Figs. 92–94.
Figs. 95–99.
The other way in which an integrated series of fronds may acquire the rigidity needful for maintaining an erect position, has next to be considered. If the successive fronds do not acquire such habit of curling as may be taken advantage of by natural selection, so as to produce the requisite stiffness; then, the only way in which the requisite stiffness appears producible, is by the thickening and hardening of the fused series of mid-ribs. The incipient axis will not, in this case, be inclosed by the rolled-up fronds; but will continue exposed. Survival of the fittest will favour the genesis of a type, in which those portions of the successive mid-ribs that enter into the continuous bond, become more bulky than the disengaged portions of the mid-ribs: the individuals which thrive and have the best chances of leaving offspring, being, by the hypothesis, individuals having axes stiff enough to raise their foliage above that of their fellows. At the same time, under the same influences, there will tend to result an elongation of those portions of the mid-ribs, which become parts of the incipient axis; seeing that it will profit the plant to have its leaves so far removed from one another, as to prevent mutual interferences. Hence, from the recumbent type there will evolve, by indirect equilibration ([§ 167]), such modifications as are shown in Figs. [92, 93, 94]; the first of which is a slight advance on the ideal type represented in Fig. [76], arising in the way described; and the others of which are actual plants—Haplomitrium Hookeri, and Plagiochila decipiens. Thus the higher Archegoniates show us how, along with an assumption of the upright attitude, there does go on, as we see there must go on, a separation of the leaf-producing parts from the root-producing parts; a greater development of that connecting portion of the successive fronds, by which they are kept in communication with the roots, and raised above the ground; and a consequent increased differentiation of such connecting portion from the parts attached to it. And this lateral bulging of the axis, directly or indirectly consequent on its functions as a support and a channel, being here unrestrained by the early-formed fronds folded round it, goes on without the bursting of these. Hence arises a leading character of what is called exogenous growth—a growth which is, however, still habitually accompanied by exfoliation, in flasks, of the outermost layers, continually being cracked and split by the accumulation of layers within them. And now if we examine plants of the exogenous type, we find among them many displaying the stages of this metamorphosis. In Fig. [95], is shown a form in which the continuity of the axis with the mid-rib of the leaf, is manifest—a continuity that is conspicuous in the common thistle. Here the foliar expansion, running some distance down the axis, makes the included portion of the axis a part of its mid-rib; just as in the ideal types above drawn. By the greater growth of the internodes, which are very variable, not only in different plants but in the same plant, there results a modification like that delineated in Fig. [96]. And then, in such forms as Fig. [97], there is shown the arrangement that arises when, by more rapid development of the proximal end of the mid-rib, the distal part of the foliar surface is separated from the part which embraces the axis: the wings of the mid-rib still serving, however, to connect the two portions of the foliar surface. Such a separation is, as pointed out in [§ 188], an habitual occurrence; and in some compound leaves, an actual tearing of the inter-venous tissue is caused by extra growth of the mid-rib. Modifications like this, and the further one in Fig. [98], we may expect to be established by survival of the fittest, among those plants which produce considerable masses of leaves; since the development of mid-ribs into foot-stalks, by throwing the leaves further away from the axes, will diminish the shading of the leaves, one by another. And then, among plants of bushy growth, in which the assimilating surfaces become still more liable to intercept one another’s light, natural selection will continue to give an advantage to those which carry their assimilating surfaces at the ends of the petioles, and do not develop assimilating surfaces close to the axis, where they are most shaded. Whence will result a disappearance of the stipules and the foliar fringes of the mid-ribs; ending in the production of the ordinary stalked leaf, Fig. [99], which is characteristic of trees. Meanwhile, the axis thickens in proportion to the number of leaves it has to carry, and to put in communication with the roots; and so there comes to be a more marked contrast between it and the petioles, severally carrying a leaf each.[12]
§ 194. When, in the course of the process above sketched out, there has arisen such community of nutrition among the fronds thus integrated into a series, that the younger ones are aided by materials which the older ones have elaborated; the younger fronds will begin to show, at earlier and earlier periods of development, the structures about to originate from them. Abundant nutrition will abbreviate the intervals between the successive prolifications; so that eventually, while each frond is yet imperfectly formed, the rudiment of the next will begin to show itself. All embryology justifies this inference. The analogies it furnishes lead us to expect that when this serial arrangement becomes organic, the growing part of the series will show the general relations of the forthcoming parts, while they are very small and unspecialized. What will in such case be the appearances they assume? We shall have no difficulty in perceiving what it will be, if we take a form like that shown in Fig. [92], and dwarf its several parts at the same time that we generalize them. Figs. [100, 101, 102, and 103], will show the result; and in Fig. [104], which is the bud of a dicotyledon, we see how clear is the morphological correspondence: a being the rudiment of a foliar organ beginning to take shape; b being the almost formless rudiment of the next foliar organ; and c being the quite-undifferentiated part whence the rudiments of subsequent foliar organs are to arise.
Figs. 100–104.
Figs. 105–106.
And now we are prepared for entering on a still-remaining question respecting the structure of Phænogams—what is the origin of axillary buds? As the synthesis at present stands, it does not account for these; but on looking a little more closely into the matter, we shall find that the axillary buds are interpretable in the same manner as the terminal buds. So to interpret them, however, we must return to that process of proliferous growth with which we set out, for the purpose of observing some facts not before named. Delesseria hypoglossum, Fig. [105], represents a seaweed of the same genus as one outlined in Fig. [40]; but of a species in which proliferous growth is carried much further. Here, not only does the primary frond bud out many secondary fronds from its mid-rib; but most of the secondary fronds similarly bud out several tertiary fronds; and even by some of the tertiary fronds, this prolification is repeated. Besides being shown that the budding out of several fronds from one frond, may become habitual; we are also shown that it may become a habit inherited by the fronds so produced, and also by the fronds they produce: the manifestation of the tendency being probably limited only by failure of nutrition. That under fit conditions an analogous mode of growth will occur in fronds of the acrogenic type, like those we set out with, is shown by the case of Metzgeria furcata, Figs. [45, 46,] in which such compound prolification is partially displayed. Let us suppose, then, that the frond a, Fig. [106], produces not only a single secondary frond b, but also another such secondary frond b’. Let us suppose, further, that the frond b is in like manner doubly proliferous: producing both c and c’. Lastly, let us suppose that in the second frond b’ which a produces, as well as in the second frond c’ which b produces, the doubly-proliferous habit is manifested. If, now, this habit grows organic—if it becomes, as it naturally will become, the characteristic of a plant of luxuriant growth, the unfolding parts of which can be fed by the unfolded parts; it will happen with each lateral series, as with the main series, that its successive components will begin to show themselves at earlier and earlier stages of development. And in the same way that, by dwarfing and generalizing the original series, we arrive at a structure like that of the terminal bud; by dwarfing and generalizing a lateral series, as shown in Figs. [107–110], we arrive at a structure answering in nature and position to the axillary bud.