POLLINATION AND STERILITY 283 



tile seeds are not produced if the pollen or male element is 

 lacking or unavailable when the flowers are in bloom. This 

 has later been observed for angiosperms in general. Other 

 subsequent investigators made contributions on this prob- 

 lem, but the work of Sprengel of Germany (1793) marked 

 an epoch when he published "The Secret of Nature in the 

 Form and Fertilization of Flowers Discovered." His con- 

 temporary in England, Thomas Andrew Knight, published 

 a number of articles bearing on the pollination question and, 

 as a matter of fact, almost discovered "Mendelism," and 

 he announced as a law that "in no plant does self-fertiliza- 

 tion occur for an unlimited immber of generations." This 

 idea found its great culmination in Darwin's work when he 

 said "nature abhors self-fertilization." And yet notwith- 

 standing the wide application which this principle may have, 

 it is by no means universal. Wheat, for example, is self- 

 fertile, likewise peach varieties in general, the cleistogamous 

 flowers of violet never open, the tomato is regularly self- 

 fertilized, and so with many other plants. 



In this country a great impetus was given to a careful 

 study of the inter-relations of varieties of any kind of fruit 

 in an orchard plantation by the works of Beach, ^ Waite,- 

 and Waugh.^ 



The problems of pollination incident to the setting of 

 fruit will be considered in this connection, but the study of 

 cross-pollination for the purpose of producing new varieties 

 will be discussed in the next chapter. 



253. Causes of unfruitfulness. — Many diverse reasons 

 or causes must be given for the failure of trees to produce 

 fruit. Some cases of barrenness still lack explanation. The 



' Beach, S. A. Rept. N. Y. State Agr. Exp. Sta. 1892, '94, '95, '98, 

 '99, 1900, '02. 

 nVaite, M.S. U. S. Dept. Agr., Div. Veg. Path. Bull. 5. 1894. 

 3 Waugh, F. A. Rept. Vt. Agr. Exp. Sta. 1896, '97, '98, 1900. 



