245 
the mechanical or molecular energy, that is at the disposition of the 
plant, but it is also the last consideration to be studied and under- 
stood. 
The temperature is the next important climatic element and that 
which has been most studied; the heat involved in temperature is 
the mechanical, molecular energy that is utilized by the vital powers 
of the plant. Each plant utilizes a fraction of the molecular energy 
that is at its disposition, according as its sunshine, temperature, and 
sap are favorable to the formation of the chemical substances that it 
can elaborate within its cells. The remaining elements important to 
the production of crops are: 
(a) The water that enters the root, which may be natural rain or 
artificial irrigation. 
(6) The chemicals dissolved in the water. 
(c) The soil that furnishes these chemicals. 
(d) The atmosphere that furnishes nitrogen, oxygen, and carbonic- 
acid gas. 
(e) The evaporation of moisture from the plant and soil, mostly 
through the influence of the wind and heat. 
Of these, only the rain water, the gases in the atmosphere, and the 
evaporation are, properly speaking, meteorological or climatic ele- 
ments not under the control of man; whereas the irrigation of the 
soil and its chemical constitutents are largely under his control. 
The quantity of water actually consumed by the plant or evapo- 
rated from its leaves and that which is daily evaporated from the soil 
or which drains away to other localities, and thus becomes useless to 
the plant, have been the subject of many experiments, some of whose 
results may be summarized as follows: 
Thus, for example, Lawes and Gilbert, at Rothamsted, England, 
from experiments in vases entirely under their control, derived the 
following numbers, showing the weight of water evaporated relative 
to the weight of grain produced per unit area of ground: 
ss Wolght 
eight jof evapo- . 
Manure. of grain.| rated Ratio. 
water. 
Grams. | Grams. 
INOS) 2S ee Sa ote iS Se ee a aes eee ee epee 9.6 7, 353 766 
MM eran Gl ZOnS mea es Ne eee eine eee ek A Se Se er 6,488 882 
Mineraliandiammoniacal fertilizers: ----- 2-222 2-1-2 2 -2e le ee 4.2 3, 627 864 
In these experiments, therefore, the ground during the wheat sea- 
son consumed water equivalent to a rainfall of from 184 to 212 milli- 
meters in order to produce a harvest of 30 hectoliters, or 80 kilograms 
in weight per hectare. 
@1Ts it not in fact the vital power of the plant?—C. A. 
