COMPARATIVE ANATOMY 331 



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. chwatum 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 leaf-insertions. 1 



FIG. 172. 



Longitudinal section through the apical cone of the stem of Lycopodiuin Sclago. X 160. 



(After Strasburger.) 



"\Vhen the above facts are taken together, it is apparent that the leaf 

 in Lycopodiuin is but an accessory appendage, and that the axis is the 

 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 

 eligulate Lycopods included under the name Lycopodites has come to hand : 

 whenever such facts are available they will provide interesting material for 

 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 are held to be more specialised and 



1 Linn. Trans., 2nd series, vol. vii., p. 19. 



