1879. | Absorption of Water by the Leaves of Plants. 21 
DeCandolle and Sachs; the explanation offered of the fact that 
withered plants revive when placed in moist air or when the 
leaves are moistened, being that transpiration is thus stopped, or 
is more than counterbalanced by the root-absorption. In his 
“ Text-book of Botany” (English edition, p. 613) Sachs says: 
“When land plants wither on a hot day and revive again in the 
evening, this is the result of diminished transpiration with the 
decrease of temperature and increase of the moisture in the air 
in the evening, the activity of the roots continuing; not of any 
absorption of aqueous vapor or dew through the leaves. Rain 
again revives withered plants, not by penetrating the leaves, but 
by moistening them and thus hindering further transpiration, and 
conveying water to the roots, which they then conduct to the 
leaves.” McNab has, however, proved that leaves do transpire, 
even in a moist atmosphere, provided they are exposed to the 
action of light. The result of recent experiments, conducted by 
Boussingault in France, and by the Rev. George Henslow, in 
England, seem to force us to return to the earlier theory held 
before the time of Duchartre. 
Boussingault’s experiments relate not only to the absorption of 
water by leaves, but also to transpiration under various atmos- 
pheric conditions. The first experiments were as to the amount 
of transpiration from the Jerusalem artichoke in sunshine, in 
shade and by night. This he found to be hourly, for every 
square metre of foliage, sixty-five grammes in sunshine, eight 
grammes in the shade, and three grammes during the night. In 
the vine the corresponding numbers were thirty-five grammes in- 
sunshine, eleven grammes in shade, 0.5 grammes by night. He 
reckoned that an acre of beet could give off,in the course of 
twenty-four hours, the enormous amount of between 8000 and 
9000 kilogrammes of water, and a chestnut tree, thirty-five years — . 
old, sixty litres of water in the same time. The next question 
investigated was whether the absorption of water by plants, and 
the ascent of the sap, are due to the force resulting from transpi- 
ration on the surface of the leaves, or whether the roots exercise 
also a certain amount of force to this end. In the case of mint, 
a plant with roots, showed an hourly evaporation per square 
metre of eighty-two grammes in the sunshine, and thirty-six — 
grammes in the shade; without roots the evaporation was six- _ 
teen grammes in sunshine, fifteen grammes in shade. ` The effects 
e 
