g water for a long time; while a similar shoot cut in the open
air, even if the end is instantly plunged under water, will wither much
sooner than the first."--Physiological Botany, p. 263.]

[Footnote 2: Physiological Botany, p. 260.]

(6) Let the leaves of a growing plant rest against the window-pane.
Moisture will be condensed on the cold surface of the glass, wherever the
leaf is in contact with it. This is especially well seen in Nasturtium
(Tropaeolum) leaves, which grow directly against a window, and leave the
marks even of their veining on the glass, because the moisture is only
given out from the green tissue, and where the ribs are pressed against
the glass it is left dry.

Sometimes the water is drawn up into the cells of the leaves faster than
it can escape into the atmosphere.[1] This is prettily shown if we place
some of our Nasturtium seedlings under a ward-case. The air in the case is
saturated with moisture, so that evaporation cannot take place, but the
water is, nevertheless, drawn up from the roots and through the branches,
and appears as little drops on the margins of the leaves. That this is
owing to the absorbing power of the roots, may be shown by breaking off
the seedling, and putting the slip in water. No drops now appear on the
leaves, but as soon as the cutting has formed new roots, the drops again
appear.

[Footnote 1: See Lectures on the Physiology of Plants. By Sidney Howard
Vines, Cambridge, England. University Press, 1886. Page 92.]

This constant escape of water from the leaves causes a current to flow
from the roots through the stem into the cells of the leaves. The dilute
mineral solutions absorbed by the roots[1] are thus brought where they
are in contact with the external air, concentrated by the evaporation of
water, and converted in these cells into food materials, such as starch.
The presence of certain mineral matters, as potassium, iron, etc., are
necessary to this assimilating process, but the reason of their necessity
is imperfectly understood, as they do not enter in the products formed.

[Footnote 1: See page 48.]

The amount of water exhaled is often very great. Certain plants are used
for this reason for the drainage of wet and marshy places. The most
important of these is the Eucalyptus tree.[1]

[Footnote 1: Reader in Botany. XII. Transpiration.]

"The amount of water taken from the soil by the trees of a forest and
passed into the air by transpiration is not s