Thus, then, is interpretable the universal connexion between monocotyledonous germination and endogenous growth; as well as the similarly-universal connexion between exogenous growth and the development of two or more cotyledons. That it explains these fundamental relations, adds very greatly to the probability of the hypothesis. § 196. While we are in this manner enabled to discern the kinship that exists between the higher vegetal types themselves, as well as between them and the lower types; we are at the same time supplied with a rationale of those truths which vegetal morphologists have established. Those homologies which Wolff indicated in their chief outlines and Goethe followed out in detail, have a new meaning given to them when we regard the phænogamic axis as having been evolved in the way described. Forming the modified conception which we are here led to do, respecting the units of which a flowering plant is composed, we are no longer left without an answer to the question--What is an axis? And we are helped to understand the naturalness of those correspondences which the successive members of each shoot display. Let us glance at the facts from our present standpoint. [Illustration: Figs. 123–129.] The unit of composition of a Phænogam, is such portion of a shoot as answers to one of the primordial fronds. This portion is neither one of the foliar appendages nor one of the internodes; but it consists of a foliar appendage together with the preceding internode, including the axillary bud where this is developed. The parts intercepted by the dotted lines in Fig. 123, constitute such a segment; and the true homology is between this and any other foliar organ with the portion of the axis below it. And now observe how, when we take this for the unit of composition, the metamorphoses which the phænogamic axis displays, are inferable from known laws of development. Embryology teaches us that arrest of development shows itself first in the absence of those parts that have arisen latest in the course of evolution; that if defect of nutrition causes an earlier arrest, parts that are of more ancient origin abort; and that the part alone produced when the supply of materials fails near the outset, is the primordial part. We must infer, therefore, that in each segment of a Phænogam, the foliar organ, which answers to the primordial frond, will be the most constant element; and that the internode and the axillary bud, will be successively less constant. This we find. Along with a smaller size of foliar surface implying lower nutrition, it is usual to see a much-diminished internode and a less-pronounced axillary bud, as in Fig. 124. On approaching the flower, the axillary bud disappears; and the segment is reduced to a small foliar surface, with an internode which is in most cases very short if not absent, as in 125 and 126. In the flower itself, axillary buds and internodes are both wanting: