470 BOTANICAL GAZETTE [JUNE 
composed of two or three slightly thickened vessels. In this 
region only a few bast fibers are present, which are very small; 
very little pith is present, which lines a central cavity. Toward 
the base a complete ring of thin-walled xylem and pith is formed. 
Sections of those tendrils under contact and under tension 
show the normal complete ring of mechanical tissue and central 
thin-walled pith. At the first examination of these sections, 
little difference could be seen in structure or areas of mechanical 
tissue, and camera sketches show no difference in thickness of 
walls of the xylem, though the greater part of those under tension 
had a tensile strength 50 per cent higher than those with contact 
alone. A closer examination of the sections with the aid of micro- 
photographs and camera sketches shows that while the xylem 
areas are approximately the same in both, in the one where ten- 
sion had been introduced the walls of the pith cells have become 
much thickened, while in the one with contact alone they are 
quite thin-walled. This thickening of walls takes place usually 
throughout the whole area of the pith of the tendril under tension, 
while in the one which had been under contact alone the pith is 
thin-walled throughout. It is worthy of note also that in sec- 
tioning, the ones under tension were much harder to cut through, 
which is no doubt due to a difference in density of cell walls. 
3. Study of sections; experiments on middle third.—Examina- 
tion of sections of those tendrils where the middle third was (1) 
with and (2) without tension shows the diameter of mechanical 
tissue to be much greater in the latter, which accounts for the 
greater outside diameter usually found in these tendrils. This 
seemingly greater area of mechanical tissue in the tendril grasping 
a support but not under tension is somewhat surprising when we 
consider that those under tension had a breaking strength nearly 
50 per cent higher. This increased strength with tension is at 
least partly accounted for by the fact that the pith walls in the 
tension-tendril are thickened (very similarly to those in the pre- 
ceding experiment under tension), while in the one not under 
tension all the pith is thin-walled. In order to be certain that this 
thickening of the pith is constant with those under tension, sec- 
tions of more than 30 tendrils in this experiment were studied 
