1912] BRUSH—MECHANICAL TISSUE 455 
use of a large number of individuals in each experiment, has, we 
believe, demonstrated that there is an actual response of the plant 
to tension, by which stronger tissues are laid down and the tensile 
‘strength of the part under tension increased, though not to such 
an extent as HEGLER’s results seem to show. 
Another line of investigation on the effect of tension has been 
followed in a comparison of the cells on opposite sides of a stem 
which has been prevented, by weighting, from responding to a 
heliotropic or negative geotropic stimulus (BALL 1, pp. 326 f.). 
In this case a thickening of cell walls occurs on the convex or upper. 
side of the stem, which has been believed by some to be due to a 
state of tension. A similar thickening occurs (BALL I, p. 339) on 
the upper side of a stem in a plaster cast placed horizontally, also 
in the concave portion of a stem which has formed a curve. In 
the last case cited, Bart demonstrated that no increase in breaking 
strength of the stem took place, a fact which he attributes to the 
concave side being built up at the expense of the convex side. 
This is substantiated by the investigations of PENNINGTON (16) on 
the effect of compression on plant stems, where he finds a reduction 
in the thickness of cell walls due to compression. 
While a number of investigators have thus studied the effect 
of mechanical strain upon tissues in stems, the investigations on 
tendrils have been almost exclusively for the explanation of external 
movements, such as the cause of coiling, etc. (For explanation 
of external phenomena see Fitt1nc 7, DEVRIEs 6, and MacDouGaL 
13.) The anatomical structure has been worked out in a compara- 
tive manner by MULieER (14) and Worcitzky (26), but only by the 
latter writer was the tendril treated in relation to its function. 
Darwin (5, p. 58) noted that in petiole climbers the petioles 
are thickened from contact, and TREUB (22, p. 65) found marked 
changes in the anatomical structure of the portion of the petiole in 
contact, consisting in a greater development of the mechanical 
system, which is borne out in a general way by the later work of 
VON DERSCHAU (24). 
MUtter (14), in his study of the tendrils of the Cucurbitaceae, 
found that contact produced earlier and greater lignification of 
sclerenchyma in the free portion on the under side (p. 127). 
