358 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 asa 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, 
