No - 443 ] MUTA TION IN PLANTS. 739 



The essential differences between the two processes appear to 

 have been originally set forth by Charles Darwin, 1 and are treated 

 at length by deVries. The formal distinctions drawn by deVries 

 appear to need some slight modification and elaboration in order 

 to make them universally applicable. Thus he holds that con- 

 tinuous, or fluctuating variability occurs only in accordance with 

 Quetelet's laws, and that it involves only the number, size and 

 weight of organs, and does not include differences in qualities. 

 Cultural experiments of various kinds during the last few years 

 have given results in which the qualities as well as the number, 

 size and structure of organs have been materially altered, but 

 such induced variations or divergences were not transmissible. 

 This particular factor in distinguishing between fluctuating and 

 mutating variability therefore becomes a safe one, when it is 

 modified to make mutating variability include only newly 

 acquired and transmissible qualities. The presence of a plant 

 or an animal parasite may not only change the mechanical fea- 

 tures of an organ but may also cause most radical alterations 

 in its physiological properties. A single example of the latter 

 may be cited in the case of the common species of Euphorbia in 

 which the affected leaves alter their geotropic sensibility in such 

 manner that thev change from diaireotropism to apo<reotronism. 



