No. 574] VARIATION IN THE APPLE 603 



and especially the moisture relation, have an important 

 bearing-, since one of the primary results of cultivation is 

 the conservation of the soil moisture. There is no doubt 

 that the nature of the soil greatly affects the crop and the 

 matter has been given much study. The intimate associa- 

 tion of other factors, however, makes it somewhat difficult 

 to pick out those influences for which the nature of the 

 soil is directly responsible. 



The influence of other organisms includes not only a 

 consideration of the effect of insect and fungus pests but 

 in the broad sense embraces such items as pollination, 

 pruning and thinning, intercrops, cover-crops and plant- 

 ing distance. Human agencies, including all operations 

 of orchard management, might properly be included here. 

 Many of these are, of course, indirect, exerting an influ- 

 ence through their effect upon some other factor. 



The Law of the Optimum 



Having enumerated the chief external influences to 

 which plants are subjected during their period of develop- 

 ment and to which variation is largely due, the question 

 naturally occurs whether there can be formulated any 

 basic principle or law which will express the manner in 

 which plants react with the environment. Such a law 

 would be of use not only in the study of variation, but 

 would shed much light on the adaptation of plants to new 

 environments. It would constitute a unifying principle 

 whereby isolated facts and disconnected observations ap- 

 pear in proper relation and perspective. Though a dis- 

 cussion of this subject might logically be delayed until 

 after the characters of the several varieties and their 

 modifications have been noted, it is thought most fitting to 

 introduce the statement at this point and examine the 

 fruit in the light of such generalizations as it has been 

 possible to make. 



A plant can live and perform its functions only within 

 certain intensities of the various factors of the environ- 

 ment. The degrees of intensity beyond which activity 



