLOGY OF SALT MARSHES

329

become covered by an efflorescence of salt. An examination of the
distribution of the various communities on the Norfolk marshes
shows that five communities are each confined to one type of habitat.
This relationship may be due to:

(a) Association with a particular phanerogamic community, e.g.
Phormidium autumnale (IX) and Ohione portulacoides.

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{h) Dependence upon certain edaphic conditions, e.g. Muddy
Chlorophyceae (Ic).

(c) Dependence upon the physical character of the environment,
for example slope, exposure, wave action, e.g. Marginal Cyano-
phyceae (III), Vernal Ulothrix (IV) and the Pan Association (XVI).

330 ECOLOGY OF SALT MARSHES

MARSHES OF NEW ENGLAND

A study of one of the New England marshes near Boston by the
present author has revealed the existence of the following algal
communities which should be compared with those in Table XVL
It will be seen that there is considerable similarity in spite of the
great distance separating the areas :

(i) The General Chlorophyceae association is divided into two
components.

(a) A Rhizocloniutn community occurs widespread in nearly all
the phaneroganic communities.

{h) A Cladophora-Enteromorpha community is present in very
wet areas and is probably equivalent to the Muddy Chlorophyceae
recorded from Norfolk.

(2) There is a Vaucheria community on the older marshes which
is dominated by V. sphaerospora with V. Thuretii locally dominant
along the banks of small creeks.

(3) A General Cyanophyceae association is spread over all the
marshes and is equivalent to the Cyanophycean element in the
Sandy Chlorophyceae found on the English marshes.

(4) Vernal Ulothrix community.

(5) The Gelatinous Cyanophyceae community is associated with
the Juncetum Gerardii.

(6) A Rivularia-Phaeococcus society is also associated with
Juncus Gerardii.

(7) Enteromorpha minima community. This is abundant in
spring and early summer along the edges of ditches and on old
plants of Spartina spp.

(8) The autumn Cyanophyceae community is present on the
higher marshes and also in the salt pans.

(9) The Limicolous Fucaceae community is dominated by one
of the following, Ascophyllum nodosum ecad Mackaii, Fucus
vesiculosus ecad voluhilis or F. spiralis ecad lutarius.

(10) Pan Flora.

An analysis of the tidal factors operating on the salt marshes of
Norfolk (England), Lynn (Mass.) and Cold Spring Harbor (L. L),
has suggested that for some of the species common to the two areas
the controlling factors must be the same.

MARSHES OF NEW ENGLAND 331

REFERENCES

England. Baker, S. M. and Blandford, M. (1912, 191 5). J. Linn. Soc.

(Bot.), 40, 275; 43, 325.
England. Carter, N. (1932, 1933). J- EcoL 20, 341 ; 21, 128, 385.
England. Chapman, V. J. (1937). jf. Linn. Soc. (Bot.), 51, 205.
England. Chapman, V. J. (1939). jf. EcoL 27, 160.
America. Chapman, V. J. (1940). jf. EcoL 28, 118.
Ireland. Cotton, A. D. (191 2). Clare Island Survey, Part XV. Sci.

Proc. R. Dublin Soc. 31.
Ireland. Rees, T. K. (1935). J. EcoL 23, 69.

CHAPTER XIII

FRESH-WATER ECOLOGY

One of the major problems in this branch of algal ecology appears
to be the establishment of a successful classification upon which
field studies can be based. Up to 193 1 the outHne given by West in
191 6 was in current use but since then a scheme proposed by
Fritsch (1931) has more or less taken its place. It would seem,
however, that neither scheme alone is wholly satisfactory, but that a
combination of the two provides a very suitable basis for workers in
this field. An outline of such a combination of the two schemes is
briefly described below.

A. Siibaerial association.

This develops at its best in the tropics although it can also be
found in temperate regions. In the latter, Protococcales and
Trentepohlia form the principal elements, whilst in the tropics the
Cyanophyceae and other members of the Trentepohleaceae
{Cephaleuros, Phycopeltis, etc.) form the dominant components.

B. Association of dripping rocks.
This can be subdivided into

(i) Permanently attached communities :

(a) On living material.

(b) On dead organic material.

(ii) Temporarily attached communities :

(a) On living material.

(b) On dead organic material.

C. Aquatic associations.

These vary from season to season and frequently have a marked
periodicity which is controlled by diverse factors. Four sub-

FRESH-WATER ECOLOGY 333

divisions of the aquatic associations can be recognized (see below)
and each one of the subdivisions can be treated as follows :
(i) Attached communities (frequently termed the Benthos):

A B

Permanently attached Temporarily attached

■^'~

(a) On living material.

(b) On dead organic material.

(d) In the silt.

(2) Floating macro-communities (Pleuston) :

(a) Originating from loose bottom forms.

(b) Originating from epiphytic forms.

(3) Loose-lying communities of the bottom.

(4) Plankton or floating communities :

(i) Limnoplankton of lakes,
(ii) Potamoplankton of slow rivers,
(iii) Cryoplankton of the eternal snows.

In this last category we have red snow due to the presence of
Chlamydomonas nivalis; yellow snow with a flora of about twelve
species all containing much fat ; green snow, principally caused by
the zoogonidia of green algae ; brown snow due to the presence of
Mesotaenium and mineral matter; black snow caused by Scotiella
nivalis and Rhaphidonema brevirostre ; and a light brownish purple
ice-bloom caused by a species of Ancyclonema.

West (191 6) divided the Aquatic Associations into the following
four major subdivisions, each of which can be further subdivided
in the manner illustrated above :

(i) Associations of rivers, rapids, and waterfalls .

This is mainly composed of fresh- water Rhodophyceae, Clado-
phora spp., Vaucheria spp., Cyanophyceae and diatoms, whilst the
flora of hot springs may also be included here. Cyanophyceae are
usually the only constituents, the various species being capable of
secreting carbonate of lime or silica to form rock masses such as
travertine and sinter, the rate of deposition sometimes being as much
as I -25-1 '5 mm. in three days. The highest temperature recorded

334 FRESH-WATER ECOLOGY

for hot springs that contain Hving plants {Phormidium laminosum)
is 87-5° C.

In slow-moving rivers there is a definite Potamoplankton
divided into :

(a) Eupotamic, thriving in the stream and its backwaters.
{h) Tychopotamic, thriving only in the backwaters.
(c) Autopotamic, thriving only in the stream.

(2) Associations of bogs and swamps.

These are very mixed associations with little or no periodicity,
probably because of the relatively uniform conditions. Zygne-
maceae, desmids and diatoms are most frequent, the desmid
element changing considerably with altitude and type of substrate,
whilst the presence of Utricularia apparently also increases the
number and variety of the desmid species.

(3) Associations of ponds and ditches.

The flora exists under very varied conditions with a regular or
irregular periodicity. In the temperate regions Protococcales,
Zygnemaceae (dominant in spring) and diatoms (dominant in
winter) form the chief elements. There is usually not enough
aeration to permit the larger filamentous forms to be present, and
for this reason the ponds and ditches can be divided into :

[a) those containing Cladophoraceae, which suggests that the
aeration is good ;

[h) those without Cladophoraceae. The substrate and fauna are
also important factors in determining the type of vegetation to be
encountered. The flora of tropical ponds contrasts sharply with
that of temperate regions for there is

(i) an excess of Cyanophyceae ;

(2) the poor aeration results in a relative scarcity of Cladophora
and Rhizoclonium together with the epiphytes associated with them,
and their place is taken by Pithophora\

(3) a scarcity of Vaucheriay Oedogonium, Xanthophyceae and
Ulotrichales ;

(4) an abundance of filamentous desmids together with Spiro-
gyra.

FRESH-WATER ECOLOGY

335

In America Transeau (191 3) concluded that fresh- water pond
algae can be divided into seven classes based on abundance, dura-
tion and reproductive season, these classes and their periodicity
being represented in fig. 198.

X±n . Feb. n^r. /^/>r Hay Tune July Aug. ^t Oct Nou. Dec.

Fig. 198. Chart showing the estimated relative importance of the different t>'pes
of algal periodicity throughout the year in the waters of E. Illinois. The irregulars
are not depicted. (After Transeau from West.)

(4) Associations of pools and lakes.

West was the first investigator of lake and pool algae who
appreciated the fact that the geology of the substrate was of pro-
found importance. He showed that the desmid flora is richest
W'here the substrate is precarboniferous, w^hilst diatoms become
abundant in younger areas or where there has been much silting
with consequent solution of mineral salts. Later workers have
greatly extended this important study, and the present treatment of
the problem is more or less summarized in the schema on p. 336.

A third type is the Dystrophic lake or pool, w^hich is to be found
on moorlands, where desmids form the most abundant part of the
flora in a water that is often highly coloured. In the course of years
Oligotrophic waters may also change into Dystrophic waters. In
sheltered lakes as compared with open lakes there is an oxygen
stratification which closely follows the bottom contours, whilst the
influence of any rivers entering the lake together with the problem
of periodic floods is yet a further factor.

Where there is a shallow littoral shore the communities are
difficult to recognize unless there is a rocky substrate, in which
case there may then be a zonation that is dependent on changes of

336

FRESH-WATER ECOLOGY

water level and wave action: this type of zonation has been ob-
served in several continental lakes. In deeper waters the com-
munities are more distinct because a zonation develops which is
primarily maintained by the light intensity factor. The Limno-

->EV0LUTI0NARY TrEND-

C Hard ancient rocks
unchanged

t/3

o
l-l

Softer more recent rocks or Soft rocks or much
-> some silting >■ silting

Generally deep

No O2 decrease with
depth at thermocline

1
Poor in dissolved

minerals

I
Rich in number of

species

►Decreasing depth

►Generally shallow

O2 decreases with
> depth at thermocline

Minerals in solution
increasing

I 1

Decreasing number of

species

Poor in actual numbers Increasing number of
of individuals individuals

►Rich in dissolved
minerals

I
Poor in number of

«■ species

Rich in actual numbers
>• of individuals

Desmids abundant -^Diatoms and-^Diatoms and -^ Aster ionella and
I desmids Eudorina Cyanophyceae

I > I I

)-EuTROPHic waters

. Oligotrophic waters -^Over many years

plankton of lakes is not usually of great bulk and is composed
principally of various members of the Cyanophyceae, Dino-
phyceae, Bacillariophyceae and Chlorophyceae, and according to
the nature of the constituents it may exhibit maxima in spring (very
commonly), spring and autumn or summer and autumn.

ASSOCIATIONS OF RIVERS AND STREAMS

Budde (1928) investigated very thoroughly the mountain streams
feeding the Ruhr river. Most of these streams and brooks are trout
streams and they can be divided into two regions:

(i) upper Hildenhrandtia region dominated by H. rivularis,
(2) lower Lemanea region dominated by L. fluviatilis.
The seasonal facies were studied and were found to be as follows :
{a) Spring period characterized by the dominance of diatoms
with Ulothrix and Hormidium as subdominants.

{h) Summer period with Chlorophyceae and desmids pre-
dominant.

ASSOCIATIONS OF RIVERS AND STREAMS 337

The most important controlHng factor is apparently tempera-
ture whilst the chemistry of the water is also significant, although
local modifications of the flora may be brought about by changes of
light intensity and oxygen concentration. When compared with the
floras of streams from other areas it is interesting to note that the
same species often occur in widely different types of habitat, thus
providing a proof of the indifference of those plants towards
habitat. The algal communities could be divided into three groups,
those occupying a vertical substrate, e.g. waterfalls, those occurring
on a horizontal substrate and those which are free-living.

A. Algal communities of vertical substrates :

(i) Those attached to stones; eight communities were distin-
guished.

(2) Epiphytic communities; four were distinguished, three of
which also occur in (i).

(3) Three spray communities.

B. Algal communities of horizontal substrates:

(i) Those attached to stones, sand or mud; three communities.
(2) Six epiphytic communities.

C. Free-living algal communities : two were distinguished.

In a study of the encrusting algae of streams Fritsch (1929)
distinguished {a) filamentous algae, {b) algae of banks and (c) sub-
merged encrusting algae: in the particular stream there appeared
to be a brief succession terminating in a mat of Phormidium
autumnale.

Interesting results have also been obtained from a study of
colonies of the blue-green alga, Rivularia haematites, growing in a
stream. It was found that the surface area of the thallus increases
greatly in proportion to the attachment area until finally the
force of the torrent becomes greater than the prehensile force
and the thallus is torn away. In fast streams the thalli are only
formed on big stones because the small stones together with the
colonies have been swept away. In such fast-flowing regions there
appears to be a relationship between size of thallus and size of
stone, but no such correlation can be demonstrated in quiet waters.

C S A 22

338 . FRESH-WATER ECOLOGY

These facts are important because they serve to indicate that
purely mechanical factors may be concerned in the distribution of
some algae, and only too often this aspect of an ecological problem
is wholly neglected.

POND ASSOCIATIONS

The literature on this subject is relatively sparse, but it is evident
that periodicity of the different species is of paramount importance,
the appearance of the different plants being controlled by a series of
factors, only one of which may be limiting for any given species.
A study of a pond near Harpenden by Fritsch and Rich (191 3)
showed that the general aspect of the flora was dependent upon
season and that four phases could be distinguished :

(a) Winter phase with Microspora, Eunotia and epiphytic
diatoms, whilst Ranunculus aquatilis and Callitriche were the domi-
nant phanerogams.

{b) Spring phase dominated by Conjugatae, Oedogonium and
Conferva^ with Ranunculus aquatilis as the most important phanero-
gam.

{c) Summer phase with Euglena, desmids and Anahaena
associated with a phanerogamic vegetation of Lemna^ Glyceria and
Bidens.

(d) Sparse autumn phase with Lynghya and Trachelomonas but
without any dominant phanerogam.

The algal periodicity is thus more or less associated with a
similar periodicity in the phanerogamic vegetation. The flora
differs from that of a similar pool near Bristol in the absence of
Cladophora and Melosira, and in their place there is a greater
development of Xanthophyceae. The two types of flora could be
regarded as distinct associations, but the difference is almost
certainly due to poor aeration in the Harpenden pool. In spite of
this the general trend of periodicity in the two pools is very similar :
a winter phase characterized by a hardy filamentous form (Clado-
phora or Microspora) and diatoms, a spring phase with Zygne-
maceae and an autumn phase with Oscillatoriaceae. The summer
phase in the two pools is very different, and this is ascribed to the
greater drying up of the Harpenden pool during that period. The
flora of pools, therefore, is very dependent not only upon general

POND ASSOCIATIONS 339

climatic conditions, such as rainfall and insolation, but also upon
what might be termed irregular microclimatic factors, e.g.
aeration in the body of water itself. In the case of many of the
species there is a profound relationship between the meteoro-
logical data and the frequency of the flora, e.g. Microspora and the
Protococcales with temperature, Oedogonium and Hormidium with
sunshine. The factors influencing the growth of aquatic algae are
(i) seasonal, (2) irregular, (3) correlated. The first group, which are
very obvious and need not be detailed, are principally of importance
for large bodies of water, but they tend to be masked by the other
two groups in small bodies of water:

(2) Irregular factors.

(i) Abnormal rainfall:

{a) Species favoured by excessive rainfall.
{h) Species favoured by drought.

(2) Abnormal sunshine :

[a) Species favoured by excessive sunshine.

{h) Species adversely affected by excessive sunshine.

(3) Abnormal temperature :

{a) Species favoured by low temperatures.

{h) Species favoured by relatively low temperatures.

{c) Species favoured by high temperatures.

(3) Correlated factors.

(i) Species depending on the enrichment of the water by decay
of other members of the flora.

(2) Forms influenced in their development by competition with
others.

(3) Forms influenced in their development by the presence of a
suitable host, e.g. epiphytic forms.

A very definite correlation can frequently be established between
the amount of sunshine and the phenomenon of reproduction, the
latter process being most frequent when there is most sunshine.
This is in accordance with experimental work by Klebs (1896) who
showed that reproduction was initiated by the presence of bright
light. An unusual concentration of the salts in the water during a

22-2

340

FRESH-WATER ECOLOGY

period of drought may, however, counteract the influence of sun-
shine.

A study of algal periodicity in some ponds near Sheffield,
together with the results of fortnightly analyses, has suggested a
correlation with the nitrate factor for some species. The maximum
for this occurs in December whilst there is a minimum in June,
and it was observed that Volvox received a severe check when the
nitrate was high and only reproduced at times of low nitrate value.
Ulothrix reappeared yearly in these ponds, whilst Euglena annually

1906

a buna.

AbuYul.

FUtheT
tATe

5e^t Oct Nou Pec. Jan Feb Mar Af:>y J^v Ju.tj£ Juiv: Au^.
20 T1TT7T 17 5 17 I 12 1 15 3 15 1 14 I 13 1 15 i

T r

T — 1 — I — I — 1 — I — I — I — I — I — I — I — I — I — I — r

T r

■ VLYUkS

iPedcOomuta
rvpEoboTLtml

Temja AU

xrATLins
•C'29'18'20'20'4' '^1 ' 10' 7 ' 6 '16' 12'I2'20' 8 '24'3r23' 33'30'28'35"

'C 28 20 15 13 7 5 9 7 1 9 12 19 22 II 24 28 16 29 24 24 33

Fig. 199. Abundance and frequencies of the most important algae in a pond
near Indiana University from 1906 to 1907. (After Brown.)

attained to a maximum between July and August soon after the
nitrate minimum.

A similar study by Brown (1908) of some pools near the Uni-
versity of Indiana, revealed the fact that the species tended to
attain their maximum abundance in autumn and spring. In one
pond (fig. 199) the phases were as follows:

Phase

Autumn
Winter
Early spring
Late spring
Summer

Dominants
Closterium, Euglena, Oedogonium
Spirogyra spp.
Spirogyra sp.

Spirogyra, Euglena, Oedogonium
No one species

POND ASSOCIATIONS 341

In another pond somewhat different phases were recorded :

Phase Dominants

Autumn Oedogonium, Chaetophora

Winter Vaucheria

Late spring Oedogonium, Protococcus

Summer Chaetophora

These observations should be compared with those from the
Harpenden pool, and it will be seen that although the spring phases
are essentially similar with either Spirogyra or Oedogonium, never-
theless there are great differences. The two ponds described above
also possessed floras that were essentially different and they must
therefore be regarded as containing two separate associations.
Furthermore, the same worker found that a sudden change in the
external conditions checked the growth of an alga and often resulted
in the development of a resting stage or else of sexual organs;
insistence upon the importance of external conditions in this respect
has also been emphasized by Fritsch and Rich in their study on the
Harpenden and Bristol pools.

LAKE ASSOCIATIONS

Only one example of the algal flora of lakes will be discussed in
these pages, and so the student must remember that lakes from
other parts of the world may exhibit differences not only in species
but also in the periodicity of the communities. A recent study by
Godward in 1937 of the littoral algal flora of Windermere in
Cumberland brought out a number of interesting facts. In the
continental lakes, some of which are of a considerable depth, many
of the algal communities are markedly limited in the depth to which
they can descend. In Windermere, however, any species of the
deeper waters is also able to exist in the surface layers, but as
only a shallow depth of water is occupied by the various communi-
ties, depth ^^r se can only be employed on a broad basis as a means
of distinguishing the communities.

Three different groups of communities were recognized :

(i) Communities growing on stones and rocks:

{a) Spray zone dominated by Pleurocapsa (May-September),

Tolypothrix and Phormidium (April-September).
{h) Zone 0-0-5 "^- Dominated by Ulothrix, diatoms and

Cyanophyceae.

342 FRESH-WATER ECOLOGY

(d) Zone 2-3-5 "^- ^ distinct community dominated by
Cyanophyceae.

(2) Epiphytic communities growing on aquatic macrophytes :

(a) On submerged plants between o and 0-5 m. This possesses
a conspicuous Chlorophycean element, e.g. Conjugales,
Chaetophorales and Ulothricales.

(b) On submerged plants between i and 3 m. dominated by
Oedogonium^ Coleochaete and diatoms.

{c) A community on submerged plants between 3 and 6 m.
which is comprised of Coleochaete^ a few diatoms and some
Cyanophyceae.

(3) Communities on dead leaves and organic debris:

{a) Between o and 12 m.: wholly Diatomaceae.
{h) Between 2 and 16 m.: four diatom species and Microcoleus
delicatulus.

The depth range of the diatoms was found to be greatest at the
time of their maximum in spring and smallest in mid-winter. It
was also discovered that the diatom frequency and light intensity
often show an opposite trend in the upper layers and a similar
trend in the lower layers of the lake. The nature of the habitat,
whether organic or inorganic, makes a considerable difference to
the behaviour of the different species, and each individual species
responds to the differences of these two environments in its own
way. In spite of these differences, however, they all exhibit an
April maximum and depth has the same influence on them all
(cf. fig. 200). A study of the plankton of Lake Windermere gave
results that were in accordance with the view that the constituents
of the floating community originate from the algae of the littoral
region.

The periodical development of the littoral algal flora can be
summarized as in Table XVII.

A study of the chemistry of the waters in the different algal
habitats around the lake is summarized in Table XVIII.

An investigation of the distribution of the algae in relation to the

LAKE ASSOCIATIONS 343

different habitats showed that the algal species clearly fall into two
main groups.

0???

Jd.R. /IjbT. Ju.ae Sc^t. Jan. Abr. Jane bc^t. Jan. A|3t. June Sejot.

—

4'5?7?.
2

1
n

1 1

7???
2

1
0

J — '•■ I

OIC morgSLnic shore

QjV ordd-ULc ^Kore

Fig. 200. Distribution of diatoms on slides suspended at different depths at
different seasons of the year off two types of shore. (After God ward.)

A. Those typical of the inner parts of reed swamps (organic shores).

B. Those typical of other habitats:

(i) Species more abundant in streams,
(ii) Species more abundant on inorganic stony shores,
(iii) Species more abundant in the outer parts of highly evolved
reed-swamp and throughout the less evolved reed swamp.

344 FRESH-WATER ECOLOGY I

A very definite gradation or succession can be traced in the algal
flora as one passes from the inner to the outer reed swamps, from
the latter to the open water or stony inorganic shores and finally

Table XVII

No. of

species

A. Occurrence of species
(i) Species present throughout the year with no distinguishable 4

maximum

(2) Species present throughout the year with a maximum at one 4
period

(3) Species present in abundance only at certain times of the 7
year

(4) Species present in some degree at certain times of the year Numerous

B. Occurrence of maximum
(i) Species with a spring maximum and smaller autumn maxi- 9

mum; diatoms predominant

(2) Species with a spring maximum only 3

(3) Species with a summer maximum only; Chlorophyceae 11
predominate

(4) Species with an autumn maximum only; Cyanophyceae 11
predominate

(5) Species with a winter maximum only; Chlorophyceae 7
predominate

C. Time of year when different species occur in abundance at their
greatest depth
(i) Species attaining greatest depth in spring; diatoms only 3

(2) Species attaining greatest depth in spring and autumn 3

(3) Species attaining greatest depth in summer; Chlorophyceae 10
predominate

(4) Species attaining greatest depth in autumn i

Table XVIII

NH3

Low or

absent

P2O5
Low

Organic
matter

Low

CO,
Low

High

Variable

High

Low

High

Very

N03

(a) Stony and Moderately

rocky shores high
(inorganic)

(b) Mouths of High

streams

(organic) high

to the mouth of streams. In other words a progressive change in
the algal flora is associated with a bottom that becomes less
and less organic in nature or as one passes from eutrophic to
oligo trophic conditions.

EPIPHYTES 345

EPIPHYTES

It is convenient at this point to consider what is known about the
distribution of algal epiphytes, and in this connexion a study of two
ponds on the outskirts of Epping Forest by Godward (1934) has
resulted in considerable advances to our knowledge. Three series of
epiphytes were distinguished.

(i) Winter forms. 16 species approx.

(2) Summer and autumn forms. 1 1 species approx.

(3) Forms existing throughout the year. 11 species approx.

An investigation of the effect of the age of the substrate upon the
epiphytic flora showed that the nature of the substrate was of great
importance. This is illustrated in fig. 201 E, where it can be seen
that, so far as the tips of the leaves are concerned, the total number
of epiphytes increases up to the third or fourth leaf from the apex,
after which there is a decline. The diatom flora, however, is an
exception to this behaviour, because they increase regularly with
the age of the substrate so that the oldest leaves bear the greatest
number of diatomaceous epiphytes. On the other hand, algal
zoospores tend to settle on the younger living leaves. There are
distinct differences in the epiphytic flora of the upper and lower
surfaces of leaves, and it was observed that in the case of the first
few leaves below the apex the upper surface was infinitely superior,
probably because of the greater light intensity. In addition to
distribution in relation to increasing age, there is also the relation
to the different parts of the phanerogamic substrate. Fig. 201 E
illustrates the distribution of epiphytes on the different parts of a
phanerogam, and it will be observed that it is only on the leaf tips
that the maximum is reached at the third or fourth leaf whilst the
leaf sheaths show a slight maximum at about the tenth leaf with a
well-marked maximum for the mid-rib at the same level. These
maxima on the lower leaves are to be associated with the diatom
flora. It will also be observed that the number of epiphytes on the
internodes remains more or less constant, but rapid growth of the
substrate, e.g. the leaf lamina, tends to prevent colonization by
epiphytes. The density of epiphytes that are attached to dead
organic material is dependent upon the habitat of the substrate,
e.g. if it is floating then there are few epiphytes, if it is attached or
submerged the epiphytes are numerous, whilst if it is lying on the

346 FRESH-WATER ECOLOGY

bottom the epiphytes will be few. The various species to be found
are all a residuum from the last living state of the material, and the
assertion that dead material bears more epiphytes than living does
not appear to be correct in this case and it can only be supposed
that it arose in the past through lack of quantitative analysis. In
some cases the appearance of epiphytes is due to change in the
host with age, e.g. old filaments of the Zygnemaceae lose their
mucilage sheath and they then become colonized by many epi-
phytes.

Experimental work and observation show that the greatest
growth and number of epiphytes are partly related to conditions
of good illumination, a feature which is illustrated by Table XIX
below.

Table XIX

Total no. of epiphytes collected on
suspended slides

Level Sandy bottom Muddy bottom

Water level 225 (no Eunotid) 262 (no Eiinotia)

5 cm. 176 (no Eunotia) 108 (102 Eiinotia)

12 cm. 176 (no Eiinotia) o

17 cm. 37 (all Eunotia) o

When considering the effect of illumination it has to be re-
membered that not only are there problems associated with the
individual plants, such as the upper and lower surfaces of leaves,
but also that the density of the host plants may be highly signi-
ficant. Fig. 201 A-D shows the distribution of various epiphytes on
plants of Equisetum lifnosiim under different conditions of spacing
and the contrast is exceedingly obvious. Where there is screening
of leaves, either on the same plant or by several plants, then the
epiphytes develop on the unscreened portion.

The interrelations of host and epiphyte are important, and it was
noticed that the epiphytes tend to develop in the depressions where
the cells of the host adjoined each other. Experiments were then
carried out with scratched slides suspended in the water, and the
results obtained from these rendered it clear that depressions in a
surface increase the number of epiphytes very considerably.

So far as the attachment organs of the epiphytes are concerned
there is no apparent relation between the nature of the substrate

EPIPHYTES

347

and the method of attachment. The differences seen above, there-
fore, must be explained by the behaviour of the motile reproductive
bodies which either come to rest in the depressions or else are
swept there by micro-currents in the water. Another interesting

No, incLhr.
120

100-

1 2*5 4*5 10 13 15 AbbTox.QjLuxniLty of LndiuLd. jsrcbcnt

Depth, of alt. Leaves in cins. ^ ^

Fig. 201. A, B, distribution of Cocconeis placentula on successive intemodes of
plants of Equisetum limosum, well separated (3 stems average). r = Iess than
5 individuals per o- 1 sq. mm.; ry = about 5; re = about 10; c = about 30; z;c = about

50. C, distribution of Cocconeis placentula ( ) and Eunotia pectinata ( )

on crowded plants of Equisetum limosum (3 stems average). D, distribution of

Stigeoclonium sp. ( ) and Coleochaete scutata ( ) on fairly crowded stems

of Equisetum limosum (2 stems average). E, distribution of total epiphytes on
successive leaves of Oenanthe fluviatilis. (After Godward.)

Epiphyte

Cocconeis
Stigeoclonium sp.
Chaetopeltis
Ulvella
Coleochaete scutata

Table XX

No. in scratches No. elsewhere

517
665
138

747
40

297
198

200

feature is the frequent association of Gomphonerna with the basal
cells of Oedogonium, but so far there is no evidence to suggest
whether this is a casual relationship or not. Ponds with muddy
bottoms have a reduced number of epiphytes probably because the
pW and the gases evolved are toxic, but so far little or no work has

348 FRESH-WATER ECOLOGY

been carried out to ascertain the effect of the host plant on the
microchemical environment. Summing up, it can be said that the
factors influencing the distribution of epiphytes are as follows :

(i) Age of substrate.

(2) Rate of growth of substrate.

(3) Light intensity.

(4) Screening.

(5) Nature of the surface.

(6) Chemical surroundings.

Of these (3) is probably the most important, although it is
difficult to separate its effects from those of (i) and (4).

REFERENCES

Ponds. Brown, H. B. (1908). Bull. Torrey Bot. Club, 35, 223.

Streams. Budde, H. (1928). Arch. Hydrobiol. Plankt. 19, 433.

General. Fritsch, F. E. (193 i). J. Ecol. 19, 233.

Ponds. Fritsch, F. E. and Rich, F. (191 3). Ann. Biol. Lac. 6, i.

Epiphytes. Godward, M. (1934). Bei. Bot. Zbl. 52 A, 506.

Lakes. Godward, M. (1937). jf. Ecol. 25, 496.

General. Klebs, G. (1896). Die Beding. der Fortpfl. ein. Algen

und Pilzen. Jena.
Ponds. LiND, E. M. (1938). jf. Ecol. 26, 257.
Ponds. Transeau, E. N. (19 13). Trans. Amer. Micr. Soc. 32, 31.
General. West, G. S. (1916). Algae, p. 418. Cambridge.
Streams. Fritsch, F. E. (1929). New Phytol. 28, 165.

CHAPTER XIV

ECOLOGICAL FACTORS, GEOGRAPHICAL
DISTRIBUTION, LIFE FORM

ECOLOGICAL FACTORS

Studies of the conditions controlling the distribution of algae on
various rocky and salt-marsh coasts has shown that although the
habitats are very different, nevertheless the controlling factors are
very similar. They may be summarized briefly as follows :

(i) The nature of the coast, whether exposed or sheltered. This
applies only to rocky shores because salt marshes always develop in
sheltered areas.

(2) Tidal rise. This factor varies considerably from place to
place, but on a rocky coast the height of the rise controls the width
of the bands, the smaller the tidal rise the narrower will be the algal
zones. This factor will also operate on salt marshes, but owing to
the great horizontal extent of the belts it is only by accurate levelling
that the effect of the factor becomes evident.

(3) Submergence and exposure operating through the daily ebb
and flow of the tide. In many cases it is probable that this factor
acts indirectly because it plays a considerable part in determining
salinity, moisture content and water loss from the algae. There
will be, however, certain species, especially the more delicate
Rhodophyceae, which require to be immersed every day or which
can only tolerate a few hours' exposure to drying.

(4) Non-tidal exposure, or the number of consecutive days
during which no tide covers the area, is a factor which principally
operates during the periods of neap tides. On the salt marshes it
may assume considerable importance, especially on the higher
marshes, and in such habitats it is noticed that the principal algae
are either Cyanophyceae or marsh fucoids, both of which are
protected against desiccation by their histological structure or by
the presence of a mucilaginous envelope. Very few Chlorophyceae
appear capable of withstanding long periods during which they are
not covered by the tide, although they may be found in salt pans on
high marshes where the presence of the water enables them to
exist. Unfortunately this factor has never been studied on a rocky

350 ECOLOGICAL FACTORS, ETC.

coast and hence it is impossible to estimate its importance, but
towards high-water mark it must operate in preventing the upward
spread of a number of species.

(5) Temperature. Rees (1935) as well as Knight and Parke
(1931) consider that changes of temperature throughout a season
are probably responsible for the upward and downward migration
of some species on the shore. Temperature would only appear to
operate in the summer on high salt marshes where there is a low-
growing vegetation, because it will then result in much evaporation
with a consequent increase in desiccating conditions together with a
rise in salinity.

(6) Salinity. This probably varies but little on a rocky coast,
except perhaps in the case of tide pools, but it is important on the
higher salt marshes in summer when the values rise so high in the
surface layers of the soil that probably only a few algae can tolerate
the conditions. The salinity of the marsh soil has also been invoked
in order to explain the origin of spirality in the marsh fucoids. On
all types of shore the incidence of fresh water flowing down from
the land always produces a local modification in the flora.

(7) Substrate. The nature of the substrate, whether solid rock,
boulders, pebbles, sand, mud or peat, is of fundamental import-
ance in connexion with anchorage, the general aspect of the flora
being largely determined by this factor. On the rocky coasts the
angle of slope may aflfect the occurrence locally of some species, or
even whole belts of vegetation (cf. p. 314).

(8) The movement of water, apart from the ebb and flow, plays a
great part in determining the luxuriance of the vegetation. In
many cases there may be considerable local currents and in salt
marshes there is the continual flow of water in the creeks which
persists even when no tide is present. The action of surf may also be
included here, and there are several species, e.g. Postelsia, which are
known to be surf-loving, whilst there are also those species which
cannot tolerate surf. In places where the water carries a heavy
load of silt there may be some modification in the flora because it is
probable that some species are not able to tolerate consistent
deposition of a muddy covering. Recent work shows that turbulence
has a depressing eflFect upon the respiration of marine algae which
is particularly marked in the case of the littoral species, and this
may indirectly be related to the zonation.

ECOLOGICAL FACTORS 351

(9) Biota. On a rocky coast this is largely concerned with over-
shadowing and the degree of epiphytism which may often reach
such proportions that the host plant is torn away with its massive
burden because of the resistance offered to the water. There may
also be animals, usually molluscs, which feed on the seaweeds, and
these can be present in such quantity as to reduce the number of
plants considerably or even to keep the area bare. An example of this
is the behaviour of Hydrohia Ulvae on Norfolk marshes where it is
present in such abundance that certain areas are kept more or less
clear of Monostroma and Ulva. In addition to the molluscs there is
the further problem of the phanerogamic vegetation on the salt
marshes. In certain cases this may provide additional shade or
lower the surface evaporation so that algae can grow at higher
levels than they would do on the open marsh, e.g. Catenella repens
around bushes of Suaeda fruticosa on the Norfolk marshes. The
density of the phanerogamic vegetation, e.g. swards of Puccinellia
7naritima or Spartina patens, may prevent any real algal vegetation
from developing. This can be seen on many west coast marshes of
England and also on the marshes of New England.

(10) Light. Measurements show that the incident light is cut
down very considerably at even a depth of i m., and hence algae
living near low-water mark will be existing under very different
light conditions to those near high-water mark. This factor is said
by many workers to be of great importance in determining vertical
range, but it is of course very difficult to disentangle its effect from
that of the other factors. In heavily silt-laden waters this factor will
probably assume even greater dimensions.

Although all these factors may be operating continually through-
out the year, it must not be forgotten that only one factor operating
at the critical period in the life history of a single species may be of
even more importance. Johnson and Skutch (1932) have stressed
this point, and they maintained that a maximum water loss during
the most active growing period may be of paramount importance in
determining the presence or absence of some species.

With this general introduction we may now turn to consider
studies dealing more specifically with zonation on a rocky shore.
Baker (1909, 19 10) carried out numerous field observations on the
algal zones found around the Isle of Wight, and also conducted
experiments in which the four principal fucoids were grown in jars

352 ECOLOGICAL FACTORS, ETC.

and treated artificially to diflFerent periods of exposure. As a result
she came to the conclusion that the essential control of zonation
was height (modified by exposure), substrate and sunshine. It is
difficult, however, to see how the effects of exposure can be
separated from those of actual height, and there would appear to
be no good reason why exposure was not the principal determining
factor. Grubb (1936) has suggested that submergence and emerg-
ence are the most important factors in determining the occurrence
of algal zones, but it would appear, however, that all these factors
really operate indirectly through the degree of desiccation that the
different species can tolerate. Since this is bound up with their
physiological economy it may be expected to have more significance
than just simply height or exposure per se, because the real control
must be related to the physiology of the plant. Gail (1920) has
declared that it is the desiccation of young plants which prevents
the appearance of algae outside their usual zones, and it is a re-
markable fact that sporelings of fucoids are usually very strictly
confined from an early stage to the zones occupied by the adult
plants. As sporelings of the fucoids are not readily identified
specifically when young, field experiments with young plants
become extremely difficult, if not impossible, to perform. Berthold
(1882) was so much impressed by the importance of this factor that
he divided the rocky shore into five zones based on the degree of
desiccation. It has been concluded that species growing high up on
the shore have a power of resisting desiccation which is not
possessed by those growing lower down, and also that those
species which resist desiccation best possess the slowest growth in
contrast to the others which do not resist desiccation and grow
more rapidly. Fig. 202 A compares the distribution of the principal
fucoids from various areas in relation to the tidal levels, and it has
been suggested that the demarcation between the Fucus spiralis f.
platycarpus and Ascophyllutn zones is probably caused by desicca-
tion, whereas the determination of the other limits may be partially
or wholly explained by one or more of the following factors :

(a) Bottom structure. Boulders are essential for the attachment
of Ascophyllutn but smaller stones will suffice for the other species.

(b) Water movements, although the evidence here is somewhat
conflicting.

ECOLOGICAL FACTORS

353

Pringsheim (1923) and Zaneveld (1937) have both shown that
the water loss of the four species is very great, especially during the
first 18 hours (cf. fig, 203). Fucus spiralis f. platycarpus loses its

TIDE
LEVELS

M.H.VV.S.
M. H.W.N.

M.L.H.W.N.
M.H.L.W.N.

M.L.W.N.
M.L.W.S.

M.E.L.W.S.-

I O WIGHT.
SM, BAKER

? A V

WEMBURY
CQLMAN

LEI DAM
ZANFFELD

TKick-ness

Thickness
100

100

V \ ^^-

\ "^ \

B 90

- \a \^ ^v.

\ ^\ ^^^^^

■ \^ \.

>\ \fv

\ x..,^^^

\ ^^^--..^^

\ ^^-,An

^■"~~-^-Fp

1 1 i 1 — _ — -

'Increasing Cong.

4 hrs.

Fig. 202. A, distribution of Fucaceae on various coasts in relation to the tide
levels. M.H.W.S. =mean high water mark spring tides, M. H. W.N. = mean high
water mark neap tides, M.L.H.W.N. =mean low high water mark neap tides,
M.H.L.W.N. = mean high low water mark neap tides, M.L.W.N. = mean low
water mark neap tides, M.L.W.S. = mean low water mark spring tides,
M.E.L.W.S. = mean extreme low water mark spring tides, M.S.L.=mean sea
level, N.A.P. = Amsterdam tide datum line. B, decrease in diameter of cell walls
when placed in sea water of increasing concentration. C, decrease in diameter of
cell walls under normal conditions of exposure. An = Ascophyllum nodosum,
Fp = Fucus platycarpus, Fs = Fucus serratus, Fv = F. vesiculosus. (After Zanefeld.)

water the slowest of all, and a definite increase in the rate of water
loss can be observed with the different species as each occupies a
successively lower zone on the shore, but it must be noted that
F. spiralis f. platycarpus ultimately loses a higher percentage of
water than the other three. Haas and Hill (1933) also showed that

C S A

354

ECOLOGICAL FACTORS, ETC.

the higher the alga grows the greater is the fat content (p. 288), and
hence the thickness of the cell wall must be of some significance.

84 96 Hours

Fig. 203 . Loss in weight of Fucoids in relation to time of desiccation. The higher
an alga grows the slower it loses water and the greater the total loss. Symbols the
same as in Fig. 202. (After Zanefeld.)

Subsequent examination has shown that the thickness of the cell
wall does bear a relation to the height at which an alga grows.

Table XXI

Fucus spiralis
Ascophyllum nodosum
Fucus vesiculosus
Fucus serratus

Thickness in
divisions of 3 /x

0-49 ±0-05
o-34±o-oi
0-23 ±0-03
0-I4 + 0-0I

These cell walls decrease in thickness when subjected to desiccat-
ing conditions, and the higher a fucoid is growing on the shore the
more the cell walls shrink on drying ; so it must be assumed that a
large part of the water lost is contained in the cell walls (cf. fig.
202 B, C). Those species which lose water most slowly will also
reabsorb it most slowly and, as a result, the growth rate of the
highest species will therefore tend to be the slowest. It would
appear from this study that the real factor controlling zonation, so
far as the fucoids are concerned, is the biochemical nature and

ECOLOGICAL FACTORS

355

properties of the cell wall, although it is also possible that these
features have appeared as a result of the habitat they occupy.

Of those members of the Fucaceae which appear in belts,
Pelvetia canaliculata, which forms the highest zone, is subject to the
greatest exposure, but the situation of the algae in relation to each
other and the density of the flora will also affect the water loss.
Actual measurements carried out in the field show that the loss of
weight curves for this alga are characteristically hollow, the
greatest loss being in the first 6 hours, whilst the total loss may be

3 4 5 6

^ Time in Hours

Fig. 204. Loss of water, as represented by loss in weight, in Pelvetia canaliculata
during intertidal exposure. (After Isaac.)

between 60 and 68 % during periods of 8-9 hours (cf. fig. 204).
Fucus spiralis f. platycarpus shows the same order of water loss as
Pelvetia but then it only occupies a slightly lower belt. Besides
being able to tolerate a considerable water loss which may enable it
to live in a relatively inhospitable habitat, the plants of Pelvetia^ in
order to succeed, must be able to reproduce under such conditions.
The two ova are not liberated from the thick-walled mucilaginous
megasporangium as they are in Fucus and so the antherozoids must
penetrate this envelope, and although this structure secures the
protection of the eggs under desiccating conditions there is
apparently no protection for the antherozoids. We do seem to be

23-2

356 ECOLOGICAL FACTORS, ETC.

arriving gradually towards a state when the factors controlling the
zonation of fucoids on the seashore are really becoming understood,
but much more still remains to be discovered, especially in respect
of the species occupying the lower belts.

Rock pools are commonly encountered on most rocky coasts, and
for this reason it is perhaps desirable that they should be mentioned
here. Klugh (1924) studied a series of six pools on the coast of
New Brunswick and concluded that the factors which may affect
the flora are (i) character of the bottom, (2) depth of the pool,
(3) amount of wave action, (4) temperature, (5) salinity, (6) ^H,
which probably depends to a large extent on the proportion of
chlorophyllous organisms in the pool, and (7) light. Many algae are
tolerant of a wide pH but there is often only a narrow range in
which they will develop to their greatest extent, whilst it is also
possible that the percentage of iron in the liquid medium or solid
substrate may at times be a limiting factor. Klugh considered that
temperature was the most important factor operating in tide pools
and Johnson and Skutch (1932) arrived at a similar conclusion.
Whilst a pool is exposed the temperature of such a small body of
water may rise considerably, and then when the tide returns the
cold sea water will lower the temperature very suddenly. An
examination of the flora showed that Rhodophyceae tended to be
more abundant in shaded pools whereas Chlorophyceae and
Phaeophyceae were relatively more abundant in the exposed pools.

Biebl (1937) has recently published the results of a study of
seven rock pools on the English south coast, in which particular
attention was paid to the influence of different concentrations of
seawater on the cells of various species of the Rhodophyceae. As a
result of this study he concluded that light, temperature, pH and
seawater concentration were the important factors operating in tide
pools, though they are subject to some qualification. Temperature is
more likely to be effective in determining the northern limits of
species rather than actually causing damage, because it was dis-
covered that warming up to 26° C. over a period of 24 hours has no
effect on most Rhodophyceae, and changes of 12° C. could occur on
a hot day without causing any damage. Spondylothamnion multi-
fidum, for example, apparently cannot tolerate temperatures lower
than +3° C. and so reaches a northern limit on the English south
coast. Most of the algae investigated tolerate a^H range of 6-8-9-6

ECOLOGICAL FACTORS 357

and the^H of most pools, except perhaps the highest, will rarely be
outside these values. Table XXII shows that the intertidal algae,
which form the principal component of the tide pool vegetation,
exhibit a greater range of osmotic tolerance than those from deep
waters, although season and time of day is important in this respect
because either may bring about changes in the concentration.

It would appear that in spite of all this careful work we are still
far from understanding the factors that control the vegetation of
tide pools because many of the algae will tolerate the range of
conditions which are likely to be found in such places. The algae of
the tide pools can be placed into one of the four following groups :

(i) Those which are sublittoral and which also occur in the tide
pools.

(2) Those which grow near the ebb line and reach their upper
limit in the pools.

(3) Those which grow in both the intertidal zone and the pools.

(4) Those confined wholly to the rock pools.

Klugh and Martin (1927) studied the growth rate of various
algae in relation to submergence by measuring plants and then
tying them to floats which were suspended in the water at different
depths. After some months the floats were pulled up and the plants
were remeasured. Light, temperature and salinity all vary with
depth, but the last two factors vary so little that it is doubtful
whether they can be of any significance. Light, however, is very
rapidly absorbed by the water, so that at about 2 m. down only
25 % of the surface light has penetrated. The curves (fig. 205) show
that maximum growth occurred between i and 2 m., and it would
seem that whilst the light was perhaps too bright at the surface,
nevertheless it soon became limiting at depths which varied for the
different species. On the basis of a rather limited number of species
and experiments it was concluded that the Rhodophyceae are no
better adapted to greater depths than the Chlorophyceae and
Phaeophyceae, but in the light of more recent experiments it would
seem that this conclusion must be revised. Using coloured Hght
under experimental conditions Montfort (1934) showed that there
was an essential confirmation of Englemann's complementary
theory, which states that the colour of an alga is complementary to
the colour of light that it absorbs (cf. also p. 293). The phycoery-

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ECOLOGICAL FACTORS

359

thrin-rich red algae assimilate far better in green light than the
green and brown algae, and this is valid for both surface and deep-
living species. In blue-green light, on the other hand, the per-
formance of the fucoxanthin-rich Phaeophyceae far surpasses that

GrowtK
In. mrrt.

200

100

GrowtK
In mm.

300r

200-

100

SCYTOSIPHON
LOMENTARIUS

100-

ECTOCARPUS

CON FERVO IDES

FUCUS
VE5ICUL0SUS

J L

LTL

2 4

Fig. 205. Rate of growth of various algae at different depths in sea water, New
Brunswick. (After Klugh.)

of the Rhodophyceae. It has also been found that the algae of all
three groups can be divided into those which are shade algae and
those which are sun algae, each possessing a structure that permits
of maximum assimilation under the respective conditions.

Table XXIII

Shade algae

Cladophora rupestris, Chlorophyceae
Dictyota dichotoma, Phaeophyceae
Rhodynienia palmata, Rhodophyceae

Sun algae

Ulva lactuca, Chaetomorpha linum
Fucus serratus, Pelvetia canaliculata
Ceramiuyn rubrum, Porphyra laciniata

Ehrke (193 1), however, divided the algae into sun and shade
groups on the basis of their compensation points, i.e. the light value
at which respiration and assimilation are equal. On this criterion
the Chlorophyceae and Phaeophyceae form one group, the sun
algae, and the Rhodophyceae behave as shade algae. It is evident
that the division into sun and shade algae is of considerable
significance, but the basis upon which the division ought to be
made would appear to necessitate further experimental work. For

36o ECOLOGICAL FACTORS, ETC.

example, it has recently been shown that, under persistently
changed conditions a fresh-water 'sun' algae can be converted
into a 'shade' area.

As a result of an exhaustive study of colour in relation to assimi-
lation, Montfort (1934) concluded that the quality and intensity of
the light form the limiting factors in determining the depth at
which an alga can live. These conclusions may be summarized as
follows :

(i) An alga may go deeper in the water the nearer its assimilation
curve approaches that of the shade type and the lower is its com-
pensation point. (Compensation point = that strength of light in
which the minimum assimilation will compensate for respiration.)
The better the protoplasmic adaptation to the strong, deep-going,
blue-green light waves, the greater is the power of colonizing the
deeper areas. Under these conditions, for example, a green shade
alga would be able to go to a low^r limit than a red sun alga.

(2) An alga will go deeper the more its colour is complementary
to the spectral composition of the light. Chromatic adaptation by
means of Phycoerythrin and Phycocyanin may enable a red alga to
have a greater energy absorption in blue-green light than a green
alga, even if under conditions of white light the green alga has a
greater light absorption than the red.

GEOGRAPHICAL DISTRIBUTION

Many of the studies of algal distribution are based on a consider-
ation of continuous or discontinuous distribution which are, for
convenience, discussed as though they were separate phenomena,
although it is clear that no distribution can be absolutely continuous.
When, however, it is found that an area in which the localities are
fairly close together is separated by the width of a continent or of
an ocean from another similar area, then we may talk of discon-
tinuous distribution. The problem is rendered more difficult by the
unreliability of earlier records and the somewhat scanty literature,
especially for tropical and sub-tropical areas. The few studies
(Svedelius, 1924; Borgesen, 1934), that have been pubHshed have
established certain general features which are briefly summarized
below :

(i) There is a general resemblance between the algal floras of
the West Indies and the Indo-Pacific. Vicarious pairs of species

GEOGRAPHICAL DISTRIBUTION 361

(two separate species closely related morphologically and yet widely
separated geographically) are known and even vicarious generic
groups. The genus Hormothamnion in the Cyanophyceae, Micro-
dictyon and Neomeris in the Chlorophyceae (cf. fig. 206), all have a
Caribbean-Indo-Pacific discontinuity, whilst there are several
vicarious pairs in the genus Udotea. The explanation of these
discontinuities which has been advanced by Murray, namely
change of climate in former epochs, would only appear to explain
certain cases, e.g. certain species in the Laminariaceae (cf. below),
whilst it is equally obvious that the factors operating at present do
not provide an adequate explanation. The only feasible hypothesis
would be to postulate migration during an earlier epoch when there
was a sea passage through the Panama isthmus, and this involves a
migration not later than the Cretaceous.

(2) There are some species which are common only to the
Western Atlantic and the western part of the Indian Ocean around
Madagascar e.g. Chamaedorus penicidum and three species of
Cladocephaliis. (Cf. fig. 207.) Although there is at present no very
adequate explanation for this distribution three possible hypotheses
may be suggested, but there does not appear to be any evidence
which supports one of them more than the others :

(a) Migration via the Cape.

{b) Migration via the Pacific and Panama, the related species
perhaps still existing in the Pacific but not yet recorded.

{c) The related species or representatives in the interzone have
died.

(3) There are some genera which are common to the Mediter-
ranean and the Indo-Pacific region, e.g. C odium Bursa group, the
vicarious pair Halimeda tuna in the Mediterranean and H.
cuneata in the Indo-Pacific, Acetahidaria mediterranea and other
species of Acetabularia in the Indo-Pacific (cf. fig. 208). In this
case also the only satisfactory explanation is the existence of a
former sea passage across the Suez isthmus. In the flora of the
northern part of the Arabian sea, out of a total of 137 species and
varieties, 22% are endemic, 52% are Indo-Pacific and 59-6% also
occur in the Mediterranean and Atlantic Ocean, the most striking
example being Cystoclonium purpureum which does not now exist
between its widely separated stations along the southern shores of

362

ECOLOGICAL FACTORS, ETC.

GEOGRAPHICAL DISTRIBUTION

363

364

ECOLOGICAL FACTORS, ETC.

GEOGRAPHICAL DISTRIBUTION 365

France and in the Northern portion of the Arabian Sea. In the case
of the Indo-Pacific species of the Arabian sea it is often found that
they are absent from the intervening tropical waters, so that their
distribution must be explained as occurring at a period when the
tropical waters had a more equable temperature.

(4) In general, the Indo-Pacific region is more probable as the
home of the various tropical and subtropical genera and they can be
classified into :

(a) genera with no Atlantic representatives,

(b) genera with a few Atlantic species, e.g. Haltmeda, Caulerpa,
Sargassum^ Dictyota, Scinaia, Galaxaura.

The following genera are probably of Atlantic origin: Dasy-
cladus, Penicillus, Cladocephalus, Batophora.

(5) Several families in the Laminariales, e.g. Laminariaceae,
Alariaceae, are of Boreal Atlantic — Pacific discontinuity. These
families must formerly have had a circumarctic distribution but
were pushed south by the onset of the Ice Age and then they re-
mained in their new habitat when the ice retreated. In this case
change of climate in a former epoch provides a satisfactory ex-
planation of the present discontinuity. Other genera, however,
e.g. Lessonia, Macrocystis, Ecklonia, are of Antipodes-Northern
Pacific discontinuity, Macrocystis in particular being primarily
circumantarctic, after which it is absent from the tropics, to re-
appear again on the Pacific coast of North America and around the
shores of South Africa. The two species of the southern hemisphere
appear to be identical with the two species in the northern hemi-
sphere so that presumably they have disappeared from the inter-
vening warm zone. Again, it must be concluded that their migra-
tion took place at a time when the temperatures of the ocean
waters were more equable, unless it is assumed that the species
have since become less tolerant towards temperature.

Apart from these facts of general distribution there is very little
further information in the literature. The Danish workers,
Borgesen and Jonsson (1905) and Jonsson (1912), have studied the
arctic and subarctic floras in some detail and their results may
properly be included here. They concluded that the component
species of the flora could be divided into a number of distinct
groups :

366 ECOLOGICAL FACTORS, ETC.

(i) The arctic group, with its southern European border in north
Norway and Iceland, although in America the group may extend as
far south as Cape Cod.

(2) A subarctic group, the species of which are common in the
Arctic sea and the cold boreal area of the Atlantic as far south as
western France.

(3) Boreal arctic group. These species are common in the
Arctic Sea and the boreal area of the Atlantic as far south as the
Atlantic coast of North Africa, some perhaps penetrating even
farther south.

(4) A cold boreal group which is of more limited distribution,
extending northw^ards from western France to south Iceland and
Finland, with outlying species penetrating in the south to the
Mediterranean and in the north to the White Sea and Sea of
Murman.

(5) A warm boreal group, the species of which extend as far
south as the Mediterranean and Atlantic coast of North America,
some perhaps even farther south. Their northern limits are to be
found in south Iceland, the Faeroes, north-west Norway and
Scotland.

Although Iceland is so far north, nevertheless the flora is pre-
dominantly boreal because 54 % belong to the last three groups. If
the different districts of Iceland are compared with neighbouring
floras it is extremely interesting to see how the floras of the various
parts of the Icelandic coast show resemblances to floras from a
number of widely separated areas.

Table XXIV

Groups

Grou

I and 2

3-5

East Greenland

Spitzbergen

West Greenland

East Iceland

Finland

South-west Iceland

South Iceland

1 3^

Faeroes

Nordhaven

Another interesting feature in geographical distribution, which
has been established by Setchell (1920), is the relation of the

GEOGRAPHICAL DISTRIBUTION 367

various species to the isotherms. The surface waters of the Oceans
are divided into zones according to the courses of the 1 0° , 1 5 ° , 20° and
25° C. isotheres. The great majority of algal species are confined to
only one zone, a considerable number occur in two, only a small
number occur in three zones, whilst the number extending over
four or five zones are very few indeed and their distribution is
usually by no means certain. In New England many of the species
are apparently separated by the 20° C. isothere which approxi-
mates closely to the position of Cape Cod, so that the flora to the
north of the Cape is essentially different to that of the south.
Those species Hmited to one zone are called stenothermal whilst the
wide ranging forms are termed eurythermal. The former species are
particularly characteristic of the warmer waters, but, even so,
many apparent eurythermal species are found on examination to be
essentially stenothermal. Monostroma Grevillei and Polysiphonia
urceolata are summer annuals in the cold waters of Greenland, but
in the southern part of their range they develop in winter and early
spring when the temperature will be the same as it is in the Green-
land summer. With the exception, then, of the temperatures
endured by the resting spores they are essentially stenothermal.
Ascophyllum nodosum, with a temperature range from o to 10° C, is
another case and in the southern part of its range the plants pass
into a heat rigor during the hotter months.

Feldmann (1937) has recently drawn attention to the pheno-
menon of seasonal alternation of generations and seasonal dimor-
phism. In Ceramium corticatulum the tetrasporic plants exist only
at the end of autumn or in the winter whilst the sexual plants are to
be found at the end of summer. This is an example of seasonal
alternation of generations in which there are ephemeral summer
haploid plants with the diploid plants occurring during the winter
and persisting over a longer period. Seasonal dimorphism is
exhibited in the Mediterranean by Cutleria multifida and C.
monoica with their sporophytes Aglaozonia parvula and A. chilosa.
The two species are almost indistinguishable morphologically, but
the former occurs in spring in shallow waters off-shore whilst the
latter occurs in summer at greater depths. Another example of
seasonal dimorphism is shown by the two morphologically similar
species Polysiphonia sertularioides and P. tenerrima, the former
occurring on exposed rocks from December to May whilst the

368 ECOLOGICAL FACTORS, ETC.

latter grows epiphytically on Nemalion helminthoides between June
and December.

A word may conveniently be said here about the behaviour of
some species in relation to fish and fisheries (Savage, 1932). One of
the most outstanding examples is Phaeocystis pouchetii, a coloured
flagellate which, when present in quantity, gives the waters of the
North Sea a muddy appearance, the so-called ''baccy juice".
Herrings are repelled by this organism when it is present in mass,
and the vernal maximum of this organism off the Dutch coast turns
the northward herring migration west towards the coast of E.
Anglia and thus brings about the spring fishery (cf. fig. 209 A, B).
The occurrence of an abnormal autumn maximum out of its usual
station may completely change the grounds of the autumn fishery
during the southward migration: such an abnormal maximum is
known to have occurred in 1927 (cf. fig. 209 C).

LIFE FORM

A study of algal ecology leads one to the conclusion that the
distribution of the diflterent types appears to be largely controlled
by the nature of the habitat, e.g. rocky shore, sandy shore or salt
marsh, although of course there may be other factors because this
will not explain the predominance of the large kelps in the colder
waters and the predominance of the lime-encrusted forms in the
warmer waters. For this reason there would seem to be a need for
some sort of Life Form classification comparable to that of Raun-
kiaer's for the flowering plants. Such a system can be used to give
a quantitative picture of the composition of the vegetation and also
to demonstrate the absence of any type, thus raising the problem as
to why they are absent. Biological spectra, similar to those em-
ployed by Raunkiaer (1905), form a convenient way, if used with
caution, of comparing floras from two diflferent areas although they
are subject to the limitation that they do not indicate the dominant
types.

Oltmanns' schema of 1905, which is one of the earliest, is
based largely upon morphological criteria, but in the light of
present knowledge it is more desirable to adopt a scheme with some
relation to habitat rather than one based on purely morphological
characters :

LIFE FORM

369

C. E

Fig. 209. Phaeocystis and herrings. A, distribution of Phaeocystis 17-24 April,
1924, normal distribution. +, Phaeocystis scarce or absent. O, stations in
Phaeocystis zone. Intensity of concentration shown by shading. -^ ^^^^"^^"l
herring migrations. B, distribution of Phaeocystis, 8-13 April, 1926. bpring
fishery interference. C, distribution of PhaeocysUs, 6-9 November, 1927.
Autumn fishery interference. ^.K". = Smith's Knoll Lightship. (After Savage.)

C S A

370 ECOLOGICAL FACTORS, ETC.

(i) Bush and tree forms (Bryopsis).

(2) Gelatinous bush forms (Diatoms).

(3) Whip forms {Himanthalia).

(4) Net forms [Hydrodictyon),

(5) Leafy forms: (a) lattice {Agarum),

(b) flag (Macrocystis),
{c) buoy (Nereocystis).

(6) Sack forms {Leathesia).

(7) Dorsiventral forms (Delesseria).

(8) Cushion, disk and encrusting forms {Ralfsia).

(9) Epiphytes, endophytes and parasites.

(10) Plankton.

(11) Symbionts.

In 1927 Funk proposed a new classification which apphed
particularly to the algae of the Gulf of Naples. He distinguished
four primary groups, all of which were capable of subdivision
according to the same principles, but unfortunately the terms that
he employed for the major groups are not particularly happy as
some of them are open to the widest interpretation :

1. Seaweeds ("Tange" in the original).

II. Lime-encrusted algae.

III. Fine algae ("Feinalgen", or algae of small proportions).

IV. Microscopic algae, including species measuring less than
I cm.

Each of these groups could be subdivided as follows, the
examples being taken in this case from the first group.

I . Sea weeds ( ' ' Tange " ) :

(a) Large algae, more than i m. in length, e.g. Laminaria.

(b) Medium algae, with a length of 0*5-1 m., e.g. Fucus.

(ii
(iii

(iv

(vii

Main axis not branched, e.g. Chaetomorpha.

Main axis branched, e.g. Gracilaria.

Thallus bushy, e.g. Gelidium.

Thallus leafy or a fohose bush, e.g. Phyllitis.

Creeping thallus, e.g. Caulerpa.

Crustaceous thallus, e.g. Ralfsia.

Thallus a hollow ball, e.g. Colpomenia.

LIFE FORM 371

Gislen in 1930 proposed another classification to include both
plants and animals, the biological types referable to the plants
being as follows :

I. Crustida (Crustaceous thallus) :

(i) Encrustida or encrusting forms, e.g. Lithothamnion.
(2) Torida or small cushions, e.g. Rivularia.

II. CoRALLiDA (lime skeleton more or less developed):

(i) Dendrida or tree-like forms, e.g. Corallina.

(2) Phyllida or leaf-like forms, e.g. Udotea.

(3) Umbraculida or umbrella-like forms, e.g. Acetahularia.

III. SiLViDA (no lime skeleton) :

{a) Magnosilvida, or forms more than i dcm. high and with
branches more than i mm. thick.

i) Graminida, e.g. Zoster a (a phanerogamic group).

2) Foliida, e.g. Laminaria,

3) Sack-form, e.g. Enter omorpha.

4) Palm form, e.g. Lessonia.

5) Buoy form, e.g. Nereocystis.

6) Cord form, e.g. Himanthalia.

7) Shrub-like form, e.g. Chordaria.

8) Sargassum form.

9) Caulerpa form.

{h) Parvosilvida (small delicate forms less than i dcm. high).

It will be seen that all these classifications are based primarily upon
morphological criteria and are therefore incomplete because they
do not take into consideration the biological requirements of the
algae.

Setchell propounded a scheme in 1926 based primarily on the
conditions found in tropical waters, with particular reference to
coral reefs. For this reason the classification is restricted because it
would require considerable extensioa if the flora of colder waters
were to be included, but at the same time it is an improvement over
the previous schemas in that its basis is largely ecological :

24-2

372 ECOLOGICAL FACTORS, ETC.

Heliophobes :

(i) Pholadophytes. Forms nestling into hollows and avoiding

much light.

(2) Skiarophytes. Forms growing under rocks or in their shade.

Heliophiles :

(3) Metarrheophytes or attached flexible forms growing in moving

water.

(4) Lepyrodophytes or encrusting forms.

(5) Herpophytes composed of small creeping algae.

(6) Tranophytes or boring species.

(7) Cumatophytes or "surf-loving" species.

(8) Chordophytes, where the thallus has the form of a cord.

(9) Lithakophytes or lime-encrusted species (Corallinaceae).

(10) Epiphytes.

(11) Endophytes.

Knight and Parke (1931) proposed a brief classification based
upon the same criteria, duration and perennation, that Raunkiaer
employed for the higher plants. They only distinguished four
groups; perennials, pseudoperennials, annuals and casual annuals,
and it would require a thorough restudy of many species in order to
determine to which group they belong. More recently (1937)
Feldmann has proposed a new scheme, based on these same criteria,
which can be regarded as the logical elaboration of Knight and
Parke's classification:

(i) ANNUALS

(a) Species found throughout the year. Spores or oospores germinate immedi-

ately.

Ephemerophyceae : Cladophora.

(b) Species found during one part of the year only.

(i) Algae present during the rest of the year as a microscopic thallus.

ECLIPSIOPHYCEAE : {o) with prothallus, Sporochnus.

(b) with plethysmothallus, Asperococcus.

(ii) Algae passing the unfavourable season in a resting stage.
Hypnophyceae — Resting stage :

{a) spores, Spongomorpha lanosa.

(b) oospores, Vaucheria.

(d) akinetes, Ulothrix pseudoflacca.

{e) spores germinate and then become quiescent,

Diidresnaya.
(/) protonema, Porphyra.

LIFE FORM 373

(2) PERENNIALS

(a) Frond entire throughout year.

(i) Frond erect. Phanerophyceae : Codium.
(ii) Frond a crust. Chamaephyceae : Hildenbrandtia.

(b) Only a portion of the frond persisting the whole year.

(i) Part of the erect frond disappears. Hemiphanerophyceae : Cystoseira.
(ii) Basal portion of thallus persists.

Hemicryptophyceae :

{a) basal portion a disk, Cladostephus.
(b) basal portion composed of creeping fila-
ments, Acetabularia.

This scheme must be regarded as a great advance on the other
classifications, but at the same time it does not seem to take
adequate account of the effect of environment and, furthermore, it
is primarily of use for the marine algae and does not take into
consideration the numerous fresh-water and terrestrial species.

Cedergren (1939) has recently published a life form scheme
based primarily upon the nature of the medium and secondarily
upon the nature of the substrate. This scheme can be considered as
excellent in so far as it classifies the algae in a more general sense.

Series A. Air-loving Algae.

(i) Terricolae (on the earth). (4) {a) Epiphytes.

(2) {a) Saxicolae (on stone). {b) Endophytes.

{h) Calcicolae (on chalk). (5) Epizoic forms (on animals).

(3) Lignicolae (on wood). (6) Succicolae (gelatinous).

Series B. Soil Algae (in the earth).

Series C. Water Algae.

(i) Nereider (river and stream (4) {a) Epizoic forms,
algae). {h) Endozoic forms.

(2) Limnaeider (lake algae). (5) Plankton (small floating algae).

(3) {a) Epiphytes. (6) Pleuston (large floating algae).
\b) Endophytes. (7) Neuston.

Of all those so far published, however, Feldmann's appears to be
the most workable. The real test will come if and when it is em-
ployed to give biological spectra, and if the spectra from different
locahties e.g. temperate and tropical regions, show a distinct

24-3

374 ECOLOGICAL FACTORS, ETC.

difference then it should prove possible to extend its use as a
means of comparing the vegetation from different regions. Such
differences may be expected to open up problems, the solutions of
vi^hich should yield us valuable information concerning the general
biology and ecology of the species concerned.

REFERENCES

Ecology. Baker, S. M. (1909, 1910). New Phytol. 8, 196; 9, 54.
Ecology. Berthold, G. (1882). Mitt. Zool. Staz. Neapel^ 3.
Ecology. BiEBL, R. (1937). Beih. bot. Zbl. 57 A, 381.
Geographical Distribution. Borgesen, F. (1934). Det. Kgl. Danske

Vidensk. Selsk. Biol. Meddel. 11, i.
Geographical Distribution. Borgesen, F. and Jonsson, H. (1905).

Botany of the Faeroes, 3. Copenhagen.
Life Form. Cedergren, G. R. (1939). Bot. Notiser, p. 97.
Ecology. Chapman, V. J. (1937). jf. Linn. Sac. (Bot.), 51, 205.
Ecology. Ehrke, G. (193 i). Planta, 13, 221,
Life Form. Feldmann, J. (1937). Rev. Alg. 10, i.
Life Form. Funk, G. (1927). Puhl. della Staz. Zool. Napoli, 7.
Ecology. Gail, F. W. (1920). Piihl. Puget Sd Biol. Sta. 2.
Life Form. Gislen, T. (1930). Skr. K. Svensk Vetensk. nos. 3, 4.
Ecology. Grubb, V. M. (1936). J. Ecol. 24, 392.
Ecology. Isaac, W. E. (1933). Arm. Bot., Lond., 47, 343.
Ecology. Johnson, D. S. and Skutch, A. F. (1928). Ecology, 9, 307.
Geographical Distribution. Jonsson, H. (191 2). Botany of Iceland,

Part I, p. 58. Copenhagen.
Ecology. Klugh, A. B. (1924). Ecology, 5, 192.
Ecology. Klugh, A. B. and Martin, J. R. (1927). Ecology, 8, 221.
Ecology. Knight, M. and Parke, M. (193 i). Manx Algae, p. 20.

Liverpool.
Ecology. MoNTFORT, C. (1934). Jh. wiss. Bot. 79, 493.
Life Form. Oltmanns, F. (1905). Morphologic und Biologic der Algen,

2, 276. Jena.
Ecology. Pringsheim, E. G. (1923). jfb. iviss. Bot. 62, 244.
Ecology. Rees, T. K. (1935). jf. Ecol. 23, 69.

Life Form. Raunkiaer, C. (1905). Acad. Roy. Sci. Let. Dan. 5, 347.
Geographical Distribution. Savage, R. E, (1932). jf. Ecol. 20, 326.
Geographical Distribution. Setchell, W. A. (1920). Amer. Nat. 54,

385.
Life Form. Setchell, W. A. (1926). Univ. Calif. Publ. Bot. 12, 29.
Geographical Distribution. Svedelius, N. (1924). Arch. Bot. 19, i.
Ecology. Zanefeld, J. S. (1937). jf. Ecol. 25, 431.

INDEX

Numbers in heavy type refer to the figures

Abe, K., 189
Aberlady, 322

Acetabiilaria, 82, 83, 84, 275, 276, 371,
373; mediterranea, 83, 318, 361;
Wettsteinii, 84
Acicularia, 171
Acinetospora, 128, 160, 162
Acinetosporeae, 127
Acrothrix, 139, 154, 259
Actinococcus subcutaneous, 241
Acton, E., 12, 17, 84
Adelophycee, 131, 141
Aegagropila, 74, 77; Sauteri, 77
Agar-agar, 224
Agardh, J., 298
Agardhiella tenera, 216
Agarum, 370
Aglaozonia, 87, 155, 156; chilosa, 367;

parvula, 367; reptans, 156
Ahnfeldtia, 241, 250, 254
Akehurst, S. C, 36
Akinete(s), 8, 20, 25, 33, 46, 48, 59,

102, 103, 117
Akontae, i, 2, 18, 98
Alaria, 178, 182, 185, 186, 306, 317;

esculenta, 185
Alariaceae, 185, 186, 365
Alaska, 167, 182, 190
America, 136, 366

Atiabaena, 7, 8, 16, 304, 338; Azollae,
297; cycadearum, 297; filiculoides,
297; oscillarioides var. terrestris, 301 ;
torulosa, 323, 327
Anabaenin, 217
Anand, p., 312, 320
Ancyclonema, 333
Androspore(s), 61, 62
Anomalae, 191, 208
Antarctic, 107
Anthoceros, 297
Antipodes, 191
Antithamnion cruciatmn, 358; plumula,

291, 292, 358; tenuisshnum, 358
Aphanocapsa, 217, 257
Aplanospore(s), 20, 35, 46, 48, 70, 95,

102, 115, 116, 117
Araceae, 87
Archeolithothamnion, 273

Archeozoon, 267
Arctic Sea, 366
Arabian Sea, 361, 365 .
Ardissone, F., 319
Arthrocladia villosa, 318
Arthrospira Jenneri, 9
Arwidsson, T., 97
Ascocyclus, 265

AscophyUum, 137, 138, 190, 197, 199,
200, 217, 306, 311, 314, 316, 352,
367; nodosum, 137, 193, 199, 200,
230, 289, 310, 353, 354, 367; ecad.
Mackaii, 324, 325, 330; ecad.
scorpioides, 325 ; var. minor, 324
Asperococcaceae, 150
Asperococcus, 150, 153, 154, 265, 372;
bullosus, 151, 152, 153; compressus,
152; fistidosus, 152
Atlantic, 361, 365, 366
Aucklands, 192
Australia, 190, 209, 267
Autospore(s), 20, 126
Auxiliary cell(s), 213, 214, 221, 223,
224, 225, 228, 229, 233, 235, 240,
242
Auxin, 109
Auxospore, 121
Azygospore(s), 102

Bacillariophyceae, 3, 98, 119, 261, 290,

336
Bahamas, 267
Baker, K. M., 244, 374
Baker, S. M., 325, 331, 35i
Bangia, 257, 262, 306, 307, 310, 316;

fusco-purpurea, 318
Bangiaceae, 216, 217, 218, 257
Batophora, 365
Batrachospermum, 215, 217, 220, 221,

244, 249, 252, 265 ; moniliforme, 220
Batrachospermaceae, 220
Behlau, J., 36

Bembridge, I.O.W., 310, 311
Berthold, G., 134, 352, 374
Bharadwaja, Y,, 17
Bidens, 338

BiEBL, R., 289, 304, 356, 374
Black, M. C, 267, 277

376

INDEX

Blackburn, K., 126

Blackler, M. C. H., 154

Blakeney, 324

Blandford, M., 331

Blasia, 297

Blepharoplast(s), 59, 60

Bliding, C, 56

BOHLIN, M., 18

Bold, H. C, 44

BoRGESEN, F., 90, 97, 207, 211, 360,

365, 374

BORNET, E., 298

Bornetella, 270

Bostrychia, 322, 323, 327

Botrydiaceae, 118

Botrydium, 118, 126, 264; divisum,

118; granulatum 118, 119; Wall-

rothii, 118
Botryococcus, 114, 126; Braunii, 114,

Boueina, 268; Hockstetteri, 269
Bower, F. O., 195, 211
Brand, F., 73
Bristol, 338, 341
Bristol, B. M., 44
Brongniartiella byssoides, 291, 358
Brown, H. B., 340, 348
Bryopsis, 88, 370; plumosa, 88
BuDDE, H., 336
Bulbochaete, 57

Bullock- Webster, G. R., 126
Bumilleria exilis, 299

Cainozoic, 273

Calcicolae, 373

California, 181, 187, 219

Calcium carbonate, 81

Callithamnion, 235, 236, 244, 265, 306;
arbuscula, 307; brachiatum, 236;
byssoides, 235; corymbosum, 318;
roseum, 215; tetragonwn var. brachi-
atum, 358

Callitriche, 338

Callose, 84, 91

Callus, 174

Caloglossa Leprieurii, 212

Calothrix, 306, 316; parietaria, 9;
ramosa, 15

Cambrian, 266

Canary Islands, 223

Canvey, 323, 326, 327

Cape Cod, 366

Cape of Good Hope, 183, 218, 361

Capitula, 1 1 1

Carboniferous, 180, 267, 269

Caribbean, 317

Carotin, 6, 69

Carpogonium(ia), 72, 212, 213, 219,

220, 221, 223, 225, 227, 228, 233,

235, 237, 240. 241, 243
Carpospores, 4, 213, 219, 220, 223,

233, 236, 237, 240, 242
Carposporophyte, 213, 223, 225
Carter, N., 56, 326, 331
Carter, P. W., 5
Carteria, 22, 297; ovata, 24
Castagnea, 139, 141, 154, 258, 265
Castletown Bay, I.O.M., 307, 309, 310,

311
Catenella, 322, 323, 327; repens, 351

Caulerpa, 54, 88, 90, 97, 365, 370. 371 ;

ciipressoides, 90, 91; racemosa, 90;

verticillata, 90
Caulerpaceae, 88, 89, 227
Cedergren, G. R., 373, 374
Centricae, 119, 121
Cephaleuros, 68, 332; virescens, 68
Ceramiaceae, 233, 235, 237
Ceramiales, 214, 229
Ceramium, 238, 265, 306; ciliatum,

358; codicola, 216; corticatulum, 367;

rubrum, 215, 359
Ceratium, 126
Chaetangiaceae, 221, 223
Chaetangium, 222
Chaetomorpha, 78, 255, 261, 370;

linum, 359
Chaetopeltis, 347
Chaetophora, 73, 341
Chaetophoraceae, 64, 65, 255, 256
Chaetophorales, 63, 129, 262, 342
Chamaedorus, 363; peniculum, 361
Chamaephyceae, 373
Chamaesiphon, 7, 9, 13
Chamaesiphonaceae, 13
Chapman, V. J., 331, 374
Chara, 109, no, in, 112, 113, 265,

273
Characiopsis, 116, 117; saccata, 116

Characium, 10, 21, 36, 116, 262;

angustatum, 37; saccatum, 36

Charales, 19, 71, 98, 108, 126, 262,

273, 279
Charophyta, 273
Chaudefaud, M., 36
Chlamydomonadaceae, 22, 24 et seq.,

261, 262
Chlatnydomonas, 20, 21, 22, 24, 29,

33, 36, 39, 252, 264; botryoides, 2^\

Braunii, 23; coccifera, 23; eradians,

INDEX

377

23; eugametos, 24; Kleinii, 23;

longistigma, 23 ; media, 23 ; monoica,

23; nivalis, 333; parietaria, 23;

pertusa, 24; reticulata, 23; sphagni-

cola, 23 ; variabilis, 24
Chloramoeba, 114, 115
Chlorella, 39, 44, 264, 279, 296;

lacustris, 39
Chlorellaceae, 39
Chlorobotrys, 264
Chlorochytrium, 22, 37, 44; Lemnae,

37. 297

Chlorococcaceae, 36 ef 5eg.

Chlorococcales, 18, 36, 41, 44, 63, 295,
296

Chlorococcum, 21, 22, 38, 39, 44, 85,
262; hiimicolum, 38, 39, 299

Chlorodendraceae, 35

Chlorodendron, 35

Chlorogoniiim oogatnum, 23

Chloroynonas, 23

Chloromonodineae, 261

Chlorophyceae, i, 2, 5, 9, 18, 22, 57,
60, 73, 79, 84, 98, 113, 116, 122, 212,
216, 217, 245, 250, 252, 254, 255,
261, 263-5, 276, 279, 281, 290, 293,
294, 297, 304, 326, 327, 330, 336,

349, 356-7, 359, 361
Chlorosaccus, 264
Chlorotheciaceae, 116
Chondria, 230
Chondrus, 215, 239, 294, 306, 317;

crispus, 215, 239, 310
Chorda, 128, 136, 167, 170, 189, 258;

Filum, 168
Chordaceae, 167
Chordaria, 136, 139, 142, 143, 258,

307, 371
Chordariaceae, 127

Chordariales, 260
Chordophytes, 372
Choreocolacaceae, 238
Choreocolax, 216, 238; polysiphoneae,

297

Choreonema, 216

Chromulina, 123, 264

Chroococcaceae, 8, 10, 11, 12, 266

Chroococcus, 7, 10, 11, 17; macro-
coccus, 7, 12; turgidus, 7, 9, 11;
varius, 7

Chroolepus, 69

Chrysocapsa, 264

Chrysococcus, 264

Chrysodendron, 264

Chrysophaera, 264

Chrysophyceae, 2, 98, 122, 255, 261,

264-5, 306, 312, 313
Chrysotila stipitata, 306
Church, A. H., 84, 128, 132, 139, i45,

148
Chylocladia, 315
Ciliata, 261
Cirrhoids, 77
Cladhymenia, 230
Cladocephalus, 361, 363, 365
Cladophora, 74, 75, 76, 77, 80, 84, 264,

307, 310, 330, 333, 334, 338, 372

flaccida, 78; flavescens, 77, 78

fracta, 74; glomerata, 77, 78

gracilis, 291, 292; pellucida, 78, 297

repens, 77, 78; rupestris, 291, 292,

306, 315, 359; Suhriana, 77, 78, 255
Cladophoraceae, 73, 74, 78, 334
Cladophorales, 73
Cladostephaceae, 158
Cladostephus, 158, 160, 247, 363, 373;

verticillatus, 159
Clare Is., 307, 308, 314, 323
Cleland, R. E., 244
Clint, B., 158, 162
Closterium, 105, 106, 107, 340;

parvulum, 106
Coccogonales, 10, 13
Cocconeis, 347; placentula, 347
Codiaceae, 92, 93, 268, 277
Codiolwn, 79, 309, 316
Codium, 92, 97, 216, 250, 251, 264,

265, 277, 306, 364, 373; Bursa, 93,

361, 364; tomentosum, 92
Coelastraceae, 43
Coelenterata, 124, 295, 296
Cold Spring Harbor, 330
Coenobia, 33
Coleochaetaceae, 71
Coleochaete, 20, 21, 71, 72, 251, 252,

256, 262, 342; scutata, 72, 347
Collenia, 267
CoLMAN, J. S., 313, 320
Colpomenia, 154, 259, 326, 370;

simiosa, 153, 154, 259
Compensation point(s), 359, 360
Conferva, 338

Conjugales, 2, 98, 251. 262, 342
Convoluta Roscoffensis, 297
Corallida, 371
Corallina, 212, 227, 244, 306, 307, 315,

371; officinalis, 228, 229; rubens,

228, 229
Corallinaceae, 226, 227, 273, 372
Cortex, 139, 171, 173, 193

378

INDEX

Corynophloeaceae, 144
Cotton, A. D., 308, 320, 331
Couch, J. N., 97
Cranwell, L. M., 309, 320
Cretaceous, 81, 268, 273, 361
Cromer, 307
Crow, W. B., 7, 17
Crustida, 371
Cr\'oplankton, 333
Cryptomonas, 264; anomala, 124
Cryptonemiales, 214, 224
Cryptophyceae, 3 , 98, 1 24, 26 1 , 264, 296
Cryptopleiira ramosa var. uncinata, 358
Cr>'ptostomata, 178, 182, 186, 194,

196, 198, 207, 259, 325
Cryptozoon, 267
Cumbrae, 307, 311, 324
Cumatophytes, 372
Cutler, Miss, 155
Ciitleria, 87, 131, 154-6, 162, 248;

monoica, 367; multifida, 155, 156,

367
Cutleriaceae, 127, 155

Cutleriales, 154, 257, 260

Cyanophyceae, i, 6, 217, 256, 257, 263,

265, 267, 290, 295, 299, 300, 302,

311, 327, 328, 330, 332 et seq., 341,

342, 349, 361
Cyanophycin, 217
Cycas, 297

Cyclocrinus, 270; porosus, 270
Cyclosporeae, 127, 128, 189
Cylindrocapsa, 48, 262
Cylindrocapsaceae, 48
Cylindrocystis, 104
Cylindrospermum, i.'j
Cymhella lanceolata, 121
Cymopolia, 270
Cystocarp(s), 222, 230, 233, 235, 236,

240, 243
Cystoclonium purpureum, 361
Cystococcus, 38, 296
Cystodinium, 264; lunare, 125
Cystophyllum, 166

Cystoseira, 158, 190, 204, 205, 307, 373
CzuRDA, v., 107

Dangeard, p., 126

Dasycladaceae, 80-2, 269 et seq., 278

DasydaduSy 80, 265, 271, 365; clavae-

f or mis, 81
De, p. K., 304
Delessariaceae, 229
Delessaria, 215, 229, 244, 284, 294,

370; sanguinea, 215, 229

Delessert, Baron, 229 /

Delf, E., 259, 277

Delophycee, 131

Dendrida, 371

Denmark, 299

Derhesia, 86, 87; marina, 86, 87, 251

Dermatolithon, 273

Desmarest, a. G., 168

Desmarestia, 168, 169, 170, 189, 265;

aculeata, 289
Desmarestiaceae, 127, 168
Desmarestiales, 260
Desmidiaceae, 98, 105, 117
Desmids, 105, 106, 334-6, 338
Desmokontae, 126
Devonian, 267, 273-5
Diatomaceae, 119, 126, 337, 342
Diatoms, 3, 126, 333 et seq., 341, 342,

345, 370
Dictyosiphon, 150, 151, 258; foeni-

cidaceiis, 152
Dictyosiphonaceae, 128, 150
Dictyosiphonales, 257, 260
Dictyota, 131, 163, 165-7, i97, 247,

248, 255, 365; dichotoma, 163, 165,

291, 292, 359
Dictyotaceae, 127, 163
Dictyotales, 127, 130, 131, 161, 163,

257, 258, 260, 277

DiLLWYN, L. W., 298

Dimorphosiphon, 268
Dinohryon, 123; sertularia, 123
Dinocloniiim, 126, 264; Conradi, 125
Dinococcales, 126
Dinoflagellates, 125, 126
Dinophyceae, 3, 98, 125, 126, 261^

264, 268, 296, 336
Dinothrix, 3, 126, 264
Diplobiont, 250
Diplont, 243, 250

Diplopora, 270, 271 ; phanerospora, 271
Dostal, R., 97
Dover, 306, 309, 312
Dovey, 323, 324, 326, 327
Draparnaldia, 21, 64, 66, 73, 262, 263,

265 ; glomerata, 65
DraparnauD; J. P. R., 64
Dry Tortugas, 279
Du Buy, H. G., 304
Dudresnay de St-Pol -de-Leon, 224
Dudresnaya, 224, 225, 372; coccineay

224
Dumontia, 215
Dumontiaceae, 224
Dunaliella, 114

INDEX

379

D'Urville, I. D., 191

Durvillea, 190, 191, 197, 258, 277, 309;

Antarctica, 191
Durvilleaceae, 190, 191

Ecklonia, 365
Eclipsiophyceae, 372
Ectocarpaceae, 132, 136, 139, 147, 254
Ectocarpales, 130, 132, 170, 246, 247,

248, 255, 257 et seq.
Ectocarpus, 129. 132, 133, 136-8, 154,

247, 258, 265; fasciculatus, 132;

Padinae, 134; secundus, 133; sili-

culosus, 77, 131 et seq., 135, 246, 248;

virescens, 134, 247, 254
Egle, K., 290, 305
Egregia, 186; Menzesii, 187
Ehrke, G., 284, 286, 287, 304, 359,

374
ElSEN, G., 186
Eisenia, 186, 188, 189, 289; bicyclis,

288
Elachista, 145, 146; fuciola, 145, 291,

292
Elachistaceae, 127, 145
Elliot, A. M., 36
Encoeliaceae, 259
Encrustida, 371
Endodernia, 22, 263, 306
Endophytes, 372, 373
England, 138, 166, 273, 351
Enteromorpha, 19, 50, 51, 56, 265, 284,

294, 307 et seq., 313, 316, 321, 330,

371; clathrata, 52, 323; compressa,

284; intestinalis, 51, 52, 306, 307;

Linza, 284, 307; minima, 323, 327,

330
Eocene, 269, 271, 273
Epichrysis paludosa, 123
Epilithon, 226, 227; membranaceum,

226, 227
Epiphytes, 345-7, 37°, 372, 373
Epitheca, 119
Ephemerophyceae, 372
Epping Forest, 345
Equisetum, 108; limosum, 346, 347
Erythrocladia, 265
Essex, 325
Eiidesme, 139, 141, 142, 145, 259;

virescens, 141, 142
Eudorina, 26, 27, 36, 262; elegans, 26;

illinoiensis , 26
Eu-Florideae, 213, 220, 252, 256, 257,

262, 265
Eiiglena, 300, 338, 340

Eugleninae, 261

Eunotia, 338; pectinata, 347

Evection, 74

Faeroes, 366

Feldmann, J., 73, 84, 319, 320, 367,

372, 374
Finland, 366
Flagellata, i, 268
Flahault, C, 298, 319
Florideae, 265
Foliida, 371
Forbes, E., 319
France, 365, 366
Fritsch, F. E., 56, 73, 132, 263, 277,

302, 304, 332, 337, 338, 341, 348
Frustule, 119
Fucaceae, 192, 198 et seq., 248, 284,

286, 355
Fucales, 127 et seq., 163, 178, 189, 190,

192, 198, 247, 248, 257 et seq., 276,

277
Fucoxanthin, 4, 18, 127, 129, 359

Fiicus, 93, 131, 145, 164, 190, 192, 194,
195, 196 et seq., 211, 248, 250, 254,
259, 279, 284, 285, 287, 289, 312,
3i3> 355, 370; ceranoides, 192, 306,
311, 324; furcatus, 316; limicola,
322, 333; serratus, 137, 192, 193,
283, 284, 285, 286-8, 306, 307, 310,
311, 314, 353, 354, 359; spiralis,
192, 193, 285, 306, 307, 310, 311,

315, 354; var. lutarius, 324, 330;
var. nanus, 324; (spiralis var.) platy-
carpus, 283-5, 310, 314, 352, 353,
355; vesiculosus, 137, 192, 193, 198,
283, 289, 306, 307, 310, 311, 314,

316, 325, 353, 354; ecad. caespitosus,
324; ecad. filiformis, 324; ecad.
muscoides, 324; ecad. nanus, 324;
ecad. suhecostatus, 324; ecad. volu-
bills, 289, 324, 330; var. evesiculosus,
306, 307, 315

Fungi, 97, 261
Funk, G., 370, 374

Gail, F. W., 352, 374

Galaxaura, 222, 365

Gamble, F. W., 305

Geitler, L., 17, 36, 126

Gelidiaceae, 223

Gelidiales, 213, 214

Gelidium, 223, 306, 370; corneum, 224

Gelose, 224

Geniculations, 10 1, 103

38o

INDEX

Geosiphon, 296

Germany, 271

Getman, M. R., 211

GiBB, D. C, 211, 320

Gijfordia secundus, 133

Gigartina, 306, 307, 310, 315

Gigartinaceae, 239, 240

Gigartinales, 214, 238

Girvanella, 267

GiSLEN, T., 374

Gleucocystis, 295

Gloeocapsd, 7, 12, 17, 262, 267, 296;

crepidinum, 7
Gloeocapsomorpha, 266
Gloeochaete, 295
Gloeocystis, 9

Gloeodiniim, 126, 264; montanum, 125
Gloeothece, 9, 267
Glycerin, 338
Glycogen, i, 6
Gobia, 151
GoDWARD, M., 341, 345, 348

GOEBEL, K., 126
GOMONT, M., 298

Gomphonema, 347

Gongrosira, 67, 95, 263

Gonidia, 7, 13, 32

Gonimoblast(s), 214, 220, 222, 223,

225, 227, 233, 236, 237
Goniolithon, 273

Gonium, 24, 36; pectorale, 24, 25
Gracilaria, 370; confervoides 238
Graebner, p., 298
Graminida, 371
Greenland, 299, 366, 367
Griffiths, Mrs, 233
Griffithsia, 215, 233, 244; corallina,

215, 233, 234; flosculosa, 358;

furcellata, 358; globulifera, 233;

opuntioides, 358
Gross, F., 126
Gross, I., 56
Groves, J., 126
Grubb, V. M., 215, 244, 320, 352,

374
Grintzesco, J., 44

Giinnera, 300

GUSSEWA, K., 62

Gymodinium, 264; aeruginosum, 125

Gymnosolen, 267

Gyrogonites, t.'J'^

Haas, P., 288, 304, 353
Haematochrome, 18, 19, 33
Haematococcus, 32, 36; pluvialis, 33

Haines, H., 302, 304
Halarachnion ligulatum, 291, 292
Halicystaceae, 86
Halicystis, 73, 86, 87, 97, 166, 261;

ovalis, 86, 87, 251
Halidrys, 158, 203, 306, 311; dioica,

203; siliquosa, 203, 204, 289
Halimeda, 93, 97, 269, 276, 277, 365;

cuneata, 361; fwna, 361
Halopteris filicina, 247
Halosaccion, 265, 317
Halosphaera, 115, 126; viridis, ii6
Halosphaeraceae, 115
Hamel, G., 136, 154
Hammerling, J., 84
Hanson, E. K., 281, 304
Hantschia amphroxys, 299
Hapalosiphon arhoreus, 9
Haplobionts, 213, 243, 250
Haplonts, 213, 250
Haplospora, 160; globosa, 161, 162
Haplostichineae, 127
Haptera, 129
Harpacticus chelifer, 168
Harpenden, 338, 341
Harper, R. A., 36, 44
Hartmann, M., 20, 36, 51, 56
Harvey, G., 238
Harveyella, 216, 238, 239, 244;

mirabilis, 238
Haustoria, 239
Hawaii, 219
Heilbron, I. M., 5
Heligoland, 291
Heliophiles, 372
Heliophobes, 372
Hemicryptophyceae, 373
Hemiphanerophyceae, 273
Herpophytes, 372
Herrings, 368
Heterocapsaceae, 114
Heterochloridaceae, 114
Heterochloridales, 114
Heterochloris, 264
Heterococcales, 114
Heterocyst(s), 8, 14, 15, 16
Heterogeneratae, 127, 128, 131, 132,

155, 167, 189, 190, 260
Heterokontae, i, 2, 18, 98
Heterosiphonales, 114, 118
Heterosiphonia plumosa, 358
Heterotrichales, 114, 262
Heterotrichy, 254, 255, 262, 277
HiGGiNS, E. M., 162
Hildenbrandt, F. E., 262

INDEX

381

Hildenbrandtia, 217, 226, 265, 306,

307. 373, pf'ototy pus, 262; rivularis,

262, 336
Hill, T. G., 288, 304, 353
Himanthalia, 195, 201, 211, 306, 307,

310, 311, 315, 370, 371; lorea, 222,

281

HODGETTS, W. J., 107
HOLDEHEIDE, W., 305
HOLLENBERG, G. J., 1 89

Holmsella, 215, 216, 238, 339; pachy-

derma, 238
Hor77iidiwn, 10, 262, 279, 303, 336,

337, 339
Horniogonales, 10, 13

Hormogone(s), 8, 13, 14, 16

Hormosira, 191, 208, 210, 211, 277;
Banksii, 209

Hormothamnion, 361

Howe, M. A., 97

HowLAND, L. J., 71, 73

HoYT, W. D., 167

Hyde, M. B., 286, 304

Hydrobia idvae, 351

Hydrodictyaceae, 40, 41

Hydrodictyon, 21, 32, 40, 41, 42, 43,
44, 370; Africanum, 41; patenae-
forme, 41, 42; reticulatum, 41

Hy drums, 2, 123; foetidus, 123

Hypnophyceae, 372

Hypnospore(s), 20, 33

Hypotheca, 190

Iceland, 366

Idioandrosporous, 61

Indiana, 340

Indian Ocean, 361

Inch, S., 211

Iodine, 171

Irish Sea, 158

Isaac, W. E., 374

ishikawa, m., 244

Isle of Man, 134, 137, 315

Isle of Wight, 306, 351

Isofucoxanthin, 121

Isogeneratae, 127, 132, 154-6, 160,

163, 260
Isokontae, i, 18, 57

Jamaica, 165, 166

Janczewski, E, de, 230

Janczezvskia, 216, 230, 231

Japan, 167, 171, 187, 190, 207, 219, 224

Johnson, D. S., 316, 320, 351, 356, 374

JoNssoN H., 365, 374

Juncetum Geradii, 330
Jurassic, 268, 271

Kanda, J., 189

Keeble, p., 305

Klebs, G., 339, 348

Klugh, a. B., 356, 357, 373

Knebel, G., 56

Knight, M., 134, 136 et seq., 154, 278,

314, 320, 350, 372, 373
Kolbe, R. W., 126
Kombu, 171
Kornmann, p., 97
Kosmogyra, 273
Kothbauer, E,, 136
Krausel, R., 275, 278
kunieda, h., 56, 154, 278
Kutzing, p. J., 194
Kylin, H., 51, 56, 127, 131, 132, 154,

189, 244, 258, 278

Lagynophora, 273

Lambert, P. D., 36

Laminaria, 170, 172, 173, 174, 177,
189, 191, 197, 247, 283, 294, 306,
307, 310, 311, 370, 371; Andersonii,
289; Cloustoni, 171, 172, 310, 315;
digitata, 172, 174, 285, 289, 310,
315; religiosa, 176; Rodriquezii, 172;
saccharina, 172, 174, 176, 310;
Sinclarii, 171

Laminariaceae, 170, 174, 177, 179,

187,. 193, 307, 361, 365
Laminariales, 127-9, 131? 163, 167,
170, 178, 189, 190, 248, 257, 260,

276, 277, 365
Laminarin, 288
Lampe, H., 287, 305
Lander, C. A., 36
Lang, W. H., 274, 276, 278
Laurencia, 215, 230, 306, 307, 310,

311, 315; pinnatifida, 307
Leathes, G. R., 144
Leathesia, 128, 144, 154, 370; dif-

f or mis, 144
Lefevre, M., 107
Lemanea, 217, 220; fluviatilis, 336
Lemna, 37, 338
Lepyrodophytes, 372
Lesson, P. de, 180
Lessonia, 180, 275, 277, 365, 371
Lessoniaceae, 180-183
Lewis, LP., 244
Liagora, 223; tetrasporifera, 249;

viscida, 318

INDEX

382

Lichina, 306, 307, 315

Lignicolae, 373

Limu Luau, 219

Limnaeider, 373

Limnoplankton, 336

Linnaeus, 214

LiND, E. M., 56, 348

List, H., 84

Lithakophytes, 372

Lithoderma, 128, 131, 255

Lithophyllum, 273

Lithothamnion, 273, 274, 306, 307, 317,

371
Lloyd, F. E., 107, 108
Lomentaria, 242, 244, 306, 307, 310,

315 ; clavellosa, 215, 243 ; rosea, 242,

243, 250, 254

LORENZ, J. R., 319

Loriformes, 190, 201

Los Angeles, 182

Lough Ine, 306, 309, 314, 322, 323

Lower California, 186, 188

Luther, 18

Lyngbye, H. C, 14, 298

Lynghya, 10, 14, 306, 338; aestiiarii, 14

Lynn, Mass., 330

Lythgoe, B., 5

Macrandrous, 60, 61, 62
Macrocystis, 172, 181, 183, 258, 289,

365, 370; pyrifera, 184
Madagascar, 361
Magnosilvida, 371
Mainx, F., 44, 62
Mangeot, G., 244
Manguin, F., 107
Mannitol, 288
Manubrium, iii
Marpolia spissa, 266
Martensia, 230; fragilis, 212
Martin, J. C, 279, 305, 357, 374
Matthias, W. T., 244
Mediterranean, 82, 89, 317, 318, 361,

366, 367

Medulla, 139, 150, 171, i73> i93
Melohesia, 215, 263, 273
Melosira, 121, 338; granulata, 120
Membranoptera alata, 291, 292, 358
Merismopedia, 12; aeruginosa, 12;

elegans, 9, 12; ichthyolahe, 12
Mesogloia, 128, 139 et seq., 154, 167,

265; Levillei, 141; vermiculata, 140,

141
Mesogloiaceae, 127, 139, 141, 142, 144,

147, 259

Mesotaenium, 104, 105, 333
Metarreophytes, 372
Me\'ER, K., 56
Microcoleus delicatulus, 342
Microcystis, 9, 10, 17; aeruginosa, 10,

II
Microdictyon, 361
Microspora, 46-8, 56, 117, 262, 338,

339; amoena, 47; Willeana, 20
Microsporaceae, 46
Miller, V., 126
Mischococcus, 264

Monospore(s), 161, 162, 216, 219, 220,
241, 242, 250

Monostro7?ia, 48, 49, 51, 56, 252, 294,
326, 351; Blytii, 49; crepidinum,
49; Grevillei, 49, 367; Li ndaueri, 4g

Monostromaceae, 49

MoNTFORT, C, 357, 360, 374

Moore, L. B., 281, 305, 309, 320

MouGEOT, J. B., 102

Mougeotia, 102, 104; tenuis, 103

Mount Desert Is., 316

Murray, G., 361

Murman Sea, 366

Mycetozoa, 261

Myrionema, 128, 146; strangulans, 146

Myrionemaceae, 146

M>'xonema, 65

]\I>Tcophyceae, 6

Myxoxanthin, 6

Nannandrous, 61, 62
Naples, 134, 165, 166, 370
Nemalion, 215, 221, 244, 249, 252, 265,

306, 307; helminthoides, 318, 368
Nemalionales, 213, 216, 220, 221, 223
Nemastomales, 214
Nemathecia, 240, 241
Nematochrysis, 264
Nematocysts, 125
Nematophyceae, i, 4, 209, 274
Nematophycus, 275
Nematophytales, 273-5, 277, 278
Nematophyton, 4, 182, 273, 274, 275,

276, 277
Nematothallus, 4, 274, 275, 276;

pseudo-vasculosa, 276; radiata, 276
Nemoderma, 131, 166, 249, 255
Neomeris, 81, 82, 84, 97, 271, 361, 362
Nereider, 373
Nereocystis, 181, 182, 289, 370, 371;

Luetkeana, 183
Neuston, 373
New Brunswick, 356

INDEX

383

New England, 327, 330, 351
New Zealand, 190, 192, 209, 309
NiCHOLLS, A. G., 305

NiENBURG, W., 211

NisizAWA, N., 305

Nitella, 71, 108, 109, no, in, 112,

113, 279; cernua, 108
Nitophyllum punctatum, 358
Nodularia Harveyana, 302
Nordhaven, 366

Norfolk, 321, 322, 324, 326 et seq., 351
North Africa, 366
North America, 167, 168, 181, 365
North Carolina, 165, 166
North Sea, 368
Norway, 366
Nostoc, 8, 15, 17, 296, 297, 303, 304;

commune, 16; muscorum, 302; Pas-

serinianum, 202; punctif or me, 300
Nostocaceae, 15, 17
Notheia, 191, 210, 211; anomala, 210,

211, 297

Obione portulacoides, 329 *

Ocelli, 125

Ochromonas mutabilis, 123

Oedocladitim, 57

Oedogoniales, 2, 18, 21, 57, 62, 262

Oedogoriiiwi, 19, 57, 58. 59. 334. 338 et

seq., 347
Oeyianthe fluviatilis, 347
Ohashi, H., 62
Olson, R. A., 304
Oltm.\nns, F., 368, 374
Oomycetes, 97

Ordovician, 266, 268, 270, 273
Oscillatoria, 7, 13, 14; brevis, 303, 304;

trrigua,g; limosa, 14; margaritifera,

9; proboscidea, 9
Oscillatoriaceae, 13, 14, 338
0\'ulites, 269

Pachytheca, 267, 268
Pacific Ocean, 183, 190, 361
Padina, 166; pavonia, 318
Palaeodasycladus mediterraneus, 271,

272
Palaeonitella, 273 ; Cranii, 273
Palaeoporella, 268; variabilis, 269
Palm, B. T., 44
Pabnella, 21

Palmellaceae, 217, 261, 262
Panama, 361
Pandoritia, 21, 25, 33, 262, 264;

ynorum, 25

Papexfuss, G., 136, 154

Paraphyses, 146, 152, 175, 186, 188,

190, 197
Paraspores, 216, 237, 238
Parke, M., 141, 154, 314, 320, 350,

372, 374

Parthenospores, 103

Par\-osilvida, 371

Pascher, a., 56, 126

Patagonia, 190

Patina pellucida, 307

Pediastrum, 40, 43, 44

Pelagophycus, 182, 183, 289

Pelvet, Dr, 198

Pelvetia, 190, 198, 211, 306, 307, 311,
314, 323; canaliculata, 198, 199,
285, 289, 310, 319, 327, 355, 359;
ecad. coralloides, 324; ecad libera,
289, 324; muscoides, 323; radians,
324; fastigiata, 198, 199; Wrightii,
199

Penicillus, 365

Pennatae, 119, 122

Peridinium, 126; angliaim, 125

Perizonium, 122

Petersen, J. B., 36, 300, 305

Petruschewsky, G., 17

Peveril Point, Dorset, 306, 309, 310,

311

Peysonielliopsis, 216

pH, 60, 65, loi, 118, 174, 281, 289,

347, 357
Phacotus, 21, 264
Phaeococcus, 124, 255, 264, 322, 323,

327, 328, 330
Phaeocxstis, 123, 369; pouchetii, 123,

368
Phaeophyceae, i, 3, 4, 5, 124, 127, 128,

149, 163, 190, 215, 245-8, 254-7,

260-3, 265, 266, 273, 275, 283, 290.

293, 294, 297, 356, 357, 359
Phaesporeae, 127, 257, 258
Phaeostroma, 128, 139; Bertholdi, 139
Phaeothamnion, 2, 123, 255; confervi-

colum, 123
Phanerophyceae, 373
Phialophore, 32
Pholadoph>tes, 372
Phormidium, 323, 341 *, autivnnale, 320,

327, 329, 337; laminosum, 334
Phycochr>"sin, 2, 122
Phycocyanin, i, 4, 6, 212, 360
Phycodrys rubens, 358
Phycoen.-thrin, i, 4, 6, 212, 281. 282,

357. 360

384

INDEX

Phycomycete, 296

Phycopeltis, 332

Phycoporphyrin, 18, 102

Phyllida, 371

Phyllitis, 149, 154, 370; Fascia, 149

Phyllophora, 240, 241, 294; Brodiaei,

240, 250; niembranifolia, 240, 250
Phyllosiphon, 87, 97; Arisari, 88
Phyllosiphonaceae, 87
Phyllospadix, 219
Phyllotaxis, 129
PiA, J., 278
Pila, 115
Pith, 173

Pinnularia viridis, 120
Pithophora, 334; oedogonia, 20
Plankton, 119, 333. 37o, 373
Plasmodesmae, 28, 212
Pleodorina, 27, 36, 262; Calif ornica,

27
Plethysmothallus(i), 132, 141, 142, I53
Pleurocapsa, 341
Pleurococcaceae, 63
Pleurococcales, 63
Pleurococcus, 63, 262, 302; Naegelii,

63, 64
Pleuston, 373

Plocamium, 216; coccineimi, 290, 292
Pliimaria, 216, 237, 244, 306; elegans,

237

Pneumatocyst(s), 192, 200

Pneuniatophore(s), 200

PococK, M. A., 36, 44

Pocillopora bulbosa, 296

PoLYANSKi, G., 17

Polygonum lapathifoliiim, 37

Polyneura Hilliae, 358

Polykrikos, 264

Polysiphonia, 212, 215, 230, 244, 290,

306, 326; Brodiaei, 315; fastigiata,

216, 230; nigrescens, 233, 291, 292;

sertularioides, 367; tenerrima, 367;

urceolata, 291, 358, 367; violacea,

215, 231, 232
Polysiphoneae, 297
Polysporangia, 237
Polyspores, 216
Polystichineae, 128
Porifera, 124
Porostromata, 267
Porphyra, 218, 219, 244, 257, 265, 287,

294, 306, 307, 310, 311, 315, 316,

318, 372; laciniata, 291, 292, 359;

naiadum, 219; umbilicalis, 218, 284,

318

Porphyridium, 244, 257; cruentuniy
217, 256, 316

POSTELS, A., 181

Postelsia, 181, 350; palmaefonnis , 181
Potamoplankton, 333, 334
PouLTON, E. M., 126
Prasinocladus, 21, 35, 36, 264
Prasiola, 9, 53, 56, 257, 262, 302, 303;
crispa, 54; japotiica, 54; stipita^

307
Prasiolaceae, 53

Prhnicorallina, 270

Pringsheim, E. G., 353, 374

Propagules, 93, 130, 162

Prothallus, 132, 150

Protista, 256

Protococcales, 332, 334, 339

Protococcus, 341

Protoderma, 21, 262, 265

Proto-florideae, 4, 212, 217, 221, 256,

257, 265
Protomastigineae, 261
Protonema, 113
Protonemata, 150
Protophyceae, 266
Protosiphon, 21, 85, 87, 97, 118, 262;

Botryoides, 85, 86
Protosiphonaceae, 85
Prototaxites, 275
Pruvot, G., 319
Pseudobryopsis myura, 318
Pseiidopringsheimia, 265
Pseudoraphe, 119
Pterygophora, 187, 188
Ptilota elegans, 290, 292; plimiosa, 358
Puccinellia maritima, 350
Punctaria, 129, 258, 265
Punctariaceae, 128
Punctariales, 260
PUMALY, A. de, 44
Pylaie, de la, 136
Pycnophycus, 196
Pylaiella, 128, 136, 154; littoralis, 131,

136, 137, 138, 306

Ralfsia, 226, 265, 306, 315, 370

Ranunculus aquatilis, 338

Raphe, 119

Raunkiaer, C, 368, 372, 374

Rees, T. K., 309, 314, 320, 331, 350>

374
Reich, K., 73
Reinschia, 115
Renfrezvia, 172
Rhabdonema, 121, 269

INDEX

38s

Rhabdoporella pachyderma, 269
Rhaphidonema hrevirostre, 333
Rhizoclonium, 78, 306, 323, 330, 334;

lubricum, 79; riparum, 306
Rhizochrysis, 123
Rhizocysts, 119
Rhodochorton, 307; endozoicum, 297;

floridulum, 291, 292, 307
Rhodochytrium, 22
Rhodo?nela, 238
Rhodomelaceae, 230
Rhodophyceae, i, 4, 72, 212, 213, 217,

245, 249 et seq., 254, 261, 265, 273,

277, 281, 290, 293, 294, 297, 333,

356, 357, 359
Rhodymenia, 215, 307, 317; palmata,

168, 359
Rhodymeniaceae, 242
Rhodymeniales, 214, 242
Riccardia Moritagnei, 216
Rich, F., 338, 341, 348
Rivularia, 15, 306, 322, 323, 327, 330,

371, 372; atra, 15, 16; hae?tiatites,

9, 337
Rivulariaceae, 15

Roach, B. M., 299, 302, 305

Robinson, W., 167

Rock Pools, 289, 350, 356, 357

Roe, M. E., 211

Ruhr, River, 336

Rothampstead, 302

Sacchorhiza, 177, 178, 182, 189, 192,

317; bulbosa, 177, 178; dermatodea,

178, 179
Sachs, G., 194
Sarcophycus, 190
Sargassaceae, 203-5, 207
Sargasso Sea, 207
Sargassum, 166, 190, 196, 205, 206,

208, 211; 365, 371; enerve, 207;

filipendula, 207; Horneri, 198;

Hystrix, 207; natans, 207; vulgare,

207
Sauvageau, C, 154
Savage, R. E., 368, 374
Saxicolae, 373
Scaphospora speciosa, 162
Scenedesmus, 43, 44, 300, 301 ; costel-

latus var. chlorelloides, 299
SciNA, D., 221
Scinaia, 221, 244, 249, 252, 365;

furcellata, 215, 221, 222, 223
Schizogoniaceae, 217
Schizogoniales, 262

Schizomeris, 46, 262

Schizothrix, 266, 267; Fritschii, 306;

purpurascens, 9
Schmidt, O. C, 97, 162
Schmitziella, 216; mirabilis, 297
Schreiber, E., 176, 189
ScHUSSNiG, B., 97, 136, 154
Scolt Head Island, 323
Scotiella nivalis, 333
Scotland, 325, 366
Scytonema, 14, 15, 17, 296
Scytonemataceae, 7, 14
Scytosiphon, 139, 150
Scytosiphonaceae, 149
Seatron, 182
Seirococcus, 200, 201
Sertularia, 138
Setchell, W. a., 171, 189, 366, 371,

374
Seural, 319

Seward, A. C, 278

Seybold, a., 283, 290, 293, 305

Shaw, W. R., 36

Sheffield, 340

Siberia, 271

Silurian, 267, 268, 270, 275

Silvida, 371

Simons, E. B., 211

Siphonales, 19, 21, 41, 56, 63, 73, 79,

80, 84, 86, 94, 97, 248, 254, 261, 262,

265
Siphonocladiales, 19, 21, 56, 63, 73,

255, 261, 262, 265
Siphonocladus, 80; pusillus, 318
Sjostedt, J., 214
Skiarrophyte, 372

Skutch, a. S., 316, 320, 351, 356, 374
Smith, G. M., 36, 44, 278
Solenoporaceae, 273
South Africa, 365
Spartina, 322, 330; patens, 351;

Tozvnsendii, 327
Spearing, J. K., 17
Spermatochnaceae, 127, 147
Spermatochnus, 147; paradoxus, 147
Spermatium(ia), 212, 215, 219, 221,

229
Spermocarp, 73
Spermothamnion, 216; Synderae, 237;

Turner i, 236
Spessard, E. a., 62
Sphacelaria, 129, 130, 157, 158, 162;

bipinnata, 158, 2^7; Harvey ana, 247;

hystrix, 247; tribuloides, 158
Sphacelariaceae, 127, 157

386

INDEX

Sphacelariales, 156, 260

Sphacella, 157

Sphaerella, 18, 29, 32, 33; lacustris, 33

Sphaerellaceae, 32

Sphaerococcales, 214

Sphaerocodium, 267

Sphaeroplea, 54, 55, 56 ; Africana, 54,

56; atinulina, 55, 56; cambrica, 55;

tenuis, 56
Sphaeropleaceae, 54
Spirogyra, 21, 98, 99. 100, 103, 107,

108, 334, 340, 341; adnata, 98
Spirulina, 13
Spitzbergen, 366

Spondyloth amnion niultifidwn, 356, 358
Spongiostromata, 266, 277
Spongo7fiorpha, 77 ; arcta, 217; lanosa,

372; spinescens, 317
Sporochnaceae, 127, 148
Sporochnales, 260
Sporochniis, 148, 372; pedunculatus,

148, 318
Sporocysts, 119
Squamariaceae, 226
Starkey, C. B., 108
Steinecke, F. von, 56
Stephanokontae, i, 2, 57
Stephanosphaera, 33
Sterrocolax decipiens, 241
Steward, F. C, 279, 305
Stichida, 159, 261
Stigeoclonium, 21, 65, 66, 73, 262, 264,

265, 347
Stigonetna, 7
Stocker, O., 283, 305
Stolons, 77

Strafforella, J., 319
Strangford, Lough, 322, 324, 325
Streblonema, 153, 263
Striaria, 326
Sturch, H. H., 244
Stypocaulaceae, 159
Stypocaulon, 129, 159, 160, 162;

scorparium, 160
Stiaeda fruticosa, 351
Subcortex, 139
Succicolae, 373
SuNESON, S., 244
SuTO, S., 154, 278

SVEDELIUS, N., 84, 213, 244, 249, 250,

252, 257, 278, 360, 374
Sweden, 132, 138
Symbiosis, 295

Synura, 123, 264; ulvella, 123
Synzoospore, 56

Tansley, a. G., 18
Taonia, 165, 167
Taylor, W. R., 127, 132
Taxaceae, 275
Temperley, B. N., 126
Terricolae, 373
Tertiary, 93, 269, 273
Tetragonidium, 124; verrucatum, 124
Tetraspora, 23, 33, 34, 36
Tetrasporaceae, 33, 295
Tetraspore(s), 130, 161, 162, 164, 165,

213, 216, 223, 226, 230, 235, 238,

240, 242, 243, 249, 250, 264
Thallassiophyllum, 179; clathrus, 179
TiLDEN, J., 18, 278
Tilopteridaceae, 127
Tilopteridales, 127, 130, 160, 162, 260
ToBLER, F., 97
Torida, 371
Tolypothrix, 341
Trabeculae, 91
Tracheloynonas, 338
Trailliella intricata, 291, 292
Tranophytes, 372
Transeau, E. N., 335, 348
Treboiixia, 38
Trentepohl, J. F., 69
Trentepohlia, 68, 69, 73, 255, 262, 296,

332; aurea, 71
Trentepohliaceae, 67-9, 332
Triassic, 269-71, 273
Tribonema, 48, 117, 118, 123, 262, 264;

bombycina, 117
Tribonemaceae, 117
Trichog>'ne(s), 212, 215, 219, 221, 228,

232
Trochiscia aspera, 299
Trumpet hyphae, 174
TsHUDY, H., 281, 282, 305
Turbinaria, 207, 208
Tylenchus fuciola, 200

Udotea, 92, 276, 277, 361, 371

Ulothrichaceae, 44

Ulothrichales, 18, 54, 73, 262, 334, 342

Ulothrix, 19, 21, 44, 48, 49, 56, 62, 66,
73, 250-2, 261, 262, 264, 307, 316,
321, 323, 327, 329, 330, 336, 337,
340, 341; flacca, 46, 318, 323;
pseudoflacca, 318, 372; subflaccida,
318; subtilis,2gg; so?tata, 45, 46

Ulva, 19, 20, 46, 49, 50, 51, 146, 265,
279, 294, 306, 351; Lactuca, 50,
291, 292, 359; Lima, 50, 291, 292;
Rhacodes, 50

INDEX

387

Ulvaceae, 50, 51, 56

Ulvales, 21, 261

Ulvella, 347

Umbraculida, 371

Urospora, 79, 251, 264, 306, 307, 309,

310, 316
Utricularia, 334

USPENSKAJA, W. J., 73

Valonia, 73, 79, 80, 261, 279-81, 305;
Macrophysa, 279-81; utricularis,
80; ventricosa, 279, 280, 281

Valoniaceae, 79, 80, 87

Vancouver Island, 181, 186, 188

Vaucher, J. P., 94, 298

Vaucheria, 19, 20, 81, 89, 94, 95, 96,
97, 264, 306, 330, 333, 334, 341, 372;
Debaryana, g4; sessilis, g']\ sphaero-
spora, 322, 330; Thuretii, 322, 330

Vaucheriaceae, 94, 97

Vaucherietum, 322

Verrucaria, 306, 307, 315, 316

ViscHER, W., 73, 262

Volvocales, 18, 19, 22, 262, 263

Volvox, 20, 21, 28, 29, 30, 31, 36, 43,
262,264,340; Africana, 22; aureus,
28, 32; globator, 28, 32; tertius, 28

Walcott, C. D., 267

Wales, 165

Webster, T. A., 281, 305

Weedia, 267

Wembury, Dorset, 306, 313

Wesley, O. C, 73

West, G. S., 62, 108, 298, 332, 333,

335, 348
West Indies, 207, 2>9, 360
Weyland, H., 278
White Sea, 306
Whitley, E., 281, 305
Wille, N., 298

Williams, J. Lloyd, 132, 167, 189
Williams, M. M., 97, 211
Windermere, Lake, 341, 342
Winkler, 282

Xanthophyceae, 2, 18, 48, 94, 97, 98,
113, 114, 122, 126, 261, 264, 334,

338
Xiphophora, 210, 309

Yabe, Y., 56

Yamanouchi, I. S., 156, 162, 244

Yendo, K., 150, 154

YoNGE, C. M., 297, 305

Zanardinia, 154, 155, 255
Zanefeld, J., 353, 374
Zimmerman, W., 36 .
ZiRKLE, C, 244
Zoochlorella, 40
Zooxanthella, 22, 40, 124, 296
Zostera, 139, 219, 371
Zygnema, 102,103, 107; pectinatum, 18
Zygnemaceae, 98, 102, 334, 338, 34^
Zygnemales, 18

Zygogonium, 102, 107, 108; ericetorum,
102, 103, 303

CAMBRIDGE: PRINTED BY

W. LEWIS, M.A.
AT THE UNIVERSITY PRESS

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