Figs. 31–36.
We might not inappropriately call this type of structure pseud-axial. It simulates that of the higher plants in certain superficial characters. We see in it a primary axis along which development may continue indefinitely, and from which there bud out, laterally, secondary axes of like nature, bearing like tertiary axes; and this is a mode of growth with which Phænogams make us familiar.
§ 185. Some of the larger Algæ supply examples of an integration still more advanced; not simply inasmuch as they unite much greater numbers of morphological units into continuous masses, but also inasmuch as they combine the pseudo-foliar structure with the pseud-axial structure. Our own shores furnish an instance of this in the common Laminaria; and certain gigantic Laminariaceæ of the Antarctic seas, furnish yet better instances. In Necrocystis the germ develops a very long slender stem, which eventually expands into a large bladder-like or cylindrical air-vessel; and the surface of this bears numerous leaf-shaped expansions. Another kind, Lessonia fuscescens, Fig. [37], shows us a massive stem growing up through water many feet deep—a stem which, bifurcating as it approaches the surface, flattens out the ends of its subdivisions into fronds like ribands. These, however, are not true foliar appendages, since they are merely expanded continuations of the stem. In Egregia branches of the thallus not only take the form of leaves, but these are differentiated into several categories in accordance with a division of labour. In any of these Laminariaceæ the whole plant, great as may be its size, and made up though it seems to be of many groups of morphological units, united into a compound group by their marked subordination to a connecting mass, is nevertheless a single thallus, which is added to by intercalary growth at the “transition place,” at the junction of the stem-like and leaf-like portions. The aggregate is still an aggregate of the second order.
Fig. 37.
Figs. 38–40.
But among certain of the highest Algæ, we do find something more than this union of the pseud-axial with the pseudo-foliar structure. In addition to pseud-axes of comparative complexity; and in addition to pseudo-folia that are like leaves, not only in their general shapes but in having mid-ribs and even veins; there are the beginnings of a higher stage of integration. Figs. [38, 39, and 40], show some of the steps. In Rhodymenia palmata, Fig. [38], the parent-frond is comparatively irregular in form, and without a mid-rib; and along with this very imperfect integration, we see that the secondary fronds growing from the edges are distributed very much at random, and are by no means specific in their shapes. A considerable advance is displayed by Phyllophora rubens, Fig. [39]. Here the frond, primary, secondary, or tertiary, betrays some approach towards regularity in both form and size; by which, as also by its partially-developed mid-rib, there is established a more marked individuality; and at the same time, the growth of the secondary fronds no longer occurs anywhere on the edge, in the same plane as the parent-frond, but from the surface at specific places. Delesseria sanguinea, Fig. [40], illustrates a much more definite arrangement of the same kind. The fronds of this plant, quite regularly shaped, have their parts decidedly subordinated to the whole; and from their mid-ribs grow other fronds which are just like them. Each of these fronds is an organized group of those morphological units which we distinguish as aggregates of the first order. And in this case, two or more such aggregates of the second order, well individuated by their forms and structures, are united together; and the plant composed of them is thus rendered, in so far, an aggregate of the third order.
Just noting that in certain of the most developed Algæ, as the Sargassum, or common gulf-weed, this tertiary degree of composition is far more completely displayed, so as to produce among Thallophytes a type of structure closely simulating that of the higher plants, let us now pass to the consideration of these higher plants.
§ 186. Having the surface of the soil for a support and the air for a medium, terrestrial plants are mechanically circumstanced in a manner widely different from that in which aquatic plants are circumstanced. Instead of being buoyed up by a surrounding fluid of specific gravity equal to their own, they have to erect themselves into a rare fluid which yields no appreciable support. Further, they are dissimilarly conditioned in having two sources of nutriment in place of one. Unlike the Algæ, which derive all the materials for their tissues from the water bathing their entire surfaces, and use their roots only for attachment, most of the plants which cover the Earth’s surface, absorb part of their food through their imbedded roots and part through their exposed leaves. These two marked unlikenesses in the relations to surrounding conditions, profoundly affect the respective modes of growth. We must duly bear them in mind while studying the further advance of composition.