POLIJNATION IN ORCHARDS. 371 



Waugh have shown that self-sterile varieties of native plums often 

 have a large per cent of pistils which are too weak to develop into 

 fruit. This could not be a general cause of self-sterility, however, 

 for self-sterile varieties can usually be made fruitful by planting 

 other varieties near them. This shows that there are enough sound 

 pistils on the tree for a good crop of fruit, provided they receive 

 the right kind of pollen. 



Again the blossoms of some varieties may produce but a small 

 amount of pollen. When these varieties are planted alone they may 

 not have enough pollen to set a good crop, even though the pollen 

 is fertile on its associated pistils. The amount of pollen which 

 flowers produce is greatly modified by weather conditions and the 

 vigor of the tree. 



Many plums are worked on Marianna and Miner stocks, two of 

 tlie most self-sterile varieties in common cultivation. It has been 

 thought that possibly there might be an influence of the stock on the 

 scion in the direction of self-sterility, but this assumption seems to 

 be without foundation. 



Finally, tlie stamens and pistils of a tree may not mature simul- 

 taneously, which would make a tree unfruitful unless pollen is sup- 

 plied from other sources. With many varieties of orchard fruits 

 the pistil of each flower matures a little before the stamens ; and 

 not infrequently the stamens mature before the pistil is ready to 

 receive the pollen. But there is usually enough variation in the 

 opening of flowers on the same tree to promote pollination with 

 each other and so prevent serious loss from this alternate ripening 

 of the sexes. Defective pistils, scanty pollen supply and the prema- 

 ture ripening of either pistils and stamens may often be important 

 in determining the fruitfulness of a tree ; but the main cause of 

 unfruitfulness in most self-sterile varieties is the failure of the pollen 

 to fertilize its associated pistils. This cause cannot be removed, but 

 its injurious results may often be prevented by a judicious selection 

 of varieties. 



A Practical Application. 



The practical bearing of the self -sterility problem is this : There 

 are certain varieties of fruit which we wish to grow largely for the 

 general market, but we find that they are not productive when 



