the latter bdng shown
In Spite of the proven duality of lichens, in longitudinal section; c, cortical
there are Various things which suggest absorptive layer; a, algal layer; /,
.... - loose internal mycelium; w, an axial
that they possess a high degree of auton- strand of densely placed hyphae>
omy or unity. It is quite conceivable
that this autonomy might ultimately be-
come so complete as to make impossible
the separate cultivation of the two symbionts. Perhaps the most strik-
ing evidence of autonomy is afforded by the soredia (figs. 1114-1116),
which are unique reproductive or-
gans consisting of a group of algal
cells invested by fungal hyphae; at
maturity the soredial structure buds
,,15 off from the lichen thallus like a
FIGS. 1114-1116. — Soredia from the gemma (p. 808), forming, perhaps,
beard lichen (Usnea barbata): 1114, a the most efficient means of repro-
simple soredium consisting of an algal ductlon pOSSessed by lichens, since
cell, surrounded by a web of fungal hy- r
phae; 1115, a soredium in which the algal the fungal spores are of value only
cell has reproduced by division; m6, a when they happen to fall among
germinating soredium in which the algae appropriate algae. This is almost
are dividing, the hyphae forming an apex
of growth; all figures highly magnified, the only case where two symbionts
— From SJHWENDENER. have a common reproductive body.1
1 The fungal symbiont of Lollum is scattered with the seeds, the mycelia occupying
a definite layer ; the bacterial galls of Ardisia also are propagated by seed.
forming the highly elastic mechan-
ical tissue of the lichen; highly
magnified. — From. SACHS.
802 ECOLOGY
Another indication of autonomy is seen in the geographic distribution of
lichens. As a class, they are among the most xerophytic and autophytic
of plants, many species growing on the driest and barest of rocks, where
few other plants can maintain themselves. Yet lichens are made up of
algae, which as a group are characteristically hydrophytic, and of fungi,
which as a group are characteristically mesophytic and dependent;
the symbiotic union of two such diverse elements appears to result in a
form widely different from either, and more resistant and independent
than is to be found in almost any other group of plants.
The nature of lichen symbiosis. — The parasitism of the fungal sym-
biont upon the algal layer is undoubted, but there are various theories
concerning the relation of the alga to the fungus.
A common theory is that the relationship is one
of helotism or slavery, the algal symbiont leing
represented as indifferent to the fungus. Another
common theory is that of reciprocal parasiiism,
FIG. 1117.— Analgal allied to which is the recently proposed theory of
cefl of a lichen (Cla- endosaprophytism, and also the older view tha. the
donia furcata), which ^ , jj. within the f g j, ^ better
is closely embraced by
hyphal filaments of the protected than when separate and thus are enabled
lichen fungus; highly to live in drier habitats. Still another view is that
~~ Fr°m the alga is merely the host of an ordinary parasite.
The parasitism of the fungus is demonstrated
clearly by a number of facts, such as the close embracement of algal
cells by fungal hyphae (fig. 1117), by the frequent entrance of hyphae
into algal cells, by the occasional development of haustoria, by the
disorganization and subsequent emptying of the contents of many
algal cells, and, perhaps, by the apparent restriction upon algal activity,
which is evidenced by increased vigor when released from symbiosis.
As for the algal symbiont, the theory of endosaprophytism on fungal
excreta seems most tenable in view of the fact that lichen algae are de-
cided mixophytes which thrive particularly on peptones; water and
salts must also be derived through the medium of the fungus.
The origin of lichens. — There is almost no experimental know edge
concerning the origin of lichens, and most of the common species appear
to be well-defined lichens without obvious indications of their evolution.
In some instances the fungal symbiont lives saprophytically on bark or
on humus or parasitically on the plant which gives it mechanical support,
as well as on its algal layer ; such species are most abundant in the tropics
SAPROPHYTISM AND SYMBIOSIS 803
(p. 659), but some forms of temperate regions (Usnea, for example)
may be partially parasitic on trees. A remarkable tropical lichen is
Cora Pavonia, in which the fungal symbiont, one of the Thelephoraceae,
may live entirely apart from algae, or symbiotically either with the alga,
Chroococcus, or with the alga, Scytonema, the body form differing in each
case. The shape of this lichen varies also with the proportional develop-
ment of the symbionts, the so-called genus Dictyonema representing
a bracket-like form in which the fnngal element dominates, while the
so-called genus Laudatea represents a felt of filaments in which the alga
(i.e. Scytonema) dominates. Botrydina vulgaris, which commonly is
regarded as an alga, is invested by fungal hyphae, and is thought by
some investigators to be a primitive lichen. A lichen such as Cora sug-
gests that the fungal symbionts of other -lichens may once have had
facultative relations with algae, and also that the body form is very
likely to have resulted from the symbiosis.
Green-celled animals. — Among the most remarkable of organisms are certain
green-celled animals (such as Spongilla, Hydra, and Convoluta), which in some
important respects resemble lichens. It has long been believed that the green cells
represent enslaved algae, though, in contrast with the algal symbionts of lichens,
separate cultivation generally is impossible. Indeed, the resemblance to algae is
much less than in lichens, consisting of little more than the presence of chlorophyll ;
usually even the nuclei are absent from the cells, and the chlorophyll may be dis-
seminated through the cell sap instead of being in plastids. Much light has been
thrown on these strange organisms by a careful study of Convoluta roscqffensis, one
of the flat-worms. This animal is colorless when hatched, but in the first few days
it becomes infected by motile algae (appearing to belong near Carteria), which seem
to exhibit prochemotactic reactions to substances in the egg capsules. At first the
animal has a mouth and feeds like other flat-worms, but soon the opening becomes
closed and the worm henceforth is completely dependent upon its symbiotic alga;
even excretory organs are wanting in the adult, it being supposed that the algae
utilize the excreta as a source of nitrogenous food. If the appropriate alga is
absent in a culture of the worms, the animals soon die, even in the continued presence
of such food as they previously have used. After imprisonment the algae lose their
motility, though active cell division takes place for some time. The new cells have
no cell walls, and eventually they become distorted ; finally the nuclei disappear and
all activity ceases. Such modified algae are unable to live apart from the worm,
and the worm cannot live apart from the algae; indeed, the algae of adult worms
cannot infect young worms, so that when they die, they leave no progeny. All
doubt as to the reality of symbiosis has been removed by synthesizing the composite
organism from pure cultures of the alga and the worm, precisely as lichens previously
were synthesized from cultures of algae and fungi. While the exact nutritive inter-
relations are not certainly known, it has been shown that the mature animal with its
green cells is prophototropic, and that starch is manufactured and oxygen given off
804 ECOLOGY
in the sunlight. Since the animal has no other source of food, it clearly is parasitic
on the alga. At first the animal uses the food (probably sugar) manufactured by
the green cells, but ultimately it destroys the cells themselves and brings on thereby
its own destruction. While it is possible that the alga utilizes substances in the
animal (in which event the relation is one of reciprocal parasitism), it is quite as likely
that the relation is to be regarded as a sort of destructive helotism, destructive be-
cause the enslaved organism is weakened and finally destroyed. In another flat-
worm, Convoluta paradoxa, the symbiotic alga is a unicellular brown species.
Here the parasitism of the animal seems less obligate than in C. roscojfensis, since
it continues to use its mouth in taking food, even after the symbiotic algae are well
established in its body. However, the animal dies if it is kept in the dark until
the algae are destroyed. While the origin of such symbiosis is unknown, it may be
noted that in Noctiluca, one of the infusorians, symbiosis with green algae is faculta-
tive rather than obligate, thus suggesting a more primitive condition. Some of the
sea anemones contain algae which are believed to be of nutritive importance to the
animals, since the reactions of the latter to light resemble the reactions of algae
rather than those of such sea anemones as are without chlorophyll. It is 1 kely
that the plants utilize the carbon dioxid given off by the animals, and that the animals
in turn utilize the oxygen given off by the plants.
CHAPTER V — REPRODUCTION AND DISPERSAL
I. REPRODUCTIVE BEHAVIOR IN THE SEEDLESS PLANTS
General remarks. — The process by which organisms give rise to
others of their kind is known as reproduction. The essential element
in reproduction is the organization of a cell or a group of cells, which,
if detached, possesses a capacity for independent development, and hence
may be called offspring. Closely associated with reproduction is dis-
persal, which makes possible the development of organisms in new
territory, and without which reproduction would be of small significance.1
Detachable structures, however produced, if capable of dispersal, may
be called disseminules, and it is obvious that upon such capacity for de-
tachability and for subsequent mobility, the effectiveness of dispersal
and therefore the success of a species must largely depend.
Most plants give rise to many new individuals within their lifetime,
but only a few of these come to maturity and have progeny. The vast
majority of plant disseminules fail to lodge in places suitable for de-
velopment, while of those that make a start, but a very few ever reach
maturity. The preemption of space by other plants, the submergence
of the weaker individuals, and untoward physical conditions cause the
destruction of most plant offspring and prevent the otherwise rapid
advance of any given species over the face of the earth. . Three kinds
of reproduction may be distinguished, each with its characteristic dis-
seminules, namely, vegetative reproduction or propagation, reproduction
by asexually formed spores, and sexual reproduction.
Vegetative reproduction. — General characteristics. — In vegetative
reproduction or propagation, new plants are formed in connection with
the vegetative organs, and the offshoots, sometimes known as propa-
gules, more or less resemble the parts from which they issue. Vegetative
reproduction is associated with periods of activity, while other forms of
reproduction commonly terminate such periods. Vegetative dissemi-
1 Dispersal without reproduction, though a conspicuous feature in animals, is relatively
rare in plants; however, it is illustrated by certain motile algae (as Chlamydomonas and
Volvox, figs. 21-29) ar>d bacteria (figs. 14-20), and also by the amoeboid movements of
myxomycete plasmodia (fig. 3).
805
806 ECOLOGY
nules differ from most other sorts in the relative absence of protective
structures.
Vegetative reproduction in the algae. — The simplest form of repro-
duction is by fission or cell division, and is well illustrated by various
unicellular algae, in which the mature cell divides, producing two or
more new cells, which, whether cohering or becoming detached, repre-
sent new. plant individuals (figs. 858, 4, 34). In most unicellular species
the entire body takes part in propagation, so that the disappearance of
the adult organism necessarily is coincident with the development of its
progeny, though the original cell wall may remain for some time. Simple
fission of this character is the only form of reproduction in unicellular
blue-green algae and in some green algae (as Pleurococcus).
Many algae are filamentous, since division takes place only in parallel
planes, and since the newly formed cells cohere (figs. 859, 6). While
such filaments have a certain individuality, the cells, at least in the
lower algae, are essentially independent, and therefore to be regarded
as potential individuals; hence as in unicellular forms, cell division may
here be called vegetative reproduction, whether or not the new cells
become detached from the old. True filamentous forms differ from
unicellular species in that the adult does not necessarily disappear r s its
vegetative progeny develops. In the higher filamentous algae, cell
coherence becomes a fixed feature, and the individuality of single cells
is less marked, so that commonly a filament as a whole rather than one
of its cells is regarded as a plant. Here the propagules rarely are single
cells, but rather pieces of filaments that become detached. Even with-
out such detachment, however, filamentous algae (such as Spirogyra)
by continued elongation may spread so as to fill a pond in a relatively
short time; branched forms like Cladophora (fig. 63) would seem partic-
ularly suited for rapid propagation of this sort. In many blue-green
algae the filaments break in rather definite places, the limits of the new
filaments, the so-called hormogonia, being determined by cells differing
from the rest, and known as helerocysts (fig. 8).
Vegetative reproduction in bacteria, fungi, and lichens. — Bacteria
reproduce only by fission, yet they increase more rapidly than does any
other group of plants. The possibility of rapid vegetative increase
among the fungi is well illustrated by the growth of molds in moist
chambers, where the hyphae spread quickly in all directions from the
original center (fig. 1078). Often the fungus mycelium dies at its
original growth center, perhaps because of the exhaustion of its food
REPRODUCTION AND DISPERSAL 807
supply, but quite as likely because of the inhibitory effect of its own
excretions. Subsequently the living mycelium forms an obvious ring
of constantly increasing circumference, but whose thickness may not
increase on account of the death of the inner hyphae pari passu with
the advance of those outside. Such rings are common on pots in moist
greenhouses. In some of the agarics, circles of toadstools, which are
known as, fairy rings, arise from subterranean mycelial rings, producing
a striking effect. Sometimes fairy rings recur from a given mycelial
center for many years; in a colony of Hydnum suaveolens that was
observed for nine years, the diameter of the ring increased from seven-
teen to twenty-one meters, whence the age of the colony was estimated
at about forty-five years. Sometimes similar circles of reproductive
bodies, arising from rings of hidden mycelia, are associated with parasitic
fungi (e.g. Puccinia Pyrolae). Lichen thalli commonly spread radially
from the original center of establishment, the advancing edge being
lobate by reason of differential growth (fig. mi); frequently the older
portions die as the newer parts spread out radially, producing living
rings or bands, as in fungi.
Vegetative reproduction in the bryophytes. — Thallose liverworts, such
as Marchantia and Riccia, spread radially from their original growth
center (figs. 742, 743), after the manner of lichens. As the branches
radiate outward and the posterior portions die, a number of individuals
may arise from one by isolation; it has been shown that liverwort frag-
ments which are only two millimeters in diameter can develop into a plant.
Essentially similar is the propagation of foliose liverworts and of creeping
mosses. Mosses increase vegetatively to a notable extent through the
activity of their prolonema, which is composed of branched alga-like
filaments that creep along the soil surface much as do rhizomes. The
protonemal filaments bear buds that grow into leafy plants, so that the
area occupied by moss colonies is subject to constant radial extension.
Mosses possess a wonderful capacity for propagation, almost any part
of the leaves or stems (either gametophyte or sporophyte) being capable
of giving rise to protonemal filaments under suitable conditions.
Vegetative reproduction in the vascular plants is of great significance, but has
been considered in connection with roots and leaves, and particularly in connection
with stems (p. 667).
Gemmation. — There is another kind of reproduction which generally
is regarded as vegetative, although it grades insensibly into reproduction
8o8
ECOLOGY
1118 1119
Fl6s. 1118, 1119. — Liver-
wort gemmae: 1118, a thallus
lobe" of Lunularia vuigaris,
bearing a crescentic cupule in
which are numerous gemmae;
1119, a single gemma from a
cupule of Marchantia polymor-
pha ; considerably magnified.
by asexual spores. Gemmation may be defined as the organization of
vegetative buds that readily become detached from the parent plant;
such disseminules are called gemmae. The
simplest gemmae are unicellular, and they
differ from asexual spores chiefly in the
absence of a protective wall and of the
resting period usually associated therewith,
although there exist all intergradations be-
tween the two. Simple gemmae of this char-
acter are found in yeast (figs. 168-173), and
also in Mucor and in Vaucheria geminala.
Some species of liverworts also have uni-
cellular gemmae on the leaf margins; in
Aneura the gemmae are two-celled, and in
Marchantia and Lunularia they are multicellular and are borne in clus-
ters in special cupules (figs. 1118, 1119).
Multicellular gemmae occur also on various mosses (as Georgia pellucid i), on
fern prothallia, and on some algae (as Sphacelaria and Chara). The soredia of
lichens (figs. 1114—1116) also may be classed with gemmae. Certain structures in
the vascular plants, such as the gemmae of Lycopodium, the leaf bulbils of ferns,
the stem bulbils of lilies, and the inflorescence bulbils of the onion (p. 902) are com-
parable to the gemmae of the lower plants.
Sclerotia. — In autumn the mycelium of the ergot fungus (Claviceps) becomes
enveloped in a dense and relatively impermeable protective laye'r of dark, thick-
walled cells, within which the vegetative hyphae remain
dormant over winter ; the entire structure, which is richly
packed with food, is called a sclerotium (Sg. 1120). In
spring the sclerotium germinates, and ordinary vegetative
activity is resumed. Somewhat similar to the sclerotia of
ergot are those of Peziza sclerotiorum and of various other
fungi. Many fungi (e.g. Sclerotinia) have subterranean
tuber-like sclerotia richly packed with food, which endure
through unfavorable periods, and other forms have tough
sclerotial strands resembling shoestrings. In the myxomy-
cetes the plasmodium or a part of it may become encysted
into a sclerotial mass and remain dormant even for years.
While strictly vegetative tissue is involved in the formation
of sclerotia, they agree with spores and seeds in being formed
at the close of vegetative periods and in being fitted for
existence in a dormant state during severe periods. Com-
parable to sclerotia are the resting cells of bacteria, the thick-
walled resting cells of Nostoc which are closely packed with food, the starchy tubers
of Chara, and the subterranean resting buds of liverworts and mosses (fig. 251).
FIG.' ii 20. — A
sclerotium (s) of the
ergot fungus (Clavi-
ceps purpurea), grow-
ing from a spikelet of
the sand-reed (Am-
mo phtia arenaria).
REPRODUCTION AND DISPERSAL Soq
Auxospores. — In the diatoms vegetative reproduction takes place by longitudinal
splitting, but each new individual often is shorter than the last, because it is formed
within the old and rigid silicious wall. In time progressive diminution ceases, and
the protoplast escapes, whereupon it enlarges to the original size and again becomes
incased by rigid silicious walls. The enlarging protoplast is called an auxospore.
The significance of vegetative reproduction. — Vegetative propagation
is the most universal kind of reproduction. Some plants (as the bac-
teria and lower algae) have no other kind, while very few plants (e.g.
some annuals, biennials, and trees) are altogether without it. Many
plants that are capable of producing spores or sex organs nevertheless
spread almost wholly by vegetative means; among such -plants are many
mosses, some liverworts (as Lunularia), and even some of the higher
plants (as the duckweeds). In far northern regions many plants are
said to reproduce as a rule only vegetatively, the summer being too short
for seed production. Even those plants that fruit regularly usually
spread much more by vegetative propagation than by spores or seeds.
Thus there can be no doubt that vegetative reproduction is the chief
factor in the maintenance of species and in the enlargement of their areas.
The chief disadvantage associated with vegetative reproduction is that
propagules rarely are fitted for distant dispersal. Hence the invasion
of new areas by this means alone is slow; the ultimate establishment of
a species by vegetative reproduction in a distant region is even im-
possible unless favorable habitats are continuous, since propagules
rarely are able to cross barriers. For example, the rhizomes of meso-
phytes (such as the Solomon's seal and various ferns) are unable to
migrate over bodies of water or dry ridges, although such migration may
be accomplished quickly by most spores and by many seeds. Some-
times, however, propagules are true disseminules, notably among the
water plants, where portions of plants may become detached and float
for great distances, thus equaling seeds in mobility and in the wide-
spread invasion of new regions. Among land plants the distant dispersal
of vegetative disseminules is confined chiefly to gemmae; the minute
gemmae of various liverworts may be scattered for some distance by
wind, and lichen soredia are scattered almost as effectively as are spores.
Reproduction by asexually formed spores. — General characteristics. —
A sexually formed spores l commonly are unicellular structures (multinu-
cleate in Vaucheria}, produced, as a rule, by specialized spore-bearing
organs (sporangia, etc.; see Part I). Generally they differ from gemmae
1 Often these are called for convenience asexual spores or simply spores.
8io
ECOLOGY
in their unicellular nature, in their production by specialized organs, and
in their capacity for endurance, which often is increased by the presence
of thick protective walls; however, hard and fast lines are not to be
drawn, since some spores are incapable of enduring severe periods, while
gemmae may be unicellular or borne by special organs.
Reproductive structures generally have one or more of three char-
acteristics: capacity for increasing the number of individuals in a species
(which is, of course, the primary feature of reproduction) ; capacity for
endurance through severe periods; and capacity for dispersal. Asexual
spores are efficient in all three respects, thus contrasting with propagules,
which have been seen to be relatively ineffective as disseminules and
often unfitted for endurance, though they are the most efficient of all
means of multiplying individuals. Asexual spores occur in nearly all
plant groups, though they are unknown in various algae (as in the Con-
jugales, Fucales, and Charales), and are practically absent in some
higher plants (as in various mosses and in the duckweeds).
Asexual spores in the algae. — The most representative asexual sp ores
among the algae are the zoospores or swarm-spores, which differ J rom
most spores in being without protective walls, and
whose chief distinguishing character is the power
of locomotion in water ; usually they move by
means of variously arranged cilia, which may be
single (as in Boirydium, fig. 92), two (as in Hy-
drodictyon, fig. 1121), four (as in Ulothrix, fig.
1133), or many (as in Oedogonium and Vawhe'ria,
figs. 76, 96). While most characteristic of green
algae, swarm-spores occur in some of the brown
algae (as in Ectocarpus, fig. 121). Zoospores are
among the most efficient of reproductive struc-
tures, partly because commonly they are produced
in large numbers, but particularly because they
differ from almost all other disseminules in ex-
hibiting directive dispersal. For example, they are
prophototactic, hence they usually move to a well-
lighted situation where they may germinate into
new plants under favorable conditions. The lack of protective walls is
hardly a disadvantage, since zoospores are not exposed to transpiration,
nor are they obliged to live over unfavorable seasons. In the red algae
true zoospores are wanting, though a few species have spores that exhibit
FIG. iiai. — A zoo-
spore or swarm spore
of the water-net (Hy-
drodictyon) ; note the
two cilia which effect
locomotion; highly
magnified. — After
TlMBERLAKE.
REPRODUCTION AND DISPERSAL
811
amoeboid movements; much more characteristic are non-motile car-
pospores and tetraspores (figs. 150, 151), which, like zoospores, are
devoid of protective walls. Non-motile spores may occur also in the
green algae (as in the aplanospores of Botrydium, fig. 93).
Asexual spores in the fungi. — Perhaps the culmination of asexual
spore development occurs in the fungi. A few forms that grow in water
or in wet places have ciliated zoospores (as in Saprolegnia, fig. 156);
in certain myxomycetes there are zoospores which swim for a time, and
then lose their cilia and creep with an amoeboid movement. In general,
however, fungus spores are not self-motile, and are invested with con-
spicuous walls. They may be borne within a
special spore-bearing organ, for example, a spo-
rangium (as in Mi4cor, fig. 1122), or an ascocarp
(as in Peziza, figs. 175, 176), or they may be
developed externally, as in the conidia of Peni-
cittium (fig. 179) and in basidios pores l (fig.
201).
Fungus spores commonly are dispersed by
wind, and their minute size and their resistance
to wetting make possible the remarkable effi-
ciency of this agent; even the very slightest
movements of the air are sufficient to initiate
dispersal. Many species of fungi are common
to widely separated regions, and it is thought
that this cosmopolitanism is due in large part to
the effectiveness with which their spores are dis-
persed by wind. The abundance of spores and the ease with which they
are carried is shown by the readiness with which cultures of various
fungi may be made anywhere by exposing to the air, bread or cheese,
properly moistened, so as to insure good conditions for germination.
Fungi surpass all other plants in the number of new individuals that
may be produced from a single plant by asexual spores. A single large
puffball (as Lycoperdon giganteum) may produce several trillion spores,
and in other large fungi their number may run well into the billions.
The production of spores in such great numbers is advantageous, since
generally only a single spore in many millions falls in a place where it
can develop into a plant.
1 Basidiospores, however, though actually external, in the mushrooms are considerably
protected by the fruit body on which they develop.
FIG. 1 122. — The spo-
rangiumof a mold (Mucor\
showing the columella (c)
and numerous spores (5);
highly magnified. — From
COULTER (Part I).
8l2
ECOLOGY
Various features of structure or habit facilitate spore dispersal. As
previously noted, many fungi bear spores externally, so that they are
readily blown away as soon as they are abstricted. In the fleshy fungi,
the spore-containing organs often are borne on conspicuous apogeo-
tropic stipes, which thus elevate the spores into a good position for wind
dispersal (figs. 1078, 2, 197). In many of these forms the spores are dis-
charged from the gills, after which they drop into positions where they
may be wafted off by air currents. In Coprinus (figs. 198, 199), which
has a cylindrical fruit body, the spores mature first in the lower part,
which then curves outward, and hence does not hinder the dispersal
of those which ripen later. Where spores are borne within sporangia
or similar organs, there often are no special features which facil tate
spore removal, it being necessary for the enveloping organs to rot away
before the spores
can be dispersed.
In some cases 1 here
is definite di'his-
cenc'e, as in Gea sler,
where the spcran-
gial wall (perid'uni)
has two layers, of
which the outer
splits into star-
shaped segments
(whence the com-
mon name, earth
star), while the in-
ner has an apical
opening (fig. 1124); in the related puffballs the outer layer breaks
irregularly. In Geaster the hyphae are arranged at right angles to the
surface in the inner (i.e. upper when open) part of the ray and parallel
to the surface in the outer part. Hence in moist weather the inne - parr
absorbs the more water and the rays open (fig. 1 124), while they else in
dry weather (fig. 1123), since the inner part loses the more water. This
hygroscopic mechanism has been thought to facilitate spore dispersal ;
the dry closed structure is bowled along by the wind like a tumbleweed,
and the rain washes out spores from the opened structure.
In a few fungi, spores are scattered by agents other than wind. In Pilobolm
(fig. 630) the columella of the sporangium ultimately bursts by reason of increasing
1123
FIGS. 1123, 1124. — An earth star (Geaster hygrometrtcus);
1123, a fructification, as seen in dry weather, the peridium
rays (r) being incurved about the spore-bearing portion;
1124, a fructification, as seen in moist weather, the peridium
rays being expanded; note the aperture (a) through which
the spores escape.
REPRODUCTION AND DISPERSAL
8*3
turgor, whereupon the escaping water tears loose the sporangium and expels it with
the enclosed spores for some distance. In a somewhat similar fashion are expelled
the conidia of Entomophthora and the ascospores of Ascobolus and of Peziza repanda.
In the ergot fungus (Claviceps) a sweetish substance, known as honey dew. is se-
creted as the conidia ripen, and insects visiting the fungus for the honey dew scatter
the'spores. In the stinkhorn fungus (Phallus impudicus) the spore-bearing portion
deliquesces into a vile-smelling
mass that attracts flies, which
scatter the spores. Doubtless
many fungus spores also ad-
here to the slimy surface of
slugs and thereby are scattered.
Flies are among the most effi-
cient scatterers of spores, which
become attached to various
parts of the body, and occur
abundantly in the excreta ; the
spores or propagules of more
than fifty species of fungi and
bacteria have been found in
a single fly speck.
Many fungus spores are able
to endure severe conditions.
For example, the spores of
Mucor and of Aspergillus have
been dried for two years, after
which they were exposed for
three weeks to a temperature
of — 180° C., and for three days
to — 253° C., without impair-
ing their capacity for germi-
nation. Desiccated bacteria
have been known to retain
their vitality for nearly a hun- FIGS. 1125-1127. — Spores of the wheat rust (Puc-
dred years. It is concluded cima graminis): 1125, uredospores; 1126, young
from such experiments that all teleutospores; 1127, mature teleutospores; note that
vital activity mav be suspended the uredospores are one-celled, and the teleutospores
for long periods of time (p. QOO). tw°-celled: h'ghly magnified; 1125, 1127 from
... , . COULTER; 1126 from CHAMBERLAIN; (Part I).
In part this endurance is due
to unexplained features in the resting protoplasm, but there are also many instances
of protective structures or habits. In most ascomycetes the spores, though thin-
walled, are protected within the ascocarps (as in lichens and mildews, figs. 181, 182),
while in many hymenomycetes the thin-walled basidiospores are protected by the
piieus ; some of the so-called bracket fungi are hard and woody and capable of
enduring the winter. In the heteroecious rusts there are borne in spring and sum-
mer basidiospores (fig. 194), aecidiospores * (fig. 196), and uredospores (fig. 1125),
1 Sometimes aecidiospores and uredospores are regarded as sexually formed spores.
8i4 ECOLOGY
all with relatively thin cell walls, while toward the close of the season there are
developed teleutospore* (figs. 1126, 1127), which are thick-walled and are capable
of enduring the winter.
Asexual spores in the bryophytes. — In most liverworts and mosses
there is a well-defined alternation of generations (p. 822), spores being
characteristic of one generation, the sporophyte, and sex organs being
equally characteristic of another generation, the gametophyle. The
spores are scattered chiefly by the wind, their minute size and the
generally stalked and therefore elevated capsules facilitating such dis-
persal (figs. 977, 231, 254). The spores contain chlorophyll, so that
independence is possible from the outset, if the sporelings (i.e. the ger-
minating spores) are exposed to light. Sometimes (as in Riccia and
Phascum) the spores are exposed to dispersing agents only upon the
decay of the capsule wall, but more commonly there is definite dehis-
cence. In the Jungermanniales and in Anthoceros the capsule wall
splits into valves (figs. 235, 239), and in the Marchantiales and in most
mosses there is a lid or operculum (fig. 250). Most moss capsules are
fringed toward the tip with a peristome (figs. 263, 264), whose hygro-
scopic teeth open when the weather is dry and close when it is moist;
these movements effect the detachment of the operculum, and probably
are of some value in facilitating the removal of spores from the capsule.
In most liverworts long, fiber-like, spirally thickened bodies, known as
elaters (fig. 230), occur among the spores, and, like the peristome teeth
of mosses, they exhibit hygroscopic movements which are thought to
facilitate spore removal. As a rule, the spores of liverworts soon lose
their capacity for germination, but the spores of mosses may retain their
vitality for a long time; cases are on record, where moss spores have
germinated, after having lain dry in a herbarium for fifty years.
Asexual spores in the ptendophytes. — In the Filicales the spores com-
monly are borne in sporangia on the backs of ordinary foliage leaves
(figs. 1128, 1129), but in some cases (as in Onoclea and Osm.mda)
special leaf regions or entire leaves are spore-bearing, while other leaf
regions or entire leaves are foliage organs; comparable to the latter are
the Ophioglossales (figs. 352-354). In Equisetum the sporangia are
borne on a special structure, the strobilus (fig. 332), and, as in liverworts,
there are elaters (figs. 337, 338) which assist to some extent in dispersal.
In Lycopodium the sporangia may be arranged in the axils of foliage
leaves (fig. 265) or in a strobilus (fig. 266). In the above ptendophytes
all the spores are alike, that is, homosporous, but in the water ferns
REPRODUCTION AND DISPERSAL
(Le. Marsilea, Salvinia, and Azolla) and in Selaginella and Isoetes there
are two kinds of spores, namely, small spores or microspores, and large
spores or megaspores; such a condition is known as heterospory (fig. 303).
Upon germination the microspores give rise to male plants and the
FIGS. 1128, 1129. — Reproduction by asexual spores in a fern (Aspidium): 1128, a
leaf segment (pinnule) with fruit dots (sori), each with a shield-shaped cover (indusium);
1 129, a cross section through a sorus, showing the indusium (z) and long-stalked sporangia
(s) ; 1 129 considerably magnified. — After WOSSIDLO.
•»
megaspores to female plants, whereas the spores of most homosporous
ferns give rise to plants that bear both male and female organs.
The spores of most pteridophytes are scattered by the wind, and they
are well fitted for such dispersal by their small size, by their resistance
to wetting (particularly in Lycopodium), and by their elevation upon
foliage leaves or special stalks (figs. 266, 332, 353). Fern sporangia
dehisce in a somewhat complicated manner (p. 351), a ring of dead
FIGS. 1130-1132. — Dehiscence of a sporangium in a fern (Polystichumacrostichoides):
1130, the sporangium cracked; a, the annulus; 1131, position of reversal, exposing the
spores; 1132, position after recoil, the sporangium having been emptied; highly magnified.
— After ATKINSON.
tissue, the annulus, springing back and releasing the spores when a
certain stage of desiccation is reached (figs. 1130-1132). Probably no
816 ECOLOGY
other vascular plants equal homosporous pteridophytes in their capacity
for dispersal; the great wealth of ferns on oceanic islands commonly is
explained by the easy dissemination of their spores by wind.
In Salvinia there is no true dehiscence, the whole sporangium being shed and the
spores germinating within. In Azolla the sporangial wall slowly decays, setting free
the spores. In Marsilea the spores are contained within a hard-walled structure,
the sporocarp; when moistened the internal mucilaginous tissue absorbs water and
it swells to such an extent as to burst the sporocarp wall and protrude into the water,
carrying with it the attached sporangial masses (fig. 411). In heterosporous pteri-
dophytes the microspores have the mobility characteristic of the spores of homo-
sporous forms, but the megaspores are much less mobile; indeed, in some species
of Selaginella mobility is entirely absent, and the megaspore no longer is a dis-
seminule (fig. 308). With regard to protection and endurance, spores may vary
from the relatively delicate chlorophyll-containing spores of Equisetum, which die
unless germination occurs at once, to the remarkably protected spores of Selagirella
(fig. 303), which commonly germinate only after a long resting period. Remark ible
resistance to severe conditions is shown by the spores of Marsilea, which have
been known to germinate when sporocarps that had been kept dry for eighteen y -ars
were placed in waier; much of this capacity for endurance is due to the impemea-
bility of the sporocarp wall, as is shown by the fact that the spores in sporocarps that
have been kept in alcohol for three years may still remain capable of germinal ion.
In the seed plants there is an extremely complicated situation (p. 256 ff.). Het-
erospory is there universal, and the microspores (better known as pollen grains)
are scattered by various agents. The megaspores, however, always are retained,
having no longer the character of disseminules. The ecological features of these
organs will be considered in connection with flowers (p. 825 ff.).
Sexual reproduction. — Significant features. — The chief feature of
sexual reproduction is the union or fusion of two cells, known as gametes,
resulting in the production of a sexually formed spore. Usually the two
gametes may be distinguished as male cells or sperms, and female cells
or eggs. The spore resulting from fusion, upon germination, deve ops
into a structure called the embryo.
Isogamy. — In those thallophytes in which sexuality seems to be just
beginning (e.g. Ulothrix, fags. 1133, 1134), the two gametes are sirrilar
in size and in structure and usually in activity; such a condition is called
isogamy; the spore resulting from the fusion of equal gametes is
called a zygospore (figs. 49, 50). Isogamous gametes may be ciliated
and actively motile (as in Ulothnx), non-ciliated but somewhat motile (as
in the diatoms), or almost inrnotile, that is, not leaving the plant body
(as in Spirogyra, fig. 109). Although isogamous gametes exhibit no struc-
tural differences, there is some evidence of unlikeness; for example, in
Ulothrix and in Acetabularia, fusion takes place only between gametes
REPRODUCTION AND DISPERSAL
817
from different gamete-producing organs (ga nictang ia) , and in Dasy-
cladus, only between gametes from different plants, though it is impossible
in any of these to distinguish male and female
characters. However, in the Conjugates, one of
the gametes often is immotile, while the other
migrates from a neighboring filament through a
passageway made by the fusion (or conjugation)
of two lateral outgrowths (figs. 107-109). From
analogy with the higher plants, the immotile
gamete may be called female and the motile
gamete, male. In some conjugating forms, as
Afucor, there is no such distinction, the two
gametes moving equally and meeting in the
passageway between the filaments (figs. 163-
166). Usually zygospores are thick- walled rest-
ing cells closely packed with food and well able
to exist over severe periods (figs. 50, no, 166).
Heterogamy. — In the great majority of plants,
including many thallophytes and all the higher
plants, the two gametes are unequal; this con-
dition is known as heterogamy, and the spore re-
sulting from the fusion of unequal gametes is
called an oospore. It is in the heterogamous
plants that one may speak of true sex differenti-
ation and of the development of male gametes or
sperms and of female gametes or eggs (fig. 1135).
In nearly all bryophytes, pteridophytes, and
heterogamous algae the sperms are relatively
small, ciliated, actively motile bodies (figs. 28,
119, 320, 349, 415), whereas in the seed plants
(except in Ginkgo and in the cycads, fig. 455),
they are non-ciliated, and exhibit but little true
locomotion (fig. 479). Eggs commonly are much
larger than sperms, and, except in the case of a
few algae where they float freely in the water,
they are essentially immotile (figs. 31, 77, 481).
Often the male and female gametes are borne in special organs, the
antheridia and the oogonia (or archegonia), respectively. In many
thallophytes the oospore is a thick- walled resting cell (fig. 70).
FIGS. 1133, 1134. —
Zoospores and isoga-
mous gametes in Ulo-
thrix : 1133, a part of a
filament from which a
4-ciliate zoospore and
several smaller biciliate
gametes are escaping ;
1134, gametes pairing
and fusing ; highly mag-
nified.— From COULTER.
1135 r
FIG. 1135. — Heterog-
amy; an egg of Fucus,
surrounded by a swarm
of biciliate sperms;
highly magnified. —
After THURET.
8i8 ECOLOGY
Of chief ecological interest in connection with heterogamy are the
factors concerned in facilitating the fusion of gametes and particularly
the fusion of gametes of different immediate ancestry. In the algae as a
group, fusion is comparatively easy, since the eggs are immersed and the
sperms capable of locomotion. In the bryophytes and pteridophytes the
difficulty is greater, because usually they are land plants. In the liver-
worts and ferns the gametophytic generation commonly grows close to
the moist soil, where there often is sufficient moisture for the swimming
of such minute bodies as sperms. In many cases the female organs con-
tain substances which occasion prochemotactic reactions in sperms, thus
greatly facilitating the fusion of gametes; among such substances, are
cane sugar (as in mosses) and malic acid (as in ferns).
Monoecism and dioecism. — When the same individual bears correl-
ative kinds of reproductive organs (e.g. antheridia and archegoni i, or
stamens and pistils), the species is called monoecious if the organs are
borne on separate branches, and hermaphroditic if the organs are t orne
together on a common branch ; if the two kinds of organs are borr e on
separate individuals, the species is called dioecious. In the het pros-
perous pteridophytes dioecism occurs regularly, male game topi lytes
developing from microspores and female gametophytes from mega-
spores. In most homosporous pteridophytes the gametophytes are
monoecious, but they are chiefly dioecious in Equisetum, and there are
many dioecious species among the algae and bryophytes. Obviously
the movement of sperms to the female organs is easier in monoecious
than in dioecious species. It has been supposed that the chief advantage
of dioecism is that it prevents close inbreeding (i.e. fusion between closely
related sex cells), it being believed oftentimes that certain advantages
are associated with the fusion of gametes of different immediate ancestry
(see p. 820). In the homosporous pteridophytes the fusion of related
gametes often is impossible, since many species are dichogamous, that
is, with the correlative organs on the same individual maturing con-
secutively; commonly the male organs develop first.
In some dioecious species there are features that facilitate the germinal on of
male and female plants in close proximity; for example, the elaters of Equisetum
(figs. 337, 338) often cause a group of spores to become intertangled and thus to
fall and germinate together, and in Azolla the microspores cohere in masses and
often have hooks, the so-called glochidia, which become caught in the projecting
filaments of the megaspores (fig. 403). However, in many cases dioecism doubtless
is disadvantageous because of the difficulties in the way of fusion between male and
female gametes. In many mosses sporophytic generations rarely are seen, partly,
REPRODUCTION AND DISPERSAL 819
perhaps, because of dioecism and partly, it may be supposed, because the elevated
female organs make more uncertain the presence of sufficient water for sperm mo-
tility. In hermaphroditic mosses dichogamy is rare, hence close inbreeding is the
rule rather than the exception.
The advantages and disadvantages of heterospory. — In heterosporous
pteridophytes the proximity of male and female gametes is a matter of
uncertainty, since it depends upon the chance of microspores and meg-
aspores lodging near one another. In most living species the difficulty,
perhaps, is slight, since the plants are so small that often the spores must
fall near the parent plant and hence near each other; furthermore,
a number of the species are hydrophytes, and hence the motile sperms
have a favorable medium. In past ages, however, there have been many
heterosporous trees among the pteridophytes, and the waste of both
microspores and megaspores must have been enormous. This is the
only known ecological group of past ages that is unrepresented among
living forms, and it well may be that its disappearance was due in part,
at least, to its disadvantageous heterospory, coupled, perhaps, with
extensive land emergence and with the- consequent lessening of habitats
favorable for the fusion of gametes. In contrast with this extinct group
are the seed plants, whose greater success probably is due in part to the
retention of the megaspores instead of their dispersal, with the enormous
consequent waste; the waste even of microspores is reduced largely in
the great group of insect-pollinated plants. However, the seed plants
do not equal the homosporous ferns and the lower plants in ease of
dispersal, as appears from the fact that the homosporous constituents of
any two widely separated floras are much more alike than are their
heterosporous constituents. Obviously, the great advantage of heteros-
pory, if such there be, must be sought along other lines, even in the
seed plants.
The significance of sexual reproduction. — In the simplest cases (as in
Ulothrix) the result of the fusion of gametes is a decrease of potential
individuals, since two cells resembling zoospores and having, perhaps,
the possibility of growing into two plants unite and form a spore that can
grow into but one plant. However, since a single algal filament may
produce a number of gametes, considerable multiplication is possible
through sexuality; this is conspicuous especially in those groups which
are without asexual spores (viz. the Conjugates, Charales, and Fucales).
But in plants with a well-defined alternation of generations (viz. in
bryophytes, ferns, and seed plants) sexual reproduction rarely results
820 ECOLOGY
in a significant increase of individuals, asexual spores or propagules
chiefly being responsible for such increase.1 Thus multiplication, which
is the feature of chief significance in other forms of reproduction, usually
is not conspicuous in sexual reproduction. Gametes and the spores
resulting from their fusion (except in the thallophytes) are among the
most delicate of plant structures, so that fitness for endurance through
severe periods is not one of their characteristics, as it is of many asexual
spores. Furthermore, neither the gametes nor the resulting spores are
particularly efficient disseminules; the female gamete, in particular,
whose position determines the place of the next generation, is for the
most part immotile. Hence none of the three features commonly as-
sociated with reproduction, namely, multiplication, endurance, and
dispersal, are of especial significance in sexual reproduction.
It is believed commonly that sexual reproduction makes possible the
advantageous merging in one individual of the qualities of two races,
hence sometimes the phenomenon is known as amphimixis. In Ulo-
thrix the advantage gained has been thought to be one of size, since plants
developing from zygospores are larger than those which develop from
gametes that fail to fuse. In other cases (apart, perhaps, from seed
plants, p. 866) the advantages of sexuality appear more hypothetical
than real, but even hypothetically the crossing of two races might fairly
be expected to introduce into a given strain disadvantages as well as
advantages. The chief reason for believing that sexuality is of no partic-
ular advantage (at least in the lower plants) is that its absence seems
to bring no disadvantage. In the bacteria and blue-green algae, in some
green algae, and in many fungi, true sexuality is absent, but no plants
are more successful than these; in many fungi (as in Saprolegnia and in
the Ascomycetes) , and in some liverworts and mosses there is excellent
evidence of diminishing sexuality (see p. 883), but none of diminishing
success.2 Even in the higher plants, where sexuality is much more
1 Such increase as there is among these plants is most conspicuous among th< bryo-
phytes, where single gametophytes may bear several (rarely many) sporophytes. Some-
times (especially in gymnospcrms) two or more embryos develop from one sexual s x>re or
from one sporiferous center of a gametophyte, a phenomenon known as polyemlryony ;
this is of little significance, however, inasmuch as but one embryo, as a rule, is able to
mature.
2 Somewhat recently there have been discovered modified forms of sexuality in the rusts
and smuts and in various .other fungi, but in these cases there is no crossing, so that
true amphimixis with its supposed advantages necessarily is excluded. In many algae,
there occurs inbreeding or automixis, which is well illustrated in Spirogyra, where
fusion may take place between gametes of adjoining cells in the same filament, and in
REPRODUCTION AND DISPERSAL 821
general, no demonstrable loss comes from its elimination in those cases
where vegetative reproduction is well-developed, and such cases con-
stitute the vast majority. In various seed plants (as the duckweeds)
sexual reproduction rarely occurs, and many economic plants (such as
the banana, the fig, and the sweet potato) have been propagated almost
from time immemorial solely by vegetative means and yet without obvious
deterioration. A second theory claims that sexuality insures rejuvenes-
cence. However, this is insured much more generally and economically
through propagation and asexual reproduction. A third theory is that
sexuality favors variation and therefore evolution through the repeated
mingling of new elements, thus giving rise to new combinations of char-
acters and hence to new species. This view seems reasonable, but there
is little positive evidence in its favor. Furthermore, variation is known
to be of frequent occurrence in the bacteria and blue-green algae and in
other sexless groups; indeed, many investigators hold that crossing
promotes fixity rather than variation, and it has been shown that in
inbred races of Spirogyra and Phaseolus the amount of variation is as
great as or greater than in cross-bred races. Sometimes the theory is
advanced that the significance of sexuality lies in the fusion of kinetic
and trophic (i.e. nutritive) elements ; the egg is regarded as having the
food necessary for development, while the sperm adds the requisite
developmental stimulus. These kinetic and trophic roles are not to be
doubted, but they furnish no clew to the significance of sexuality, giving
rather an explanation of embryo development. In propagules and in
asexual spores both kinetic and trophic elements are present in sufficient
degree to insure development, so that in these respects sexuality adds
nothing new. At present no theory as to the role of sexuality has much
support. It is not impossible that it is a necessary accompaniment of
evolution but without particular significance, although in the entire
plant kingdom there probably is no other equally widespread phenomenon
which is without conspicuous advantage. The most that can be said
with certainty concerning the advantage of sexual reproduction among
the lower plants is that it supplements the other and more successful
kinds.1 As to the plants above the thallophytes, there remains to be
considered the alternation of generations.
Viva, where fusion may take place between sister gametes arising from a common cell.
In some ferns (as Lastrea) and in various fungi, the fusing structures are thought to be
vegetative rather than sexual ; in contrast to amphimixis and automixis such fusion
has been termed pseudomixis.
1 In Paramoecium, one of the infusorians, individual animals reproduce ordinarily by
822 ECOLOGY
The significance of alternating generations. — The chief advantage
in the alternation of generations has been supposed to be that one genera-
tion, the sporophyte, produces asexual spores in great abundance, thus
facilitating multiplication and dispersal, while the other generation, the
gametophyte, produces gametes, thus facilitating the merging of char-
acters of different individuals. The advantages of the sporophytic
generation are obvious enough, but those of the gametophytic generation
are less apparent, depending solely upon such advantages as may inhere
in sexuality. There are some obvious disadvantages in alternation;
for example, in certain mosses, as previously noted, the conditions for
the fusion of gametes often are lacking, hence development is impossible
for the sporophyte with its numerous asexual spores, however well-
fitted they may be for multiplication and dispersal. In the ferns, where
considerable moisture is required by the gametophytic generatior and
particularly for the fusion of gametes, the sporophytic generation, which
often is well-suited for xerophytic situations, can grow only wh;re a
gametophytic generation has preceded it.1 In the seed plants, the
alternation of generations means that seed formation, in addition to
favorable conditions in the soil and the climate, depends upon polli-
nation, and therefore upon various pollinating agents, such as wind
and insects.
Apogamy and apospory. — In some cases one of the alternating gener-
ations in whole or in part is eliminated; if the eliminated structure or
process is gametophytic, the phenomenon is called apogamy; if sporo-
phytic, it is called apospory. In Pteris cretica, Nephrodium molle, and
in many other ferns the sporophyte may develop from a bud on the
simple fission. After a time, individuals of one line of ancestry conjugate with those of
another, and there appears to be an exchange of substance between the individuals, which
later separate and again reproduce by fission. Since cultures of Paramoecium in which
conjugation does not take place show a gradual decrease in size and activity after many
generations, it has been urged that sexuality thus is shown to be advantageous, at L-ast in
animals. Indeed, in the older experiments, cultures in which conjugation was pn vented
could not be maintained for more than 140 generations (i.e. about three months), al hough
parallel cultures with conjugation remained vigorous indefinitely. More recently it has
been shown that it is not lack of conjugation which causes death, but probably some
deterioration in the culture media, since by varying the media from time to time, 1500
generations have been secured without conjugation and with no loss of vigor. It is now
believed that cultures of Paramoecium thus can be kept indefinitely without conjugation,
and it is to be noted that the changes introduced in the culture media probably arc much
less than are those which occur in natural habitats.
1 Of course such a plant as Pteris may migrate from its place of origin by rhizome
propagation.
REPRODUCTION AND DISPERSAL 823
gametophyte; here gametophytes but not gametes appear to be necessary
for sporophyte development. In Coelebogyne (one of the seed plants)
certain cells of the sporophyte nucellus are able to develop into embryos,
the entire gametophytic generation thus being unnecessary for seed
production. Apogamy is now recognized in a number of seed plants,
as in Euphorbia, Allium, Elatostema, and Balanophora; in Coelebogyne,
in Balanophora, and in Euphorbia dulcis there is no necessity for
sexual fusion; in Elatostema there is no egg, and in Balanophora
globosa even the staminate flowers are wanting. Apospory is illustrated
in certain ferns, in which the gametophytic generation may develop
from sporangia (as in Asplenium) or even from vegetative parts of the
leaf (as in Polystichum). There are some varieties of ferns and of seed
plants which exhibit both apogamy and apospory, their reproduction
being wholly vegetative. The chief advantages of apogamy arid of
apospory would appear to be that they eliminate the disadvantages of
alternating generations.
Parthenogenesis. — The development of a gamete into a plant without
fusing with another gamete is known as parthenogenesis. The egg is
much more likely to develop parthenogenetically than is the sperm, prob-
ably because of its greater size and more abundant food supply.1 In
such forms as Ulothrix, where a gamete almost indifferently either may
fuse with another gamete or develop independently, or in Viva,, where
small gametes commonly fuse and certain of the larger gametes usually
develop without fusion, parthenogenesis probably represents the retention
of a primitive character; perhaps the same is true in Zygnema (fig. 112).
The significance is quite otherwise in Saprolegnia, where parthenogenesis
is accompanied by all stages in the abortion of the male organs, from
almost complete development to the entire absence of the antheridium.
Other plants in which parthenogenesis has been reported are Chara
crinita, Marsilea, Thalictrum, Alchemilla, Wikstroemia, Hieracium,
Antennaria, and Taraxacum, the last six being seed plants, and the
final three being Compositae. In these cases also parthenogenesis
undoubtedly involves the loss of a character formerly present; in Wik-
stroemia and Hieracium the pollen often is imperfectly formed or im-
potent. Parthenogenesis occurs in many animals, as in rotifers and
in a number of insects and crustaceans. Like apogamy, partheno-
1 However, male parthenogenesis has been reported in the brown alga, Ectocarpus
silicuiosus, though here the sperms are relatively large and the plants into which they
develop relatively small.
824 ECOLOGY
genesis is advantageous in that it eliminates the disadvantages of alter-
nating generations. The elimination of sexual fusion, though often
regarded as a sign of degeneracy, is quite as likely to be a sign of pro-
gressive evolution. Furthermore, the theory which holds that sexuality
leads to variability has little support from the facts of parthenogenesis,
since no plant genera are more variable than are Taraxacum and
Hieracium.
Concluding remarks. — So far, at any rate, as the seedless plants
are concerned, the significance of sexual reproduction is in doubt, as
has been indicated in the preceding paragraphs. The obvious advan-
tages appear to be subsidiary, and not at all commensurate with the
amount of energy and material that is involved. The appearance of
dioecism, together with that of alternating generations and of he.eros-
pory, multiplies disadvantages and introduces no conspicuous < orre-
sponding advantages, unless it should be discovered that amphimixis
is inherently advantageous ; in this event dioecism, alternating g< nera-
tions, and heterospory are highly beneficial, since they increase the
chance of fusion between gametes that differ in immediate ancestry.
In the seed plants, through the marked subordination of the j;ame-
tophytic generation, through the retention of the megaspore, and
through the dispersal of the embryo (seed), the chief disadvantages of
alternation and heterospory are eliminated. To a small extent the dis-
advantages of alternation are eliminated through apogamy, apospory,
and parthenogenesis, but the elimination of disadvantage has come
chiefly through vegetative reproduction, which in the great majority of
plants insures the perpetuation of species, regardless of the presence or
absence of sexual reproduction. The almost unlimited capacity for
vegetative reproduction in the gametophyte generation of bryophytes
and of the sporophyte generation of ferns and seed plants doubtless
has been the means of preserving many species that otherwise would
have perished. Thus it is not to be assumed that the progress of
evolution necessarily is advantageous, and that heterospory and alterna-
tion must be an improvement over homospory and lack of alternation.
Probably the decadence of the heterosporous pteridophytes and of many
groups of animals is due to disadvantageous trends in evolution. Even
in the seed plants, supremacy is due, not so much, probably, to heteros-
pory and alternation, as to various features which eliminate their disad-
vantages and most of all to their high capacity for vegetative reproduc-
tion, for foliage display, and for the development of secondary wood.
REPRODUCTION AND DISPERSAL
825
2. FLOWERS
General characteristics of flowers. — The parts of a representative
flower. — Ecologically speaking, a. flower is an organ whose role is pol-
lination, which is the
initial process of seed
production. Struc-
turally, a flower is
a shortened shoot
with spore-bearing
organs, which usually
(though not neces-
sarily ) are subtended
by one or more leaf-
like structures.1 In
a representative
flower the outermost
whorl of floral leaves
is known as the calyx,
the individual leaves
being termed sepals
(s,k, figs. 1 136, 1 137).
Next within this is the
corolla, which may or
, , , FIG. 11-56. — An inflorescence of a syringa (Philadelphus).
may not be made up u . ., ,, , r
» r showing the floral organs of .a hypogynous, monoclmous, poly-
of Separate leaves, petalous flower; note the calyx with its individual sepals (5)
known as 'petals (b an<^ tne corolla with its individual petals (/»), the calyx and
corolla together forming the perianth ; note also the stamens,
» o • 3°> "37/' each composed of a filament (/) and an anther (a), and the
The calyx and corolla pistil, of which there are here to be seen the style (<) and four
together form the per- st'gmas (&) ; ^is inflorescence is a cyme, the terminal flower
blossoming first.
ianth. Next within
the corolla are the stamens, each of which consists usually of a slender
stalk, the filament (f, fig. 1136), and a spore-bearing body, the anther (a,
figs. 1136, 1137) , the spores being known as microspores (fig. 1145). At
1 The latter statement groups the strobilar organs of many pteridophytes with flowers,
there being no sharp line structurally between strobili and certain floral shoots or inflores-
cences (see p._ 180); however, since the r&le of gymnosperm and pteridophyte strobili is
fundamentally different, in the following pages gymnosperms, but not pteridophytes, wil'
be regarded as true flower-producing plants.
826
ECOLOGY
the center of the flower is the pistil (or pistils); a simple pistil or one
member of a compound pistil is called a carpel (g, fig. 1137). Commonly
a pistil is composed of an enlarged basal portion, the ovary (o, figs. 1 180,
1181), and a slender upper portion, the style (/, fig. 1136), which is sur-
mounted by the somewhat enlarged and sticky stigma (or stigmas, g,
fig. 1136).
Inside of the ovary are ovules (figs 581-584), which represent incipient
seeds, and within each ovule is the megaspore or embryo sac (figs. 582,
FIG. 1137. — A longitudinal section through the flower of a peony (Paeonia), showing
the calyx with its sepals (&), the corolla with its petals (c), numerous stamens with their
filaments and anthers (a), and the pistils or carpels (g) ; the broadened end of the axis
just below the carpels is the receptacle. — From STRASBURGER.
589) , which develops into the minute female gametophyte that is char-
acteristic of seed plants (figs. 590-594). The entire life of the fe nale
gametophyte is passed within the ovule, and after the fusion of the
gametes, the sexually produced sp'ore (oospore) germinates into the em-
bryo, whose subsequent development is the most conspicuous feature
of seed formation (figs. 600-613). Usually the minute male gameto-
phyte begins to develop from the microspore within the anther, foming
a structure of two or more cells which with the persisting microspore
wall forms the mature pollen grain (fig. 1146). The pollen grains,
lodging upon the stigma, germinate, developing elongated -structures,
known as pollen tubes, which penetrate the pistil to the female gameto-
REPRODUCTION AND DISPERSAL 827
phyte, thus permitting the migrating male cells to reach the neighbor-
hood of the egg (figs. 533, 599).
The more or less broadened terminal part of the axis, which bears the
floral organs, is the receptacle (fig. 1137). Most flowers are subtended
by leaflike organs, known as bracts (b, fig. 1141), into which foliage
leaves often grade imperceptibly; a group of whorled or closely arranged
bracts is called an involucre (figs. 1193, 1194). Although flowers often
are solitary, they more commonly are grouped into an inflorescence
(fig. 1141).
Differences in floral structure. — While the sort of flower described
above is as representative as any, there are divergences in almost all
respects, and since these divergences are relatively fixed, whatever the
environmental conditions, they have been made the chief basis for sep-
arating seed plants into subdivisions. The kind of flower that is most
fundamentally different from the one above pictured is that of the
gymnosperms, which has no ovary, style, or stigma, the ovules being
exposed directly to falling pollen. Any one of the parts of a flower may
be wanting or even all the parts except either stamens or pistils. Often
there is but one kind of floral leaves which in the dicotyls is arbitrarily
regarded as the calyx (figs. 1159, 1160), but which in the monocotyls is
termed the perianth ; sometimes there are no floral leaves, as in the cat-
tails, peppers, and hazels (fig. 1161), and in most gymnosperms.1 Even
where the perianth is lacking, one or more bracts commonly are present.
The simplest flower is that of the duckweeds, in which the only organ
present is a single stamen or pistil. In the dicotyls the corolla may be
made up of separate petals (figs. 1136, 1137), or the parts may be united
(as in the Sympetalae, fig. 1185). Most flowers are monodinous, that
is, with pistils and stamens occurring in the same flower (figs. 1136,
1137), but some are diclinous, that is, with stamens and pistils occurring
in separate flowers ; diclinous species may be monoecious, having the
two kinds of flowers on the same plant (fig. 1161), or dioecious, having
the two kinds on separate plants (fig. 1165).
While the floral whorls commonly are sharply delimited, the calyx
and corolla often are much alike, as in many monocotyls and in some
dicotyls (e.g. Poly gala). A striking instance of intergrading parts is
found in the white water lily (Castalia), where the stamens pass gradually
into petals, suggesting to some observers that stamens are transformed
1 Spikes or catkins of such flowers do not differ essentially in structure from pterido-
phyte strobili, though their r61e is radically different.
828
ECOLOGY
petals and to others that petals are transformed stamens, neither view
having adequate support. When the calyx, corolla, and stamens are
inserted on the receptacle below the ovary, the flower is called hypogynous
(figs. 1137, 1138); when the corolla and stamens are inserted on the
calyx at the level of the ovary, the flower is called perigynous (fig. 1139);
and when the calyx appears to be inserted on the ovary, the flower is
called epigynous (fig. 1140). A determinate inflorescence is one in which
1139
FIGS. 1138-1140. — Diagrams, showing the position of the floral organs in hypogy-
nous (1138), perigynous (1139), and epigynous (1140) flowers; in 1138 the calyx, c >rolla,
and stamens are attached to the receptacle; in 1 139 the corolla and stamens are at ached
to the calyx tube; in 1140 the other floral organs appear to be attached to the ovary. —
From GANONG.
the terminal flower blossoms first, while an indeterminate inflorescence
is one in which the lateral flowers blossom first, so that a shoot may con-
tinue to bloom somewhat indefinitely (fig. 1141). Cymes are a represen-
tative form of determinate inflorescence (fig. 1136), and common forms
of indeterminate inflorescences are spikes (fig. 1163), catkins (fig. 1161),
racemes (fig. 1199), corymbs (fig. 1173), umbels (figs. 1196, 1197), pan-
icles (fig. 1162), and heads (fig. 1193).
The significance of differences in floral structure. — The floral diver-
gences heretofore noted are of great convenience in classification, because
they are relatively invariable, but they appear to have had little or no
significance in determining the success or failure of plants. It is believed,
for example, that the trend of plant evolution has been toward epi^yny,
but there is practically no evidence that epigyny is more advantageous
than hypogyny. Monocliny or dicliny and the presence or absence of
a perianth may be of greater consequence, and they will be considered
later, but it appears that floral evolution has taken place in large part
without relation to role or to ecological advantage, especially in those
structures most used in classification. In many other respects, how-
REPRODUCTION AND DISPERSAL
829
ever, flowers possess conspicuous advantages, and these will now be
considered.
The role of flowers and the essential organs involved. — Pollination. —
Pollination, that is, the transfer of pollen grains to the stigma (or to the
ovule in gymnosperms) is the chief activity
associated with flowers. When pollen is trans-
ferred from a flower of one plant to a flower of
another, the phenomenon is termed crass polli-
nation or xenogamy, and when pollen is trans-
ferred from tKe~anTHers to the stigma of the same
flower, it is termed close pollination or autogamy.
Geitonogamy, in which pollen is transferred from
one flower to another on the same plant, is
intermediate between xenogamy and autogamy,
and often is classed with the former, but in reality
it is much closer to the latter. In many species
autogamy is the only kind of pollination possible,
and in other species (probably a greater number)
only xenogamy is possible, but it is probable that
in the great majority of plants both autogamy (or
geitonogamy) and xenogamy are possible, though
usually it is believed that the latter is the more
advantageous. In all cases xenogamy is possible
only through the action of external agents, of
which wind and insects are the most important.
In geitonogamy and autogamy (especially the
latter) pollination may occur through the direct
contact of anther and stigma, but gravity, wind,
and insects often effect autogamy and geitonog-
amy as well as xenogamy; in some cases in-
sects are as necessary for autogamy as for
xenogamy (as in Yucca).1
The dehiscence of the anthers. — When the
pollen grains are mature, the anther dehisces,
usually by longitudinal slits (fig. 1142), but sometimes by transverse
slits, by valves (fig. 1176), or by terminal pores (as in Solanum and
FIG. 1141. — A com-
pound raceme of Coleus,
the individual flowers
being arranged in paired
cymes; c, calyx; c' , the
sympetalous bilabiate co-
rolla, composed of an as-
cending upper lip (i>) and
a boat-shaped lower lip
(/) ; note the partially ex-
serted stamens (a) and
style (*); the developing
cymes are subtended by
caducous bracts (6).
1 On this account the term self-pollination, if used, should be restricted to contact polli-
nation, rather than be made synonymous with autogamy in general.
83o
ECOLOGY
1142
1143
1144
FIGS. 1142-1144. — Stamens of angiosperms,
showing methods of anther dehiscence: 1142,
ordinary stamens with longitudinal dehiscence;
1 143, a stamen of Solatium with dehiscence by a
terminal slit or pore; 1144, a stamen of Vaccinium
with tubular prolongations of the pollen sacs. —
From KEENER.
in the Ericaceae, figs. 1143,
1144). Dehiscence is occa-
sioned by tissue desiccation.
Beneath the epidermis is a
layer with unequally thick-
ened fibers, in which strains
arise when the water content
lessens; rupture then occurs
along the lines (or at the
spots) of weakness, where-
upon the pollen may be
shaken out by such agents
as wind and insects.
Commonly anther desiccation
is due to the great transpir ition
to which open flowers are exposed.
Some anthers, however, opi n in
the bud or in moist weather and it has been claimed that this is due to the absorption
of water from the anther by adjoining nectarhs or by other tissues rich in s igar.
Dehiscence occurs when anthers are placed in contact with a cane sugar solution,
though much more slowly
than in dry air. Light and
the pressure of growing pol-
len also appear to facilitate
dehiscence.
1145
The pollen. — The
pollen grains are borne
in pollen sacs within the
anther, where they com-
monly are produced in
fours (tetrads'). Usually
the grains break apart at
maturity, scattering in-
dependently, but in some
plants they cohere in
. . PIGS. 1145-1148. — Different stages of pollen gram
groups (as m Mimosa), development in a rosin-weed (Silphium) : 1145, a micro-
while ill Others they spore, representing the one-celled stage of a developing
cohere in large and defi- P°1Ien gram5 ','46' a matu/e ?°lle" g™in; II47' f"4*'
0 germinating pollen grains, shoving the first stages of pol-
nite masses, known as len tube development; note the thick and spiny outer coat
pollinia (as in the milk- (exine); highly magnified. — From MERRELL.
REPRODUCTION AND DISPERSAL
831
weeds and the orchids).
Pollen grains commonly
have a thick outer layer,
the exine, and a delicate
inner layer, the inline
(figs. 1145-1148); in
cases where there is a
single layer, it may be
thick and cutinized (as
in Senecio) or thin and
permeable (as in sub-
mersed aquatics). Pol-
len grains differ con-
siderably in shape, the
common forms being spher-
ical or ellipsoidal (figs.
1149-1157), and also in
size, those of some mallows
being a hundred times as
1157
1156
1154 1155
FIGS. 1149-1157. — Pollen grains: 1149, grains of
Euphorbia splendens, both dry (a) and moistened (b) ;
1150, angular grain of the nightshade (Solanum
nigrutri) ; 1151, grains of a croton (Codiaeum varie-
gatum), both dry (a) and moistened (b); 1152, a
germinating pollen grain of Oxalis ; 1153, ellipsoid
grain of Impatiens Sultani; 1154, grain of Cuphea
ignea with processes at the angles; 1155, grain of a
nasturtium (Tropaeolum) with prominent angles;
1156, spiny pollen grain of Bidens ; 1157, grain of
Hibiscus with prominent spiny processes; note the
relatively gigantic size ; all equally magnified.
large as the grains of many
other plants (fig. 1157); they differ also in surface sculpturing, most
grains being smooth, but some being spiny, as in
the composites and the mallows (figs. 1156, 1157).
Many pollen grains have thin spots which upon
germination determine the position of the develop-
ing pollen tubes; in some cases the tube forces off
a part of the spore coat as a lid.
The stigma. — The essential elements of the
pistil are the ovary and the stigma, the style often
being short or wanting, though its presence may
be advantageous through its elevation of the stigma
into a region of optimum exposure to pollen.
When mature, the stigma secretes mucilaginous
substances, which, together with its papillate or
spinescent surface, facilitate the adherence of pollen
(fig. 1158). Stigmas also secrete substances which
facilitate the germination of pollen grains, and in
some cases they secrete very specialized substances
which stimulate the germination of pollen from
FIG. 1158. — Stig-
matic region of Hibis-
cus; t, the upper part
of a style branch with
scattered hairs; g, the
stigma with its hairy
surface to which pol-
len grains (p) are ad-
hering; highly mag-
nified.
832 ECOLOGY
flowers of the same or nearly related species, but which have either
no effect or a detrimental effect upon other pollen.
The pollen tube. — Under suitable conditions pollen grains adherent
to the stigmatic surface germinate, and the developing pollen tube,
which is the bearer of the male cells, penetrates the style and enters
the ovary; ultimately it may reach the female gametophyte in-
side an ovule, where the fusion of the gametes takes place. Usually
the pollen tube enters the ovule through the micropyle (m, fig. 594),
which is a narrow channel at the ovule apex, where the enveloping
integuments have not quite grown together. In some species the
pollen tube penetrates the pistil so rapidly that the gametes fuse
a few hours after pollination, while in other species a number of
months elapse between pollination and gamete fusion (as in the )aks
and pines).
The secluded position of the female gamete and the usual non-mo ility
of the male gametes make the pollen tube an organ of the first impor-
tance in the facilitation of sexual reproduction in most seed plants, .'ince
it bears the male cells (sometimes for an almost incredible distance)
in its fungus-like course through the pistil tissues, from which it derives
food parasitically. This method of bringing the male gametes into the
proximity of the egg seems especially suited to land plants, since it rlim-
inates the necessity of a liquid medium, such as is required by motile
sperms. Pollen grains germinate readily in various liquid media, swell-
ing rapidly and sending out tubes for a short distance. In respect to
conditions favoring germination, pollen grains show wide diversity,
especially in their osmotic relations with the medium. The pollen of
a number of species germinates readily in distilled water, but in other
cases this medium causes the grains to burst; Canna grains, for exariple,
burst in water, but not in a 2 per cent cane sugar solution. Most p >llen
germinates in cane sugar solutions, that of some species requiring high
concentration, while that of others germinates readily in solutions o low
concentration. Pollen grains that are difficult to germinate (as those of
the grasses) send out tubes if they absorb water slowly. Some p alien
(as in certain umbellifers and composites) has never been seen to germi-
nate except on stigmas. Probably because of the presence of the proper
stimulating substance at the proper degree of concentration, germina-
tion usually takes place more readily on stigmas than in artificial media,
and complete development does not occur unless germination has taken
place on the stigma of the proper plant (viz. of the same or of a closely
REPRODUCTION AND DISPERSAL
833
related species) *; in some species pollen is essentially impotent on the
stigma of the flower in which it was produced (p. 854). Pollen grains
usually retain their vitality for a number of days, but those of Hibiscus
Trionum live scarcely more than three days, while those of some species
(as the date palm) may live for several months, especially if kept dry.
Usually pollen grains that have been moistened and subsequently dried
die quickly, but some pollen is so resistant that submergence for a num-
ber of hours does not impair its vitality. The pollen of vernal flowers
is especially resistant, not only to moisture, but also to low temperatures.
FIGS. 1159, 1160. — The dioecious wind-pollinated flowers of the box elder {Acer
Negundai): 1159, iascicles of drooping staminate flowers borne on long stalks or pedicels
(/>); note the prominent anthers (a); 1160, ascending racemes of pistillate flowers from
another tree; note the perianth, consisting only of a calyx (c), and also the two prominent
stigmas (t) ; note also the transition between the bud scales (6) and the ordinary foliage
leaves (/), the intermediate leaves having a prominent flattish petiole (0) and a small
trifoliate blade (£').
In general, the stigmas are more sensitive to harmful factors than are the
pollen grains.
Pollen tubes usually take a more or less direct course toward the
ovary. Commonly the central region of the style is composed of delicate
elongated cells, or sometimes, even, it is hollow, so that the direct course
is the easiest; in the grasses, however, the region traversed by the pollen
tube seems no more easily penetrable than do the adjoining tissues.
After leaving the style and entering the ovary, the pollen tube commonly
1 Often germination, but not the later stages of development, may take place on the
stigmas of unrelated plants; the pollen tubes of Ranunculus, a dicotyl, have been seen
penetrating to the micropyle of Scilla. a monocotyl. It may be noted in this connection
that the sperms of ferns swim into the archegonia of many species indifferently, but that
fusion with the egg takes place only in the same or in a closely related species. "
ECOLOGY
follows the inner wall, and it may pursue a tortuous course, or it may
grow directly toward a micropyle; pollen tubes* have been shown to
exhibit prochemotropic reactions toward certain carbohydrates and pro-
teins, including those that are secreted by stigmas.
Wind pollination. — Features that favor the scattering of pollen. —
The simplest form of pollination and the one most closely related
to spore dispersal in the lower
plants is wind pollination,1
and wind-pollinated plants
have many features which re-
semble those of the fungi,
bryophytes, and pteridopl iy tes
rather more than they do
those of the insect-pollinated
seed plants. In many c ases
the .staminate flowers are
arranged in catkins, which
usually are slender, peidu-
lous inflorescences that yield
gracefully to breezes (fig.
1161). Catkins suited for
wind pollination areespec ially
characteristic of many trees
/^ twm**^- • ^*«S*a and shrubs (notably the pop-
\iyr ^^*1|^!W ^% lars, oaks, birches, and other
I /I ^"&§^te^ Amentiferae, and also most of
the conifers), which perhaps
is advantageous in view of
the relative exposure of such
plants to wind; in most of
these plants, alsp, the flowers
develop before the leives,
thus further facilitating ex-
posure to wind. The j istil-
late flowers sometimes are in catkins (as in poplars and birches), but
often they are not (as in oaks and hickories) ; such arrangement, appar-
ently, is of no particular advantage.
1 Species with wind pollination often are called anemophilous, a term that should be
discarded, together with other humanistic words as applied to plants.
FlG. 1161. — -A flowering twig of a hazel
(Corylus omericana), a shrub which has monoe-
cious wind-pollinated flowers; note that the stam-
inate flowers are lowermost and are in catkins
(c) which sway in the breeze, the pollen grains
(p) often appearing in clouds ; s, scale leaves which
protect the flower buds in winter; the pistillate
flowers develop from scaly buds (&), and at anthe-
sis the stigmas (g) are exscrted.
REPRODUCTION AND DISPERSAL
835
In many species with wind pollination, and especially in those without
catkins that move readily in the wind, the stamens have long and slender
filaments, which so expose the anthers that they are shaken in the
gentlest air movements (as in the grasses and the box elder, figs. 1159,
1162, 1163). In the nettles the pollen is discharged
into the air by a sudden move-
ment of the filaments. In many
plants pollen that falls in quiet
weather accumulates in pockets
of one 'sort or another, whence
it is scattered readily by the
first breeze. In most wind-
pollinated species (not, how-
ever, in most grasses and
sedges) the pollen is produced
in great abundance; this is a
matter of much advantage in
view of the great waste. The
abundance of pine pollen re-
sults sometimes in the so-called
sulfur showers, and the abun-
dance of ragweed pollen in the
air is thought to be a factor in
causing hay fever.
Wind-scattered pollen com-
monly is smooth, light, and dry, and hence easily
blown about (fig.
1161), and in the
pines, dispersal is
facilitated further
by the presence
of a wing on each
side of the grain
(fig. 1164). In
wind-pollinated
species the pollen
grains are not
easily wetted, thus further resembling the spores of fungi and ferns;
this is highly advantageous, since moistening might prevent wind
FIG. 1162. — A
panicle branch of the
meadow fescue (Fcs-
tuca elatior), a plant
with monoclinous
wind-pollinated flow-
ers ; note the unopened
spikelets above with
their imbricated
scales; below to the
right is a spikelet in
which two of the lower
flowers have openecl,
each disclosing two
plumose stigmas and
three stamens whose
long and slender fila-
ments expose the an-
thers to the wind.
FIG. 1163. — The
upper part of a plan-
tain spike (Plantago),
illustrating protogyny
in monoclinous wind-
pollinated flowers;
note that the conspic-
uous plumose stigmas
(g) appear before the
stamens are evident ;
in the older flowers
note the long and
slender filaments (/)
and the triangular an-
thers (a) ; c, calyx ; cf,
corolla.
FIG. 1164. — A pollen grain of a
pine (Pinus), showing the two wings
which aid in its dispersal by wind;
highly magnified. — From COULTER
and CHAMBERLAIN.
836 ECOLOGY
dispersal and lead to premature germination. Even among the
submersed aquatics there are some species (e.g. many pondweeds) that
at anthesis develop aerial flowering shoots, which produce light pollen
that is not easily wetted and that is scattered by wind.1 The fact that
anthers dehisce chiefly when dry is of much significance in the pro-
tection of pollen from moisture.
Features that favor pollen reception. — In wind-pollinated species the
stigmas commonly are large and conspicuously exserted (figs. 1160,
1161), and sometimes they are feathery plumose (as in the grasses, figs.
1162, 1163), the "silk" of corn being a familiar and conspicuous
example of these characters. In the conifers, where there is no stigma,
the pollen may be caught in a drop of mucilaginous liquid exuied
from the ovule.
Features that favor cross pollination. — While wind-pollinated flovers
structurally are relatively simple, they are on the whole as well fii ted
for cross pollination as are insect-pollinated species, and they exhibit
even many of the specialized features which are regarded as nore
characteristic of the latter. In the first place, many and perhaps most
wind-pollinated plants are diclinous, and in these, of course, there can
be no autogamy. A large number of the diclinous forms are dioec ous
(e.g. the poplar, ash, box elder, juniper, date palm, and meadow rue),
and their pollination necessarily is xenogamous (figs. 1159, u6o).
Among common monoecious forms are the oaks, hickories, birches,
alders, pines, nettles, and most of the sedges (fig. 1161); while geito-
nogamy as well as xenogamy might occur in such plants, the chance of
it is minimized in the many cases in which the pistillate flowers are
higher than the staminate (as in the hazel, the pine, and in many
sedges). Furthermore, in monoecious species the pistillate flowers of
a given individual blossom before the staminate, and sometimes several
days before, as in some alders and cattails. Even in dioecious pi ints
the pistillate flowers commonly mature before the staminate.
In monoclinous wind-pollinated flowers (as in the grasses and plan-
tains, figs. 1162, 1163), cross pollination commonly is favored by the
consecutive maturity or dichogamy of the anthers and stigmas. In the
plantain (fig. 1163) the stigmas mature first, exhibiting a phenomenon
known as protogyny, while the earlier maturation of the anther is known
1 This phenomenon is especially striking in Myriophyllum, since the hitherto flaccid
and submersed main stem axis becomes at the tip rigidly erect and emersed just before
anthesis.
REPRODUCTION AND DISPERSAL
837
as protandry; both protandry
and protogyny are seen in
maize. The most special-
ized means of preventing
close pollination, namely,
that in which the pollen is
impotent on the stigma of
the same flower, is illustrated
in rye, though in wheat and
barley, and probably in most
monoclinous species, close
pollination is not necessarily
excluded.
Miscellaneous features of wind-
pollinated flowers. — Wind-polli-
nated flowers usually contrast with
those that are insect-pollinated
in their lack of showiness, odor,
and nectar, though some of them
are conspicuously colored (as in
the cottonwood and field sorrel).
The perianth mostly is inconspicu-
ous (either through its greenish
or brownish color o r its small size)
and often it is absent; when pres-
ent, it consists commonly of a
calyx, the corolla being rarely in
evidence. None of these features
would occasion comment, but for
the corresponding presence of
showiness, odor, and nectar in
insect-pollinated flowers and for
the consequent assumption that
in the latter these features prob-
ably are advantageous. The dis-
tribution of species with wind-pol-
linated flowers has been thought
to differ somewhat from that of
other seed plants. For example,
the percentage of the former is
greater in windy habitats than
elsewhere (as on small islands and
along shores), while the flowers
FIG. 1165. — Pollination in the tape grass
(Vallisneria spiralis); s, staminate plant; p, pis-
tillate plant; the staminate flowers are borne in a
spike (k) ; upon detachment they rise (a) to the
surface, open out (6), and float on the water; the
pistillate flowers (/) are borne in spathes (d) on
long scapes (e), just reaching the water surface,
where the floating staminate flowers may come in
contact with them (c) ; note also the vertical ribbon-
like leaves (/) and the stolon (r), o representing a
new potential plant or offset. — After KERNER.
838 ECOLOGY
of our northern trees contrast with those of tropical trees in being predominantly
wind-pollinated.
The advantages and disadvantages of wind pollination. — The ques-
tion of advantage is here largely one of speculation. Undoubtedly a
great disadvantage in wind pollination is the enormous waste of pollen.
Probably not more than one out of a thousand or even out of many thou-
sand grains ever reaches the proper stigma. Perhaps, on the other hand,
the chance of a favorable wind is greater than that of a visit by the proper
insect. The dominance of wind pollination in such plants as the oaks,
pines, grasses, and sedges at once suggests that wind pollination certainly
is not detrimental. However, the great abundance of such plants
(especially the grasses and sedges) is quite as likely to be due to vegeta-
tive as to reproductive organs.
Water pollination. — Pollination through the agency of water is a relatively rare
occurrence hut it is of much interest. In plants that are completely subrm rsed
(as in several of the Potamogetonaceae and Najadaceae) the pollen grains are fila-
mentous structures that are as heavy as water or heavier, and the thick exine < har-
acteristic of aerial pollen is lacking; such pollen grains upon release float belov the
surface and may come into contact with the long exserted stigmas.
In the tape grass( Vallisneria) and in some of its relatives, pollination takes place
at the water surface. Vallisneria (fig. 1165) is a dioecious plant, whose pist Hate
flowers are single and are borne on long scapes that bring the flower at the tine of
stigmatic maturity just to the water level. The staminate inflorescences at maturity
become detached from their short scapes and rise to the surface; upon the opening
of the bract (spalhe), the individual flowers also become detached and float about on
the water as miniature boats, the perianth opening and exposing the stamens. The
floating staminate flowers, like any small particles, swirl readily into the slight de-
pressions formed about the pistillate flowers, as about other objects on the water,
and come into contact with the stigma. After pollination the scape of the pistillate
flower coils up into a spiral, thus withdrawing the ovary below the surface, where the
fruit develops. In essential respects pollination in the water weed (Elodea) is com-
parable to that in Vallisneria.
General characteristics of insect-pollinated flowers. — Monocliny and
its advantages. — Were it not so common, the symbiotic relation ex:5 ting
between flowers and insects would be regarded as most marvelous.
From the standpoint of evolution, no great facts of nature are more
remarkable than that in many plant species the flowers remain unpolli-
nated unless they are visited by insects in search of nectar or pollen,
and that in a much greater number of species visiting insects are the chief
agents of pollination. Insect- pollinated flowers * are in great part mono-
1 Insect-pollinated flowers often are inaptly called enlomophilous, that is, insect-loving.
REPRODUCTION AND DISPERSAL 839
clinous (figs. 1136, 1137), though a few are diclinous; for example, the
willows are dioecious, and many composites are monoecious. Dicliny
has been thought to be advantageous in wind-pollinated flowers because
it increases the probability of cross pollination; however this may be,
monocliny would seem to have a distinct advantage in insect-pollinated
flowers in that it makes possible double the amount of pollination for
a given number of insect visits. Furthermore, pollen-gathering insects
would not visit pistillate flowers, and nectar-gathering insects would visit
both pistillate and staminate flowers only in case each were nectar-
bearing, thus involving two nectaries in one act of pollination.
Pollen. — The stamens of insect-pollinated flowers rarely are promi-
nently exserted and the filaments often are short; also the inflorescences
are relatively inflexible in the wind. The pollen, instead of being dry
and powdery, commonly is adhesive through the possession of spines or of
other protuberances (figs. 1156, ii5'/), or through the presence of viscid
substances (as in Oenothera), so that the grains often cohere in masses.
The shape of the grains is more likely to be elliptical than spherical, the
latter shape being especially characteristic of the grains in wind-polli-
nated flowers. Such pollen grains are not easily blown about by the
wind, and they adhere readily to visiting insects and to stigmatic surfaces.
In species with wide-open flowers, which therefore are exposed to insects
of all kinds, including pollen-gathering insects, the pollen often is almost
as abundant as in wind-pollinated species; sometimes also the stamens
in such flowers are very numerous (as in the roses and buttercups). In
tubular or otherwise partly closed flowers, where the stamens are con-
cealed, the latter commonly are few in number and the pollen is relatively
sparse (as in the phloxes and mints). As a rule, the stigmas are smaller
and otherwise less conspicuous than in wind-pollinated flowers.
Features supposed to be attractive to insects. — The most noticeable
single feature of insect-pollinated flowers is their showiness, which is due
to the color of the flowers, or to their size, position, or arrangement.
Many insect-pollinated flowers are fragrant, and many also possess
nectar. It is rare that a flower which is pollinated regularly by insects
is neither showy, fragrant, nor nectar-producing, and some insect-polli-
nated flowers have all these features.
Most insect-pollinated plants north of the tropics are of low stature, but in warm
countries many trees have insect-pollinated flowers. An odd phenomenon, com-
monest in the humid tropics (but characteristic also of our northern redbud, Cercis
canadensis), is cauliflory (i.e. stem flowering), the tree trunks often being covered
840
ECOLOGY
with flowers ; this habit is without obvious advantage, though it has been suggested
that trunk flowers are well protected from torrential rains. Cauliflory appears to
be stimulated by an excess of moisture; it has been induced in the grape also by
wounding and in the orange by defoliation. In some tropical trees and shrubs (as
in Ficus geocarpa) flowers break through the soil from subterranean stems. Tran-
sitions between wind-pollinated and insect-pollinated flowers sometimes are seen,
as in the ericads, where the pollen which commonly is scattered by insects ultimately
becomes dry and powdery and thus may be scattered by the wind ; chestnut flowers
which usually are wind-pollinated are fragrant and attract insects. Ephedra cam-
Pylopoda is interesting as b'e-
ingan insect-pollinatedgym-
nosperm, the flowers, which
are much frequented by in-
sects, exhibiting nectar and
sticky pollen which co icres
in masses. Before consider-
ing in detail the features
that attract insects to flo vers,
it is necessary to consider
the pollinating organisms
themselves.
Pollinating insects. —
.y, ,.v^ General remarks. — Ths
\\ vast majority of efficient
FIGS. 1166-1169. — Flowers of Salvia, illustrating pollinating animals are
pollination by bees :, 166, a floy* of Salvia glutinosa .in jn particularly fly-
longitudinal section, the arrow indicating the direction
taken by visiting bees; s, style; a, anther; 1167, a simi-
lar section, showing the lower arm of the connective
lever pushed back, as by an entering bee, the pollen-
bearing anther (a) thus being deflexed in such a way as
to rub pollen over the insect; 1 1 68, a Salvia flower into
which a bee has entered, the anther (a) being in contact pollen en route.
with the bee; 1169, an older flower, showing the stigma .. a •
tnp nvino*
(g) in such a position as to come into contact with an
entering bee; 1 168 and 1 169 show that Salvia is prqtan- that visit flowers :-egU-
drous. — 1166 and 1167 from KERNER; 1168 and 1169 larly for nectar or pollen
from AVEBURY (LUBBOCK).
are the most impoitant.
Flowers with exposed nectar and pollen are visited by most of the
flower-frequenting species, but flowers with hidden nectar or pollen,
especially those with long corolla tubes or whose nectar accumulates
in long spurs (fig. 1171), are pollinated only by highly specialized in-
sects with elongated mouth parts.
Bees. — The most important pollinating insects belong to the Hymen-
optera, a group which includes the bees, wasps, and ants. The honey-
insects, since those
which crawl from flower
tQ flower are \[\^e]y to
brush off most of the
Among
' hnip
REPRODUCTION AND DISPERSAL
841
bee (Apis) and the bumblebee (Bombus) are the most efficient of all
pollinating insects, because of their remarkable and continued activity
from the opening to the close of the flowering season, because of their
precision, which insures the successive and rapid pollination of many
individuals of the same species, and because they visit flowers for pollen
as well as for nectar. Their hairy legs are well suited for carrying pollen,
and their long probosces enable them to secure nectar in partially closed
or tubular flowers (figs. 1166-1169). Among the flowers that are almost
entirely dependent upon bees for pollination are those with irregular
(zygomorphic) corollas, as in the legumes, the violets, and many of the
mints; in certain instances (as in the clovers and aconites) the natural
distribution area is confined to those parts of the world frequented by
bees. The bees are diurnal insects and visit only diurnal flowers, and
it commonly is thought that they have a high color sense and a keen sense
of smell which aid them in detecting the presence of flowers. The wasps
are of minor importance as pol-
linating insects, though some
flowers are pollinated chiefly by
them (as in the figwort).
Moths and butterflies. — The
butterflies and certain moths
(classed in the Lepidoptera) are
nectar-feeders, and they possess
greatly elongated and special-
ized mouth parts, known as
maxillary laminae. The butter-
flies, like the bees, are diurnal
insects and are able to get nectar FIG- "7°- — A hawk moth (Phlegethonius
from deeply hidden parts of the -f «* to) ^.f8 *e fl?w,,er °flf Pelunia;A nKote, the
r . . long corolla tube of the flower and the long
flower; as a rule, they visit mouth parts of the insect. — After FOLSOM.
showy and fragrant flowers (such
as various honeysuckles and pinks). Most remarkable, perhaps, are the
hawk moths (Sphingidae) , a group consisting chiefly of nocturnal insects
with maxillary laminae of great length (up to 80 mm.), which are coiled
when not in use (fig. 1170). The nocturnal hawk moths visit flowers
rapidly and with the precision of bees, thus contrasting with the more lan-
guid and haphazard movements of the butterflies; they are attracted espe-
cially to heavily scented white nocturnal flowers with long corolla tubes
(e.g. Nicotiana alata, which becomes fragrant as it opens in the dark).
842 ECOLOGY
Flies and beetles. — As a class the flics (Diptera) are not very important polli-
nating insects, largely because of their absence of precision in making floral visits.
Some, however, notably the drone flies (Syrphidae), have extended probosces and
depend largely upon flowers for food, and thus are important pollinating agents.
Most flies pollinate only flowers with exposed nectar and pollen (as in Euonymus).
Color seerns to have but little attractive significance, but odors (especially those
offensive to human nostrils) attract numerous flies, particularly carrion flies and
dung flies, which may thus be important pollinating agents in ill-smelling flowers
like Rafflesia. The pollination of Arum and Arixtolochla is thought to be effected
largely by small flies, which are able to crawl through the narrow apertures. Beetles
are still less important than flies, though some species with narrow elongated heads
are of some significance; as a class their floral visits result in more harm than benefit.
Pollinating animals other than insects. — Apart from insects the most important
pollinating animals are birds, especially those with long slender bills and protrusile
tongues, such as the humming birds, which visit honeysuckles, trumpet flower;, and
other long-tubed blossoms containing nectar; in some cases birds visit flowers in
search of nectar-feeding insects. Bird pollination is much commoner in the tropics
and in the southern hemisphere than in northern latitudes; in parts of South Amer-
ica, humming birds almost equal insects in importance as pollinating agent;, and
in South Africa the sunbirds and their relatives are even more important, polli lating
insects being much less conspicuous than in the northern hemisphere. The struc-
ture of bird-pollinated flowers does not differ from that of flowers which are polli-
nated by insects with elongated mouth parts. A few instances of pollinat on by
bats have been reported, but they are not regarded as important. Pollinat on by
slugs or snails is of possible importance in a few cases, as in Calla and in other aroids
with numerous blossoms close together near the ground.
The food of pollinating insects. — Pollen. — Pollinating insects visit
flowers to obtain pollen, nectar, or sap, and sometimes for shelter, and
it is while they are engaged in one or more of these activities that pollina-
tion takes place incidentally. Bees obtain nectar, which they store for
future use, and pollen, which is in large part utilized more immediately
by the larvae, while butterflies and moths obtain only nectar and that for
immediate use; it is largely because of this that the bees are more useful
pollinators than are the more highly specialized butterflies. Ir some
flowers there is little or no nectar (as in Papaver, Hypericum, and Sola-
tium) and insect visits are made mainly for pollen, which usually is pro-
duced in considerable abundance. The insects presumably get most
of the pollen, but some of it is pretty certain to be rubbed off on the
stigmas. Nectarless insect-pollinated flowers commonly are regular
(actinomorphic) and wide open, with the anthers prominently exposed.
Sometimes there are two kinds of stamens (as in Cassia), one which the
insects visit for pollen and another which sprinkles pollen over the insects
REPRODUCTION AND DISPERSAL
843
in such a way that it is likely to come into contact with the stigmas.1
Commonly pollen-gathering insects are relatively non-specialized (ex-
cept in the case of bees), corresponding in general to the lack of speciali-
zation in the flowers, in which the pollen is so exposed that it may be
taken readily by any insect that visits it.
Nectaries and nectar. — Nectar-secreting .flowers commonly are more
specialized than are nectarless flowers, and
the nectar-gathering insects are the most
specialized of pollinating insects. How-
ever, there are many simple actinomor-
phic flowers with exposed nectar (notably
among the umbellifers) or with nectar but
slightly concealed (as in the crucifers), which
are frequented by flies and by other insects
with short probosces. From these simple
nectar-producing flowers there are grada-
tions in the degree of concealment of nectar
to the highly specialized and often zygomor-
phic forms in which it is concealed at a
considerable depth at the base of long co-
rolla tubes or in elongated Spurs (fig. 1171), section through
where as a rule only the most specialized flower (TroP^lum
showing the spurs (s) with nec-
msects with long mouth parts can obtain tar (») collected in its lower por-
it. In most cases it is difficult or even tion; this flower is hypogy-
impossibls for insects getting the nectar nous and zygomorphic. -From
BARNES (Part II).
to avoid rubbing against anthers and
stigmas, thus facilitating pollination. As a rule, the pollen in nectar-
bearing flowers is not abundant, and in long-tubed and zygomorphic
flowers it commonly is concealed.
Most arctic and alpine flowers and also most vernal flowers of temperate climates
are comparatively simple in structure and have their nectar supply relatively ex-
posed. On the other hand, many tropical flowers and a large number, of estival
flowers of temperate climates have more specialized structures, their nectar supply
being hidden in spurs or at the base of long corolla tubes. With the former there
may be associated the general prevalence of insects with short probosces, charac-
terizing climates or seasons of low temperature, and with the latter there may be
1 The high degree of specialization here present is shown by the fact that the pollen
which is used for food does not readily germinate on account of the absence of the proper
enzym ; when this is supplied artificially, it germinates as readily as does the pollen from
the other stamens.
FIG. 1171. — A longitudinal
nasturtium
844
ECOLOGY
associated the more specialized estival or tropical insects with long probosces. The
attraction of pollinating insects is not the only advantage derived frorrt floral nectar;
there has been previously noted the possibility that nectar associated with the sta-
mens may withdraw water from anthers, causing their dehiscence. It has been
suggested also that nectar may play some part in the maturation of fruit, and that it
may help to protect flowers from desiccation; the last-named role seems especially
evident in the case of flowers with water calyxes.
Nectar is secreted by special structures, known as nectaries, and there
exist all gradations between those which are composed of undifferen-
tiated nectar-secreting tissue and those which are specialized glandular
hairs of complex structure. Usually they are
associated with the corolla, but they may be con-
nected with the stamens or with any other floral
part, even with the involucre (as in the poin-
settia, Euphorbia pulcherrima), or they may
occur on vegetative organs, where they are called
exlrafloral nectaries (p. 858) ; in the poinse' tia,
insects may get the abundant nectar without
pollinating the flowers, and in the case of the
FIG. 1172. — A longi- extrafloral nectaries the visitors rarely are effi-
tudinal section through a cient pollinating agents. The secreting regions
floral nectary of the poin- are composed of epidermal cells rich in cyto-
scttia (Euphorbia pulcher-
rima), showing palisade- plasm; commonly they are long and narrow and
closely packed in palisade-like rows (fig. 1172).
Nectaries differ from other glands chiefly in
secreting sugar ; the process is not well under-
stood, although the presence of sugar outside the cell causes the with-
drawal of water from within and the consequent formation of a drop
of nectar, of which sixty to eighty-five per cent usually is water.
Sometimes the nectar forms in sufficient quantity to drip from the
secreting surface, and in some such cases it collects in protected pou< hes
or sacs, which usually are corolla structures known as spurs (fig. 1171).
In most cases the secretion of nectar occurs only at anthesis, though
it may continue for some time after pollination, as in the tulip and
the quince. The secretion of water, but not of sugar, is greater in
humid than in dry weather, quite as with hydathodes. Indeed,
there exist all gradations between nectaries and hydathodes. Espe-
cially interesting transitional forms are seen in certain tropical
flowers, whose glandular hairs secrete but little sugar, though exuding
like secretory cells (a)
which are rich in cyto-
plasm; highly magnified.
REPRODUCTION AND DISPERSAL 845
large quantities of water, which accumulates in the outer floral organs,
giving rise to the term water calyx.
In a few cases insects visit flowers for other kinds of food than pollen or nectar,
as in certain orchids (e.g. Maxillaria), where there occur on the lip of the corolla
fragrant hairs rich in fatty and albuminous foods. Some flowers and inflorescences
develop a considerable degree of heat at anthesis, and it has been claimed that cer-
tain insects visit them for nocturnal shelter and warmth. Since most of the con-
spicuous heat-producing flowers and inflorescences are found among tropical palms
and aroids, this view seems untenable.
Floral features accessory to pollination. — Color. — The role of the
pistil and the stamens is very obvious; the protective and synthetic role
of the calyx also is obvious (p. 869), but the role of the corolla is far less
evident. The corollas of flowers, taken as a whole, are ephemeral
organs whose evanescence is due to their extreme delicacy and conse-
quent easy wilting, and to their early abscission, much .after the manner
of deciduous foliage leafes. Corollas present a most bewildering luxu-
riance of form, color, and marking without parallel elsewhere among
plant organs. Most colors except black and green occur commonly,
and flowers therefore contrast sharply with the foliage. Reds and blues
are due to anthocyans dissolved in the cell sap, the former indicating
maximum acidity and the latter minimum acidity; indeed, certain flowers,
as in Lychnis, vary in color with the varying acidity of the cell sap.
Some yellow flowers owe their color to pigments related to the antho-
cyans and like them dissolved in the cell sap. Orange colors and many
yellow colors are due to plastids colored with carotin, xanthophyll,
or with related pigments (fig. 755). Brown colors are due commonly
to a combination of plastid and sap pigments. Flower pigments are
believed to be oxidation products, and whiteness, which denotes the
absence of pigment, arises where the necessary oxidizing ferment (oxi-
dase) is absent, or, if present, is neutralized by reducing agents. The
peculiar color-like effect of white flowers is due to the presence of air in
the petals or to unequal reflection and refraction. Nocturnal flowers
especially are likely to be white, and many species, whose flowers com-
monly vary from blue to red, may produce white sports, known as albinos.
The more or less fundamental distinction between the anthocyan (or
cyanic) flowers and the yellow (or xanthic) flowers is shown by the fact
that species and even genera rarely change from one to the other; for
example, hepaticas and asters, with all their variations, are not yellow,
or goldenrods and sunflowers cyanic. The cyanic colors would seem to
846
ECOLOGY
FIG. 1173. — A flowering
shoot of the yarrow (Achillea
Millefolium), illustrating the
massing of flowers into heads
(A), and the massing of heads
into a compact corymb ; r, ray
flowers; »', involucre.
be the more specialized, since they contrast
more sharply with the foliage, not only in
aspect but fundamentally, inasmuch as most
yellow petals resemble foliage leaves in hav-
ing plastids.
In many flowers showiness is increased by the
presence of party-colored effects. Sometimes the
two halves or lips have different colors (as in Col-
linsia and Viola pedata bicolor), but more commonly
the variegation is due to spots or lines on a back-
ground of another color. In some plants with vernal
flowers (as Hepalica) a group of individuals may
exhibit a number of colors, varying from white
through pink to blue, thus greatly increasing the
showiness of the plant group as a whole. Often
flowers that are inconspicuous individually are so
CL
massed into compact inflorescences as to produce a showy
effect; such a condition is seen in the umbellifers and even
more in the composites, where the inconspicuous central or
disk flowers often are surrounded by showy outer or ray
flowers, giving the effect of a large simple flower (fig. 1173).
The inflorescences of Hydrangea consist similarly of incon-
spicuous central and of showy outer flowers, the latter being
sterile. In some plants the calyx is the showy organ (as in
Abronia and Mirabilis), and in some species of Caslilleja,
Euphorbia, and Monarda the bracts, or even the upper
leaves, are much showier than are the relatively insignificant
flowers. In some dogwoods the involucre is much showier
than are the flowers, and in the willows where there is no
perianth, the staminate catkins often are showy by reason
of the conspicuous stamens.
Zygomorphy. — Zygomorphy or irregularity in the corolla
often adds to the conspicuousness of flowers. Many flowers
are labiate or lipped (as in the mints and the legumes), the
lower lip commonly protruding farther than the upper (fig.
1174); the culmination of lip development and zygomorphy
is found in the orchids, whose flowers are noted for their
bizarre shapes. The projecting lower lip is of obvious
advantage as a landing place for pollinating insects, notably
the bees. Often, as in the flowers of many legumes, the
weight of the insect presses down the lip sufficiently to
expose the anthers and the stigma. Floral lips are of no
advantage for the hawk moths and for similar insects, which
hover before the flowers without alighting. As a class, acti-
nomorphic flowers are erect, contrasting with the generally
FIG. 1 1 74. — Flow-
ers of Coleus, illus-
trating zygomorphy;
the calyx (c) and the
sympetalous corolla
(c1) are bilabiate, the
latter having an as-
cending upper Ic be (b)
and a descending boat-
shaped lower lobe (a),
from which th-' sta-
mens and the style
are partially exserted;
the lower lobe of one
flower (/) is held to
one side, so as to show
more clearly the up-
turned style (/) with
its two-lobed stigma
(g) and the four sta-
mens (s).
REPRODUCTION AND DISPERSAL 847
lateral display of zygomorphic flowers (fig. 1174), which thus are well suited for in-
sects that alight on lips or hover before the flowers.
Odor. — The attractiveness of flowers to insects is in large part due to
their fragrance. Most fragrant flowers are also showy (as in the lilacs,
roses, crabs, and water-lilies), but some very fragrant flowers are incon-
spicuous (as in the grape and mignonette), just as some very showy
flowers are without appreciable odor (as in the poppy). In many plants
,(as in Smilax herbacea and Trillium erectum) the odor, though offensive
to human nostrils, attracts certain insects. Some flowers that are rela-
tively odorless by day are very fragrant at night (as in species of Silene).
Flower fragrance commonly is due to the escape of volatile oils into the
atmosphere. A remarkable case of floral dimorphism is seen in Renan-
thera, a tropical orchid; most of the flowers are white and inodorous, but
at the base of the inflorescence are two fragrant yellow Sewers which
bloom first and remain fresh and fragrant until all the other flowers
have gone.
The sensitiveness of pollinating insects to color and to odor. — It is
believed commonly that odors and bright colors in flowers are of great
importance as indicators (or " signals ") to insects of the presence of
nectar or pollen, and some observers even go so far as to suppose that
these features have arisen through natural selection, the insects preferring
the more fragrant and showy flowers, while others go unpollinated, so
that the plants bearing them have no progeny. There is no evidence
whatever for the selection theory of the prevalence of showiness and odor,
and even the theory that insects are attracted by color and by fragrance
rests too little on experiment and too much on the untenable assumption
that the theory must be true, because nobody knows any other role for
these floral features. It is a tenable hypothesis that such features are
without value to the flowers possessing them, and the " signal " theory
deserves support only as it is proven experimentally.
It is not certain that insect attraction is the only possible role of colored corollas;
it has been suggested that they may play an important part in the chemistry of fruit
maturation. Pigmented plastids may be important in food making, and pigmented
cell sap may indicate the formation of useless by-products. It is to be noted that
some wind-pollinated flowers are very showy, as in the larch and the red maple.
Corollas also are of some importance as protective organs for the pollen and stigmas,
especially in flowers whose corollas close at night and in stormy weather.
The possession of a keen sense of odor by pollinating insects is un-
doubted, inconspicuous fragrant flowers being visited much more than
448 ECOLOGY
are showy odorless flowers. The readiness with which flies are drawn
to sources of nauseous odors is well known, and they frequent ill-smelling
flowers in a similar fashion. Hawk moths have been found to be able
to detect at a distance of several meters the presence of fragrant but in-
visible nocturnal flowers, and bees have been seen to fly directly toward
honey artificially hidden. Indeed, there are reasons for believing that
many insects are able to detect odors that are inappreciable to human
nostrils.
The possession of a keen sense of color is much less certain. Even the
ardent supporters of the " signal " theory hardly postulate it except for
the more specialized insects, such as butterflies and bees. The best ex-
periments indicate that insects are very short-sighted, none being able to
see distinctly for more than sixty centimeters, and bees very much less
than that. Objects in strong contrast (such as large light and dark
bodies in juxtaposition, or bodies in motion) appear to be seen rruch
farther than are other objects, certain Lepidoptera seeming to be ible
to see thus vaguely for a meter and a half, and bees for a half meter.
The only insects in which color perception has been definitely demon-
strated are the honeybees (Apis). These highly organized insects often
have been seen to visit gaudy but nectarless artificial flowers, and some-
times they attempt to get at showy natural flowers that are under glass.
Frequently they visit colored, unopened buds and wilted flowers, the
latter being at times approached, even after they have fallen to the ground.
Apiarists rather generally believe that honeybees are able to perceive
color differences, and hence they sometimes paint their hives in different
colors, so as to aid the bees in recognizing their abode. To the extent
that color is perceived by insects, it is a much more reliable " signal "
than odor, since the latter often is affected by the wind or masked by
other odors. Probably the characteristic forms of flowers serve as in-
dices to nectar, especially in the case of flowers that are conspicuous
by their shape or by their size; some observers think that form is
even more important than color as an insect " signal."
Some investigators believe that honeybees not only perceive colors, but that they
have marked color preferences. Experiments with honey on colored papers seem
to show that bees tend to visit a particular color, even if others are more conveniently
situated, and elaborate theories have been worked out on the assumption that bees
dislike yellow and prefer blue, whence it seems to some observers an easy postulate
that the day of yellow flowers is waning and that of blue flowers is in the ascendant.
Such conclusions certainly are unwarranted. The constancy of the honeybee to
REPRODUCTION AND DISPERSAL
849
a given color, such as blue, does not mean a preference for blue as such, but the asso-
ciation of nectar or pollen with that color. If a bee commences its activities on a
red flower, or on honey placed on a red paper, it is constant to red. In visiting
flowers, bees are constant not only to color, but also to form, flying from flower to
flower of the same species. This constancy to a given plant species for a certain
period is of great advantage to the plant, since it means a minimum waste of pollen.
It is equally of advantage to the bees, since the nectar or pollen is all of the same
quality, and since time and energy are saved in that exactly the same process is
repeated in each flower that is visited. The collapse of the color preference theory
is well shown in those cases in which different individuals of a given plant species
have flowers of different colors. In such species bees soon learn the essential like-
ness of the differently colored flowers, going from one color to another indifferently.
In other words, bees learn to ignore differences in color that are unaccompanied by
differences in nectar or pollen. Even if bees prove to be the only insects with a
FIG. 1175. — A colony of morning glories (Calystegia Soldanella) in dune sand; note
the striking contrast in tone between the flowers and the foliage, illustrating the possibility
of floral showiness even for color-blind insects ; New Zealand. — From COCKAYNE.
color sense, other insects certainly are able to appreciate differences in tone, as they
appear in a photographic print (fig. 1175), where whites and various colors come
into sharp contrast with the darkness of the foliage. Similarly, the prevalent white-
ness of nocturnal flowers makes them more conspicuous than would any pigment
color.
Memory and instinct. — When bees are taken to a new feeding ground,
their first flights are more or less misdirected and haphazard, resembling
850 ECOLOGY
i
the habitual movements of such insects as the flies. Soon they appear
to be attracted by various odors or colors, and after some days they
show their accustomed rapid and precise movements. That is, memory
appears to replace both odor and color as the directive stimulus of first
importance. Probably many discordant results of various observers
can be harmonized if the memory factor is taken into account. There
are some cases where instinct seems to be the controlling factor, as in
the pollinating insects of the figs and the yuccas (pp. 860, 864).
Many experiments with bees show the importance of the memory factor. When
showy flowers are deprived of their corollas, the number of visiting bees at first IF
small, but after a time the insects become accustomed to the new conditions and
visits become numerous. In some such experiments the seed production is less thar
in flowers with corollas, but this may be due to lessened protection of the ovary os
to a less effective dusting of the stigmas with pollen. If flowers are artificially
hidden by leaves, bees soon learn the new conditions, and the visits which at first
are few soon become frequent. Similarly bees soon learn to visit wind-pollir ated
flowers if there is placed on them honey and water, or sugar and a fragrant volatile
oil. The ability of bumblebees to learn is shown by Bombus terrestris, which has
a proboscis too short to get honey from Aquilegia vulgar in; after vain attempts to
reach the nectar in the ordinary way, it has been seen to bite a hole in the spu and
suck it out, repeating the process thenceforth. Similar holes are bitten in the -;purs
of Tropaeolum by Bombus hortorum.
Concluding remarks. — As a directive stimulus, insuring the visitation
of flowers by insects, odor seems to be more important than color, be-
cause it is distinguished from a much greater distance and by a much
larger number of insects; in the higher insects, notably among the bees,
which do most of the pollinating, memory seems to be a still more im-
portant factor. In the majority of flies and in most lower insects it is
doubtful if either color or memory plays a very conspicuous part, the
odor sense here being regarded as the most important. Odor is of par-
ticular significance where flowers grow in masses. So far as color and
form play a part, it is only in the immediate vicinity of the flower a id in
the most general way. The elaborate theories which assign a distinct
role for each floral form and for each shade of color, which regard the
lines and spots on the corolla as guides to the nectar,1 and which relate
the showiness of alpine flowers to the paucity of insects have no support
from exact observation and experiment.
Features favoring the sprinkling of insects with pollen. — In most
flowers, especially in those that are open and actinomorphic, the anthers
1 Striking spots or lines may occur in such nectarless flowers as that of the poppy.
REPRODUCTION AND DISPERSAL
851
are exposed so conspicuously and the pollen is so abundant that visiting
insects scarcely can avoid getting more or less pollen on their bodies,
even if they are searching only for nectar; of course, much pollen must
adhere to all pollen-gathering insects. In the composites the dense
massing of flowers into heads greatly facilitates pollen removal, since the
visiting insects necessarily crawl over numerous flowers with their ex-
serted stamens. In many flowers, especially in
those which are zygomorphic or which contain
but little pollen, there often are specialized features
that facilitate pollen removal. Certain parts of
the body (chiefly about the head) may receive the
pollen somewhat locally; in certain flowers exserted
stamens often are grasped by the alighting insect in
such a way that the under parts of the body receive
the pollen.
a
Some flowers with introrse anthers (i.e. opening in-
wards), as in the gentians, have nectar to the interior of
the stamens, while some flowers with extrorse anthers (i.e.
opening outwards), as in 7m, have nectar to the exterior of
the stamens. Often the stamens grow rapidly just before
dehiscence (as in Parnassid), assuming a position corre-
sponding to that of the stigma. In a number of instances
the insect occasions the release of the pollen, as in the
legumes, where the alighting of a bee causes the anthers to
protrude suddenly from the enclosing petal and to sprinkle
pollen over the visitor. In Pyrola and Kalmia the anthers
are held in unstable equilibrium, and the sudden release
coming with the insect visit causes the pollen to be shaken
out. In various ericads with pendulous flowers the sta-
mens have appendages, which are likely to be struck by
visiting insects in such a way as to result in the scattering
of the pollen. Sensitive mechanisms occur also in Lopezia,
where a petal-like structure holds the single stamen in un-
stable equilibrium, in Berberis, where the stamen itself is
sensitive to contact (fig. 1176), in Galeopsis, where contact
causes the anther lids to fly open, and in Crucianella,
where the style is held in unstable equilibrium until the
flower is touched, whereupon the style is suddenly released, bringing out with it a
shower of pollen. In Salvia, in which there is a swinging anther, an entering bee so
presses against the lower arm of the lever as to dust himself with pollen from the
upper arm (figs. 1166—1169). In orchids the pollen masses (pollinia) have an ex-
posed adhesive disk, which sticks to the head parts of a visiting insect. The some-
what similar pollinia of milkweeds have clips that fasten about the feet of the insect.
FIG. 1176. — A bar-
berry flower (Berberis
Thunbergii) with the
calyx and corolla re-
moved, so as to show
the pistil (p) and the
hypogynous stamens,
which at maturity lie
back upon the inner
surface of the petals;
when an insect comes
in contact with the base
(b) of the filament (/),
the latter flies forward,
assuming the position
of the stamen at the
right, and pollen is
dusted on the insect and
on the stigma (g) ; note
that the filaments (/)
broaden toward the
apex, and that the an-
ther valves (a) open
upwards, being hinged
at the filament apex.
852 ECOLOGY
Features favoring the deposition of pollen on stigmas. — Stigmas, as
previously noted, secrete sticky substances, and their hairy or papillate
surfaces still further favor pollen reception (fig. 1158). In many plants
the stigmas at maturity have essentially the same position as that of the
mature anthers (as in the figwort, figs. 1178, 1179), so that the part of
the insect which is covered by pollen is likely to touch the stigma. In
many other cases (as in the violets) the stigma projects beyond the an*
thers, so that it is likely to receive pollen from the entering proboscis.
In Cenlaurea mechanical irritation (as from a visiting insect) causes the fila-
ments to contract, thus exposing the stigma to pollination by the visitor. The most
remarkable situation is in the orchids, where the pollinia above noted, after removal
from the flower, move into such a position that they are likely to come into c mtact
with the stigma of the next flower visited. The orchid stigmas remain recef live a
remarkably long time if potent pollen fails to come in contact with them, t lough
they wither soon after the proper pollen begins to germinate. The corollas also
remain fresh on unpollinated flowers some days or even weeks longer than on polli-
nated flowers.
Features which impede close pollination and facilitate cross pcllina
tion. — Mechanical features impeding dose pollination. — In a vast
number of flowers close pollination is difficult or even impossible. Fre-
quently the stigma projects beyond the anthers (as in certain lilies and
evening primroses), so that pollen cannot fall upon the stigma of the same
flower (fig. 1174); in pendulous flowers, of course, the stamens would
have to project beyond the stigma to have a like result. Sometimes, as
in Iris, the receptive surface of the stigma is so oriented that the insect
rubs against it upon entering, but not upon leaving the flower, thus
facilitating cross pollination and preventing close pollination. Close
pollination is difficult in flowers with extrorse anthers. In orchids it is
almost impossible for the pollinia to come into contact with the stigma
of the same flower; in some lady's slippers (as in Cypripediun Cal-
ceolus) the insect enters and leaves the flowers by different routes, brush-
ing the stigma upon entering and the anthers before leaving.
Dichogamy. — The commonest floral feature that facilitates cross
pollination and makes close pollination difficult is dichogamy, or the
consecutive maturity of anthers and stigmas, contrasting with simulta-
neous maturity or homogamy. Dichogamy may be complete, that is,
the pollen may be shed before the stigma matures, or the stigma may
wither before the pollen sheds; more commonly it is incomplete, that isr
there is a partial overlapping of the periods of stigmatic receptiveness
REPRODUCTION AND DISPERSAL
853
and of the shedding of pollen. Stigmas commonly remain receptive
(especially when unpollinated) for a longer time than that required for
the shedding of the pollen, hence cross pollination
is more likely to result when flowers are protan-
drous (i.e, with the anthers maturing first) than
when they are protogynous (i.e. with the stigmas
maturing first); however, pollination of some
kind, either cross or close, is more likelv to result
when the flowers are protogynous, because of
the greater likelihood of overlap in the latter case.
Probably the number of protandrous and protogynous
species is about equal, though there are a greater number
of conspicuously protandrous forms, such as the saxifrage
(fig. 1177), the evening primrose
(in which the anthers may shed
before the corolla opens), the
composites, and the umbelli-
fers, than there are of conspic-
uously protogynous forms, such
as the figwort (figs. 1178, 1179)
and the crucifers. A striking
case of protogyny occurs in
Aristolochia Clematitis, where
the interior of the narrow calyx
tube is lined with reflexed hairs.
Insects enter easily and crawl
over the mature stigma, but on
account of the stiff hairs they
cannot leave until the anthers
mature, when they become dusted with pollen; the subse-
quent withering of the calyx hairs permits their exit, and
upon their entering another flower, cross pollination takes
place.
In most dichogamous flowers the stigmas and the an-
thers, though usually occupying the same position consecu-
tively, nevertheless are out of the way of the one, when the
other is mature. In the mallows the anthers at first hide
the stigmas, but later bend back and expose them, while
in Salvia the style which at first is short grows out after
the pollen is shed, assuming a favorable position for pollen reception by the stigma.
In the figwort, which has a protogynous flower, the style bends back over the lip
after maturity (fig. 1179). In Parnassia one stamen after another assumes a posi-
tion where visiting insects are likely to come into contact with them. In most
dichogamous flowers (but not in Aristolochia) two insect visits are necessary if both
FIGS. 1178, 1179. —
Flowers of the figwort
(Scrophularia marilan-
dica), -illustrating pro-
togyny: 1178, a young
flower with a promi-
nently exserted style (t)
and a receptive stig-
matic surface (g); 1179,
the same flower a day or
two later, with its style
(t) declined and out of
the way of visiting in-
sects, the stamens hav-
ing grown sufficiently to
expose the anthers (a)
to such pollinating
agents; note that the
sympetalous corolla (cf)
is bilabiate; c, calyx.
FIG. 1177. — Flowers
of a saxifrage (Saxi-
fraga sarmentosa), illus-
trating protandry; in
the younger (upper)
flower, the anthers (a)
are mature and the pis-
tils (s) immature ; in the
older (lower) flower, the
anthers (a') have shed
their pollen and the pis-
tils ($') have become
mature; note that the
corolla is zygomorphic,
three of the petals (/>)
being short and two
long (/>').
ECOLOGY
pollen and stigma are to play a part in pollination; this is an apparent disadvan^
tage as compared with homogamous flowers.
Heterostyly. — A highly specialized condition that opposes close
pollination and favors cross pollination is that known as heterostyly, in
which the stigmas and the anthers in different flowers occupy different
positions. Most such flowers are actinomorphic, and they are illustrated
by the primrose, flax, forget-me-not, and bluets. In the primrose some
plants have flowers with long styles, the stamens being attached toward
the base of the corolla tube, while other plants have flowers with short
styles and with the sta-
mens attached toward the
upper part of the corolla
tube (figs. 1180, 1181).
In LylhrumSalicaria there
are three kinds of flowers,
one with long styles and in-
termediate and shon sta-
mens, another with short
styles and intermediate
and long stamens, and a
third with intermediate
styles and long and short
FIGS. 1180, 1181. — Longitudinal sections through
flowers of the Chinese primrose (Primula sinensis),
illustrating heterostyly: 1180, a flower with a long
style (/) and with stamens (5) inserted near the base
of the corolla tube (cf)\ 1181, a flower with a short
style (t) and with stamens (5) inserted at the median
part of the corolla tube (c7); that part of a visiting
insect which strikes the anthers of one flower will be
.likely to strike the stigma of the other, thus effecting
cross pollination; note that the corolla tube of 1181
is dilated where the stamens are inserted ; these
flowers illustrate perigyny; c, calyx; o, ovary.
stamens. The same part
of the insect that comes in
contact with the lower sta-
mens will touch the stigma of a short-styled flower, while pollen from
the upper stamens will come in contact with the stigma of a long-styled
flower, thus insuring cross pollination.
Commonly the upper stamens of heterostyled flowers have large pollen grains
corresponding to the large long-haired stigmas of the long-styled flowers, while
the lower stamens have small grains corresponding to the small smooth stig nas of
the short-styled flowers; the corollas and other organs also may differ considerably.
Some investigators regard the large size of the pollen grains of the upper stamens
as advantageous, since their pollen tubes have to traverse a greater distance upon
germination; this view, which is doubtful a priori on account of their parasitic
nourishment, has been experimentally disproven.
Impotent and prepotent pollen. — So far as the prevention of close
pollination is concerned, the most specialized flowers are those in which
the pollen of a given flower is impotent (i.e. unable to initiate seed produc-
REPRODUCTION AND DISPERSAL 855
tionj on the stigma of the same flower. Complete impotence is compara-
tively rare, well-known cases being found in Corydalis cava, Hemerocallis
fulva (day lily), Fagopyrum esculentum (buckwheat), Secale cereale
(rye),1 and also in several of the Leguminosae, but there are many plants
in which foreign pollen (i.e. pollen from other flowers) is prepotent
(i.e. more or earlier effective) on a given stigma than own pollen 2 (i.e.
pollen from the same flower); foreign pollen that is sown on a stigma
several hours after own pollen often gains the ascendency in a very short
time. The acme of impotence is found in various orchids, in which own
pollen actually is prejudicial to the stigmas (or vice versa), appearing
to behave like a poison. In the Leguminosae own pollen is much more
potent in the annual species than in the perennials. In some legumes
(as Cytisus Laburnum) the usual impotence of own pollen is due to the
fact that the pollen tube cannot penetrate the cuticle of the stigma;
when this is ruptured artificially, own pollen is potent. In Corydalis
cava own pollen frequently germinates, but the pollen tube is unable to
penetrate to the ovules.
In nearly every case pollen from a given flower is no more potent on
other flowers of the same plant than on the stigma of the flower that pro-
duced it, thus showing in a most striking way that geitonogamy is essen-
tially the same as autogamy and should not be classed with xenogamy.
In a number of cases own pollen appears sometimes to be impotent, and
sometimes variously potent (as in Eschscholtzia and in Brassica Rapa),
possibly by reason of varying external conditions. From the viewpoint
of pollen potency, therefore, there are three classes of plants: (i) those
in which own pollen is as potent as foreign pollen, forming a class with
numerous representatives (as Oenothera and most crucifers); (2) those
in which foreign pollen is prepotent, also forming a class of large size;
and (3) those in which own pollen is impotent, forming a comparatively
small class.
Among the most remarkable examples of impotence are those afforded
by heterostyled flowers, own pollen being completely impotent in Linum,
and slightly potent in Primula? The most extraordinary feature of
these plants, however, is that cross pollination between the anthers and
Even in rye geitonogamy may occur.
2 However, the pollen must not be too foreign, as from another genus or family. Thus
impotence is found at the extremes of relationship, that is, where pollination occurs be-
tween anthers and stigma in the same flower or in flowers of distantly related plants.
2 Some observers report the complete potency of own pollen in some species of Primula.
856 ECOLOGY
stigmas of different position in separate plants is quite as ineffective as
is close pollination, thus showing clearly that the cause of impotence is
not closeness of relationship, but something as yet unknown. A possible
advantage of this peculiar phenomenon is seen in the fact that the prog-
eny of individuals which are close pollinated, or of individuals where
there is cross pollination between anthers and stigmas of different posi-
tion, usually is made up of plants with but one kind of flower. If cross
pollination is advantageous (see p. 866), the combination of heterostyly
and impotence in own pollen would seem to be particularly advantageous,
since own pollen is scarcely likely to be deposited on a stigma, and if it
should chance to lodge there, it would not initiate seed production.
Dicliny. — Dicliny, which commonly is regarded as a primitive floral
feature, is more characteristic of wind-pollinated than of insect-pollinated
plants, but it is far more common in the latter than formerly was supposed,
and there is almost certain proof of a strong evolutionary tendency from
monocliny to dicliny, as in the figs and in many composites. Among
the diclinous insect-pollinated species that probably are primitive, the
best known are the willows, which are dioecious. Some investigators
doubt whether as many as half of the plants that appear to be monoc inous
are so in fact. A large number of species have both monoclinots and
diclinous flowers on the same or on different plants; the maples illus-
trate this condition, some of them (as the box elder) appearing to have
become completely dioecious. Certain cultivated varieties of. the straw-
berry exhibit similar features.
Asparagus appears to have become essentially dioecious, since the stamens of
some plants and the pistils of others appear to play no part in pollination. Many
species (as in the grape and the horse chestnut) have been found to possess impotent
pollen in some flowers, and non-receptive stigmas in others. Rhamnus lance^lata, a
heterostyled species, seems to be approaching dioecism, since the short-styled flowers
produce the most seed, while the long-styled flowers have but little pollen a id that
small-grained. Many plants have organs occupying the position of stamens which
now play no direct part in pollination, whatever may have been the case formerly;
notable illustrations are the so-called sterile stamens of Parnassia (now aectar-
secreting organs) and of Pentstemon.
Much the most significant tendency toward dicliny is seen in the comjx>sites,
which commonly are regarded as the highest family of plants. In this family there
are three common floral conditions, that in which all the flowers are actinomorphic
and inconspicuous (as in Eupatorium), that in which all the flowers have conspicu-
ous strap-shaped (ligulate), zygomorphic corollas (as in the dandelion, fig. 1182),
and that in which there are actinomorphic and inconspicuous disk flowers, sur-
rounded by petal-like zygomorphic ray flowers; the third group is much the largest
REPRODUCTION AND DISPERSAL
857
and includes the asters, goldenrods, and sunflowers (fig. 1173). Perfect monocliny
is confined essentially to the second and to a part of the first group, so that a greater
or less amount of dicliny characterizes the majority of this great family. In the
third or ray-flowered group there are three common conditions : (i) that in which
all flowers are seed-producing, the disk flowers being monoclinous and the ray
flowers pistillate (as in Aster); (2) that in which the disk
flowers are monoclinous and the ray flowers reduced essen-
tially to corollas (as in Helianthus); and (3) that in which
the disk flowers are monoclinous but with the pistils sterile,
while the ray flowers are pistillate (as in Polymnia); Sil-
phium belongs in the last group, but it seems to have pro-
gressed still more toward dicliny, since the styles of the disk
flowers do not even fork into stigmas.
Composites without ray flowers show as much diversity
as do the ray-flowered forms, though the latter are much
more numerous. Some forms have all flowers alike and
monoclinous (as in Eupatorium). In Artemisia the heads
in some species consist of monoclinous and pistillate flowers,
while in other species they consist of monoclinous but sterile
disk flowers and of pistillate marginal flowers. Iv a exhibits
monoecious dicliny, the inner flowers being staminate and the
outer pistillate. Ambrosia and Xanthium also are monoe-
cious, but the two kinds of flowers are in separate heads.
The evolution of dioecism from monoecism appears to be
illustrated by Petasites, for though all heads have monocli-
nous but sterile central flowers and pistillate marginal
flowers, some plants have heads with many staminate and
few pistillate flowers, while other plants exhibit the reverse
condition. Gnaphalium alpinum is essentially dioecious,
since in some plants the stamens do not shed pollen, while in
others the pistils are sterile. Complete dioecism is illus-
trated by the related Antennaria. Some composites, notably
Ambrosia, are wind-pollinated, as well as diclinous. The
possible significance of the remarkable floral diversity of
the Compositae will be considered elsewhere (p. 877).
The protection of flowers from deleterious insects. — Deleterious kinds of insects.
— Crawling insects, such as the ants, are disadvantageous floral visitors, since the
pollen they carry is likely to be brushed off as they crawl from flower to flower.
Even among the flying insects, where such pollen losses are reduced to a minimum,
those that fly about in a haphazard manner, visiting various plant species in succes-
sion, are far less beneficial than are such insects as the bees which on any given
day visit individuals of the same plant species with consistency. It has often been
supposed that various floral features are highly advantageous because they exclude
certain insects, but the evidence for this view in many cases is more imaginary than
real. In any case, it is not to be supposed that the development of such features
has had any relation to deleterious insects; so far as they have value in this connec-
tion, it must be regarded as purely incidental.
10
FIG. 1182. — A
flower from a dande-
lion head (Taraxacum
officinale), illustrating
epigyny; note the
achene (o), the capil-
lary pappus (p) rep-
resenting the calyx,
the strap-shaped, five-
toothed, sympetalous
corolla (c), the tubular
column of syngene-
sious anthers (a) sur-
rounding the basal
portion of the style
(/), and the two re-
curved stigmas (g).
ECOLOGY
Hairs and glandular surfaces. — Stiff bristly hairs have been thought to serve
as barriers against various crawling animals, especially snails. Glandular hairs
doubtless are still more effective, and it is noteworthy that they abound on floral
stems more than elsewhere. Perhaps the most undoubted instance of such protec-
tion is in Silene, some species of which (as S. antirrhina) develop just at anthesis
an extensive glandular surface on the upper stem internodes; insects are caught by
these plants so frequently as to have led to the common name of catchflies.
Extrafloral nectaries. — On many plants there occur extrafioral nectaries (i.e.
nectar-secreting organs apart from inflorescences), as in various legumes, and in
Ricinus and Passiflora (figs. 1183, 1184). Usually they are most abundant on' the
upper side of the petioles and on the under side of the leaf blades. Ants frequently
1184
FIGS. 1183, 1184. — Extrafloral nectaries on the leaf of a passion flower (Passiflora).
1183, a palmately five-lobed leaf with nectaries (n) on the petiole and also on the blade;
1 184, a single nectary (n) with a large drop of nectar (d) ; considerably magnified.
visit these nectaries for food, and it commonly has been supposed that the organs
thus art advantageous to plants, the view being that the insects are satisfied with
what they obtain from the extrafloral nectaries and thus keep away from the flowers,
where the rifling of the floral nectaries might endanger cross pollination. It has
even been held that nectar-feeding ants are combative and keep off leaf-cutting
ants and other harmful insects. There is no valid evidence for these fanciful
theories, and recent careful experiments in which plants have been deprived of extra-
floral nectaries without affecting seed production or other plant activities would
seem to make them untenable. Indeed, the greater frequency of the visits of ants
to the nectary-bearing individuals has been shown to lead to more flower-rifling than
in the plants deprived of nectaries. In some cases, as in Vicia, bees have been ob-
served to visit extrafloral nectaries in preference to floral nectaries; in such a case
also extrafloral nectaries are a positive disadvantage to plants. Furthermore, in
REPRODUCTION AND DISPERSAL
859
many plants the chief secretion of nectar occurs before and in others after anthesis;
rarely, if ever, is there any exact correlation with this period, as in the case of floral
nectaries. The theory that these nectaries have no r61e of importance is more ten-
able than the theory of protection from ants. This view of the case is supported by
the fact that extrafloral nectaries occur in flowerless plants, as in Pteris and in various
other ferns.
Flower structure. — Many flowers are so constructed that certain flying insects, as
well as ants, are unable to disturb the pollen or nectar ; this is most obvious in flowers
with long corolla tubes and in zygomorphic flowers.
In flowers with long corolla tubes (such as the petunia,
fig. 1185), or with long spurs, only such insects as
various Lepidoptera, which have corresponding elon-
gated mouth parts, can reach the nectar; in some
cases the corolla tubes are lined with bristly hairs
which still further tend to keep out small insects,
though they offer practically no obstruction to long
probosces. In a number of zygomorphic flowers (as
in the snapdragon and in various legumes) it is diffi-
cult for small and weak insects to force their way to
the nectar or pollen. Among the features which tend
to exclude undesirable insects, floral zygomorphy, long
corolla tubes, and spurs are much the most impor-
tant. Since insects with long mouth parts can get
freely exposed nectar, the chief value of zygomorphy
and tubular corollas would seem to be the exclusion
of undesirable insects. While such structures may
have some evolutionary connection with insect visita-
tion, the connection is too complex to be understood
at present. It would seem much better for a flower to be pollinated in any manner
than to run the chance of no pollination if the proper insect were not present. The
significance of flower structure, here as elsewhere, is an unsolved enigma.
Some instances of specialized cross pollination. — General remarks. — The con-
sideration of cross pollination cannot be concluded without a short account of some
of the more striking instances of extreme specialization. In some of the cases to be
mentioned the dependence of the flower upon the insect (and often of the insect
upon the flower) is absolute, and therefore to be regarded as illustrating obligate
symbiosis. When such specialized forms are taken to other countries for cultiva-
tion, they may not produce seed, unless the insects also are transported.1
Silene and the orchids. — Some of the night-blooming catchflies (Silene) are
visited by nocturnal Lepidoptera, especially Dianthoecia, whose movements in
getting nectar incidentally effect pollination; later the moth deposits eggs in the
ovary with its long ovipositor, and the developing larvae feed upon the ovules. In
1 An excellent illustration of this is afforded by the orchid, Vanilla, whose fruits fur-
nish commercial vanilla; the absence of the proper pollinating insect in certain regions
makes artificial pollination a necessity for profitable cultivation. The day lily (Hemero-
callisfulva) never fruits in Europe, probably because of the absence of the proper insect-
FIG. 1185. — A flower of
Petunia ; note the long tube
(/) of the sympetalous corolla
(c/), well-fitted for pollina-
tion by moths with long
mouth parts; c, calyx of
five sepals.
86o ECOLOGY
such an instance symbiosis is more obligate for the insect than for the flower. The
orchids, as a class, show the most extreme floral specialization, dependence upon
some particular insect often being obligate.
Arum. — In Arum, although entirely different structures are involved, there is
much to recall the cross pollination of Aristolochia (p. 853). The inflorescences
are composed of staminate flowers above and of pistillate flowers below, which are
arranged on a club-shaped central axis, the spadix, and enclosed within a large
bract, the spathe, which, though enlarged below, is considerably constricted above.
At anthesis the flowers give forth a nauseous odor that attracts numerous small
flies, whose exit for a time is said to be barred by reflexed hairs between the pistillate
and staminate flowers and in the narrow passageway above. The pistillate flowers
mature first, and when the staminate flowers mature, the lower ring of hairs dies,
permitting the insects to crawl over the stamens, where they become covere'l with
pollen. Very soon the upper ring of hairs withers also, permitting escape to the
exterior. If the insects visit another inflorescence at once, it is evident that the
mature pistillate flowers are likely to be cross-pollinated. Since Arum, as veil as
most other aroids with similar features, is monoecious, it is obvious that a highly
specialized mechanism of this sort prevents not autogamy, but geitonogamy. Re-
cent observations call most of this familiar account in question, especially as to
Arum maculatum. In this species it is claimed that the exit for the visiting lies is
not barred, since the hairs are not stiff enough to impede the insects and often are
not even long enough to fill the passageway. So far as the insects are held in the
spathe, it is said to be due to drugging by the plant, and it is claimed tha: there
is a sufficient amount of overlapping in the periods of maturity of the staminate and
pistillate flowers to result in geitonogamy. Furthermore, there is no adequate proof
that the flies which manage to escape visit other inflorescences sufficiently soon to
effect cross pollination.
The fig. — The most remarkable of the known cases of cross pollination whose
evolutionary development cannot even be imagined, is that of the figs, a group of
plants which, like the aroids, are diclinous and commonly placed low in the scale
of seed plants. The inflorescence, known as a synconlum, is unique; the numerous
flowers line the walls of a chamber (representing the receptacle) and are entirely
hidden (figs. 1186, 1187). Entrance to the flowers is possible only through \\ small
apical orifice (as in the India-rubber tree, Ficus elastica), which is lined with scales.
AH species of figs are diclinous, some having the two kinds of flowers soriewhat
indiscriminately mixed; in most species, however, the staminate flowers are oward
the top and the pistillate flowers toward the base of the synconium. Some species
approach dioecism, certain trees having pistillate and others monoecious synconia.
The fig of commerce (Ficus Carlca) is essentially dioecious, the pistillate :lowers
of apparently monoecious synconia being sterile; in rare cases, however, the stam-
inate trees bear some pistillate flowers and even ripen seeds.
The commercial fig is pollinated by a small wasp, Blastophaga glossorum. whose
life cycle is most extraordinary. The females force their way through the synconial
orifice, and this is so difficult of accomplishment that usually they lose their wings
in the process; after laying their eggs within the young ovules, they die within the
growing fig. Those females that chance to enter the pistillate synconia (which
become the figs of commerce) have no progeny, since the flowers have styles of such
REPRODUCTION AND DISPERSAL
86l
length that the insects are unable to lay their eggs in the right spot (fig. 1188). But
in the staminate figs, known as caprifigs, there are short-styled rudiments of
pistillate flowers (often called gall flowers, fig. 1189), in which eggs may be placed
properly, later hatching into wasps. Some stimulus exerted by the insect causes
the ovary primordia to develop into seedless galls. After a time the males hatch,
eating their way out
of the galls in which
they developed and
into the galls occu-
pied by developing
females ; copulation is
followed by the death
of the males within
the caprifig. The fe-
males thereupon
escape (fig. 1190),
crawling over the
staminate flowers of
the caprifig and be-
coming dusted with '"1189 1187 n 1188 1186 1190
pollen; those that FIGS. ri86-ii9o. — Pollination of the fig (Ficus Carica):
1 1 86, a synconium cut longitudinally, showing gall flowers pro-
duced by the fig wasp (Blastophaga grossorum); near the mouth
of the cavity is a female fig wasp, which has escaped from one
of the galls; 1187, a similar synconium with seed-producing
pistillate flowers; near the mouth of the cavity are two female
fig wasps, one of which has already crept inside; 1188, a long-
styled seed-producing flower; 1189, a short -styled gall flower;
1190, a fig wasp escaping from a gall flower. — From KERNER.
chance to visit figs
incidentally pollinate
the stigmas therein,
but have no progeny,
while those that go
to caprifigs have prog-
enyi, but are of no
service in pollination.
One of the strangest features of a process strange throughout is that the pistil-
late flowers mature two months before the staminate flowers; however, by the time
the latter are mature, another crop of synconia has developed with stigmas ready for
pollination, so that stigmas of a given generation are pollinated from inflorescences
of the preceding generation. In southern Italy there are three such crops of figs
and caprifigs each year (viz., in April, June, and August), and three corresponding
generations of wasps. This symbiosis between Ficus and Blastophaga has been
denominated mutualism, but surely it is a somewhat destructive form of mutualism,
where death without progeny comes to such a large proportion of the symbionts on
each side, namely, to the female insects that enter the figs and to the pistillate flowers
of the caprifigs.
Centuries before the process of pollination was discovered, the ancients cultivated
the commercially valueless caprifigs, and placed branches with maturing synconia
on fertile fn; trees ; this process, known as Caprification, makes it easy for the female
wasps (which fly weakly, though possessing wings) upon emergence from the capri-
figs to enter and pollinate the figs. Caprification and pollination are quite unneces-
sary for reproduction commercially, since figs always are propagated from cuttings.
Caprification is not always necessary, even for the production of commercial figs,
862 ECOLOGY
there being some varieties which mature edible fruits (though without viable seeds)
without caprification or pollination. Other varieties, notably the Smyrna fig, re-
quire pollination for their best development, the caprified fruits surpassing those
that are not caprified in plumpness, juiciness, and flavor.
Geitonogamy. — General remarks. — In spite of the numerous and
remarkable features which facilitate cross pollination, geitonogamy (or
pollination between different flowers of the same plant) and close polli-
nation are very common and, taken together, perhaps are more common
in monoclinous flowers than is cross pollination. Furthermore, there are
cases in which the features facilitating these kinds of pollination are about
as specialized as any that have been previously noted in connection with
cross pollination. In most species a number of flowers bloom at once on
the same plant, so that dichogamy does not prevent geitonogamy, es-
pecially because insects usually visit all the flowers on a given plant before
flying to another. The first flower on each plant visited may gel only
foreign pollen, but the chance of geitonogamy increases with the ni mber
of flowers visited, though it should be remembered that the general
prepotency of foreign pollen greatly favors the latter in initiating seed
production. Geitonogamy is commonest in plants with compa t in-
florescences, especially where these are umbels, spikes, or heads, as in
the umbellifers (the highest of the polypetalous dicotyls) and in the
composites (the highest of plants). While such floral massing does not
exclude cross pollination, it so greatly facilitates geitonogamy that the
latter probably is the chief method of pollination. Although the almost
habitual dichogamy of these plants prevents autogamy and peculiarly
facilitates geitonogamy, there is no adequate evidence that the latter is
perceptibly more advantageous than the former.
Illustrations of geitonogamy in the composites and umbellifers. — The culmination
of conditions favorable to geitonogamy occurs in the composites, a group that is
notably protandrous. The outer flowers bloom first, but the stigmas remain recep-
tive until the flowers next within shed pollen. After the stamens mature, the style
elongates, pushing up through the surrounding tube of united anthers and sw ibbing
out some of the pollen, which adheres to the style bristles. Later the style forks,
exposing the receptive stigmatic surfaces (fig. 1182), which with the ac herent
pollen commonly come into contact with the stigmatic surfaces of adjoining
flowers. Entangled style branches of this sort are especially conspicuous in Eupa-
torium. In contrast to pollination by wind or insects, this may be termed contact
pollination. Geitonogamy by contact is especially characteristic of the milky-
juiced composites (such as the dandelion, figs. 1193, 1194), since the heads open by
day and close at night and in rainy weather, insuring to an unusual degree the
contact of stigmas and pollen-covered organs of adjoining flowers.
REPRODUCTION AND DISPERSAL 863
In composites with conical or columnar receptacles (such as Rudbeckia and
Lepachys), there occurs what may be termed gravity pollination, pollen from the
upper flowers dropping upon the stigmas of the lower and older flowers. In An-
thentis, as the flowers develop, the disk elongates in such a way that the stigmas of
the older flowers are exactly under the shedding stamens of the younger flowers. It
will he recalled that in those composites that tend toward dicliny, it is the outer
(older) flowers that ordinarily are pistillate, and the inner (younger) flowers that
ordinarily are staminate. So far as geitonogamy is concerned, such a condition is
economical, since stamens would be useless in the outer flowers and pistils in the
inner flowers, though they might be of value in case of cross pollination by insects.
The structural facilitation of geitonogamy in the umbellifers is almost as marked
as in the composites. In Eryngium the flowers are in dense heads that facilitate
contact pollination between adjoining flowers. In Sanicula there are monoclinous
and staminate flowers, the long styles of the former bending over and bringing the
stigmatic surfaces into contact with the latter. In such wind-pollinated composites
as Ambrosia, geitonogamy is likely to take place between flowers of different heads,
as the staminate heads are uppermost.
Close pollination or autogamy. — General remarks. — Autogamy,
that is, pollination between anthers and stigmas of the same flower, once
thought to be relatively rare, is now known to be extremely common. In
some cases the features that facilitate autogamy are quite as striking
as are those previously mentioned features that impede or prevent it.
Autogamy by contact may be called self pollination, a term often incor-
rectly made synonymous with autogamy in general ; as with geitonogamy,
close pollination occurs chiefly by contact or through the agency of grav-
ity, though wind and insects also may be operative. In many cases,
especially where foreign pollen is prepotent over own pollen, autogamy
probably is effective only in the absence of cross pollination; in many
other cases both appear to be equally effective, and in a number of in-
stances autogamy is the only form of pollination possible. Careful
study has shown that in alpine and arctic regions autogamy habitually
exceeds xenogamy.
Illustrations of autogamy. — Simple cases of autogamy occur in Trillium and in
Geranium, the anthers and stigmas being in close juxtaposition; most such flowers
are slightly protogynous, so that cross pollination may occur before there is a chance
for contact pollination. Somewhat more complex are the many cases in which
anthers and stigmas come into contact through growth movements, as in the stamen
movements of Circaea and of many crucifers, and in the style movements of Epilo-
bium and of many other plants; such flowers also are as a rule slightly dichogamous.
In the composites autogamy as well as geitonogamy may take place through style
inflections and through the closing and the opening of heads; similarly, many flowers
that open and close daily may exhibit autogamy (as in Gentiana). Pollen drops
directly on the stigma, illustrating gravity pollination in many erect flowers with
364 ECOLOGY
styles shorter than the stamens (as in the lilac), and in many pendulous flowers with
styles longer than the stamens (as in Dodecatheon). In Pedicularis the growing
corolla rubs over the anthers, so that the pollen falls upon the stigma underneath.
In Cyclamen and Moneses the growth curvatures of the flower stalk at the time ol
anther maturity cause the pollen to drop out upon the stigma.
Yucca. — The flowers of Yucca commonly are pendent (fig. 1192), and though
the stigmas extend beyond the anthers, pollination by gravity is unlikely if not im-
possible. The flowers are nocturnal, blossoming but once, and are pollinated by
a small moth, Pronuba. The females pierce the ovaries with their ovipositors and
lay eggs among the ovules, before and after which they deliberately take pollen
from the anthers, holding it in their specially constructed maxillary palps, and ram
it into the stigma. As a result of this astounding process the ovules develop; each
larva eats about twenty, and the rest develop into seeds. This symbiosis is be-
lieved to be obligate for each symbiont; in any event, Yucca is seedless in the ab-
sence of Pronuba. The mode of evolution of an instinct that impels an insert to
stuff a stigma with pollen cannot even be imagined.1
Cleislogamy. — The culmination of structures facilitating close pol-
lination is found in flowers that never open, since in these with rare ex-
ceptions autogamy alone is possible. Flowers that regularly open, .< uch
as those heretofore considered, are termed chasmogamotts, while flo'vers
that never open are called cleistogamous . Cleistogamy may be habitual
(as in the subterranean flowers of Viola and Poly gala), or it may be
facultative, depending upon definite external factors. Conspicuous cases
of facultative cleistogamy are found in Oxalis, Specularia, Impatiens,
and Lamium. Subjection to low temperature is believed to be the chief
cause of such cleistogamy; in Lamium, for example, the spring and
autumn flowers are cleistogamous, while the summer flowers are chas-
mogamous or sometimes cleistogamous in cold or rainy weather. In
Viola sepincola the aerial flowers are chasmogamous in the sunshine and
cleistogamous in the shade, indicating that light as well as heat may be
a factor. The submersed flowers of Alisma also are cleistogamous.
Habitual cleistogamy is well illustrated by the subterranean and
generally colorless flowers of Viola cucidlata (and many other violets),
Amphicarpaea, and Polygala polygama (fig. 1191); in Viola cucitllata
they appear some weeks or months after the showy aerial flowers, but
in Polygala the two kinds of flowers are nearly synchronous. In all
three cases they are much more productive than are the showy and pre-
sumably cross-pollinated aerial flowers. In the rock rose (Helianthe-
1 While Yucca is here considered as illustrating close pollination, the moths, after
gathering pollen, often fly to other flowers or even to other plants, so that they may effect
autogamy, geitonogamy, or xenogamy.
REPRODUCTION AND DISPERSAL
865
mum) both the open and the closed flowers are aerial, the former being
large and showy, while the latter appear later and are much smaller.
In Leersia oryzoides, the conspicuous open monoclinous flowers rarely
fruit, though the cleistogamous flowers hidden within the leaf sheaths
are fertile. Few if any species have exclusively cleistogamous flowers^
though this condition has been reported
for Myrmecodia echinata, Salvia cleis-
logama, Ophrys apifera, Ammannia
latifolia, and several grasses.
The cleistogamous and chasmoga-
mous flowers of the same species differ
widely in structure, though they agree
in some important respects, as in the
potency of own pollen. In general ths
former are much the smaller, and in
some respects they resemble early
stages in the development of the latter;
this is especially true as to the corolla,
which either is entirely lacking or exists
in the form of protuberances (e.g. in
Specidaria), as in the bud of a chas-
mogamous flower. The development
of the stamen and pistil is not arrested,
as is that of the corolla, though the
stamens may
r be fewer in
1 number, and
usually the
pollen grains are much less numerous
than in chasmogamous flowers. In
Helianthemum the closed flowers have
but three to ten stamens, as contrasted
with the numerous stamens of the open
flowers; sometimes the stamens are
reduced to a single anther, and the pol-
len grains may number only a dozen
or even less in each chamber (as in
Oxalis). Occasionally the pistil exhibits reduction; for example, in
Helianthemum the open flowers have many ovules, while there are only
FIG. 1191. — Fruiting shoots of
Poly gala polygama ; the aerial stem (a)
bears showy racemed flowers which
open and may be cross pollinated; on
underground stems (M) are borne
numerous cleistogamous flowers (/),
which are close pollinated and give
rise to abundant seed pods (/>).
866 ECOLOGY
a few in the closed flowers. Perhaps the most striking case of reduction
is where, instead of a stigma, there is an open passageway to the ovules,
recalling gymnosperms. Pollination, of course, is by direct contact, but
there are some remarkable cases (as in Oxalis and Impatiens) in which,
strictly speaking, there is no pollination at all, since the pollen grains ger-
minate within the anther, putting forth their tubes which grow out toward
the stigma.1 In iMmium and in Viola odorata the anthers do not even
dehisce, so that the pollen tubes have to penetrate the anther walls; in
Viola the anther wall is devoid of the usual thickening, and the pollen
tubes pass readily through permeable spots of small plasmatic cells. As
might be expected, cleistogamous flowers do not exhibit marked dichog-
amy. Allied to cleistogamy is bud pollination (as in Oenothera and in vari-
ous orchids), where autogamy occurs in ordinary flowers before they cpen.
The fact that the open flowers of plants which possess also cleistogamous fl< >wers
usually produce but few seeds has led to the theory that the failure of cross polli-
nation probably has resulted in the evolution of cleistogamy. This theory h is no
evidence in its favor. In Viola biflora there are cleistogamous flowers, although
the showy open flowers fruit abundantly. Furthermore, close pollination can take
place quite as readily in the open as in the closed flowers of a given species, since own
pollen is as potent in one case as in the other. As noted above (see also p. 901),
cleistogamy is in part associated with arrested development, and usually is due to
definite external conditions, which are unfavorable for chasmogamy; for example,
in Lamium amplexicaule the first flowers in spring and the last flowers in autumn
are cleistogamous, while the summer flowers are open and showy. Cleistogamy
is advantageous in that closed flowers are well protected from rain and from the
visits of pollen-gathering insects. Subterranean cleistogamy is advantageous in
that the seeds are self-planted and are well-protected from many seed-eating ani-
mals, such as birds.
The comparative advantages of cross pollination and close pollination.
— Introductory remarks. — Usually it is believed that cross pollination
must be highly advantageous because it is so common, and particularly
that the diverse and sometimes extraordinary features which impede
or even prevent close pollination are prima facie evidence of the value
of xenogamy. The usual reason for regarding cross pollination as supe-
rior to close pollination is either that it facilitates the merging of d, verse
racial characters 2 or that it promotes variability or racial vigor. The
1 The germination of pollen grains within the anther has been reported occasionally
even in chasmogamous flowers.
* If this conception is true, it still further emphasizes the essential difference between
true cross pollination and geitonogamy, since there is no such merging in the latter, the
Sowers of a single individual having a common immediate ancestry.
REPRODUCTION AND DISPERSAL 867
problem of cross pollination is essentially a phase of the problem of
sexuality, which has been considered previously.
Experimental data. — It long has been believed by plant breeders
that occasionally crossing is necessary if the individuals of a species
are to be kept in a state of the highest vigor, inbreeding (i.e. breeding
between closely related forms, as in close pollination) often having been
shown to result in diminished luxuriance in the progeny. Careful ex-
periments conducted many years ago on a number of species, though
yielding rather discordant results, tended to show that cross pollination,
on the whole, is more advantageous than close pollination. In some
cases diminished vigor, which is made evident by smaller size and les-
sened seed production, is obvious in the progeny of the first generation (as
in Ipomoea purpurea and Mimidus luteus) ; in a much larger number of
cases it is obvious only after continued inbreeding for several genera-
tions, and in still other cases inbreeding appears never to result in
deterioration.
Recent experiments made on Indian corn show that close pollination
results in the first generation in reduced height and yield; in the gener-
ations following there is further reduction, but the decrement becomes
less each time until about the fifth generation, after which continued
close pollination makes no appreciable change in the offspring. Such
dwarfs may reproduce as such indefinitely, but if crossed the offspring
of the first generation has the original full size. In the earlier experiments
geitonogamy usually resulted in quite as much weakening as did autog-
amy, showing conclusively that it is much more nearly related to close
pollination than to cross pollination. Progeny from crossed individuals
were found to flower first and to suffer less in crowded cultures than
did progeny from inbred races; this fact has been taken to be of great
significance, since it suggests the likelihood of the submergence of inbred
races by cross-pollinated races in the " struggle for existence " in nature.
The possible advantages of cross pollination. — The experiments cited
may mean either that cross pollination is in some way advantageous,
or that close pollination is in some way disadvantageous. The support-
ers of the first theory have held that cross pollination is advantageous,
because it insures the mingling of two lines of ancestry with their sup-
posedly different beneficial characters, or because it promotes varia-
bility or racial vigor. There is very little in favor of this view and very
much against it. In the first place, experiments on the same species
at different times and places vary widely in their results; sometimes own
868 ECOLOGY
pollen may be quite impotent, sometimes imperfectly potent, and some-
times as potent as foreign pollen. Usually these variations are due to
external factors, such as differences in moisture, light, etc. Many of
the very species that have been supposed to demonstrate the benefits of
cross pollination grow under certain conditions quite as well when
close pollinated. Similarly, the results of cross pollination vary widely
under different external conditions. In case of crossing between un-
related individuals of inbred ancestry grown under similar external con-
ditions, the progeny if grown under like conditions are scarcely more
vigorous than with close pollination, showing that the vigor supposed
to result from crossing may result rather from favorable external con-
ditions. Comparable phenomena are known also in the animal king-
dom.
The data from heterostyled plants oppose the usual theory of rross
pollination, since weak individuals or none at all result from crossing be-
tween plants with styles of unequal length, irrespective of nearne;s or
remoteness of relationship; in such cases the progeny is weak, regar Hess
of the mingling of " diverse racial characters." Dichogamy is cit< d as
affording a priori evidence that cross pollination must be beneficial , yet
dioecious plants (in which close pollination is of course impossible) ex-
hibit the earlier maturation of anthers or of stigmas almost as habitually
as do monoclinous plants.
The chief reasons for disbelieving that occasional cross pollination is
necessary in order to prevent the deterioration of plant species are
afforded by the phenomena of autogamy and vegetative reproduction.
Usually the argument for the value of cross pollination is built on those
cases in which foreign pollen is prepotent or own pollen impotent, omit-
ting the equally numerous and significant cases where own pollen is fully
potent. Apart from the vast number of plants that exhibit frequent
autogamy, there are many in which autogamy is habitual, particularly
the numerous species with cleistogamous flowers; in these there is no
deterioration, in spite of repeated close pollination. In the experiments
cited above, autogamy in several forms (as in Petunia, Eschschcltzia,
and Nicotiana) resulted in progeny that was essentially as vigorous
as when xenogamy was employed. Even in the two species (viz. Ipemoea
purpurea and Mimulus luteus) which were thought most clearly to
demonstrate the benefits of cross pollination, luxuriant sports arose in the
autogamous cultures, which had as vigorous progeny when close polli-
nated, as did cultures from cross-pollinated individuals. Perhaps most
REPRODUCTION AND DISPERSAL 869
significant of all is the prevalence of vegetative reproduction in nearly
all the higher plants. There is not a single case known in which the
indefinite continuance of vegetative reproduction Causes the slightest
deterioration. Indeed, there are many plants in which crossing has
essentially disappeared (as the duckweeds and the horn worts). Nor
is there any evidence of deterioration in the species that are persistently
parthenogenetic. It is to be noted that the plants which in experiments
have shown benefit from crossing are chiefly garden varieties that have
been much hybridized. No benefit from crossing has been shown in
natural species. that have long been pure. Nor is there good evidence
that crossing promotes variability. Among the most variable of plants
are the cleistogamous violets and such parthenogenetic genera as Hiera-
cium and Taraxacum.
The possible disadvantages of close pollination. — Even if there are no
conspicuous advantages in cross pollination, there may be disadvantages
in close pollination. Such a view seems particularly plausible in the
case of those orchids in which own pollen seems to be prejudicial and
possibly even poisonous to the stigma (or the stigma to the germinating
own pollen), perhaps in a way analogous to the excreta of root hairs,
except that here the deleterious effects concern only individuals of a
species, and not the species as a whole. If this conception is valid, it
may account for the occasional impotence of own pollen; in species
where foreign pollen is prepotent, the deleterious influence of own pollen
may be considered to be less marked. In the large number of species
with potent own pollen it may be supposed that such deleterious effects
are wanting. The benefits of cross pollination and the disadvantages
of close pollination have been too much emphasized. Close pollination
and its essential equivalent, geitonogamy, are extremely common in
nature, nor must it be forgotten, also, that many of the important plant
and animal races utilized by man have reached their present state of
commercial perfection by the most careful inbreeding.
The protective features of flowers. — The calyx. — Flowers are among
the most delicate of plant organs, the ephemeral petals and the stamens
and pistils with their gametophytes and embryos being particularly
sensitive. Foremost among the protective organs is the calyx (fig. 1136),
which, during development, often is the only exposed floral organ, and
commonly is much less delicate than are the structures it encloses.
Among the dangers to which the developing corolla and the essential
organs might otherwise be exposed are those arising from rain, drought,
Byo ECOLOGY
heat, cold, and wind.1 Since the calyx commonly is composed of green
and leaflike sepals, it probably plays an important part in food manufac-
ture, as well as in protection, and occasionally it replaces the corolla in
the matter of showiness. Sometimes bracts supplement or replace the
calyx as protective structures, as in Desmodium. In the composites
a calyx-like involucre is the chief protective organ (fig. 1194). In the
aroids the large spathe protects the entire inflorescence in flower as well
as in bud. Even the corolla may be a protective organ, as in the grape,
where it falls as soon as the bud opens, and in flowers which close at
night and in stormy weather.
The duration of flowers. — From the standpoint of protection,
flowers may be divided into those that remain open throughout anthesis
and those that close once or more during that period. Of those which
remain open, many are ephemeral, and hence need but little protection,
particularly as opening usually comes only in favorable (i.e. warm and
sunny) weather. Among the latter are the flower-of-an-hour (Hib.scus
Trionum), which has the most ephemeral of flowers, rarely lasting for
more than three hours, the day lily (Hemerocallisfulva), and the n ght-
blooming cereus. In striking contrast with these are certain oichid
flowers which may remain open for fifty to eighty days if unpollinated
(as in Odontoglossum)? On the whole, plants with ephemeral flowers
are more likely to have a large number of blossoms in a season than are
plants with long-lived flowers.
The protection of non-closing flowers. — Long-lived, non-closing
flowers would seem to need some protection, the greatest dangers, per-
haps, being the waste of pollen through rain, the drying of the stigmatic
surface through drought, and injury from low temperature or frost.
Pollen is not readily wetted, which is itself a matter of considerable pro-
tective importance; also it will be recalled that some pollen, such as that
of exposed vernal flowers, is not readily injured by wetting or by low
temperature. Many plants have nodding flowers, notably the ericads,
and also Bryophyllum and Yucca (fig. 1192), and thus are well protected
from injury by rain. Many of the features that protect flowers from
crawling insects also protect them from rain; among these are contracted
corolla throats (-as in Arctostaphylos and in various borages), and «ygo-
1 An illustration of calyx protection is afforded by the peach and strawberry, in which
unopened buds are much less subject to injury from frost than are buds that are partly
or fully opened.
* The early withering of orchid corollas may be induced not only by pollination, but
also by the mechanical irritation of the stigma.
REPRODUCTION AND DISPERSAL
87I
morphic corollas, which not only are more or less closed, but commonly
are swung laterally, as in the labiates (fig. 1174) and in the orchids.
The protection of flowers by closing. — Closure during anthesis usually
involves a single flower, but sometimes it involves an entire inflorescence,
FIG. 1192. — Yucca flowers: A, an inflorescence of Yucca patens with numerous
pendulous flowers : B, flowers of Yucca Treculeana ; note the perianth, the stamens with
recurved anthers, and the stigmas. — From TRELEASE.
as in the heads of the milky-juiced composites (figs. 1193, 1194). Most
such flowers or inflorescences open in sunshine and close at night and in
cloudy weather, though in a few cases this order is reversed, the 'flowers
opening at night and in cloudy weather and closing in the sunshine.
Some flowers open for two successive days (as in the poppy), others
for three to five days (as in Eschsch oltzia) ; the flowers of Crocus and
Hepatica may open daily for more than a week. Usually nocturnal
872
ECOLOGY
flowers are more ephemeral than are diurnal flowers (as in the night-
blooming cereus), but the flowers of evening primroses, although opening
but once, remain open for some hours after sunrise; the flowers of some
species of Silene open for at least three successive nights. Flowers
often have a longer period of anthesis in
spring and autumn than in summer; even
ff T"*'TJiijO?5^ filfffl ^ suc^ ePnemeral flowers as those of Hibiscus
Trionum and Hewerocallis fidva may open
toward the beginning or the end of the
flowering period on three successive days.
Flowers cannot be classed simply as diurnal or
nocturnal, since most hours of the day and night
are marked by the opening or closing of the f owers
of some species. The opening and the closing
hours of a given species vary widely with the lati-
tude and the season, but in general the > arlier
diurnal flowers open about sunrise (as in the
chicory and the morning glory) and the later about
noon (as in Mesembrianthemum). The noc;urnal
series begins about dusk (as in Silene) and closes
about ten (as in Cereus). The times of < losing
are much less definite than are those of opening,
and they extend over most of the twenty-four
hours; this is partly because flowers open much
more rapidly than they close, the former process
sometimes being sudden, as in Oenothcra. Begin-
ning with the early forenoon (as in salsify and
chicory) each hour until after sunset is marked by
FIGS. 1193, 1194. — Floral
opening and closing in the dan-
delion (Taraxacum officinale):
1193, an open head as seen in
full sunlight; 1194, the same
head as seen at night; the invo-
lucre (f) is double, consisting of
short outer bracts (b) and a
single row of long inner bracts
(/>') ; opening and closing are due
chiefly to the movements of the
inner bracts, the position of the
outer bracts shifting but slightly.
the closing of some diurnal flowers. Similarly
nocturnal flowers may close at any time from midnight (as in Cereus) to suniise, or
even during the following forenoon (as in the evening primrose). The entire
scheme, as above outlined, may be disarranged in cloudy weather.
The factors involved in the opening and the closing of flowers. — - The
mechanism of opening and closing and the factors controlling this mech-
anism are in part unknown. It has been shown in a few cases, and it is
believed to be true in others, that these movements are true movements of
ffcowth, opening being due to epinasty, and closing to hyponasty, in the
segments of the perianth or involucre. Probably the chief single factor
causing epinasty, and hence opening, is an increase of temperature.
Opening as a result of an increase of temperature has been proven ex-
perimentally in a number of cases, notably in the tulip and the crocus;
REPRODUCTION AND DISPERSAL 873
a crocus flower opens in two to four minutes, when the temperature is
raised suddenly fifteen or twenty degrees, and it is sensitive to a change
of half a degree. Successive alternations of cold and warmth may induce
several successive closings and openings within an hour. It is believed
by many investigators that changes in turgor are responsible for some
cases of opening and closing, as in the tulip. Possibly some flowers open
and close autonomously; the flowers of Calendula arvensis, for example,
open in the dark without any change in temperature.
In some cases light, independently of temperature, has been shown to cause
opening (as in the day lily and in the gentians). In nature both light and heat prob-
ably cooperate, especially in opening; the early closing of far northern flowers, in
spite of long daylight, probably is a matter of temperature alone. In a few cases
opening and closing are due to moisture changes; so sensitive is the head of Carlina
to such changes that it has been used as a hygroscope, closing in moist air and open-
ing in dry air. Obviously the explanation of the movements of flowers that do not
open rather promptly at sunrise or close at sunset presents certain difficulties. While
these have not as yet been solved experimentally, it is likely that certain species
require a greater amount of heat or light for opening or a less amount for closing
than do others, or that they are less quickly sensitive. Most remarkable are the
nocturnal flowers, in which opening is caused by decreased temperature and light
instead of by increased temperature and light, as in diurnal flowers.
The advantages of flower closing. — In general, flowers that close
during anthesis are open at the time when their special pollinating in-
sects are most active. Most of the latter (especially the bees and butter-
flies) are diurnal, and are affected by the same factors that influence
flower opening, such as increased heat and light and low atmospheric
humidity (or at least absence of rain). Nocturnal flowers are pollinated
almost exclusively by nocturnal insects, such as certain moths. How
such a remarkable correlation of flower opening and insect activity may
have arisen is altogether unknown. It is probable that closure when
the insects are not active is likely to be of considerable protective impor-
tance ; in diurnal flowers pollen probably is conserved by closure in
rainy weather, and in all cases closure for a part of the time would seem
to favor the lengthening of the period of stigmatic receptivity, particu-
larly in nocturnal flowers, which are freed from the desiccating influence
of sunlight. In addition, as previously noted, opening and closing favor
close pollination and even geitonogamy in the composites.
Protective movements other than floral opening and closing. — In some
flowers the anther valves are hygroscopic (as in Alchemilla and Laurns),
closing in moist weather; usually they are closed much more quickly
874
ECOLOGY
than the closure of perianth segments is effected by low temperature,
a few seconds commonly sufficing. Such closure is of direct value in
protecting pollen from rain. It may be recalled, also, that in anthers,
dehiscence generally is caused by desiccation, so that the first opening
is unlikely to occur in wet weather. Many plants show movements of
the flower axes (pedi-
cels) or inflorescence
axes (peduncles). .In
certain instances the
buds are erect and
the flowers pendent (as
in AquUegia). More
closely related to floral
movements are those
cases in which the
flower is erect by day
and pendent by night
and in rainy weather
(as in Geranium and
Anemone); in the um-
bellifers there is a simi-
lar movement of the
entire inflorescence.
Such movements are
due to growth inequali-
ties in the pedicels or
the peduncles, and the
advantages therefrom
would appear to be in
1197
FIGS. 1195-1197. — Growth movements accompany-
ing flower and fruit development in a wild onion (Allium
cernuum) : 1 195, a flower bud, showing the nodding posi-
tion of the young peduncle (p); b, spathe; 1196, a shoot
in full bloom ; note that the peduncle (/>) has straightened
out except at the tip (<) ; the flowers are arranged in an
umbel, the pedicels (/>') being oriented in various direc-
tions; note the exserted stamens (5); 1197, a shoot in
fruit, showing the conspicuous capsules (c) ; note that the
pedicels (/>') have become erect or ascending.
exposure to pollinating
insects in sunshine, and in protection from rain and cold during the
night or in rainy weather. In the poppy the buds are nodding, b it the
flowers are erect, the pedicels becoming apogeotropic. Movements of
much complexity, but without obvious advantages, are seen in Allium
cernuum (figs. 1195-1197), where the nodding bud becomes erect by
differential growth in the peduncle, while later the fruits become erect
by similar growth in the pedicels.
Protection during fruit development. — Usually the developing ovules
are delicate. Ultimately the enveloping ovary walls and seed coats
REPRODUCTION AND DISPERSAL
875
become thick protective structures of considerable value to the embryos,
but often during the earlier stages of growth some protection from desic-
cation and other dangers is afforded by special structures or habits.
After anthesis the calyx may close about the developing fruits (as in
Physalis), appearing much as in the bud, thus once more serving as an
organ of protection, as well as continuing to manufacture food. In
the composites the protective and synthetic activity of the calyx is re-
placed by that of the involucre. Many fruits which
are edible when ripe, during immaturity are more
or less protected from predatory animals by un-
pleasant flavors, hardness, and spinescence (as in
Opuntia and Ribes), and perhaps by inconspicu-
ousness, since their green color is similar to that of
the leaves.
In many plants, especially in hydrophytes, the
developing fruit-stalks exhibit striking 'growth cur-
vatures. In most cases the previously erect stalk
recurves (as in Pellandra and Nymphaea), causing
the downward orientation and consequent submer-
gence of the fruit. Some land plants show similar
reactions, notably Phry-ma (fig. 1198), whose fruits
become strongly reflexed, whence the common
name, lopseed; in the peanut, stalks that are erect
or ascending until anthesis, later recurve and force
the developing nut into the ground. Probably in
all of the above cases pedicels or peduncles orig-
inally apogeotropic become progeotropic after an-
thesis, but no explanation of such peculiar behavior
has been given, nor is there any obvious advantage, except perhaps a
slight one, in connection with planting (p. 929).
The origin of floral structures. — The flower is the most complicated
of plant structures, and the organs concerned directly or indirectly with
pollination form the most complicated part of the flower. An adequate
theory of flower structure, however, must explain not alone this complex-
ity, but also the evolution of the mouth parts of the flower-visiting in-
sects (notably those of the bees, butterflies, and moths), which appear
to be so obviously related to the flowers. Capping all, and most diffi-
cult of all to explain, are the cases of obligate reciprocal- symbiosis, of
which the fig and the yucca are the most remarkable.
FIG. 1 198. — A flow-
ering spike of the lop-
seed (Phryma Lepto-
stachya) ; at anthesis
the flowers (/) with
their bilabiate corollas
(c) are horizontal, but
subsequent epinastic
growth causes the
fruits (/') to become
strongly reflexed.
876 ECOLOGY
Since the abandonment of the theory of special creation, a common
hypothesis has been that floral structures and specialized mouth parts
have arisen pari passu by reciprocal natural selection. This theory
implies that those flowers and insects of each generation that happen to
exhibit the greatest reciprocal specialization will be the ones to have
progeny, while the more generalized forms will be so handicapped that
they will be submerged in the " struggle for existence." Upon analysis
this theory seems almost inconceivable; furthermore, there are few if any
facts to support it, and many facts to controvert it.
In the first place, many floral features, such as the kind of inflorescence
the position of the various organs, the forms and markings of the corolla,
and the association of dichogamy with dioecism, have no known ad-
vantage, much less an advantage great enough to make their possessors
better adapted than are their neighbors.1 Nor is there the remctest
evidence that generalized flowers are less successful than those that are
specialized. Indeed, the orchids, which have the most specialized of
flowers, appear to be on the way toward extinction, because of this
very specialization; they represent a case of " over-adaptation," and
therefore present a condition that is contrary to the fundamental
postulates of natural selection. In some other groups of plants the
flowers are so strongly protandrous that pollination rarely takes p'ace,
because insect visits occur either after the pollen is shed or before
the stigmas mature. In contrast to the orchids, the grasses and the
catkin-bearing trees are dominant and widely successful groups of
plants, although they possess generalized flowers, which are diclinous
and wind-pollinated.
Actinomorphic flowers with exposed pollen and nectar often are
visited in preference to long-tubed or zygomorphic flowers, even by
such specialized insects as the bees, and it has been noted that the former
usually set seed more regularly than do the latter. That dichogamy
is not due to natural selection seems to be indicated by the fact that < >ften
it is modified by external factors ; for example, in the sunshine the
flowers of Parnassia are protandrous and those of Biscutella and Thlaspi
are protogynous, but all alike become homogamous in the shade or in
cloudy weather. In many cases the postulated intimate and exact
relation between a specific insect and a specific flower may well be
doubted for a number of reasons: the same flowers are pollinated by
1 The association of protandry and geitonogamy, which is very common in the com-
posites, would also seem to be without advantage.
REPRODUCTION AND DISPERSAL 877
very different insects in different countries; naturalized flowers seem
to be pollinated by the insects of the new country quite as successfully
as their congeners are pollinated by the insects of their native country ;
insects sometimes are held captive and even are killed by specialized
floral structures, as in Asdepias and Hedychium; bees that are unable
to reach the honey in long floral spurs frequently bite holes at the side,
thus getting the nectar without effecting pollination.
Probably the chief reason for not holding to natural selection as a
factor of prominence in the origin of floral structures is that flowers,
though they are the most diversified and specialized of plant organs,
probably have played a comparatively minor role in determining the
success of plant groups. It is likely that the success of the grasses and
the catkin-bearing trees is due less to the floral features above noted
than to vegetative reproduction in the former and to the tree habit in
the latter.
Perhaps the best evidence in support of the view that flowers contribute
largely to the success of plants is found in the composites, though even
here it is likely that vegetative reproduction and the wind dispersal of
seeds play a larger part. Even the composite flower owes its advantage
not so much to floral specialization as to the massing of inconspicuous
and relatively non-specialized flowers into compact heads, which greatly
facilitates pollination. Furthermore, it is to be remembered that the
composites, forming supposedly the highest of plant groups and certainly
the largest in number of species and one of the largest in display of indi-
viduals, are notable for their geitonogamy and autogamy, for their rela-
tively actinomorphic flowers (the disk flowers being strictly actino-
morphic) with their pollen exposed for any insects that may visit them,
and for their tendency toward dicliny; it may be significant, also, that
the greatest display of parthenogenesis among seed plants is among the
composites.
The preceding paragraphs appear to show that the fundamental pos-
tulate of natural selection, namely, that the trend of evolution is along
the line of maximum advantage, is untenable, at least so far as flowers
are concerned. The evolution of the orchids beyond the point of maxi-
mum advantage, the phenomenal success of the groups with generalized
flowers, and the probable dominance of the vegetative over the repro-
ductive factors in determining success in the majority of groups, all
appear to indicate that some other factor than natural selection has
determined the diversity of floral structures. Though the theory of
878 ECOLOGY
natural selection seems to explain such structures quite as inadequately
as did the old and discredited theory of special creation, it is not possi-
ble as yet to put one which is adequate in its place. Perhaps the most
tenable theory is that of orthogenesis. This theory postulates a definite
trend in the course of evolution, regardless of the influence of selection.
It would assume that the specialized features of flowers and also of
insects are organization characters that are more or less inherent in the
species. According to this conception the insects and flowers are not
adapted to each other, but insects in their floral visits select those flowers
whose structures happen to be suited to their mouth parts. It is obvious
that this still leaves unanswered the most fundamental question of all,
namely, the cause of floral structures. In the present state of knowledge,
it is not possible to say whether the evolution of floral structures has been
determined chiefly by external factors or by factors that we call internal.
This subject, in so far as it has to do with external factors, belongs
properly to the following section.
3. THE INFLUENCE OF EXTERNAL FACTORS
UPON THE
DEVELOPMENT AND FORM OF REPRODUCTIVE ORGANS
Introductory remarks. — Variations in the development and form
of reproductive organs are less common than are similar variations in
vegetative organs, but they are much more common than has been sup-
posed. Their relative invariability has long made differences in repro-
ductive structures the chief basis of classification. For this very reason
variation is nowhere more significant, since, if the present theories of
classification are correct, the study of reproductive variations, however
few or inevident they prove to be, may lead to the interpretatic n of
evolution. The possibilities of experimentation in this field are well
shown by a recent study of the fungus, Saprolegnia; from a single my-
celium there have been derived by appropriate changes in the media
the forms of asexual reproduction that have been held to be charac-
teristic of six different genera.
Reproductive variation in the seedless plants. — Experimental data
from the algae and the fungi. — In the seed plants it is common to speak
of two contrasting states, namely, the vegetative and the reproductive,
but in many algae there are three such states, characterized respectively
by vegetative activity, by asexual reproduction, and by sexual reproduc-
REPRODUCTION AND DISPERSAL 879
tion. It is believed that the inception of each of these states depends
upon definite external factors, though vegetative activity necessarily must
antedate the others, since it is the stage of food accumulation.
Vegetative activity may be prolonged~mdefinitely, being favored by
the continued uniformity of optimum vegetative conditions. The most
important single factor favoring such activity appears to be the constant
presence of sufficient water to keep the cell sap dilute, and to facilitate
active growth. Another important factor seems to be a uniform and
moderately high temperature, chiefly, perhaps, because of its effect upon
the absorption of water. Under uniformly high temperatures, Bacillus
anthracis and other bacteria have been kept in a state of continued vege-
tative activity, with no tendency to develop resting cells (" spores ").
Saprolegnia has been kept for six years in a purely vegetative condition,
and brewers' yeast probably has been kept essentially vegetative for
centuries.
Both sexual reproduction and asexual reproduction are induced by
changes in external conditions, and particularly by changes that are
detrimental to optimum vegetative activity. Although species differ
quantitatively and qualitatively as to -the precise external factors that
are involved in the initiation of reproductive activity, it has been shown
in many cases that the development of reproductive structures is induced
by desiccation, by increased concentration of the medium, by very high
and by very low temperatures, by intense illumination, by decreased
food supply, and by the presence or absence of specific chemical sub-
stances. It is scarcely possible as yet to distinguish sets of factors which
commonly initiate sexual reproduction as opposed to asexual reproduc-
tion, although in some cases (notably in- the molds) the development of
asexual spores is favored by those factors which are most opposed to
vegetative activity, namely, desiccation, food impoverishment, low tem-
perature, high concentration of the medium, and strong illumination;
zygospore formation, on the other hand, is favored in the molds by con-
ditions which more closely resemble those favoring vegetative activity,
namely, moisture, rich food supply, high temperature, low concentration
of the medium, and darkness. However, sexual reproduction is favored
by strong illumination in Vaucheria, by low temperature, and by food
impoverishment in Saprolegnia, and by desiccation in Spirogyra. In
some cases it seems as if almost any alteration of previous conditions
serves to initiate reproductive activity, and in other cases there seem to
be certain individuals or strains predisposed to continued vegetative
88o ECOLOGY
activity, while other individuals or strains appear to develop reproduc-
tive organs, almost regardless of external conditions. From the repro-
ductive standpoint one of the most plastic of plants is Vaucheria (figs.
94-100), which in poorly illuminated running water may be kept in a
vegetative condition for several years, while in standing water it pro-
duces zoospores if weakly illuminated, and sex organs if well-illuminated
or if grown in media poor in food l; zoospores may be formed also when
the food is scanty, and desiccation may result in the formation of non-
motile, thick- walled resting spores (aplanospores) .
In the lichens shade and moisture favor the formation of the soredia, while light
and desiccation favor the development of the organs concerned in asexual repro-
duction (apothecia). In Botrydium, zoospores develop in water, but when the plants
are desiccated, there develop aplanospores comparable to those of Vauchtria.
Saprolegnia is quite as plastic as is Vaucheria, vegetating indefinitely if well n >ur-
ished, but developing zoospores if grown in distilled water ; the developmen; of
sex organs is favored by growth on solid substrata, by low temperatures, by food
impoverishment, and by the addition of specific salts to the media. In Spiror,yra
zygospore formation is facilitated by high temperature as well as by desiccation;
there is a striking contrast between the dark green vegetative filaments of dilute
fresh water and the yellowish reproductive filaments of ponds that are drying up.
In Oedogonium, zoospore production is favored by depriving the media of nitrates
and phosphates, by growth in darkened distilled water, and by transfer from a rich
to a poor nutrient solution. In Botrylis there is a reciprocal relation betweesi the
sclerotia and the conidia, the former being favored bf good vegetative conditions,
while spore formation is favored by desiccation, by poor nutrition, and by high
concentration of the medium. Species differ widely as to the effect of increased
concentration of the medium; in Stigeoclonium, and perhaps in most forms, low
concentrations favor zoospore production, but in Tetraspora, zoospores continue to
develop at high concentrations, and in Vaucheria, concentration seems to make
but little difference. In Basidiobolus low concentrations favor zygospore production,
and high concentrations facilitate the development of thick-walled resting spores.
A reduced supply of oxygen appears to induce reproduction in Ulothrix. Me nas,
one of the infusorians, reproduces vegetatively or sexually at 20° C., but by asexual
spores at temperatures between i° C. and 4° C.
Comparatively little is known concerning reproductive reactions to external con-
ditions among the higher fungi, though in Coprinus, Stereum, and Xylaria reproduc-
tive activity is favored by illumination, by poor nutiition, and by partial desiccation
In the rusts the development of teleutospores is hastened by refrigeration, as in alpine
cultures. It has been found also that in Uromyces Vtratri, similar aecidios|>ores
produce the uredo generation if sown on young leaves, and the teleuto generation
if sown on old or wounded leaves, suggesting that the kind of spore that is formed
may be related to nutrition. In some cases external fac'.ors not only initiate periods
1 In Hydrodictyon intense light favors zoospore production, and in Ulothrix light seems
to be without influence in this connection.
REPRODUCTION AND DISPERSAL 881
of reproductive activity, but they influence the character of the reproductive struc-
tures ; for example, in some of the rusts the spore walls are thicker in xerophytic
situations than elsewhere, and in Bornelina the size, shape, and sculpturing of the
spores vary with the culture media and with the illumination.
In certain marine algae, as Dictyota dichotoma, there is a remarkable periodicity,
which seems to be related to external conditions. In England the sex organs de-
velop at fortnightly periods, the gametes being liberated at a fixed interval after the
highest spring tide. In North Carolina there also is a relation to the tides, but the
production of sex organs occurs monthly rather than fortnightly. Similar phenom-
ena have been observed at Naples, and in Japan a fortnightly period of gamete
liberation has been discovered for Sargassum. The most probable causative stimu-
lus of reproductive periodicity is the increased illumination that is associated with
the fortnightly recurrence of extreme low water ; at Naples the liberation of gametes
appears to be on the day when low water occurs nearest to midday. Factors which
modify the tides, such as wind or change of atmospheric pressure, also affect the time
of gamete liberation.
The influence of external factors upon reproductive activity in animals appears
to be much less obvious than in plants. However, in Paramoecium and in other
infusorians the continuance of favorable nutritive conditions seems to cause con-
tmued vegetative^ activity, whereas conjugation is due chiefly to changes in the
media. In the water-fleas (Daphnia) there are two kinds of generations, one being
composed of males and females, and the other being composed solely of partheno-
genetic females. It has been ascertained that parthenogenetic generations result
when the conditions for nutrition are favorable, whereas bisexual generations result
from conditions unfavorable for nutrition, such as increased concentration of the
medium, desiccation, high or low temperature, the accumulation of excreta, and
starvation. In nature the bisexual generation is especially to be seen in shallow
pools, and in autumn in ordinary ponds. The conditions for the development of
parthenogenetic and bisexual generations are very similar in certain other animals,
such as rotifers (Hydatina), plant lice (aphids), and the grape-louse (Phylloxera).
Under favorable nutrient conditions there may be many successive parthenogenetic
generations without any intervening bisexual generations.
The origin of sexuality. — There is little experimental evidence
bearing upon the origin of sexuality, although there exist a number of
forms with facultative gametes, and even with facultative gamete-produc-
ing organs (gametangia). In Ulothrix (figs. 1133, 1134) there are
intergradations (e.g. zoospores of intermediate size with two or four cilia)
between the large quadriciliate zoospores and the small biciliate gametes,
suggesting the possible origin of gametes from zoospores; indeed, it is
known that without fusion gametes sometimes develop into plants, quite
as do zoospores. In Hydrodictyon similar primordia produce gametes
in some media and zoospores in others. In Zygnema stellinum there are
intergradations between the isogamous, the heterogamous, and the
parthenogeoetic gametes, and in the sea lettuces, Ulva and Entero-
882 ECOLOGY
morpha, there are small conjugating gametes and large parthenogenetic
gametes. The auxospores of diatoms may develop vegetatively, may
form asexual spores, or may conjugate sexually. In Ectocarpus there
are transitions between sporangia and gametangia, the same structures
producing either zoospores or isogamous or heterogamous gametes,
thus suggesting the possible origin of sex as well as of sexuality. In all
of these cases the exact determinative factors remain to be discovered,
although it has been suggested from their small size in comparison with
zoospores and parthenogenetic gametes that conjugating gametes
represent poorly nourished spores.
Artificial parthenogenesis. — The most important experimental evi-
dence concerning parthenogenesis is derived from animals, and in view
of its great significance, it must be cited here. Parthenogenesis is ob-
served somewhat frequently in a number of animals, such as bees, w;isps,
and plant lice; in the latter it occurs especially at high temperatures or
when the host plant is very watery. It has been demonstrated tha the
eggs of the sea urchin (Arbacid) develop into larvae in the absem e of
sperms, if they are placed in somewhat concentrated solutions of mag-
nesium chlorid and sea water. Comparable results were obtained with
other salts, and all were at first referred to the increased osmotic pres sure
occasioned by their presence. Hence it was suggested that the stimulus
necessary for egg development is the extraction of water. Later ex-
periments have demonstrated that artificial parthenogenesis can be
brought about in many other ways than by exposing eggs to increased
osmotic pressure, and it is becoming evident that the explanation of the
process is by no means simple; a feature of recent experiments is the
emphasis that has been placed upon chemical factors.
Experiments with similar results have been made upon the eggs of other echino-
derms than the sea urchin (e.g. those of the starfish, Asterias), and also those of
certain annelids (as Chaetopterus and Polynoe) and mollusks (as Sottia). Ii the
starfish, eggs develop parthenogenetically when they are exposed for several hours
to temperatures below 7° C. It was found some time ago that the eggs of Chae-
topterus develop parthenogenetically by the addition to the medium of potassium
ions in an amount too small to produce an osmotic effect, and more recently vrrious
acids and alkalis have been seen to act similarly. Treatment with a fatty acid
(as acetic acid) before placing in a concentrated solution greatly stimulates develop-
ment, because it causes the formation of a membrane, just as when a sperm fuses
with the egg. The mechanical agitation of eggs sometimes causes their partheno-
genetic development. Indeed it would seem that almost any disturbance may serve
to stimulate the development of certain eggs. Probably a large factor in the case
is the permeability of the egg to the substances it needs for its development, and it
REPRODUCTION AND DISPERSAL 883
may be that the various stimulating influences have as their chief r6le the establish-
ment of increased permeability. In most cases artificially parthenogenetic animals
die in an early stage of development, but in at least one instance mature sea urchins
have been secured by this means.
Few similar experiments have been performed with plant eggs, though
parthenogenesis has been induced in Spirogyra and in Chlamydomonas
by growing ptants in concentrated (6 per cent) solutions of cane sugar;
in these experiments plasmolysis occurred, indicating the extraction of
water, as in Arbacia. Artificial parthenogenesis has been reported for
other plants, such as Protosiphon and Marsilea, high temperature seem-
ing to be the stimulating factor. The experiments on artificial partheno-
genesis seem to suggest that the role of the sperm is less that of a carrier
of necessary hereditary substance than that of a growth excitant, which
by physical or chemical means makes the egg permeable to the sub-
stances which bring about development.
Sexuality in the fungi. — The sexual relations of the fungi are very
suggestive of modifications resulting from saprophytic or parasitic modes
of life, although confirmatory experimental evidence is largely lacking.
In Saprolegnia and Achlya (figs. 155-157) there are all gradations be-
tween completely developed male sexual organs and the absence of such
organs. Some forms have apparently complete sexual organs but the
eggs develop parthenogenetically; other forms have antheridial tubes
which reach the egg but remain closed or merely pierce the oogonium
wall without reaching the egg; still other forms have no antheridial tube,
and some forms have no antheridium. There may be considerable
variation also within a given species; for example, antheridia are rarely
present in Saprolegnia Thureti, as often absent as present in 5. mixta,
and usually present in S. hypogyna; they are always present in S.
monoica. In no case are the female organs absent, so that Saprolegnia
forms a striking instance of parthenogenesis by reduction.1 In the
zygomycetes there are gradations between heterogamy and isogamy;
suggesting the evolution of the latter from the former by reduction, and
in the ascomycetes there appear to exist many stages in the reduction of
sexuality. In comparatively few fungi does there appear to be a fusion
of ordinary gametes, though a number of apparently modified forms of
1 Cases of reduction are known also in animals; for example, some rotifers have small
and functionless males or none at all, and in some crustaceans (as Limnadia Hermanni)
and ostracods (as Cypris reptans) only females are known, though they still retain the
sperm sac.
884 ECOLOGY
sexuality have recently been discovered in the rusts and smuts and in
certain other groups. Even the formation of asexual spores appears to
have ceased in some fungi, as in the internal fungus of Lolium, which
probably is a smut, and it is rare in others, as in most mycorhiza fungi.
It commonly has been supposed that the reduced or modified sexuality
of the fungi is in some way associated with their saprophytic or para-
sitic mode of life; since well-nourished plants reproduce sexually less
than do poorly nourished plants, it is possible that the good nutritive
conditions of the group in part account for the character of its sexual
development.
Reproductive variations in the bryophytes and pteridophytes. — Light
favors the development of sex organs in liverworts, mosses, and ferns.
In Marchantia, weak light or an excess of moisture favors ordinary veg-
etative reproduction; an increase of illumination favors the development
of gemmae, and strong illumination favors the development of sex c rgans.
In weak light, fern gametophytes develop into filaments resembling
moss protonemata instead of producing sex organs. If in Salvir, ia the
sperms and eggs fail to fuse, the female gametophyte, whose growth
commonly is checked at such fusion, continues to grow vegeta lively,
producing new female organs; thus embryo development seems in some
way to check gametophytic vegetative activity. Similarly, the gameto-
phytes of Osmunda are long-lived, if fusion does not take place. While
most fern gametophytes are monoecious, producing first male organs
and then female organs, gametophytes that are poorly nourished (having,
for example, a small supply of nitrogen) or are exposed to strong illumina-
tion, may produce male organs only, as though the food supply were
insufficient for complete development; in rare instances, vigorous, well-
nourished gametophytes bear only archegonia. In the ostrich fern
(Onoclea Struthiopteris) the gametophytes commonly are dioecious, the
larger plants being female, and the smaller plants being male; in or-
dinary cultures one to twelve per cent of the plants are monoecious.
Under certain culture conditions, as when female plants are trans ferred
to rich nutrient media, at least fifty per cent of the plants may become
monoecious. Similar phenomena occur in some monoecious mosses,
antheridia being the only sex organs developed when the nutritive con-
ditions are poor ; in some dioecious forms (as in Dicranum) the male
plants are smaller than the female plants. In the ferns it often is easy
to induce apogamy and apospory, especially where the illumination is
weak, or where the soil is dry or poor in food materials.
REPRODUCTION AND DISPERSAL 885
A somewhat remarkable situation occurs in Equisetum; though it is a
homosporous genus, some of its ancestral relatives were heterosporous,
and even now the gametophytes usually are dioecious, though arising
from approximately similar spores. However, the smaller and more
poorly nourished gametophytes usually bear male organs and the larger
gametophytes, female organs. Furthermore, the smaller spores are
likely to give rise to male gametophytes and the larger spores to female
gametophytes ; in the true ferns, however, there appears to be no re-
lation between the size of the spore and the sex of the gametophyte that
comes from it. Occasionally, the gametophytes are monoecious, the
female organs appearing last, as in ordinary ferns. In Marsilea, one of
the heterosporous pteridophytes, the development of fruiting organs
(sporocarps) may be incited by partial desiccation (as in the drying up
of a pond), by increased illumination, or by high temperature ; on the
other hand, fruiting may be retarded or prevented by placing the plants
under water in weak light, at low temperatures, or in crowded cultures.
In the microsporangia there are sixty-four primordia which develop
commonly into microspores, but of the sixty-four megaspore primordia,
only one develops, and that at the expense of the others nutritively. It
has been shown that by subjecting developing Marsilea -sporocarps to
spraying by cold water, no megaspore primordia develop, but that struc-
tures resembling megaspores may be made to develop from microspore
primordia under optimum nutritive conditions, growth being at the. ex-
pense of other primordia, as in the development of ordinary mega-
sporangia; sometimes such spores are sixteen times as large as ordinary
microspores. Thus it is suggested that heterospory may have arisen
from homospory through the influence of optimum nutrition on develop-
ing sporangia.
Some ferns show interesting transformations of reproductive primordia into
vegetative organs; for example, in Osmunda and Botrychium there often are leafy
organs in the reproductive region, and in Onoclea the removal of the foliage leaf is
followed by the development of another foliage leaf from the primordium of the
reproductive shoot.
Reproductive variations in the seed plants. — Vegetative and repro-
ductive periods. — In the seed plants it is convenient to distinguish as
the reproductive phase all of the complex phenomena, both sporophytic
and gametophytic, from the inception of the flower to the maturation
of the seed, contrasting this with the vegetative phase of the sporophyte.
As in the lower plants, a vegetative period always antedates the period of
886 ECOLOGY
reproduction. The length of this initial vegetative period differs widely,
varying from a few weeks in certain xerophytic annuals to a number of
years in the century plant and in most trees. There may be but one
reproductive period following this initial vegetative period, as in annuals
nd biennials, and in such perennials as the century plant ; in these
•:'orms, which are known as monocarpic plants, death follows fruit matura-
jon. In most perennials, however, reproduction either continues in-
definitely after its inception or more often recurs at certain periods,
vegetative activity also continuing indefinitely or periodically ; such
forms are termed polycarpic plants. The most representative poly-
ca*pic plants are trees and shrubs, in which the shoots do not die down
after flowering; in rhizomatous and bulbous plants, however, each shoot
dies soon after flowering, much as in annuals, and new shoots arise by
vegetative reproduction.
Probably in the majority of the perennials of temperate climates, the
vegetative and reproductive periods, to some extent at least, alt -mate
with one another, the flowering period being rather sharply defined, and
often of short duration. Excellent cases of such alternating periods occur
among plants with vernal flowere, as in the willows and poplars, und in
such herbs as Hepatica and Sanguinaria, which bloom before vegeta-
tive activity begins. In such plants reproductive activity merely ap-
pears to antedate vegetative activity, early flowering being possible
only because of the food accumulated during the previous season; for
that matter, the reproductive period in such plants begins in the spring
or in the summer previous to flowering, and in some cases (as in the
alder and hazel) the buds are fully formed before winter begins (fig.
1234). In many plants the spring and summer are periods of vegeta-
tive activity (as in the goldenrods, asters, gentians, and witch-hazel),
while the reproductive period falls in the late summer or in autumn.
In some plants (as Satureja and Lechea) there are strongly marked vege-
tative periods in spring and in autumn, separated by the summer repro-
ductive period. An unusually sharp contrast between vegetative and
reproductive activity is afforded by the wild leek (Allium triccccum)
and by the meadow saffron (Colchicum autumnale), in which the leaves
appear in spring and soon die down, while the flowers do not appear
until summer or autumn.
There are many species in which the vegetative and reproductive
periods are essentially coincident. In a few instances (as in Dicentra
and Claytonia) the two periods not only are more or less coincident, but
REPRODUCTION AND DISPERSAL 887
they are of short duration; in most cases, however, as in the chickweed
(Stellaria media), the periods of vegetative and of reproductive activity
are coincident and of long duration; such plants may be called ever-
Uoomers. As might be expected, everbloomers flourish particularly
in uniform tropical climates. While in temperate climates each month
or even each week from spring to autumn is characterized by the anthesis
of particular species, in uniform tropical climates almost any species
may bloom at almost any time, and a large number of species are true
everbloomers, being in flower at all times. Even those species which
are strictly periodic in temperate climates may be everbloomers in the
tropics (as in the grape and the Virginia creeper). In many species of
tropical everbloomers there is a suggestion of periodicity, since some
branches bloom at one time and some at another; for example, in the
grape one shoot on a given vine may be putting forth leaves and another
flowers, while still another is bearing ripe fruit. In such species the
phenomena exhibited by an individual branch are periodic, but taking
the plant as a whole the phenomena may be termed spasmodic. The
most representative everbloomers are plants with unbranched trunks,
such as Cocos or Carica, for in them there is essential continuity in both
vegetative and reproductive activity in a given shoot; new leaves are
found at all times, as well as flowers and fruits in all stages of develop-
ment. In many tropical plants, on the other hand, flowering is of rela-
tively rare occurrence, several years or even many years elapsing between
periods of anthesis. The most remarkable case of this sort is afforded
by a bamboo, Dendrocalamus strictus, which is said to flower regularly
at thirty-year intervals. Some tropical plants and even some plants
of high latitudes (as the duckweed) are almost never seen in blossom,
their reproduction being essentially vegetative.
The relation of anthesis to meteorological factors. — While the occurrence
and the duration of flowering periods often have been regarded as due
to inherent causes, it always has been known in a general way that
climatic factors may hasten or retard such periods and modify their
length. If vernal plants bloom sooner than usual, the season is called
" early," while delayed anthesis causes a season to be called " late."
The observation of meteorological phenomena in connection with the
periodic activities of plants, and particularly of temperature in relation
to anthesis, is known as phenology. In a general way it is known that
temperature, among other factors, bears an important relation to flower-
ing, which is facilitated by high temperatures and retarded by low
888 ECOLOGY
temperatures. Phenological observers, however, often have regarded
temperature as of such controlling importance that they have prepared
tables showing the total amount of heat necessary for flowering in the
various species. Such tables are almost worthless, since they fail to
include the many other factors involved, some of which, as soil moisture
or atmospheric moisture, equal or surpass temperature in importance.
Further, in preparing tables, temperatures below o° C. commonly are
ignored, although they are certainly of considerable significance in some
plants (as in those of arctic regions), while the temperatures just above
o° C. may be without significance in other plants (as in palms).
The difficulties involved in discovering the factors that determine the
inception of anthesis are best illustrated in those species which form
flower buds early in the season previous to flowering. Some buds, as
in the lilac and the white birch, begin to develop a year before the/ come
into bloom, and in most vernal species the flower buds are in evid( nee by
midsummer. The insufficiency of the phenological method in tlie case
of such plants is most striking, since certain buds (as in the alder and
the hazel) that withstand days and even weeks of warm weather in the
autumn1 without blooming require but a few days of warm v eather
in spring to induce anthesis.1 Years ago it was shown that sumir er and
autumn temperatures have little or no influence upon the flower buds of
the cherry (Prunus Avium), though the buds are evident as early as
July; however, shoots taken into a hothouse in the middle of December
bloomed in twenty-seven days, whereas those taken in the middle of
January, in early March, and in early April, bloomed, respectively, in
eighteen days, in twelve days, and in five days. In some recent com-
prehensive experiments with nearly three hundred species of woody
plants, more than half of the twigs which were brought into a greenhouse
in November started to grow within two weeks; the twigs of seventy
species began to grow in February, and those of thirty-six species did
not become active until March. These results make it very obvious that
the influence of temperature or of other external factors upon anthesis
depends entirely upon the condition of the buds at the inception of the
experiment. While buds in February look much as they do in Decem-
ber, in reality they are different, one determinable change being that in
1 Occasionally vernal species flower in autumn (as in the violet, strawberry, and apple),
particularly if favorable temperature and moisture conditions are long continued. The
wonder is that such phenomena are relatively rare, especially since some buds seem to
be f ully formed by early autumn (as in the alder and the hazel).
REPRODUCTION AND DISPERSAL 889
winter there is a gradual increase of available food in the embryonic
organs ; probably this relative absence of available food is one of the chief
reasons why autumnal buds open so tardily or remain closed, when they
are exposed to favorable temperatures.
It is probable that buds undergo progressive changes other than those
related to the food supply, though the nature of such changes is unknown.
Recently it has been shown that the development of buds can be greatly
stimulated by various methods of treatment during the early part of the
resting period. For example, the subjection of resting buds to anes-
thetics, to freezing temperatures, to warm water baths, or to various
methods of chemical treatment, results in a material shortening of the
rest period, provided the plants are brought subsequently into conditions
suitable for bud development; as might be expected, this artificial
hastening of development has proven to be of great commercial advan-
tage in the " forcing " of bulbs, and of lilacs and other ornamental
shrubs. The exact effect of these methods is unknown, although it is
believed that the stimulation of development in potato tubers that have
been subjected to low temperatures is due to the fact that at such
temperatures there is a rapid accumulation of diastase, which results
in the transformation of starch into sugar, and also to the probability
that the cell membranes are more permeable than at higher tem-
peratures.
Flowering periods in arid and in frigid climates, — In uniform trop-
ical climates, the flowering of plants does not characterize any one season
more than another, many species even being everbloomers. In most tem-
perate climates, flowers appear at all seasons that are in any way favor-
able; estival flowering occurs chiefly at the expense of food accumulated
in spring, but the earlier vernal flowers utilize the food accumulated
during the previous vegetative period. In respect to anthesis, arid and
frigid climates present certain features of marked contrast to temperate
climates and to uniform tropical climates. In arid climates the incep-
tion of the rainy period is marked by vegetative activity, but this is
checked at the beginning of the next dry period. Flowering, however,
is to a large extent associated with the dry period; indeed, in many cases,
anthesis is as definitely associated with the dry season as is vegetative
activity with the rainy season. In the monsoon district of eastern Java,
where the year is about equally divided into two periods, one of con-
siderable rain and the other of drought, more than 60 per cent of the
species bloom solely in the dry period, while only 8 per cent bloom
890 ECOLOGY
solely in the wet period; the remaining 30 per cent are either everbloomers
or forms which overlap the two periods.
In alpine and in arctic climates the flowering period is very short, often
not lasting more than two or three months. Plants that bloom in the
lowlands in April (as Erythronium and Claytonia) may not bloom until
June in alpine meadows, because of the long-continued cold and the
tardy melting of the snow at high altitudes. Strangely enough, however,
the alpine season soon catches up with that of the lowlands, so that by
July similar forms may be blooming at all altitudes, and in August the
alpine season actually is ahead of that of the lowlands; for example,
goldenrods and gentians commonly blossom sooner in the mountains
than at lower altitudes. Similarly, spring is much later and autumn
much earlier in high than in low latitudes; the farther grain grows from
the equator, the shorter is its maturation period, barley, for example,
ripening in ninety days in northern Norway, but requiring one hu idred
days in southern Sweden. In part this surprising phenomenon may be
due to the fact that alpine and arctic species are different from the low-
land species, and therefore, perhaps, inherently characterized by shorter
periods. That this is a minor matter in the explanation, however, is
shown by the fact that some of the species are common to high and to
low altitudes (as the yarrow and the harebell), but particularly by the
fact that alpine plants grown in the lowlands, or lowland plants grown
in alpine districts, behave in each case precisely like the indigenous
plants. Obviously, also, the usual phenological assumption that low
temperatures retard anthesis is the very reverse of the fact, for the heat
sums are much greater in low than in high altitudes and latitudes.
It has been suggested that the greater intensity of alpine light and the
greater duration of arctic light, respectively, account for the "hurrying
up " of the seasons at high altitudes and high latitudes, enabling slants
to make the food necessary for anthesis in a shorter time. The experi-
ments about to be cited give another suggestion, namely, that those fac-
tors that are detrimental to vegetative activity and which, therefore,
cause its early cessation, are at the same time favorable to reproductive
activity. Among the factors in alpine habitats that tend to check op-
timum vegetative activity are low nocturnal temperatures,* great tem-
perature differences between day and night, high transpiration in pro-
portion to absorption, and, perhaps, intense light.
The experimental determination of vegetative and of reproductive
periods. — Adequate experimental study has shown that the length of
REPRODUCTION AND DISPERSAL 891
the initial vegetative period (i.e. the period from germination to anthesis)
and the length of the reproductive period are subject to wide modifica-
tion through the operation of external factors, and it has been found
possible also to extend the initial vegetative period indefinitely by the
inhibition of reproduction. These experiments shed much light upon the
phenomena cited in the preceding pages. Many interesting facts con-
cerning reproductive periods have long been recognized, because of their
important practical bearing. For example, crops like peas, tomatoes, and
sweet corn may mature some days or even some weeks sooner on dry,
well-lighted slopes than in rich, moist lowlands, so that the profit from
the former is often much the greater; but in crops where the vegetative
organs are marketed, the rich, moist habitat often is preferable, because
of the greater luxuriance of the foliage. Comparable phenomena are
exhibited by trees, Pinus silvestris maturing fruit in fifteen years if stand-
ing alone in dry soil, but requiring thirty to forty years in a grove ; simi-
lar differences are seen in the beech and in many other trees.
Early reproduction, which often is of great practical benefit, frequently
is brought about by various mechanical means. Picea excelsa, which
commonly flowers in thirty to forty years, may be induced to flower in
four to ten years by transplanting, ' especially if the roots are injured.
Orchard trees often fruit much better, if some of the roots are removed.
Girdling sometimes induces flowering in apple trees that otherwise
exhibit only vegetative activity. Shoots of a young tree grafted on
an old tree bloom much sooner than those that are left on the young tree.1
Cuttings bloom long before seedlings, a matter of the highest economic
importance. Of much interest is the fact that a cutting from an old plant
blooms much sooner than one from a young plant, though the cuttings
may be of equal size and similar aspect. This phenomenon is most
strikingly displayed in leaf cuttings (as in Begonia or Achimenes), in
which the young shoot flowers almost at once if the leaf is taken from
a flowering plant, but only after a long time if taken from a young plant.
This phenomenon has been explained by postulating the accumulation
of flower-forming substances in plants approaching maturity, but this
assumption needs explanation as much as do the facts which it attempts
to explain. Furthermore, there are some cases, as in Torenia, where
leaf cuttings flower at once almost regardless of the age of the part of the
1 For example, when a twig from an apple sapling is grafted on an old stock, ft may
fruit in a year or two instead of in ten or fifteen years, while a twig from an old stock
grafted on a sapling does not fruit for many years.
892 ECOLOGY
plant from which the cutting is taken. At any rate, the maturity
of the flowering plant^seems to be in some way transmitted to the
propagule.
It has been proved conclusively that plants may be kept in a vegetative
state indefinitely, and that the usual successive stages in a plant's life
history are reversible. For example, when the ground ivy (Nepeta
hederacea), which commonly has a reproductive period intercalated
between the vegetative periods of spring and autumn, is grown in a green-
house under uniform conditions of moderate temperature and consid-
erable moisture, vegetative activity continues uninterruptedly. How-
ever, flowering may be induced at any time by transferring the plants to
a dry, well-lighted situation. Similarly, by exposure to proper ex.ernal
conditions, " winter buds " have been induced in Utricularia at any
season, hyacinths have been induced to flower twice without an inter-
vening period of rest, and Parietaria has been kept in constant bloom.
Annuals have been transformed into biennials or perennials by keeping
them under constant conditions favorable to vegetative activity and
Echium, which usually is a biennial, has been known to grow in the
tropics ten years without flowering.
When the annuals, Poa annua and Senecio vulgaris, are transferred to alpine
habitats, the season is too short for fruit maturation and the plants become biennial.
Many garden annuals may be transformed into biennials by removing the flower
buds as they appear ; in this manner the mignonette may be transformed even into
a woody perennial. In monocarpic species life may be shortened by hastening
reproduction, as well as lengthened by promoting vegetative activity ; thus the castor
bean, a tropical perennial, has been transformed into an annual in temperate cli-
mates, where the conditions facilitate early reproduction. The closeness with which
death follows reproduction in monocarpic species is well illustrated in hemp, i dioe-
cious annual; the staminate plants die immediately after anthesis, while the pistil-
late plants live until the fruit has matured, several weeks later. A number of plants
which display vigorous vegetative reproduction (as the yam, the potato, aid the
sweet potato) rarely produce seeds, hence it has been suggested that seed proc uction
and vegetative production may be more or less mutually exclusive; however, there
are many plants in which both kinds of reproduction are vigorous (as in the < lahlia,
the strawberry, and the willow).
The usual succession of events from the inception of vegetative ac-
tivity to the maturation of fruit is so familiar that it has often been mis-
takenly referred to as normal, thereby implying that any change in the
order of events is abnormal. It has been shown that the order is re-
versible at almost any point. In certain species of Veronica, for example,
REPRODUCTION AND DISPERSAL
893
>P
'f an inflorescence is cut off and allowed to strike root in a moist chamber
the tip grows into a vegetative shoot (figs. 1199, 1200). The oldest
buds develop into the usual
flowers, while younger buds de-
velop into cleistogamous flowers
without prominent corollas; still
younger buds develop only the
calyx, and the very youngest
lateral buds, as well as the ter-
minal bud, develop vegetative
shoots. If a flowering shoot of
Myriophyllum heterophyllum is
transferred from a pond to a
covered aquarium, it becomes
transformed into a vegetative
shoot. Ajuga reptans has three
phases, a rosette, a flowering
shoot, and a stolon, and any of
the three can be induced at any
time by supplying the requisite ex-
ternal conditions. If the flowers
of Opuntia are removed from the FlGS 1 199 I200 _ Var,ation in the flower.
plant and placed in the soil, they ing shoots of Veronica Chamaedrys: 1199, an
soon strike root and give rise to °rd'nai7 flowering shoot with buds, flowers,
. . and young fruits; 1200, a similar shoot that
vegetative shoots. Most striking was placed in moist air at the inception of
of all, perhaps, are the reactions anthesis; note the metamorphosis of the upper
of the xerophyte, Semperuivum, Part into a Ieafv shoot- ~ From KLEBS-
which also has three phases, comparable to those of Ajuga; vegetative
activity may be made to continue indefinitely, stolon formation may be
eliminated, and phenomena unusual in nature (such as rosette for-
mation at the stem apex, and the transformation of the inflorescence
into a vegetative shoot) may be induced at will.
Reversibility of stages is not confined to the seed plants; if a fruiting shoot of
Selaginella lepidophylla is cut off and placed in the soil of a moist hothouse, it be-
comes transformed into a vegetative shoot, even developing rhizophores. Reversi-
bility can be induced also in animals, for if a polyp of the hydroid, Campanularia,
is brought into contact with a solid body, it gradually becomes undifferentiated and
finally develops into a stolon, whereas removal to the original habitat soon results in
a transformation back to a polyp. It may be noted finally that reversibility is the
usual thing in the pineapple, a vegetative shoot developing at the apex of the fruit.
fl99
1200
894 ECOLOGY
The above experimental evidence compels the abandonment of the
notion that only the usual succession of events in a plant in nature is
to be considered normal. One thing alone is fixed, namely, that plants
must at the outset have a period of vegetative activity, but whether this
continues through life, or whether the reproductive period begins early
or late or not at all, is a matter that is determined by external conditions,
and one series of events is quite as normal as another. It is quite as
normal for a bulrush to live in deep water and to vegetate indefinitely
as to live in shallow water and to flower annually. Indeed it is permis-
sible to regard anything that a plant ever does as normal, since in every
case its particular reactions are due to its life conditions. Thus it is
demonstrated that plants as a rule do not possess an inherent rhythm,
since external factors are the dominant determining causes of repn>duc-
tive periods; if plants flower regularly, the requisite conditions may
be supposed to recur regularly.
The exact analysis of the reproductive factors remains to be deter-
mined, though the data now available are sufficient to show thai suc-
cessive stages imply successive causes (i.e. changed conditions), while
uniform phenomena imply uniform or unchanged conditions. Br >adly
speaking, the conditions commonly termed hydrophytic and mesophytic
seem especially to favor the continuance of vegetative activity, while
xerophytic conditions favor reproduction. Hence it is not unlikely that
the amount of available wate*r may be a dominant specific factor, es-
pecially as its presence in abundance is known to be a primary requisite
for optimum vegetative development.
Another important reproductive factor is light. Careful experiments on Mimulus
show that light of high intensity favors flower production. In the giant cactus
there is a ring of flowers about the stem, and anthesis begins on the side toward the
sun. In the teasel (Dipsacus) there is a tendency for the basal flowers to biossom
first, but usually the lighting is better toward the upper part of the inflorescence;
as a resultant of the two factors concerned, the first flowers to appear usua ly are
those near the middle. High temperature also favors flowering. It has been shown
that if radishes are grown in concentrated (10 per cent) solutions of glucose, they
bloom earlier than otherwise; this may explain why light favors flowering, since
it facilitates the production of carbohydrates; also the girdling of trees, which hastens
flowering, would tend to cause the accumulation of carbohydrates in the upper parts
of the plant. It has been claimed that a minimal supply of food salts (especially
nitrates and phosphates) at times favors flower production. Defoliation occasioned
by storros, by insects, or by freezing sometimes causes flower production, but the
reason is not obvious. The influence of parasites upon flower production varies;
the black rot appears to stimulate autumnal flowering i.-? the apple ; in other cases
REPRODUCTION AND DISPERSAL 895
parasites cause flower primordia to develop into vegetative organs, as in the golden-
rod (fig. 1097).
The influence of external factors upon sex determination. — The great
majority of plants appear to be strictly monoclinous or diclinous, the
latter being for the most part strictly monoecious or strictly dioecious.
As previously noted, however, there are a number of species, which vary
between monocliny and dicliny or between monoecism and dioecism.
It is particularly among such plants that experimentation has been car-
ried on regarding sex determination. When maize (Zea Mays} is grown
under favorable vegetative conditions, the plants commonly are monoe-
cious, but when it is grown in dry, sterile soil or is exposed to weak light,
a small unbranched plant develops, which produces only a staminate
inflorescence. Even under ordinary growth conditions, the staminate
flowers originate first, and it has been suggested that the pistillate flow-
ers come later when the nutritive conditions are more favorable.1 When
the nutritive conditions are very favorable, or when the primordia are
parasitized by smut, pistillate flowers may be induced in staminate
inflorescences. Most of the catkin-bearing trees resemble maize in that
the primordia of the staminate flowers develop earlier than do the pri-
mordia of the pistillate flowers. In Picea and in some other conifers,
pistillate flowers occur only on the more vigorous and better nourished
shoots, while the staminate flowers occur on weaker shoots. When
Satureja horlensis is grown in rich soil and is well illuminated, 79 per
cent of the flowers are monoclinous, the remainder being pistillate ;
in poor soil and under poor illumination only 13 per cent are monoclinous,
the remaining 87 per cent being pistillate only.
It is a general belief that in dioecious plants xerophytic conditions (or conditions
of food impoverishment) facilitate the development of staminate plants. The ex-
periments cited above favor this view, but there certainly are other factors con-
cerned. For example, the hemp (Cannabis saliva), which is a representative
dioecious plant, may grow in rich, alluvial soil, where it displays great vigor, or in
dry and sterile waste soil, where the plants are weak and impoverished, but in all
cases both staminale and pistillate plants are found, if there is a large number of
individuals. It has been claimed that in spinach certain salts (as those of sodium
and calcium) favor the development of staminate plants, while other salts (as those
of potassium or phosphates) favor the development of pistillate plants, but this is
very doubtful. It has been suggested also that culture solutions which have a high
osmotic pressure tend to favor the development of an unusually large number of
1 The opposite condition is seen in Humulus, in which pistillate plants sometimes
develop staminate flowers late in summer.
896 ECOLOGY
females. In dioecious species it has been claimed that large seeds are likely to
develop into pistillate plants, and small seeds into staminate plants.
There may be noted some interesting cases of correlation, whose explanation is
not as yet forthcoming. Immediately after flowering it often is possible to distin-
guish at some distance pistillate from staminate mulberry trees by their much
smaller leaves, as though the constructive material in the former were utilized
chiefly in fruit development, and in the latter, in leaf development. Simifarly,
in the box elder the leaves on flowering branches often are much smaller than
on vegetative branches. Later in the season, both in the mulberry and the box
elder, the leaves are equally large on pistillate, on staminate, and on vegetative
shoots.
Among dioecious perennials (such as the box elder, poplars, and willows) the
same individual usually bears the same kind of gametophytes, regardless of external
conditions (even when transplanted as a whole or in the form of a cutting into a
very different habitat), so that two individuals which appear alike when not in liou LT
really are different, the one transmitting male attributes, and the other female
attributes. The gametophytes of Marchantia, for example, have been cult vated
vegetatively for thirty generations without undergoing any change of sex. There
are on record, however, some noteworthy cases of change of sex on the part of in-
dividual plants. Perhaps the best authenticated cases are those in which the sex
has been changed by wounding (traumatism). The primordia of pistillate in lores-
cences of maize have been subjected to torsion and thereby changed to star linate
inflorescences; also staminate inflorescences have been changed to pistillate in-
florescences. By injuring the terminal bud of a staminate plant of Carica Papaya,
the plant has been stimulated to produce pistillate flowers which have matured into
fruits. Pulicaria dysenterica commonly has monoclinous flowers, but when the
subterranean organs are infested by Baris analis, an insect parasite, the species
becomes dioecious. The pistillate flowers of Lychnis dioica have stamen primordia
which rarely develop into mature stamens ; if these primordia are infested by a
smut ( Uslilago violacea), the stamens develop to a considerable size, though they
contain spores of the smut instead of pollen grains. A staminate grape used as a
stock for a monoclinous scion has been known to become monoclinous and to mature
fruit. In the strawberry, ordinary vegetative reproduction has been known to be
accompanied by sexual changes; in an imperfectly dioecious variety with mono-
clinous and pistillate individuals, the vegetative progeny of each kind of individual
has been known to develop into the other. A very remarkable change without
change of conditions has been reported for Aucuba japonica, in which a plant
that had been staminate for some years became monoecious and finally mono-
clinous. Similar changes have been reported for the lower plants, parti< ularly
for Vaucheria, in which female branches have been transformed into bisexual
branches.
Recent experimentation has resulted in a material change of view regarding the
significance of the influence of external conditions upon sexual development and
upon the change of sex, as noted in the preceding paragraphs. It is now generally
believed that in most plants the sex of an individual is not due to the external con-
ditions to which the individual itself may be subjected, but that sex is determined
much earlier than had been supposed. In the liverwort, Sphaerocarpus, sex is
REPRODUCTION AND DISPERSAL 897
determined at the time of spore formation, since two spores in each tetrad give rise
to male plants and two spores to female plants. In heterosporous pteridophytes
and in seed plants, the sex of the gametophyte is determined long before spore forma-
tion, since it depends upon the kind of sporangium that is produced by the preceding
sporophyte. On the other hand, in homosporous pteridophytes and in monoecious
mosses, sex determination appears to come much later than spore formation, and
to depend in part, at least, upon the conditions to which the gametophyte is exposed.
A remarkable situation has been disclosed in the dioecious mosses, in which sex is
determined at spore formation, half of the spores giving rise to male plants and half
to female plants, as in the liverwort, Sphaerocarpus. Pieces of the sporophyte may
give rise vegetatively to gametophytes, and such gametophytes are bisexual, whereas
gametophytes that develop from spores are unisexual. Hence it appears that the
supposedly non-sexual sporophyte is in reality bisexual.
In the seed plants it seems probable that the sex of the gametophyte is determined
far back in the history of the preceding sporophyte, at least as far back as the seed.
In this event, ordinary sporophytes are as characteristically sexual as are the gameto-
phytes to which they give rise, so that it is proper to call a staminate plant male and
a pistillate plant female. There is some evidence in favor of the view that in the
seed plants the sex of the gametophyte is determined farther back than the seed,
perhaps as far back as the gametes of the preceding gametophyte, or even as far back
as the spores from which the latter gametophytes arise. In Bryonia dioica and in-
Cannabis saliva, experiments seem to show that the eggs have a female potentiality
and that half of the sperms have a male potentiality and half of them a female
potentiality. In case a sperm with a male potentiality fuses with an egg, the de-
veloping sporophyte is male, because the sperm dominates over the egg. If a sperm
with a female potentiality fuses with an egg, the resulting sporophyte is female.
An alternative hypothesis postulates that sperms have either strong male potential-
ities or weak male potentialities, the former dominating over the egg, and the latter
being subordinate to the egg. In any event it would seem that in dioecious plants
the sex of a given sporophyte or of the subsequent gametophyte depends upon the
sexual potentiality of the preceding sperm or of the still more antecedent pollen grain.
In all of these phenomena, external factors seem to have no part, unless, perhaps,
they affect in some unknown way the sex tendency of pollen grains ; in any case it
seems clear that external factors operating upon a sporophyte can have no influence
upon the sex of the subsequent gametophyte. Supplementary evidence in favor
of the female potentiality of the egg is afforded by the fact that in Chora crinita
and in Antennaria, eggs which develop parthenogenetically always give rise to
female plants. Further data are afforded also by Mercurialis annua, a dioecious
species whose pistillate plants bear occasional staminate flowers ; in the event of
geitonogamy, these plants nearly always have female progeny; conversely the occa-
sional female flowers of male plants with geitonogamous pollination have male
progeny.
In animals, as in plants, sex determination appears to be unrelated to obvious
external factors, the sex potentiality of the gametes being predetermined. The exact
factors involved in such predetermination are unknown, but it has been suggested
that in those animals in which the female possesses one more chromosome than does
the male, the extra chromosome may be the sex determinant. Formerly it was
898 ECOLOGY
believed that external factors act as sex determinants in daphnids, grape lice, aphids,
and rotifers, but it is now realized that such factors determine only whether a
given generation is to be sexual or parthenogenetic (p. 881); in the case of the sexual
generation, the maleness or the femaleness of the different individuals is predeter-
mined. In some cases at least, the sex of the progeny is determined before egg
formation and possibly as a result of external factors ; in one of the rotifers (Hyda-
tina), the better nourished females lay large eggs, which develop parthenogeneti-
cally into females, and the more poorly nourished females lay small eggs, which
develop parthenogenetically into males. That the sex potentialities of animal
gametes may differ from those of plant gametes is shown by the fact that in most
cases, eggs which develop parthenogenetically grow into male animals (as in ants,
bees, and wasps) ; in those cases in which certain eggs develop parthenogenetically
into females (as in rotifers and grape lice), there are other and smaller eggs, which
develop parthenogenetically into males. In bees, in rotifers, and in grape lieu, fer-
tilized eggs develop with equal certainty into females.
If it is to be concluded from the above data that sexuality but not sex is deter-
mined by external conditions, some further explanation is needed to accou it for
the change in sex noted above for such plants as Zea, Car tea, Pulicaru , and
Lychnis. These cases seem best explained by assuming that all of these forr is are
potentially bisexual and that external factors either may cause the suppression of
one of the sexes (as in Zea and Pidicaria, and also in most homosporous ferns) or
may stimulate to development a sex that commonly is suppressed (as in Caries and
Lychnis, and also in Onoclea) , in the latter case acting as releasing stimuli. This
view is supported by the fact that Carica and Equisetum are known sometimes to
be monoecious, and also by the fact that the staminate and pistillate flowers of
Piper Betel may under proper conditions become monoclinous. To what extent
other supposedly dioecious species are thus potentially bisexual is unknown; it may
be noted that even the willows, which commonly are thought to be strictly dioecious,
occasionally have monoecious individuals and, still more rarely, monoclinous flowers.
Variations in flower color, — The most variable character of flowers
is that of color. Many cases of color variation in flowers of the same
species clearly are due to external factors, particularly in those flowers
in which colors are due to anthocyan *; such variations may be quanti-
tative, involving differences in intensity only, or they may be qualitative,
involving differences in wave length. Light seems to be the most im-
portant factor determining variations in color intensity. It was dis-
covered long ago that when bulbs (as in the tulip or hyacinth) are grown
in the dark, they develop colored flowers much as in the light, though
the color intensity is less, sometimes being much less, as in blue hya-
1 However, there are some striking cases of color variation in flowers whose color is
due to chromoplasts, as in Tropaeolumand in Castilleja coccinea; the latter is more likely
to have scarlet flowers in rich soil, where the plants are vigorous, and lemon-yellow
flowers in pea.:/ soil, where the plants are impoverished.
REPRODUCTION AND DISPERSAL 899
cinths. Coloration takes place in the dark in some non-bulbous plants,
such as Lychnis, Hydrangea, and Papaver. In striking contrast to
bulbous plants are Antirrhinum and Prunella, where the food necessary
for anthesis does not accumulate during the previous season, but is
manufactured just before the period of floral development. In such
plants the flowers do not become fully colored when the entire plant is
grown in the dark, as in the tulip, although they become colored if the
vegetative shoots are grown in the light and the floral shoots in the dark.
Even tulip flowers do not become colored in the dark unless the previous
leaf generation is exposed to sunlight. Thus the influence of light upon
color intensity appears to be in part direct, as is indicated by the deep
shades of hyacinth flowers that are grown in the sunlight and by the
high intensity of color of alpine flowers. However, to an equal or greater
extent the light influence is indirect, as is well shown by those flowers
that become colored only when the leaves are in the light. The color
in this case and in the dark cultures of bulbous plants seems to be asso-
ciated with a rich food supply, a fact which is quite in harmony with the
sugar theory of anthocyan formation. Probably in the majority of flow-
ers, direct exposure to sunlight is required to produce full coloration,
although a certain amount of pigmentation occurs in darkness. Yellow
colors are much less weakened by darkness than are the anthocyan
colors. Heat as well as light affects coloration, the intensity often being
heightened at low temperatures.
Variations in the quality or kind of color are much less understood
than are variations in color intensity, though it is known that the cell
sap of red anthocyan flowers is more acid than is the cell sap of blue
anthocyan flowers; hence it is to be supposed that factors which cause
variations in the acidity of the cell sap cause variations in color also.
The flowers of Hydrangea hortensis, which usually are red, become blue
when the plants are grown in soil containing a considerable amount of
the sulfates or of other salts of aluminum and potassium. Aluminum
salts frequently change lilac-colored flowers to blue, whereas potassium
salts may change them to green. Acids, on the other hand, frequently
change flower colors to red. White roses have been changed to red by
adding potassium salts to the soil. Heat affects the quality as well as
the intensity of flower color, low temperatures, for example, sometimes
causing white geraniums to become red or rose geraniums to become
carmine. The flowers of harebells and morning-glories also vary with
the temperature in respect to color quality (see also p. 845).
900 ECOLOGY
There are some instances where color variation may not be due to external
factors. Hepatica plants, in apparently similar conditions, exhibit various colors
from pink to blue. Perhaps the most probable instance of " inherent " color char-
acters is in the albinos, which seem to have white (i.e. unpigmented) flowers in any
habitat; such albinos are known in many plants (as in Lupinus and Sisyrinchium) ,
and in some cases there are comparable variegated flowers (as in Viola cucullata).
Albinos commonly are regarded as sports or mutants, but the possibility of external
determining factors even here is suggested by the reported pigmentation of Trillium
albinos that have been transplanted to a new habitat.
Variations in the size and number of floral organs. — When plants
are grown in very poor nutritive conditions, the number of flowers on
each individual is much reduced, and sometimes (as in the poppy; the
size of the flower also is reduced. Weakened illumination may cause
a decrease in flower size, particularly in the size of the corolla (j.s in
Mimulus); by contrast it is to be noted that in xerophytic alpine liabi-
tats, in spite of the marked reduction of other organs, there is no marked
reduction in flower size, probably because of the intense illumination
(figs. 1051, 1052). In poorly nourished specimens of Agrimonia, the
stamen number may be reduced from about twenty to five. Wher. the
poppy is grown in dense cultures, the number may be reduced rom
thirty or forty to six; this result is most significant, since in this group
the large number of stamens is an important taxonomic character.
Equally significant is the carpel variation in the poppy; in well-nourished
individuals there may be one hundred and fifty carpels, but in poorly
nourished individuals the number may be reduced to four. In Chrysan-
themum and in some other composites, the number of ray flowers varies
with the nutrition, well-nourished plants having the largest number of
such flowers. A remarkable situation is presented in Sempervivum,
in which there have been produced all gradations between flowers and
vegetative shoots; some flowers lack corollas, others lack stamens and
pistils, and even the calyx, which usually is the most certain of devjlop-
ment of floral organs, sometimes is absent, in which event the bracts
alone represent the floral organs. In this genus also it is possible
to induce the transformation of stamen primordia into carpels, or
of carpel primordia into stamens. Similar results have been obtained
in Veronica (p. 892).
Variations in flower form. — Perhaps the most significant of all the
variations in reproductive structures are those involving changes in form,
since they have to do with the very fundamentals of classification, and
therefore are likely to shed important light upon the processes of evolu-
REPRODUCTION AND DISPERSAL 901
tion. It was shown long ago that various zygomorphic flowers owe to
gravity their peculiar shape, since they become actinomorphic when its
influence is equalized. In Mimulus, zygomorphy is reduced in weak
illumination. Floral symmetry has been modified also by cutting con-
ductive strands that lead to the flowers and by otherwise changing the
nutritive conditions. Of particular interest are the experimental data
on cleistogamy, which involves marked change of form and structure,
especially in the corolla. In Stellaria, light is required for the opening
of the flowers, and in Linaria, flowers that usually are chasmogamous
in weak light are cleistogamous. In Lamium the vernal and autumnal
but not the estival flowers are cleistogamous. In Impatiens noli-tangere
the first flowers usually are cleistogamous and spurless, while the later
ones are chasmogamous and spurred; but if such a plant is transferred
to sterile sandy soil, only cleistogamous flowers are produced, indicating
that poor nutrition favors cleistogamy. Chasmogamous flowers may
be produced by Stellaria even in weak light, if the plants are supplied
with glucose. Parasites may induce cleistogamy; mildewed plants of
Impatiens produce only closed flowers, and Biscutella produces such
flowers, when the plants are attacked by cecidomyid insects. In Viola
mirabilis the primordia of chasmogamous flowers develop into closed
flowers in extreme conditions, as in dry, sterile soil and in a warm,
humid greenhouse, and in V. odorata the primordia of the cleistogamous
flowers develop into showy open flowers in dryish, sunny habitats.
A most interesting floral modification is that seen in the so-called
double flowers (figs. 1201, 1202). Where the phenomenon is one of
the replacement of other floral organs (especially stamens and pistils)
by petals, it may be denominated petalody or petalization.1 There
are varying degrees of petalody; for example, the buttercups, which
commonly have five petals, may have the number doubled or otherwise
increased even to complete petalization. In the white water lily (Cas-
talia), in which there are many petals disposed in several rows, the inner
members become smaller and narrower, and show all transitions to sta-
mens. Passing outward from the center, the stamen filaments become
broader and more petaloid, while the anthers gradually become effaced,
suggesting the possible origin of petals from stamens or of stamens from
petals; the first theory is the more reasonable, but there is no valid
1 In the composites, however, doubling is due to the replacement not of stamens by
petals, but of disk flowers by ligulate flowers (as in double sunflowers and chrysanthe-
mums), so that one should speak of double heads rather than of double flowers.
ECOLOGY
evidence for either. The exact cause of petalization is unknown, but
in many cases it appears to be inherent, double flowers usually being
regarded as sports or mutants, since they often may be reproduced by
seed as well as by cuttings.1 In other cases, petalization clearly is due
to external factors, notably in a number of species in which plants whose
roots are infested with certain parasitic fungi (as Heterodera radicicola)
develop double flowers. Saponaria sometimes has double flowers
when the roots are infested with Fusarium. In the tulip, petalody
is facilitated by
good nutrition,
especially if there
is an abundance
of nitrogenous sub-
stances in the soil.
\ fK I2°5
enclosed by one or two
integuments arising from
the basal region (chalaza)
just above the funiculus;
the integuments do not
close tightly about the
nucellus, but leave a
slender canal, the mi-
cropyle, through which
the pollen tube usually makes its way. The ovules may be erect on the
funiculus (orthotropous), partially pendent (campylotropous), or more
1204
FIGS. 1204, 1205. — 1204, cross and longitudinal
sections of a seed of Canna, showing the seed coat or
testa (0, the perisperm (e), and the embryo (f>); 1205,
longitudinal and cross sections of a bean (Phaseolus),
showing the seed coat or testa (<), the cotyledons (c).
and the plumule (p).
906 ECOLOGY
r
commonly completely pendent (anatropous) ; in the last case the close
application of the funiculus to the integument causes a suture, the
raphe (figs. 582-584). 1 The young sporophyte or embryo at first grows
vigorously, usually becoming differentiated at seed maturity into the
embryo root (radicle), the embryo stem (hypocotyl), one, two, or more
seed-leaves (cotyledons), and the embryo shoot (plumule)? The seed
also contains foods that are utilized by the young sporophyte during its
second phase of activity, commonly called germination? These foods
may accumulate within the cotyledons (as in peas and beans, fig. 1205),
which in that event occupy most of the space within the testa, or they
may accumulate in a tissue surrounding the cotyledons (as in most
monocotyls, fig. 1204), this tissue being called endosperm if arising
within the embryo sac, and perisperm if arising from the nucellus.
Most seeds mature in the season of anthesis. Some plants with autumnal
flowers, such as Hamamelis and Colchicum, mature seeds the folk wing
season, and in some plants with vernal flowers, such as the pines
and certain oaks, maturation comes in the second season.
The rdle of seeds. — Primarily seeds are disseminules, and ma ly of
their chief structural features are associated with dispersal. Of almost
equal importance in many plants, especially in annuals and bienni ils, is
their protective r61e, since in no other form is the seed plant so immune
to danger as in the seed. Though they are often so regarded, seeds are
in no sense reproductive organs. The reproduction of which the seed
is the result, takes place previously within the flower, while the seed rep-
resents in a state of arrested development the protected offspring of that
reproduction. Thus the unique feature of the seed plants is the sepa-
ration of reproduction from protection and dispersal; post-reproductive
disseminules, the seeds, take the place of reproductive disseminules, the
asexual spores.
The protective structures and relations of seeds. — The protection oj
developing seeds. — Developing seeds are protected from transpiration
and from other dangers by the ovary wall, which thickens and hardens
into the fruit wall or pericarp. It has been thought that grazing ani-
mals might eat the young fruits, so that the sourness, bitterness, or hard-
ness of fruits that later become edible have been regarded as advantageous
in protecting them from such dangers. In some cases, as in the jimson
1 Most of these terms apply also to seeds.
* In the orchids and in some parasites the embryo remains undifferentiated.
* Little or no food is found in minute seeds, as in those of the orchids.
REPRODUCTION AND DISPERSAL 907
weed (Datura), the chestnut, and certain gooseberries (as Ribes Cynos-
bati), the fruits are spinescent.
The prickly pear (Opuntia) is especially interesting from this viewpoint, since
the unpleasant bristles of the young fruits fall off as the fruit ripens, from which it
has been inferred that the young fruit is protected from the fruit-eating animals
which later scatter the ripe seeds. Such views are misleading in their implications,
since most young fruits are not especially attractive to animals. Their unpalata-
bility is a sign of immaturity rather than of protection.
The protective structures of mature seeds. — Seeds as a class are the
most xerophytic of plant structures, since not alone in xerophytes, but
also in mesophytes and even in hydrophytes, they generally are covered
with hard and impermeable coats. So universal is the xerophytism of
the seed that usually it is impossible to determine from its structure the
habitat in which it grew. This xerophytism consists in three features:
the thick and impermeable coat, the compactness of the tissues within
the testa, and the small amount of water. The testa, or seed coat, com-
monly is single, being derived from the ovule integument (from the outer
integument, in case there are two) through thickening, hardening, and
other modification. In some seeds there is a second coat within the
testa, and in others there is a structure outside the testa, which is known
as an aril (e.g. in the water lily). The testa at maturity usually is hard
and bony, being composed of several or more layers of cells with greatly
thickened walls; -in the hickory nut it is made up of a number of layers
of sclereids. Sometimes the testa is so hard that it is difficult to cut it
with a knife, as in Gymnocladus. In most one-seeded fruits,Nsuch as the
grains of cereals (fig. 1211) and the achenes of the composites, the fruit
wall or pericarp remains closed about the seed at detachment, an J often
is the chief protective layer, especially where it is hard and bony (as in
Lithospermum). In some instances seeds are essentially without a pro-
tective outer layer; this is the case particularly in the Amaryllidaceae,
where the outer integument or the endosperm may become fleshy and
green (as in Hymenocallis and Crinum).
The advantages of seed protection. — The chief dangers which beset
seeds are premature germination, loss of viability, and destruction by
herbivorous animals. Adequate protection is especially important in
monocarpic species, above all in annuals, since the maintenance of the
species depends absolutely upon the viability of its seeds. For months
at a time annuals may be non-existent over vast tracts of country
"xcept in the form of seeds. While most trees, as the pines, spread
9o8 ECOLOGY
only through the agency of seeds, the situation is different, since the
same individual produces seeds a number of times. The adequacy
of seed protection is well illustrated by the abundant annual recur-
rence of such weeds as the ragweeds, pigweeds, purslane, and Russian
thistle.
Seed protection in relation to animals. — Many seeds are used as food
by herbivorous animals. Often, as in the nuts that are eaten by squirrels
and in the many small seeds that are eaten by birds, the protective coats
are insufficient to give adequate protection, the survival of the species
depending upon those seeds that chance not to be eaten. The likeli-
hood of such survival is not so slight as it might seem, since most species
produce many more seeds than would commonly be eaten, and many
seeds fall to the ground and become hidden by leaves. The seeds of
edible fruits might be thought to be in especial danger, but in most c ases
they pass through the digestive tracts unharmed. The smooth and
slippery surfaces and the pointed ends of most such seeds make it p rob-
able that they will be swallowed whole rather than masticated, anc the
thick and hard testa prevents the destructive action of digestive juices
upon the living contents. Sometimes the seeds, as in the grape, an- en-
closed by a mucilaginous pulp that is likely to be swallowed whole, and
sometimes they are protected by special structures, such as the car-
tilaginous layers within the apple.
The vitality of seeds. — The amount of protection exhibited by seeds
is shown in no other respect so well as by their remarkable longevity.
While some seeds (as in the willow and the cacao) die unless they
germinate almost immediately, most seeds retain their viability for
several months or even years, and a few may remain alive for many
years.
There is a popular belief in the possession of extreme longevity by
certain seeds. For example, it often is asserted that the reason for the
development of a totally new kind of vegetation when a forest is ck ared
is that seeds which have lain dormant for years or even for centimes
then for the first time have a chance to germinate; a much simpler ex-
planation, however, is found in the ease of seed dissemination. Many
people have believed that wheat buried many centuries ago with the
Egyptian mummies has germinated in recent times when properly
planted. While stories of this character are without foundation, never-
theless it is true that under proper conditions certain seeds may remain
alive for many years. Probably the longest-lived seeds are those of the
REPRODUCTION AND DISPERSAL 909
legume family. In experiments made with dried seeds of considerable
age (none under 25 years old), involving over 500 species belonging to
30 families, those of 23 species distributed among 4 families proved
viable, 18 of these being among the legumes; in these experiments the
oldest viable seeds had an age of 87 years. Other experiments indicate
the maximum retention of viability by legume seeds to be 150 to 250
years. Other long-lived seeds are those of the water lilies, the mallows,
and some of the mints.
In the phenomena of longevity there are three features of special in-
terest: the status of the living cells during this long period, the features
of the seed that cause the retardation of death, and the nature of the
factors that ultimately cause death. There have been two theories con-
cerning the status of the living cells, namely, that they manifest very
slight respiratory activity, and that they are in a state of suspended ani-
mation.
It is not possible at present to determine which theory of cell life is the more valid,
but recent experiments seem to give strong support to the theory of suspended ani-
mation. There is no adequate evidence of respiratory gas exchanges nor of any
other metabolic activity in dry seeds; the very small gas exchanges that have been
noticed are quite as characteristic of dead seeds as of living seeds, and in the latter
they are fully as prominent in the dead testa as in the embryo. Furthermore, the
theory of suspended animation best accounts for the wonderful resistance of seeds
to extreme temperatures; indeed, seeds can endure a temperature so low that
activity of any kind under such conditions seems quite impossible. The likeli-
hood that activities may take place in seeds has been suggested from the fact that
recently matured seeds of certain species germinate poorly, if at all, while, without
any obvious structural change or physiological activity, they germinate readily
after a lapse of some months. However, this theory has become less tenable in
view of the discovery that differences in the germinability of seeds are due chiefly
to changes in the permeability of the dead seed coat. In any case the life processes
of seeds, if present, are intracellular and anaerobic and are exceedingly minute in
amount.
It is practically certain that the chief feature of seeds which retards
premature germination and facilitates longevity is the impermeability
of the enveloping coat, especially of the testa. In many instances the
seed coats of desiccated seeds have been found to be nearly impermeable
to water and to gases; the most impermeable of such envelopes are
those of legume seeds, which, as has been noted, are the longest-lived of
all. The compact structure and the low water content of seeds are un-
favorable to activity, and hence facilitate lo'ngevity. Certain short-lived
910 ECOLOGY
seeds live longer in the soil than when dried, possibly because, unlike
most seeds, they are unable to withstand prolonged desiccation.
It is not unlikely that in some cases longevity is due to much less obvious features
than to seed coats. Seeds similar in structure and with envelopes equally imper-
meable vary widely in longevity. Still more striking in this respect are the minute
asexual spores of the seedless plants. While the spores of Equisetum die if they
fail to germinate almost immediately, moss spores that have lain dry in a her-
barium for fifty years have been known to retain their viability; it seems improb-
able that such differences can be accounted for by differences in the spore chat,
which is not noticeably dissimilar in the two cases. In the liverworts, however, it
has been observed that the spores of xerophytic species may withstand desiccation
for two years, whereas the thin-walled, green spores of semi-hydrophytic species
lose their vitality very quickly.
The causes of the death of seeds are in part known and in part open
to question. While water is necessary for the initiation of germint tion,
it often is absorbed by seeds under conditions that are unfavorable for
the continuance of the germinative processes. This is the case with
many seeds which fall into the water, or which are subjected tc low
temperatures or to desiccation, immediately after the absorption of
water has begun. Such seeds soon decay, or at any rate lose their
vitality. Submergence in water for a month results in the death of the
seeds of many land plants, such as rye, oats, and maize. However, the
seeds of many water plants (such. as Alisma and Sagittaria) can with-
stand submergence for some years, probably because of the extreme
resistance offered by the seed coats to the penetration of water. Even
when seeds are kept in ordinary rooms, the changes in atmospheric
humidity probably are sufficient to reduce longevity seriously, because
•of the hygroscopic properties of the integuments. Most seeds die
within three months if they are continuously exposed to saturated air,
the longevity increasing somewhat regularly as the percentage of hu-
midity is reduced. Parsnip seeds die within two months at a humidity
of 70 per cent, although they may be kept alive for three years when
desiccated and placed in a vacuum. Apparently, then, the exemption
of seeds from conditions that tend to incite water absorption, respira-
tion, or activity of any kind is a necessity for longevity. It is probably
for this reason that most seeds retain their vitality best when they are
stored where conditions are uniformly cool and dry. Experiments show
that certain seeds retain their longevity for a very long time when they
are buried in the soil, though not so long as in dry storage. For ex-
ample, seeds of mustard, dock, and purslane have been known to retain
REPRODUCTION AND DISPERSAL gn
their vitality for twenty-five years, when buried at a depth of fifty centi-
meters. It has been shown also that deep burial insures greater longev-
ity than does shallow burial. Under such conditions, longevity would
seem to depend largely upon the resistance of the seed coats to water.
If the seeds are deeply buried, the conditions are relatively favorable for
longevity, because of the uniformly low temperature and because of
comparative freedom from exposure to air and to alternations of wetness
and dryness in the soil.
While the amount of water in seeds is small, a portion of this amount
is essential to life.1 Hence, it is probable that any seed would die, if
it is exposed to evaporation for a sufficient length of time, but the time
may vary with the species from a few hours or days to hundreds of years.
If continued respiration takes place in dry seeds, however slowly, it is
obvious that death must sooner or later ensue. Conditions which are
fatal to most other plant organs often have no deleterious influence
upon seeds. For instance, dry seeds can be kept for some time with-
out injury at a temperature of — 210° C., even if the testa is perforated,
and a long sojourn in a vacuum or in an atmosphere of carbon dioxid or
nitrogen is not injurious. Extremely high temperatures also may be
withstood without harm, but with them there is a recognizable limit,
as is not the case with low temperatures. Most desiccated seeds can
withstand for one or two hours a temperature of 100° C., and alfalfa
seeds can withstand a short exposure to a temperature of 120° C., even
when placed in water. Perhaps the severest test yet made has been
with the seeds of alfalfa, mustard, and wheat, whose coats had been
perforated and thus made permeable; these seeds germinated after
having been subjected to desiccation for six months, and then placed in
a vacuum for a year, and finally subjected for three weeks to a tem-
perature of — 190° C., and for three days to a temperature of — 250° C. It
may well be wondered why seeds should ever die if they can withstand
such severe conditions. The seeds which have been reported to have
retained their vitality for more than two centuries were subjected during
this time to constant changes of humidity and temperature. It is im-
possible to conjecture how long they might have lived, had they been
stored under conditions of uniform desiccation and refrigeration.
Seeds as organs of food accumulation. — Introductory statement. — If
has been seen elsewhere that foods accumulate in various organs, par-
1 When seeds are placed in a desiccator, they retain six per cent or more of their water
for weeks; when at last this hygroscopic water evaporates, death ensues.
912
ECOLOGY
ticularly in stems and in roots, but it is in seeds that food accumulation
is most conspicuous, so that the chief discussion of plant foods has been
reserved for this place. Seeds are filled with food more generally than
is any other plant organ, and the kinds of foods reach here their greatest
diversity in composition, form, and distribution. The foods in seeds
and in other organs may conveniently be divided into those without
nitrogen (such as the carbohydrates) and those containing nitrogen
(notably the proteins).
Starch. — Starch probably is more generally accumulated in seeds
and in other plant organs than is any other kind of food, being particu-
larly well known in the grains, in peas and beans, and in potato tubers.
Starch grains differ widely in size, in shape, and in structure, these dif-
ferences serving often to characterize particular species, genera, or
families (figs. 1206, 1211). As previously noted, starch grains are pro-
duced through the activity of plastids; in seeds the plastids conce-ned
are the colorless leucoplasts, the sugar that enters the developing seeds
being transformed by them into starch. As the starch accumulates \\ the
plastid, the peripheral portion of the latter expands until finally the pro-
toplasm consists merely of a thin film
enveloping the starch grain (see figs.
660-662).1
The most obvious
structural features of
starch grains are their
lines of stratification,
which are due to alter-
nating layers of different
density (fig. 1207). In
the large grains of Pel-
lionia and probably else-
where the dense layers
have been thought to
represent accumulation
by day when the sugar
layers representing accumulation by night
Thus the layers of starch
FIG. 1206. — Cortical cells of a
potato tuber (Solatium tuberosum),
showing starch grains (j) of differ-
ent sizes, and also a protein crystal
(c); highly magnified.
FIG. 1207. — A
starch grain from
the -cortex of a
potato tuber (50-
Itinum tuberosum),
showing the eccen-
tricdevelopm -ntof
the ringsof growth;
very highly mag-
nified.
is abundant, the other
when the sugar supply is less (fig. 660).
1 All gradations between ordinary chloroplasts and chloroplasts which are reduced to a
thin enveloping film often may be seen in the stems of various water plants, as Myrio-
phyllum.
REPRODUCTION AND DISPERSAL
9*3
grains appear comparable to the growth rings of trees, like them being
caused by alternations in growth conditions.1 Differences in size and
shape are due partly to growth conditions in the plastid. Commonly
growth starts in the center (as in peas and beans), and the rings are paral-
lel to the plastid periphery, the resulting grain being a symmetrical
spheroid or ellipsoid. Sometimes growth begins at one end, resulting
in eccentric rings (as in the potato tuber, fig. 1207). Sometimes more
than one grain forms in a plastid, resulting in a compound grain through
mutual crowding in growth (as in oats and rice) ; crowded grains often
are polyhedral in shape.
The minute structure of starch grains is in doubt. One view is that they are
spherocrystals, that is, structures composed of a vast number of needle-like crystals
or trichites, radiating in all directions from the growth center. This conception is
based upon their behavior in polarized light, which is comparable to that of inulin
when precipitated by alcohol (see below). Another view is that starch is an amor-
phous colloid; formerly this view was supposed to be supported by the fact that
starch grains readily absorb stains and swell as they absorb water, but certain un-
doubted crystals exhibit similar phe-
nomena. Starch grains, because of
strains arising from desiccation or
otherwise, often exhibit cracks radi-
ating from the center (fig. 1017).
The exact chemical formula of starch
is not known, but it is generally
written n (CeHioOs) (see p. 358).
Variousnon-nitrogenous foods.
— Scarcely second in impor-
tance to starch among the non-
nitrogenous foods in seeds are
the fats or glycerids, which are
compounds of fatty acids and
glycerin, and are well illustrated
in the seeds of the castor bean,
cotton, and sunflower, and in
many nuts. The fats usually
exist as drops of oil in the cell
lumina. A third form in which non-nitrogenous food Accumulates is
the so-called reserve cellulose or hemicellulose, which makes up the
FIG. 1208. — A section through part of the
endosperm of a persimmon seed (Diospyros
virginiana), showing greatly thickened walls
of "reserve cellulose"; the lines traversing
the cell walls indicate the paths of communi-
cation between adjacent cells; highly magnified.
1 However, starch grains exhibit some stratification when exposed during development
to continuous illumination.
ECOLOGY
greater part of the thickened endosperm walls of vegetable ivory and
of the seeds of the persimmon (fig. 1208) and the date; it is "reserve
cellulose" that gives the characteristic horny hardness
to these and to similar seeds.
Although they rarely if ever accumulate in quantity in
seeds, a word may be said as to sugars and similar substances.
Sugar (particularly saccharose ') frequently accumulates in
quantity in stems (as in sugar
cane) and in roots (as in beets),
being in solution in the cell sap.
Related to sugar is inulin, a car-
bohydrate occurring in solution in
the roots of composites and of
various other plants. When these
roots are immersed in alcohol,
the inulin is precipitated in solid
bodies with concentric stratifica-
tion layers, as in starch, and also
with lines radiating in all directions
from the center, suggesting the
trichites that characterize sphere-
crystals (fig. 1209). As with
starch, the behavior of these bodies
in polarized light is that of sphere-
crystals, yet some investigators
root cell of the ele- sti11 re8ard them ** amorphous
campane(/HW/a He- colloids.
lenium) taken from
FlG.
1209
— A
FIG. 1210. — An endo-
sperm cell from a s 'ed of
the castor bean (Aicinus
communis), showing protein
grains (/>) made up of amor-
phous proteins, crystalline
proteins (c), and globular
compounds of protein with
calcium and magnesium.
•HT-J j- j -NT- the globoids (#); highly
a specimen pre- NUrogCHOUS foods. - Nl- magjified. _ j^ B£NH,
served in alcohol; trogenoUS foods, Such as the (Part II).
note the sphcntes pr0teins are much less abun.
of inulin with their x
growth rings and dant m seeds than are starches and fats, but they are
with cracks radiat- universally distributed and of much significance. The
ing from the center; ordinary protoplasm of the living cells is, of course,
note also that where ' 1
growth begins at the nitrogenous; during seed development it is active, but
wall, only half of a jt enters a period of comparative quiescence at ma-
sphente is formed; turjty again becoming active at germination. N trog-
highly magm6ed. J
enous substances also develop from vacuoles rich
in nitrogenous materials and later hardening into aleurone grains (fig.
1210). In the wheat grain, as in grasses generally, most of the endo-
sperm cells are packed with starch, but the peripheral layer, often called
1 The sugar of onion bulbs is dextrose.
REPRODUCTION AND DISPERSAL
915
the gluten layer, is filled with small aleurone grains (fig. 1211); in most
other seeds the aleurone grains are scattered among the starch grains
or the drops of fat. Some-
times, as in Ricinus (fig. 1210),
the protein grains are large and
contain inclusions, such as pro-
tein crystals and globoids, the
latter composed in part of
calcium-magnesium phosphate.
Protein crystals also may lie
free in the cell sap, as in the
cortex of the potato tuber (fig.
1 206) ; such crystals differ from
inorganic crystals in being able
to take stains and to swell in
certain media. In the algae,
nitrogenous foods occur in the
pyrenoids (fig. 106).
The distribution of foods in
seeds and the associated advan-
tages. — In nearly all seeds
there occur nitrogenous and
non-nitrogenous foods, the lat-
ter always dominating in amount. Commonly one form of non-nitrog-
enous food dominates in any given case, so that one may speak of
starchy, oily, or horny seeds. The percentage of carbon in fat is about
77 per cent, as compared with 44 per cent in starch, yet because of its
greater density, a given volume of starch contains about as much carbon
as does the same volume of fat. The chief advantage of fatty seeds
would seem to be that their relative lightness facilitates dispersal, while
on the other hand starchy seeds are better fitted for quick germination,
since the amount of oxygen required to make starch available for growth
is much less than that for fat. Thus it is distinctly advantageous that
large seeds generally are starchy ; where they are not starchy, germination
is very slow (as in the coconut). Among the seeds that are slow to
germinate are those with " reserve cellulose," as in the date.
The influence of external factors upon the formation of accumulating foods. —
Moderately high temperatures are favorable for seed maturation and also for maxi-
mum starch production, the optimum temperature for the latter being, in general,
FIG. 1211. — A cross section through the outer
part of a wheat grain (Triticum sativum), show-
ing the husk (h) whose outer part is the peri-
carp and whose inner part is the testa, the aleu-
rone or gluten layer (a) whose cells are filled
with protein grains, and a part of the starch
region (b) which makes up the body of the grain ;
highly magnified. — From COBB.
9i6 ECOLOGY
in the neighborhood of 25° C. In cold weather, sugar is not readily transformed
into starch; indeed, the reverse process often takes place (as in the sweetening of
potatoes). An important variable in starch production is the supply of available
sugar; if the sugar concentration is high, starch forms more rapidly and at lower
temperatures than usual, even at o° C. Indirectly, light favors starch formation in
that it induces a considerable production of sugar, from which, starch can be made,
but the latter can form in the darkness as well as in the light. The influence of
external factors upon the accumulation of fats, proteins, and " reserve cellulose "
is not known. For a consideration of food accumulation in tubers and galls, see
pp. 719, 782. The r61e of food in seeds will be considered in connection with
germination (p. 934).
The structure of the food-containing cells. — Practically all cells in which food
accumulates, whether in the endosperm, perisperm, or cotyledons (and also in galls
and tubers), are parenchymatic and also are thin-walled except in those cases where
the food accumulates in the walls rather than in the lumina as in " reserve cellu-
lose "). There are protoplasmic connections between adjoining endosperm cells,
and where the walls are thick, as in " reserve cellulose," the canals containing the
connecting protoplasmic threads are quite conspicuous (fig. 1208).
Variations in seeds and fruits in relation to external factors. — The
fusion of gametes in relation to fruit development. — Were the phe-
nomenon not so universal, it would seem amazing that large fruits are
able to develop as a result of so slight an external stimulus as thut in-
troduced by a male gamete upon fusion with an egg. In general those
pistils in which this fusion takes place develop into fruits, while other
pistils show no such changes, soon dropping off, as do the stamens.
In the simpler cases fruit development involves only the enlargement
or elongation of the ovary, but in other cases various organs may be in-
volved, for example, the calyx and the receptacle (as in the apple).
Sometimes the stimulation appears greater in the case of xenogamy than
of autogamy; for example, in Cheiranthus the fruits are twice as large,
and the seeds are heavier and more numerous on the cross-pollinated
individuals.
There is much in common in the formation of fruits and galls, and in eac i case
it has been held by some investigators that the growth arises solely through the
influence of a momentary stimulus at the inception of the process, and by others
that the activities within the growing structures afford constant stimuli for f irthcr
development. Of interest in this connection is the fact that staminate flowers
may be transformed into galls if stimulated by the proper insects (as in the ash).
In this event, instead of dropping off, they enlarge and remain for a year or more.
Here a foreign stimulus given by the gall insect causes the retention and the further
development of the staminate flowers much as another foreign stimulus given by
the male gamete more commonly causes the retention and the further development
of the pistillate flowers. The precise nature of the fruit-forming stimulus varies
REPRODUCTION AND DISPERSAL 917
considerably with the species. In many cases the act of pollination forms a stimu-
lus of sufficient intensity to inaugurate continued development; this condition is
well illustrated in certain orchids in which fruit development has been started by
dead pollen or pollen extract placed upon the stigma. In other cases it is the grow-
ing pollen tube which initiates fruit development, as in Geranium and in various
orchids. In the Cucurbitaceae the fusion of gametes is necessary for complete fruit
development, although pollination alone stimulates considerable growth. In still
other cases the growing ovules are an important stimulus, as in the grape, where
the size of the fruit increases with the number of seeds.
Parthenocarpy. — In striking contrast to ordinary fruit production
is parthenocarpy, or the development of fruit without the fusion of
gametes. Familiar illustrations of parthenocarpy are afforded by a
number of seedless varieties of cultivated fruits, as in oranges, grapes,
and bananas; while only certain varieties of grapes and oranges are
seedless, the cultivated banana never produces seeds.1 In some cases
of parthenocarpy, pollination seems to be necessary for fruit development,
but it is quite unnecessary in certain figs, where fruit development occurs
without the aid of any known external stimulus. The most striking
case of all is in Balanophora, a plant which is quite without functional
pistillate flowers, but which produces fruits containing viable seeds. In
this genus the pistillate flower is reduced to a protuberance with rudi-
ments of a style and an embryo sac. One species (B. globosa) lacks
staminate flowers, and even in those species which produce pollen, it is,
of course, entirely useless, affording one of the best illustrations of
the retention by a plant of a useless organ. Balanophora is a holopara-
site, and it may be that there is some connection between its parasitism
and its loss of sexuality.
In recent years the number of plants which are known to be able to develop
parthenocarpic fruits has been considerably increased; among such plants are the
persimmon, gooseberry, hop, and certain varieties of the apple and the pear. It is
also becoming clear that in most cases neither pollination nor any other known
external stimulus is necessary to secure fruit development. In several cases, as in
the gooseberry and the persimmon, the seedless fruits mature earlier than do the
seed-bearing fruits. Obviously fruit production without seeds is wholly useless so
far as the perpetuation of the plant is concerned.
Variations in the size and the structure of fruits and seeds. — Probably
no other plant organs are as invariable as are fruits and seeds, and for
this reason the few variations which are known have an unusual interest.
Seeds which develop singly or which are not crowded during develop-
1 Plants with parthenocarpic fruits are, of course, propagated vegetatively ; it is sup-
posed commonly that they originated suddenly as mutants.
918 ECOLOGY
ment are likely to be spherical, while crowded seeds commonly are
angular. In the two-seeded fruits of Xanthium and Cakile, each seed
differs considerably in shape and in size from the other. In certain
composites the achenes of the ray flowers and of the disk flowers differ
strikingly in shape. In the parasitic Scrophulariaceae it has been dis-
covered that vigorous plants give rise to larger seeds than do weak plants,
and that the large seeds give rise in turn to more vigorous plants than do
the smaller seeds; furthermore, the larger seeds are more likely than are
the others to grow into autophytic individuals, while the plants coming
from small seeds in order to thrive, apparently must be parasitic.1
The achenes of hemp vary considerably in size and in weight, those pro-
duced in moist habitats being larger and heavier than those produced
in dry habitats. The larger achenes germinate more quickly than do
the others, forming stronger plants. Similar differences have been ob-
served in the seeds and seedlings of tobacco. In a crowded groip of
natural seedlings such a difference in size might be of great significance,
since the stronger seedlings would tend to crowd out the others. The
influence of grafting upon the character of fruits has been noted elsew'iere,
but it may be recalled that changes in the size and in the flavor of c ulti-
vated fruits often result from the reciprocal influence of the stock and
the scion. Pollination may affect the character of the fruit; for example,
when the flowers of watermelons are pollinated by cucumber pollen,
the resulting fruit is very poor in sugar.
Seed variations manifested in behavior. — Seeds of the same species,
though apparently alike in structure, in reality may be very different.
Perhaps the best instance of this is seen in a comparison of seeds raised
in different climates. Farmers in the United States have long known
that northern-grown seeds produce crops that ripen earlier than do crops
raised from seeds grown farther south. It appears as if the progeny of
the northern plants have inherited from them their short maturation
period, thus furnishing evidence in favor of the theory of the inheritance
of acquired characters (p. 947). After a few years, however, it is neces-
sary once more to use northern seeds, since the progeny of northern-
grown plants come to have the same period that is characteristic ol the
climate to which they are transferred.
1 In this connection it is of interest to note that various parasites (as Hydnora, Rafflesia,
and Balanophora) and mycophytes (as Monotropa and the orchids) have minute seeds
with rudimentary un differentiated embryos and almost no food, nutritive dependence
upon other plants being necessary very early, in most cases even in the earliest stages of
germination.
REPRODUCTION AND DISPERSAL 919
The dehiscence of fruits. — Fruits that open on maturity, thus per-
mitting the ready scattering of seeds, are known as dehiscent, while those
that do not open are called indehiscent. Dehiscent fruits are illustrated
by capsules (figs. 1213, 1214) a.ndpods (fig. 1212), while berries (fig. 1222),
rtone fruits (drupes}, and acorns (fig. 1223) represent indehiscent fruits.
Many indehiscent fruits are one-seeded, and may easily be mistaken for
^eeds; among such are the small, dry fruits, known as achenes, especially
characteristic of the composites (figs. 1217, 1220), and also grains, nuts,
and acorns. In the umbellifers the fruits are known as schizocarps, the
one-seeded carpels splitting at maturity but not dehiscing (fig. 1221).
Although the habit seems relatively useless, dehiscence occurs in some
one-seeded fruits, as in the nutmeg.
Usually the opening of dehiscent fruits is due to an unequal contrac-
tion of the pericarp tissues, resulting from desiccation, some cells or tis-
sues losing more water than do others; often the cells are arranged trans-
versely on the concave side, and longitudinally on the convex side.
Capsules usually open in such a way as to expose as many valves as there
are carpels, the splitting taking place along the separating walls or along
the middle line of the individual carpels. Characteristic pods are illus-
trated by the crucifers and legumes; among the latter the valves not only
are separated by longitudinal splitting, but there may be torsion within
the individual valves (fig. 1212). Some fruits dehisce through pores,
as in the poppy; and in others the lines of dehiscence are transverse
rather than longitudinal, resulting in the detachment of the top like a
lid, as in Porhdaca and Plantago. At the time of dehiscence the seeds
readily become detached from the carpel wall and are exposed to dispers-
ing agencies; the scar left on the seed at the point of attachment is known
as the hilum. In the pines and in other conifers the cone scales commonly
separate from one another at maturity, exposing the winged seeds to the
wind; in some species the persistent cones may remain closed for many
years (as in Pinus Banksiana), the seeds thus retaining their viability
much longer than otherwise. In some of these trees, extreme desicca-
tion, such as is caused by forest fires, seems necessary to effect the open-
ing of the cones. In some indehiscent fruits there is an outer dehiscent
envelope, as in the involucre of the chestnut and the hickory nut and
in the aril of the bittersweet.
The dispersal of fruits and seeds. — Dispersal by propulsion. — In
dehiscent fruits it is generally the seed, and in indehiscent fruits, the
fruit as a whole, that is scattered. In some cases the act of dehiscence
Q20
ECOLOGY
is so sudden and violent that the seeds are expelled at the same time.
At dehiscence the seeds of the violet and lupine are shot out several
centimeters (sometimes nearly a meter), while those of the witch-hazel
are expelled much more violently, and may be scattered for several
meters. In the lupine the seeds are expelled spirally by reason of the
torsion of the valves (fig. 1212).
In Geranium the carpels separate from the central axis, coiling upwards and
discharging the seeds. In Hum crepitans the dehiscence is so violent that the
seeds are discharged with an explosive report. In Ecballium
the seeds are squirted out, together with some of the fruit
tissues, whence the name, squirting cucumber. In Impatiens
the fruit tissues are in a state of such delicate balance that
a mere touch causes violent dehis-
cence and dispersal, whence tlie sig-
nificance of the scientific name as
well as of the common name, i ouch-
me-not. In a western mis'letoe,
Arceuthobium occidenlale, th( ripe
fruits explode, ejecting the seeds for
several meters; as in other nistle-
toes, the seeds adhere read.ly to
leaves or bark.
1213 1214]
FIGS. 1212-1214. — 1212, an opening pod or
legume of the lupine (Lupinus perennis), illus-
trating violent dehiscence through the torsion
of the valves (v) when desiccated; the seeds
(s) are mechanically expelled for some distance;
In many cases there is no vio-
lent dehiscence, but the seeds lie
1213, 1214, dehiscence of the capsules of the in such a position that a mechan-
evening primrose (Oenothera biennis): 1213, a jcal impact causes scattering.
mature capsule in which the four valves (v) are
beginning to split at the apex; 12 14, a later stage Most capsules (as m Oeno/Iiera,
in which desiccation has caused the valves to figS. 1213, 1214, and Pediculdris)
spread apart, exposing the seeds (5) in such a and m dg
way that they may readily be shaken out.
w;th
.
valves open, and the wind may
shake out the seeds^ or animals may brush them out. In many mints,
if one presses down a calyx having mature nutlets, the latter shoot
out upon release. Of especial interest is Polygonum virginicnum,
whose achene is fastened to an elastic cushion of tissue in such a way
that, when pressed back, it bounds off upon release for a distance of
three or four meters. Obviously the dispersal of seeds by propulsion
is relatively ineffective, since at best the seeds are scattered but a few
meters from the parent plant, and commonly much less.
Dispersal by wind. — With seeds, as with spores, the most effective
of dispersal agents is the wind, especially from the standpoint of the
REPRODUCTION AND DISPERSAL
921
FIGS. 1215, 1216. — Winged fruits:
1215, a samara or key fruit of the moun-
tain maple (Acer spicatum); 1216, a
number of disseminules carried. As
seeds are much larger than spores, the
distances covered are much less, though
in the case of small seeds, as in the
orchids, it is possible that the dis-
tances may be very great. Among
the commonest of wind-scattered dis-
seminules are those with wings, as in
the seeds of the catalpa, and in the
fruits (known as samaras) of the
maple (fig. 1215), hop tree (fig. 1216),
and elm. Such disseminules com-
monly are much flattened, and hence samara of the hop tree (Ptdea trifoliate);
are unlikely to fall rapidly to the ™' Wi
ground; furthermore, the wings are light, often containing air spaces of
considerable size. The wings may be terminal, as in the ash and the
maple, or they may form a margin about the seed-bearing portion, as in
the hop tree, elm, and bugseed. Similar winged disseminules are found
in the pines and birches. In the linden
there are relatively heavy, globular, in-
dehiscent fruits, but they are borne on
a large, membranous bract attached to
the peduncle.
Many wind-scattered disseminules are
crowned with hairs. Perhaps the most
representative of these are found in the
composites, especially in those with milky
juice; in the latter, at maturation, the
involucre falls back once more as at an-
thesis, exposing the achenes, with their
crowns of hairs (known as the pappus)
spread out in such a way that the entire
structure resembles a parachute (fig. 1217) ;
as in parachutes, also, the resistance to
the air in falling is considerable, so that
wind currents are apt to scatter the
achenes for some distance. Dispersal is
facilitated still further, if there is a long,
slender process (known as the beak) sepa-
FIG. 1217. — A fruiting head of
the prickly lettuce (Lactuca scari-
ola), from which all of the mature
fruits but one have been blown
away; note the reflexed involucral
scales (s) and the achene (a),
which is prolonged into a beak (6)
and is crowned with pappus com-
posed of capillary bristles (/>); r,
receptacle; P', peduncle.
922 ECOLOGY
rating the achene from the pappus, as in the lettuce and the dandelion,
or if the pappus hairs are branched, as in the thistle.
In the milkweed (Asclepias) the seeds bear a crown of long, silky hairs at the
hilum end, which enables them to float in the air much as do the achenes of the
composite. Similar hairs facilitate dispersal in the willows and poplars, the cotton-
wood deriving thus its common name. Commercial cotton is derived from the
copious hairs that are attached to the seeds of the cotton plant (Gossypium); similar
cottony hairs are attached to the fruits of some anemones and of the cotton grass
( Eriophorum) .
A remarkable instance of wind dispersal is afforded by the tumbleiveeds,
a class of plants that at maturity break off from the roots as a whole or
FIG. 1218. — A general view of mature plants of the winged pigweed (Cycloloma alripli-
cifolium), a representative tumbleweed; Gary, Ind. — Photograph supplied by MEYERS.
in part, whereupon they are tumbled along over the ground by the wind,
scattering seeds as they go (fig. 1218). Among such plants, which are
especially common on the prairies and plains, are the winged pigweed
(Cycloloma), the Russian thistle (Salsola Kali tenuifolia), and Amaran-
Ihus graecizans; these generally break off entire, but in the old witch
grass (Panicum capillare) and in some other plants, portions break off
and blow about alone or attached to other tumbleweeds.
Dispersal by water. — Water, though less effective than wind in the
uumoer of seeds carried to places where they can germinate and grow,
is none the less a dispersal agent of great importance, particularly be-
cause It may carry disseminules for long distances. Sometimes the dis-
REPRODUCTION AND DISPERSAL 923
seminules move in definite directions, as in rivers and in the better-
defined ocean curren s, but in ponds and lakes the direction of move-
ment commonly varies with the winds. All seeds are heavier than air,
and hence are incapable of indefinite propulsion in that medium, but
many seeds and fruits are lighter than water, and hence may be carried
for great distances; among the latter are the fruits of many water plants
and swamp plants, such as Sagittaria and Sparganium, whose lightness
is due largely to the presence of prominent air chambers in the pericarp
or testa. Many seeds, however, sink in water, some rapidly and others
more slowly, so that the distance they may traverse is more or less limited,
as with wind-scattered seeds; among the seeds which sink at once are
included those of such pronounced hydrophytes as Ceratophyllum and
Subularia.
Of great significance in connection with water dispersal is the degree
of resistance to the entrance of water offered by floating seeds and fruits.
Many seeds capable of floating soon lose their vitality through the en-
trance of water, which thus institutes decay. Particularly is this the
case if the water is rough, and more particularly if it is salt as well as
rough. For example, the coconut, whose fruit often is seen floating on
tropical seas, loses its vitality within a few days through infiltration, so
that it is doubtful if it could populate a new land at a great distance,
though no illustration of water dispersal is quoted more frequently.
In contrast with the coconut are such fruits as that of Suriana maritima,
a common plant of tropical strands; these have been shown experimen-
tally to be uninjured after floating for 143 days in rough salt water, and
the seeds of Hibiscus tUiaceus similarly have been shown to be capable
of floating for 121 days without injury. The presence of air chambers,
especially in the pericarp, greatly retards water infiltration. In Barring-
tonia the resistance to infiltration is so great that broken pieces of the
fruit float for more than twenty weeks in a 3 per cent salt solution. The
seeds of Asparagus may retain their vitality when soaked in water for
a year, and in many water plants (as Sagittaria and Proserpinacd)
the seeds may retain their vitality at the bottom of ponds for several
years. It can hardly be doubted that in all cases the retention of vitality
in immersed seeds is due to the resistance of the various coats to infiltra-
tion.
Dispersal by animals. — Many fruits, mainly indehiscent, are scat-
tered involuntarily by animals, particularly the bur fruits and others with
hooked appendages. Unpleasantly familiar fruits of this character are
924
ECOLOGY
those of the cocklebur (Xanthium, fig. 1219), burdock (Arclium), beggar-
ticks (Bidens, fig. 1220), hound's-tongue (Cynoglossum), sweet cicely
(Osmorhiza, fig. 1221), and bur grass (Cenchrus). These and similar
fruits are scattered abundantly by man and by domestic animals, and
some plants (as Xanthium) have thus made a rapid in-
vasion of all continents.
An interesting class of fruits from the standpoint of
dispersal consists of those which are fleshy and possess a
more or less juicy and edible pulp
(fig. 1222). Birds and other ani-
mals commonly eat such fruits
abundantly, often aiding in the
scattering of the seeds. Some
birds eject the seeds immediately
after divesting them of the eJible
portion of the fruit, but the ma-
jority of fruit-eating animals ] >rob-
ably swallow the seeds, espet ially
those that are small; even s ones
as large as those
of the cherry are
swallowed by ani-
mals as small as
the raven. In
some cases, as in
the dove and the
domestic fowl, the
1219 1220
FIGS. 1219-1221. — Fruits with append-
ages which become fastened to animals
and thus dispersed: 1219, a fruit of the
cocklebur (Xanthium), whose body is cov-
ered with stiff recurved prickles; 1220, an
achene of the bur marigold (Bidens),
crowned with two sharp and stiff teeth or
awns (a) which are covered with reflexed
barbs (6); 1221, a mature fruit (schizo-
carp) of the sweet cicely (Osmorhiza loii-
gistylis), consisting of two one-seeded car-
pels (c) which separate along the inner
face, remaining delicately suspended on
slender prolongations of the axis, the car-
pophore ((/) ; the carpels readily adhere to
passing animals by means of the barbs (6).
seeds commonly
are destroyed in
passing through
the alimentary tract. The most useful animals from
the standpoint of dispersal are such birds as the
robins, thrushes, and blackbirds, which eat fleshy
fruits in abundance, swallowing the seeds, and void-
ing them without harming them in the alimentary
tract. Obviously such birds are likely to carry the seeds to some dis-
tance from the parent plant, as would not be the case with those that
reject the seeds while eating.
Fleshy, edible fruits when ripe usually are conspicuous by reason of
FIG. 1222. — An
aggregate fleshy
fruit of the mul-
berry (Mfirus) ;
such fruits are
eaten by animals,
the seeds p issing
undigested th rough
the alimeitary
tract.
REPRODUCTION AND DISPERSAL 925
their color, though green and relatively inconspicuous when immature.
Of the showy fruits of this sort some are white (as in the snowberry and
mistletoe), others red (as in the holly, bittersweet, and cherry), others
blue (as in the red cedar and blueberry), and still others black (as in the
blackberry and black haw), or yellow (as in various Solanaceae).
Though showy fruits doubtless attract fruit-eating animals and thus
facilitate seed dispersal, it is likely that the advantage of such showiness
has been overestimated. Probably the animals would find the fruits if
they were not highly colored; indeed, some edible fruits, as in Asimina
and Ribes Cynosbati, are green at maturity. Furthermore, species with
fleshy fruits doubtless would become dispersed, even if all fruit-eating
animals should disappear (see below concerning nut dispersal). Showi-
ness, therefore, probably is merely an accompaniment of ripening, in-
dicating the occurrence of certain chemical changes; incidentally they
also are of some advantage in that animals thereby are attracted. Some
fruits, as the blackberry, are showiest when red and immature, and some
showy fruits (as in Physocarpus) are quite dry and inedible.
Doubtless various large wading birds, such as the herons, carry seeds in the mud
that adheres to their feet, thus accounting, perhaps, for the wide distribution of
some swamp plants. Fruit-eating animals do not always
facilitate dispersal. For example, in autumn, birds feed abun-
dantly on the fruits of various plants (such as the ragweeds,
sunflowers, and certain grasses, as wild rice), eating the seeds,
and thus preventing rather than advancing dispersal. Recently
it has been shown that ants play an important part in the
dispersal of many small seeds, particularly where the seeds
have oily appendages which the ants utilize as food. Cer-
tain heavy seeds (such as the nuts and acorns, fig. 122?)
• , , , •" nut (acorn) of the
usually are not scattered in any of the above ways; further- black oak (<2«er««
more, they are gathered and eaten in large numbers by squirrels. veliUina), partially
Occasionally nuts that are carried off by animals are not eaten, enclosed by its cup
and thus may germinate, but at best such a means of dispersal (c), which has de-
is rather precarious. veloped from the
involucre ; note the
The relative efficiency of the various means of dis- imbricated scales
7 TH, -J *• U • 1 J- Of the CUp (5).
persaL — I hree considerations seem to be involved in
successful dispersal: the number of disseminules transported, the
distance they are taken, and the degree of precision with which
they lodge in places favorable for germination and for subsequent
development. Some seeds and fruits are not transported at all, the
most notable examples being those that ripen under ground, as in
926 ECOLOGY
the peanut and in the fruits of the cleistogamous flowers of Polygala
(fig. 1191) and Viola; though this habit might seem disadvan-
tageous, no seeds are better placed for germination. A vast number
of seeds and fruits have no regular means of dispersal apart from drop-
ping to the ground beneath the plant that bore them; among such are
the nuts, the acorns, and many other heavy fruits or seeds. Scarcely
more effective are the numerous cases of mechanical propulsion from de-
hiscent fruits. However, in all these cases the seeds are likely to lodge
in places that are relatively fit for germination.
The effective agents of distant dispersal are water, wind, and animals.
Water probably is the most likely to carry disseminules for great: dis-
tances, but the number of seeds which fall into the water is limited; a
great many of these seeds also are injured in transit, and still more fail
to lodge in a suitable habitat. Water, however, is of the utmost impor-
tance as a transporter of the fruits and seeds of plants which grew in
the water, or in swamps, and along shores, since deposition is like ly to
be in a place that is fit for subsequent growth.1 Temporary streams,
such as torrents following heavy rains, and permanent streams in imes
of flood, are highly important agents in the dispersal of the seeds of land
plants. Wind is the most likely of all agents to pick up and transport
seeds and fruits in great numbers from all habitats, but it is also the most
indiscriminate of scatterers, depositing all kinds of seeds in all kinds of
places, so that the waste of disseminules is enormous. Seeds and fruits
scattered by animals may or may not be carried far, but they are likely
to lodge in a favorable situation, since animals of a given species tend to
frequent similar habitats; wading birds, for example, fly from swamp
to swamp, and grazing animals scatter hooked fruits in places similar
to those in which they were gathered.
Probably, in spite of its wastefulness, wind is the most efficient of dis-
persing agents. On newly formed islands the pioneer plants of the in-
terior portions are mainly those whose disseminules are scattered by
wind, a smaller number being scattered by birds, while the shore plants
are brought largely by water currents. For example, on the island of
Krakatoa, whose vegetation was entirely destroyed by a volcanic eruption
in 1883, the first plants were thallophytes and bryophytes with wind-
borne spores, and the first higher plants to reappear in abundance were
1 However, the water may carry seeds so far Ihat the new climate is unsuited for de-
velopment, as in the West Indian seeds carried to the shores of Norway by the ocean
currents.
REPRODUCTION AND DISPERSAL 927
ferns, whose spores are readily scattered by wind. Fifteen years after
the eruption, fifty-three species of seed plants had reached the island,
and of these it was estimated that 60 per cent, chiefly shore species, were
brought by ocean currents, 32 per cent by wind, and 8 per cent by animals.
The dispersal of epiphytes is of interest because of the difficulties attending the
lodgment of disseminules in places fit for germination. Most epiphytes have wind-
scattered disseminules, as in the spores of the lichens, mosses, and ferns, or the
seeds of the orchids and bromelias. Most such disseminules are minute, and,
while many are wasted, a few find lodgment in bark crevices. The seeds of some
epiphytes are scattered by birds, as is the case also with many of the pseudo-epi-
phytes of temperate climates, which occur in soil in the crotches of trees (as the
raspberry, gooseberry, and nightshade). Mistletoe, which is parasitic on trees,
is also scattered by birds; after eating the enveloping fleshy rind, the slimy seeds
which often stick to their bills may be wiped off upon the limbs where they are
perched, and hence in places suitable for germination.
A study of the geographic distribution of plants shows that some
species, which are known as endemic, are confined to restricted areas,
and that other species, which are known as cosmopolitan, are almost
world- wide in distribution; the members of a third class, embracing a
much greater number of species, occupy relatively large but not world-
wide areas. It might be supposed that the size of the area occupied by
a species is determined by its means of dispersal, but this is not obviously
the case. While many mobile species (i.e. those with easily scattered
disseminules) are widely distributed (as in the "willows and cat-tails),
and while some immobile species are endemic (as in Torreya), there are
many cases in which the reverse is true; for example, the immobile oaks
and beeches are among the most widely distributed trees, while the
wonderfully mobile orchids furnish many cases of endemism.
In explaining the distribution of species, many factors other than the
mobility of disseminules are to be considered. An important element
in the problem is time. For example, even though the oak or beech
in a century might be able to migrate only a few meters, in contrast with
as many kilometers in the case of the willows, such a difference is of
little consequence in the eons of geological time. Hence it may be
stated as a somewhat general truth that the rapid occupation of a new
area depends largely upon the mobility of plant disseminules,1 but that
1 There are some cases of rapid migration, where the disseminules are not conspicu-
ously mobile, as in Galinsoga parviflora and Artemisia Stelleriana, two composites without
the usual hairlike pappus, which have spread over the world in a comparatively few years.
Apparently such cases are associated in some way with man, whose various means of
928 ECOLOGY
this is usually a matter of small moment in determining the ultimate
population.1 Geological history shows that the endemism of Torreya,
noted above, is in no wise due to disseminule immobility, for it was once
widely distributed, a fact that suggests that the most important of all
factors in distribution may be the fitness of a species to exist under the
given conditions.
The origin of seed structures. — Nothing is known concerning the
factors involved in the origin of the manifold features of seeds and frurts
which fit them for the role of disseminules. It has been suggested that
these features have arisen through natural selection, but such a hypothe-
sis seems incredible in view of the obvious difficulty in grouping seed
plants in the order of successfulness in such a way as to show a definite
relation to their kinds of disseminules. Even in the annuals, wiich
depend most upon seeds, there is no obvious relation in most cases be-
tween mobility and success. Nor is anything definitely known as to the
factors involved in seed formation, except that it is the final proce.ear
a definite relation to the causative factors of the reproductive processes.
The planting of seeds. — While gardeners are particular as to the
depth at which seeds of various sizes are planted, there is no such soiling
in nature. Large and small seeds alike fall to the ground and gradually
become covered by falling leaves, by decaying herbage, or by soil that
is deposited by winds or waters. Doubtless many small seeds become
buried too deeply to permit of successful germination; such a fate is
rarer with large seeds, except, perhaps, where they are covered by the
deep alluvium of streams. While superficial planting doubtless is more
favorable for small seeds than for large seeds, the latter may none the less
germinate successfully at the surface; perhaps the chief danger in the
shallow planting of large seeds is that there may not be sufficient water
for germination.
transportation seem to have made up, in the case of many species, for any natural lack of
disseminule mobility.
1 Where similar habitats are discontinuous, as in oceanic islands, the flora m;iy be
made up for a much longer time than elsewhere of plants with mobile disseminules;
the preponderance of ferns in many such places probably is thus explained. Yet even on
Krakatoa, a quarter of a century has been long enough for the invasion of a number of
species with apparently immobile disseminules, whose mode of migration is unknown.
It is to be noted that one seed, however extraordinary its mode of migration, may be
sufficient to populate a new area with an abundant vegetation.
d
REPRODUCTION AND DISPERSAL 929
Seeds that pass through the alimentary tracts
of large animals, such as cattle, are planted most
advantageously in their excrements, where, upon
germination, the young seedlings find an excellent sup-
ply of food materials. Nuts buried by animals, if they
• chance to escape being eaten, often are favorably placed
for germination; it is to be recalled also that some fruits
mature in the ground (as in the peanut and the violet), so that
favorable planting is sure to result. Some seeds and fruits
have features enabling them to remain attached to their posi-
tion on the ground, notably in such hooked fruits as those of
the cocklebur and the burdock; in the seeds of flax and mustard
the outer layer becomes mucilaginous when moistened, facili-
tating adherence to the substratum.
A remarkable seed-planting mechanism is seen in certain
hygroscopic fruits, notably in the porcupine grass (Stipa, fig.
1224). Here the fruit is prolonged below into a sharp spine
that is clothed except at the tip with hairs that point upward,
while above there is a long awn whose basal portion coils into a
close spiral when exposed to desiccation, and uncoils when
moistened, the tissues being so constructed that the evaporation
and the absorption of water are unequally distributed. If the
spine-tipped base sticks into the ground, the repeated twisting
and untwisting of the awn serve to bury it deeper and deeper
in the soil, the upward-pointing hairs preventing any move-
ment in the reverse direction. These fruits
FIG. 1224. A mature are sucn efficient penetrating mechanisms that
fruit of the porcupine grass they work readily through clothes or through
(Stifa spartca), showing the envelopes in which they are siored and pene-
seed-bearing portion (d) and . . /
the long, spirally twisted awn trate even into the flesh of grazing animals,
(a); the basal portion or When the fruits of Stipa lie horizontally on
callus (c) is stiff and sharp, the g^^H changes of moisture result in a
and is clothed with bristles .'
(6) which point upward. slow creeping movement along the surface.
Hygroscopic fruits simitar in character to
those of Stipa are found in various grasses (as Aristida and Avena) and
in Erodium, a relative of the geraniums.
CHAPTER VI — GERMINATION
Seed characters that facilitate or retard germination. — Introductory
statement. — Probably most seeds are able to germinate at maturity, if
suitable conditions are present. However, there are many seeds which,
under ordinary natural conditions, require the lapse of a longer or
shorter period before germination is possible. Such delayed germina-
tion may be due to a lack of actual maturity in spite of appeara ices,
or, more commonly, to enveloping structures that retard the germin itive
processes. There are some seeds which ge minate in natural condi-
tions the moment that maturity is reached, the best illustration of such
a habit being afforded by viviparous plants.
Vivipary. — Viviparous plants are those in which the embryo con-
tinues in a state of uninterrupted development from the outset. Since
a period of rest between two periods of sporophyte activity is the jhief
distinguishing feature of the seed, it is obvious that viviparous plants are
essentially seedless, and hence do not in the usual sense exhibit germina-
ation. The best examples of vivipary are the mangroves (especially
Rhizophora and Bruguiera). In the American mangrove (Rhizophora
Mangle) the " seedlings " develop a large, green, pointed structure,
mainly a greatly enlarged hypocotyl, which protrudes from the fruit (figs.
1225, 1226), and which finally becomes so heavy that the "seedling"
drops into the mud beneath; since this structure is heaviest toward the
lower end and is much more massive than the plumule, the " seedling "
falls right side up into the mud and continues growing, soon striking root
and exhibiting vigorous plumule development (fig. 1227). Vivipary has
been regarded as advantageous to the mangroves, since ordinary ;;eeds
might not be able to germinate in the oozy slime beneath the trees.
Currents frequently bear the fallen " seedlings " to neighboring shores,
so that the viviparous habit also facilitates dispersal ; as the young
plants float in an erect position, they readily lodge in places which
are suitable for further growth.
Some alpine plants exhibit vivipary, notably species of Poa and Polyganum,
but the advantage, if any, is not evident. Somewhat comparable to vivipary is the
930
GERMINATION
931
early germination of seeds within the fruit
(as in the lemon). The cause of vivipary is
unknown, though if seed formation results
from increasing xerophytism or from decreas-
ing nutrition, vivipary may be due to the
continuance of conditions favorable to vege-
tative development or to the inception of
such conditions at fruit maturity. This idea
seems to be favored by the fact that various
grasses exhibit vivipary in wet autumns, and
that peas and beans, when vegetative con-
ditions are favorable, often exhibit uninter-
rupted embryo development. Approaching
such vivipary is the germination of seeds while
still within the fallen fruits of Typha and
A ndropogon.
Seed maturity and germination. — A
number of seeds are capable of germi-
nation as soon as they are shed; among
such are those of the willows, the sen-
sitive plant, and many cycads, crucifers,
and grasses.1 It is a matter of com-
mon belief, however, that most seeds
require a resting period of some weeks
or months before they are capable of
germination, and that in temperate and
in cold climates germination ensues
only after a period of rest in the
ground, coupled with exposure to low
temperatures. In many seeds under
ordinary conditions the germinative
capacity may improve with age, a cer-
tain percentage being capable of germi-
nating after the first winter, a larger
percentage after the second winter, and
in a few instances a still larger percent-
age after the third winter; it is said
that the seeds of certain conifers are
1 It will be recalled that willow seeds soon lose
their vitality, especially if desiccated; seeds of
the sensitive plant, however, have been known
to retain their vitality for sixty years.
FIGS. 1225-1227. — Vivipary in the
mangrove (Rhizophora Mangle) :
1225, a mature fruit attached to the
tree, the basal portion of the embryo
(r) just emerging; 1226, a later stage
in which the young plant has become
so heavy that it falls from the parent
tree; note the plumule (p) and the
greatly enlarged basal portion of the
embryo (r) ; 1227, a stage still later,
in which the young plant has rooted
freely in the mud (r'), the plumule
meanwhile having grown vigorously
932
ECOLOGY
incapable of germination for several years. There are some plants (as
the cocklebur, red clover, and black locust) in which some of the
seeds appear ordinarily to require a longer time than do others before
they are capable of germination. It is probable that in most of these
cases delay in germination is due to the impermeability of the testa (see
below). Yet it is conceivable that in seeds, as in buds, various maturing
processes take place after the attainment of apparent maturity ; detach-
ment from the carpel and apparent rest may not mean the cessation x»f
maturing activities. Possibly the delayed germination of the hawthorn
(Crataegus) is to be thus explained, since the removal of the testa and
exposure to good germination conditions seems for a certain period in-
effective.1 The most remarkable of all cases of delayed germination is
afforded by the spores of Lycopodium, which seem
to require a rest of three to fifteen years before :hey
are able to develop.
The relation of the testa to delayed germination.—
The common cocklebur (Xanthium canadense) has
two seeds in each fruit, differing somewha: in
shape and in position (fig. 1228), and it has been
found that the seed nearest the base usually germi-
nates the first spring after maturation, while the
klebur fruit (Xan- upper seed commonly does not germinate until
thium) in longitudinal the second spring. In many species some seeds
section, showing the . . , . . ... ,
position of the two germinate long before others, and it is not unlikely
that in some cases the seeds of a given crop may
germinate over a period of three or more years.
Such a condition seems advantageous, especially in
annuals, since it insures the persistence of a species,
the fruit begins to de- eyen though certain seasons prove unfavorable for
cay at (c) ; p, hooked . . , , .
prickles which aid in see(^ development. In Xanthium, it has been shown
dispersal.— After that the delayed germination of the upper seed is
CROCKER (drawn from due to the fact that jts tegta Js jess permeable to
a photographic repro- .
duction). oxygen than is that of the lower seed. In nature
the lower seed is exposed first to good germin;itive
conditions, because that end of the fruit disintegrates first. In various
plants (as Abutilon, Iris, and Axyris) the testa (or endosperm) delays
germination because it excludes the necessary water. The upper
FIG. 1228. — A coc-
seeds ; note that the
lower seed (/) is larger
than the upper seed
(w) and better placed
for germination, since
1 Even in Crataegus, germination has been brought about in two months through the
removal of the testa, though in natural conditions it usually requires a year and a half.
GERMINATION 933
seed in Xanthium may be made to germinate early by exposing it to
high temperatures (32° C. to 34° C.), probably because the absorp-
tion of oxygen and water is thus facilitated; if the testa is removed
from the upper seed, it germinates as readily as does the lower seed,
and at as low a temperature (22° C. to 24° C.).
It has been seen elsewhere that the testa is chiefly responsible for
prolonged vitality in seeds, and it is here seen to be responsible for most
cases of delayed germination. Longevity obviously is advantageous,
and to a certain extent delayed germination also may be advantageous,
especially in annuals. There is reason to believe, however, that some
seeds, especially among xerophytes, are overprotected, the pericarp or
testa being so impermeable that death is likely to occur before the water
and the oxygen necessary for germination have an opportunity to enter.
The relation of external factors to germination. — It is a matter of com-
mon observation that water and moderately high temperatures are nec-
essary for the germination of seeds, and very simple experiments show
the equal necessity of oxygen. Nor are any one or two of these factors
sufficient. Seeds on a dry shelf never germinate, in spite of favorable
temperatures, nor will they germinate in an atmosphere without oxygen
or at low temperatures, whatever the other conditions. Oxygen and
water appear to be directly necessary for germination,1 the oxygen being
necessary to combine with the accumulated foods, thus making energy
for further activity available, and the water being necessary to give the
requisite dilution to the cell contents to permit of growth. High tem-
peratures, however, probably are of value only as they facilitate the
absorption of water and of oxygen.
The vigorous respiration of developing seedlings is in striking contrast to the
weak respiration of seeds, germination soon ceasing in closed chambers from lack
of oxygen. The favoring influence of high temperatures is well shown in the date,
whose seeds germinate in a few days in a hot greenhouse,, otherwise requiring weeks
or even months. Until recently it had been supposed that germination takes place
equally well in light and in darkness. Probably it is true that many seeds are indif-
ferent to the presence or absence of light, but a few seeds require light for germina-
tion (as in Viscum and in several species of Rhododendron) ; a number of seeds ger-
minate better in light than in darkness (as in Poa pratensis and Veronica peregrina).
On the other hand, there are some seeds whose germination is retarded by light
(as in Phacelia tanacetifolia'). In certain mycophytes and parasites, as previously
seen, there is a fourth condition necessary for germination, namely, contact with a
1 In a few cases, as in rice and in the water hyacinth, no oxygen fs required for ger-
mination.
934 ECOLOGY
suitable host. The exact factor here concerned is not known, though it may be
chemical in nature ; in certain orchids concentrated solutions may replace the usual
symbiotic fungus. Spores in general require germinative conditions similar to those
of seeds. Where the spores are green, as in mosses and ferns, light generally is re-
quired for germination, though in most cases germination may be induced in the
darkness by proper chemical stimulation. Many fungus spores, especially those of
parasites, germinate readily in water, but the spores of many saprophytes require
for germination the presence of a nutrient medium.
The germinative processes of seedlings. — Initiatory activities. —
In those seeds in which the outer layer becomes transformed into muci-
lage upon the absorption of water (as in flax and mustard), this is the
first germinative phase to be observed. Very soon the seed swells notice-
ably, owing to the large amount of water absorbed. The outermost
cells become active, as soon as their contents become sufficiently dilute;
diastase or other enzyms are secreted, and the digestion of the accu mu-
lated food begins. When the water and the transformed foods r^ach
the embryo and incite it to its second and final period of activity, ger-
mination proper may be said to have begun.
The digestion and the absorption of foods. — The digestive processes
are observed readily in the grains of cereals, as in wheat. The aleurone
layer (fig. 1211), which is rich in protein, first shows signs of life, the cells
becoming large and vacuolated and the protoplasm manifesting activity.
Soon the secretion of diastase begins, and the starch next to the aleurone
layer is the first to be digested. In wheat, maize, and other grasses there
is a specialized structure, the scutellum (structually the cotyledon),
which greatly facilitates the germinative processes, since it serves as a
path of transfer for the digested foods from the endosperm to the devel-
oping embryo; often on the side next to the endosperm there are hair-
like absorptive cells. In many monocotyls the tip of the cotyledon
remains in the seed in contact with the food and may be regarded as an
absorptive organ (fig. 1229). In the date the tip of the cotyledon
enlarges into a disk, presenting a large absorptive surface to the e ido-
sperm (fig. 1230). In those seeds in which the accumulated foode are
in the cotyledons, specialized absorptive structures are less likely to be
present.
The amount of food in seeds may vary from almost none (as in many parasites
and mycophytes) to such large quantities as are found in the coconut and the
avocado (Per sea gratissima). In cases like the latter, much or little of the food max
be utilized, depending upon the conditions to which the seedling is exposed
upon emergence from the testa. If the radicle has ready access to moisture and
GERMINATION
935
s'
the plumule to light, most of the food is unnecessary and
may gradually decompose in the ground; but if condi-
tions for autophytic nutrition are less favorable, much or
even all of the food may be used by the seedling.
Aspects of germination external to the seed. — The
earliest conspicuous external index of germination
is the rupture of the testa and the protrusion of
the embryo. The time necessary for such pro-
trusion, after the seeds have been exposed to
proper germinative conditions, varies from one or
two days (as in lettuce or mustard) to some weeks
or months (as in the date). Small seeds germinate
more quickly, as a rule, than do large seeds, prob-
ably because the foods are digested more quickly
through easy access to water and oxygen. Starchy
seeds commonly germinate more quickly than do
fatty seeds, and much more quickly than do
seeds with " reserve cellulose." The rupture of
the testa, which usually becomes much softened
and weakened by the absorbed water, may be
effected by the growing radicle or by the coty-
ledon, as in many monocotyls. Sometimes the
embryo emerges through thin spots, as in the
coconut, or pushes out a loosely fastened plug
of tissue. Usually the radicle is the first part of
the embryo to protrude, and this is doubtless ad-
vantageous, since most seeds contain enough food
for considerable growth, while all of the water
must come from without. Often (as in the
cocklebur) the radicle is so situated that it is the
first part of the embryo with which the entering
water comes in contact,1 and the absorption of
water from the soil by the young root system
usually is well initiated before the external de-
velopment of the plumule becomes prominent.
In some cases the cotyledons remain in the soil, especially where
these organs are the chief seat of accumulated food, as in oaks and
1 The significance of the position of the radicle in the seed of the cocklebur is seen from
the fact that if a bit of the testa IB removed near the tip of the cotyledons, growth begins
at that point, the radicle then being the last part to react.
Monocotyl seedlings :
1229, an onion seed-
ling (Allium Cepa), il-
lustrating epigaean
germination ; note the
curvature of the coty-
ledon (c) whose tip re-
mains within the seeu
(s), acting as an ab-
sorptive organ; 1230,
the seedling of a date
palm {Phoenix dactyl-
ifera) in longitudinal
section ; note the re-
markable cotyledon (c)
whose axis elongates
upon germination ; one
end of the cotyledon
(s') remains within the
seed, acting as an ab-
sorptive organ ; the
other end continues
for a time to enclose
the plumule (/>) and
the radicle (r). — From
KERNER.
936
ECOLOGY
peas, and also in the cereals, where the scutellum represents the coty-
ledon (figs. 703, 706) ; such germination is termed hypogaean. In other
cases the cotyledons emerge from the seed with the plumule and come
into the light, usually turning green, as in the beech (fig. 1231), pump-
kin, and mustard (fig. 700); such germination is termed epigaean.
In hypogaean species the cotyledons die, as soon as the foods are re-
moved, or the seedling is well established as an autophyte. The same
is true in some epigaean species, as in various beans, but in many other
epigaean forms the cotyledons enlarge and doubt-
less manufacture considerable food. Cotyledons
are much more uniform in shape than are ordi-
nary leaves, perhaps because of the relatively
uniform conditions in which they are developed.
Usually they are undivided, though occasio tally
divided, as in Tilia. In the monocotyls th< en-
closure of the delicate part of the plumule within
its older sheathing leaves prevents injury in
breaking through the soil. In many epigaean
dicotyls the cotyledons adhere at the tips, pro-
tecting the plumule until it emerges fron the
ground. In other cases (as in the pumpkin) the
hypocotyl elongates considerably, while the coty-
ledons remain within the seed, resulting in an
arching of the young stem and in the pulling
of the delicate tip from the seed and through the
ground instead of pushing. Where seeds germi-
nate too near the surface, the contraction or other
movement of the growing root exerts a pull on
the shoot, so that the position proper to the species
is eventually acquired.
The germinative processes of buds. — The structural features of buds. —
Buds commonly are divided into two classes, active and resting. Active
buds are associated with all seasons in uniform climates and with the
vegetative seasons of periodic climates, while resting buds are associated
with the unfavorable seasons of periodic climates. Germinative processes
are conspicuous as resting buds develop into active buds. The resting
buds of shrubs and trees, commonly called winter buds in temperate
and in cold climates, are protected by tough and impermeable bud
scales, whose structure and role have been considered elsewhere (p. 643).
FIG. 1 23 1 . — A seed-
ling of a dicotyl, the
beech (Fagus grandi-
folid), illustrating epi-
gaean germination ; c,
cotyledons; /, first foli-
age leaves.
GERMINATION
937
Within the bud scales are delicate embryonic leaves that are most eco-
nomically arranged as to utilization of space, a particular kind of
arrangement, known as -vernation, often being characteristic of special
plant groups.
Leaves in the bud may be plane ; conduplicate, or folded inward, as in the bean;
plicate, or folded in pleats, as in the beech; crumpled, as in the poppy; involute,
or with the halves rolled inward, as in the violet; revolute, or with the halves rolled
outward, as in the dock; convolute, or with the leaf rolled from one margin to the
other, as in the canna ; or circinate, that is, with the leaf rolled inward on itself from
the apex downwards, as in ferns (fig. 382). It is believed that the kind of verna-
tion is due in part at least to the limitations of space within the bud ; by experi-
mentally restricting this space, the leaves of Prunus which usually are flat become
crumpled, and by cutting the stipules, the leaves of Magnolia become flat.
External factors in relation to bud germination. — The germination of
winter buds is associated largely with spring, and, as with seeds, it takes
place when the temperature becomes sufficiently high to permit water
to enter the embryonic shoot in abundance and to incite it to activity.
Buds differ from seeds in being em-
bryonic shoots rather than embryonic
plants, and usually also in remaining
attached to the plants that bear them
(except in the winter buds of w,ater
plants), the water used in germina-
tion thus coming from the plant rather
than from the ground. As shown
elsewhere, buds often appear to be
mature before they are capable of ger-
mination, the maturation process seem-
ing to consist in part in the accumula-
tion of food. Germination may be
hastened by placing a plant or even
a branch indoors in winter. Only local
stimuli are needed for germination, as
is shown by growing a single branch
detached from a plant and placed in
water indoors, or by training a branch
from a tree into an adjoining house,
or even by supplying favorable temperatures locally to a part of a
plant outside, as by bending a willow branch into a sheltered position;
1232
1233
FIGS. 1232, 1233. — Eulblings of
Cicuta bulbifera; the first bladeless
leaves or phyllodes (/>) are followed
by foliage leaves (/) ; b, bulbil.
ECOLOGY
because of locally favorable
temperatures, buds germinate
soonest on sunny slopes, and
willows on the tundra come
into flower while their roots
are still in frozen soil. The
resting buds of herbaceous
plants, represented by bulbs
and bulbils (the latter de-
veloping into bulblings, figs.
1232, 1233), and by the buds
of tubers (fig. 1037), of rhi-
zomes, and of multicipital
herbs, germinate under con-
ditions similar to those that
incite activity in the resting
buds of trees and shrubs.
Climate and bud developm ?«/. —
As a rule, trees and shrubs with
large buds (as the poplars, wallows,
and alders, fig. 1234) develop vig-
orous shoots early in spring, while
FIG. 1234. — A branch of the European alder species with small buds (as the ca-
(AlHus glutinosa) in its winter aspect ; 5, the buds ta,pa an(J the honey Iocust) devdop
of staminate inflorescences ; p, the buds of pistil-
late inflorescences ; /, fruit cones of the preceding
year; /, leaf buds.
much later. Such differences seem
to follow from the fact that in the
former the embryonic shoots are
much the more advanced while still within the resting bud. Many of the large-
budded forms which thus develop at the inception of spring are northern species,
and such habits seem very advantageous for far northern plants, owing to the short-
ness of the vegetative season. On the other hand, where the vegetative season is
long, small buds seem advantageous, owing to their exemption from the develop-
ment of extensive protective structures. Some large-budded trees, such as the
hickory, are both late in germinating and relatively southern in distribution.
CHAPTER VII — PLANT ASSOCIATIONS
Definition. — In the preceding chapters, plants have been considered
as individuals having certain relations to their physical surroundings,
or to each other. While plants sometimes occur as isolated individuals,
they are associated far more commonly in more or less definite groups.
So true is this that when one who is familiar with nature sees a given
species in the field, he comes almost instinctively to look for other species
that he has seen associated with it. If he is observant, he looks a
step further and finds that these associated species also are associated
with a definite kind of habitat. For example, pitcher plants, sundews,
cranberries, and peat moss grow together in imperfectly drained swamps
cnownLas bogs or~~moqrs ; bffrach pelts, sea rocket, and beach grass
grow together on sandy coasts; beech, maple, beech fern, and beech-
drops grow together in mesophytic forests. A group of plants in its
entirety occurring in a common habitat is known as a plant association.
Sometimes the association of specific plants is obligate, as in the case
of the beechdrops, which grows parasitically on the beech, while the
beech in turn appears to require certain fungi in the soil. More
commonly, however, the association is purely facultative. Pitcher
plants and sundews or maples and beeches grow together, because
they thrive in similar conditions; so far as is known, the presence
or the absence of one is a matter of no particular consequence for
the other, except as it occupies or leaves a certain amount of space.
The kinds of associations. — Plant associations have been variously
classified, the simplest grouping being based on the water relation, and
the large divisions being termed hydrophytic^ mesophytic, andjwrpjrfiylic,
while these in turn are subdivided into various groups of association's.
This classification, though advantageous because of its ready applica-
tion, has the great disadvantage of grouping together associations that are
entirely unrelated in origin, such as those of bogs and ordinary swamps,
while separating closely related associations, such as those of bogs and
of the coniferous forests into which they commonly develop. Though
it is more difficult to apply, there are many advantages in a genetic classi-
939
940 ECOLOGY
firation, that is, one which groups associations in a series in their order
of development.
Succession. — The basis of a genetic classification is the principle of
succession, namely, that in the physiographic development of a region the
various habitats pass through a series of more or less definite stages,
owing chiefly to the processes of erosion and deposition, supplemented
by the accumulation of humus. The primitive associations, that is,
those of new lands or waters, are likely to be either xerophytic or hydrp-
phytic. In a region with a mesophytic climate, the primitive associa-
tions become displaced by others that are slightly more mesophytic,
and they in turn by others, until the series finally culminates in the
most mesophytic association of which the region as a whole is capable.
For example, in the eastern United States an upland of rock, sane!, or
clay, whose original flora is xerophytic, becomes gradually more and
more mesophytic, either through land denudation or humus accumula-
tion or both, until it becomes clothed with the ultimate plant association
of the region, namely, a deciduous mesophytic forest. A pond in the
same region gradually becomes filled through humus accumulatio i or
through stream and shore deposition, or both, so that the original aquatic
vegetation becomes displaced by a swamp vegetation, and this in turn
through further humus accumulation becomes displaced by a forest
quite comparable to that which marks the final stage in an upland suc-
cession. In arid or semi-arid climates it is obvious that the final stage
could not be mesophytic, but would necessarily be an association which
is much nearer the primitive xerophytic association of the region.
The scope of this book forbids any attempt at a detailed classification
of plant associations. The general principles enunciated above must
suffice. In the remaining paragraphs of this chapter there will be pre-
sented some of the more striking features of a few of the more irrpor-
tant plant associations, especially of those that are found in the United
States, but no attempt will be made to bring »ut genetic relation* hips
or to make exhaustive analyses.
Pond associations. — Perhaps the most representative fresh-vater
associations are those of ponds, and these are among the most interest-
ing of all associations, partly because they are more likely to remain
natural than are most habitats in densely populated districts, but espe-
cially because they show obvious and rapid stages in succession between
the primitive aquatic associations and the various sorts of swamps. The
vegetation of ponds consists visually of free-floating forms (including
PLANT ASSOCIATIONS 941"
many algae and some higher plants) and of forms attached to the bot-
tom ; of the latter some forms are submersed, some have floating leaves,
and still others are in part emersed. Aquatic plants or hydrophytes,
especially those that are submersed, have many noteworthy structural
peculiarities that have been separately noted on previous pages, but
which may here be summarized.
The root systems commonly are reduced, both in length and in amount
of branching, and root hairs are absent, at least in the water. True
water roots are hairless, and may possess root pockets. The chloren-
chyma is spongy and but slightly differentiated, and usually the plastids
are large and motile. The leaves of submersed plants are very thin and
often are finely dissected. Air chambers are capacious, often exceeding
the tissues in actual volume. Stomata are absent in submersed leaves
and are present only on the upper surfaces of floating leaves; where
present in floating or in emersed leaves, they have but slightly cutinized
walls and are almost always open. Protective features are few or want-
ing; for example, cutin and cork rarely are developed below the water
surface, hairs are scarce, and the cell sap has a low osmotic pressure;
the absence of protective structures is not disadvantageous, since ab-
sorption is easy, and below the water level, transpiration is slight or even
absent. Leaves equal or surpass roots in importance as absorptive
organs. Submersed organs usually are slime-covered, the slime har-
boring commensalistic communities of bacteria and other low organ-
isms. The aerial surfaces of floating organs usually are wax-coated and
thus are not readily wetted. Conductive and mechanical tissues are
greatly reduced. Vegetative reproduction is highly developed, both
through the fragmentation of ordinary shoots, and through the develop-
ment of winter buds. In the algae, reproduction and dispersal are
facilitated by zoospores and by motile gametes. Among the higher
plants, flowers and seeds are less abundant than in most habitats.
Swamps. — Various swamp stages in turn follow the primitive pond
associations, bulrushes, cat-tails, and reeds often being among the first
emersed plants, and sedges are often prominent later. In mesophytic
climates, thickets (as of willows and alders) soon appear, and they in
turn are replaced by forest vegetation. The structural features of swamp
plants are in part like those noted above, especially in the matter of re-
duced root systems and prominent air chambers, but in general they are
not unlike those seen in mesophytes; particularly is this true of leaf
thickness, stomata, chlorenchyma, and protective structures. The
942 ECOLOGY
roots frequently are horizontal or even ascending rather than descend-
ing. Rhizomes are greatly developed, accounting in large part for the
rapid invasion of ponds by swamp plants. A somewhat remarkable
feature is the abundant development of vertical chlorophyll-bearing
organs, whether leaves (as in the flags) or stems (as in the rushes).
Among the most plastic of plants as to leaf form and structure are the
amphibious plants; in view of the rapid transformation of ponds into
swamps, such plasticity permits certain species to dominate in two dis-
tinct successional stages.
Bogs or moors. — The mature vegetation of a peat bog contrasts most
strikingly with that of an ordinary swamp, although the early stages may
be quite the same in both cases. While a number of plants are common
to swamps and bogs, there are many kinds of plants which are more or less
peculiar to bogs, the most noteworthy being those with such xerophytic
features as prominent palisade tissues and cutin, dwarfness of habit,
and high osmotic pressure. Among the bog xerophytes are many eric ads
(such as the cranberry, leather leaf, and Labrador tea) and conifers;
that some of the bog plants are true xerophytes is shown by the fact 1 hat
a number of species are common to bogs and to dry rocky cliffs. The
peat moss (Sphagnum) is especially characteristic of bogs, as are many
orchids, and it is in bogs that most carnivorous plants are found. As
compared with mesophytic habitats or with ordinary swamps, bogs
present conditions that are deleterious for the majority of plants ; indeed,
some of the plants which are characteristic of bogs (notably the tamarack)
thrive much better elsewhere, suggesting that they " tolerate " bogs
rather than " select " them. f An analysis of the bog problem is beyond
the scope of this book, but some points that bear on the matter have been
suggested elsewhere (p. 537).
Maritime associations. — Plants that grow in salt water or in salty soil
have been denominated halophytes. The submersed halophytes are
chiefly algae, which sometimes reach gigantic size, and which differ in
color, being green, red, or brown. Most of the larger algae are attached
to rocks by anchoring organs, namely, the holdfasts or rhizoids; some
rise and fall with the tide, bladders filled with air often facilitating t icir
buoyancy. Salt marshes show stages in succession comparable to those
of ponds, but the species involved are very different. Emersed halophytes
are strikingly xerophytic in their characteristic features, palisade tissue
being prominently developed, and often the epidermis is highly cutinized.
The most striking feature of salt marsh halophytes, taken as a class,
PLANT ASSOCIATIONS 943
is their succulence, which is accompanied by a very high osmotic pres-
sure. In temperate regions the most representative salt marsh plants
are herbaceous, but in the tropics extensive mangrove forests are found
in similar conditions ; few plants show more marked xerophytic features
than do the mangroves, which have evergreen leaves with water tissue,
prominent palisade cells, and thick cutin. Often there is a network of
prop roots above the water line, and in some cases there are ascending
"knees" (fig. 726).
Xerophytic associations. — The characteristic features of xerophytes. -. —
In most respects xerophytes are the reverse of hydrophytes in their
structural features. The roots frequently are strongly developed (though
not in cacti), possessing either considerable length or great size; roots of
the latter class accumulate large amounts of water and fr>r>r* In some
extreme xerophytes the root hairs extend to the root tips, and in certain
cases they possess rigid thickened walls. Palisade tissue is jatrongly
developed, and the chlorenchyma in the leaves and stems commonly is
deeply sunken, giving them a pale tint as viewed from without; usually
the plastids are small and relatively immotile.
Protective features are remarkably developed both in amount and in
kind, and their advantage is undoubted, owing to the great exposure of
to— trjtnspjration. The transpiring surface usually is
tively reduced, the Wws-hping flrpall and thick. Many species are^
leaHess, tnecylindrical stems exposing a relatively small surface to trans-
piration, while their vertical orientation affords some protection from
the intense rays of light at midday ; species with vertical leaves are sim-
ilarly protected. In many cases there is a temporary reduction of sur-
face, as in the involute leaves of grasses, as in those legumes whose leaves
close in dry weather, and as in the " resurrection plants." -Temporary
reduction of surface is exhibited also by plants which shed their leaves
or stems during dry periods ; annuals, which die at the beginning of dry
periods, represent the culminating form of such behavior. Dwarf-
ness of habit is a prominent xerophytic feature, the resulting compactness
in arrangement of branches and leaves and the closeness to the ground
affording considerable protection.
The more minute structural features of xerophytes are no less signifi-
cant than are the more obvious characters. Commonly the egidenms
is thick aj^hjghly_c_utinized^except in succulent xerophytes), and often
fTis~superficially coated with wax, resin, or varnish. In woody stems
there is a prominent bark development, thVcbrE in particular being of
944 ECOLOGY
high significance in checking transpiration. The leaf and stem sur-
faces frequently are covered with hairs; spinescence also is common,
though its protective signiEcance~may not be important. The stomata
occur mainly on the more protected (chiefly the under^-surfaces, and
often are at the basesoTpTts~and specially protected by hairy coats or by
cutinized walls; as a rule, they are not wide open. Many xerophytes are
succulent, containing large amounts of colorless sap or of latex ^oils_and
resins often are abundantly developed. The osmotic pressure of the_cett
sap often is.. very high, especially in shrubs and in plants of alkaline soil.
The conductive tracts are prominent, the vessels being larger and longer
and the walls thicker than in most plants; lignification is prominent,
and annual rings are well developed. BastJJK&rsrand other mechanical
elements reach their highest development i_n_xerophytes.
Some xerophytes, particularly the lichens, appear wanting in prominent
xerophytic structures, seeming able to withstand prolonged desiccation
without injury. Apart from the lichens and mosses, absorption through
aerial organs is relatively rare in xerophytes, though some of the*e >i-
phytic leaf-absorbing bromelias grow in dry climates. Tubers, corns,
and bulbs especially characterize arid climates, and it is obvious that
their ability to develop rapidly at the inception of a rainy season is. a
character of great advantage. Xerophytic conditions usually are be-
lieved to favor the formation of flowers and fruits. ^
Characteristic xerophytic associations. — Perhaps the most repre-
sentative xerophytic_region is the desert, .and it is here that the features
above mentioned reach their most pronounced development. In general
the severity of the desert conditions increases as the rainfall decreases,
it being common to distinguish half deserts, such as the sagebrush plains,
from the more extreme deserts, such as those in which such plants as
the cacti, or the creosote bush, are dominant forms. Still more extreme
are the alkali deserts, in which excessive climatic aridity is supplemented
by a soil in which concentrated salts make absorption difficult. Suc-
culents with sap of high osmotic pressure seem best fitted for existence
under such conditions. There are habitats where the alkalinity is so
great that plant life is almost if not quite excluded.
Many, but not all, alpjne and arctie-bftk&ats have plants whose struc-
tures are chiefly xerophytic. Even though there is an abundant supply
of water, the soil often is so cold that absorption is difficult; conse-
quently plants without xerophytic structures are poorly fitted for such
habitats, except in alpine meadows or in similar situations, where the
PLANT ASSOCIATIONS 945
protective mantle of snow prevents the loss of water by transpiration in
the seasons during which absorption is impossible. The leaves of alpine
and arctic xerophytes are largely sclerophyllous, hairy leaves being rather
less abundant than in deserts. The perennial habit is almost universal,
the shortness of the season scarcely permitting annuals to complete their
life cycle. Anthocyans are prominently developed, resulting in showy
leaves and flowers. Palisade tissue is more pronounced in alpine than
in arctic xerophytes.
In climates where the rainfall and the temperature are such as to facili-
tate their development, there are many habitats where the local condi-
tions prevent for a time the appearance of mesophytes. Among such
habitats rocj^jLand sandvjtreas are of first importance. In rocky regions
the pioneer plants often are lichens, which are able to grow on the bare
rock surface ; with these come many crevice plants. By the accumula-
tion of humus, the growth of other plants is made possible, and after a
time a thicket develops, and later a forest, in which pines and junipers
may play an important part. In sandy regions the instability of the
soil usually inhibits the development of a pioneer lichen stage, whereas
xerophytic herbs and shrubs make their appearance sooner than on
rocks. The subsequent stages on rock and sand are much the same.
Mesophytic associations. — Mesophytes in their structural Charac-
teristics are in many respects intermediate between hydrophytes and
xerophytes. There is a prominent developmenTof progeotropic roots
with abundant root hairs. The foliage reaches a maximum -A*v«1np-
ment, and the leaveTare relatively^large and thin^ while the thin trans-
paren^epidermis and the abundant chlorophyll together cause the leaves
to appear dark green. Stomata usually occur on both leaf surfaces, ex-
cept in trees, and the guard cells possess a rnaxinuim capacity-Jox move-
njgnt. Cutimzation is moderate, except in such evergreens as the hem-
lock and the India-rubber tree. The unaerjgpidermijM^mmonlyJjas
wavy lateral walls, contracting thus with th* straight walls in hydrophytes
Tlje most representative mesophytic associations are, on the one hand,
vanous_forests, and, on the other, certain_grasslands.^ The mesophytic
forests as a class are the culminating vegetation of any region where they
grow, since they represent not only the most luxuriant kind of plant
association, but also because they form the terminal member of the suc-
cessional series in such climates as are humid enough to support them over
extensive areas. It is in the mesophytic forests that humus accumula-
946 ECOLOGY
tion above the water level reaches its maximum, and on this account the
soil is more uniformly moist than in other land habitats. Furthermore,
the rich supply of humus makes possible a wealth of saprophytic fungi
and bacteria, leading to mycosymbiosis and to other symbiotic relations.
The most luxuriant of mesophytic forests is the rain forest of the
tropics, which is characterized by the dense crowding of individual
plants, resulting in the maximum occupation of space. Not only are
there ordinary trees, shrubs, and herbs, but lianas often are abundanj,
while epiphytes cover the limbs and even develop on the leaves of many
trees. The trees often are slender and smooth-barked, and the leaves
are characteristically evergreen. The_epiphytes include ferns and
orchids^ the latter with characteristic abscgpjive roots and xerophytic
Jeaves or stems ; the leaves of many of the trees appear xerophytic also.
In north temperate regions there are extensive mesophytic forests that
either are deciduous, as in the eastern United States, Europe, and Japan,
or evergreen, as in the northwestern United States. In these forests the
tree species are relatively few in number, as compared with the trop cal
forests, and the trees often are large and rough-barked. The epiphytic
vegetation consists chiefly of lichens, liverworts, and mosses. Fre-
quently mesophytic areas are treeless, as in some prairies and in alpine
meadows, where grasses and herbage dominate the landscape.
The influence of man upon vegetation. — Man is the most destructive
of animals. He has cleared vast tracts of forest for lumber, and for the
building of cities and the development of farms, and has destroyed other
tracts through forest fires. Man also is responsible for distributing
through the world most of the "weeds " which burden the farmer and
throng the roadsides. Such plants as the Russian thistle, cocklebur,
burdock, and Canada thistle once were somewhat restricted in area,
and they owe their present widespread distribution directly or in-
directly to man. Plants of this sort that inhabit fields and wsste
places are known as ruderals. Often there are ruderal associations, such
as those that develop on cultivated land that is left fallow. The pioneer
associations that follow in man's destructive train, such as the rud implying
that a plant responds only in an advantageous direction and in advan-
tageous amount, endows the plant with a power of choice, and almost
imagines it to survey the situation and to determine upon a course of
action. It implies the possession of an inherent power to contravene the
ordinary laws of nature. It presupposes a vital mechanism that holds
adaptations in readiness for conditions that have not as yet occurred.
And yet man himself possesses no such power of adaptation ; he cannot
" by taking thought add one cubit to his stature," though he can (as a
plant cannot) study the laws of nature and place himself in such con-
ditions as to facilitate desired reactions.
The theory of fortuitous variation. — The preceding considerations
appear to show that protoplasm is not inherently adaptive. Disad-
vantageous structures (such as the food layers of galls or the enlarged
conductive tracts of parasitized plants) or indifferent structures (such
as cork wings and many hairs and spines) are quite as normal ex ses-
sions of protoplasmic behavior as are the more numerous advantageous
structures. The theory of fortuitous variation, which is based upon the
laws of chance, postulates that newly developing structures are cf all
kinds : some advantageous, some disadvantageous, and some indifferent.
The supporters of this theory are aligned in two general schools; the
one school holds that new structures arise chiefly through the influence
of external factors, while the other holds that factors residing within the
plant itself are more important. Many investigators maintain that
external factors are more important in some instances and intp>rnn1 fac;
tors in others; this composite view, embracing opinions of the two
opposing schools, seems best able to explain the facts as they are now
understood.
In the first place, variations are of frequent occurrence, though :heir
supposed rarity once was given as an argument against the theoiy of
evolution. Scarcely any species or any structure that has been studied
carefully has been found to be invariable, and in some cases the amount
of divergence from a supposed type is enormous. Not only do the indi-
viduals of a species as found in nature often differ from each other in
many particulars, but the same is true of individuals whose ancestry
is known to be identical. An excellent illustration of such variation is
seen in water cultures of various seedlings; while in such cultures maize
ADAPTATION 951
-oots commonly are hairless and wheat roots hairclad, certain maize
roots may be hairclad or wheat roots hairless, though the conditions
appear to be the same.
Congenital and reaction structures. — Structures (such as cork or
cutin) which arise through reaction to environmental changes may be
called reaction structures.1 If a plant, when placed in xerophytic con-
ditions, happens to develop as a reaction thereto such xerophytic features
as cutinization, succulence, or dwarfness, it may be called a reaction
xerophyte; similarly, there may develop reaction mesophytes or reaction
hydrophytes. It is obvious, however, that many reaction structures
cannot be classed as hydrophytic, mesophytic, or xerophytic ; especially
is this true of those that do not happen to be advantageous. Contrasting
with reaction structures are those structures that are born with the
species in whatever habitat it is developed (as in the case of mutations),
and which are not lost if the species is grown in other habitats. Such
structures may be termed congenital structure^/ If a species happens to
be born with such xerophytic features as succulence or dwarfness, it may
be called a congenital xerophyte; similarly, there may develop congenital
hydrophytes or congenital mesophytes; or plants that cannot be thus
classed.2 Thus any given structure, as a cutinized epidermal wall or
a succulent cortex, may be a reaction structure in one species and a con-
genital structure in another species. Furthermore, any species may at
the same time possess both variable reaction structures and fixed con-
genital structures, otherwise called adaptation and organization charac-
ters, careful experiment alone determining which is which. According
to this theory, each plant association is composed of certain species that
are fit because their critical features are the product of the habitat, and
of other species that happen to have been born fit and thus enabled to
survive.
Even congenital structures and organs are influenced by external
factors. It has been seen that most stomata, hairs, and spines, if present
at all, have a definite and fixed structure, and thus may be regarded as
congenital rather than reactive. However, their presence or absence is
determined by external agents, whence the latter are called determinative
1 Too much emphasis cannot be placed upon the fundamental distinction between the
word reaction and such words as adaptation, adjustment, accommodation, or regulation.
The latter words imply an inherent power to change advantageously, while the word
reaction implies no such power.
2 Reaction and congenital xerophytes, mesophytes, and hydrophytes also may be termed,
especlively, facultative and obligate xerophytes, mesophytes, and hydrophytes.
952 ECOLOGY
Sometimes reference is made instead to releasing factors, the
structure in question being regarded as potentially present, though its
manifestation is inhibited until the proper factor enters in place of or
in addition to the inhibiting factor. In the case of reaction structures
it is believed that the form assumed, as well as the time and the place of
appearance, is due to external agents, wherefore the latter are called
formative factors.
While there is no doubt of the reality of reaction structures, because
they are so readily capable of experimental production, there are many
investigators who disbelieve in the reality of congenital structures. An
alternative hypothesis is that all plant structures are or have been plastic.
The so-called rigid or congenital structures may in some ancestral form
have been as plastic as are the reaction structures of to-day; in this event
the fixation of reaction structures has resulted in structures that now are
congenital. It is equally possible that the supposedly rigid congenital
structures really are plastic, but to an imperceptible degree, as comp ared
with the leaf plasticity of amphibious plants; if this is true, all >lant
structures are plastic, some obviously and rapidly, and others so slightly
or slowly that only experiments of long duration can reveal plasticity.
While in the present state of imperfect knowledge, it is convenient and
not necessarily incorrect to contrast reaction structures and congenital
structures, it is possibly more correct to subdivide plant structures into
those that certainly are plastic and those that apparently are rigid.
The survival of advantageous structures through natural selection. —
While the origin of structures through reaction or through mutation can-
not account for the present preponderance of advantageous structures
and advantageous behavior, the theory of natural selection is in this re-
spect as satisfactory as it has proven unsatisfactory from the standpoint
of causation.1 Of the species with new congenital structures, only
those are likely to survive that happen to be suited for existence in the
habitat in which the new structures develop, or that are able to migrate
to a suitable habitat; a mesophyte that happens to originate in a desert
or a plant suited to warm climates that happens to originate in a cold
region cannot survive. Of the more plastic species only those are likely
1 Against natural selection as a causative theory there may be urged: (i) the existence
fn many species of a capacity for advantageous regeneration, though the opportunity for
such regeneration rarely if ever occurs in nature; (2) the existence of complicated struc-
tures (such as stinging hairs, digestive glands, and extra-floral nectaries) whose value to the
plants possessing them is slight; and (3) the existence of "overadaptation," as in the
Sowers of orchids and in the seeds of certain. xeropuytes.
ADAPTATION 953
to survive which are able to react advantageously. Thus in the course of
time it is to be expected that the plants that are congenitally unfit and
the plants that react disadvantageously will largely be eliminated. Only
occasionally would there survive plants with structures that are slightly
disadvantageous. Somewhat more abundant might be the number
of plants with indifferent or with only slightly useful structures. Many
more individuals are born than have a chance to live, since severe physi-
cal conditions and crowding by other plants cause the elimination of the
unfit and the survival of the fit; consequently advantageous structures
must ultimately dominate, whatever the nature of the primitive struc-
tures. Such an explanation of the predominance of advantageous struc-
tures seems far more tenable than does the theory of origin through
adaptation.
APPENDIX
SUPPLEMENTARY LITERATURE
THE following references are inserted in order to enable any who so desire 'to
obtain more detailed information concerning the subjects treated in Part III. A
complete bibliography is quite out of place, but it is hoped that the most useful
references may be found in the appended list. In all cases the references chosen
are those which generally will be found relatively accessible in libraries containing
the leading botanical and biological journals. No attempt is made to giv2 the
older references, chief attention being paid to those of recent date, in which allu-
sion to older treatises may generally be found.
Following each topic is the page of Part III to which reference is made, as (491).
The authors to whom no reference is made other than by name are those men-
tioned in the General List, as HABERLANDT and GOEBEL I. The arrange ment
of authors under each topic is alphabetical, and the abbreviations of the various
journals will be intelligible to any librarian or to any one acquainted with boU nical
and biological journals.
GENERAL
DE BARY, Comparative Anatomy, Oxford, 1884; GOEBEL I, Organography
of Plants, Part I, Oxford, 1900; GOEBEL II, Experimentelle Morphologic der
Pflanzen, Leipzig, 1908; HABERLANDT, Physiologische Pflanzenanatomie, 4th
edition, Leipzig, 1909; JOST, Lectures on Plant Physiology, Oxford, 1907; KER-
NER, Natural History of Plants, New York, 1895; SCHIMPER, Plant Geography,
Oxford, 1903; WARMING, Oecology of Plants, Oxford, 1909.
CHAPTER I
ROOTS AND RHIZOIDS
Root hairs (491): HABERLANDT; SNOW, Bot. Gaz. 40, 12-48, 1905.
Absorption (493): FITTING, Zeit. Bot. 3, 209-275, 1911; HILL, New Ph/t. 7,
133-142, 1908; JOST.
Root excretions (493): LIVINGSTON, etc., Bulls. 28, 36, U. S. Bur. Soils, 1905,
1907; REED, Pop. Sci. Mon. 73, 257-266, 1908; SCHREINER and REED,
Bull. 40, U. S. Bur. Soils, 1907; Bull. Torr. Bot. Club 34, 279-303, ^07;
Bot. Gaz. 45, 73-102, 1908; STOKLASA and ERNST, Jahrb. Wiss. Bot.
46, 55-102, 1908.
Root structure (496): FREIDENFELT, Flora 91, 115-208, 1902; TSCHIRCH,
Flora 94, 68-78, 1905; VON ALTEN, Bot. Zeit. 67, 175-199, 1909.
Direction of growth (499) : JOST.
954
APPENDIX 955
Root contraction (504): RIMBACH, Bot. Gaz. 30, 171—188; 33, 401-420; 1900,
1902.
Water and root form (505): CANNON, Carnegie Inst. Publ. 113, 59-66, 1909;
FITTING, Zeit. Bot. 3, 209-275, 1911; SPALDING, Bot. Gaz. 38, 122-138,
1904.
Absorptive air roots (511): LEAVITT, Rhodora 2, 29 ff., 1900; NABOKICH,
Bot. Cent. 80, 331 ff., 1899.
Anchoring air roots (513): WENT, Ann. Jard. Bot. Buit. 12, 1—72, 1893.
Prop roots (514): BESSEY, Mo. Bot. Gard. Rep. 1908, 25-33.
Rhizoids (516): BACHMANN, Jahrb. Wiss. Bot. 44, 1-40, 1907; BENECKE,
Bot. Zeit. 61, 19-46, 1903; HABERLANDT; PAUL, Bot. Jahrb. 32, 231-
274, 1903; SCHOENE, Flora 96, 276-321.
CHAPTER H
LEAVES
Chloroplasts and chlorophyll (521): GRIFFON, Ann. Sci. Nat. Bot. VIII, 10,
1-123, 1899; HABERLANDT; JOST; SCHIMPER, Jahrb. Wiss. Bot.
16, 1-247, r885.
Albescence (523): BAUR, Ber. Deutsch. Bot. Ges. 22, 453-460, 1904; Biol. Cent.
30, 497-514, 1910.
Chloroplast movements (524): HABERLANDT; STAHL, Bot. Zeit. 38, 297 ff.,
1880; SENN, Leipzig, 1908.
Synthesis of carbohydrates (525): BLACKMAN, New Phyt. 3, 33-38, 1904;
GIBSON, Ann. Bot. 22, 117-120, 1908; JOST; McPHERSON, Science 33,
131-142, 1911; USHER and PRIESTLY, Proc. Roy. Soc. 77, 360-376; 78,
318-327, 1905, 1906.
External factors and carb hydrate synthesis (526): BEIJERINCK and VAN
DELDEN, Cent. Bakt. 10, 33-47, 1903; BLACKMAN, New Phyt. 3, 237-
242, 1904; BROWN and ESCOMBE, Proc. Roy. Soc. 76, 20-112, 1905;
JOST; STAHL, Jena, 1909.
Anthocyan (528): HABERLANDT; KERNER; OVERTON, Jahrb. Wiss.
Bot. 33, 171-231, 1809; WHELDALE, Proc. Roy. Soc. ,81, 44-60, 1909;
Prog. Rei. Bot. 3, 457~473> JQio.
Chlorenchvma structure (530): HABERLANDT; HEINRICHER, Jahrb. Wiss.
Bot. 15, 503-567, 1884.
Chlorenchyma plasticity (5 3): ARESCHOUG, Flora 96, 329-^36, 1906; CLEM-
ENTS, Trans. Amer. Mic. Soc. 1905, 19-102; HABERLANDT; HABER-
LANDT, Sitz. Wien Akad. in, 69-91, 1902; PICK, Bot. Cent, n, 400 ff.,
1882; STAHL, Bot. Zeit. 38, 868-874, 1880.
Leaves and light (539) : BERGEN, Bot. Gaz. 48, 450-461 ; BLACKMAN, New
Phyt. 6, 270-2-9, 1907; KERNER; WIESNSR, liol. Cent. 23, 2091!., 1903.
Phyllotaxy (549): KERNER; WINKLER, Jahrb. Wiss. Bot. 36, 1-79; 38, 501-
544, 1901, 1903.
Air chambers (551.): HABERLANDT; SCHENCK, Jahrb. Wiss. Bot. 20, 526-
574, 1889.
956 APPENDIX
Structure of stomata (555): HABERLANDT.
Movements „£ stomata (562): COPELAND, Ann. Bot. 16, 327-364, 1902; DAR-
WIN, Phil Trans. 190, 531-621, 1898; HABERLANDT,
Role of stomata (563): BLACKMAN, Phil. Trans. i85, 503-562, 1895; BROWN
and ESCOMBE, Phil. Trans. 193, 223-292, 1900; HABERLANDT;
LLOYD, Carnegie Inst. JPubl. 82, 1908; STAHL, Bot. Zeit. 52, 117-146,
1894.
Transpiration (565): BURGERSTEIN, Jena, 1904; RENNER, Flora no, 451-
547. JQio-
Epidermis (567): HABERLANDT; KERNER.
Hairs (572): BAUMERT, Beitr. Biol. 9,83-162, 1907; HABERLANDT; KER-
NER; RENNER, Flora 99, 127-155, 1908.
Leaf movements (579): JOST; PFEFFER, Leipzig, 1907; TSCHIRCH, Jahrb.
Wiss. Bot. 13, 544-568, 1882.
Leaf fall (582): LEE, Ann. Bot. 25, 51-106, 1911; WIESNER, Ber. Deutsch.
Bot. Ges. 22, 64 ff., 1904; 23, 49 ff., 1905.
Protective features in the cell sap (587): BARTETZKO, Jahrl>. Wiss. Bot. 47,
57-98, 1909; BLACKMAN, New Phyt. 8, 354-363, 1909; LIDFCRSS,
Lund, 1907.
Variations in leaf form (589): GOEBEL I; GOEBEL II.
Variations in algae (591): BRUNNTHALER, Sitz. Wicn 118, 501-573, 1909;
LIVINGSTON, Bot. Gaz. 30, 289-317, 1900; 32, 292-302, 1901; Bull.
Torr. Bot. Club 32, 1-34, 1905.
Leaf variations in amphibious plants (593): BURNS, Ann. Bot. 18, 579-587,
1904: McCALLUM, Bot. Gaz. 34, 93-108, 1902; SHULL, Carnegie Publ.
36> ^OS-
Recapitulation (596): DIELS, Berli , 1906; DUFOUR, Rev. Gen. Bot. 22, 369-
384, 1910; GRIGGS, Amer. Nat. 43, 5-30, 1909.
Leaf variations in land plants (598) : BONNIER, Ann. Sci. Nat. Bot. VII, 20,
217-360, 1895; GOEBEL, Flora 82, 1-13, 1896.
Asymmetry and anisophylly (607): FIGDOR, Leipzig, 1909; CENTNER,
Flora 99, 289-300, 1909.
Absorption in water plants (609): POND, U.S. Fish Commission Report, 1905;
SNELL, Flora 98, 213-249, 1907.
Absorption in lichens and mosses (610) : HABERLANDT; KERNER; LEAVITT,
Rhodora 2, 65-68, 1900; MULLER, Jahrb. Wiss. Bot. 46, 587-508, icog.
Leaf absorption in land plants (613): HALKET, New Phyt. 10, 121-139, 1911;
HABERLANDT; KERNER; SPALDING, Bot. Giz. 41, 262-282, i n>.
Leaf absorption in epiphytes (615): ASO, Flora 100, 447-^49, 1910; HAI'.ER-
LANDT; MEZ, Jahrb. Wiss. Bot. 40, 157-229, 1904; SCHIMPER, Bot.
Cent. 17, 192 ff., 1884.
Carnivorous plants (616): DARWIN, Insectivorous plants, New York, 1895;
HABERLANDT; KERNER.
Water exudation (620): HABERLANDT; JOST; LEPESCHKIN, Flora 90,
42-60, 1902; SPANJER, Bot. Zeit. 56, 35-81, 1898.
Secretion and excretion (622): DETTO, Flora 92, 147-199, 1903; HABER-
LANDT; JOST.
APPENDIX 957
Water accumulation in leaves (627): FITTING, Zeit. Bot. 3, 209-275, 1911;
HABERLANDT; MACDOUGAL and SPALDING, Carnegie Publ. 141,
1910.
[ntumescences (633): COPELAND, Bot. Gaz. 33, 300-308, 1902; DOUGLAS,
Bot. Gaz. 43, 233-250, 1907; KUSTER, Ber. Deutsch. Bot. Ges. 21, 452-
458, 1903; Flora 96, 527-537, 1906; VON.SCHRENK, Mo. Bot. Card.
Rep. 16, 125-148, 1905.
Reproduction by leaves (636) : GOEBEL I; GOEBELII; GOEBEL, Biol. Cent.
22, 385 ff., 1902; STINGL, Flora 99, 178-192, 1909.
Scale leaves (642): GOEBEL II; LUBBOCK (Avebury), Buds and Stipules,
London, 1899; MACDOUGAL, Bull. Torr. Bot. Club 30, 503-512, 1903;
MOORE, Bull. Torr. Bot. Club 36, 117-145, 1909; THOMAS, Rev. Gen.
Bot. 12, 394 ff., 1900; WIEGAND, Bot. Gaz. 41, 373-424, 1906.
STEMS
Stem branching (646): KERNER; BARANETZKY, Flora 89, 138-239, 1901.
Lianas (651); FITTING, Jahrb. Wiss. Bot. 38, 545-634, 1903; 39, 424-526,
1904; JOST: SCHENCK, Jena, 1893, 189 ; SCHIMPER; WOODHEAD
and BRIERLY, New Phyt. 8, 284-298, 1909.
Epiphytes (657): FITTING, Ann. Jard. Bot. Buit., Supplement III, 505—518,
1910; SCHIMPER; SCHIMPER, Jena, 1888.
Lenticels (660): DEVAUX, Ann. Sci. Nat. Bot. VIII, 12, 1-240, 1900; HABER-
LANDT.
Rhizomes (667): FRANCOIS, Ann. Sci. Nat. Bot. IX, 7, 25-58, 1908; LID-
FORSS, Jahrb. Wiss. Bot. 38, 343-376, 1903; MASSART, Bull. Soc. Bot..
Bela;. 41, 67—79, I9°35 RAUNKIAER, Bull. Acad. Sci. Denmark, 1904;
SPERLICH, Flora 96, 451-473, 1906.
Runners (672): MAIGE, Ann. Sci. Nat. Bot. VIII, n, 240-364, 1900.
Tubers (^74): MOLLIARD, Bull. Soc. Bot. France, 56, 42-45, 1909.
Bulbs (674): BLODGETT, Bot. Gaz. 50, 3 0-373, r910; RIMBACH, Bot. Gar
30, 171-188; 33, 401-420, 1900, 1902; ROBERTSON, Ann. Bot. 20, 420-
440, 1906.
Conduction arid conductive tissues (678): COPELAND, Bot. Gaz. 34, 161 ff.,
1902; DE BARY; EWART, Ann. Bot. 24, 85-105, 1910; GROOM, Ann.
Bot. 24, 241-269, 1910; HABERLANDT; JOST; MANGHAM, Sci. Prog.
5, 256 ff., 1910, 1911; OVERTON, Bot. Gaz. 51, 28 ff., 1911; WIEGAND,
Amer. Nat. 40, 400-453, 1905.
Conduction in bryophytes (685): TANSLEY and CHICK, Ann. Bot. 15, 1-38,
1901.
Vascular variations in primary tissues (686): CANNON, Bot. Gaz. 39, 397-408,
1905; FREUNDLICH, Jahrb. Wiss. Bot. 46, 137-206, 1908; MOLLIARD,
Compt. Rend. 144, 1063-1064, 1907; SCHUSTER, Ber. Deutsch. Bot. Ges.
26, 194-237, 1908; SIMON, Jahrb. Wiss. Bot. 45, 351-478, 1907; WINK-
LER, Jahrb. Wiss. Bot. 45, 1-82, 1907.
958 APPENDIX
Variations in growth rings (689): BONNIER, Compt. Rend. 135, 1285—1230,
1902; HABERLANDT; SIMON, Ber. Deutsch. Bot. Ges. 20, 229-249,
1902; URSPRUNG, Bot. Zeit. 62, 189-210, 1904.
Tyloses (695): VON ALTEN, Bot. Zeit. 67, 1-23, 1909.
Stereids (696): DE BARY; HABERLANDT.
Influence of external factors on mechanical tissues (699): BALL, Jahrb. Wiss
Bot. 39, 305-341, 1003; BORDNER, Bot. Gaz. 48, 251-274, 1909; HABER-
LANDT; HIBBARD, Bot. Gaz. 43, 361-382, 1907; METZGER, Nat. Zeit.
6, 249-273, 1908; PENNINGTON, Bot. Gaz. 50, 257-284, 1910; SONN-
TAG, Jahrb. Wiss. Bot. 39, 71-105, 1903; WIEDERSHEIM, Jahrb. Wiss.
Bot. 38, 41—69, 1902.
Role of mechanical tissues (700): AMBRONN, Jahrb. Wiss. Bot. 12, 473—541,
i83i; COHN, Jahrb. Wiss. Bot. 24, 145-172, 1892; HABERLANDT;
SCHWENDENER, Leipzig, 1874; SONNTAG, Flora 99, 203-259, 1909;
TSCHIRCH, Jahrb. Wiss. Bot. 16, 303-335, 1885.
Cork (705): HABERLANDT.
Water accumulation in stems (718): MACDOUGAL and SPALDING, Carnegie
Publ. 141, 1910; SPALDING, Bull. Torr. Bot. Club 32, 57-68, 1905.
Food accumulation in stems (719): COMBES, Rev. Gen. Bot. 23, 129-164, 1911;
HARTER, Plant World 13, 144-147, 1910; LE3LERC DU SABLON Rev.
Gen. Bot. 16, 341, ff.; 18, 5 IT., 1904, 1906; NIKLEWSKI, Bei. Bot. Csnt.
19, 68-117, 1905; SCIIELLENBERG, Ber. Deutsch. Bot. Ges. 23, ; 6-45,
1905.
Latex (720): BRUSCHI, Annali di Botanica 7, 671-701, 1909; GAUCHER Ann.
Sci. Nat. Bot. VIII, 12, 241-260, 1900; HABERLANDT; KNIEP, Flora 94,
120-205, 1905; MOLISCH, Jena, 1901; PARKIN, Ann. Bot. 14, 193-214,
1900.
f Resin ducts, etc. (723): HABERLANDT; TSCHIRCH, Leipzig, 1908.
'Stem variation (725): GOEBEL I; GOEBEL II.
Stem elongation (725): BRENNER, Flora 87, 387-439, 1900; LAURENT, Rev.
Gen. Bot. 19, 129-160, 1907; MACDOUGAL, Mem. N. Y. BJ . Gard., 1903;
SELBY, Bull. Torr. Bot. Club 3,3, 67-76, 1906.
Stem dwarfing (730): BONNIER, Ann. Sci. Nat. Bot. VII, 20, 217-360, 1895.
Spinescence (741): LOTHELIER, Rev. Gen. Bot. 5, 480 ff., 1893; LOTHELIER,
Lille, 1893; ZEIDLER, Flora 102, 87-95, 1911.
Tuberization (744): BERNARD, Rev. Gen. Bot. 14, 5 ff., 1902; JUMELLE,
Rev. Gen. Bot. 17, 49-59, '1905; MOLLIARD, Bull. So7. Bot. Frame 50,
631-633, 1903, and Rev. Gen. Bot. 19, 241 ff., 1907; VOCHITNG, Org\n-
bildung, Bonn, 1878, 1884, also Jahrb. Wiss. Bot. 34, 1-149, 1889, anci Bat.
Zeit. 60, 87—114, 1902.
Regeneration (748): COULTER and CHRYSLER, Bot. Gaz. 38, 45^-453,
1904; GOEBEL I and II, also Biol. Cent. 22, 385 ff., 1902, and Bull. Torr.
Bot. Club 30, 197—205, 1903; McCALLUM, Bot. Gaz. 40, 97 ff., 1905;
MORGAN, New York, 1901.
Polarity (749): KLEBS, Jena, 1903; KUSTER, Jahrb. Wiss. Bot. 40, 279-302,
1904; KUPFER, Mem. Torr. Bot. Club 12, 195-241, 1907; MORGAN,
Bull. Torr. Bot. Club 30, 206-213, IOO3> 3*i 227-230, 1904, and Science 20.
APPENDIX 959
742-748, 1904; VOCHTING, Organbildung, Bonn, 1878, 1884, and Bot.
Zeit. 64, 101-148, 1906; WINKLER, Jahrb. Wiss. Bot. 35, 449-469, 1900.
CHAPTER IV
SAPROPHYTISM AND SYMBIOSIS
Symbiosis (752): JOST; KERNER; WARMING.
Myrmecophytes (753): BUSCALIONI and HUBER, Bei. Bot. Cent. 9, 85-88,
1900; FIEBRIG, Biol. Cent. 29, i ff., 1909; VON IHERING, Bot. Jahrb.
39, 666-714, 1907; RETTIG, Bei. Bot. Cent. 17, 89-122, 1904; RIDLEY,
Ann. Bot. 24, 457-483, 1910; SCHIMPER; ULE, Bot. Jahrb. 37, 335-352,
1906.
Saprophytism in fungi (754): KUNZE, Jahrb. Wiss. Bot. 42, 357—393, 1906.
Saprophytism in algae (756): ARTARI, Ber. Deutsch. Bot. Ges. 20, 172-201,
1002; Jahrb. Wiss. Bot. 40, 593-613, 1904; TREBOUX, Ber. Deutsch. Bot.
Ges. 23, 432-441, io°5-
Saprophytism in seed plants (757): LAURENT, Rev. Gen. Bot. 16, 14 ff., 1904.
Parasitism (761): HABERLANDT; JOST; KERNER.
Parasitic fungi (762): BULLER, Jour. Econ. Biol. i, 101-138, 1906, and Sci.
Prog- 3, 361-378, 1909; DUGGAR, Plant Diseases, Boston, 1909; DUYSEN,
Hedwigia 46, 25-56, 1906; SMITH, Bot. Gaz. 33, 421-436, 1902.
Heteroecious fungi (764): ERIKSSON, Ann. Sci. Nat. Bot. VIII, 15, 1-160, 1902,
also Ann. Bot. 19, 55—59, 1905, and Biol. Cent. 30, 618-623, I9lo> JAC—
ZEWSKI, Zeit. Pflanzenkrank 20, 321-359, 1910; KLEBAHN, Berlin, 1904;
WARD, Ann. Bot. 19, 1—54, 1905.
Physiological species and specialization (765): FISCHER, Berne, 1907, and
Geneva, 1908; KLEBAHN, Berlin, 1904; NEGER, Flora 88, 333-370,
1901; 90, 221—272, 1902; REED, Trans. Wis. Acad. Sci. 15, 135—162, 1905;
15, 527-547, 1907; and Bull. Torr. Bot. Club 36, 353-388, 1909; SALMON,
Bei. Bot. Cent. 14, 261—315, 1903, also Phil. Trans. Roy. Soc. London, 197,
107—122, 1904; 198, 87—97, I9°S5 New Phyt. 3, 55 ff., 1904; 4, 217—221,
1905; Ann. Bot. 19, 125-148, 1905; WARD, Ann. Bot. 16, 233-315, 1902.
Origin of parasitism in fungi (766) : FULTON, Bot. Gaz. 41, 81-108, 1906; MAS-
SEE, Phil. Trans. Roy. Soc. London, 197, 7—24, 1904.
Immunity (768): BROOKS, Ann. Bot. 22, 479-487, 1908.
Parasitic seed plants (769): HABERLANDT; KERNER; MIRANDE, Bull.
Sci. 34, 1-280, 1901; PEIRCE, Ann. Bot. 7, 291-327, 1893; 8, 53-118, 1894.
Partially parasitic seed plants (772): BRAY, Bull. 166, U. S. Bur. PI. Ind., 1910;
GAUTIER, Rev. Gen. Bot. 20, 67-84, 1908; HEINRICHER, Jahrb. Wiss.
Bot- 31, 77~I24, 1898; 32, 389-452,1898; 36,665-752, 1901; 37, 264-337,
1902; 46, 273-376, 1909; 47, 539-587, 1910; YORK, Bull. Univ. Tex. 120,
1909.
Origin of parasitism in seed plants (775): MACDOUGAL, Carnegie Inst. Publ.
129, 1910; Plant World 13, 207—214, 1910; PEIRCE, Bot. Gaz. 38, 214—217,
1904; WHITE, Amer. Nat. 42, 98-108, 1908.
960 APPENDIX
Reciprocal influence of stock and scion (778): DANIEL, Compt. Rend. Paris
Acad. Sci. 141, 214-215, 1905; 148, 431-433, 1909; GRAFE and LINS-
BAUER, Ber. Deutsch. Bot. Ges. 24, 368-371, 1906; GRIFFON, Bull. Hoc.
Bot. France, 57, 517 ff., 1910; GUIGNARD, Ann. Sci. Nat. Bot. IX, 6, 261-
305, 1907; McCALLUM, Plant World 12, 281-286, 1909; MEYER and
SCHMIDT, Flora 100, 317-397, 1910; RAVAZ, Compt. Rend. Paris Acad.
Sci. 150, 712, 1910.
Graft hybrids and chimeras (779): BAUR, Zeit. Abst. Vererb. i, 330-351, 1909;
Ber. Deutsch. Bot. Ges. 27, 603-605, 1910; Biol. Cent. 30, 497-514, 1910;
BUDER, Ber. Deutsch. Bot. Ges. 28, 188-192, 1910; CAMPBELL, Amer.
Nat. 45, 41-53, 1911; COWLES and CHAMBERLAIN, Bot. Gaz. 51, 147-
153, 1911; GRIFFON, Bull. Soc. Bot. France 55, 397-405, 1908; 56, 203 ff.,
1909; STRASBURGER, Jahrb. Wiss. Bot. 44, 482-555, 1907; Ber. Deutsch.
Bot. Ges. 27, 511-528, 1909; WINKLER, Ber. Deutsch. Bot. Ges. 25, 568-
576, 1907; 263, 595-608, 1908; 28, ii6-n8, 1910; Zeit. Bot. i, 3I5-.145,
1909; 2, 1-38, 1910.
Galls (780): HOUARD, Ann. Sci. Nat. Bot. 20, 280-384, 1904; KERNKR;
KUSTER, Flora 87, 117-193, 1900; Biol. Cent. 20, 520-543, 1900; J< na,
1903; Biol. Cent. 30, 116-128, 1910; MOLLIARD, Bull. Soc. Bot. France
57, 24-31, 1910; TUBEUF, Nat. Zeit. 8, 349-351, 1910.
Fasciation (786): HUS, Amer. Nat. 42, 81-97, T9°8; plant World 14, 88-96, ion;
KNOX, Carnegie Inst. Publ. 98, 1909; WORSDELL, New Phyt. 4, 55-74,
1905.
Root tubercles (787) : BUCHANAN, Cent. Bakt. 23, 59-91, 1909; FISCHER,
Ber. Deutsch. Bot. Ges. 28, 10-20, 1910; GAGE, Cent. Bakt. 27, 7-48, 1010;
MOORE, Bull. 71, U.S. Bur. PI. Ind., 1905; PEIRCE, Proc. Cal. Acad.
Sci. Ill, 2, 295-328, 1902.
Nitrogen fixation and nitrification (789): BLACKMAN, New Phyt. 3, 125-129,
1904; HALL, Science 22, 449-464, 1905; JACOBITZ, Cent. Bakt. 7, 783 ff.,
1901; JAMIESON, Aberdeen, 1905; LIPMAN,.Pop. Sci. Mon. 62, 137-
144, 1002; LOHNIS, Cent. Bakt. 14, 582 ff., 1905; PRINGSHEIM, Biol.
Cent. 31, 65-81, 1911; REINKE, Ber. Deutsch. Bot. Ges. 21, 371 «., 1903;
22, 95-100, 1904; STEVENS and WITHERS, Cent. Bakt. 27, 169-186,
1910; VOGEL, Cent. Bakt. 15, 33 ff., 1905.
Mycosymbiosis (791): BERNARD, Rev. Gen. Bot. 16, 405 ff., 1904; Ann. Sci.
Nat. Bot. IX, 9, 1-196, 1909; BURGEFF, Jena, 1909; GALLAUD, Rev.
Gen. Bot. 17, i ff., 1905; GRUENBERG, Bull. Torr. Bot. Club 36, 165-
169, 1909; MACDOUGAL, Ann. Bot. 13, 1-47, 1899; PEKLO, Ber. Deutsch.
Bot. Ges. 27, 239-247, 1909; SARAUW, Rev. Myc. 25, 157 ff., 1903; STAHL,
Jahrb. Wiss. Bot. 34, 539-668, 1900.
Rdle of root fungi (794): ARZBERGER, Mo. Bot. Gard. Rep. 21, 60-102, i)io;
BERNARD, Bull. Pasteur Inst., 1909; FROHLICH, Jahrb. Wiss. Bot. 45,
256-302, 1908; HEINZE, Ann. Mycol. 4, 41-63, 1906; LATHAM, Bull.
Torr. Bot. Club 36, 235-244, 1909; MAGNUS, Jahrb. Wiss. Bot. 35, 205-
272, 1900; PEKLO, Cent. Bakt. 27, 451-579, 1910; PENNINGTON, Bull.
Torr. Bot. Club 38, 135-139, 1911; SHIBATA, Jahrb. Wiss. Bot. 37, 643-
684, 1902; TERNETZ, Jahrb. Wiss. Bot. 44, 353-408, 1907; ZACH, Sitz
APPENDIX 961
Wien Akad. 117, 973-983, 1908; 119, 307-330, 19x0; Oest. Bot. Zeit. 60,
49-55. 191°-
Lichens (800): ELENKIN, St. Petersburg, 1906; FITTING, Ann. Jard. Bot.
Buit. 1910, 505-518; PEIRCE, Proc. Cal. Acad. Sci. Ill, i, 207-240, 1899;
TOBLER, Ber. Deutsch. Bot. Ges. 29, 3-12, 1911.
Green-celled animals (803): KEEBLE and GAMBLE, Quar. Jour. Mic. Soc. 51,
167-219, 1907; 52, 43J-479. I9o8-
CHAPTER V
REPRODUCTION AND DISPERSAL
Reproduction (805): JOST; KUSTER, Leipzig, 1906; MOBIUS, Jena, 1897.
Fairy rings (807): MASSART, Ann. Jard. Bot. Buit. 1910, 583-586; MOLLIARD
Bull. Soc. Bot. France 57, 62-69, 1910.
Fungus spores (8n): BECQUEREL, Compt. Rend. Paris Acad. 150, 1437-
1439, 1910; BULLER, Researches in Fungi, London, 1909.
Sexual reproduction (816): BLAKESLEE, Science 25, 366-372, 1907; GUILLIER-
MOND, Bull. Sci. 44, 109-196, 1910; HARPER, Amer. Nat. 44, 533-546,
1910; HOYT, Bot. Gaz. 49, 340-370, 1910; LILLIE, Science 25, 372-376,
1907; WILSON, Science 25, 376-379, 1907.
Significance of sexual reproduction (819): BLACKMAN, New Phyt. 3, 149-158,
1904; BUHLER, Biol. Cent. 24, 65 ff., 1904; DANGEARD, Le Botaniste
it, 1-311, 1910; SCHULTZ, Biol. Cent. 25, 465-473, 1905; WOODRUFF,
Amer. Nat. 42, 520-526, 1908; Biol. Bull. 17, 287-308, 1909.
Apogamy and parthenogenesis (822): BLARINGHEM, Bull. Sci. 43, 113-170,
1909; KIRCHNER, Ber. Deutsch. Bot. Ges. 22, 83-97, 1904; TREUB,
Ann. Jard. Bot. Buit. 15, 1-25, 1898; WINKLER, Prog. Rei. Bot. 2, 293-
454, 1908.
Flowers (825): KERNER.
Anther dehiscence (829): BURCK, Rev. Gen. Bot. 19, 104-111, 1907.
Pollen (830): LIDFORSS, Jahrb. Wiss. Bot. 29, 1-38, 1896; 33, 232-312, 1899;
PFUNDT, Jahrb. Wiss. Bot. 47, 1-40, 1909.
Stigma (831): LUTZ, Zeit. Bot. 3, 289-348, 1911.
Pollen tube (832): JOST, Ber. Deutsch. Bot. Ges. 23, 504-515, 1906 i Bot. Zeit.
65, 77-116, 1907; LIDFORSS, Zeit. Bot. i, 443-496, 1909.
Wind pollination (834) : KERNER.
Insect pollination (838): DARWIN (various works); KERNER; KNUTH,
Oxford, 1906-1909; MUELLER, Fertilization of Flowers, London, 1883.
Pollinating insects (840) : KNUTH, Oxford, 1906-1909; MUELLER (as above).
Nectaries (843) : HABERLANDT.
Role of color and odor in flowers (845): ANDREAE, Bei. Bot. Cent. 15, 427-470,
1903; DETTO, Flora 94, 424-463, 1905; GILTAY, Jahrb. Wiss. Bot. 40,
368-402, 1904; 43, 468-499, 1906; LOVELL, Amer. Nat. 35, 197-212, 1901;
36, 203-242, 1902; 37, 365 ff., 1903; 43, 338-349» *9°9; 44, 673-692, 1910;
PLATEAU, Bull. Roy. Acad. Belg. 1895-1910; Biol. Cent. 16-23, 1896-
1903; Bull. Soc. Bot. Belg. 46, 339-369, 1909.
962 APPENDIX
Extrafloral nectaries (858): VON UEXKULL-GULDENBANDT, Ann. Jard
Bot. Buit. II, 6, 195-327, 1907.
Pollination in figs. (860): LECLERC DU SABLON, Rev. Gen. Bot. 20, 14 ff.,
1908; 22, 65-69, 1910; TSCHIRCH, Ber. Deutsch. Bot. Ges. 29, 83-96,
1911.
Cleistogamy (864): GOEBEL, Biol. Cent. 24,673 ff., 1904; HACKEL, Oest. Bot.
Zeit. 56, 81 ff., 1906; LECLERC DU SABLON, Rev. Gen. Bot. 12, 305-
318, 1900; RITZEROW, Flora 98, 163-212, 1908.
Significance of cross pollination (866): BURCK, Biol. Cent. 28, 177-195, 1908;
DARWIN, Cross-and Self- Fertilisation, London, 1876; SHULL, Proc. Ame'r.
Breeder's Assoc., 1908.
Flower duration (870): FITTING, Zeit. Bot. i, 1-86, 1909; 2, 225-267, 1910.
Closing of flowers (871): BURGERSTEIN, Oest. Bot. Zeit. 51, 185-193, 1901;
FARMER, New Phyt. i, 56-58, 1902; STOPPEL, Zeit. Bot. 2, 369-453,
1910.
Origin of floral structures (875): ROBERTSON, Bot. Gaz. 37, 294-298, 1904.
Reproductive variation in the algae and fungi (878) : COPELAND, Bot. Gaz. 47,
9-25, 1909; DANFORTH, Mo. Bot. Gard. Rep. 21, 49-59, 1910; FREUND,
Flora 98, 41-100, 1908; HOYT, Bot. Gaz. 43, 383-392, 1907; KAUFFMAN,
Ann. Bot. 22, 361-387, 1908; KLEBS, Jena, 1896; Jahb. Wiss. Bot. 35, 80-
203, 1900; KNIEP, Jahrb. Wiss. Bot. 44, 635-724, 1907; LEWIS, :k>t.
Gaz. 50, 59-64, 1910; MORGENTHALER, Cent. Bakt. 27, 73-92, icio;
WAKEFIELD, Nat. Zeit. 7, 521-551, 1909; WILLIAMS, Ann. Bot. 19,
S^-S60. 1905-
Artificial parthenogenesis (882): DAUDIN, Bull. Sci. 43, 297-372, 1909; DE-
LAGE, Compt. Rend. Paris Acad. 147, 553 ff., 1908; DONCASTER, Sci.
Prog. 3, 40-52, 1908; 4, 90-104, 1909; LOEB, Am. Jour. Physiol. 1899,
1900; New York, 1906; Arch. Ges. Physiol. 118, 181 ff., 1907; Arch. Entwickl.
23, 479-486, 1907; Boston Zool. Congress, 1908.
Reproductive variation in pteridophytes (884): MOTTIER, Bot. Gaz. 50, 209-
213, 1910; SHATTUCK, Bot. Gaz. 49, 19-40, 1910; WORONIN, Flora
98, 101-162, 1907; WUIST, Bot. Gaz. 49, 215-219, 1910.
Vegetative and reproductive periods in seed plants (885): DIELS, Berlin, 1-506;
FISCHER, Flora 94, 478-490, 1005; GOEBEL I and II; HOWARD, Halle,
1906; JOHANNSEN, Jena, 1906; KLEBS, Jena, 1903; Biol. Cent. 24,
257 ff., 1904, Halle, 1906; SELIBER, Rev. Gen. Bot. 21, 420 ff., 1909,
Jahrb. Wiss. Bot. 42, 155-320, 1905.
Sex determination (895): BITTER, Ber. Deutsch. Bot. Ges. 27, 120-126, 1709;
BLARINGHEM, Bull. Sci. 41, 1-248, 1907; CORRENS, Jahrb. Wiss. Bot.
44, 124-173, 1907; 45, 661-700, 1908; Berlin, 1907; Amer. Nat. 42, -Sn-
824, 1908; Ber. Deutsch. Bot. Ges. 26a, 686-701, 1908; DARLING, Bull.
Torr. Bot. Club 36, 177-199, 1909; DONCASTER, Sci. Prog. 3, 40-52,
1908; 4, 90-104, 1009; GOEBEL, Biol. Cent. 30, 657 ff., 1910; HARPER
Science 25, 379-382, 1907; IORNS, Science 28, 125-126, 1908; JOHNSON,
Jour. Exper. Zool. 9, 715-749, 1910; JORDAN, Pop. Sci. Mon. 74, 540 ff.,
1909; Amer. Nat. 44, 245-252, 1910; McCLENDON, Amer. Nat. 44, 404-
411, 1910; MOBIUS, Biol. Cent. 20, 561-572, 1900; MOLLIARD, Rev.
APPENDIX 963
Gen. Bot. 21, 1-7, 1909; MORGAN, Amer. Nat. 44, 449-496, 1910; SCHAFF-
NER, Proc. Ohio Acad. Sci. 5, 327-350, 1910; STRASBURGER, Biol. Cent.
20, 657 ff., 1900; Jena, 1909; Zeit. Bot. i, 507-524, 1909; Jahrb. Wiss. Bot.
48, 427-520, 1910; THOMSON, Jour. Roy. Mic. Soc. 1911, 141-159; WIL-
SON, Science 25, 382-384, 1907; 29, 53-70, 1909; WUIST, Bot. Gaz. 49,
215-219, 1910.
Variations in flower color (898) : FISCHER, Flora 98, 380-385, 1908 ; KRAEMER,
Science 23, 699, 760, 1906; 29, 828, 1909; WHELDALE, Proc. Roy. Soc.
81, 44-60, 1909.
Variations in flower form (900): GOEBEL I and II; Biol. Cent. 24, 673 ff., 1904;
MOLLIARD, Compt. Rend. Paris Acad. 133, 548-551, 1901 ; ORTLEPP, Flora
98, 406-422, 1908; VOCHTING, Jahrb. Wiss. Bot. 17, 297-346, 1886.
Bud variation (904): CRAMER, Haarlem, 1907; EAST, Plant World n, 77-83,
1908.
Fruits and seeds (904): AVEBURY (Lubbock), Jour. Roy. Mic. Soc., 1909, 137-
166; KERNER.
Seed vitality (908): BEAL, Bot. Gaz. 40, 140-143, 1905; BECQUEREL, Ann.
Sci. Nat. Bot. IX, 5, 193-311, 1907; Compt. Rend. Paris Acad. 148, 1052-
1054, 1909; COKER, Amer. Nat. 43, 677-681, 1909; DIXON, Ann. Bot.
16, 590-591, 1902; DUVEL, Bull. U. S. Bur. PI. Ind. 58, 83, 1904, 1905;
EWART, Proc. Roy. Soc. Victoria, 1908; SCHNEIDER-ORELLI, Flora
100, 305-311, 1910.
Starch (912): FISCHER, Bei. Bot. Cent. 18, 409-432, 1905; HABERLANDT;
KRAEMER, Bot. Gaz. 34, 341-354, 1902; REINHARD and SUSCHKOFF
Bei. Bot. Cent. 18, 133-140, 1904.
Fruit and seed variations (916): FITTING, Biol. Cent. 29, 193 ff., 1909; GUIG-
NARD, Ann. Sci. Nat. Bot. VII, 4, 202-240, 1886; LECLERC DU SABLON,
Rev. Gen. Bot. 20, 14-24, 1908; MOLLIARD, Bull. Soc. Bot. France 50,
135 ff., 1903.
Fruit and seed dispersal (919): BIRGER, Bei. Bot. Cent. 21, 263-280, 1907;
KERNER; OSTENFELD, Svensk. Bot. Tid. 2, i-n, 1908; RIDLEY,
Ann. Bot. 19, 351-363, 1905; SERNANDER, Stockholm, 1906.
CHAPTER VI
GERMINATION
Delayed germination (932): CROCKER, Bot. Gaz. 42, 265-291, 1906; 44, 375-
380, 1907.
External factors and germination (933) : HEINRICHER, Ber. Deutsch. Bot. Ges.
z6a, 298-301, 1908; KINZEL, Ber. Deutsch. Bot. Ges. 25, 269-276, 1907;
26a, 105-115, 1908; 27, 536-545, 1909; LAAGE, Bei. Bot. Cent. 21, 76-115,
1907; LIFE, Mo. Bot. Gard. Rep. 18, 109-122, 1907; SCHULZ, Bei. Bot.
Cent, ii, 81—97, 1901.
Seedlings (934) : LUBBOCK (AVEBURY), New York, 1892.
Buds (936) : ARNOLDI, Flora 87, 440-478, 1900.
964 APPENDIX
CHAPTER VIII
ADAPTATION
Adaptation (947): BLARINGHEM, Paris, 1908; BORDAGE, Bull. Sci. 44, 51-
88, 1910; BOURNE, Science 32, 729-742, 1910; BUTSCHLI, Leipzig, 1901;
DETTO, Jena, 1904; DRIESCH, Leipzig, 1901; FARMER, New Phyt. 2,
193 ff., 1903; GANONG, Science 19, 493-498, 1904; HENSLOW, London,
1908; KLEBS, Heidelberg, 1909; MACDOUGAL, Science 33, 94-101,
1911; Amer. Nat. 45, 5-40, 1911; Bot. Gaz. 51, 241-257, 1911; MORGAN,
Science 14, 235-248, 1901; 31, 201-210, 1910; New York, 1903; SCOTT,
Nature 81, 115-118, 1909; WENT, Biol. Cent. 27, 257-271, 1907; WETT-
STEIN, Ber. Deutsch. Bot. Ges. 18, 184-200, 1900.
INDEX
{Figures in italics indicate pages upon which illustrations occur.]
Abies, 648.
Absciss layer, 582, 583.
Absinthin, 626.
Absorption, epiphytes, 014, 615; external
factors, 493; foods, 616, 752 &., 934;
hairs, 613, 615, 617; land plants, 610;
leaves, 608, 6og, 613, 616; rhizoids, 517,
518; roots, 491, 493, 5/0, 511; transpi-
ration, 565; water and salts, 491, 493,
517, 518, 565, 608, 614, 615; water
plants, 6oQ.
Abutilon, albescence, 524, 536.
Acacia, thorns and food bodies, 754.
Acceleration, period of, 690.
Accommodation, 487, 760, 951.
Accumulation, foods, 500, 627, 628, 719,
720, 911, QIZ, 9/3, 914, 915; leaves, 627,
628, 629, 630, 631, 632, 633; roots, 500;
seeds, 911, 912, 913, 914, 915; stems, 718,
719, 720, 721, 723, 724, 725; waste, 623,
624, 625, 626, 718, 723, 724, 725; water,
627, 628, 629, 630, 631, 632, 633, 718.
Acer, flowers, 833; fruit, 92 1; galls, 7^2 ;
habit, 584; hairs, 572; leaves, 542; stem
section, 685; twigs, 646.
Achene, 857, 919, 921, 924.
Achillea, flowers, 846.
Acorn, 919, 925.
Acquired characters, 947,
Actinomorphic flowers, 825, 842, 859.
Active buds, 936.
Adapt; adaptation; adaptation characters;
adaptive response, 487, 947, 949, 951.
Adhesive disks, 652, 653.
Adjustment, 487, 951.
Adult stages, 596, do/, 725.
Advantage; advantageous reactions, 487,
488.
Adventitious roots, 501, 502, 503, 510, 5/2,
5/4, 637, 668, 671, 673, 675, 730, 744.
Aecidial stage; aecidiospores, 764, 813.
Aeranthus, root, 5/2.
Aeration, 551, 660.
Aerenchyma, 553.
Aerial rhizoids, 518.
Aerial roots, 5/0, 511, 5/2, 513, 514, 575.
Aerial stems, 719, 725.
Agave, bast fibers, 696; epidermis, 568;
habit, 628; palisade cells, 551.
Agelaea, 656.
Aggregate fruit, 924.
Air cavities, stomatal, 555, 556, 55*, 559;
chambers, 55/, 552, 553, 557, 561, 687,
688, 703, 718 ; leaves, 590, 592, 593, 595;
pore, 557; reservoirs, 554 (see also Air
chambers) ; roots, 5/0, 511, 512, 5/3, 5/4,
5/5; spaces, 534, 55/, 552, 553, 554, 621.
Albescence, 523, 524, 536.
Albino, 845.
Albugo, haustoria, 7*53.
Alburnum, 685.
Alder, buds, 938.
Aleurone, 914, 9/5.
Algae, absorption, 609, 610; asexual spores,
810; chlorenchyma, 533; conductive
tissues, 685 ; form variations, 591, 593 ;
lichens, #00, 801, 802 ; reproductive varia-
tion, 878, 881 ; rhizoids, 5/9, 520; sapro-
phytism, 756; sexual reproduction, 816,
817; vegetative reproduction, 806.
Alkaloids, 626.
Allium, bulbils, 003; flowers, £74; seed-
ling, 935-
Alnus, buds, 938.
Aloin, 626.
Alpine plants, 590, 648, 730, 731, 732, 738,
890, 944.
Alternation of generations, 814, 822.
Alternative parasitism, 787.
Ammophila, leaf section, 581.
Amphibious plants, hairs, 574, 575; leaf
variation, 500, 502, 593, 594, 505, 597-
Amphigastria, 608.
Amphimixis, 820.
Amphivasal bundles, 683.
Anaptychia, #00.
Anastomosing veins, 638.
Anatropous ovules, 906.
Anchoring roots, 497, 498, 5/2, 5/3.
Andromeda, leaf, 57$.
Anemophilous flowers, 834.
Angelica, bud protection, 641.
Angiosperms (see also Seed plants), vessels,
68 1.
Animals (see also Ants, Birds, Carnivorous
plants, Galls, Insects), dispersal by, 923;
green-celled A., 803 ; parthenogenesis,
882; protection from, 743, 908; repro-
INDEX
ductive variation, 881; sex determina-
tion, 897.
Anisophylly, 608.
Annual, 501, 714, 715.
Annual rings, 685, 689, 690, 691.
Annular vessels, 680, 681.
Annulus, 815.
Anther, 825, 826, 829, 830, 833, 835, 840,
851, 853, 854, 857, 871.
Anthericum, stomata, 560.
Antheridia, 817.
Anthesis, 887.
Anthocyan, 522, 528, 529, 571, 845, 898.
Ant plants, 753, 754.
Ants, 753, 754, 858.
Aplectrum, mycorhiza, 793.
Apogamy, 822.
Apospory, 822.
Apostrophe, 524.
Apothecia, 880; 659, 800.
Aquatic plants, 509, 609, 676, 678, 715, 719,
728, 8 10, 837, 838, 940.
Arabis, root contraction, 504.
Aralia, 546.
Arbor vitae, leaf variation, 60 1.
Archegonia, 817.
Arctic plants, 733, 890, 944.
Arid climates, 889.
Aril, 907.
Aristolochia, pollination, 853.
Arm palisades, 532.
Arrowhead, leaf variation, 590.
Artemisia, 770.
Artichoke, bud, 642.
Arum, pollination, 860.
Asarum, leaf section, 531.
Ascocarp, 811.
Asexual reproduction, 809, 810, 811, 812,
813, 814, 815.
Ash, twigs, 736.
Aspidium, sporangia, 815.
Asplenium, reproduction, 636.
Association, plant, 939, 940, 942, 943, 944,
945-
Asymmetry, 607, 734, 735.
Atrophy, 782.
Atropin, 626.
Autecology, 485.
Autoecious parasites, 764.
Autogamy, 829, 863 (see also Close pollina-
tion).
Automixis, 820.
Autoparasitism, 786.
Autophyte, 752.
Autotrophic plants, 752.
Autumn wood, 689, 690.
Auxospore, 809.
Awn, 924, 929.
Azotobacter, 789.
Bacillus, 787, 788, 790.
Bacteria, carbohydrate synthesis, 526; in-
fection threads, 787, 788; parasitism,
762, 766; root tubercles, 787, 788, 789,
790; saprophytism, 754; vegetative re-
production, 806.
Bacteroids, 787.
Balanophora, parthenocarpy, 917.
Bald cypress, 508.
Banana, 585.
Banyan, prop roots, 515.
Barb, 654, 924.
Barberry, flower, 851 ; leaf variation, 604.
Bark, 685, 704, 708, 709.
Bartramia, 611. ^
Basidiospores, 764, Shi.
Bast fibers, 696, 699,^01.
Beak, 921.
Bean, regeneration, 748; root hairs, 491;
seed section, 905; twining, 651.
Beech, mycorhiza, 791; seedling, 936.
Bees, 840, 848.
Beet, 500.
Beetles, 842.
Begonia, collenchyma, 697; leaf sec:ion,
533-
Behavior, 488.
Berberis, flower, 551; leaf variation, ('04.
Berry, 919.
Beta, 500.
Betula, lenticels, 662.
Bicollateral bundles, 683.
Bidens, fruit, 924; pollen, 831.
Biennials, 501, 714.
Bifacial leaves, 629.
Bilabiate flowers, 829, 846, 853, 875.
Birch, lenticels, 662.
Birds, 842, 9-24.
Black knot, 784.
Bladder, 555, 579, 618, 619, 678.
Bladderwort, 618, 619, 678.
Blade, 521.
Blastophaga, 861.
Bleeding, 622.
Bloom, 568, 570.
Bog plants, 486, 537, 734, 942.
Boneset, cork, 705.
Bordered pits, 680, 690.
Botrychium, mycorhiza, 793.
Botrydium, 520.
Box elder, flowers, 833; stem section, 68$.
Bract, 827, 731, 829f 872.
Branch fall, 726. /
Branching, £46, 647, 649.
Brasenia^glands, ^623.
Brassica^ intumescences, 633; root hairs,
491. ""*•
Bridging hosts, 765.
Bristles, 921, 929.
INDEX
Bromeliaceae, 614, 615, 657, 658.
Brooms, witches', 783.
Bryophyllum, lenticels, 660; regeneration,
637.
Bryophytes (see also Liverworts, Mosses),
absorption, 6og, 610, 611, 612, 613, 614;
asexual reproduction, 814; reproductive
variation, 884; vegetative reproduction,
807, 808.
Bryopsis, reversal of polarity, 750.
Bud, 646, 668, 669, 674, 678, 736, 874, 833,
893, 938; active, 936; germination, 936,
937, 938; lateral, 646; pollination, 856;
protection, 640; resting, 936, 938;
scales, 642, 643, 644, 646, 833; terminal,
646; variation, 904; winter, 555, 572,
646, 678, 736, 936, 938.
Budding, 776, 777.
Bufiei- cells, 563.
Bulb, 674, 675, 676, 746.
Bulbil, 636, 675, 902, 903, 937.
Bulbling, 636, 937, 938.
Bulrush, 665.
Bundle, arrangement, 695, 701, 702, 703;
bicollateral, 683; collateral, 683; com-
pound, 682; concentric, 683; conduc-
tive, 530, 531, 533, 534, 536, 680, 682,
683, 684, 6S6, 687, 688, 695 ; fibrovascu-
lar, 682; radial, 683; sheath, 530, 581,
63?, 683; vascular (see conductive).
Buoyancy, 554.
Bur fruit, 923, 024; oak, 649; reed, 545.
Buttercup, laaf variation, 590.
Butterfliss, 841.
Butterwort, 618.
Buttressed trunks, 508, 509.
Cactus, 7/1, 725, 740.
Caducous bud scales, 642.
Calamovilfa, 497.
Calamus, 656.
Callus, 682, 706, 929.
Calystegia, 849.
Calyx, 731, 825, 826, 828, 829, 833, 835,
840, 853, 854, 857, 859, 869; water, 845.
Cambium, 680, 684, 769; cork, 661, 705;
f ascicular 684 ; interfascicular, 684 ; ring,
684.
Campanula, leaf section, 598; leaf varia-
tion, 598, 600, 601, 602.
Campylotropous ovules, 905.
Canna, seed section, 905.
Cap, root, 497.
Capitate hairs, 623.
Caprificatim ; caprifig, 861.
Capsule, 656, 814, 874, 919, 920.
Carbohydrates, 521, 525, 912 (see also
Starch, Sugar); conduction, 694; syn-
thesis, 525, 526, 527, 563, 588, 660, 665.
Carbon dioxid, 527.
Carnation, mechanical cylinder, 701;
stoma, 55o, 592, $93, 505,' air spaces, 534,
55', 552, 553, 554, 557, 561, 688; al-
bescence, 523, 524, 536; anisophylly,
608; anthocyan, 528, 529; asymmetry,
607; carnivorous, 617, 618, 619; chloren-
chyma, 530, 531, 532, 533, 534, 535, 536,
538, 5<57, 5*7, 629, 630, 631, 632, 688,
724; chlorophyll, 521, 523; chloroplasts,
521, 524, 531, 533, 534, 53S, 6, 603, 607, 647;
mechanical tissue, 639; mosaic, 543, 544,
607; motile, 579, 580, 582; orientation,
540, 541, 542, 543, 544, 547, 548; pali-
sade cells, 530, 532, 533, 534, 535, 536,
538, 561, 567, 574, 630, 639, 642; petiole,
501, 540, 541, 542, 640, 641; phototro-
pism, 539, 540, 542, 544, 546, 547,'
phyllotaxy, 510, 542, 543, 549, 6*1, 647;
pigments, 522, 528; plasticity, 596;
position, 549 ; protection, 565, 5^,1', • pro-
tective hairs, 536, 572, 573, 574, 5; 5, 576,
577,578; regeneration, 636, 637; repro-
duction, 636, 637; roots, 504, 637 ; run-
off, 636; scale, 641, 642, 643, 641, 669,
670, 675, 733, 736, 768, 834; scar. 583,
044, <557, 660, 708, 736; sclerophyll, 570,
578, 588; secretion, 620, 622, 85^; sec-
tion. 521, 530, 531, 532, 533, 534, 535,
536, 561, 567, 574, 581, 612, 620, 630,
631, 632, 639, 642, 688, 724; sinus, 637,
639; small, 550; spines, 604; sponge
tissue, 530, 533, 534, 535, 536; stem
display, 542, 543, 546, 548, 645, 646, 650 ;
stipules 640, 641 ; stomata, 530, 531, 534,
INDEX
535, 536, 555, 556, 55^, 559, 560, 561,
562; subterranean, 642; teeth, 621; ten-
drils, 640, 641; transpiration, 565, 577,
578, 588; variation, 544, 575, 589, 590,
592, 593, 594, 595, 597, 59#, 599, 600, 601,
602, 603, 604, 605, 606, 607, 608, 673,
726 ; 727, 731, 741, 784; veins, 638,
639; vertical, 546, 547, 549, 578 ; water,
590,592, 593, 595, 837 ; water accumula-
tion, 627, 628, 629, 630, 631, 632, 633;
water exudation, 620, 622; water tissue,
533, 629, 630, 631, 632, 635.
Leafless stem, 664, 665, 725, 733, 740; tree,
710, 711.
Leaflet, 579, 640, 652, 783.
Leaner, 654.
Ledum, leaves, 578.
Legume, 920.
Leguminosae, root tubercles, 787, 788, 790.
Lenticel, 660, 661, 662, 663, 736; water,
663.
Leonurus, leaf variation, 606.
Lepidium, leaf section, 530; roots, 498;
rosette, 582.
Leptocentric bundle, 683.
Leptome, 530, 630, 639, 682, 683, 689.
Lettuce, achene, 921; conductive vessels,
679; latex tubes, 721; leaf section, 535;
leaves, 547; root hairs, 493.
Leucobryum, leaf section, 612.
Leucoplast, 528, 721.
Level, law of, 668, 670.
Lianas, 651, 652, 653, 654, 655, 656; con-
ductive tissue, 689; mechanical tissue,
703 ; roots, 512, 513.
Libriform, 697.
Lichens, absorption, 610, 611, 615 ; chloren-
chyma, 533; epiphytism, 659; protec-
tion, 588; rhizoids, 520; symbiosis, 800,
801, 802; vegetative reproduction, 806.
Lid, 814.
Light, carbohydrate synthesis, 526; chloro-
phyll, 523; elongation, 726; flower color,
898; leaf form, 594, 5995 leaf position,
603, 607; leaves, 501, 539, 540, 541, 542,
543, 544, 546, 547, 548, 549, 550, 647;
palisade cells, 535 ; stomata, 563.
Lignification ; lignin, 679, 693
Ligustrum, lenticel, 661.
Lilium; lily, bulbils, 675; leaf section,
532; stoma, 555.
Linear migration, 669, 670, 671.
Lip, 829, 846
Lithophyte, 659.
Liverworts, chlorenchyma, 533, 557; con-
ductive tissue, 685 ; gemmae, 808; rhi-
zoids, 516, 517.
Locust, Tioney, 777.
Longevity of seeds, 908.
Long moss, 614.
Lonicera, leaves, 543.
Loosestrife, leaf section, 642; leaves, 641.
Lopseed, flowering shoot, 875.
Lunularia, gemmae, 808; rhizoids, 517.
Lupine; Lupinus, pod, 920.
Lycopodium, tuber, 746.
Lysimachia, leaf section, 642 ; leaves, 641.
Maize, roots, 494, 499, 514.
Man, 946.
Mangrove, roots, 515; vivipary, 930, 931.
Mantle, water, 533, 631, 632.
Maple, fruit, 921; gall, 782; habit, 584;
leaves, 542 ; scale hairs, 572 ; twigs, 646.
Marchantia, gemmae, 808; rhizoids, 517;
thallus section, 557.
Margo, 68 1.
Maritime associations) 942.
Marshes, salt, 942. v
Marsilea, reproductive variation, 885.
Maturity, period of, 690.
Maxillary laminae, 841.
Meadow fescue, flowers, 835.
Mechanical tissues, 513, 581, 639, 696, 697,
698, 699, 700, 701, 702, 705, 705.
Medullary ray, 684, 685.
Megaspore, 815, 826.
Melilotus, root tubercles, 788.
Memory, 849.
Mermaid weed, leaf sections, 534; leaf
variation, 592, 593, 595.
Me^ophyll, 521, 642, 688.
'Mesophyles, 486, 939, 945 ; chlorenchyma, \-
530,531,534; congenital, 951 ; faculta-
tive, 951; forest, 656; leaf sections,
530,598; obligate, 951 ; reaction, 951.
Mestome, 682, 683.
Meteorological factors, 887.
Micellae, 697.
Micropyle, 832, 905.
Microspore, 815, 82',, 826, 830.
Midrib, 638.
Migration, 027; linear, 669, 670, 671;
radial, 669, 672.
Milfoil, water, stem section, 551.
Mimosa, leaves, 579.
Mint, mountain, stem section, 702.
Mistletoe, 771.
Mixophyte, 754.
Mobility, 927.
Moisture and stomatal movement, 563.
Mold, habit, 755; sporangium, Si I.
Monocarpic plants, 886.
Monocliny, 825, 826, 827, 828, 835, 838.
Monocotyls, chlorenchyma, 530, 531 ; seed-
lings, 935; stomata, 560 (see also Seed
plants).
Monoecious plants, 818, 827, 834,
INDEX
Monotropa, mycorhiza, 792.
Moors, 042.
Morning glory, flowers, 849.
Morphological ecology, 485.
Morus, fruit, 924; leaf variation, 603.
Mosaic, leaf, 543, 544, 607.
Mosses, absorption, 610, 611, 612, 613;
chlorenchyma, 533; chloroplasts, 521;
conductive tissue, 685, 686; rhizoids,
5/7, 518; vegetative reproduction, 807.
Motherwort, leaf variation, 606.
Moths, 841.
Motile leaves, 579, 580.
Mountain sheep, 738.
Movements, chloroplasts, 524; leaves, 579,
580; stomata, 562; zoospores, 810.
Mucilage; mucilage glands, 517, 622, 624,
632, 723.
Mucor, habit, 755; sporangium, 811.
Mulberry, fruit, 924; leaf variation, 603.
Mullein, hairs, 572.
Multicipital stems, 504, 676, 677.
Musa, leaves, 585.
Mustard, root hairs, 491,
Mutualism, 786.
Mycelium, 755.
Mycophyte, 794.
Mycoplasm, 764.
Mycorhiza, 791, 792, 793, 796; ectotrophic,
79i, 792; endotrophic, 792, 793, 796.
Mycosymbiosis, 791, 792, 793, 794, 796,
798, 799.
Myriophyllum, stem section, 551.
Myrmecophily ; myrmecophyte, 753, 754.
Nanism, 734.
Narcissus, flowers, 902.
Nasturtium, flower, 843; phototropism,
540; plastids, 522 ; pollen, 831; stomata
620.
Natural selection, 743, 876, 952.
Nectar, 843, 858.
Nectary, 843, 844; extrafloral, 844, 858.
Nematus, gall, 7^2.
Neottia, mycorhiza, 796.
Nepenthes, 618.
Nephrolepis, 653.
Nereocystis, 519.
Nerium, leaf section, 567.
Nettle, stinging hairs, 577.
Nicotiana, 548.
Nightshade, pollen, 831.
Nine-bark, exfoliation, 709.
Nipa, stoma, 558.
Nitrification, 789.
Nitrogen fixation, 789, 797.
Nitrogenous food, 914.
Node, 489, 501, 660.
Non-nitrogenous food, 913.
1
Normal plants, 892, 894.
Nucellus, 905.
Nut, 919, 925.
Nutrition and leaf form, 594, 605.
Nutritive hair, 7*2; layer, 7^2, 783; root,
497, 513-
Nyctitropic movements, 579,
Nymphaea, leaf section, 561.
Nyssa, 508.
Oak, acorn, 925; cork, 705; gall, 7^2;
habit, 649; leaf variation, 602.
Obligate dwarfs, 734; epiphytes, 660;
hydrophytes, 95 1 ; mesophy tes, 95 1 ;
parasites, 762 ; saprophytes, 762; xero-
phytes, 951.
Oblique palisades, 535, 536.
Ocelli, 531.
Odor, 847.
Oenothera, capsule, 920; rosette, 714.
Offset, 672, 837.
Offspring, 805.
Oil, 522, 623, 624, 723; gland, 624.
Oleander, leaf section, 567.
Olive, sclereid, 639.
Onion, inflorescence, 874; seedling, 9.15.
Oogonium, 817.
Oospore, 817, 826.
Opening of flowers, 872.
Operculum, 814.
Opuntia, 725.
Orange, oil gland, 624.
Orchids, mycorhiza, 793, 795, 796; polli-
nation, 860; roots, 5/0, 5//T5/2T
Organization characters, 951.
Orobanche, 770. ~"^
Orthostichy, 549, 595, 642.
Orthotropic organs, 748.
Orthotropous ovules, 905.
Osmanthus, sclereid, 639.
Osmorhiza, fruit, 924.
Osmotic pressure, 493, 627.
Osmunda, 550.
Ovary, 826, 828, 854.
Ovule, 826, 905.
Own pollen, 855.
Oxalis, leaves, 579; pollen, 831.
Paeonia, flower, 826.
Palisade cells, 550, 532, 533, 534, 535, 536,
538, 539, 551, 56i, 567, 574, 630, 631,
639, 642, 724, 781; arm,. 532; oblique,
535, 536.
Palmate leaves, 676, 858.
Palmetto, habit, 645, 653.
Panax, corm, 676.
Panicle, 828, 835.
Pappus, #57, Q2i.
Paramoecium, 821.
parasitism, 752, 761, 762, 763,
764; 766, 768, 760, 770, 771, 775, 780,
786 ; alternative, 787 ; conduction, 688 ;
facultative, 762 ; obligate, 762 ; partial,
761, 771; reciprocal, 752, 786; water,
762, 771.
Parenchyma, conductive, 682 ; sheath, 683.
Parthenocarpy, 917.
Parthenogenesis, 823 ; artificial, 882.
Partial parasites, 761, 771; saprophytes,
754-
Passage cells, 683.
Passiflora; passion vine, nectaries, 858.
Pathogenic bacteria, 762.
Pea, leaves, 640; root tubercles, 788;
stipules, 641.
Peat bogs, 942.
Pedicel, 833, 874.
Peduncle, 874, 921.
Peg rhizoids, 516.
Pelargonium, glandular hairs, 623.
Pellionia, cystolith, 626.
Pendulous organs, 657, 703.
Pentarch bundles, 683.
Penthorum, rhizomes, 670.
Peony, flower, 826.
Peperomia, leaf section, 632.
Peppergrass, leaf section, 530; roots, 498;
rosettes, 582.
Perennial, 501 ; herbs, 713, 716.
Perianth, 825, 826, 827, 833, 871, QO2, 903.
Pericambium, 683.
Pericarp, 906, 915.
Periclinal chimera, 780.
Pericycle, 683, 696.
Periderm, 705, 706.
Peridium, 812.
Perigynous flowers, 828, 854.
Period, acceleration, 690; maturity, 690;
retardation, 690.
Periodicity, 735, 736, 737, 881, 885, 889,
890.
Peripheral chlorenchyma, 630, 631; water
tissue, 533, 632, 635.
Perisperm, 905, 906.
Peristome, 814.
Peronospora, haustoria, 763.
Persimmon, endosperm, 913.
Petal, 825, 826, 853.
Petalization ; petalody, 901, 902.
Petiole, 507, 521, 540, 541, 542, 579, 640,
641, 728, 833-.
Petunia, flower, 859; pollination, 841.
Phaeophyll, 522.
Phagocytosis, 798.
Phaseolus, regeneration, 748; seed section,
905; twining stem, 651.
Phelloderm, 705
Phellogen, 553, 661, 705.
Phenology, 887.
Philadelphus, flowers, 825.
Philodendron, roots, 512, 513.
Phlegethonius, 841.
Phloem, 530, 630, 682, 683, 684; second-
ary, 684; sheath, 683.
Phloeoterma, 683.
Phoenix, seedling, 935.
Phoradendron, 777.
Phosphorescence, 760.
Photeolic movements, 579.
Phototropism, 539, 540, 542, 544, 546, 547.
Phryma, flowering shoot, 875.
Phylloclade, 666.
Phyllode, 590, 640, 937.
Phyllotaxy, 510, 542, 543, 549, 595, 641,
642, 647, 714.
Physcia, 800.
Physiographic ecology, 485.
Physiological ecology, 485 ; species, 765.
Physocarpus, exfoliation, 709.
Picea, 648.
Pigments, 522, 528.
Pigweed, winged, 922.
Pilea, stoma, 559.
Pine, habit, 584; leaf section, 724; second-
ary wood, 690.
Pinguicula, 618.
Pinna, 754.
Pinnule, 636.
Pinus, habit, 584; leaf section, 724}
secondary wood, 690.
Pistil, 825, 826, 851, 853.
Pistillate flowers, 833, 834, 837, 861.
Pisum, root tubercle, 788.
Pit, 679, 680, 68 1 ; bordered, 680, 690;
stomatal, 557, 558, 567, 724.
Pitcher plant, 618.
Pith, 680, 685, 769.
Pitted vessel, 681.
Plagiotropic organs, 748.
Plane rhizoids, 516; vernation, 937.
Plank roots, 509.
Plantago; plantain, flowers, 835.
Plant associations, 939, 940, 942, 943, 944,
945-
Planting of seeds, 928.
Plastid, 522 (see also Chloroplast).
Platanus, bud protection, 640.
Plate, sieve, 681, 682.
Plicate vernation, 937.
Plowrightia, gall, 784.
Plumule, 905, 906, 931, 935.
Plurivore, 765.
Poa, leaf section, 531 ; stoma, 556.
Pocket, root, 510.
Pod, 865, 919, 920.
Poinsettia, nectary, 844.
Polarity, 749, 750; reversal, 750, 751.
INDEX
Pollen, 826, 830, 831, 834, 836, 830, 842,
850, 852; foreign, 855; impotent, 854;
own, 855 ; prepotent, 854, 855 ; sac, 830;
tube, 826, 830, 832.
Pollination, 829, 845 ; bud, 866; close, 829,
852, 863, 866, 869; contact, 862; cross,
829, 836, 852, 854, 859, 866, 867 ; gravity
863 ; insect, 838, 839, 840, 841, 842, 847,
850, 857 ; self, 829, 863 ; water, 837, 838 ;
wind, 833, 834, 835, 837, 838.
Pollinium, 830, 851.
Polyarch bundles, 683.
Polycarpic plants, 886.
Polyembryony, 820.
Polygala, 865.
Polygonatum, rhizome, 671.
Polygonum, leaf variation, 575.
Polypetalous flowers, 825, 826.
Polystichum, sporangia, 815.
Polytrichum, habit, 613; vascular bundles,
686.
Pond associations, 540, 940.
Pondweed, leaf section, 531.
Poplar; Populus, absciss layer, 583; leaf
section, 574; twigs, 736.
Porcupine grass, fruit, 929.
Pore, 612, 68 1 ; air, 557.
Portulaca, leaf section, 533.
Potamogeton, leaf section, 531.
Potato, starch, 912; tuber, 674, 720; varia-
tion, 727.
Predisposition, 768.
Prepotent pollen, 854, 855.
Pressure, osmotic, 493, 627 ; turgor, 566,
621.
Prickle, 742, 783, 932.
Prickly pear, 725.
Primary conductive tissue, 680, 682, 683,
686 ; root, 491, 496, 498, 500.
Primrose, flowers, 854; hydathode, 621;
evening, capsules, 920; rosettes, 714.
Primula, flowers, 854; hydathode, 621.
Privet, lenticel, 661.
Proboscis, 841.
Procambium, 684.
Profile position, 546, 547, 578.
Progeotropism, 499.
Progressive variability, 759.
Prohydrotropism, 499.
Pronuba, 864.
Propagation, 502, 503, 505, 805 ; root, 505.
Propagule, 805.
Prop root, 514, 515.
Propulsion, 919.
Prosenchyma, 679, 686, 696.
Proserpinaca, leaf section, 534; leaf varia-
tion, 592, 593, 595.
Prostrate plants, 647.
Protandry, 837, 840, 853.
Protection, bud, 640; flower, 857, 869, 870,
871, 872, 873; fruit, 874; leaf, 565, 566,
567, 572, 587, 588; seed, 906, 907, 908;
stem, 704.
Protective hairs, 567, 572, 573, 574, 575,
5?6, 577, 578, 623; sheath, 617, 683;
tissue, 704, 705, 709.
Protein, 521, 912, 914, 915; conduction,
694; grains, 914, 915; synthesis, 528.
Proteinoplast, 721.
Proteolytic enzym, 616.
Protogyny, 835, 836, 853.
Protonema, 807.
Prunus, gall, 784; shoot, 643.
Psedera, leaf variation, 605; leaves, 544;
pendulous stems, 657; tendrils, 65?.
Pseudoepiphyte, 659.
Pseudomixis, 821.
Pseudotsuga, 755.
Ptelea, crystals, 625; fruit, 921; vascular
bundle, 639.
Pteridophytes, asexual reproduction, $14,
£75; reproductive variation, 884 (see
also Ferns).
Puccinia, 813.
Pulvinus, 579.
Purslane, leaf section, 533.
Pycnanthemum, stem section, 702.
Pyrenoid, 522, 915.
Pyrus, spines, 739.
Quercus, acorn, 925; cork, 705; galls, 782 ;
habit, 649; leaf variation, 602.
Raceme, 828, 829, 833, 865.
Radial bundles, 683; migration, 669, ^72;
symmetry, 489, 629, 630.
Radicle, 006, 935.
Ranunculus, leaf variation, 590.
Raoulia, 738.
Raphe, 906.
Raphides, 625.
Rattan palm, 656.
Ray flower, 846; medullary, 684, 685
Reaction, 487, 951; advantageous, ^87;
hydrophyte, 951; mesophyte, 932; death, 910; dispersal, 919;
food accumulation, 911; germination,
93O, 931 ; protection, 906, 907, 908; sec-
tion, 905, 913, 915; vitality, 908, 923.
Seedless plants, reproduction, 805 (see also
Algae, Bacteria, Fungi, Lichens, Liver-
worts, Mosses, Pteridophytes).
Seedling, 491, 492, 494, 499, 506, 934, 935,
936.
Seed plants, leaf reproduction, 637; para-
sitism, 769, 775; reproductive variation,
885 ; saprophytism, 757 (see also
Dicotyls, Monocotyls, etc.).
Selaginella, chloroplasts, 521; conductive
bundle, 683; habit, 608; rhizophore,
5/2, 516.
Selection, natural, 743, 876, 952.
Self-parasitism, 786.
Self-pollination, 829, 863.
Self-pruning, 726.
Sempervivum, reproductive variation, 900 ;
stem variation, 726.
Senecio, habit, 541; hairs, 573; leaf sec-
tion, 631 ; leaves, 629.
Sensitive plant, leaves, 579.
Sepal, 825, 826, 859.
Sepalody, 902.
Separation layer, 709.
Seta, 667.
Sex determination, 895.
Sexuality, fungi, 883; origin, 881.
Sexual reproduction, 816, 817, 819.
Shade plants, chlorenchyma, 530, 531, 535.
Sheath, bundle, 530, 581, 639, 683 ; paren-
chyma, 683 ; phloem, 683 ; protective,
617, 683 ; starch, 683.
Shield-budding, 777.
Shoot-forming substances, 750.
Shrubs, 646, 650, 717.
Sieve plate, 681, 682 ; tube, 681, 694.
Signal theory, 847.
Silene, pollination, 859.
Silky hairs, 572.
Silphium, pollen, 830.
Simple leaves, 605.
Single flowers, 902.
Sinistrorse twiners, 651, 656, 768.
Sinus, leaf, 637, 639.
Sleep movements, $79.
Slime gland, 561, 623.
Slit, stomatal, 555, 556, 562, 571.
Small leaves, 550.
Smilacina, rhizome, 668.
Snow protection, 588.
Soil exhaustion, 493 ; roots, 491, 492, 496,
497, 498, 500, 501; toxins, 494.
Solanin, 626.
Solanum, pollen, 831 ; stamen, 830 ; stz rch,
912; tuber, 674, 720; variation, 727.
Solidago, galls, 784.
Solomon's seal, rhizome, 671.
Soredia, 801.
Sorus, 636, 815.
Spadix, 860.
Sparganium, 545.
Spartina, rhizomes, 669.
Spasmodic habit, 737, 887.
Spathe, 837, 838, 860, 874, 903.
Special creation, 947.
Specialization, 765.
Sperm, 816, 817.
Sphagnum, cells, 612; habit, 666.
Spherite, 914.
Spherocrystal, 913, 914.
Spike, 828, 835, 837.
Spikelet, 835.
Spikenard, wild, 546.
Spine ; spinescence, 604, 72$, 739, 740, 741,
742, 743.
Spiral thickening, 612; vessel, 680, 68 1.
Spiranthes, mycorhiza, 793.
Spirodela, 509.
Sponge tissue, 530, 533, 534, 535, 536, ;,6it
567, 574, 642, 781.
Sporangium, 755, 809, 811, 815.
Spore, asexual, 809, 810, 811, 813, 814, Hi 5.
Sporeling, 814.
Sporocarp, 816.
Sporophore, 755.
Sporophyte, 666, 814.
Springwood, 689, 690.
Spruce, 648 ; Douglas, 735.
Spur, 840, 843, 844.
Stamen, 825, 826, 828, 829, 830, 833, 835,
846, 851, 853, 854, 871, 874.
INDEX
Staminate flowers, 833, 834, 837, 038.
Starch, 522, 528. 687, 912, 015; sheath, 683.
Stellate cells, 561 ; hairs, 572.
Stem, 488, 489, 645 ; aeration, 660 ; aerial,
719, 725; air chambers, 551, 553, 703,
718; aquatic, 719; asymmetry, 734,735;
bark, 704, 708, 700; branching, 646, 647,
649; chlorophyll, 660, 66 1; climbing,
651, 652, 6 53, 654, 655, 656; conductive
tissue, 678, 679, 680, 681, 682, 683, 684,
685, 686, 687, 688, 689, 690, 692, 693;
cork, 705, 706, 707 ; correlation, 747 ;
creeping, 501, 672, 673; display, 542,
543, 546, 548, 645, 646, 648, 649, 650,
666, 667; duration, 717; dwarfing, 730,
7Ji, 732, 733, 734 ! elongation, 646, 648,
725, 726, 727, 728, 729, 736; epidermis,
704; erectness, 647 ; excretion, 722, 723,
724; fall, 713; food accumulation, 719;
habit, 584, 585, 588, 709, 71 1, 715; len-
ticels, 660, 661, 662, 663 ; mechanical
tissue, 696, 697, 699, 700, 701, 702, 703;
multicipital, 504, 676, 677; pendulous,
657; periodicity, 735, 736, 737 ; polarity,
749; protection, 704; regeneration, 748;
reproduction, 667, 668, 669, 670, 671,
672, 673, 674, 675, 676, 677, 678; running,
672, 673; section, 557, 685, 701, 702, 703,
769; spines, 739, 740, 741, 742, 743;
subterranean, 667, 668, 669, 670, 671,
674, 675, 676, 719, 744, 746, 865; syn-
thesis, 660, 665 ; twisting, 543, 647, 648,
673; variation, 375, 725, 726, 727, 731,
732, 733, 735, 736, 739. 741, 742,' water
accumulation, 718; waste accumulation,
723-
Stereid, 561, 696, 697, 700.
Stereome, 639, 682, 698, 705.
Stigeoclonium, 591.
Stigma, 825, 826, 831, 833, 834, 835, 840,
846, 851, 852, 853, 854, 857, 871.
Stinging hairs, 577.
Stipa, fruit, 929.
Stipe, 667, 812.
Stipule, 502, 575, 579, 640, 641, 643, 783.
Stock, 776, 777. 778.
Stolon, 672, 837.
Stomata, 530, 53*. 534, 535, 536, 555, 556,
558, 550, 560, 561, 562, 563, 564, 566,
567, 571, 574, 581, 620, 629, 66 1, 724;
water, 620, 621.
Stone cell, 697, 698; fruit, 919.
Stonecrop, ditch, 670.
Storage, 487 ; tracheid, 536, 631.
Strand, conductive, 680, 682, 683, 684, 686,
687, 688, 695.
Strangling fig, 5/4, 515.
Strength, columnar, 704 ; compression,
704; flexile, 702; tensile, 700, 703.
Strigula, habit, 659.
Strobilus, 733, 814, 825.
Stroma, 521, 784.
Structure, 488.
Struggle for existence, 487.
Strychnin, 626.
Style, 825, 826, 829, 831, 840, 846, 853, 854.
857.
Suberin, 706.
Submersed stems, 703.
Subsidiary cells, 555, 556, 558, 724.
Substitute hydathodes, 622.
Subterranean flowers, 864, 865 ; leaves, 642 ;
stems, 667, 668, 669, 670, 671, 674, 675,
676, 719, 744, 746, 865.
Succession, 940.
Succulerure. 533, 627, 628, 629, 630, 631,
~532To337634, 635, 747.
Sugar, 528, 914; conduction, 694.
Sundew, glandular hairs, 616, 617.
Sun plants, chlorenchyma, 535; leaves,
547-
Surplus food, 718.
Swamp plants, 506, 507, 508, 509, 941 (see
also Bog plants).
Swarm spores, 810.
Sweet cicely, fruit, 924.
Sweet pea, leaves, 640.
Switch plants, 664, 666.
Sycamore, bud protection, 640.
Symbiont, 752.
Symbiosis, 752.
Symbiotic saprophytism, 757, 798.
Symmetry, 489; dorsiventral, 489, 629.
630; radial, 489, 629, 630.
Sympetalous flowers, 829, 846, 853, 857.
85Q.
Synconium, 860, 861.
Syncyte, 679.
Synecology, 485.
Syngenesious anthers, 857.
Synthesis, carbohydrate, 525, 526, 527, 563,
588, 660, 665 ; protein, 528.
Syringa, flowers, £25.
Systrophe, 524.
Tactile spot, 653.
Tannin, 724.
Tape grass, pollination, £37.
Tap root, 496, 677.
Taraxacum, flower, #57 ; flower movements,
572; roots, 677; variation, 599.
Taxodium, 508.
Taxonomic variation, 486.
Teeth, leaf, 621.
Telentospore, 764, 813, 814.
Teleology, 947.
Temperature, 523, 527, 587.
Tendril, 640, 641, 652, 699.
INDEX
Tensile strength ; tension, 699, 700, 703.
Terminal bud, 646, 736.
Termo, 486.
Testa, 905, 907, 909, 9/5, 932.
Tetrad, 830.
Tetrarch bundles, 683.
Tetraspore, 811.
Thallophytes, absorption, 614; conductive
tissues, 685 ; form variations, 501 (see
also Algae, Bacteria, Fungi, Lichens).
Thallus, 488, 516, 557, 659, 678, 800.
Thistle, Russian, leaf section, 630.
Thorn, 730, 741, 754.
Thorn climber, 654.
Thread, infection, 787. 788.
Thuja, leaf variation, 601.
Tillandsia, absorptive scales, 615; habit,
614, 658.
Tomentum, 578.
Torsion, dehiscence by, 920.
Torus, 680.
Toxins, soil, 494.
Trachea, 679, 680, 681, 692.
Tracheid, 617, 621, 631, 639, 679, 680, 692 ;
storage, 536, 631.
Transfusion cells, 511.
Transpiration, 536, 564, 565, 566; leaf
form and, 595 ; protection from, 565, 577,
578, 588.
Transplanting and root form, 498, 501.
Transverse phototropism, 539, 540, 542,
544, 546.
Traumatism, 896.
Trees, 645, 648, 649, 650, 709, 710, 717;
deciduous, 583, 584, 649, 710; evergreen,
709, 710; leafless, 710, 711; origin, 737;
rosette, 645; stomata, 560; tropical,
709; variation, 725.
Triarch bundles, 683.
Trichome hydathode, 622.
Triticum, grain section, 915; root hairs,
492.
Tropaeolum, flower, 843; phototropism,
540; plastids, 522; pollen, 831; sto-
mata, 620.
Tropical forest, 656, 946; trees, 709.
Tsuga, dwarfed, 732.
Tube, bacterial, 787, 788; corolla, 859;
latex, 721; pollen, 826, 830, 832; sieve,
68 1, 694.
Tuber; tuberization, 671, 674, 675, 720,
744, 745, 746.
Tubercle, root, 787, 788, 700.
Tumbleweed, 922.
Tupelo, 508.
Turgor pressure, 566, 621.
Twiners, 651, 656, 768.
Twisting, stem, 543, 647, 648, 673.
Tyloscs, S59, 695.
Ulex, variation, 741.
Ulmus, 655.
Ulothrix, reproductive organs, 817.
Umbel, 676, 828, 874, 903.
Umbelliferae, geitonogamy, 862.
Underground stem, 667, 668, 669, 670, 671,
674, 6?5, 676, 719, 744, 746, 865.
Undergrowth, forest, 545, 546, 550.
Univore, 765.
Uredospore, 764, 813, 814.
Urtica, stinging hairs, 577.
Use, 948.
Usnea, 801.
Utricularia, bladders, 618, 619; winter
buds, 678.
Vaccinium, stamen, 830.
Vallisneria, pollination, 837.
Valves, 814, 920.
Variation, 486 : air spaces, 553 ; bud, 904 ;
chlorenchyma, 533, 534, 535, 538; con-
ductive tissues, 686, 689, 690, 692 ; cork,
706; ecological, 486; fortu tous, 950;
fruit, 916, 917 ; hairs, 573, 57 .\, 575, 576;
leaf form, 589, 500, 592, 59^, 594, 595,
597. 598, 599, 600, 601, 602 603, 604,
605, 606, 607, 608; mechanical tissue,
699; parasitic fungi, 765; reproductive
organs, 878, 884, 885, 893, 89!;, 900, 902,
903; rhizoids, 516, 518; root:., 505, 506,
507, 508, 513; root hairs, 494. 495, 496;
saprophytic fungi, 759; seeds, 917, 918;
stems, 725, 726, 727, 729, 730, 731, 732,
733, 735, 736, 730, 741, 744! stomata,
550. 557. 559, 561; taxonomic, 486.
Variegation, 523.
Vascular bundles, 530, 531, 533, 534, 53<$,
581, 617, 621, 680, 682, 683, 684, 686,
687, 688, 695, 701, 702, 703; elements,
679, 680, 681; plants, 609, 61,5 (see also
Dicotyls, Ferns, Monocotyls, Pterido-
phytes, Seed plants) ; tissue, 51.?, 551,
630, 638, 639, 678, 679, 680, 681, 682,
683, 684, 685, 686, 687, 688, 689, 690,
692, 693, 769.
Vaucheria, reproductive variation, 880.
Vegetative periods, 885, 800; reproduc-
tion, 505, 509, 516, 591, 636, 637, -667,
668, 669, 670, 671, 672, 673, 674, 675,
676, 677, 678, 805, 806, 807, 8i>8, 809.
Veins, 521, 638, 639.
Velamen, 511.
Venation, 638.
Ventral scale, 516, 517; wall, 555, 556, 562.
Verbascum, hairs, 572.
Verbena, hairs, 623.
Vernation, 937.
Veronica, reproductive variation, 893.
Vertical leaves, 546, 547, 549, 578.
INDEX
Vervain, hairs, 623.
Vessels, 67 Q, 680, 68 1; annular, 680, 68 1;
latex, 721; pitted, 681; reticulated,.68o ;
scalariform, 679, 680; spiral, 680, 681.
Vestibule, 555, 556, 574.
Viburnum, leaf gall, 781; shoots, 644.
Vicia, root hairs, 491.
Viola ; violet, leaf section, 530:
Virginia creeper, pendulous stem, 657.
Vitalism, 048.
Vitality, seed, go8, 923.
Vitis, gall, 576; habit, 655.
Vivipary, 933, 931.
Wall, dorsal, 555, 556, 558, 562; ventral,
555, 556, 562.
Wandering Jew, crystals, 625.
Wasp, fig, 86 1.
Waste, accumulation of, 623, 624, 625, 626,
718, 723, 724, 725.
Water, 565 ; absorption, 491, 493, 517, 518;
565, 608, 614, 615; accumulation, 627,
628, 629, 630, 631, 632, 633, 718; calyx,
845; dispersal, 922, 926; exudation, 620,
622; hyacinth, 510; leaf form and, 599;
leaves, 590, 592, 593, 595, 837; lenticels,
663 ; lily, 540, 561 ; net, 810 ; parasites,
762, 771; plants, 609, 676, 678 (see also
Hydrophytes); pollination, 837; reten-
tion, 627; roots, 509, 510, 610; roots
and, 499, 502, 503, 504, 505, 506 ; shield,
623 ; stem elongation and, 727 ; stomata,
620, 621; synthesis and, 527; tissue,
533, 629, 630, 631, 632; vascular de-
velopment and, 686.
Waterweed, vascular bundle, 687.
Wax, 568, 570; plant, 697.
Wheat, grain section, 915; root hairs, 492;
rust, 813.
Willow, epidermis, 570; habit, 584;
polarity, 749; roots, 503.
Wind dispersal, 920, 926; pollination, 833,
834, 835, 837, 838.
Winged fruits, 921.
Winter bud, 555, 572, 646, 678, 736, 936,
938.
Witches' brooms, 783.
Withered leaves, 581.
Wolrfia, 678^
Wood, autumn, 689, 690; fibers, 697;
secondary, 684, 685, 689, 690; spring,
689, 690; waste accumulation in, 725.
Woolly hairs, 572, 573, 574.
Wound cork, 707.
Xanthic colors, 845.
Xanthium, fruit, 924; fruit section, 932.
Xanthophyll, 522.
Xenogamy, 829 (see also Cross pollination).
Xerophile, 487.
Xerophytes, 486, 930, 943 ; absorption,
613; associations, 943, 944; bog, j£6,
537, 942 ; cnTorenchyma, 53 2^5-33 ," con-
genital, 9_gj ; cutinigation. 568: dwarfs.
733 ; f acloltative, 95 1 ; leaf sections, 533,
535, 567, 581, 598, 629, 630, 631, 639,
724; leaves, 520, 578; obligate, 951 ;
reaction, 951 ; foots, 505, 506^50? ; .salt
marsh, 486, 942 ; stomata, 557, 55*. 5^T ;
succulgHCe, 627, 628, 629, 630, 631, 635 ;
waicoats, 570.
Xylem, 530, 630, 680, 682, 683, 684, 685;
secondary, 684, 685.
Xylocentric bundles, 683.
Yarrow, flowers, 846.
Yucca, flowers, 871; habit, $88; pollina-
tion, 864.
Zea, prohydrotropism, 499; prop roots,
514; roots, 494.
Zebrina, crystals, 625.
Zoogloea, 787.
Zoospores, 810, 817.
Zygadenus, roots, 505.
Zygomorphy, 841, 843, 846, 853, 859.
Zygospore, 816.
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