351] B. E. Livingston 153 



and for any given complex of external conditions may readily 

 be determined. All the various standard nutrient solutions 

 heretofore employed have contained at least four salts (besides 

 the trace of iron), and have been correspondingly more com- 

 plex and difficult to handle and interpret. This work applies 

 to many phases of the art of fertilizer practice, as employed in 

 agriculture, and it furnishes a method by which we may now 

 begin to study the salt relation as influenced by the condi- 

 tions of the water relation, the temperature relation, etc. 



The effects of some other inorganic ions upon plants have 

 begun to receive attention here, also the effects of variations 

 in the oxygen content of the soil. 



The relation of plants to climatic conditions. The main 

 climatic conditions that affect plants are air temperature, 

 atmospheric evaporating power, and the effective intensity If 

 solar radiation. Other climatic -conditions generally affect 

 plants only indirectly; for example, rainfall influences the 

 water-supplying power of the soil. 



The studies thus far undertaken in this laboratory have 

 dealt with an attempt to find out in what manner and to what 

 degree the annual march of the complex of climatic condi- 

 tions may be related to the corresponding annual or seasonal 

 march of plant growth-rates. From these studies has been 

 developed -a method by which it appears possible to compare 

 climates (of different places at the same time or of the same 

 place at different times) in terms of the growth-rates of a 

 standard plant. The plant is thus employed as an auto- 

 matically weighting and integrating instrument. 



This general relation is of great importance to agriculture 

 and forestry and the point of view here taken (that of the 

 conditional control of plant processes) is attracting the atten- 

 tion of investigators in these subjects. The problems are 

 exceedingly complex, but progress is being slowly made. 



The reader will be able to form a somewhat more concrete 

 conception of what has thus far been accomplished, by refer- 

 ence to the list of publications from this department, which 

 follows the present paper. 



