98 ENVIRONMENTAL CONDITIONS. 



conditions that make up the environment. Considerable time is 

 required for most responses, and a momentary alteration in the envi- 

 ronment may often pass without apparent effect upon the plant. 

 Thus, outside of the time factor, the necessary and sufficient conditions 

 for the production of those changes in manner of growth that are 

 termed etiolation are present every night, but these conditions are 

 not effective over a long enough period of time to result in visible 

 responses. In the study of any external factor or complex of factors 

 it is logically necessary and inevitable that the time element enter 

 seriously into consideration. 



Physiologists have found it advantageous to analyze the environ- 

 ment into its component conditions or factors. While some factors 

 have so far received but a minimum of attention, a large amount of 

 reliable information is already at hand bearing upon the effects pro- 

 duced by the action, over various time intervals, of different intensi- 

 ties of heat, light, oxygen-supply, etc. The method of such deter- 

 minations has been to hold all factors but one as nearly constant as 

 possible, and to cause various selected intensities of this one factor to 

 register their effects upon the plant, in the form of alterations in 

 growth or other activity. 



But this study of the simple component factors of the environment 

 is only the learning of the alphabet, and the task of really reading 

 the book of plant phenomena in the light of cause and effect still rests 

 with the future. We are already well aware, in a general way, that 

 the responses brought about in the organism by a certain quality, 

 intensity, and duration of any external factor are totally dependent 

 upon the nature of the other concomitant factors which are comprised 

 in the environmental system. For example, a given increase in the 

 rate of water-supply may fail to produce any marked acceleration 

 of growth in certain forms existing under excessive drought conditions, 

 but if the increase in soil-moisture be accompanied by a decrease in 

 the evaporating power of the air, growth response may be immediate 

 and definite enough.^ Again, the agriculturist is well aware that with 

 many soils an increase of the nitrate content is without full effect unless 

 other salts are simultaneously added. In such cases the result of 

 these several increases together is not generally a simple summation 

 of the results obtained by the single additions separately. 



While a large amount of laboratory experimentation of the most 

 refined physical sort will be required before we shall even approach 

 an adequate knowledge of the influence of single conditions upon 

 plants, the far more difficult study of the complex environmental 

 system of which these single conditions are always components has 

 already begun to attract attention. It seems safe to predict tliat the 

 line of work thus started will rapidly gain in prominence, and it is 



^Livingston, B. E., Evaporation and Plant Development, Plant World, 10: 269-276, 1907. 



