FACTORS INFLUENCING REPRODUCTION 849 



The growth of corn, wheat, and clover, measured at five-day inter- 

 vals, is shown graphically in figures 3 and 4. The plants were growing 

 on the same flats as the coniferous transplants, and consequently were 

 under half shade until they reached about 15 centimeters in height. 



These graphs show that the mild humus was by far the most favor- 

 able for all plants concerned ; in fact, the corn grew so well on this soil 

 that it had to be cut by the 15th of July to prevent interference with 

 the rest of the experiment. But aside from the favorableness of the 

 neutral humus, growth does not appear to be directly correlated with 

 acidity except in the case of clover. For example : With corn, the min- 

 eral soil, though considerably less acid, shows up more poorly than the 

 partly decomposed humus ; and with wheat the mineral soil, though 

 least acid, is the poorest of all. The poorness of the mineral soil for 

 these plants may be due to the lack of organic matter and the unavail- 

 ability of its mineral nutrients, its "rawness." If this is the case, the 

 availability of nitrogen seems to be the important factor for these 

 plants, although the use of the nitrogen, where it is available, is limited 

 by the presence of acids. 



The efifect of acidity upon growth appears to be more marked in 

 clover than in the others. On all three acid soils it gradually died and 

 disappeared, but persisted longest on the mineral soil. 



The following table gives the dry weight per plant of the wheat and 

 corn grown on the difl:'erent soils, except for the corn on the mild 

 humus. The figures give a quantitative idea of the crop-producing 

 possibilities of these soils. 



T.\BLE 2 



Dry weight per plant, cut 

 September 15, 1916 



Grams 



Corn Wheat 



Mild humus Cut July 15 .53 



Mineral soil .13 .03 



Partly decomposed humus .49 .03 



Undecomposed humus .09 .08 



The corn and wheat afforded an opportunity of observing the effect 

 of soil upon drought resistance. Ordinarily we would expect the in- 

 fluence of drought to be directly related to the physical properties of 

 the soils affected. In this case, however, the effects of drought and 

 recovery from drought were correlated with soil fertility. The con- 

 trolling factor here was the vigor of the plants, which in turn was de- 

 termined by the fertility of the soil. Thus in both corn and wheat the 

 smallest percentages affected by the drought were on the mild humus ; 



