FORMATION OF FOOD MATERIAL 



III. MANUFACTURE OF FOOD MATERIAL BY 

 PLANTS. 



Carbohydrates. It has been pointed out above 

 that the green parts of plants, under certain 

 conditions, actually absorb carbon dioxide gas and 

 give off oxygen gas, hence this process is called 

 "carbon dioxide assimilation," or "carbon fixation." 

 This process is now known to be due to the green 

 part of the plant (chlorophyll) having the power, under 

 the influence of sunlight, of splitting up the carbon 

 dioxide gas contained in the air, retaining the carbon 

 and liberating the oxygen ; the carbon is at the same 

 time combined with water in the leaves to form carbo- 

 hydrates, generally starch (CgH^Og). 1 



The formation of carbohydrates only proceeds as 

 long as there is a suitable supply of minerals which 

 the plant can secure from the soil, more especially 

 potash and a little iron. These are absorbed and 

 carried up to the leaves in solution, as mentioned 

 above. 



But how does the air get into the leaves ? To 

 understand this, one should know something of the 

 structure of a leaf, which in flat leaves has on the under- 

 side a very large number of pores (stomata) which admit 

 air, and at the same time allow gases and water-vapour 

 to escape into the atmosphere. These pores admit air 

 to the many cavities inside the leaf, and it is here that 

 the carbon dioxide in the air dissolves in the cell sap. 



1 Professor Sachs found in a few cases that sugar, or even a 

 fat, may be the first detectable organic product. 



