17b EXPERIMENT STATION. [Jan. 



The " standard deviation " shown in the right-hand cohimn 

 above will be easily understood by students of i:)lant variation. 

 Eoughly, it may be taken as the average deviation from the 

 mean. 



The figures show, therefore, that the marginal serrations on 

 Milton jilum leaves are much larger and much more variable 

 when Milton plum is worked on Americana stocks than when it 

 is worked on Wayland, Marianna or peach. On Marianna the 

 serrations are finest and least variable. 



Now, leaf margin characters are generally taken by botanists 

 to be of special importance in determining species and varieties. 

 That is, they are thought to be relatively constant. Moreover, 

 leaf margin characters are of special significance in the genus 

 Prunus, to which all the true plums belong. These curious 

 modifications, therefore, due almost certainly to the influence of 

 the various stocks, show conclusively that such influences may 

 be profound and far-reaching. 



Rate of Growth. 



The horticulturist will be more interested in the rate of 

 growth than in marginal serrations. Here, also, the influences 

 of the various stocks are traceable. Unfortunately, our measure- 

 ments include only three of the stocks in the experiment, the 

 peach being omitted. The average annual groAvth, taken in 

 centimeters, together with the standard deviation, is shown 

 below : — 



Annual Growth. 



Average 

 (Centimeters). 



Staiiilanl Deviation 

 (Centimeters). 



On Americana 103.45 35.62 



On Wayland 106.14 56.54 



On Marianna 149.30 57.90 



It is seen at once that the annual growth of Milton plum on 

 IMarianna stocks is much greater than on Americana or Way- 

 land, with somewhat greater variation. These averages, with 

 their respective deviations, may Ik; more convincingly shown, 

 at least to students used to modern plant breeders' methods, by 



