THE GENERAL SHAPES OF PLANTS.
§ 217. Among protophytes those exemplified by _Pleurococcus vulgaris_
are by general consent considered the simplest. As shown in Fig.
1, they are globular cells presenting no obvious differentiation
save that between inner and outer parts. Their uniformity of figure
co-exists with a mode of life involving the uniform exposure of all
their sides to incident forces. For though each individual may have
its external parts differently related to environing agencies, yet the
new individuals produced by spontaneous fission, whether they part
company or whether they form clusters and are made polyhedral by mutual
pressure, have no means of maintaining parallel relations of position
among their parts. On the contrary, the indefiniteness of the attitudes
into which successive generations fall, must prevent the rise of any
unlikeness between one portion of the surface and another. Spherical
symmetry continues because, on the average of cases, incident forces
are equal in all directions.
[Illustration: Figs. 1, 2, 3.]
Other orders of _Protophyta_ have much more special forms, along with
much more special attitudes: their homologous parts maintaining, from
generation to generation, unlike relations to incident forces. The
_Desmidiaceæ_ and _Diatomaceæ_, of which Figs. 2 and 3 show examples,
severally include genera characterized by triple bilateral symmetry.
A _Navicula_ is divisible into corresponding halves by a transverse
plane and by two longitudinal planes--one cutting its valves at right
angles and the other passing between its valves. The like is true
of those numerous transversely-constricted forms of _Desmidiaceæ_,
exemplified by the second of the individuals represented in Fig.
2. If now we ask how a _Navicula_ is related to its environment,
we see that its mode of life exposes it to three different sets of
forces: each set being resolvable into two equal and opposite sets.
A _Navicula_ moves in the direction of its length, with either
end foremost. Hence, on the average, its ends are subject to like
actions from the agencies to which its motions subject it. Further,
either end while moving exposes its right and left sides to amounts
of influence which in the long run must be equal. If, then, the two
ends are not only like one another, but have corresponding right and
left sides, the symmetrical distribution of parts answers to the
symmetrical distribution of forces. Passing to the two edges and the
two flat surfaces, we similarly find a clue to their likenesses and
differences in their respective relations to the things around them.
These locomotive protophytes move through the entangled masses of
fragments and fibres produced by decaying organisms and confervoid
growths. The interstices in such matted accumulations are nearly all
of them much longer in one dimension than in the rest--form crevices
rather than regular meshes. Hence, a small organism will have much
greater facility of insinuating itself through this _débris_, in which
it finds nutriment, if its transverse section is flattened instead of
square or circular. And while we see how, by survival of the fittest, a
flattened form is likely to be acquired by diatoms having this habit;
we also see that likeness will be maintained between the two flat
surfaces and between the two edges. For, on the average, the relations
of the two flat surfaces to the sides of the openings through which
the diatom passes, will be alike; and so, too, on the average, will be
the relations of the two edges. In desmids of the type exemplified by
the second individual in Fig. 2, a kindred equalization of dimensions
is otherwise insured. There is nothing to keep one of the two surfaces
uppermost rather than the other; and hence, in the long succession
of individuals, the two surfaces are sure to be similarly exposed to
light and agencies in general. When to this is added the fact that
spontaneous fission occurs transversely in a constant way, it becomes
manifest that the two ends, while they are maintained in conditions
like one another, are maintained in conditions unlike those of the
two edges. Here then, as before, triple bilateral symmetry in form,
co-exists with a triple bilateral symmetry in the average distribution
of actions.
[Illustration: Figs. 4, 5, 6.]
Still confining our attention to aggregates of the first order, let us
next note what results when the two ends are permanently subject to
different conditions. The fixed unicellular plants, of which examples
are given in Figs. 4, 5, and 6, severally illustrate the contrast
in shape arising between the part that is applied to the supporting
surface and the part that extends into the surrounding medium. These
two parts which are the most unlike in their relations to incident
forces, are the most unlike in the forms. Observe, next, that the part
which lifts itself into the water or air, is more or less decidedly
radial. Each outward-growing tubule of _Codium adhærens_, Fig. 4, has
its parts disposed with some regularity around its axis; the upper stem
and spore-vessel of _Botrydium_, Fig. 5, display a lateral growth
that is approximately equal in every direction; and the stems of the
_Mucor_, Fig. 6, shoot up with an approach to evenness on all sides.
Plants of this low type are naturally very variable in their modes of
growth: each individual being greatly modified in form by its special
circumstances. But they nevertheless show us a general likeness between
parts exposed to like forces, as well as a general unlikeness between
parts exposed to unlike forces.
Respecting the forms of these aggregates of the first order, it has
only to be added that they are asymmetrical where there is total
irregularity in the incidence of forces. We have an example in the
indefinitely contorted and branched shape of a fungus-cell, growing as
a mycelium among the particles of soil or through the interstices of
organic tissue.
§ 218. Re-illustrations of the general truths which the forms of
these vegetal aggregates of the first order display, are furnished by
vegetal aggregates of the second order. The equalities and inequalities
of growth in different directions, prove to be similarly related to
the equalities and inequalities of environing actions in different
directions.
Of spherical symmetry an instance occurs in _Eudorina elegans_.
The ciliated cells are here so united as to produce a small,
mulberry-shaped, hollow ball which, being similarly conditioned on all
sides, shows no unlikenesses of structure. An allied form, however,
_Volvox globator_, presents a highly instructive, though very trifling,
modification. It is not absolutely homogeneous in its structure and
is not absolutely homogeneous in its motions. The waving cilia of
its component cells have fallen into such slight heterogeneities of
action as to cause rotation in a constant direction; and along with a
fixed axis of rotation there has arisen a fixed axis of progression. A
concomitant fact is that the cells of the colony exhibit an appreciable
differentiation in relation to the fixed axis. There is an incipient
divergence from spherical uniformity along with this slight divergence
from uniformity of conditions.
Vegetal aggregates of the second order are usually fixed: locomotion
is exceptional. Fixity implies that the surface of attachment is
differently circumstanced from the free surface. Hence we may expect
to find, as we do find, that among these rooted aggregates of the
second order, as among those of the first order, the primary contrast
of shape is between the adherent part and the loose part. Sea-weeds
variously exemplify this. In some the fronds are very irregular and
in some tolerably regular; in some the form is pseudo-foliar and in
some pseud-axial; but differing though they do in these respects,
they agree in having the end which is attached to a solid body unlike
the other end. The same truth is seen in such secondary aggregates
as the common Agarics, or rather in their immensely-developed organs
of fructification. A puff-ball, Fig. 192, presents no other obvious
unlikeness of parts than that between its under and upper surfaces. So
too with the stalked kinds that frequent our woods and pastures. In the
types which Figs. 193, 194, 195, delineate, the unlikenesses between
the rooted ends and the expanded ends, as well as between the under
and upper surfaces of the expanded ends, are obviously related to this
fundamental contrast of conditions. Nor is this relation less clearly
displayed in the sessile fungi which grow out from the sides of trees,
as shown at _a_, _b_, Fig. 196. That which is common to this and the
preceding types, is the contrast between the attached end and the free
end.
[Illustration: Figs. 192–196.]
From what these forms have in common, let us turn to that which they
have not in common, and observe the causes of the want of community.
A puff-ball shows us in the simplest way, the likeness of parts
accompanying likeness of conditions, along with the unlikeness of parts
accompanying unlikeness of conditions. For while, if we cut vertically
through its centre, we find a difference between top and bottom, if
we cut horizontally through its centre, we find no differences among
its several sides. Being, on the average of cases, similarly related
to the environment all round, it remains the same all round. The
radial symmetry of the mushroom and other vertically-growing fungi,
illustrates this connexion of cause and effect still better. But now
mark what happens in the group of _Agaricus noli-tangere_, shown in
Fig. 195. Radially-symmetrical as is the type, and radially symmetrical
as are those centrally-placed individuals which are equally crowded
all round, we see that the peripheral individuals, dissimilarly
circumstanced on their outer sides and on their sides next the group,
have partially changed their radial symmetry into bilateral symmetry.
It is no longer possible to make two corresponding halves by _any_
vertical plane cutting down through the pileus and the stem; but there
is only _one_ vertical plane that will thus produce corresponding
halves--the plane on the opposite sides of which the relations to the
environment are alike. And then mark that the divergence from all-sided
symmetry towards two-sided symmetry, here caused in the individual by
special circumstances, is characteristic of the race where the habits
of the race constantly involve two-sidedness of conditions. Besides
being exemplified by such comparatively undifferentiated types as
certain _Polypori_, Fig. 196, _a_, _b_, this truth is exemplified by
members of the genus just named. In _Agaricus horizontalis_, Fig. 196,
_c_, we have a departure from radial symmetry that is conspicuous only
in the form of the stem. A more decided bilateralness exists in _A.
subpalmatus_, shown in elevation at _d_ and in section at _d′ _. And
_Lentinus flabelliformis_, of which _e_ and _e′_ are different views,
exhibits complete bilateralness--a bilateralness in which there is the
greatest likeness of the parts that are most similarly conditioned,
and the greatest unlikeness of the parts that are most dissimilarly
conditioned.
Among plants of the second order of composition, it will suffice
to note one further class of facts which are the converse of the
foregoing and have the same implications. These are the facts showing
that along with habitual irregularity in the relations to external
forces, there is habitual irregularity in the mode of growth. Besides
finding such facts among Thallophytes, as in the tubers of underground
fungi and in the creeping films of sessile lichens, which severally
show us variations of proportions obviously caused by variations in
the amounts of the influences on their different sides, we also,
among Archegoniates of inferior types, find irregularities of form
along with irregularities in environing actions. The fronds of the
_Marchantiaceæ_ or such _Jungermanniaceæ_ as are shown in
Figs. 41, 42, 43, illustrate the way in which each lowly-organized
aggregate of the second order, not individuated by the mutual
dependence of its parts, has its form determined by the balance of
facilities and resistances which each side of the frond meets with as
it spreads.
§ 219. Among plants displaying integration of the third degree, and
among plants still further compounded, these same truths are equally
manifest. In the forms of such plants we see primary contrasts and
secondary contrasts which, no less clearly than the foregoing, are
related to contrasts of conditions.
That flowering plants from the daisy up to the oak, have in common the
fundamental unlikeness between the upward growing part and the downward
growing part; and that this most marked unlikeness corresponds with the
most marked unlikeness between the two parts of their environment, soil
and air; are facts too conspicuous to be named were they not important
items in the argument. More instructive perhaps, because less familiar,
is the fact that we miss this extreme contrast in flowering plants
which have not their higher and lower portions exposed to conditions
thus extremely contrasted. A parasite like the Dodder, growing in
entangled masses upon other plants, from which it sucks the juices, is
not thus divisible into two strongly-distinguished halves.
Leaving out of consideration the difference between the supporting
part and the supported part in phænogams, and looking at the supported
part only, we observe between its form and the habitual incidence of
forces, a relation like that which we observed in the simpler plants.
Phænogams that are practically if not literally uniaxial, and those
which develop their lateral axes only in the shape of axillary flowers,
when uninterfered with commonly send up vertical stems round which the
leaves and flowers are disposed with a more or less decided radial
symmetry. Gardens and fields supply us with such instances as the Tulip
and the Orchis; and, on a larger scale, the Palms and the Aloes are
fertile in examples. The exceptions, too, are instructive. Besides the
individual divergences arising from special interferences, there are
to be traced general divergences where the habits of the plants expose
them to general interferences in anything approaching to constant ways.
Plants which, like the Foxglove, have spikes of flowers that are borne
on flexible foot-stalks, have their flowers habitually bent round to
one face of the stem: an unlikeness of distribution probably caused
by unlikeness in the relation to the Sun’s rays. The wild Hyacinth,
too, with stem so flexible that its upper part droops, shows us how a
consequent difference in the action of gravity on the flowers, causes
them to deviate from their typically-radial arrangement towards a
bilateral arrangement.
[Illustration: Figs. 197–199.]
Much more conspicuous are these general and special relations of form
to general and special actions in the environment, among phænogams
that are multiaxial. That when standing alone, and in places where
the winds do not injure them nor adjacent things shade them, shrubs
and trees develop with tolerable evenness on all sides, is an obvious
truth. Equally obvious is the truth that, when growing together in
a wood, and mutually interfered with on all sides, trees still show
obscurely radial distributions of parts; though, under such conditions,
they have tall taper stems with branches directed upwards--a difference
of shape clearly due to the different incidence of forces. And almost
equally obvious is the truth, that a tree of this same kind growing
at the edge of the wood, has its outer branches well developed and
its inner branches comparatively ill-developed. Fig. 197, which
inaccurately represents this difference, will serve to make it manifest
that while one of the peripheral trees can be cut into something like
two similar halves by a vertical plane directed towards the centre of
the wood--a plane on each side of which the conditions are alike--it
cannot be cut into similar halves by any other plane. A like divergence
from an indefinitely-radial symmetry towards an indefinitely-bilateral
symmetry, occurs in trees that have their conditions made bilateral
by growing on inclined surfaces. Two of the common forms observable
in such cases are given in Fig. 198. Here there is divisibility into
parts that are tolerably similar, by a vertical plane running directly
down the hill; but not by any other plane. Then, further, there is the
bilateralness, similar in general meaning though differently caused,
often seen in trees exposed to strong prevailing winds. Almost every
sea-coast has abundant examples of stunted trees which, like the one
shown in Fig. 199, have been made to deviate from their ordinary equal
growth on all sides of a vertical axis, to a growth that is equal only
on the opposite sides of a vertical plane directed towards the wind’s
eye.
From among vegetal aggregates of the third order, we have now only to
add examples of the entirely asymmetrical form which accompanies an
entirely irregular distribution of incident forces. Creeping plants
furnish such examples. They show, both when climbing up vertical or
inclined surfaces and when trailing on the ground, that their branches
grow hither and thither as the balance of forces aids or opposes;
and the general outline is without symmetry of any kind, because the
environing influences have no kind of regularity in their arrangement.
§ 220. Along with some unfamiliar facts, I have here set down facts
which are so familiar as to seem scarcely worth noting. It is because
these facts have become meaningless to perceptions deadened by infinite
repetitions of them, that it is needful here to point out their
meanings. Not alone for its intrinsic importance has the unlikeness
between the attached ends and the free ends been traced among plants
of all degrees of integration. Nor is it simply because of the
significance they have in themselves, that instances have been given
of those varieties of symmetry and asymmetry which the free ends of
plants equally display: be they plants of the first, second, third,
or any higher order. Neither has the only other purpose been that of
showing how, in the radial symmetry of some vegetal aggregates and
the single bilateral symmetry of others, there are traceable the same
ultimate principles as in the spherical symmetry and triple bilateral
symmetry of certain minute plants first described. But the main object
has been to present, under their simplest aspects, those general laws
of morphological differentiation which are fulfilled by the component
parts of each plant.
If organic form is determined by the distribution of forces, and the
approach in every case towards an equilibrium of inner actions with
outer actions; then this relation between forms and forces must hold
alike in the organism as a whole in its proximate units, and in its
units of lower orders. Formulas which express the shapes of entire
plants in terms of surrounding conditions, must be formulas which also
express the shapes of their several parts in terms of surrounding
conditions. If, therefore, we find that a plant as a whole is radially
symmetrical or bilaterally symmetrical or asymmetrical, according as
the incident forces affect it equally on all sides of an axis, or
affect it equally only on the opposite sides of one plane, or affect
it equally in no two directions; then, we may expect that, in like
manner, each member of a plant will display radial symmetry where
environing influences are alike along many radii, bilateral symmetry
where there is bilateralness of environing influences, and unsymmetry
or asymmetry where there is partial or entire departure from a balance
of surrounding actions.
To show that this expectation is borne out by the facts, will be the
object of the following four chapters. Let us begin with the largest
parts into which plants are divisible; and proceed to the successively
smaller parts.