166 FOUNDATIONS OF BOTANY 



the raw materials which are afforded by the earth and air 

 and all the steps of the processes by which these foods are 

 used in the life and growth of the plant are together known 

 as its nutrition. When we think more of the chemical 

 side of nutrition than of its relation to plant-life, we call 

 any of the changes or all of them metabolism, which means 

 simply chemical transformation in living tissues. There 

 are two main classes of metabolism — the constructive kind, 

 which embraces those changes which build up more com- 

 plicated substances out of simpler ones (Sect. 179), and the 

 destructive kind, the reverse of the former (Sect. 184). A 

 good many references to cases of plant metabolism have 

 been made in earlier chapters, but the subject comes up in 

 more detail in connection with the study of the work of leaves 

 than anywhere else, because the feeding which the ordinary 

 seed-plant does is very largely done in and by its leaves. 



177. Details of the Work of the Leaf. — A leaf has four 

 functions to perform: (1) Starch-making; (2) assimilar 

 tion ; ^ (3) excretion of water ; (4) respiration. 



178. Absorption of Carbon Dioxide and Removal of its 

 Carbon. - — Carbon dioxide is a constant ingredient of the 

 atmosphere, usually occurring in the proportion of about 

 four parts in every 10,000 of air or one twenty-fifth of one 

 per cent. It is a colorless gas, a compound of two simple 

 substances or elements, carbon and oxygen, the former 

 familiar to us in the forms of charcoal and graphite, the 

 latter occurring as the active constituent of air. 



1 In many works on Botany (1) and (2) are both compounded under the 

 term assimilation. Many botanists (most of the American ones) apply the 

 name photosynthesis or photosyntax to the starch-making process, but these 

 names are not wholly satisfactory, and perhaps it is as well (as suggested by 

 Professor Atkinson) to name the process from its result. 



