OF MOVEMENT IN PLANTS. 79 
period, all the vascular elements are in a nascent state (Plate V. Fig. 6 a). As soon, 
however, as the leaflets unfold and movement begins, the various vascular elements are 
found to have become strongly developed (Fig. 6 B), and from that time onward they 
continue to increase in their character as permanent structure. It thus appears true that 
no movement can occur until the woody tissue reaches a certain stage of maturity. 
SENSITIVENESS.—The leaves of Robinia are not sensitive in any marked degree. Inei- 
sions and other strong irritations of the pulvinus have, with us, produced no effect. 
Simple irritations, such as would produce an immediate effect in the tendril of Cucurbita 
unless very much prolonged, are also without effect, and response appears to be gained 
from nothing less violent than percussion. Several determinations of the effect of percus- 
sion were made. In each case the base of the leaf just above the pulvinus was given a 
short, sharp blow with a pencil. The following results were obtained (in each case the 
degrees given represent the depression of the leaflets two minutes after percussion) :— 
(a) 10.05 a.m.—No. 1=87°. No. 2=87°. No. 3=60°.—Recuperation complete at 10.25.—Time required=20 m.— 
Plant in shade. 
(b) 10.12 a.m.—No. 1=87°. No. 2=50°.—Recuperation complete at 10.30.—Time required=18 m.—Plant in shade. 
(e) 10.17 a.m.—No. 1=50°. No. 2=45°.—Recuperation complete at 10.30.—Time required=13 m.—Plant in sun. 
(d) 10.20 a.mw.—No. 145°. No. 2=45°-80°.—Recuperation complete at 10.31.—Time required=11 m.—Plant in 
the sun. 
(e) 10.32 a.m—No 1=0°. No. 2=0°. No. 3—50.—Recuperation complete at 10.42.—Time required=10 m.—Plant 
in the sun. 
(f) 10.388 a.m.—No. 1=50°. No. 2=45°. No. 3—45°.—Recuperation complete at 10.53—Time required—15 m.— 
Plant in the shade. 
(g) 10.42 a.m,—No. 1=55°. No. 2=45°-50°. No. 3—45°-60°.—Recuperation complete at 10.59.—Time required=17 m. 
—Plant in the shade. 
(h) 10.47 a.m.—No. 1=0°. No. 2=0°. No, 3=2°-3°.—Plant in the sun. 
In all these observations it was noticed that the basal leaflets, hence those nearest 
the percussion, responded first and most strongly; also, that the effect of percussion did 
not appear until fifteen or twenty seconds had elapsed, after which the motion became 
an accelerating one until the maximum of change was reached ; the time for recuperation, 
as indicated above, thus embraces both depression and subsequent elevation to normal 
position. In all these cases one fact is conspicuous, viz., the relation of recuperation to 
direct action of the sunlight. Whenever the plant was in the sun the leaves were much 
less depressed from percussion, and their recuperation was much more rapid as compared 
with leaves in the shade. 
NyctTITROPISM.—The nyctitropic, or true sleep movement, is that which essentially 
characterizes the leaves of Robinia. They may also manifest during the day, under the 
influence of bright sunshine, a paraheliotropic movement, during which the general 
tendency is for the edges to be turned upward to the sun, as if to check its influence. As 
Darwin has already pointed out,’ the object of this movement is totally different from 
that of sleep movement, and is doubtless designed to lessen the destructive influence of 
too intense sunlight upon the chlorophyll. 

' Movements of Plants, 355, 445. 
