APPENDIX, 199 



varieties are jjlanted near, they are often made fruitful. A self-sterile va- 

 riety is one which is unable to set fruit when alone : in order to be produc- 

 tive, it must be planted near some other variety. Two varieties very com- 

 monly self-sterile are Miner and Wild Goose plums. Large blocks of the 

 Kieffer pear and some of the oreat prune orchards on our Pacific Coast have 

 been unprolitable from this cause. Besides these striking examples, there 

 is reason for believing that much of the unsatisfactory fruiting of orchards 

 all over the country is due to the isolation or indiscriminate mixing of va- 

 rieties. 



THE MAIN CAUSE OF SELF-STERILITY. 



In general, the cause of self-sterility is that the pollen of a variety is un- 

 able to fertilize the pistils of that same variety. That is, if pollen from a 

 Wild Goose blossom falls on a Wild Goose pistil, whether on the same tree 

 or any other Wild Goose tree, no fruit will result, as a rule. The pollen of 

 a self-sterile variety may be, and generally is. iiroduced in abundance, and is 

 well formed. Wild Goose generally bears pollen freely, although it is one 

 of the most self-sterile varieties in cultivation. The Bartlett pear is often 

 self-sterile, yet its pollen is perfect. (Fig. 80. i The pollen of a self-sterile 

 vai'iety also has vitality, for it will fertilize the pistils of 

 other varieties. For example, plant together trees of the 

 two self-sterile varieties, Miner and Wild Goose, and both 

 will often be made fruitful, because the pollen of each, 

 though infertile on itself, is fertile on the other. It is not 

 so —Pollen of Bart- ^^"^"^^'^ i° what way this infertility is usually shown, but 

 iett. Much mag- with Wild Goose, at least, the pollen grain actually germi- 

 nates and the pollen tube passes down to the ovule. Why 

 the two sexes are unable to unite after having got thus far. the embryologist 

 has not yet told us. 



MINOR CAUSES OF SELF-STERILITY. 



Aside from the impotency of pollen, the main cause of self-sterility, there 

 may be several other incidental causes. Gott" and 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 fruit- 

 ful 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 w-orked on Marianna and Miner stocks, two of the most 

 self-sterile varieties in common cultivation. It has been thought that possi- 

 bly 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.- 



