1912] BRUSH—MECHANICAL TISSUE 473 
thickened, is larger in amount than in the last, and the central 
cavity is much smaller. 
If we now compare the structure of the tendrils in the two 
methods of experimentation, it becomes very evident that the 
additional amount of xylem in the tension-free tendrils (as com- 
pared to the tension-tendrils) of the first set is due to the extra 
contact-pressure introduced, and the thick-walled pith found in 
the same tendrils, which has not before appeared in tendrils except 
when tension was introduced, is due to the stimulus of tension 
conducted to the basal part from the portion in tension between 
the two ligatures. That this thickening of the pith which was 
caused by only a small portion of the tendril being under tension 
did not appear in the former experiments on the middle third of 
the tendril is no doubt due to the fact that the tension in the latter 
case was in the contact portion of the tendril, which is not so sensi- 
tive to the stimulus of tension as is the lower two-thirds of the 
tendril. 
5. Study of sections; experiments on contact portion.—Examina- 
tion of sections through the contact region of tendrils which had 
been put under (comparatively) great pressure by a column of 
mercury failed to detect any difference in anatomical structure 
when compared with those which had been in contact only. Sec- 
tions through the middle of the tendrils, however, where there was 
no tension and where the break usually occurred, show marked 
differences between the two sets of tendrils in the amount of xylem 
present. The area of xylem in the ones which had been under 
pressure, in an average tendril, was approximately twice as great 
as the area of xylem in the ones which had been in contact only. 
No differences in the pith could be detected; it was thin-walled 
alike in both sets of tendrils. 
In those cases where a pressure of 20 grams was obtained by 
laying a weight on the tendril, no difference could be observed 
between these tendrils and those under mere contact, though the 
former had a slightly greater average breaking strength. Bo 
had the usual ring of xylem and the pith was thin-walled. 
Sections were not made of tendrils which were ligatured in 
different regions. 
