INTRODUCTORY 203 



Some morning there appears a film of green over fields that 

 were brown the day before. It is the green of the first 

 leaves, the promise of the harvest. Whether it be the 

 green of the grass blades, or the green of the stirring leaves 

 in the tree tops ; whether it be the velvet green of lawns 

 or the pale green of leaves that grow under the shadow of 

 forests — always this green delights us. It rests our eyes. 

 We respond to it with pleasure. Everywhere it is this 

 green of plants which dominates the summer landscapes. 

 It fills the fields of the farms and the parks of the cities. 

 It covers the hills and the valleys ; the whole world seems 

 green. And this green is always the green of leaves ; the 

 green of numberless millions of leaves. In the sunlight of 

 each growing season these myriads of leaves are at their 

 work, and upon that work all familiar life depends. 



B. Points already Noted. — Blade and petiole, vein and 

 midrib, axil and stomate, are terms which have to do with 

 leaves, and with these you are already familiar. You 

 know in general what photosynthesis is, and that it occurs 

 principally in leaves. You know that the loss of water by 

 evaporation is a very important phenomenon in plant life, 

 and that it, too, occurs principally in leaves ; you know that 

 this process is called transpiration (see page 108), and 

 that the water which ascends from the roots to the leaves 

 is called the transpiration stream. You know what the 

 mesophyll is, and that it is usually composed of palisade 

 tissue and spongy parenchyma. (See page 84.) You know 

 that air which comes in through the stomates diffuses in 

 the intercellular spaces of the spongy parenchyma, and that 

 with this air in the intercellular spaces the plant makes 

 various exchanges of gases — oxygen, carbon dioxide, and 



