EXHALATION OF VAPOR BY TREES. LST 
do not know that the exudation of water into the earth, through 
the bark or at the extremities of these latter organs, has been 
proved, but the other known modes of carrying off the surplus 
do not seem adequate to dispose of it at the almost leafless 
period when it is most abundantly received, and it is possible 
that the roots may, to some extent, drain as well as flood the 
water-courses of their stem. Later in the season the roots 
absorb less, and the now developed leaves exhale an increased 
quantity of moisture into the air. In any event, all the water 
derived by the growing tree from the atmosphere and the 
ground is parted with by transpiration or exudation, after 
having surrendered to the plant the small proportion of matter 
required for vegetable growth which it held in solution or 
suspension.* The hygrometrical equilibrium is then restored, 
so far as this: the tree yields up again the moisture it had 
drawn from the earth and the air, though it does not return it 
culation of the two kinds of sap; and no crude sap exists separately in any 
part of the plant. Evenin the root, where it enters, this mingles at once 
with some elaborated sap already there.”—Gray, How Plants Grow, § 273. 
* Ward’s tight glazed cases for raising and especially for transporting plauts, 
go far to prove that water only circulates through vegetables, and is again 
and again absorbed and transpired by organs appropriated to these functions. 
Seeds, growing grasses, shrubs, or trees planted in proper earth, moderately 
watered and covered with a glass bell or close frame of glass, live for months, 
and eyen years, with only the original store of air and water. In one of 
Ward's early experiments, a spire of grass and a fern, which sprang up in a 
corked bottle containing a little moist earth introduced as a bed for a snail, 
lived and flourished for eighteen years without a new supply of either fluid. 
In these boxes the plants grow till the enclosed air is exhausted of the gascous 
constituents of vegetation, and till the water has yielded up the assimilable 
matter it held in solution, and dissolved and supplied to the roots the nutvri- 
ment contained in the earth in which they are planted. After this, they con- 
tinue for a long time in a state of vegetable sleep, but if fresh air and water be 
introduced into the cases, or the plants be transplanted into open ground, 
they rouse themselves to renewed life, and grow vigorously, without appear- 
ing to have suffered from their long imprisonment, The water transpired by 
the leaves is partly absorbed by the earth directly from the air, partly con- 
densed on the glass, along which it trickles down to the earth, enters the roots 
again, and thus continually repeats the circuit. See Aus der Natur, 21, B. 
8. 557. 
