most important phenomena of
vegetation. It is principally absorbed by the roots, and passes into the
tissues of the plant, where a part of it is decomposed, and goes to the
formation of certain of its organic compounds; while by far the larger
quantity, in place of remaining in it, is again exhaled by the leaves.
The extent to which this takes place is very large. Hales found that a
sunflower exhaled in twelve hours about 1 lb. 5 oz. of water, but this
quantity was liable to considerable variation, being greater in dry, and
less in wet weather, and much diminished during the night. Saussure made
similar experiments, and observed that the quantity of water exhaled by
a sunflower amounted to about 220 lb. in four months. The exhalation of
plants has recently been examined with great accuracy by Lawes. His
experiments were made by planting single plants of wheat, barley, beans,
peas, and clover, in large glass jars capable of holding about 42 lb. of
soil, and covered with glass plates, furnished with a hole in the centre
for the passage of the stem of the plant. Water was supplied to the soil
at certain intervals, and the jars were carefully weighed. The result of
the experiments, continued during a period of 172 days, is given in the
following table, which shows the total quantity of water exhaled in
grains:--

Wheat 113,527
Barley 120,025
Beans 112,231
Peas 109,082
Clover, cut 28th June 55,093

It further appears, that the exhalation is not uniform, but increases
during the active growth of the plant, and diminishes again when that
period is passed. These variations are shown by the subjoined tables, of
which the first gives the total exhalation, and the second the average
daily loss of water during certain periods.

TABLE I.--_Showing the Number of Grains of Water given off by the Plants
during stated divisional Periods of their Growth._

+-----------+--------+--------+--------+--------+--------+--------+---------+
|Description|9 Days. |31 Days.|27 Days.|34 Days.|30 Days.|14 Days.| 27 Days.|
| +--------+--------+--------+--------+--------+--------+---------+
|of Plant. | From |From |From |From |From |From | From |
| | Mar. 19|Mar. 28 |Apr. 28 |May 25 |June 28 |July 28 | Aug. 11 |
| | to |to |to |to |to |to | to |
| |Mar. 28.|Apr. 28.|May 25