384 VEGETABLE PHYSIOLOGY 



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. In cases of such complex changes of position 

 there necessarily must be a very delicate co-ordination of 

 the stimulation received by so many cells in different parts 

 of the organ concerned, each contributing some small part 

 of the movement of the whole. 



The nocturnal movement may be merely a relaxation of 

 the effort involved in maintaining the strain of tone induced 

 by the light of daytime, in a certain sense implying a con- 

 dition 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 mani- 

 festation 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 



