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]