218 



STRUCTURE AND VITAL PROCESSES OF PLANTS 



surface. It is in these green granules that the all-important 

 work is done of decomposing the carbonic acid gas into its con- 

 stituents, carbon and oxygen, and of forming ne\\: substances, 

 which are used as food by phmts in the same manner as animals 

 use the materials they eat. This process is called pJiotosynthesis 

 (or c&.rhon-assimilation) . To allow the air to reach the inner 

 parts of the leaf, there are the stomata (S.) on the under surface 

 of the leaf, which lead into the spaces between the cells of the 

 spongy tissue. And as the walls of cells, in which the chemical 

 changes are going on, are exceedingly thin, they require protec- 

 tion. This is afforded by the epidermis. 



(c) The Substances formed by Photosynthesis or Assimilation — 

 The products of this process, that is, of converting the raw 

 food from the soil and the air into food necessary for building 

 up new tissue in the plant, are chiefly two, namely, starch and 

 protein. Of these the more important is starch, a compound of 

 carbon, hydrogen, and oxygen. This substance abounds within 



Fin. I'.i'.i. — Stai'(;li in Potato. 



the cells of many parts of various plants in the shape of distinct 

 grains, as in the potato and all cereal grains, and is also the 

 principal constituent of arrow-root and sago. Protein is neces- 

 sary for the formation of life-substance of tlie plant, the proto- 

 plasm (see page 199), and is a compound of nitrogen. Other 

 organic products formed in the cells, the little laboratories of 



