10 INTRODUCTORY 



and live in a single medium, the atmosphere, or in 

 water, most of the Higher Plants are fixed, and live 

 partly in the soil (or in water) and partly in the 

 atmosphere. The roots buried in the soil absorb 

 water and certain essential mineral salts, especially 

 those of nitrogen, but not of carbon. These raw 

 materials are passed on to the protoplasmic machinery 

 from cell to cell. This function is known as absorp- 

 tion. 



On the other hand, the leaves perform the function 

 known as assimilation. They can withdraw carbon 

 dioxide from the atmosphere. This gas passes 

 through into the leaf by means of numerous minute 

 pores, which the botanist terms stomata. This simple 

 raw food material, with the addition of water derived 

 from the soil by means of the roots, is first of all 

 converted into a complex substance, such as sugar, 

 by the aid of the special portions of the protoplasm 

 of the leaf, which contain the green colouring matter 

 known as chlorophyll. It is the chlorophyll which 

 gives the green coloration to the majority of plants. 

 Chlorophyll is able, in the presence of light and other 

 favourable physical conditions, to absorb energy, 

 which gives it the power to convert the simple gases 

 of the atmosphere into complex sugars, which are 

 passed on in turn to the protoplasm of the growing 

 cells. 



The protoplasm thus receives, and incorporates 

 into its own substance, raw food material from two 

 sources : the atmosphere, which suppHes the carbon 



