A STUDY OF RECIPROCAL CROSSES. 87 



It is generally conceded that the descent of the pollen tube is 

 largely dependant on the nature of the stigmatic fluid as a 

 medium for the germination of pollen. During the descent of 

 the tubes certain enzymes are developed ; and while the exact 

 nature and importance of these is not worked out, it is thought 

 that they exert a considerable influence on the growth of pollen 

 tubes. If this influence is correct, it follows that unless these 

 nutrients are such as to develop proper conditions for germina- 

 tion, the penetration by pollen tubes cannot be effected. To what 

 extent this will account for failure in making reciprocal crosses 

 is still an open question. It is difficult to understand why trouble 

 should occur in the nourishment of the germ tubes only when 

 certain pollen is used, and not in all cases, and yet a slight dis- 

 turbance of the sexual organs may be of considerable import- 

 ance in this connection. 



B. Non-fusion of Nuclei. 



Whatever reasons there may be for failure in fertilization, it 

 is certain that fecundation cannot be effected unless there is a 

 fusion of male and female nuclei. The fusion of the nuclei takes 

 place only when the pollen tubes develop in such a way as to 

 allow the male nucleus to pass into the oosphere. Pollination 

 and development of pollen tubes may take place and yet fertili- 

 zation fail to occur. A few instances from other investigations 

 will bear out this statement. Mr. John Scott 1 observed that in 

 the case of Oncidium sphacelatum self pollination did not pro- 

 duce capsules, although the stigmas were penetrated by pollen 

 tubes. 



Furthermore, in another species, Oncidium microhilum, the 

 pollen was good, for with it he fertilized two distinct species ; he 

 found its ovules good, for they could be fertilized with another 

 plant of the same species, but this species could not be 'fertilized 

 with its own pollen, although the pollen tubes penetrated the 

 stigma. 



In 1900 Fletcher 2 noted that self-fertile varieties of plums 

 — miner and wild goose — were infertile on each other. In the 

 case of the wild goose plum the pollen germinates, and the tubes 

 pass down to the ovules, but for some reason the two sexes fail 



1. Scott, Journal of Proc. Linn. Soc, Vol. III. 



2. S. W. Fletcher, Cornell Univ. Exp. Sta., Bui. 181. 



