CHAPTER IV. 



THE LEAVES. 



The leaves are highly vascular organs, in Tvhicli are 

 performed some of the most important fmictions of a 

 plant. They are very general, but not absolutely 

 necessary organs, since the branches sometimes per- 

 fonn their offices ; such plants, however, as naturally 

 possess them, are destroyed, or greatly injured by 

 being deprived of them. The duration of a leaf is, 

 in general, but for a year, though in some plants, 

 they survive for t^^'ice or thrice that period. These 

 organs are generally of a green colour. Light seems 

 to have a powerful influence in causing this, since, 

 if kept in the dark, they become of a pale yellow, or 

 even white hue, unless uncombined hydrogen is 

 present, in which case they retain theii' verdure 

 though light be absent. Hence their etiolation would 

 seem to arise from their being unable to obtain this 

 gas, ujider ordinaiy circumstances, except when light is 

 present. Now, the only source from which they can 

 obtain hydrogen, is by decomposing water; and how 

 light assists in the decomposition may perhaps be 



