Starting, as before, with the proliferous form shown in Fig. 111, it is clear that if the strength required for maintaining the vertical attitude, is obtained by the rolling up of the fronds, the primary frond will more and more conceal the secondary frond within it. At the same time, the secondary frond must continue to be dependent on the first for its nutrition; and, being produced within the first, must be prevented by defective supply of light and air, from ever becoming synchronous in its development with the first. Hence, this infolding which leads to the endogenous mode of growth, implies that there must always continue such pre-eminence of the first-formed frond or its representative, as to make the germination monocotyledonous. Figs. 111 to 115, show the transitional forms that would result from the infolding of the fronds. In Fig. 116 (a vertical section of the form represented in Fig. 115) are exhibited the relations of the successive fronds to each other. The modified relations that would result, if the nutrition of the embryo admitted of anticipatory development of the successive fronds, are shown in Fig. 117. And how readily the structure may pass into that of the monocotyledonous germ, will be seen on inspecting Fig. 118; which is a vertical section of an actual monocotyledon at an early stage--the incomplete lines at the left of its root, indicating its connexion with the seed.[13] Contrariwise, where the strength required for maintaining an upright attitude is not obtained by the rolling up of the fronds, but by the strengthening of the continuous mid-rib, the second frond, so far from being less favourably circumstanced than the first, becomes in some respects even more favourably circumstanced: being above the other, it gets a greater share of light, and it is less restricted by surrounding obstacles. There is nothing, therefore, to prevent it from rapidly gaining an equality with the first. And if we assume, as the truths of embryology entitle us to do, an increasing tendency towards anticipation in the development of subsequent fronds--if we assume that here, as in other cases, structures which were originally produced in succession will, if the nutrition allows and no mechanical dependence hinders, come to be produced simultaneously; there is nothing to prevent the passage of the type represented in Fig. 111, into that represented in Fig. 122. Or rather, there is everything to facilitate it; seeing that natural selection will continually favour the production of a form in which the second frond grows in such way as not to shade the first, and in such way as allows the axis readily to assume a vertical position.