POLLINATION AND STERILITY 299 



the latter may be sterile or fertile depending on the seed 

 relationship. 



266. Sterility not a constant factor. — Since the time of 

 Darwin it has been known that self-fertility and -sterility 

 of varieties of fruit may not be a constant characteristic, 

 but that it may vary with the age of the trees, health and 

 vigor, climate, general ecological conditions, and the like. 

 This led to many disputes and apparent misstatements until 

 the dual behavior of a variety was disclosed. The same tree 

 may be self-sterile at one time and self-fertile at another. 

 Vincent ^ reviews this question and calls attention to the 

 case of the Yellow Newtown apple which is listed as self-ster- 

 ile in one place and self-fertile in another, while the same 

 variation is recorded for the Rhode Island Greening and 

 Grimes Golden. Garcia reports the Bartlett pear as self- 

 fertile in New Mexico while most other observers record it 

 as quite self-sterile. Gardner ^ reports a similar variation 

 with sweet cherries in different parts of Oregon. 



267. Causes of sterility. — Kraus ^ divides the causes of 

 sterility into two general groups: (1) morphological and (2) 

 physiological. Several of the causes belonging to the first 

 group are rather well known, while those of the second are 

 more complicated and intangible. Among the more im- 

 portant of the former may be listed the following: (1) a 

 lack of germinability of pollen which in some cases may 

 amount to as much as 1 to 100 per cent; (2) imperfect pollen 

 or pollen in which some of the structures have degenerated; 

 (3) more complete abortive pollen as occurs with some of the 

 parthenocarpic fruits; (4) imperfection of the ovules which 

 is frequent in many kinds of fruit ; (5) a physical impossibil- 

 ity of self-fertilization as with dioecious plants; (6) various 



1 Better Fruit, Feb., 1920. 



2 Ore. Agr. Exp. Sta. Bull. 116. 



^ The self -sterility problem. Jour. Heredity, Dec. 1915. 



