. The origin of a land flora, a theory based upon the facts of alternation. Plant morphology. COMPARATIVE ANATOMY 33i strands become differentiated in the intervening tissue, which form a connec- tion with the central cylinder: upon this they are inserted laterally. It is thus clear that in the ontogeny of the shoot the leaf is an accessory which arises after the stele is already in existence. Its relative unimportance is not only apparent from this late origin, but also from the fact that the arrangement of the leaves upon the shoot does not dominate the number or position of the protoxylem-g
. The origin of a land flora, a theory based upon the facts of alternation. Plant morphology. COMPARATIVE ANATOMY 33i strands become differentiated in the intervening tissue, which form a connec- tion with the central cylinder: upon this they are inserted laterally. It is thus clear that in the ontogeny of the shoot the leaf is an accessory which arises after the stele is already in existence. Its relative unimportance is not only apparent from this late origin, but also from the fact that the arrangement of the leaves upon the shoot does not dominate the number or position of the protoxylem-groups of the stele. It has long been known that the number of the xylem-rays is independent of the position of the leaves. In L. clavatum Jones has found that though in shoots with simple leaf-arrangement it is usual for the protoxylems to correspond to the leaf-insertions, still, where the number of protoxylems is beyond six, there is no apparent relation between them and the Fig. 172. Longitudinal section through the apical cone of the stem of Lycopodium Sclago. X 160. (After Strasburger.) "When the above facts are taken together, it is apparent that the leaf in Lycopodium is but an accessory appendage, and that the axis is the â¢dominant feature of the shoot. This conclusion probably applies for Lycopods at large, and it has its important bearing on the relation of leaf to axis, discussed in Chapter XI. Hitherto no definite knowledge of the anatomy of the smaller fossil â eligulate Lycqpods included under the name Lycopodites has come to hand : whenever such facts are available they will provide interesting material for â¢comparison with the modern species of Lycopodium. The ligulate and hetero- sporous forms would be equally important for comparison with Selaginella. The discussion of the external morphology of the latter genus has led to the /recognition of the radial type as relatively primitive, while those species with dorsiventral shoots ar
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