THE SPERMATOPHYTA : GENERAL INTRODUCTION 647 



views of this relationship, (i) That the axis pre-existed and that the leaves 

 originated as lateral outgrowths from it. (2) That branches and leaves both 

 originated from the differentiation of a system whose divisions were originally 

 all alike. (3) That the leaf was the primitive structure, in the form of a 

 thalloid body, the basal part of which assumed the form of an axis bearing the 



leafy portion. 



The first view may be called the classic scheme of plant morphology, and 

 it is that upheld by Bower himself. He considers the leaves to have arisen 



Fig. 652. — Lycopodium selago. Complete plant 

 with sporangia in the axils of undifferen- 

 tiated leaves. Half natural size. 



by direct outgrowth from a primordial leafless axis, in a manner similar to 

 their actual origin at the growing point of a living plant. 



The second view assumes that dichotomy, or division of the shoot into 

 equal portions at each branching point, is primitive, and that the development 

 of the condition in which there is a principal axis bearing laterals (which is 

 normal in the higher plants) has been arrived at by a process of " overtoppmg." 

 This implies the unequal growth of the two segments of a dichotomy, whereby 

 one segment assumed a leading position and the other segment took the 

 character of a mere appendage to the leader, becoming eventually, in many 

 cases, dwarfed to the condition of a lateral bud, which might or might not grow 

 out into a branch (Fig. 653). It is true that dichotomy does seem to have 

 been the earliest type of branching, at least in vascular plants, and it is possible 

 that this scheme may truly represent the evolution of the higher monopodial 

 system of branching, but it is doubtful whether this view has anything to do 

 with the evolution of leaves as distinct organs, except perhaps, as we shall 

 see later, in the Ferns. 



