No. .374] 



VARIATION IN THE APPLE 



cm 



changes induced are pathological in nature and the plant 

 can not survive. 



Perhaps the most important factor to which plant life 

 is subjected is the moisture relation. This may be deter- 

 mined by the amount of moisture actually present or by 

 the modifying influence of other coexistent factors which 

 interfere with the availability of the moisture and the 

 capacity of the plant to make use of it. Among such in- 

 direct influences may be noted the modifying effect of 

 temperature upon the rate of absorption and transfer of 

 moisture, the presence in the soil of certain salts or humic 

 acids which interfere with the osmotic activity of the 

 roots, and certain atmospheric conditions favorable to 

 rapid transpiration. In such cases care is necessary to 

 determine which is the direct and which the indirect cause 

 of the modifications. If it is borne in mind that many 

 factors cause variation through their influence on the 

 moisture supply confusion may often be avoided. 



The temperature relation is much more obscure than 

 the moisture relation in its effect upon plant growth. 

 Heat, being a molecular phenomenon, acts directly upon 

 the protoplasm and its effects are therefore physiological. 

 It is now pretty well understood that heat alone is in- 

 capable of modifying plant structure, but acts indirectly 

 through other factors and the functions of the plant. The 

 direct effect of temperature is limited very largely to its 

 influence upon the rate and amount of development. A 

 slight difference in the average temperature of the grow- 

 ing season influences greatly the relative development of 

 apple varieties. The accompanying table gives the mean 

 monthly temperature during the growing season at 

 Geneva, N. Y., and Pullman, Wash., since the establish- 

 ment of the experiment stations at those points, as well as 

 the mean for two years at White Salmon, Wash. 



