Guar. X. SENSITIVENESS OF THE LEAVES, 233 
facts we may conclude that drops of syrup placed on 
the backs of leaves do not act by exciting a motor 
impulse which is transmitted to the tentacles; but 
that they cause reflexion by inducing exosmose. 
Dr. Nitschke used the secretion for sticking insects 
to the backs of the leaves; and I suppose that he 
used a large quantity, which from being dense pro- 
bably caused exosmose. Perhaps he experimented on 
cut-off leaves, or on plants with their roots not supplied 
with enough water. 
As far, therefore, as our present knowledge serves, 
we may conclude that the glands, together with the 
immediately underlying cells of the tentacles, are 
the exclusive seats of that irritability or sensitiveness 
with which the leaves are endowed. The degree to 
which a gland is excited can be measured only by 
the number of the surrounding tentacles which are in- 
flected, and by the amount and rate of their move- 
ment. Kqually vigorous leaves, exposed to the same 
temperature (and this is an important condition), 
are excited in different degrees under the following 
circumstances. A minute quantity of a weak solu- 
tion produces no effect; add more, or give a rather 
stronger solution, and the tentacles bend. Touch 
a gland once or twice, and no movement follows; 
touch it three or four times, and the tentacle becomes 
inflected. But the nature of the substance which is 
given is a very important element: if equal-sized par- 
ticles of glass (which acts only mechanically), of 
gelatine, and raw meat, are placed on the discs ot 
several leaves, the meat causes far moro rapid, ener- 
getic, and widely extended movement than the two 
former substances. The number of glands which are 
excited also makes a great difference in the result: 
place a bit of meat on one or two of the discal 
