5 



is rencierea exceedingly complex ty the nurrber cf these conditjons 

 sna their continue! verif-tion, fs well ss by the ch^ngin/x inter- 

 n&l conditions of the plant itself, e detailed analysis of the 

 control of plant growth in terrr.s of effective climatic conditions 

 is very oifficult, but, as Livingston and iicLean suggest, the 

 rate of growth *of any plant is itself an expression of the sum 

 total ail the effects of the external conditions acting during 

 the growtii period, so that a standard ^olant rright be eir.ployed 

 as &n eutorsa tically weighting, integrating, and recording in- 

 str'acent foj' the cor.parative neasurenent of growth conditions as 

 these act on plants. I'.ius tc;veral environments i^.sy be measured 

 ana compareu in ternns of their several capacities for producing 

 grovith in the standard plant. This method of rr.easur in*"' environ- 

 ment in terr.s of plant growth can be appliec. only when it rcay 

 be assumeu that all the standard plants are alike at the begin- 

 nings of the several periods of exposure. In the present study 

 the requirement just stated was fulfillea by employing the seed 

 as the starting point for the plants of the various cultures. 

 It was apparent that if the cultures were always started from 

 the seed the plants migh" be considered as more nerrly alike 

 at the beginning of the several culture periods than woula 

 have been the case if an attempt haa been made to obtain. like plants 

 in any other phase of their uevelcpment. The internE 1 conditions 

 of tiie plants change continually, however, during growth, and 

 no two of the cultures were the same at the end of the culture 

 periods. Tiiis phase of th« problem will receive attention later. 



As in other problems in which a num.ber of" conditions 

 enter into the control of a process, the' relat ions between 



