370 Bulletin 181. 



are usually in solid blocks, or at a distance from any other variety. 

 Planting near them other trees of the same variety does not make 

 them fruitful ; but if trees of other varieties are planted near they 

 are often made fruitful. A self-sterile variety is one which is 

 unable to set fruit when alone / in order to he productive^ it iiiust 

 he planted near some other variety. Two varieties very commonly 

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

 Kieffer pear and some of the great prune orchards on our Pacific 

 coast have been unprofitable from this cause. Besides these strik- 

 ing examples, there is reason for believing that much of the unsat- 

 isfactory fruiting of orchards all over the country is due to the iso- 

 lation or indiscriminate mixing of varieties. 



The Main Cause of Self-Sterility. 



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

 variety is unahle to fertilize the pistils of that same varnety. 

 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 produced 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.) The pollen of a 

 self-sterile variety also has vitality, for it will fer- 

 tilize 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 



80. -Pollen of infertile on itself, is fertile on the other. It is not 

 Bartlett. Much . . . ^ .i • • <• ^-i-^ • ^^ 



maonified. known m what way this miertility is usually 



shown, but with Wild Goose at least, the pollen 

 grain actually germinates 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 Cases of Self-Sterility. 



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

 sterility, there may be soverfvl other incidental causes, Goff and 



