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 tipon 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 Z. 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 leaf-insertions.1 
Fic. 172. 
Longitudinal section through the apical cone of the stem of Lycopodium Selago. 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 
‘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 Se/aginel/a. 
The discussion of the external morphology of the latter genus has led 
to the fecognition of the radial type as relatively primitive, while those 
Species ‘with dorsiventral shoots are held to be more specialised and 
1 Zinn. Trans., 2nd series, vol. vii., p. 19. 
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