THE SHAPES OF VEGETAL CELLS.
§ 237. We come now to aggregates of the lowest order. Already something
has been said (§ 217) concerning the forms of those morphological units
which exist as independent plants. But it is here requisite briefly to
note the modifications undergone by them where they become components
of larger plants.
[Illustration: Fig. 254.]
Of the numerous cell-forms which are found in the tissues of the higher
plants, it will suffice to give, in Fig. 254, representing a section
of a leaf, a single example. In this it will be seen that the cells
forming the upper and lower surfaces, _a_ and _b_, have differences of
shape related to differences in the incidence of forces: they are more
or less flattened in relation to the environment. The underneath cells
at _c_, form a class which, similarly exposed to light at their outer
ends, and, as we may assume, largely developed in adjustment to their
active assimilative functions, are, by mutual pressure, made to grow
more in the direction of their lengths than in the direction of their
breadths. Then on the other side we see that the cells _d_, next above
the outer layer, while approximately similar, become more and more
dissimilar as they diverge from the surface, and are quite irregular
in the interior _e_, where there is no definiteness in the conditions
to which they are exposed. Thus the divergences of these cells from
primordial sphericity are such as correspond with unlikenesses in their
circumstances. And throughout the more complex modifications which the
cells of other tissues exhibit, the like correspondences hold.
[Illustration: Figs. 32–35.]
Among plants of a lower order of aggregation, we have already seen
how cells become metamorphosed as they become integrated into masses
having definite organizations. The higher _Algæ_, exemplified in Figs.
32, 34, 35, show this very clearly. Here the departure from the simple
cell-form to the form of an elongated prism, is manifestly subordinated
to the contrasts in the relations of the parts. And it is interesting
to observe how, in one of the branches of Fig. 32, we pass from the
small, almost-spherical cells which terminate the branchlets, to the
large, much-modified cells which join the main stem, through gradations
obviously related in their changed forms to the altered actions their
positions expose them to.
[Illustration: Figs. 19–23.]
More simply, but quite as conclusively, do the inferior _Algæ_, of
which Figs. 19–23 are examples, show us how cells pass from their
original spherical symmetry into radial symmetry, as they pass from a
state in which they are similarly-conditioned on all sides, to a state
in which two of their opposite sides or ends are conditioned in ways
that are like one another, but unlike the ways in which all other sides
are conditioned.
Still more instructive are the morphological differentiations of
those protophytes in which the first steps towards a higher degree of
integration are shown. In Fig. 10, representing one of the transitional
forms of _Desmidiaceæ_, it is to be noted that besides the difference
between the transverse and longitudinal dimensions, which the component
units display in common, the two end-units differ from the rest:
they have appendages which the rest have not. Once more, where the
integration is carried on in such ways as to produce not strings but
clusters, there arise contrasts and correspondences just such as might
be looked for. All the four members of the group shown in Fig. 12,
are similarly conditioned; and each of them has a bilateral shape
answering to its bilateral relations. In Fig. 14 we have a number of
similarly-bilateral individuals on the circumference, including a
central individual differing from the rest by having the bilateral
character nearly obliterated. And then, in Fig. 15, we have two central
components of the group, deviating more decidedly from those that
surround them.[39]
[Illustration: Figs. 7–17.]