THE SHAPES OF BRANCHES.
§ 221. Aggregates of the first order supply a few examples of forms
ramified in an approximately-regular manner, under conditions
which subject their parts to approximately-regular distributions
of forces. Some unicellular _Algæ_, becoming elaborately branched,
assume very much the aspects of small trees; and show us in their
branches analogous relations of forms to forces. _Bryopsis plumosa_
may be instanced. Fig. 200 represents the end of one of its lateral
ramifications, above and beneath which come others of like characters.
Here it will be seen that the attached and free ends differ; that the
two sides are much alike; and that they are unlike the upper and under
surfaces, which resemble one another. The more highly developed members
of the same group of _Algæ_, the _Siphoneæ_, show a marked radial
symmetry co-existing with very elaborate branching, _e.g._, _Neomeris_,
_Cymopolia_, and others.
[Illustration: Fig. 200.]
§ 222. Fig. 201 shows us how, in an aggregate of the second order,
each proximate component is modified by its relations to the rest;
just as we before saw a whole fungus of the same type modified by
its relations to environing objects. If a branch of the fungus here
figured, be compared with one of the fungi clustered together in Fig.
195, or, still better, with one of the laterally-growing fungi shown
in Fig. 196, there will be perceived a kindred transition from radial
to bilateral symmetry, occurring under kindred conditions. The portion
of the pileus next to the side of attachment is undeveloped in this
branched form as in the simpler form; and in the one case as in the
other, the stem is modified towards the side of attachment. A division
into similar halves, which, as shown in Fig. 196 _e_, might be made of
the whole fungus by a vertical plane passing through the centre of the
pileus and the axis of the supporting body, might here be made of the
branch, by a vertical plane passing through the centre of its pileus
and the axis of the main stem. Among aggregates of this order, the
_Algæ_ furnish cases of kindred nature. In the branches of _Lessonia_,
Fig. 37, may be observed a substantially-similar relationship. As their
inner parts are less developed than their outer parts, while their two
sides are developed in approximately equal degrees, they are rendered
bilateral.
[Illustration: Fig. 201.]
§ 223. These few cases introduce us to the more familiar but more
complex cases which plants of the third degree of aggregation present.
At _a_, _b_, _c_, Fig. 202, are sketched three homologous parts of the
same tree: _a_ being the leading shoot; _b_ a lateral branch near
the top, and _c_ a lateral branch lower down. There is here a double
exemplification. While the branch _a_, as a whole, has its branchlets
arranged with tolerable regularity all round, in correspondence with
its equal exposure on all sides, each branchlet shows by its curve
as much bilateral symmetry as its simple form permits. The branch
_b_, dissimilarly circumstanced on the side next the main stem and
on the side away from it, has an approximate bilateralness as a
whole, while the bilateralness of its branchlets varies with their
respective positions. And in the branch _c_, having its parts still
more differently conditioned, these traits of structure are still more
marked. Extremely strong contrasts of this kind occur in trees having
very regular modes of growth. The uppermost branches of a Spruce-fir
have radially-arranged branchlets: each of them, if growing vigorously,
repeats the type of the leading shoot, as shown in Fig. 203, _a_,
_b_. But if we examine branches lower and lower down the tree, we
find the vertically-growing branchlets bear a less and less ratio to
the horizontally-growing ones; until, towards the bottom, the radial
arrangement has wholly merged into the bilateral. Shaded and confined
by the branches above them, these eldest branches develop their
offshoots in those directions where there is most space and light:
becoming finally quite flattened and fan-shaped, as shown at Fig.
203, _c_. And on remembering that each of these eldest branches, when
first it diverged from the main stem, was radial, we see not only that
between the upper and lower branches does this contrast in structure
hold, but also that each branch is transformed from the radial to
the bilateral by the progressive change in its environment. Other
forces besides those which aid or hinder growth, conspire to produce
this two-sided character in lateral branches. The annexed Fig. 204,
sketched from an example of the _Pinus Coulterii_ at Kew, shows very
clearly how, by mere gravitation, the once radially-arranged branchlets
may be so bent as to produce in the branch as a whole a decided
bilateralness. A full-grown _Araucaria_, too, exhibits in its lower
branches modifications similarly caused; and in each of such branches
there may be remarked the further fact, that its upward-bending
termination has a partially-modified radialness, at the same time that
its drooping lateral branchlets give to the part nearer the trunk a
completely bilateral character.
[Illustration: Figs. 202–204.]
Now in these few instances, typical of countless instances which might
be given, we see, as we saw in the case of the fungi, that the same
thing is true of the parts in their relations to the whole and to one
another, which is true of the whole in its relations to the environment
at large. Entire trees become bilateral instead of radial, when exposed
to forces that are equal only on opposite sides of one plane; and in
their branches, parallel changes of form occur under parallel changes
of conditions.
§ 224. There remains to be said something respecting the distribution
of leaves. How a branch carries its leaves constitutes one of
its characters as a branch, and is to be considered apart from
the characters of the leaves themselves. The principles hitherto
illustrated we shall here find illustrated still further.
The leading shoot and all the upper twigs of a fir-tree, have their
pin-shaped leaves evenly distributed all round, or placed radially;[32]
but as we descend we find them beginning to assume a bilateral
distribution; and on the lower, horizontally-growing branches, their
distribution is quite bilateral.[33] Between the Irish and English
kinds of Yew, there is a contrast of like significance. The branches
of the one, shooting up as they do almost vertically, are clothed with
leaves all round; while those of the other, which spread laterally,
bear their leaves on the two sides. In trees with better-developed
leaves, the same principle is more or less manifest in proportion as
the leaves are more or less enabled by their structures to maintain
fixed positions. Where the foot-stalks are long and slender, and where,
consequently, each leaf, according to its weight, the flexibility and
twist of its foot-stalk, and the direction of the branch it grows from,
falls into some indefinite attitude, the relations are obscured. But
where the foot-stalks are stiff, as in the Laurel, it will be found, as
before, that from the topmost and upward growing branches the leaves
diverge on all sides; while the undermost branches, growing out from
the shade of those above, have their leaves so turned as to bring them
into rows horizontally spread out on the two sides of each branch.
[Illustration: Fig. 205.]
A kindred truth, having like implications, comes into view when we
observe the relative sizes of leaves on the same branch, where their
sizes differ. Fig. 205 represents a branch of a Horse-chestnut, taken
from the lowermost fringe of the tree, where the light has been to
a great extent intercepted from all but the most protruded parts.
Beyond the fact that the leaves become by appropriate growths of their
foot-stalks bilaterally distributed on this drooping branch, instead of
being distributed symmetrically all round, as on one of the ascending
shoots, we have here to note the fact that there is unequal development
on the upper and lower sides. Each of the compound leaves acquires a
foot-stalk and leaflets that are large in proportion to the supply
of light; and hence, as we descend towards the bottom of the tree,
the clusters of leaves display increasing contrasts. How marked these
contrasts become will be seen on comparing _a_ and _b_, which form one
pair of leaves that are normally equal, or _c_ and _d_, which form
another pair normally equal.
Let us not omit to note, while we have this case before us, the proof
it affords that these differences of development are in a considerable
degree determined by the different conditions of the parts after they
have been unfolded. Though those inequalities of dimensions whence
the differentiations of form result, may be in many cases largely due
to the inequalities in the circumstances of the parts while in the
bud (which are, however, representative of inequalities in ancestral
circumstances); yet these are clearly not the sole causes of the
unlikenesses which eventually arise. This bilateralness resulting from
the unequal sizes of the leaves, must be considered as due to the
differential actions that come into play after the leaves have assumed
their typical structures.
§ 225. How, in the arrangement of their twigs and leaves, branches tend
to lapse from forms that are approximately symmetrical to forms that
are quite asymmetrical, need not be demonstrated: it is sufficiently
conspicuous. But it may be well to point out how the tendency to
do this further enforces our argument. The comparatively regular
budding out of secondary axes and tertiary axes, does not usually
produce an aggregate which maintains its regularity, for the simple
reason that many of the axes abort. Terminal buds are some of them
destroyed by birds; others are burrowed into by insects; others are
nipped by frost; others are broken off or injured during gales of wind.
The environment of each branch and its branchlets is thus ever being
varied on all sides: here, space being left vacant by the death of some
shoot that would ordinarily have occupied it; and there, space being
trenched on by the lateral growth of some adjacent branch that has had
its main axis broken. Hence the asymmetry, or heterogeneity of form,
assumed by the branch, is caused by the asymmetrical distribution of
incident forces--a result and a cause which go on ever complicating.
§ 226. One conspicuous trait in the shapes of branches has still to be
named. Their proximal or attached ends differ from their distal or free
ends, in the same way that the lower ends of trees differ from their
upper ends. This fact, like the fact to which it is here paralleled,
has had its significance obscured by its extreme familiarity. But
it shows in a striking way how the most differently conditioned
parts become the most strongly contrasted in their structures. A
phænogamic axis is made up of homologous segments, marked off from
one another by the nodes; and a compound branch consists of groups of
such segments. The earliest-formed segments, alike of the tree and of
each branch, serve as mechanical supports and channels for sap to the
successive generations of segments that grow out of them; and become
more and more shaded by their progeny as these increase. Hence the
progressively-increasing contrasts which, while mainly due to the
unlikenesses of bulk accompanying differences of age, are in part due
to the unlikenesses of structure which differences of relation to the
environment have caused.
§ 227. Thus, then, it is with the proximate parts of plants as it is
with plants as wholes. The radial symmetry, the bilateral symmetry, and
the asymmetry, which branches display in different trees, in different
parts of the same tree, and at different stages of their own growths,
prove to be all consequent on the ways in which they stand towards the
entire plexus of surrounding actions. The principle that the growths
are unequal in proportion as the relations of parts to the environment
are unequal, serves to explain all the leading traits of structure.