206 HOW CROPS FEED. 



completely dry, or tliey had been for a long time acted 

 upon by a i)Owerful sun. Since, however, the soil was 

 moist, the wilting could only arise from the inability of 

 the roots to absorb water as rapidly as it exhaled from 

 the leaves, owing to the low temperature. Further ex- 

 periments showed that warming the soil in which thj 

 wilted plants stood, restored the foliage to its [jroper tur- 

 gidity in a short time, and by surrounding the soil of a 

 fresh plant with snow, the leaves wilted in three or four 

 hours. 



Cabbages, winter colza, and beans, similarly circum- 

 stanced, did not wilt, showing that diiferent plants are un- 

 equally affected. The general rule nevertheless appears to 

 be established that within certain limits the root absorbs 

 mere vigorously at high than at low temperatures. 



The Amount of Loss of Water of Vea^etation in Wilt- 

 ing has been determined by Ilesse ( Vs. St., I, 248) in 

 case of sugai'-beet leaves. Of two similar leaves, one, 

 gathered at evening after several days of dryness and sun- 

 shine, contained 85.74°!^ of water; the other, gathered 

 the next morning, two hours after a rain storm, yielded 

 89.57° |„. The difference was accordingly 3.8° |„. Other 

 observations corroborated this result. 



Is Exhalation Indispensable to Plants] — It was for 



a long time supposed that transpiration is indispensable 

 to the life of plants. It was taught that the water which 

 the i^lant imbibes from the soil to replace that lost by ex- 

 halation, is the means of bringing into its roots the min- 

 eral and other soluble substances that serve for its nutri- 

 ment. 



There are, however, sti'ong grounds for believing that 

 the current of water Avhich ascends through a plant moves 

 independently of the matters that may be in solution, 

 either without or within it ; and, moreover, the motion of 

 soluble matters from the soil into the plant may go on, 



