584 Journal of Agricultural Research vol. xx.No. 7 



some error which would tend to underweight the coefficients of deter- 

 ' mination between transpiration and the known factors and so over- 

 weight the apparent degree of determination by outstanding factors. In 

 certain cases, however, the residue is so small, in one case actually com- 

 ing out negative, that it is probable that this is not an important source 

 of error. The residual determination is greatest for the crops which 

 were cut twice during the season — namely alfalfa and amaranthus. 

 There were considerable periods following each cutting during which the 

 absolute value of the transpiration was small. 



Wind velocity has about the same relative value as a factor in deter- 

 mining transpiration as it has in determining wet-bulb depression. Its 

 relative importance is a little greater for determining evaporation from 

 the shallow tank. 



Temperature is somewhat more important than absolute humidity in 

 determining the variations in wet-bulb depression and rate of evapora- 

 tion from day to day. It is very much the most important factor in 

 determining the rate of transpiration in all the plants. 



Radiation is an important factor in evaporation, coming out equal to 

 wind velocity and only slightly less important than absolute humidity. 

 In the plants, on the other hand, it is almost a negligible factor. 



Comparing transpiration in the average plant with evaporation in the 

 sun from a shallow tank, we find that the former is influenced relatively 

 much more by temperature, to about the same degree by absolute 

 humidity, somewhat less by wind velocity, and very much less by radia- 

 tion. The four factors are much more nearly equal in importance in the 

 case of evaporation (g?e-t = o.3o, <f E . H = o.i9, d E . w =o.i6, d E . E = o.i6) than 

 in the case of transpiration (c/ x .t = 0.55, d x . H = o.i8, g? x . w = 0.09, <i x . E = 0.04). 

 In comparing the importance of these factors it should be added that 

 radiation has an importance somewhat in excess of its direct influence, 

 in that its variations are correlated with those of temperature. Humidity 

 has reduced importance, since, though correlated with temperature, it 

 affects evaporation and transpiration in the opposite direction. 



OTHER APPLICATIONS 



The method of analysis presented here can readily be applied to the 

 problem of the relative importance of heredity and environment. An 

 application of this kind to the case of the piebald pattern of guinea pigs 

 has already been published (9), and one to the resistance of the same 

 animal to tuberculosis is in press. 1 The method can be applied also to 

 such a problem as the determination of the effects of various systems 

 of mating, such as inbreeding, line breeding, and assortative mating on 

 the genetic composition of an originally random-bred stock. 2 



1 Wright, Sewall, and Lewis, Paul A. factors in the resistance of guinea pigs to tuberculosis 

 with special regard to inbreeding and heredity. In Amer. Nat., v. 55. 1921. In press. 

 'Wright, Sewall. systems of mating, i to v. In Genetics, v. 6. 1921. In press. 



