36 HOW CROPS FEED. 
on the suppression of evaporation, which is a consequence 
of the saturation of the surrounding air with water. 
Unger, and more recently Duchartre, have found, Ist, 
that plants lose weight (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 other hand, that leaves, either 
separate or attached to twigs, gain weight by continued 
immersion in water, and not only recover what they may 
have lost by exposure, but absorb more than they orig- 
inally contained. ( Versuchs-Stationen, 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 great 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. Sz, II,7.) A young camellia, 
having several fresh leaves,:was taken from the loose 
soilof the pot inwhich 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 
glase 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 
