351] B. EH. Lwwingston 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 
bégin 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 of 
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 progross 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. 
