77. 



must "be sought in the different changes in size of the cells 

 of the two sides of the leaf as it dries, These are indicated 

 "by measurements made of the cells in curled auid expsmded 

 leaves. 



Measurements of cells of the upper epidermis, of the 

 palisade and lower epidermis from three different regions 

 of the leaf seem to show that the lower epidermal region 

 shrink least in width, while those of the same region in the 

 upper epidermis shrink most in v/idth.On the other hand, the 

 shrinkage in length is practically the same in the cells 

 of both the upper and lower epidermis near the midrib. 

 It is further evident that the shrinkage of; the palisade 

 cells in width corresponds more to that of the cells 

 of the upper epidermis than to those of the lov/er epidermis. 

 Furthermore, the shrinkage in v;idth of the cells :^ of ::the 

 lower '.epidermis in the intermediate region is much greater 

 than in those at the same epidermis near the midrib, while 

 at the margin the shrinkage in width is practically the same 

 in both the upper and lower epidermis, Correlating these ; 

 measurements with the loss of water and the curling of the 

 leaf the following explanation may be offered for the 

 characteristic curling of the leaf, While the cellc rf uh3 

 lower epidermis are decreasing in size ov/ing to the loss of 

 v^ater, their osmotic concentration is undoubtedly becoming 

 greater than that of the mesophyll cells; this causes , therefore 

 a withdrawl of the water from the mesophyll cells by osmosiaw' 



