140 ELEMENTS OF BOTANY 
166. Assimilation. — Assimilation means the transforma- 
tion of food into the tissues of the plant. From the starch 
in the leaf, grape-sugar or malt-sugar is readily formed, and 
some of this in turn is apparently combined on the spot with 
nitrogen, sulphur, and phosphorus. These elements are de- 
rived from salts taken up by the roots and transported to the 
leaves. The details of the process are not understood, but 
the result of the combination of the sugars or similar sub- 
stances with suitable (very minute) 
proportions of nitrogen, sulphur, 
and phosphorus is to form com- 
plex nitrogen compounds. These 
are not precisely of the same com- 
position as the living protoplasm 
of plant-cells or as the reserve 
proteids stored in seeds (Sect. 24), 
stems (Sect. 91), and other parts 
of plants, but are readily changed 
Fic. 96. Leaf of Tropeolum into protoplasm or proteid foods 
partly covered: with Disks: 45 necessiiy may demand, 
of Cork and exposed to Nie aoe nae 
Sunlight. Assimilation is by no means 
confined to leaves; indeed, most 
of it, as above suggested, must take place in other parts 
of the plant. 
167. Excretion of Water and Respiration. — Enough has 
been said in Sect. 159 concerning the former of these 
processes. Jtespiration, or consuming oxygen and giving 
off carbon dioxide, is an operation which goes on con- 
stantly in plants, as it does in animals, and is necessary to 
their life. For, like animals, plants get the energy with 
which they do the work of assimilation, growth, reproduc- 
tion, and performing their movements from the oxidation 
