472 BOTANICAL GAZETTE [JUNE 
view especially to ascertaining the cause of the difference in results 
obtained in breaking strength under the. two different methods of 
experimentation (see tables IV and V). Sections of tendrils in 
the first set of experiments (where two ligatures were used in the 
one not under tension) showed the ring of mechanical tissue in 
the one not under tension to have a greater outside diameter than 
that in the one under tension. Xylem and thickened pith are 
present in both tension and tension-free tendrils. A well marked 
difference could be observed, however, between the amounts of 
xylem and thickened pith in the two sets of tendrils. In the one 
not under tension xylem was present in greater quantity than in 
the one which had been under tension, while in the tension-tendril 
thickened pith was in much greater quantity than in the one without 
tension. In a typical tendril the comparative amounts of xylem 
and thick-walled pith were as follows: 
Pith Pith 
Xylem thick-walled thin-walled an tissue 
Brees Len 6 6. Oo. 5 6.0 4:7 (r17) | 10.7 
RONSON Hee. onc. 7.2 1.8 (3.6) | 9.0 
i 
The fact that, nothwithstanding the smaller area of mechanical 
tissue, the breaking strength of the ones not under tension was 
practically the same as in those under tension, is no doubt due to 
the fact that much of the thick-walled pith in the tension-tendril 
does not possess as thick walls as does the xylem; hence does not 
give as much strength to the tendril as does the latter. An exami- 
nation of sections in the second series of experiments on the basal 
third, where contact-pressure was applied to the tension-free 
tendril by means of two loops of cord pulling against each other, 
shows a very different appearance from that just described when 
tension and tension-free tendrils are compared. In this case the 
area of xylem is practically the same in both, being about equal 
to the amount found in those under tension in the above experi- 
ments on the basal third; in the ones not under tension the pith 
is thin-walled throughout, is small in amount, and has a large 
cavity in the center; in the ones under tension the pith is much 
