<?7 



duration introduces several experimental difficulties not 

 met with in the shorter experiments of previous investi- 

 gators and which difficulties it has been necessary to 

 overcome. She three most important of these difficulties 

 are the maintenance of a permanent seal about the plant 

 stem, the supply of water without breaking the seal, 

 and the control of the soil atmosphere without consum- 

 ing an impossibly large quantity of the needed gases, A 

 seal which is sufficiently permanent has been devised and 

 will be described in detail. The difficulty of water 

 supply has been met by the use of the porous cup auto- 

 irrigator as devised by Livingston. In the experiments 

 of Xosaroff and other previous investigators the control 

 of the composition of the soil atmosphere was secured 

 by passing through the pot containing the sealed-in soil 

 a stream of gas having the desired composition. This 

 method is impracticable in experiments running over weeks 

 or months because of the large quantities of gas which 

 would be required, the cost of this being prohibitive for 

 all gases except air. There has been devised, therefore, 

 a static method of controlling the soil atmosphere which 

 will be described in detail and which does not require the 

 passage of a stream of gas. By this method the consumption 



" T. Livingston,- Plant World 11: 39-40 (iy08); 

 Hawkins,- Plant Jorld 13: 220-227 (1910); Livingston and 



Hawkins,- Gar. Inst, of "./ash.. Pub. IIo. 204: 5-48 (1915). 



