376 VEGETABLE PHYSIOLOGY 



frequently serves 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 itself 

 upon its mid-rib as an axis, so as to hide completely the 

 ventral face. 



It may be merely a relaxation of the effort involved in 

 maintaining the strain of tone induced by the light of day 

 time, in a certain sense implying a condition of rest. 



Movements which bear a striking superficial resem- 

 blance to the nyctitropic movements of leaves are those of 

 the opening and closing of certain flowers, which take 

 place with astonishing regularity and precision at certain 

 hours of the morning and evening. Though they seem 

 to be influenced by the alternation of light and darkness, 

 it is more probable that they are really stimulated by the 

 changes of temperature which accompany such alternation. 

 These variations, to be effective, must lie, however, within 

 the range already indicated as being necessary for the 

 manifestation of irritability at all. The movement is due 

 to rhythmically varying turgescence of the cells upon the 

 two faces of the growing zone of the floral leaves, which is 

 in these cases a narrow transverse band situated near 

 their bases. This change of the turgescence is followed 

 in many cases by actual growth, and as the latter is not of 

 prolonged duration the flower can only open and close a 

 few times while it is attaining its maturity. 



Besides the general reactions of protoplasm to varia- 

 tions in those features of the environment which bring 

 about modifications of its general tone, and which thus 

 affect more or less the whole plant, we find instances of 

 special sensitiveness in various parts to influences which 

 are not appreciated by the whole of the living substance. 

 Of these the most prominent are lateral light, gravity, 



