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 
