114 THE FIRST BOOK OF FARMING 



will be seen collecting and rising to the surface. If 

 a mass of weed like the green scum of fresh water 

 ponds or green sea lettuce be used, the bubbles of 

 gas will become entangled in the mass and will 

 cause it to rise to the surface of the water. At the 

 same time prepare another jar of the weed and place 

 it somewhere out of the sun ; very few bubbles will 

 be seen to rise and the weed will settle to the bot- 

 tom of the jar (Fig. 65). 



All of the food of the plant, whether taken from 

 the air or from the soil is digested in the leaves, 

 and sunlight and air are necessary for this work. 



Another function of leaves then is to digest food 

 for the plant. 



Important functions of leaves then are : 



To transpire moisture sent up by the roots. 



To manufacture starch by combining some of the 

 water sent up by the roots with carbon taken from 

 the air. 



To digest the starch and food sent up by the 

 roots. 



To do these things well leaves must be connected 

 with a strong, healthy root system and must have 

 plenty of light and air. 



We are now ready to give reasons for the facts 

 about leaves mentioned in the first part of the chap- 

 ter (see page 109). 



Leaves are green because the green coloring mat- 

 ter is necessary for the leaf to do its work. 



Leaves are flat and thin and broad in order that 

 they may present a large surface to the air and sun- 

 light. 



