228 The Principles of Fruit-growing. 



mental conditions. It is probable that varieties may 

 sometimes be self -fertile and at other times self- 

 sterile. The strawoerry flowers in Fig. 22 show the 

 marked influence upon pollen -bearing which is ex- 

 erted by different conditions. The flowers are of the 

 same variety, and were grown under glass. The 

 lower one shows the small development of stamens 

 in a long cloudy spell, and the upper one shows 

 the profusion of stamens which appeared in other 

 flowers after two or three days of sunshine. It is 

 probable that pollen is more profuse and more potent 

 in some years than in others. 



There is very little positive knowledge concerning 

 the inter -pollination of fruits, and no subject con- 

 nected with pomology is in greater need of study. 

 We chiefly know that the most productive orchards 

 are usually those of many varieties, and that some 

 varieties sometimes refuse to fertilize themselves. The 

 most positive knowledge respecting the impotency of 

 pollen amongst our common fruits is in connection 

 with the plums of the Wild Goose type. The safest 

 practice, therefore, is to plant no more than two 

 or three rows of any one variety together of fruits in 

 which self -sterility is a frequent characteristic (pears, 

 plums, apples). The chief point to be observed in se- 

 lecting the varieties is that they shall bloom together. 

 All pears and apples, and nearly or quite all plums, 

 seem to be inter-fertile when they bloom simultaneously. 



The following lists of self -sterile and self -fertile 

 fruits are summaries of our present knowledge upon 

 the subject: 



