XL] LEAVES, AND LEAF-DISEASES. 247 



vapour especially being quicker on a dry, hot, sunny 

 day. 



Inside the cells between which these tortuous 

 passages run, are contained structures which have 

 much to do with these changes. Each of the cells I 

 am considering contains a lining of protoplasm, in 

 which a nucleus, and a number of small protoplasmic 

 granules, coloured green, and called chlorophyll- 

 corpuscles, are embedded : all these are bathed in a 

 watery cell-sap. 



Now, putting together in a general manner some of 

 the chief facts which we know about this apparatus, it 

 may be said that the liquid sap inside the cells gives 

 off water to replace that which escapes through the 

 damp cell-walls, and evaporates into the above-named 

 passages and out through the stomata, or at the sur- 

 face. This evaporation' of the water is in itself the 

 cause of a flow of more water from behind, and this 

 flow takes place from the vascular bundles forming the 

 so-called venation of the leaf, coming directly from 

 the wood of the stem. The course of this water, then, 

 is from the soil, through the roots, up the young wood 

 and into the venation of the leaf, and thence it is 

 drawn into the cells we are considering. But this 

 water is not pure water : it contains in solution small 

 quantities of salts of lime, potash, magnesia, nitric, 



