272 PLANT HYBRIDIZATION BEFORE MENDEL 



no reason why we may not assume that each cell of the future plant has 

 representative molecules in the apparently simple egg." {ib., p. 276.) 



The general matter of fertility or sterility in the case of crosses 

 is briefly epitomized in the following statement: 



"To sum up present-day experiences, it may be said that crosses be- 

 tween species that are nearly related in structure and habit can readily 

 be effected, and the offspring may be largely fertile, at least among cer- 

 tain genera. Crosses between species that differ considerably in form, 

 flower color, and habit, are more difficult to perform, and the hybrids 

 are largely sterile, while crosses between such divergent species or 

 genera as Dianthus alpinus and barhatus, Saxifraga geum and Aizoon, 

 Lapageria and Philesia are almost wholly sterile." {ib., p. 277.) 



And again : 



"if we return now to hybrid production of the more extreme types, 

 though in virtue of the attraction which exists between sexual elements, 

 the original male and female cells from parents of different species — 

 in the absence of cells from the same species — may be capable of uniting, 

 and, in the process, of overcoming the repulsion due to dissimilar co- 

 relative molecules in each, when the attempt is made by all the herma- 

 phrodite cells of the resulting hybrid organism to concentrate repre- 

 sentative hermaphrodite groups of molecules, many cases will occur in 

 which these will blend imperfectly, owing to difference in the composi- 

 tion and amount of chemical substances present, or interference and can- 

 celling effects due to unequal propagation pf waves of motion between 

 the molecules. Thus many groups of molecules will break down or fail 

 to reach their destination, so that gaps or vacancies will occur in the 

 organic completeness of the pollen or egg cell. It will then have the 

 shrivelled half-empty look so characteristic of hybrid sex-cells that are 

 sterile. In hybrids from more nearly related species the interfering or 

 cancelling effects will be reduced in proportion, and a larger number of 

 sex cells will have a chance to mature." {ib., p. 281.) 



The last paper of Macfarlane's dealing with the histological 

 details of plant hybrids, is entitled "Observations on some hy- 

 brids between Drosera filiformis and D. intermedia,'' published in 

 1899. (6g.) 



The investigation was conducted upon a natural hybrid between 

 the above species, discovered near Atco, New Jersey. A group of 

 eleven plants was found, intermediate in form and color between 

 the two above local species. These were removed to the green- 

 houses of the Botanical Garden of the University of Pennsyl- 

 vania, where a histological examination was made of the two 

 parent species and of the hybrid. The comment is made that 



"The phenomenon which the writer terms 'bisexual heredity' receives 

 several striking exemplifications. Where two more or less diverse growths 

 have occurred, one on either parent, these have been shown to be re- 



