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 Bawhinia 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, 
