204 HOW CROPS FEED. 
lieved that the air surrounding the plants in these experi- 
ments was saturated with vapor of water, and concluded 
that heat was developed within the plant, which caused 
vaporization. More recently, Boehm (Sitzungsberichte 
der Wiener Akad., XLVIII, 15) has made probable that 
the air was not fully or constantly saturated with moist- 
ure in these experiments, and by taking greater precau- 
tions has arrived at the conclusion that transpiration abso- 
lutely ceases in air saturated with aqueous vapor. 
d. The condition of the tissues of the plant, as depend- 
ent upon their age and vegetative activity, likewise has a 
marked effect on transpiration. Lawes* and Knop both 
found that young plants lose more water than older ones. 
This is due to the diminished power of mature foliage to 
imbibe and contain water, its cells becoming choked up 
with growth and inactive. 2 
é. Lhe character of the medium in which the roots are 
situated also remarkably influences the rate of transpira- 
tion. This fact, first observed by Mr. Lawes, in 1850, Joe. 
cit., was more distinctly brought out by Dr. Sachs at a 
later period. (Vs. S¢., I, p. 203.) 
Sachs experimented on various plants, viz.: beans, 
squashes, tobacco, and maize, and observed their transpi- 
ration in weak solutions (mostly containing one per cent) 
of nitre, common salt, gypsum, one-fifth per cent solu- 
tion) and sulphate of ammonia. He also experimented 
with maize in a mixed solution of phosphate and silicate 
of potash, sulphates of lime and magnesia, and common 
salt, and likewise observed the effect of free nitric acid 
and free potash on the squash plant. The young plants 
were either germinated in the soil, then removed from it 
and set with their rootlets in the solution, or else were 
kept in the soil and watered with the solution. The glass 
* Experimental Investigation into the Amount of Water given off by Plants 
during ther Growth, vy J. B. Lawes, of Rothamstead, London, 1850. 
