LEAF STRUCTURE 9 



pound of dry weight produced by the plant. More water is required 

 in a dry climate than in a humid climate. At the Nebraska Experi- 

 ment Station corn was grown in two greenhouses. In one the air was 

 kept very dry, and in the other very humid. In the dry greenhouse it 

 required 340 pounds of water to one pound of dry weight produced, 

 while in the humid house only 191 pounds were required. The amount 

 of water has been determined for several crops at various times with 

 general results about as follows : 



Amount of Water Lost by Evaporation and Transpiration for Each Pound of 



Dry Matter 1 



Oats 402-665 pounds 



Red Clover 249-453 pounds 



Barley 262-774 pounds 



Corn 233-400 pounds 



Wheat 225-650 pounds 



LEAVES AND THEIR FUNCTIONS 



The principal function of the leaf is to manufacture the raw food 

 elements taken into the plant from soil and air, into " plant foods " 

 or compounds that can be utilized by the plant in building up 

 tissues. 



Leaf Structure. Examine a.section of leaf under a microscope, 

 and it will be seen to have several rather distinct parts (Fig. 3) : 

 (a) An outer covering, or " skin," called an epidermis. This is 

 practically air- and water-proof, (b) Several layers of cells, a part 

 being rather loosely grouped together, so as to leave air-spaces, and 

 thus provide for the free interpassage of air among them, (c) 

 Stomata, or air-holes, in the epidermis or skin. These stomata allow 

 the free passage of outside air into the interior of the leaf, (d) The 

 veins or circulating system. When the water solution is taken up 

 from the soil, it passes up the stem through small vessels, which ex- 

 tend to all parts of the leaf. When the solution reaches the leaves, 

 the water is quickly evaporated, leaving the plant food elements in 

 the leaf to be manufactured. (e) The chlorophyll bodies are 

 small green bodies scattered through the leaf, and these have the 

 power of absorbing energy from the sunlight. The work of the leaf 

 requires much energy, and this is all derived from the sun. No food 



1 American Society of Agronomy, vol. iii, p. 261 (1911). 



