Figs. 41–44.
Figs. 45–49.
The class of plants to which we now turn—that of the Archegoniatæ—is nearly related by its lower members to the classes above dealt with: so much so, that some of the inferior liverworts are quite licheniform, and are often mistaken for lichens. Passing over these, let us recommence our synthesis with such members of the class as repeat those indications of progress towards a higher composition, which we have just observed among the more-developed Algæ. The Jungermanniaceæ furnish us with a series of types, clearly indicating the transition from an aggregate of the second order to an aggregate of the third order. Figs. [41] and [42], indicate the structure among the lowest of this group. Here there is but an incomplete development of the second order of aggregate. The frond grows as irregularly as the thallus of a lichen: it is indefinite in size and outline, spreading hither or thither as the conditions favour. Moreover, it lacks the differentiations required to subordinate its parts to the whole: it is uniformly cellular, having neither mid-rib nor veins; and it puts out rootlets indifferently from all parts of its under surface. In Fig. [43], Pellia epiphylla, we have an advance on this type. There is here, as shown in the transverse section, Fig. [44], a thickening of the frond along its central portion, producing something like an approach towards a mid-rib; and from this the rootlets are chiefly given off. The outline, too, is much less irregular; whence results greater distinctness of the individuality. A further step is displayed in Metzgeria furcata, Fig. [45]. The frond of this plant, comparatively well integrated by the distribution of its substance around a decided mid-rib, and by its comparatively-definite outlines, produces secondary fronds. There is what is called proliferous growth; and occasionally, as shown in Fig. [46], representing an enlarged portion, the growth is doubly-proliferous. In these cases, however, the tertiary aggregate, so far as it is formed, is but very feebly integrated; and its integration is but temporary. For not only do these younger fronds that bud out from the mid-ribs of older fronds, develop rootlets of their own; but as soon as they are well grown and adequately rooted, they dissolve their connexions with the parent-fronds, and become quite independent. From these transitional forms we pass, in the higher Jungermanniaceæ, to forms composed of many fronds that are permanently united by a continuous stem. A more-developed aggregate of the third order is thus produced. But though, along with increased definiteness in the secondary aggregates, there is here an integration of them so extensive and so regular, that they are visibly subordinated to the whole they form; yet the subordination is really very incomplete. In some instances, as in Radula complanata, Fig. [47], the leaflets develop roots from their under surfaces, just as the primitive frond does; and in the majority of the group, as in J. capitata, Fig. [48], roots are given off all along the connecting stem, at the spots where the leaflets or frondlets join it: the result being that though the connected frondlets form a physical whole, they do not form, in any decided manner, a physiological whole; since successive portions of the united series, carry on their functions independently of the rest. Finally, the most developed members of the group, whether lineally descended from the less developed or from an early type common to the two, present us with tertiary aggregates which are physiologically as well as physically integrated.[5] Not lying prone like the kinds thus far described, but growing erect, the stem and attached leaflets become dependent upon a single root or group of roots; and being so prevented from carrying on their functions separately, are made members of a compound individual: there arises a definitely-established aggregate of the third degree of composition.
The facts as arranged in the above order are suggestive. Minute aggregates, or cells, the grouping of which we traced in [§ 182], showed us analogous phases of indefinite union, which appeared to lead the way towards definite union. We see here among compound aggregates, as we saw there among simple aggregates, the establishment of a specific form, and a size that falls within moderate limits of variation. This passage from less definite extension to more definite extension, seems in the one case, as the other, to be accompanied by the result, that growth exceeding a certain rate, ends in the formation of a new aggregate, rather than an enlargement of the old. And on the higher stage, as on the lower, this process, irregularly carried out in the simpler types, produces in them unions that are but temporary; while in the more-developed types, it proceeds in a systematic way, and ends in the production of a permanent aggregate that is doubly compound.
Must we then conclude that as cells, or morphological units, are integrated into a unit of a higher order, which we call a thallus or frond; so, by the integration of fronds, there is evolved a structure such as the above-delineated species possess? Whether this is the interpretation to be given of these plants, we shall best see when considering whether it is the interpretation to be given of plants which rank above them. Thus far we have dealt only with the Cryptogamia. We have now to deal with the Phanerogamia or Phænogamia.
CHAPTER III.
THE MORPHOLOGICAL COMPOSITION OF PLANTS, CONTINUED.
§ 187. That advanced composition arrived at in the Archegoniatæ, is carried still further in the Flowering Plants. In these most-elevated vegetal forms, aggregation of the third order is always distinctly displayed; and aggregates of the fourth, fifth, sixth, &c., orders are very common.
Our inquiry into the morphology of these flowering plants, may be advantageously commenced by studying the development of simple leaves into compound leaves. It is easy to trace the transition, as well as the conditions under which it occurs; and tracing it will prepare us for understanding how, and when, metamorphoses still greater in degree take place.