MORPHOLOGICAL DIFFERENTIATION IN PLANTS.
§ 212. While, in the course of their evolution, plants and animals
have displayed progressive integrations, there have at the same time
gone on progressive differentiations of the resulting aggregates,
both as wholes and in their parts. These differentiations and the
interpretations of them, form the second class of morphological
problems.
We commence as before with plants. We have to consider, first, the
several kinds of modification in shape they have undergone; and,
second, the relations between these kinds of modification and their
factors. Let us glance at the leading questions that have to be
answered.
§ 213. Irrespective of their degrees of composition, plants may, and
do, become changed in their general forms. Are their changes capable of
being formulated? The inquiry which meets us at the outset is--does a
plant’s shape admit of being expressed in any universal terms?--terms
that remain the same for all genera, orders, and classes.
After plants considered as wholes, have to be considered their
proximate components, which vary with their degrees of composition,
and in the highest plants are what we call branches. Is there any law
traceable among the contrasted shapes of different branches in the same
plant? Do the relative developments of parts in the same branch conform
to any law? And are these laws, if they exist, allied with one another
and with that to which the shape of the whole plant conforms?
Descending to the components of these components, which in developed
plants we distinguish as leaves, there meet us kindred questions
respecting their relative sizes, their relative shapes, and their
shapes as compared with those of foliar organs in general. Of their
morphological differentiations, also, it has to be asked whether they
exemplify any truth that is exemplified by the entire plant and by its
larger parts.
Then, a step lower, we come down to those morphological units of
which leaves and fronds consist; and concerning these arise parallel
inquiries touching their divergences from one another and from cells in
general.
The problems thus put together in several groups cannot of course be
rigorously separated. Evolution presupposes transitions which make all
such classings more or less conventional; and adherence to them must be
subordinate to the needs of the occasion.
§ 214. In studying the causes of the morphological differentiations
thus divided out and prospectively generalized, we shall have to bear
in mind several orders of forces which it will be well briefly to
specify.
Growth tends inevitably to initiate changes in the shape of any
aggregate, by altering both the amounts of the incident forces and
the forces which the parts exert on one another. With the mechanical
actions this is obvious. Matter that is sensibly plastic cannot be
increased in mass without undergoing a change in its proportions,
consequent on the diminished ratio of its cohesive force to the force
of gravitation. With the physiological actions it is equally obvious.
Increase of size, other things equal, alters the relations of the parts
to the material and dynamical factors of nutrition; and by so affecting
differently the nutrition of different parts, initiates further changes
of proportions.
In plants of the third order it is thus with the proximate components:
they are subject to mutual influences that are unlike one another and
are continually changing. The earlier-formed units become mechanical
supporters of the later-formed units, and so experience modifying
forces from which the later-formed units are exempt. Further, these
elder units simultaneously begin to serve as channels through which
materials are carried to and from the younger units--another cause
of differentiation that goes on increasing in intensity. Once more,
there arise ever-strengthening contrasts between the amounts of light
which fall upon the youngest or outermost units and the eldest or
innermost units; whence result structural contrasts of yet another
kind. Evidently, then, along with the progressive integration of cells
into fronds, of fronds into axes, and of axes into plants still more
composite, there come into play sundry causes of differentiation which
act on the whole and on each of its parts, whatever their grade. The
forces to be overcome, the forces to be utilized, and the matters to
be appropriated, do not remain the same in their proportions and modes
of action for any two members of the aggregate: be they members of the
first, second, third, or any other order.
§ 215. Nor are these the only kinds and causes of heterogeneity which
we have to consider. Beyond the more general changes produced in the
relative sizes and shapes of plants and their parts by progressive
aggregation, there are the more particular changes determined by the
more particular conditions.
Plants as wholes assume unlike attitudes towards their environments;
they have many ways of articulating their parts with one another; they
have many ways of adjusting their parts towards surrounding agencies.
These are causes of special differentiations additional to those
general differentiations that result from increase of mass and increase
of composition. In each part considered individually, there arises
a characteristic shape consequent on that relative position towards
external and internal forces, which the mode of growth entails. Every
member of the aggregate presents itself in a more or less peculiar way
towards the light, towards the air, and towards its point of support;
and according to the relative homogeneity or heterogeneity in the
incidence of the agencies thus brought to bear on it, will be the
relative homogeneity or heterogeneity of its shape.
§ 216. Before passing from this _à priori_ view of the morphological
differentiations which necessarily accompany morphological
integrations, to an _à posteriori_ view of them, it seems needful to
specify the meanings of certain descriptive terms we shall have to
employ.
Taking for our broadest division among forms, the regular and the
irregular, we may divide the latter into those which are wholly
irregular and those which, being but partially irregular, suggest
some regular form to which they approach. By slightly straining the
difference between them, two current words may be conveniently used
to describe these subdivisions. The entirely irregular forms we may
class as _asymmetrical_--literally as forms without any equalities of
dimensions. The forms which approximate towards regularity without
reaching it, we may distinguish as _unsymmetrical_: a word which,
though it asserts inequality of dimensions, has been associated by
use rather with such slight inequality as constitutes an observable
departure from equality.
Of the regular forms there are several classes, differing in the number
of directions in which equality of dimensions is repeated. Hence
results the need for names by which symmetry of several kinds may be
expressed.
The most regular of figures is the sphere: its dimensions are the same
from centre to surface in all directions; and if cut by any plane
through the centre, the separated parts are equal and similar. This is
a kind of symmetry which stands alone, and will be hereafter spoken of
as _spherical symmetry_.
When a sphere passes into a spheroid, either prolate or oblate, there
remains but one set of planes that will divide it into halves, which
are in all respects alike; namely, the planes in which its axis lies,
or which have its axis for their line of intersection. Prolate and
oblate spheroids may severally pass into various forms without losing
this property. The prolate spheroid may become egg-shaped or pyriform,
and it will still continue capable of being divided into two equal
and similar parts by any plane cutting it down its axis; nor will the
making of constrictions deprive it of this property. Similarly with the
oblate spheroid. The transition from a slight oblateness, like that of
an orange, to an oblateness reducing it nearly to a flat disc, does
not alter its divisibility into like halves by every plane passing
through its axis. And clearly the moulding of any such flattened oblate
spheroid into the shape of a plate, leaves it as before, symmetrically
divisible by all planes at right angles to its surface and passing
through its centre. This species of symmetry is called _radial
symmetry_. It is familiarly exemplified in such flowers as the daisy,
the tulip, and the dahlia.
From spherical symmetry, in which we have an infinite number of axes
through each of which may pass an infinite number of planes severally
dividing the aggregate into equal and similar parts; and from radial
symmetry, in which we have a single axis through which may pass an
infinite number of planes severally dividing the aggregate into equal
and similar parts; we now turn to _bilateral symmetry_, in which the
divisibility into equal and similar parts becomes much restricted.
Noting, for the sake of completeness, that there is a sextuple
bilateralness in the cube and its derivative forms which admit of
division into equal and similar parts by planes passing through the
three diagonal axes and by planes passing through the three axes
that join the centres of the surfaces, let us limit our attention to
the three kinds of bilateralness which here concern us. The first
of these is _triple bilateral symmetry_. This is the symmetry of a
figure having three axes at right angles to one another, through each
of which there passes a single plane that divides the aggregate into
corresponding halves. A common brick will serve as an example; and of
objects not quite so simple, the most familiar is that modern kind of
spectacle-case which is open at both ends. This may be divided into
corresponding halves along its longitudinal axis by cutting it through
in the direction of its thickness, or by cutting it through in the
direction of its breadth; or it may be divided into corresponding
halves by cutting it across the middle. Of objects which illustrate
_double bilateral symmetry_, may be named one of those boats built for
moving with equal facility in either direction, and therefore made
alike at stem and stern. Obviously such a boat is separable into equal
and similar parts by a vertical plane passing through stem and stern;
and it is also separable into equal and similar parts by a vertical
plane cutting it amidships. To exemplify _single bilateral symmetry_
it needs but to turn to the ordinary boat of which the two ends are
unlike. Here there remains but the one plane passing vertically
through stem and stern, on the opposite sides of which the parts are
symmetrically disposed.
These several kinds of symmetry as placed in the foregoing order, imply
increasing heterogeneity. The greatest uniformity in shape is shown
by the divisibility into like parts in an infinite number of infinite
series of ways; and the greatest degree of multiformity consistent
with any regularity, is shown by the divisibility into like parts in
only a single way. Hence, in tracing up organic evolution as displayed
in morphological differentiations, we may expect to pass from the one
extreme of spherical symmetry, to the other extreme of single bilateral
symmetry. This expectation we shall find to be completely fulfilled.