THE ROLE OF GREEN PLANTS 



267 



again after a brief period of exposure to sunliglit, when it will be found 

 to have lost weight. This loss ol' water takes place through the sto- 

 mata and to some extent through the lenticels of the stem, a loluniom- 

 enon closely associated with the process of i)hotosynthesis, for which 

 a relatively enormous amount of water is required. The reasons for 

 this are that living matter is largely composed of water ; that the pro- 

 cess of food making cannot take place in plants unless the interior of 

 their leaves are moist ; and, in the third place, because water is one of 

 the raw materials used in making sugar. The amount of water given 

 off by plants through transpiration is very great. Early in the eight- 

 eenth century Stephen Hales (see p. 241) estimated that an average 

 crop of cabbages loses from three to four tons of water per day per acre 

 in warm weather. An acre of pasture grass is said to give off over 

 100 tons of water on a hot, dry day. A medium-sized tree will evapo- 

 rate about five to six tons of water on a hot day. One writer, von 

 Hohnel, estimated that an acre of large beech trees would transpire 

 30,000 barrels of w^ater in one summer. Such figures show that a 

 green plant loses water very rapidly during hot, dry days. 



The amount of water lost differs greatly under different conditions. 

 If the air is humid, or if the temperature is lowered, or if the tempera- 

 ture of the plant becomes low, the rate of transpiration is greatly 



Diagrammatic cross section through a stoma to show movement of guard cells. 

 The dotted lines show the closed position. Closure is brought about by the 

 guard cells becoming more elongated and flattened, while the outer wall (w) 

 remains in place, the ventral wall (/) and dorsal wall (V/) assume the positions (/') 

 and id') moving toward the central slit (s) of the opening of the stoma. This 

 movement is largely brought about through the change in position of the base 

 or hinge {h) {h') of the guard cell. (After Schwendener.) 



reduced. The stomata tend to close under certain conditions, thus 

 helping to prevent evaporation. The opening and closing of the 

 stomata depend on changes in turgor of the guard cells. The stomata 



