36 HOW CROPS FEED. 



on the suppression of evaporation, whidi is a consequence 

 of the saturation of the surrounding air with water. 



linger, and more recently Duchartre, have found, 1st, 

 that plants lose weiglit (from loss of water) in air that is 

 as nearly as possible saturated with vapor, when their 

 roots are not in contact with soil or liquid water. Du- 

 chartre has shown, 2d, that plants do not gain, but some- 

 times lose weight when their foliage only is exposed to 

 dew or even to rain continued through eighteen hours, al- 

 though they increase in weight strikingly (from absorption 

 of water through their roots,) when the rain is allowed to 

 fall upon the soil in which they are planted. 



Knop has shown, on the otlier hand, that leaves, either 

 separate or attached to twigs, gain weight by continued 

 imynersion in water, and not only recover what they may 

 have lost by exposure, but absorb more tlian they orig- 

 inally contained. ( Versuchs-Statlonen^ VI, 252.) 



The water of dews and rains, it must be remembered, 

 however, does not often thoroughly wet the absorbent sur- 

 face of the leaves of most plants ; its contact being pre- 

 vented, to a grent degree, by the hairs or wax of the 

 epidermis. 



Finally, 3d, Sachs has found that even the roots of 

 plants appear incapable of taking up watery vapor. 



To convey an idea of the method employed in such 

 investigations, we may quote Sachs' account of one 

 of his experiments. ( V. St., II, 7.) A young camellia, 

 having several fresh leaves, was taken from the loose 

 soil of the pot in which it had been growing ; from 

 its long roots all particles of earth were carefully remov- 

 ed, and its weight was ascertained. The bottom of a 

 glass cylinder was covered with water to a little depth, 

 and the roots of the camellia were introduced, but not in 

 contact with the water. The stem was supported at its 



