OF MOVEMENTS IN PLANTS. 83 
Being in a state of positive tension, any release of tension in the surrounding pulvinus at 
once permits contraction in the bast. Under irritation, and possibly other influences, this 
may also be increased by loss of water, as occurs in a more marked degree in the collenchyma 
of Cucurbita. The direction of bending, as determined by this contraction, depends upon 
the distribution of the bast in the organ. In the case of the leaflets, we have seen the bast 
to be so disposed below the axis of the petiolule, that the latter can only curve downward, 
this being facilitated, moreover, by the vascular structure being open along the upper side. 
In the main pulvinus, the closed cylinder of vascular structure preserves a condition of 
rigidity in all parts except at the extreme base, where we find the vascular structure to 
become branched in such a way as to produce a true joint in connection with an intercel- 
lular cavity. 
EXPLANATION OF PLATES. 
Puate III. 
Figure, half natural size, showing movement of tendril tip. The figure is seen as if the observer were at 
at the base of the tendril looking toward the tip. 
Prare IV. 
Fig. 1.—Half section of tendril arm of Cucurbita pepo x 66. 
(a) Vibrogen. 
(b) Collenchyma. 
(c) Active parenchyma. 
(d) Bast. 
(e) Vascular bundles. 
(f) Central pith parenchyma. 
“ 2.—Bast cells x 266. 
A. During active period. 
B. After coiling of tendril. 
3.—Collenchyma x 266. 
4.—Continuity of Protoplasm in collenchyma x 266. 
“ 5.—Vibrogen x 133. 
PLATE V. 
Fig. 1—Half cross-section of tendril of Vitis cordifolia, Michx., x 66. 
cl. collenchyma. 
pr. parenchyma. 
w. wood. 
p. pith. 
2.—v. vibrogen tissue x 266. 
3.—cl. collenchyma x 266. 
“ 4,—wood tissue x 266. 
A. During activity of the tendril. 
B. After cessation of motion. 
5.—Cross section of smaller pulvinus of leaflet, Robinia pseudacacia x 66, showing relation of central 
vascular structure to the pulvinus tissue. 
6.—Structure of vascular zone x 266. 
A. Before nyctitropic activity. 
B. During nyctitropic activity. 
