WATER STORAGE 163 



a storage tissue in each leaf, but a withdrawal of water from 

 the older leaves by the younger. 



Some of the suction force measurements of Ursprung 

 and Blum {igiSa, h) have an application in this connection. 

 In a detailed examination of the different tissues of the leaf 

 in the ivy and beech, they found that the suction force of 

 the epidermal cells was less than that of the mesophyll. 

 Thus in the beech, the upper epiderm had a suction force 

 of 7 atmos. and the palisade of 16 atmos., while the values 

 for the lower epiderm and the spongy parenchyma were 6 

 and II atmos. respectively. For the ivy, values for the 

 upper epiderm and the palisade were 8*3 and 15' 6. This 

 is regarded as showing that in these leaves, too, the epiderm 

 functions as a water store, which may be drawn on by the 

 mesophyll. If this is so, then the storage function of the 

 epiderm may be general. In view of the small fall in water 

 content which occurs at wilting in such leaves, it cannot be 

 said to provide a large working margin (cp. also Haberlandt). 



We see, then, that the possession of water storage tissue 

 provides many plants with a considerable water balance, 

 with the help of which they can withstand prolonged spells 

 of drought. How assimilation is affected in such plants by 

 a loss of water short of the danger point, we do not know. 

 In the ordinary plant water storage is at a minimum, the 

 available margin is small, and a short period of drought is 

 sufficient to cause wilting or more serious consequences. 

 With the onset of wilting assimilation may practically cease. 



We may here note the water storage capacity of the 

 trunks of deciduous trees, which is useful in another con- 

 nection. Craib (19 18) has shown that the water content of 

 the trunk of the maple increases greatly during the winter, 

 a store being formed which supplies the enormous demands 

 of the expanding, and rapidly transpiring young leaves in 

 spring. 



§ 20. Atmospheric Conditions and Transpiration 



The transpiration of a leaf exposed to the air is greatly 

 influenced by external conditions. Just as rise of air 



