I 



646 A TEXTBOOK OF THEORETICAL BOTANY 



organization. The vascular system of the Ferns belongs essentially to the 

 axis or stem. Its development is correlated with the size of the stem, and 

 the strands which supply the leaves are only subsidiary branches from the 

 cauline {i.e., axial) system. The vascular bundles of the Spermatophytes 

 are, on the contrary, all part of a system " common " to both stem and leaf. 

 The cauline vascular system has disappeared and every strand in the stem 

 is destined sooner or later to form part of the supply passing to a leaf.^ It 

 is even doubtful whether there is any cauline vascular tissue in the apical 

 meristem, and it is probable that in all normal cases the earliest appearance 

 of vascular tissue at the apex is in connection with the leaf rudiments. This 

 is a very significant difference, which invalidates any direct comparison of the 

 vascular structure in a Pteridophyte stem and Spermatophyte stem. The 

 leaves, which are the principal evaporating surfaces of the plant, have thus 

 become linked directly to the roots, which are the sources of the water supply, 

 and the amount of vascular tissue formed is proportionate to the develop- 

 ment of the leaves rather than to the development of the stem. In other 

 words, the development of the stem itself is dependent on the development 

 of the leaves, and it is no longer, as in the Pteridophytes, an independent 

 entity, and hence the plant gains in physiological efiiciency. 



The Strobilar Theory 



There are two main theories of the evolution of the cormophytic type of 

 body ; but they are not mutually exclusive, for one may be said to extend 

 the other. The older theory is called the strobilar theory, and it has been 

 chiefly expounded by Bower. He defined a strobilus as a spike of fertile 

 appendages, borne in a terminal position on the shoots. The vegetative 

 region below is so similar in structure to the strobilus that it is to be regarded 

 as derived from it by the abortion of the sporangia in the lower regions of 

 the shoot. This idea is illustrated by Lycopodhim selago (Fig. 652), in which 

 sporangia are formed in recurrent zones on the shoot, from the base upwards, 

 while the sterile zones between show abortive sporangia in the leaf axils. 



Thus on Bower's view the most primitive condition of the vascular plant 

 is one with an axis bearing leafy appendages which all carried sporangia, the 

 whole plant being one continuous strobilus. 



Although the most typical strobili are characteristic of the small-leaved 

 Pteridophytes, it is claimed that the large-leaved Ferns and their allies also 

 conform to the above theory, the spiral succession of leaves upon the main axis 

 constituting, in effect, a large strobilus, in which sometimes all, sometimes 

 only certain groups of the leaves are sporangiferous. Such a view obviously 

 does not recognize any essential difference, except in size, between the Fern 

 leaf and the Lycopod leaf. 



Bower then goes further to speculate on the primitive relationship of the 

 axis and its appendages. He points out that there are at least three possible 



^ To avoid any possible misconception it might be well to point out that the conventional 

 way of expressing a vascular connection as "passing" from one organ to another does not 

 imply any actual motion, any more than when we say a road " runs " from A to B. 



