40 Dr. W. C. Williamson on 



The further question has arisen, how far has the ordinary- 

 growth of a branch exercised any influence upon, or borne 

 any relation to, the varying dimensions of the Primary 

 Xylem cylinder of the stele, and upon the number of its 

 component tracheids? I think my measurements will be 

 found to give some decided answers to these questions. By 

 what modus operandi these results have been brought about 

 was hitherto an open problem. Unlike what occurs amongst 

 the living Lycopods, amongst the Carboniferous Lepidodendra 

 we find, as we descend from the uppermost and youngest 

 shoots, that there is a regular progressive enlargement of the 

 branches below each succeeding dichotomy ; and, what has 

 an important bearing upon my present methods and ultimate 

 conclusions, these enlargements are accompanied by a similar 

 though less conspicuous enlargement of the cylinder of 

 Primary Xylem, and also in the number of its component 

 tracheids. 



In measuring the relative sizes of these branches I 

 confine myself to the periphery of the Prosenchymatous or 

 Periderm Zone of the cortex. I do so because the leaves 

 are an uncertain boundary as well as being so frequently 

 absent, which is rarely the case with the tissue referred to. 

 In some of my larger sections even this peripheral outline 

 has broken away. Nevertheless, I have still measured what 

 remains of the cortex, since its large dimensions shew how 

 much the branch from which it is taken must have advanced 

 in age. 



The Type of Lepidodendron Selaginoides. 

 In this type a peculiarity exists which prevents my 

 measuring the Primary Xylem of the stele beyond recording 

 the diameter of its periphery. The latter margin is sharply 

 defined, but its internal one is not so. Unlike all the other 

 types the tracheids, which are numerous and densely 

 aggregated at the periphery, become fewer in number and less 



