6 



conditions and process rate are more easily detected tfee 

 smaller is the nuinber of conditions involved, and conditions 

 may be left out of consideration if they are the same in sev- 

 eral experiments. Just as the internal conditjor.s of the 

 standard plant are left out of the arguniemt by the simple 

 device of having them all alike at the beginning of the ex- 

 ■posure period f the instrument being set at zero of its scale, 

 in the v/ords of livings-^on and Tv'cLean) , so selected ones of the 

 surroundings may be left out of consideration by having them 

 alike throughout all of the periods. According to this -nrin- 

 ciple all of tlie environmental conditions that acted on the plants 

 belovf the soil r^urface were kept practically constant at all 

 times and at all stations. Assuming that the artificial 

 control of the subterranean environmental co^.ditions T/as thus 

 practically constant, -^iie differences observed in the grov;th 

 of the standard plants vv'ere taken to be related almost entire- 

 ly to the aerial conditions of the surroundings. These are 

 the ones referred to by McLean afc climatic , and this terir; v/ill 

 be used '>vith the same meaning in the present paper. To a.ccom- 

 plish this control of the subterranean conditions, the soil 

 was always the same at the beginning of all cultures and its 

 moisture content was generally kept approximately the same 

 throughout all culture periods, by means of the Livingston auto- 

 irrigator. "^he arrangement and its operation have been de- 

 Gcribed by I'^o'Lea.n. and will receive some attention below. 



The grov/th rates of the plants were meastirXsd and com- 

 pared in terir.- "'' their size ani weight. Zr?.cih cultue consist- 

 ed of six plants grown for a leriod o:'' four weeks from the 

 se=d. Cultures were started approximately every two weeks 



