PHOTOSYNTHESIS 43 



its weight is carbon, which is approximately the proportion of 

 carbon in the total plant tissue. The CO 2 of the atmosphere 

 finds its way, with the other constituents of the air, through 

 the stomata of the epidermis, into the intercellular spaces of 

 the leaf. From here it passes through the cell walls of the 

 mesophyll by osmose and is then by photosynthesis converted 

 into starch, the free oxygen passing out of the cell, also by 

 osmose, to the air of the intercellular spaces and thus out of 

 the leaf. This process must not be regarded as assimilation, 

 since the substances absorbed have not been converted into 

 living protoplasm nor built up into the structural elements of 

 the plant. The starch is simply food material which has been 

 manufactured by the plant from a substance which is not food. 

 For CO 2 cannot be directly assimilated by protoplasm. 



90. Starch is practically insoluble in water at ordinary 

 temperature, yet it quickly disappears in an active cell when 

 photosynthesis is not going on. There is an active principle 

 called a ferment present in the protoplasm, which corrodes the 

 starch grain, wearing away the surface until finally it goes to 

 pieces and disappears. In place of the starch a form of sugar 

 is found, dissolved in the cell sap. This is formed directly from 

 the starch by the addition to the molecule of a molecule of water, 

 thus starch (C 6 HioO 5 )+H 2 = sugar (C 6 Hi 2 O 6 ), a substance 

 readily soluble in water. This soluble food substance may be 

 directly assimilated by the protoplasm of the cell in which it was 

 formed, or it may be transferred to other cells by osmose. 



Respiration 



91. The fact that oxygen is liberated from the plant during 

 photosynthesis must not be interpreted to mean that the 

 plant does not need oxygen. As has been noted elsewhere, 

 oxygen is necessary to the germination of seeds and it is as 

 necessary to the growing plant. During photosynthesis more 



