90 THE POPULAR SCIENCE MONTHLY. 



matter have been looked upon as the sole factors requiring atten- 

 tion, and a simplicity in organic processes has been assumed that 

 is not warranted by our present knowledge of the conditions that 

 have a decided influence on the nutrition and well-being of plants 

 and animals. 



An approximate quantitative estimate of the expenditure of 

 energy in certain processes of Nature involved in growing a field 

 crop will serve to illustrate its importance in biological science 

 and farm economy, and a preliminary review of some of the 

 salient points in the economy of plants will simplify the problem 

 we have to deal with. 



A growing crop, in common with other living organisms, re- 

 quires certain conditions of environment for the healthy and vig- 

 orous exercise of its vital activities, among which may be enu- 

 merated as essential, a suitable temperature, a certain supply of 

 moisture, and a sufficient food-supply; and to these must be 

 added soil conditions that promote an extended root development 

 and distribution. 



Plants differ as to the temperature required for active growth, 

 but there is for each a minimum, below which growth ceases ; a 

 maximum, above which life is destroyed ; and between these an 

 optimum temperature which is most favorable for the activity of 

 the processes of nutrition. The temperature of the atmosphere, 

 which is an incident of seasons, need not be noticed here, but it 

 may be remarked that it is of less practical importance than soil 

 temperatures, which depend on conditions that, to some extent, 

 may be controlled. 



Plants obtain their supply of water from the diffused moisture 

 of the soil, which is retained by capillary attraction. In fertile 

 soils this capillary water is kept in constant circulation by the 

 drafts made upon it by growing plants, and by the evaporation 

 which takes place from the surface soil, and an equilibrium is 

 thus maintained in the distribution of soluble soil constituents, 

 and in the processes of soil metabolism.* 



To say nothing of other important considerations, it is evident 

 that soil conditions favorable for the extended distribution of the 

 roots of plants are necessary to enable them to obtain their needed 

 supplies of water from the comparatively limited amount present 

 in the soil. As the water evaporated from the surface soil is re- 

 placed from below by capillary attraction, its influence on soil 

 metabolism and the transportation of soluble soil constituents to- 

 ward the surface strata should receive attention as a factor in the 



* The series of chemical, physical, and biological changes taking place in the soil, or in 

 the processes of vegetable and animal nutrition, are conveniently expressed by the general 

 term metabolism, and they are frequently designated as metabolic processes. 



