122 PLANT PHYSIOLOGY 
of barberry, the rapid closing of the leaf halves of 
Dionaea, the closing of the leaflets and dropping down- 
ward of the leaves of Mimosa are responses to the stimulus 
of contact. In the case of the sundew the movement of 
the tentacles may take place both in response to contact 
or to the presence of certain chemicals such as ammonium 
sulphate, proteins, etc. It is worthy of note that the 
stimulus may be applied at a distance even of several 
centimeters from the point where the change in turgor 
occurs, i.e. the plant tissues are able to transmit a stimu- 
lus for a considerable distance. None of these move- 
ments will take place except under the proper degrees of 
temperature, moisture, etc. 
Laboratory Studies. (a) Observe a plant of Desmodium 
gyrans at a temperature of between 20° and 30° C. The 
rapidity of the rotation of the leaflets will be found to vary 
with the temperature, degree of illumination and other factors. 
(b) Observe clover and Oxalis leaves by night and by day. 
Compare also the leaves of Mimosa, Robinia, etc., in light and 
darkness. 
(c) Touch one of the three bristles on the surface of a leaf 
half of Venus fly-trap (Dionaea). Note the sudden closing of 
the leaf. The temperature and humidity must be rather high 
or it will not respond well. 
(d) Touch a leaf of a sensitive plant (Mimosa pudica) at 
the under side of the pulvinus. Touch or slightly pinch other 
leaves of the same plant at various points. Apply the flame 
of a match to the end of one of the leaflets. Note in this case 
the progressive closing of the leaflets followed by the dropping 
of the whole leaf and in many cases of the nearest leaves above 
and below. 
(e) Place a grain of sand on the tip of a tentacle of a leaf of 
sundew (Drosera). Note the degree of movement in the sur- 
rounding tentacles. On a tentacle on another leaf place a tiny 
piece of meat or a very small crystal of ammonium sulphate and 
note the movements of the adjacent tentacles. 
181. Growth Movements. Many plant movements 
are the result of unequal growth on opposite sides of an 
LS 
