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.
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]
Figs. 7–17.
CHAPTER XII.
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.