CH. IX] DROOPING OF THE LEAF 87 



vesicle, like a bladder, tensely distended by the water it 

 contains, and a closely packed system of such distended 

 bladders would be very rigid. 



There are differences, however, in the relative rigidity 

 of the upper and lower parts of the mesophyll of one and 

 the same leaf, and also of the mesophyll at different 

 periods ; the former being due to differences in the rela- 

 tive volumes of the interspaces between the cells nearest 

 the upper or the lower surfaces respectively of the leaf 

 or, what is the same thing, differences in the closeness 

 of the packing of the cells while the latter is due to the 

 quantity of water at the disposal of the mesophyll-cells. 



Everyone knows that if a soft thin leaf, like that of a 

 Lilac or Elder, is plucked from the plant and carried in 

 the hot hand, its fresh and rigid condition soon gives 

 place to a state known as drooping : the leaf becomes 

 limp and flaccid because water has been evaporating 

 from the cells, and they collapse from their previously 

 distended or turgid condition, as may be seen under the 

 microscope. The loss of water may readily be observed 

 by placing such a fresh leaf on a balance and carefully 

 equipoising it : the scale containing the leaf soon rises, 

 because the cells lose water, and since we have torn the 

 leaf from the stem broken the water pipes no more 

 can flow in to replace that which is lost. 



The arrangement of the mesophyll-cells is best seen 

 in a section of a leaf such as that shown in Fig. 25. 



Neglecting for the moment the epidermis (b and d) 

 which surrounds the whole section, and the cut vascular 

 bundles (a, c and e) which run through the mesophyll, we 

 see that the mesophyll in the upper moiety of the section 

 consists of cylindroidal cells standing vertically, and so 

 closely in contact with each other at the sides, and with 

 the epidermis at their upper ends, that the intercellular 



