claims our attention. The typical leaf has its upper surface built
of strong, closely placed cells, to offer a stout resistance to
rain and hail, and to frost or overpowering sun-heat. In hot, dry
weather, when great evaporation is taking place, the plant can
close up all its stomata--shut down, so to speak, all the sluices
by which the water employed to convey dissolved salts from root
to leaf is allowed to escape, and thus retain an abundant water
supply in spite of parching heat. But in arid ground, such as sandy
wastes or sea-beaches, further protection against overtranspiration
may be desirable, and this is frequently effected by impervious
varnish-like layers on the upper surface of the leaves, or by dense
coverings of hairs. Layers of impermeable corky cells in the
epidermis or skin of the leaves are also frequently to be found
in plants liable to excessive transpiration. Such impermeable
leaves are beautifully developed in plants like the stone-crops,
which, growing in dry ground and on rocks, and being liable to
long-continued drought, store up in their leaves a copious water
supply. Such reservoir-leaves are greatly developed in the plants of
desert countries. Protection against the often fatal effect of frost
is likewise afforded by a thickening of the cuticle of leaves, and
especially by felt-like coverings of hairs. In some noteworthy cases
protection against cold is effected by means of movement on the part
of the leaves. The most familiar examples occurring among our native
plants are furnished by the trifoliate leaves of many of the clover
family. As evening approaches, the clovers and their allies fold
their three leaflets together by means of an upward movement; the
juxtaposition of the leaflets retards loss of heat, and the vertical
position which they thus assume has the same effect, tending to check
the radiation of heat to the cold sky overhead. The wood sorrel,
which, though of a quite different order, has leaves which resemble
those of the clovers, effects the same object by folding its leaflets
_downward_.
Wet, which by lying on the leaves might hinder transpiration, must
also be guarded against; a danger which in many species is obviated
by means of a waxy excretion, especially on those parts of the leaves
where the stomata are situated; on which, as on an oily surface,
water will not lie.
Another danger to which plants are exposed, and one which we might
think they would be powerless to meet, is the attacks of browsing
animals--animals of all sizes, from minute insects up to great
munching cattle. But to note how perfectly such defence may be
provided for we need only look at our common gorse, which boldly
invades the pasture, protected by its impenetrable chevaux-de-frise.
This plant, indeed, seems to have put so much of its vital energy
into the production of spines that it has none left with which to
produce leaves, and the making of plant-food has to be carried on
by the green and much-branched stems. The beautiful tribe of the
thistles naturally comes to our minds in this connection. Armed with
innumerable spines of the most exquisite structure, sharper and
more delicate far than needles, the spear thistle and marsh thistle
raise their tall and graceful forms untouched amid the close-browsed
herbage, and without fear of molestation--save from man, with his
implements of iron--open their flower-heads to the sun and the
insects, and scatter their numberless winged fruits to the wind. In
the thistle the spines are borne alike on the stems, leaves, and
involucres or outer whorls of the heads of flowers. The holly is an
interesting case. In low bushes the edges of the leaves are provided
with strong spines; but when the bush grows into a tree, and bears
leaves far above the reach of browsing animals, the unnecessary
spines disappear, and the edges of the leaves are entire. In the
blackthorn and hawthorn, the strong spines are modified branches;
and we may observe that they are much more numerous in young plants
than in old bushes. A more complicated mode of protection is found
in the nettles. They are furnished with hollow hairs, filled with a
virulent fluid, and bent at the tip. A slight pressure causes the
curved extremity to break across, leaving a slender tube, tapering to
an extremely fine point, which easily enters the flesh and discharges
a portion of its venomous contents.
So far we have considered leaves as fulfilling their normal functions
of producing plant-food by means of chlorophyll cells. In conclusion,
brief reference may be made to various exceptions; for the production
of plant-food is not necessarily carried on by leaves, nor is the use
of leaves altogether limited to the production of plant-food. First,
leaves may be dispensed with, as we have already seen in the case of
the gorse. The stem may be modified to supply the place of leaves,
as in the butcher’s broom, whose flattened “leaves” are really
branches, as we see when we find flowers and fruit borne on these
flat leaf-like structures.
In climbing plants the leaves, or a portion of them, are frequently
converted into tendrils, often endowed with a marvelous sense of
touch, for grasping supports and thus aiding the plant in its upward
climb through surrounding herbage to the light. This is seen in
many of the vetches, the upper end of whose leaves are modified in
this fashion. In the yellow vetchling (Lathyrus aphaca) a further
modification has taken place. The whole leaf is converted into a
tendril, while the stipules (the usually small pair of leaf-like
appendages that often grow at the point where a leaf joins a
stem) are enlarged into a very respectable pair of “leaves,” and
manufacture food while the true leaf helps the plant to climb.
WIND-FERTILIZED FLOWERS
--ALEXANDER S. WILSON
As an agent in cross-fertilization, the wind performs an
indispensable service to many plants. Flowers which depend on its
agency for the transport of their pollen are termed anemophilous;
those adapted to insects, entomophilous. Wind-fertilized blossoms
are all of small size, obscurely colored, and, even when clustered
together in catkins, inconspicuous; hence they escape observation
more readily than their entomophilous neighbors, which are adorned
with bright colors to allure visitors. Although anemophilous flowers
do not exhibit the variety of curious contrivances found in the
entomophilous class, they yet present a number of highly interesting
characters, and are well worthy of examination. Wind-fertilization
is universal in the lower or gymnospermous division of flowering
plants, of which we have examples in the pine, larch, cedar, and
other coniferous trees. The apetalous dicotyledons or Incompletæ form
another large group in which wind-fertilization prevails extensively.
In this sub-class are included the various species of dock,
sorrel, nettle, pellitory of the wall, dog’s-mercury, goosefoot,
boxwood, hop, mulberry, elm, and catkin, bearing trees such as
the oak, hazel, beech, poplar, birch, alder, walnut, and willow,
all of which are wind-fertilized. Anemophily is not so common in
dicotyledons belonging to the sub-classes; it occurs, however,
in the ash, plantain, wormwood, mare’s-tail, and meadow-rue. The
number of wind-fertilized monocotyledons far exceeds those adapted
to insects, both as regards individuals and species. The extensive
order of grasses, the sedges, carices, and rushes, together with
the arrow-head, arrowgrass, bur-reed, and bulrush, are all without
exception anemophilous. It thus appears that wind-fertilization
occurs in many different and widely separated families. Certain
negative characters are common to all the wind-fertilized class;
no honey is secreted, no perfume emitted, and conspicuous colors
are wanting. On flowers of this description it is difficult for a
large insect like a bee to obtain a footing; there is no corolla
that can serve as a landing-stage for insects to alight. For these
reasons anemophilous blossoms are almost entirely neglected by bees
and other flower-hunting insects; only in exceptional instances
do visitors have recourse to them in search of pollen, but this
is so dry and has so little cohesion that it must be difficult
indeed for a bee to collect an appreciable quantity of anemophilous
pollen. Wind-fertilized flowers thus offer little or no attraction
to insects, and are in no way adapted to derive benefit from
their visits. On the other hand, there exists in them a number
of provisions which admirably adapt them for cross-fertilization
through atmospheric agency. The most important of these is abundant
pollen; always more than in insect-fertilized blossoms, the quantity
produced by some plants of the wind-fertilized class is enormous.
The so-called showers of sulphur, occasionally reported in the
newspapers, are really great deposits of pollen blown from the male
cone of the Scotch fir. It has been known to fall on ships at sea,
and has been swept up in bucketsful from their decks. The common
ash discharges an immense quantity from its innumerable flowers, so
much so that a person shaking a branch when the tree is in bloom is
dusted from head to foot with the dry, powdery pollen. That of the
elm is also very abundant, and this is more or less characteristic
of all plants which depend for cross-fertilization on the wind. At
certain seasons, the air may be said to be literally charged with the
pollen of anemophilous plants. In the beginning of May, I exposed on
the window-sill for forty-eight hours a microscopic slide smeared
with syrup, and on examining it afterward detected upward of fifty
pollen-grains belonging to various trees, some of which are not to be
found within a radius of two miles. The efficiency of the wind as a
fertilizing agent is, therefore, much greater than one might suppose.
The pollen grains of insect-fertilized flowers are frequently, as
in the harebell, colt’s-foot, and mallow, studded over with little
projecting points; these cause them to adhere readily to each
other or to the hairs of an insect. In other cases the pollen is
viscid, and the granules are difficult to separate. This cohesive
character obviously renders them ill-adapted for transference by
means of the wind; accordingly, the pollen of wind-fertilized
plants is excessively light and dry, the granules are smooth,
they do not stick together, and this incoherence facilitates their
wide dispersion. A special provision exists in the pine, whereby
its pollen is rendered lighter and more easily wafted by the wind;
the extine or outer membrane of each granule is inflated into two
globular air-sacs, which reduce its specific gravity so that it can
keep longer afloat in the air.
Although there are wind-fertilized species to be found in bloom all
the year round, a large number, especially of trees, blossom early
in the season; the hazel comes into bloom in February, the elm,
poplar, and willow following in March or April. The little flowers of
the willow are already developed within the bud at the beginning of
winter; in spring they merely expand. It is, therefore, probable that
trees of this class originally flowered toward the end of the year,
but ultimately became so belated that the opening of their flowers
had to be delayed over winter. During the dry, windy days of spring,
when the farmer sows his seed-corn, the flowers of our anemophilous
trees are in perfection. At this early period, when so few insects
are abroad, these unattractive blossoms are not likely to be visited.
A marked peculiarity of anemophilous trees is the appearance of the
flowers before the foliage; the blossoms of the elm, poplar, ash,
and willow, for example, are put forth while as yet the branches
are entirely leafless. This arrangement is clearly advantageous;
the foliage would protect the flowers from the wind, preventing its
gaining access to the stigmas and interfering with the removal of the
pollen.
The fir does not shed its leaves in autumn, as deciduous trees do,
but its needle-like foliage interferes as little as possible in the
way indicated; nevertheless, the male and female cones are developed
on the branches of the fir in the most exposed positions. A good
illustration of the manner in which wind-fertilized plants secure the
exposure of their blossoms is seen in the dog’s-mercury (Mercurialis
perennis). This plant, common in most districts, has rather large
leaves; they expand before the flowers, and would be a great
hindrance to wind-fertilization were it not that the little staminate
flowers are elevated on long, slender stalks which spring from the
axils of the leaves and entirely overtop the foliage. The male catkin
of the oak is an inflorescence of the same description, not erect,
however, but pendulous, and so flexible that it swings freely in the
lightest breeze. After the flowering period, the ground under the
oak, poplar, and other trees is strewn with their male catkins; these
are caducous, falling off soon after they have shed their pollen; the
catkins of female flowers are necessarily persistent, though a few
may occasionally be broken off by the violence of the wind.
In reeds and grasses, the entire plant, being flexible, is easily
shaken by the wind, and the ripe pollen is readily dislodged from the
anthers; but where the stem is more rigid either the flower stalks
are slender or the stamens have thin, thread-like filaments; or the
entire inflorescence is mobile; in any case provision is made in the
structure of the flower for the agitation of the anthers by the wind.
Slender flower stalks are seen in the dock and in the quaking grass
(Briza). The ribwort plantain (Plantago lanceolata) and a great many
grasses have their anthers borne on long, excessively thin stalks, so
that they quiver in the slightest breeze. Broad and leaf-shaped, the
anther itself in plantago is clearly adapted, like the seed-vessels
of some crucifers, to be set in motion by the wind. On a calm and
warm day in summer the gentlest touch is sufficient to make many
grasses, such as the foxtail, cock’s-foot or timothy, emit a little
cloud of pollen. Some grasses even appear to eject the pollen with
force either by the explosion of the pollen-sacs or by a sudden
jerking of the stamens. The nettle and pellitory have each four
elastic stamens; when the flower opens, these are bent inward toward
the centre in a constrained position; later on the tension is removed
and the liberated stamens suddenly straighten out, scattering their
pollen like little puffs of smoke. The object of this liliputian
artillery is to throw the pollen away quite clear of the plant by
which it was produced.
Petals in ordinary flowers are intended to secure the attention
of insects; to wind-fertilized blossoms, having no occasion for
visitors, they are unnecessary. So far from an advantage, the
presence of a corolla would exclude the wind from the essential
organs. Accordingly, petals are either absent altogether or reduced
to rudimentary proportions. The calyx is also much reduced, and
in some flowers is dispensed with entirely. Comparatively few
anemophilous flowers possess both sets of floral envelopes.
Plantago is, however, dichlamydeous, but its chaffy petals afford
incontrovertible evidence of degeneration from the entomophilous
condition.
The stigma in the wind-fertilized class is highly specialized, and
much larger relatively to the other parts of the flower than is
the case with entomophilous blossoms. It is commonly penicillate,
consisting of a tuft of hairs, as in nettle; feathery, as in grasses;
or elongated and thread-like, as in plantago and the rushes. The
spirally twisted stigmas of the last-mentioned flowers are beautiful
objects when examined with a pocket lens. The larger the surface
which the stigma presents to the wind, the greater are the chances
of pollination. Its fine fringes of papillose hairs are also well
calculated to entangle the pollen-grains, while the viscid secretion
serves to retain them when caught. This adaptation may be seen in the
common rye grass; each tiny blossom as it expands hangs out its two
white, feathery stigmas from the sides of the spikelet, reminding
one of a fisherman spreading out his nets, or a sailor his studding
sails to catch the favoring breeze. At the time of fertilization the
dock, too, thrusts out its three little brush-like stigmas between
the lobes of the perianth. It is instructive to compare these
wind-fertilized flowers of Rumex with those of the nearly allied
genus Polygonum, which is entomophilous. The perianth of the latter
is rose-colored; the stigmas are included within it, never exserted
as in the dock--they are not at all brush-like or feathery, but in
the form of little knobs; the stamens and flower-stalks are rigid;
moreover, the various species of Polygonum secrete nectar and are
frequented by many different insects. Stigmas are entirely absent in
the gymnospermous division, but in most Coniferæ the ovule at the
time of flowering secretes a drop of liquid, and the pollen-grains
caught on it are, as the fluid gradually evaporates, stranded on
the nucleus of the ovule. The ovule of the larch is provided with
elongated papillæ, functionally equivalent to a stigma.
A flower is said to be hermaphrodite or monoclinous when, as in
the elm, both stamens and pistils are present in the same blossom.
With insect-fertilized flowers this is mostly the case, though
there are some exceptions, such as the cucumber and begonia, which
are unisexual or diclinous, stamens and pistils being produced in
separate blossoms. The diclinous condition is exceedingly common
in the wind-fertilized class. The staminate or male, and the
pistillate or female, flowers are sometimes found growing on the
same individual plant, which is then termed monœcious, as in the
oak, hazel, birch, pine, etc. The poplar, willow, yew, juniper,
nettle, and dog’s-mercury, on the other hand, are diœcious; their
staminate and pistillate flowers grow on separate plants. This
separation of the sexes renders self-fertilization impossible, and
secures whatever benefit may arise from the physiological division
of labor. Anemophilous species in general show a marked tendency in
the direction of separation. Self-fertilization may be prevented
in monoclinous flowers by the stamens and stigmas maturing at
different times. This arrangement, known as dichogamy, occurs in
both insect and wind-fertilized blossoms, but while the former
usually have the stamens in advance of the stigmas, in the latter
the reverse order is much more frequent. There are thus two kinds of
dichogamy--protandrous, when the stamens are in advance; protogynous,
if the pistils are first developed. Protogyny is characteristic of
wind-fertilized flowers, and may be easily observed in the rush
and plantain. In the first or female stage of the flower of the
rush, the thread-like stigma protrudes from the top of the still
unopened perianth, while the stamens, as yet immature, are completely
concealed. In the second stage, the pollinated stigmas have begun to
shrivel, the perianth has now spread out, disclosing the six stamens
which are ready to discharge their pollen. The same two stages are
equally apparent in plantago. All our readers must be familiar
with the black heads of this plant, which are to be seen in every
pasture, bending and waving in the wind. In the first stage, the
head appears black, but on looking into it we see projecting from
each little unopened floret a white thread-like stigma. Later on,
the lower part of the spike or head is seen to be encircled by a
wreath of tiny white bodies, and closer inspection shows that these
are the stamens, four of which project like little banners from
each of the newly opened florets. The protogynous character belongs
in the bur-reed to the plant itself rather than the individual
flowers. Its pistillate flowers, which are lowermost, expand first;
only when their stigmas have withered do the male florets higher up
begin discharging their pollen. In this case, it is evident that
the flowers on any plant must be fertilized with pollen from another
in more advanced condition. A social habit is highly characteristic
of wind-fertilized plants--pines, grasses, sedges, nettles, etc.,
usually grow together in considerable numbers. Entomophilous plants
have a much more sporadic character, and admit of a greater degree
of isolation; their guests, doubtless, maintain the necessary
communication between members of the species. This social habit
partly explains the tendency toward the diœcious condition, for a
complete separation of the sexes is hardly possible, except in plants
of social habit. From the gymnosperms, the oldest flowering plants,
being all wind-fertilized, it has been inferred that such must also
have been the case with the primitive angiosperms. It is not certain,
however, that any of their representatives remain, for many of our
existing wind-fertilized flowers appear to be merely degraded forms.
Anemophilous species appear in families, the rest of which are highly
specialized in relation to insects. Some species of plantago are
adapted to insects; others, as we have seen, to the wind. Most of
the sub-classes with incomplete flowers, from which so many of our
examples are taken, also exhibit striking marks of degeneration,
and the same may be said of the grasses and other anemophilous
monocotyledons. We also find some flowers in an intermediate
condition, such as the vine and certain willows, which secrete honey
and are visited by insects. Facts of this description are held by
some to show that all existing anemophilous species, with the
exception of the gymnosperms, are descended from bright-colored,
insect-fertilized ancestors.
Wind-fertilization has, in some instances, been rendered highly
efficient, but in any case it is far from economical, for the vast
amount of pollen miscarried represents an enormous loss to plants;
neither does this method admit of the same certainty and precision as
the other. A wind-fertilized bears to an insect-fertilized blossom
very much the relation which an æolian harp bears to a pianoforte.
MOVEMENTS OF PLANTS
--DAVID ROBERTSON
Scarcely any one can have failed to notice that many plants close
their flowers when evening approaches, others again at various
periods of the day, while some close their flowers when the sky is
overcast; foliage leaves also are in many cases subject to periodic
movements.
The movements of different plants are dependent on various causes.
Some of these movements are solely mechanical, and caused by the
tissues being affected, owing to the condition of the surrounding air
and to varying states of turgidity and exhaustion.
Other movements are apparently due to physical causes, but can not be
fully explained by attributing them to these causes.
Movements in plants also depend upon the contractile quality of the
protoplasm in the cells, and on the passage of the protoplasm from
cell to cell. The property of the protoplasm gives rise to movements
caused by the plant itself, which are not at least directly due to
any external exciting cause. These movements can be compared with the
movements of the lower animals, and to the ciliary motion found in
certain tissues belonging to the most highly organized animals.
The periodic movements, such as the “waking” and “sleeping” condition
of leaves, the closing of flowers, etc., are manifested only when the
organs are fully matured, and when the peculiarity of their internal
structure which gives rise to the phenomena of periodic movements is
fully developed.
These movements are to be carefully distinguished from those due to
unequal growth, such as movements of nutation. In this case there is
no special structure upon which the movements depend.
The bursting of seed-vessels, anthers, etc., is due partly to the
fact that the condition of the tissues, as regards the amount of
liquid they contain from their possessing unequal power of imbibing
moisture, is not equally elastic. For this reason, when the less
elastic portions of tissue are subjected to strain they are torn
apart or bent in various ways, owing to unequal contractions and
expansions, caused by an access or withdrawal of moisture.
These cases can scarcely be regarded as vital phenomena, but should
rather come under the category of what is in ordinary language
named “warping.” They are simply caused by particular modes of the
destruction of dead tissue due to conditions brought about by
variations in the structure of the tissues in question.
Movements in plants which take place periodically, such as sleeping
and waking, or those movements that take place when they are touched
or otherwise affected by certain kinds of exciting stimulus, can
not be attributed to mechanical causes. The slightest mechanical
stimulus on the sensitive plant Mimosa pudica causes the leaflets to
fold together. Such movements are not proportional to the external
stimulus, but depend on the internal structure of the plant.
To this class of movements have been added the very remarkable
movements which give rise to the twining condition of certain stems.