economy of plant growth that is closely related to that presented "by the water absorbed by the roots of plants and exhaled by their leaves.
Energy has been defined as "the power of doing work, or overcoming resistance," and its varied transformations into heat, motion, electricity, etc., without gain or loss, are expressed by the general term conservation of energy. In the nutrition and growth of plants an expenditure of energy is evidently required in the work involved in a number of distinct, but correlated, processes, the most important of which are—constructive metabolism, or the building of organic substance; the exhalation of water by the leaves, which is constantly taking place in their processes of nutrition; the evaporation of water from the surface soil; and the warming of the soil to provide optimum conditions of temperature.
The energy expended in constructive metabolism, or tissuebuilding, is stored up as potential energy, and reappears as heat when the plant is decomposed by any process, as, for example, when it is burned. The mechanical force exhibited by growing plants is a phase of the constructive process that has often been noticed. President Clark's squash raised a weight of 4,120 pounds in its processes of growth. Sprouts from the roots of a tree pushing their way through an asphalt pavement have been observed by myself, and many similar exhibitions of the force exerted by growing plants are often seen.
These obvious manifestations of energy in constructive metabolism are, however, so familiar that they require but a passing notice, and we will proceed to consider the much larger expenditures of energy involved in vaporizing the water exhaled by the leaves of plants and evaporated from the surface soil, as these unobtrusive and incidental processes, as they might be termed, are quite as significant factors in plant growth as the direct work of building organic substance, to which the attention of physiologists is more particularly directed. In field experiments the results obtained with manures must largely depend on the expenditure of energy, under the prescribed conditions, in the work of exhalation by the plants and the evaporation of water from the surface soil. The supply of plant food in the manure may, in fact, be a matter of secondary importance to the growing crop.
Experiments at Rothamsted, England, and on the continent by Hellriegel, on the exhalation of water by a variety of farm crops, including wheat, oats, peas, beans, and clover, show that about three hundred pounds of water are exhaled by the leaves for each pound of dry organic substance formed by the plants. It was estimated by Lawes and Gilbert that the average annual exhalation from the wheat grown on some of the experimental plots