THE TIGETATIVE OEGANS OF PLANTS. 287 



number upon, the upper surfaces of land plants, ayIucIi are 

 exposed to the heat of the sun, while they exist in great 

 nunihers 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 be- 

 long in 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 moist- 

 ure. On the other hand, in rapidly growing plants adapt- 

 ed 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 atmospihere. 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 opening 

 with a cork, through which weighed quantities of water 

 are added from time to time, as required. The amount 

 of exhalation during any given interval of time is learned 

 with a close approach to accuracy by simply noting the 

 loss of weight which the plant and pot together suffer. 

 Hales, who first experimented in this manner, found that 

 a sunflower, whose foliage had an aggregate surface of 

 39 square feet, gave off 3 lbs. of water in a space of 24 

 hours. Knop observed a maize-plant to exhale, between 



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