POLLINATION AND STERILITY 305 



pistils are receptive (proterandrous), while in other cases 

 the pistils mature before the pollen is ripe for pollination 

 (proterogynous). 



It is true, however, as has been stated, that the Japan- 

 ese and American plums are generally inter-fertile both 

 within their respective species and also in hybridizing be- 

 tween the species. P. americana is less inclined to hybridize 

 than some other species of American plums. 



The Domestica plums (European species) are variable, 

 some varieties being self-fertile and others self-sterile,^ but 

 as a class they may be considered inter-fruitful.- They are, 

 however, largely inter-sfcerile with Japanese and American 

 plums but may be inter-fertile to some degree, as indicated 

 by Hendrickson.^ 



He has shown also that the French prune is abundantly 

 self-fertile in California if bees are present to work over 

 the blossoms, but that the Imperial prune is very much 

 less so. His work with these varieties well illustrates the 

 value of the common honey bee as an agent in prune pollin- 

 ation.^ The following data which he obtained are self- 

 explanatoiy: 



1 Bailey, L. H. Principles of Fruit-Growing, 20th Ed. p. 158. 1915. 



2 Marshall, Roy E. Proc. Amer. Soc. Hort. Sci. 1919. p. 42. 



3 Hendrickson, A. H. Proc. Amer. Soc. Hort. Sci. 1919. p. 50. 



* As an interesting side light on the application of these pollination 

 studies, the following excerpt is quoted from W. L. Howard, letter, 

 dated March 11, 1920. "Our investigations in the Santa Clara Valley, 

 zarried on for five years, showed that bees are an important factor in 

 prune pollination. Before our investigations were started, bee men paid 

 the fruit growers from 50c to .$1.00 an acre for the privilege of pasturing 

 their bees in the orchards during the blooming season. Since our ex- 

 perimental findings were published, three years ago, there has been a 

 complete change in the situation. This sea.son bee men have been able 

 to rent every hive they have to fruit growers for from S2.00 to $3.00 

 per hive, depending upon whether they are bunched in one place or 

 scattered over the orchard." 



