Joined with the facts previously specified, these facts help us to conceive how, in the evolution of flowering plants in general, the morphological components that were once distinct, may become extremely disguised. We may rationally expect that during so long a course of modification, much greater changes of form, and much more decided fusions of parts, have taken place. Seeing how, in an individual plant, the single leaves pass into compound leaves, by the development of their veins into mid-ribs while their petioles begin to simulate axes; and seeing that leaves ordinarily exhibiting definitely-limited developments, occasionally produce other leaves from their edges; we are led to suspect the possibility of still greater changes in foliar organs. When, further, we find that within the limits of one natural order, petioles usurp the functions and appearances of leaves, at the same time that in other orders, as in Ruscus, lateral axes so simulate leaves that their axial nature would by most not be suspected, did they not bear flowers on their mid-ribs or edges; and when, among Cactuses, we perceive that such metamorphoses and re-metamorphoses take place with great facility; our suspicion that the morphological elements of Phænogams admit of profound transformations, is deepened. And then, on discovering how frequent are the monstrosities which do not seem satisfactorily explicable without admitting the development of foliar organs into axial organs; we become ready to entertain the hypothesis that during the evolution of the phænogamic type, the distinction between leaves and axes has arisen by degrees.
With our preconceptions loosened by such facts, and carrying with us the general idea which such facts suggest, let us now consider in what way the typical structure of a flowering plant may be interpreted.
§ 192. To proceed methodically, we must seek a clue to the structures of Phanerogams, in the structures of those inferior plants that approach to them—Archegoniatæ. The various divisions of this class present, along with sundry characters which ally them with Thallophytes, other characters by which the phænogamic structure is shadowed forth. While some of the inferior Hepaticæ or Liverworts, severally consist of little more than a thallus-like frond, among the higher members of this group, and still more among the Mosses and Ferns, we find a distinctly marked stem.[8] Some Archegoniates (or rather Rhizoids) have foliar expansions that are indefinite in their forms; and some have quite definitely-shaped leaves. Roots are possessed by all the more-developed genera of the class; but there are other genera, as Sphagnum, which have no roots. Here the fronds are formed of only a single layer of cells; and there a double layer gives them a higher character—a difference exhibited between closely-allied genera of one group, the Mosses. Equally varied are the developments of the foliar organs in their detailed structures: now being without mid-ribs or veins; now having mid-ribs but no veins; now having both mid-ribs and veins. Nor must we omit the similarly-significant circumstance, that whereas in the lower Archegoniates the reproductive elements are immersed here and there in the thallus-like frond, they are, in the higher orders, seated in well-specialized and quite distinct fructifying organs, having analogies with the flowers of Phænogams. Thus, many facts imply that if the Phænogamic type is to be analyzed at all, we must look among the Archegoniates for its morphological components, and the manner of their integration.
Already we have seen among the lower Cryptogamia, how, as they became integrated and definitely limited, aggregates acquire the habit of budding out other aggregates, on reaching certain stages of growth. Cells produce other cells endogenously or exogenously; and fronds give origin to other fronds from their edges or surfaces. We have seen, too, that the new aggregates so produced, whether of the first order or the second order, may either separate or remain connected. Fissiparously-multiplying cells in some cases part company, while in other cases they unite into threads or laminæ or masses; and fronds originating proliferously from other fronds, sometimes when mature disconnect themselves from their parents, and sometimes continue attached to them. Whether they do or do not part, is clearly determined by their nutrition. If the conditions are such that they can severally thrive better by separating after a certain development is reached, it will become their habit then to separate; since natural selection will favour the propagation of those which separate most nearly at that time. If, conversely, it profits the species for the cells or fronds to continue longer attached, which it can only do if their growths and subsequent powers of multiplication are thereby increased, it must happen, through the continual survival of the fittest, that longer attachment will become an established characteristic; and, by persistence in this process, permanent attachment will result when permanent attachment is advantageous. That disunion is really a consequence of relative innutrition, and union a consequence of relative nutrition, is clear à posteriori. On the one hand, the separation of the new individuals, whether in germs or as developed aggregates, is a dissolving away of the connecting substance; and this implies that the connecting substance has ceased to perform its function as a channel of nutriment. On the other hand, where, as we see among Phænogams, there is about to take place a separation of new individuals in the shape of germs, at the point where the nutrition is the lowest, a sudden increase of nutrition will cause the impending separation to be arrested; and the fructifying elements, reverting towards the ordinary form, thereupon develop in connexion with the parent. Turning to the Archegoniates, we find among them many indications of this transition from discontinuous development to continuous development. Thus the Liverworts give origin to new plants by cells which they throw off from their surfaces; as, indeed, we have seen that much higher plants do. “According to Bischoff,” says Schleiden, “both the cells of the stem (Jungermannia [now Lophocolea] bidentata) and those of the leaves (J. exsecta) separate themselves as propagative cells from the plant, and isolated cells shoot out and develop while still connected with the parent plant into small cellular bodies (Metzgeria furcata), which separate from the plant, and grow into new plants, as in Mnium androgynum among the Mosses.” Now in the way above explained, these propagative cells and proliferous buds, may continue developing in connexion with the parent to various degrees before separating; or the buds which are about to become fructifying organs may similarly, under increased nutrition, develop into young fronds. As Sir W. Hooker says of the male fructification in Metzgeria furcata,—“It has the appearance of being a young shoot or innovation (for in colour and texture I can perceive no difference) rolled up into a spherical figure.” On finding in this same plant, that sometimes the proliferously-produced frond buds out from itself another frond before separating from the parent, as shown in Fig. [46], it becomes clear that this long-continued connexion may readily pass into permanent connexion. And when we see how, even among Phænogams, buds may either detach themselves as bulbils, or remain attached and become shoots; we can scarcely doubt that among inferior plants, less definite in their modes of organization, such transitions must continually occur.
Figs. 73–76.
Let us suppose, then, that Fig. [73] is the frond of some primitive Archegoniate, similar in general characters to Pellia epiphylla, Fig. [43]; bearing, like it, the fructifying buds on its upper surface, and having a slightly-marked mid-rib and rootlets. And suppose that, as shown, a secondary frond is proliferously produced from the mid-rib, and continues attached to it. Evidently the ordinary discontinuous development, can thus become a continuous development, only on condition that there is an adequate supply, to the secondary frond, of such materials as are furnished by the rootlets: the remaining materials being obtainable by itself from the air. Hence, that portion of the mid-rib lying between the secondary frond and the chief rootlets, having its function increased, will increase in bulk. An additional consequence will be a greater concentration of the rootlets—there will be extra growth of those which are most serviceably placed. Observe, next, that the structure so arising is likely to be maintained. Such a variation implying, as it does, circumstances especially favourable to the growth of the plant, will give to the plant extra chances of leaving descendants; since the area of frond supported by a given area of the soil, being greater than in other individuals, there may be a greater production of spores. And then, among the more numerous descendants thus secured by it, the variation will give advantages to those in which it recurs. Such a mode of growth having, in this manner, become established, let us ask what is next likely to result. If it becomes the habit of the primary frond to bear a secondary frond from its mid-rib, this secondary frond, composed of physiological units of the same kind, will inherit the habit; and supposing that the supply of mineral matters obtained by the rootlets suffices for the full development of the secondary frond, there is a likelihood that the growth from it of a tertiary frond, will become an habitual characteristic of the variety. Along with the establishment of such a tertiary frond, as shown in Fig. [74], there must arise a further development of mid-rib in the primary frond, as well as in the secondary frond—a development which must bring with it a greater integration of the two; while, simultaneously, extra growth will take place in such of the rootlets as are most directly connected with this main channel of circulation. Without further explanation it will be seen, on inspecting Figs. [75 and 76], that there may in this manner result an integrated series of fronds, placed alternately on opposite sides of a connecting vascular structure. That this connecting vascular structure will, as shown in the figures, become more distinct from the foliar surfaces as these multiply, is no unwarranted assumption; for we have seen in compound-leaved plants, how, under analogous conditions, mid-ribs become developed into separate supporting parts, which acquire some of the characters of axes while assuming their functions. And now mark how clearly the structure thus built up by integration of proliferously-growing fronds, corresponds with the structure of the more-developed Jungermanniaceæ. Each of the fronds successively produced, repeating the characters of its parent, will bear roots; and will bear them in homologous places, as shown. Further, the united mid-ribs having but very little rigidity, will be unable to maintain an erect position. Hence there will result the recumbent, continuously-rooted stem, which these types exhibit: an embryo phænogam having the weakness of an embryo.[9]
A natural concomitant of the mode of growth here described, is that the stem, while it increases longitudinally, increases scarcely at all transversely: hence the old name Acrogens. Clearly the transverse development of a stem is the correlative, partly of its function as a channel of circulation, and partly of its function as a mechanical support. That an axis may lift its attached leaves into the air, implies thickness and solidity proportionate to the mass of such leaves; and an increase of its sap-vessels, also proportionate to the mass of such leaves, is necessitated when the roots are all at one end and the leaves at the other. But in the generality of Acrogens, these conditions, under which arises the necessity for transverse growth of the axis, are absent wholly or in great part. The stem habitually creeps below the surface, or lies prone upon the surface; and where it grows in a vertical or inclined direction, does this by attaching itself to a vertical or inclined object. Moreover, throwing out rootlets, as it mostly does, at intervals throughout its length, it is not called upon in any considerable degree, to transfer nutritive materials from one of its ends to the other. Hence this peculiarity which gives their name to the Acrogens, now called Archegoniates, is a natural accompaniment of the low degree of specialization reached in them. And that it is an incidental and not a necessary peculiarity, is demonstrated by two converse facts. On the one hand, in those higher Acrogens which, like the tree-ferns, lift large masses of foliage into the air, there is just as decided a transverse expansion of the axis as in dicotyledonous trees. On the other hand, in those Dicotyledons which, like the common Dodder, gain support and nutriment from the surfaces over which they creep, there is no more lateral expansion of the axis than is habitual among Acrogens or Archegoniates. Concluding, as we are thus fully justified in doing, that the lateral expansion accompanying longitudinal extension, which is a general characteristic of Phanerogams as distinguished from Archegoniates, is nothing more than a concomitant of their usually-vertical growth;[10] let us now go on to consider how vertical growth originates, and what are the structural changes it involves.
§ 193. Plants depend for their prosperity mainly on air and light: they dwindle where they are smothered, and thrive where they can expand their leaves into free space and sunshine. Those kinds which assume prone positions, consequently labour under disadvantages in being habitually interfered with by one another—they are mutually shaded and mutually injured. Such of them, however, as happen, by variations in mode of growth, to rise higher than others, are more likely to flourish and leave offspring than others. That is to say, natural selection will favour the more upright-growing forms. Individuals with structures which lift them above the rest, are the fittest for the conditions; and by the continual survival of the fittest, such structures must become established. There are two essentially-different ways in which the integrated series of fronds above described, may be modified so as to acquire the stiffness needful for maintaining perpendicularity. We will consider them separately.
