310 HOW CROPS GROW. 



marked by irregular double lines. The round bodies in the guard- 

 cells of the pores are starch-grains, often present in these cells, when 

 not existing in any other part of the leaf. 



The stomata are, with few exceptions, altogether want- 

 ing on the submerged leaves of aquatic plants. On 

 floating leaves they occur, but only on the upper surface. 

 Thus, as a rule, they are not found in contact with 

 liquid water. On the other hand, they are either absent 

 from, or comparatively few in number upon, the upper 

 surfaces of the foliage of land plants, which are exposed 

 to the heat of the sun, while they occur abundantly on 

 the lower sides of all green leaves. In number and size 

 they vary remarkably. Some leaves possess but 800 to 

 the square inch, while others have as many as 170,000 to 

 that amount of surface. About 100,000 may be counted 

 on an average-sized apple-leaf. In general, they are 

 largest and most numerous on plants which belong to 

 damp and shaded situations, and then exist on both sides 

 of the leaf. 



The epidermis itself is most dense consists of thick- 

 walled cells and several layers of them in case of leaves 

 which belong to the vegetation of sandy soils in hot cli- 

 mates. Often it is impregnated with wax on its upper 

 surface, and is thereby made almost impenetrable to 

 moisture. On the other hand, in rapidly-growing plants 

 adapted to moist situations, the epidermis is thin and 

 delicate. 



Exhalation of Water-Vapor. A considerable loss 

 of water goes on from the leaves of growing plants when 

 they are freely exposed to the atmosphere. The water 

 thus lost exhales in the form of invisible vapor. The 

 quantity of water exhaled from any plant may be easily 

 ascertained, provided it is growing in a pot of glazed 

 earthen or other impervious material. A metal or glass 

 cover is cemented air-tight to the rim of the vessel, and 

 around the stem of the plant. The cover has an open- 



