Pollination 283 



transplanted in this way. The cause was seldom dis- 

 covered; but in many cases it was doubtless due to the 

 fact that the tree had been growing with many others 

 and had been well cross-pollinated in its old home on 

 the river bank, but when isolated in the garden its 

 flowers were not fertilized. 



Self -sterility. This condition of self-sterility is 

 very common among plums. It is well-nigh universal 

 with the native species, and the Japanese plums seem 

 to have the same character. The old European or 

 Domestica varieties seem to be less subject to this dif- 

 ficulty, though there is less experimental evidence on 

 record touching their case. 



A variety is spoken of as being self-sterile when 

 the pollen from its blossoms fails to fecundate its own 

 ovules. Unless the ovule of the fruit is fertilized, 

 or fecundated, it does not develop into a seed; and 

 unless the seed develops, the fruit fails to grow. The 

 crop of fruit thus depends in many cases entirely on 

 proper pollination. This subject has been thoroughly 

 studied in recent years, and our knowledge of it, 

 though still limited, is much greater than it was a 

 decade ago. It will not be necessary here to go into 

 the details of the experiments which have been directed 

 to the study of this question. The practical results 

 are all that are now required. 



Cause of self -sterility. Self-sterility of any va- 

 riety or tree may be due to any one of several 

 different causes. The most important of these are 

 (a) defective pistils, (b) insufficient pollen, (c) differ- 

 ence in time of maturity between the stigma and the 

 pollen, and (d) impotency of pollen. 



Defective pistils. It has been found that many 

 trees and varieties, especially of native plums, often 

 have imperfect pistils. These imperfections or de- 



