NUTRITION 77 



2. Recombining these parts into carbohydrates. 



3. Giving off the waste material (chiefly oxygen). 

 Taking in the Raw Materials. — We have seen in Chapter 



IV that the air spaces between the green cells of a leaf 

 are in direct connection with the outside air through the 

 stomata. In the presence of sunlight the formation of 

 carbohydrates begins in the green cells. This uses up 

 the carbon dioxide in the cells, and the supply is renewed 

 from the intercellular spaces. The gas passes through 

 the cell-walls and the layer of protoplasm in solution in 

 water. This results in reducing the amount (and thus 

 the pressure) of this gas in the intercellular spaces, and as 

 a result more carbon dioxide passes by diffusion through 

 the stomata to the intercellular spaces. Thus, as fast 

 as the gas is used the supply is renewed from without. 



76. Photosynthesis. — Within the cell, the carbon di- 

 oxide and water (or simple combinations of these) are 

 finally, hy a series of steps, recombined by the chlorophyll 

 in the presence of sunlight, into a carbohydrate — probably 

 some form of sugar. It is this series of steps that is called 

 photosynthesis} N6T: all of the oxygen contained in the 

 water (H2O) and carbon dioxide (CO2) used, enters into 

 the composition of the carbohydrate. The unused por- 

 tion is set free, and is either utilized in other processes, 

 or diffuses out through the stomata- to the surrounding 

 air. Thus, the taking in of carbon dioxide and the 

 giving off of oxygen are outward indications that photo- 

 synthesis is going on inside the green cells. 



77. Starch-making., — The sugar made by photosynthe- 

 sis is soluble in the cell-sap, and if it were not removed 



1 The word means combining {synthesis) in the presence of, or by means 

 of, light {photos). 



