fore and after each filling of the jar with fresh solution. It is de- 

 sirable to employ the weighing method for such work as this, but 

 experience suggests that many experimenters prefer the volu- 

 metric method. Either will serve our purpose, however. In 

 either case the desideratum is to have a record of the approxi- 

 mate amount of solution removed from each jar during each 

 three and one-half day period. 



Experiment Location and Exposure. 



The cultures are to be exposed generally in a greenhouse, but 

 at some stations they may be out of doors. In any event, they 

 are to receive the climatic light (with shade, as of painted glass, 

 etc., only when this seems necessary in order that the plants may 

 thrive), but they must not receive any rain. For the colder 

 months at most stations the temperature will be artificially 

 much above the climatic temperature. When artificial heat is 

 not considered as necessary it will of course be omitted ; in gen- 

 eral, any greenhouse in which other plants are kept growing 

 should be suitable for these cultures. It is planned to carry out 

 these tests with a large number of climatic complexes, such as 

 will be obtained by employing a large number of geographical 

 locations and various seasons of the year at each station. 



The special problem of uniform exposure to the aerial complex 

 of conditions (uniform for all cultures of any comparable series) 

 is of considerable importance, and it is hoped that all experi- 

 menters will use rotating tables. This point has been discussed 

 by Shive (1915) , who describes one way to build a rotating table 

 for this sort of work. It is desirable that all cultures that are to 

 be comparable should stand on the table in a single circle; two 

 circles of them form two series with slightly different aerial 

 surroundings, although the different cultures of any one circle 

 are themselves comparable. (If a rotating table cannot be em- 

 ployed, the cultures should be shifted on the bench so as to pass 

 through each position of exposure every few days; it. is well to 

 shift them daily according to a definite plan.) 



Rotating tables built on motor-cycle wheels can be supplied by 

 the Plant World, but it is probably best for each cooperator to 

 superintend the building of his own. Tripod bases can be sup- 

 plied at $15.00 apiece, just the casting. Five-ply wood circles, 

 painted, four feet in diameter, can be supplied at $12.00 apiece, 

 packing included. The bearing portion may be built of a bicycle 

 hub, etc., in various ways that will suggest themselves, and 

 second-hand wheels may perhaps be obtained in some places. 



20 , 



