490 MANUAL OF BOTANY 



The peculiar movements which the leaves perform in response 

 to the stimulus are brought about by different mechanisms in dif- 

 ferent cases. In young leaves they are attendant upon growth, 

 and are brought about by variations of turgescence upon the two 

 sides of the leaf or its petiole, which are frequently followed by 

 growth. We have seen that during growth the internal turgescence 

 varies a good deal and leads to the curious movements of nutation 

 or circumnutation. The actual nyctitropic movement is in these 

 cases a modification of the extent of the circumnutation and 

 takes place in response to the stimulus. Those leaves which 

 show it can be seen by careful observation to be circumnutating 

 during the day. When they assume their nocturnal position it 

 is generally effected by their describing a much longer ellipse 

 than during their ordinary movement. In some cases onl^^ a 

 single ellipse is described during the twenty-four hours ; in 

 others two ellipses, the nyctitropic one being much the greater 

 in amplitude. In yet other cases several ellipses may be de- 

 scribed in the same time. Adult leaves which show this move- 

 ment do so by virtue of a special pulvinus, or development of the 

 hypopodium. This becomes turgid alternately on its two faces, 

 causing the leaf to droop and to rise accordingly. These leaves 

 generally show the movement for a much longer period than 

 those in which it is brought about by variations of turgescence 

 followed sometimes by growth. This naturally follows from 

 the fact that the growth of leaves is not as a rule very prolonged. 



That these movements are essentially dependent on the 

 power of the protoplasm to receive impressions from without can 

 be seen from a study of the conditions under which they are per- 

 formed. When the soil is too dry, or when from any other cause 

 the protoplasm in the cells is not supplied with water in suf- 

 ficient quantity, they cease. When the temperature is too low 

 they are interfered with. Violent disturbance of the protoplasm 

 by shaking the plant will in some cases prevent their occurrence 

 for one or two nights. 



The purpose of the movement is probably to protect the 

 delicate leaves from excessive radiation, which affects them very 

 prejudicially. Their upper surfaces are especially liable to be 

 injured in this way, and it is noteworthy that in all cases these 

 surfaces are most sheltered when they take up their nocturnal 

 positions. Often the upper surfaces of leaflets are then closely 

 approximated together ; in Bauhinia the leaf folds upon its 

 midrib as an axis, so as to hide completely the ventral face. 



The relation of the plant to temperature may next be con- 



