120 GENERAL BOTANY 



seeds and the growth of the embryo. Fats and oils, therefore, 

 like nitrogenous foods, are primarily derived from the products 

 of photosynthesis. All organic food for both plants and animals 

 is thus composed of the products of photosynthesis, or of food 

 substances which have been built around these products. In a 

 similar manner the skeletal and supporting structures of the 

 plant in the form of cell walls are immediately derived from 

 sugar and starch formed by photosynthesis. The cellulose which 

 forms the bulk of the cell-wall substance of plant tissue is closely 

 related to starch in chemical composition and is undoubtedly 

 constructed from sugar molecules by the living protoplasm of 

 plant cells. This cellulose framework of the plant body also 

 comprises the bulk of the fuel, in the form of wood, coal, and 

 combustible oils, upon which mankind depends, either directly 

 or indirectly, for heat and light. We see, therefore, that a large 

 part of the food supply for the organic world, the skeletal struc- 

 tures of plants, and the energy derived from fuel in the form of 

 light and heat are dependent upon the process of photosynthesis 

 by green plants. 



RESPIRATION 



The process by which living organisms secure energy by oxi- 

 dation for carrying on their life activities is termed respiration. 

 Unlike photosynthesis, respiration is not confined to the cells of 

 the plant body which contain green chlorophyll, but takes place 

 in every living cell of the organism. On account of its similarity 

 to combustion the respiratory process is most easily understood 

 by beginning students when compared with the burning of wood 

 or coal in a stove or a furnace. When thus compared it is found 

 that respiration and combustion are alike in that free oxygen is 

 absorbed and energy is liberated, together with certain waste 

 products which depend for their complexity upon the nature of 

 the substance oxidized. In the burning of coal the oxygen unites 

 directly with the pure carbon of the coal, and the products are 

 heat energy and carbon dioxide, which may be represented by 

 the following general equation : 



Coal (C) + oxygen (O 2 ) = carbon dioxide (CO 2 ) 4- energy 



