CHAPTER VII 



THE VASCULAR SYSTEM OF THE AXIS 



(A) THE PROTOSTELE AND ITS IMMEDIATE DERIVATIVES 



The Vascular System provides the most constant structural characters avail- 

 able for comparison in Land-living Plants. As vascular tissue is frequently 

 well preserved in the fossils it gives a basis for comparison between ancient 

 and modern Pteridophytes. But in dealing with anatomical facts it must be 

 remembered always that in any progressive evolution vascular structure 

 follows, it does not dictate, external form. All the evidence which it yields 

 is necessarily ex post facto evidence. On the other hand, the structural effect 

 of a certain development may remain after the formal characters with which 

 it was primarily bound up have been altered, or even wholly removed. 

 Anatomical characters are apt tardily to follow evolutionary progress, and 

 thereafter to persist as vestigia. In them we see illustrated what may be 

 described as phyletic inertia. This gives them a special value for comparison. 



The Filicales show a very wide latitude of vascular structure. In no Class 

 of living plants is there so great a complication of the primary vascular tracts. 

 With one or two insignificant exceptions, the vascular system of all Ferns is 

 primary. Secondary thickening is a rare occurrence in them. The physio- 

 logical problem of their progress to large size has then been to provide, 

 through a primary development, a conducting system sufficient for an en- 

 larging plant-body. Little wonder then that with means so restricted the 

 Ferns have never achieved the largest dimensions. 



The scientific basis for a comparative treatment of the vascular system 

 of such a Class should be founded on (i) the facts of actual structure and 

 distribution of the vascular tissues in the adult of the various types ; (ii) the 

 facts derived from the related fossils, presumably also in the adult state ; 

 (iii) the facts derived from the ontogeny of living and possibly also of fossil 

 types. Historically these three avenues of enquiry have been taken up in 

 the order above stated, and frequent misunderstandings have arisen as a 

 consequence of the long delay in developing the last of them. The best 

 method will be to carry along all of these lines of observation and comparison 

 simultaneously. In particular, the study of the ontogeny should always take 

 quite as prominent a place as either of the other two. It is only when these 

 three sources of knowledge have been combined, aided it may be by actual 

 experiment by means of starvation-cultures which may reverse the steps of 



