54 BOTANICAL GAZETTE [ JANUARY 
regarded as a phenomenon of growth. Similar changes in the 
membrane have been shown by Strasburger to accompany 
growth in the formation of new branches of Cladophora. 
Whether the shortening that has been frequently observed on 
the concave side of curving grass nodes is due to compression 
merely or to active shortening has not been demonstrated. 
The climbing hooks of various tropical plants investigated 
by Ewart in Java (20) are examples of organs which respond to 
stimuli by increased cambial growth and cell division. Here 
also the mechanical relations of the tissues are such that neither 
mere changes of turgor on opposite sides nor any plastic or 
elastic stretching could produce the response to stimuli which 
these organs show. The curve of the hook may be increased by 
a greater growth on the convex side; or the hook may be made 
to clasp a support merely by an increase in the transverse diam- 
eter which diminishes the diameter of the circle already formed 
by the normally curved hook. In either case the response takes 
the form of growth. 
Noll has reported experiments with Hippuris stems which are 
perhaps of more importance than he supposed (50, p. 51). The 
stems were laid in a horizontal position in air not saturated with 
moisture and they were not in contact with liquid water. Never- 
theless they curved upward in a U-shape; and the lower convex 
side was smooth, shiny and turgid, while the upper concave side 
became wrinkled and limp. Since the stem could get no water 
from the outside, any increase in the amount of water in the cells 
on the lower side must have come from the cells on the upper 
side. The water could not have been forced out of the cells on 
the upper side by mechanical pressure from the lower side, 
because that pressure could have been exerted only after the 
lower side had obtained more water. Either the cells on the 
lower side withdrew water from the cells on the upper side by 4 
greater osmotic power, in which case the water would not pass 
into the intercellular spaces; or the cells on the upper side, 
affected directly by the stimulus, allowed the water to pass into 
the intercellular spaces, and the water was then taken up by the 
