THE VASCULAR SKELETON 691 



It is naturally the primary developments, however, which are of im- 

 portance in the present comparisons : and sufficient has been said to show 

 that the anatomical evidence, combined with that from embryology, has 

 a very direct bearing on the theory of the strobilus. The uniform reference 

 of the stelar structure to a protostele, and the actual existence of this 

 structure in the young seedlings of the most diverse types, points clearly 

 to its early existence in the race. Its continuity up to the apex of the 

 axis in the more primitive of the living, small-leaved types is a further 

 fact of importance : while the attachment of the foliar traces to the outer 

 surface of the cauline core indicates not only the priority of the latter, 

 but also the subsidiary character of the former. Lastly, the correlative 

 reduction of the axis in the embryos of the larger-leaved forms, consequent on 

 their precocious development of the first leaf accounts on well-known prin- 

 ciples for their structure : it explains the fact that in them the evidence of 

 early existence of the cauline core is not so prominent as it is in the 

 smaller-leaved forms, which are on our hypothesis the nearer to an 

 original type. The general conclusion from comparative study of the 

 vascular skeleton, combined with the facts of embryogeny, will therefore 

 be that it supports the priority of the axis over the leaf: it shows that 

 the axis was from the first traversed by a conducting core, upon which 

 the conducting strands of the leaves became attached. But that both the 

 stele and the leaf-trace were susceptible of amplification and disintegra- 

 tion as a consequence of the enlargement of axis and leaf, and of the 

 increasing proportional influence of the latter : in fact, the leaf in certain 

 forms became at last the dominating feature of the shoot, and conse- 

 quently its influence also controlled the internal vascular structure of the 

 whole shoot. This condition, which is that characteristic of those forms 

 which have been designated " phyllosiphonic," is believed to have been 

 of secondary origin, and the structural progress shown in the individual 

 life would appear to indicate with special clearness that it was so. 



