296 BOTANICAL GAZETTE [APRIL 
of 35° C. to 40° C. the leaves frequently responded to one downward — 
thrust without a release. 
When a sensitive hair was bent down and held mechanically in 
that position for 5 minutes, the leaf nearly always failed to respond. 
This experiment was repeated many times, but response followed in 
only a very few cases, and then only a few seconds after the contact. 
These exceptional closures seem to have been due to the downward 
thrust. 
In order to determine the effect of the upward movement of the 
hair, the latter was bent down carefully against the leaf without 
causing closure, and held in that position for a period of 5 minutes 
in order that the effect of the compression stimulus might disappear. 
The hair was then allowed to spring back to its original position by 
two successive upward movements. This experiment was repeated 
many times, but the leaves never exhibited any response. The same 
operation was then repeated with the addition of alternate bendings 
and releases subsequent to the two upward movements, at intervals 
of 15 seconds. In no case did the leaf respond to a release, but 
always to a downward movement. The leaf is thus seen to respond 
only to a downward bending of the hair, and it seems probable that 
stimulation is due to a compression of the cells at its base, and that 
this compression is analogous to the compression of the cells of the 
blade when the latter is squeezed with forceps. 
That the bending of the cells at the base is alone sufficient to cause 
response was shown in the following manner. A hair was cut off 
near the base and the leaf allowed ample time to recover from any 
possible shock produced by the cutting. The remaining portion was 
then pressed with a needle. In this operation it could be seen by 
means of a lens that bending took place only in the region where 
lie the cells shown by HaBerLaAnpr to be especially suited to a com 
pression stimulus. In such cases the leaves responded normally. 
That simple bending of the sensitive hair, without contact with 4 
hard object, is sufficient to cause closure was shown by directing jets. 
of air against it, when the leaf responded as if stimulated by contact. 
We have seen that all parts of the leaf are sensitive to mechanical 
pressure, and that the hair probably functions as a mechanism for 
the compression of certain cells. It next remains to be seen whether 
