THE SHAPES OF LEAVES.
§ 228. Next in the descending order of composition come compound
leaves. The relative sizes and distributions of their leaflets, as
affecting their forms as wholes, have to be considered in their
relations to conditions. Figs. 206, 207, represent leaves of the
common _Oxalis_ and of the _Marsilea_, in which radial symmetry is
as completely displayed as the small number of leaflets permits.
This equal development of the leaflets on all sides, occurs where
the foot-stalks, growing up vertically from creeping or underground
stems, are so long that the leaves either do not interfere with one
another or do it in an inconstant way: the leaflets are not differently
conditioned on different sides, as they are where the foot-stalks grow
out in the ordinary manner. How unlikeness of position influences the
leaflets is clearly shown in a Clover-leaf, Fig. 208, which deviates
from the Oxalis-leaf but slightly towards bilateralness, as it deviates
from it but slightly in the attitude of its petiole; which is a little
inclined away from the others borne by the same procumbent axis.
A familiar example of an almost radial symmetry along with almost
equal relations to surrounding conditions, occurs in the root-leaves
of the Lupin, Fig. 209 _b_. Here though we have lateral divergence
from a vertical axis, yet the long foot-stalks preserve nearly erect
positions, and carry their leaves to such distances from the axis,
that the development of the leaflets on the side next it is not much
hindered. Still the interference of the leaves with one another is, on
the average, somewhat greater on the proximal side than on the distal
side; and hence the interior leaflets are rather less than the exterior
leaflets. In further proof of which influence, let it be added that, as
shown in the figure, at _a_, the leaves growing out of the flowering
stem deviate towards the two-sided form more decidedly. Two-sidedness
is much greater where there is a greater relative proximity of the
inner leaflets to the axis, or where the foot-stalk approaches towards
a horizontal position. The Horse-chestnut, Fig. 205, already instanced
as showing how the arrangements and sizes of leaflets are determined
by the incidence of forces, serves also to show how the incidence of
forces determines the relative sizes and arrangements of leaflets.
Fig. 210, which shows a leaf of the _Bombax_, further illustrates this
relation of structure to conditions.
[Illustration: Figs. 206–210.]
[Illustration: Figs. 211, 212.]
Compound leaves that are completely bilateral, present us with
modifications of form exemplifying the same general truth in another
way. In them the proximal and distal parts have none of that
resemblance which we see in those intermediate forms just described.
The portion next the axis and the portion furthest from the axis
are entirely different; and the only likeness is between the wings
or leaflets on opposite sides of the main foot-stalk or mid-rib. On
turning back to Fig. 65, it will be seen that the compound leaf there
drawn to exemplify another truth, serves also to exemplify this truth:
the homologous parts _a_, _b_, _c_, _d_, while they are unlike one
another, are, in their main proportions, severally like the parts with
which they are paired. And here let us not overlook a characteristic
which is less conspicuous but not less significant. Each of the lateral
wings has winglets that are larger on the one side than on the other;
and in each case the two sides are dissimilarly conditioned. Even in
the several components of each wing may be traced a like divergence
from symmetry, along with a like inequality in the relations to the
rest: the proximal half of each leaflet is habitually larger than the
distal half. In the leaves of the Bramble, previously figured, kindred
facts are presented. How far such differences of development are due to
the positions of the parts in the bud; how far the respective spaces
available for the parts when unfolded affect them; and how far the
parts are rendered unlike by unlikenesses in their relations to light;
it is difficult to say. Probably these several factors operate in all
varieties of proportion. That the habitual shading of some parts by
others largely aids in causing these divergences from symmetry, is very
instructively shown by the compound leaves of the Cow-parsnip. Fig.
211 represents one of these. While the leaf as a whole is bilaterally
symmetrical, each of the wings has an unsymmetrical bilateralness: the
side next the axis being larger than the remoter side. How does this
happen? Fig. 212, which is a diagrammatic section down the mid-rib
of the leaf, showing its inclined attitude and the positions of the
wings _a_, _b_, _c_, will make the cause clear. As the wings overlap,
like the bars of a Venetian blind, each intercepts some light from the
one below it; and the one below it thus suffers more on its distal
side than on its proximal side. Hence the smaller development of the
distal side. That this is the cause is further shown by the proportion
that is maintained between the degree of obscuration and the degree
of non-development; for this unlikeness is greater between the two
sides _a_ and _a′_, than between _b_ and _b′_ or _c_ and _c′_, at the
same time that the interference is greater in the lower wings than in
the upper. Of course in this case and in the kindred cases hereafter
similarly interpreted, it is not meant that this differentiation
is consequent solely, or even chiefly, on the differential actions
experienced by the individual plant. Though there is good reason to
believe that the rate of growth in each part of each leaf is affected
by the incidence of light, yet contrasts so marked and so systematic as
these are not explicable without taking into account the inheritance
of modifications either functionally caused or caused by spontaneous
variation. Clearly, the tendency will be towards the preservation of a
plant which distributes its chlorophyll in the most advantageous way;
and hence there will always be a gravitation towards a form in which
shaded parts of leaves are undeveloped.
§ 229. From compound leaves to simple ones, we find transitions in
leaves of which the divisions are partial instead of total; and in
these we see, with equal clearness, the relations between forms and
positions that have been traced thus far. Fig. 213 is the leaf of a
Winter-aconite in which, round a vertical petiole, there is a radial
distribution of half-separated leaflets. The _Cecropia_-leaf, Fig. 214,
shows us a two-sided development of the parts beginning to modify, but
not obliterating, the all-sided arrangement; and this mixed symmetry
occurs under conditions that are intermediate. A more marked degree of
the same relation is presented in the leaf of the Lady’s Mantle, Fig.
215. And then in the Sycamore and the Vine, we have a cleft type of
leaf in which a decided bilateralness of form co-exists with a decided
bilateralness of conditions.
[Illustration: Figs. 213–215.]
[Illustration: Figs. 216–218.]
The quite simple leaves to which we now descend, exhibit, very
distinctly, a parallel series of facts. Where they grow up on long and
completely-independent foot-stalks, without definite subordination
to some central vertical axis, the leaves of water-plants are
symmetrically peltate. Of this the sacred Indian-bean, Fig. 216,
furnishes an example. Here there is only a trace of bilateralness in
the venation of the leaf, corresponding to the very small difference
of the conditions on the proximal and distal sides. In the _Victoria
regia_, Fig. 217, the foot-stalks, though radiating almost
horizontally from a centre, are so long as to keep the leaves quite
remote from one another; and in it each leaf is almost symmetrically
peltate, with a bilateralness indicated only by a seam over the
line of the foot-stalk. The leaves of the _Nymphæa_, Fig. 218,
more closely clustered, and having less room transversely than
longitudinally, exhibit a marked advance to the two-sided form; not
only in the excess of the length over the breadth, but in the existence
of a cleft, where in the _Victoria regia_ there is merely a seam.
Among land-plants similar forms are found under analogous conditions.
The common _Hydrocotyle_, Fig. 219, which sends up direct from its
roots a few almost upright leaf-stalks, has these surmounted by peltate
leaves; which leaves, however, diverge slightly from radial symmetry in
correspondence with the slight contrast of circumstances which their
grouping involves. Another case is supplied by the Nasturtium, Fig.
220, which combines the characters--a creeping stem, long leaf-stalks
growing up at right angles to it, and unsymmetrically peltate leaves,
of which the least dimension is, on the average, towards the stem.
But perhaps the most striking illustration is that furnished by the
_Cotyledon umbilicus_, Fig. 221, in which different kinds of
symmetry occur in the leaves of the same plant, along with differences
in their relations to conditions. The root-leaves, _a_, growing up
on vertical petioles before the flower-stalk makes its appearance, are
symmetrically peltate; while the leaves which subsequently grow out of
the flower-stalk, _b_, are at the bottom transitionally bilateral,
and higher up completely bilateral.
[Illustration: Figs. 219–221.]
That the bilateral form of leaf is the ordinary form, corresponds with
the fact that, ordinarily, the circumstances of the leaf are different
in the direction of the plant’s axis from what they are in the opposite
direction, while transversely the circumstances are alike. It is
needless to give diagrams to illustrate this extremely familiar truth.
Whether they are broad or long, oval or heart-shaped, pointed or
obtuse, the leaves of most trees and plants will be remembered by all
as having the ends by which they are attached unlike the free ends,
while the two sides are alike. And it will also be remembered that
these equalities and inequalities of development correspond with the
equalities and inequalities in the incidence of forces.
§ 230. A confirmation that is interesting and important, is furnished
by the cases in which leaves present unsymmetrical forms in positions
where their parts are unsymmetrically related to the environment. A
considerable deviation from bilateral symmetry may be seen in a leaf
which habitually so carries itself, that the half on the one side of
the mid-rib is more shaded than the other half. The drooping branches
of the Lime, delineated in Fig. 222, show us leaves so arranged and
so modified. On examining their attitudes and their relations one to
another, it will be found that each leaf is so inclined that the half
of it next to the shoot grows over the shoot and gets plenty of light;
while the other half so hangs down that it comes a good deal into the
shade of the preceding leaf. The result is that having leaves which
fall into these positions, the species profits by a large development
of the exposed halves; and by survival of the fittest, acting along
with the direct effect of extra exposure, this modification becomes
established. How unquestionable is the connexion between the relative
positions of the halves and their relative developments, will be
admitted on observing a converse case. Fig. 223 represents a shoot
of _Strobilanthes glomeratus_. Here the leaves are so set on the stem
that the inner half of each leaf is shaded by the subsequently-formed
leaf, while its outer half is not thus shaded; and here we find the
inner half less developed than the outer half. But the most conclusive
evidence of this relation between unsymmetrical form and unsymmetrical
distribution of surrounding forces, is supplied by the genus _Begonia_;
for in it we have a manifest proportion between the degree of the
alleged effect and the degree of the alleged cause. These plants
produce their leaves in pairs, in such ways that the connate leaves
interfere with one another, much or little according as the foot-stalks
are short or long; and the result is a correlative divergence from
symmetry. In _Begonia nelumbiifolia_, which has petioles so long that
the connate leaves are not kept close together, there is but little
deviation from a bilaterally-peltate form; whereas, accompanying the
comparatively marked and constant proximity in _B. pruinata_, Fig. 224,
we see a more decidedly unsymmetrical shape; and in _B. mahringii_,
Fig. 225, the modification thus caused is pushed so far as to destroy
the peltate structure.[34]
[Illustration: Figs. 222–225.]
§ 231. Again, then, we are taught the same truth. Here, as before,
we see that homologous units of any order become differentiated in
proportion as their relations to incident forces become different. And
here, as before, we see that in each unit, considered by itself, the
differences of dimension are greatest in those directions in which the
parts are most differently conditioned; while there are no differences
between the dimensions of the parts that are not differently
conditioned.[35]