FUNCTIONS 221 



the weight of food which they produce, and as to other 

 properties. 



Thus the need of plants for water is a thing, which con- 

 cerns our Uves as well as theirs. The reason that they 

 need so much is that they transpire so much. Nearly all 

 the water that goes into them evaporates out again. Evi- 

 dently, from the standpoint of the farmer as well as from 

 the standpoint of the plant, this loss by transpiration is a 

 great waste of good water unless such loss is in some way 

 necessary to the life of the plant. It is for us to consider 

 in what ways this constant loss appears to be a good thing 

 for the plant, or in what ways it appears to be a bad thing. 



a. The disadvantages of transpiration are more evident 

 than the advantages. More plants die on account of it 

 than on account of anything else. That is, water evaporates 

 from them more rapidly than they can replace it, and so 

 they die. Much that we see in the structure of plants is 

 explained by the need to protect against this loss of water. 

 By bark and by cutin, by hairiness and by waxy coatings, 

 by reduced foKage and by thick walls, and in many other 

 ways plants are protected from water loss. To get neces- 

 sary gases, however, air must be let in, and as surely as air 

 gets in, water gets out. The carbon dioxide and the oxygen 

 of the air are necessary to the plant, but any structure 

 which permits them to enter will surely at the same time 

 permit water to escape. The former is necessary to the 

 life of the plant, but the latter may cause its death, and 

 they must occur together. So we are not surprised to 

 find leaves so constructed that, though the entrance of air 

 is permitted, the exit of water is restricted. The plant 

 appears to be trying to let in air and keep in water at the 

 same time. 



