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. 



(6) 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 



