POLLINATION AND FERTILIZATION 175 



be able to compare the set of fruit obtained as the result of 

 cross-pollination with that secured under natural conditions. 

 Blossoms are counted on twigs similar to those cross-polli- 

 nated and the twigs are tagged. The effect of cross-polli- 

 nation can then be estimated correctly by comparing the set 

 of fruit on the twigs artificially pollinated with that on twigs 

 pollinated under natural conditions. 



273. Choosing varieties for pollination purposes. By 

 extensive experimenting, the kinds of pollen can be found 

 which give the best set of fruit for any particular variety. 

 In order that an abundance of fruit may set, the grower must 

 in many cases inter-plant with trees of the proper variety for 

 supplying suitable pollen. It is important to know that some 

 varieties are inter-sterile, that is, will not set fruit when cross- 

 pollinated. For example, the Bing and Napoleon (Royal 

 Ann) sweet cherries are inter-sterile. The Bing will not set 

 fruit when pollinated with the Napoleon pollen, and neither 

 will the Napoleon set fruit when pollinated with Bing pollen. 

 In like manner, Nonpareil and IXL. almond varieties are 

 inter-sterile. 1 On the other hand, some varieties are inter- 

 fertile, that is, their pollen will fertilize each other. The 

 Napoleon and Black Republican varieties of sweet cherries 

 are examples. 



Some fruit-trees are self -fertile, ,or will set fruit well even 

 when self -pollinated. Most peach trees are self -fertile. In 

 setting out an orchard, in addition to selecting good com- 

 mercial varieties, the grower must select kinds which are 

 either self- or inter-fertile. In the past, many growers have 

 planted large blocks of a single desirable commercial variety 

 of fruit, only to learn after a number of years that the sort 

 would not produce well because it was self -sterile. Indeed, 

 it is not sufficient to know that a variety is generally self- 

 sterile or self -fertile, but the grower should know the facts 

 with reference to his own section of the country. For ex- 



Bull. 306, Calif. Exp. Sta. 



