262 SOILS. 



was of good, normal size; while the trees in the uncultivated 

 ground made barely three inches of growth, and the fruit was 

 stunted and wholly unsaleable. It may be added that when, 

 instructed by the season's experience, the owner of the " un- 

 cultivated ' orchard cultivated deeply the following season, 

 his trees showed as good growth and fruit as his neighbor's. 



EVAPORATION THROUGH THE ROOTS AND LEAVES OF PLAXfS. 



Undesirable as is the evaporation from the surface of the 

 soil, under all but exceptional conditions the evaporation from 

 the leaves of plants is one of the essential functions of veg- 

 etable development. Not only because water serves as the 

 vehicle of the plant-food absorbed by the roots and to be or- 

 ganized by and redistributed from the leaves, and the aeration 

 occurring in the latter must of necessity result in a certain 

 degree of evaporation; but largely because the conversion of 

 liquid water into vapor serves to prevent an injurious rise of 

 temperature in the leaves under the influence of hot sunshine 

 and dry air. It is undoubtedly for the latter purpose that 

 the greater part of the enormous amount of water required, as 

 above stated (chap. 11 ) for the production of one part of 

 dry substance, is actually used. When sufficient water to 

 supply the required evaporation through the leaves cannot be 

 brought up from the soil, the plant begins to wilt; or in the 

 case of some plants with very thin and soft leaves the blade 

 normally begins to droop during the hottest hours of the day; 

 thus escaping excessive exposure to the sun's rays, and re- 

 covering their turgor later in the afternoon. 



The amount of \vnter actually evaporated from orchard trees has un- 

 fortunately not been directly determined, the investigations made in 

 this respect having borne mainly upon forest trees. The Austrian 

 Forest Experiment Station made a series of elaborate investigations on 

 this subject in iSyS. and the following data (quoted from the Report 

 of the U. S. Dep't of Agriculture for 1889) convey some idea of the 

 results. 



It was found that the surface-areas of the leaves do not give reliable 

 results, but that these depend very largely upon the thickness (mass) of 

 the leaves. The dry weight of the latter was found, as in the case of 



