35§ BOTANICAL GAZETTE [may 



manner of formation of vascular elements by stelar tissues varies 

 with the species, but is relatively constant in any one species un- 

 der various conditions. 



Examination of the various structures shows that they differ 

 mainly in the tissue producing them and in amount of development. 

 That is, these individual variations are differences of position and 

 quantity of vascular elements, and as such their explanation is to be 

 looked for among the physiological factors operating at the time of 

 their development. From this point of view, vascular structures 

 produced as a result of injury may readily show ancestral characters; 

 but such characters are to be considered, not as the repetition of a 

 definite stage of the phylogenetic development of the form, but 

 rather as an indication of the recurrence of certain conditions of 

 development. 



It seems well to insist at this point that vascular strands are 

 secondary structures as compared with the tissues which they 

 traverse. The formation of an organ creates a physiological de- 

 mand to which the vascular strand is a response; and uniformity 

 of the structure which results is only an indication of uniformity 

 of demand and of uniform conditions of development. In this 

 view, the vascular connections of the branches are determined 

 by factors of the same character as those controlling wound 

 reactions. 



The vascular supply must be contributed by stelar tissues ca- 

 pable of growth; these tissues are the ones already enumerated, 

 pith, cambium and adjacent parenchyma, and pericycle, together 

 with the parenchyma between the leaf trace and the leaf gap. The 

 manner of formation of the vascular elements of the branch supply 

 is more or less restricted in any species to the particular method of 

 that species. Thus in B. ramosum, in which secondary wood forma- 

 tion is relatively slight, no renewed cambial activity occurred in 

 connection with the formation of a branch; while in B. obliquum, 

 in which secondary wood formation is very marked under usual 

 conditions, not only does the cambium begin active growth in every 

 case of branching, but similar cambial activity is sometimes set up 

 in both pith and pericycle. The physiological demand likewise 

 varies; in the branch of B. ramosum represented in figs. 4 and 8, 



