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) assimila- 
tion ;1 (8) 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. 
1Jn 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. 
