ENERGY AS A FACTOR IN AGRICULTURE. 91 



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 over- 

 coming 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 nu- 

 trition ; the evaporation of water from the surface soil ; and the 

 warming of the soil to provide optimum conditions of tempera- 

 ture. 



The energy expended in constructive metabolism, or tissue- 

 building, 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 push- 

 ing 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 metab- 

 olism are, however, so familiar that they require but a passing 

 notice, and we will proceed to consider the much larger expendi- 

 tures of energy involved in vaporizing the water exhaled by the 

 leaves of plants and evaporated from the surface soil, as these un- 

 obtrusive 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 ex- 

 halation from the wheat grown on some of the experimental plots 



