POLLINATION AND FERTILIZATION 169 



low germination test. In fact, pollen from trees and shrubs 

 which depend on wind for distribution is seldom high in ger- 

 mination percentage. 



In case of pollen from common fruit-trees, the germination 

 test in sugar solutions indicates the viability of the pollen, for 

 grains which will not germinate in the sugar solutions will 

 not germinate and fertilize the ovules when applied to the 

 stigmas. In fact, germination media are known with which 

 to test the viability of any sort of pollen, although no one 

 medium has been found which works equally well with all 

 varieties. 



263. Viability of pollen affected by various factors. In 

 addition to the factors already mentioned, climate, moisture 

 supply, physiological conditions of the tree, and earliness of 

 bloom affect the viability and production of pollen. How- 

 ever, some species and varieties of trees produce better pollen 

 than others. This characteristic seems to be inherited and 

 remains true (but is varied by other conditions) from year to 

 year. The Combination plum is an example of a variety 

 which produces pollen of poor viability and also in very scant 

 amounts. Its pollen is not sufficient in quantity and via- 

 bility to fertilize other trees successfully. In general, it may 

 be said that some species of trees naturally produce pollen of 

 higher viability than others, and may also yield pollen in 

 larger quantities. The apple produces pollen in abundance 

 and of high viability. Varieties of trees differ in the amount 

 of pollen borne. Individual trees also differ in this regard. 

 Many hybrids, that is, plants resulting from the crossing of 

 species, produce only a scant quantity of pollen which is fre- 

 quently not viable. 



264. Inter-planting for pollination. Inasmuch as pollina- 

 tion bears directly on fruit production, every orchardist is 

 vitally interested in planting the correct mixture of varieties. 

 Many pollination problems are still unsolved. Much experi- 

 menting is being done to determine which varieties are 



