1 84 CAUSES OF VARIATION 



the center of the embryo sac to form the endosperm. It will 

 be remembered, also, that the pollen nucleus undergoes a second 

 division during its progress down the pollen tube and before 

 uniting with the egg nucleus. 



Of this divided nucleus one portion unites with the single 

 functional member of the female group, making the germ, and 

 in cases such as are now under consideration the other joins 

 with the minor members concerned with the development of 

 the endosperm. In this way, by means of this kind of double 

 fertilization, the endosperm is itself affected and the crossing is 

 evident the first year. 



Of course this visible effect upon the endosperm is of itself 

 purely transitory, having no influence upon the line of descent. 

 The real effect of pollination is manifestly, as in all other ferti- 

 lization, confined to the germ. 



Whether the two fertilizations are similar as to comparative 

 influence of the two parents no one knows, nor does it greatly 

 matter. The effect upon the endosperm enables us to detect 

 the cross, if it is capable of detection, and to remove the con- 

 taminated seeds if we desire to retain purity. If the object be 

 to secure crossing, we shall of course subsequently deal with 

 the products of the cross-bred germ, which only are significant 

 from the breeder's standpoint. 



Just what species indulge in this double fertilization is not 

 well known. It is, however, well established in a large number, 

 and the process is supposed to be common rather than unusual. 



Effect of crossing upon fruit in general. What the layman calls 

 fruit is commonly not the endosperm that has been under dis- 

 cussion but the thickened and much developed fleshy receptacle 

 on which the seeds are borne. It has been claimed that these 

 parts are directly influenced the first year by crossing, so that the 

 character of strawberries, apples, pears, melons, squashes, etc., 

 depends much upon the source of the pollen used in fertilization. 



This claim has never been well substantiated by direct experi- 

 ment. Dr. Burrill, of the University of Illinois, tells me that he 

 crossed Crescent strawberries both with the Sharpless and with 

 a wild berry especially selected for its insignificant, worthless 

 fruit. Nobody was able to detect the difference in the resulting 



