30 THE ESSENTIALS OF AGRICULTURE 



A small part of the water, the nitrogen and the mineral ele- 

 ments, are left in the plant and later made into new plant 

 tissues. In the surface layer, or epidermis, of the leaves of 

 plants there are many small pores through which the water 

 passes to the outside air. These pores are known as stomata 

 (stoma, singular, meaning " mouth "). The stomatal opening is 

 surrounded by a pair of surface cells, known as guard cells. In 

 dry weather the guard cells are usually affected so as to close 

 the opening, and in damp weather, so as to leave it open. 



The number of stomata on the leaves varies on an average 

 from about 24,000 to 180,000 a square inch, although there 

 are plants, such as the olive tree, in which the number runs 

 as high as 375,000 a square inch, and in rape the enormous 

 number of 429,600 a square inch is reached. It is said that an 

 average leaf of rape contains as many as 1 1 ,000,000 stomata 

 and that a large sunflower leaf has 13,000,000. 



It is estimated that if all the water given off by the plants 

 of a wheat field in the growing season could be put back on 

 the land again, it would cover the ground to a depth of about 

 four and a half inches, while that given off from the plants of a 

 field of oats would cover the ground to a depth of five inches. 

 It is estimated that from one twelfth to one eighth of the total an- 

 nual rainfall in the Central states passes out through evaporation 

 from the soil. This shows that a wide margin of safety exists for 

 the farmer who handles his land so as to conserve the water. 



34. Weeds waste water. It is estimated that an acre of land 

 covered with sunflowers will lose in a season, through these 

 plants, 392,040 gallons of water, which would cover the ground 

 to a depth of one and one-fifth feet and would be sufficient to 

 irrigate about one acre of alfalfa through the season. In every 

 farming region thousands of acres of weeds are constantly send- 

 ing the precious ground water into the air throughout the grow- 

 ing season, and this water is often needed by our farm crops. 



35. How organic matter is made. About 20 per cent of 

 the weight of green plants is organic matter. The two prin- 

 cipal groups are the carbohydrates and the proteins. The 



