76. 



Relation of Water Content of Leaf to Leaf-movement, 



It appears obvious both from the physiological study of the 

 le^ euid from the study of its internal gtrucure that 

 evaporation takes place more rapidly from the lov/er surface. 

 The peculiar curling of the leaf which exposes this surface pjrd 

 conceals the upper one, and the presence of an abimdance of 

 scales on the lower surface are both concerned in producin^f 

 the striking change that occurs in the tissues of the leaf 

 under dry conditions, When the leaf is placed in the dessicator 

 v/hile still expanded the scales stand out somewhat so that 

 there is a space between the wing of the scale and the surface 

 of the leaf,When traoispiration begins the lov/er surface containing 

 the stomata naturally loses more water than the upper one 

 which is devoid of stomata, A stage then is reached when the 

 cells of the lowdr epidermis begin to shrink due to the loss 

 of water, At about this time the scales begin to be drawn ■.: 

 close to the surface due to: the shrinkage of the cells of the 

 loY.er 3pidermis,Such a shrinkage would naturally be expected 

 to cause a curling of the leaf v/ith the lo7/er epidermis on the 

 inside and the upper one on the outside, A curling of this sort 

 v.'ould seem the leg,! •'.''. iitcome of the known facts^'since the 

 lower surface loses the greater amount of water, In reality 

 quite the contrary occurs, It is the upper epidermis that is 

 found inside in the curled leaf, while the lower one is alv/ays 

 on the outside .The explanation for this unexpected curling 



