CHANGES OF SHAPE OTHERWISE CAUSED.
§ 238. Besides the more special causes of modification in the shapes of
plants and of their parts, certain more general causes must be briefly
noticed. These may be described as consequences of variations in the
total quantities of the matters and forces furnished to plants by their
environments. Some of the changes of form so produced are displayed by
plants as wholes, and others only by their parts. We will glance at
them in this order.
§ 239. It is a familiar fact that luxuriant shoots have relatively-long
internodes; and, conversely, that a shoot dwarfed from lack of sap,
has its nodes closely clustered: a concomitant result being that the
lateral axes, where these are developed, become in the one case far
apart and in the other case near together. Fig. 255 represents a branch
to the parts of which the longer and shorter internodes so resulting
give differential characters. A whole tree being in many cases
simultaneously thus affected by states of the earth or the air, all
parts of it may have such variations impressed on them; and, indeed,
such variations, following more or less regularly the changes of the
seasons, give to many trees manifest traits of structure. In Fig. 256,
a shoot of _Phyllocactus crenatus_, we have an interesting example
of a variation essentially of the same nature, little as it appears
to be so. For each of the lateral indentations is here the seat of
an axillary bud; and these we see are separated by internodes which,
becoming broader as they become longer, and narrower as they become
shorter, produce changes of form that correspond with changes in the
luxuriance of growth.
[Illustration: Figs. 255, 256.]
To complete the statement it must be added that these variations of
nutrition often determine the development or non-development of lateral
axes; and by so doing cause still more marked structural differences.
The Foxglove may be named as a plant which illustrates this truth.[40]
§ 240. From the morphological differentiations caused by unlikenesses
of nutrition felt by the whole plant, we pass now to those which are
thus caused in some of its parts and not in others. Among such are
the contrasts between flowering axes, and the axes that bear leaves
only. It has already been shown in § 78, that the belief expressed by
Wolff in a direct connexion between fructification and innutrition,
is justified inductively by many facts of many kinds. Deductively
too, in § 79, we saw reason to conclude that such a relation would be
established by survival of the fittest; seeing that it would profit
a species for its members to begin sending off migrating germs from
the ends of those axes which innutrition prevented from further
agamogenetic multiplication. Once more, when considering the nature of
the phænogamic axis, we found support for this belief in the fact that
the components of a flower exhibit a reversion to that type from which
the phænogamic type has probably arisen--a reversion which the laws of
embryology would lead us to look for where innutrition had arrested
development.
Hence, then, we may properly count those deviations of structure
which constitute inflorescence, as among the morphological
differentiations produced by local innutrition. I do not mean that the
detailed modifications which the essential and subservient organs of
fructification display, are thus accounted for: we have seen reason
to think them otherwise caused. But I mean that the morphological
characters which distinguish gamogenetic axes in general from
agamogenetic axes, such as non-development of the internodes and
dwarfing of the foliar organs, are primarily results of failure in the
supply of some material required for further growth.[41]
§ 241. Another trait which has to be noticed under this head, is the
spiral, or rather the helical, arrangement of parts. The successive
nodes of a phænogam habitually bear their appendages in ways implying
more or less twist in the substance of the axis; and in climbing plants
the twist is such as to produce a corkscrew shape. This structure is
ascribable to differences of interstitial nutrition. Take a shoot which
is growing vertically. It is clear that if the molecules are added
with perfect equality on all sides, there will be no tendency towards
any kind of lateral deviation; and the successively-produced parts
will be perpendicularly over one another. But any inequality in the
rate of growth on the different sides of the shoot, will destroy this
straightness in the lines of growth. If the greatest and least rates
of molecular increase happen to be on opposite sides, the shoot must
assume a curve of single curvature; but in every other case of unequal
molecular increase, a curve of double curvature must result. Now it is
a corollary from the instability of the homogeneous, that the rates of
growth on all sides of a shoot can never be exactly alike; and it is
also to be inferred from the same general law, that the greatest and
least rates of growth will not occur on exactly opposite sides of the
shoot, at the same time that equal rates of growth are preserved by the
two other sides. Hence, there must almost inevitably arise more or less
of twist; and the appendages of the internodes will so be prevented
from occurring perpendicularly one over another.
A deviation of this kind, necessarily initiated by physical causes
in conformity with the general laws of evolution, is likely to be
made regular and decided by natural selection. For under ordinary
circumstances, a plant profits by having its axis so twisted as to
bring the appended leaves into positions which prevent them from
shading one another. And, manifestly, modifications in the forms,
sizes, and insertions of the leaves, may, under the same agency, lead
to adapted modifications of the twist. We must therefore ascribe this
common characteristic of phænogams, primarily to local differences of
nutrition, and secondarily to survival of the fittest.
It is proper to add that there are some Monocotyledons, as _Ravenala
madagascariensis_, in which this character does not occur. What
conditions of existence they are that here hold this natural tendency
in check, it is not easy to see.[42]