140 



ESSENTIALS 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 

 into protoplasm or proteid foods 

 as necessity may demand. 



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. Respiration, 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 



Fig. 96. Leaf of Tropaeolum 

 partly covered with Disks 

 of Cork and exposed to 

 Sunliglit. 



