594 AGRICULTURE. 



and it got into the plant by being dissolved in the soil, by the 

 water we dried off. 



How Plants get Food from the Air. Take a large seed, 

 like a Lima bean ; press it, eye downwards, into a box of moist 

 soil, in a sunny window, and watch it sprout and grow. You 

 will notice that this bean is in two parts, inside of an outer skin. 

 In a short time it swells, a little stem starts from the eye, and 

 makes little roots in the soil. Then the two thick halves burst 

 the skin and rise on the stem, and gradually spread out into two 

 broad leaves. They are thinner now than when they merely 

 formed the two parts of the bean, for they have given part of 

 their material to form the little stem and roots, before they 

 turned green. This turning green is a very important matter. 



Suppose, instead of planting one bean in a sunny window, we 

 had simply stuck it in moist sand in a warm place, and covered 

 it over so that no light could reach it. It will swell and germi- 

 nate, but the two halves will not turn green. Now take it and 

 dry it, and you will find that it has not gotten any heavier, though 

 apparently larger than when put in the moist, warm place ; but 

 take the bean which has been in the sunlight, and dry it, and 

 you will find that it has already increased in weight. The bean 

 in the dark did not grow, but only changed some of the food 

 stored up in its thick halves into a little stem of rootlets. The 

 one in the sunlight added something else. Now let us see how 

 this was done. We have seen that, when the halves of the bean 

 spread out into broad leaves, they became green. This green is 

 caused by a substance formed in the leaves of plants which are 

 in sunlight, and not in those that are kept in the dark. This 

 substance is called leaf-green. It is found in little boxes in the 

 leaf, which are called cells. These cells are placed side by side, 

 somewhat like a honeycomb, and are so small that we cannot 

 see them without a microscope, but between them there are lit- 

 tle vacant places, still smaller than the cells. Opening into these 

 spaces there are little holes in the leaf, particularly on the under 

 side, which open and shut like little mouths with a pair of lips. 

 These are really the mouths of the leaves, and through them 

 the plant takes in all the food it gets from the air, and through 

 them it also puts out some things it does not want, especially 

 what water it does not need. These little mouths are so small 



