238 GENETICS IN RELATION TO .AGRICULTURE 



For a few particular characters, Jesenko was able to establish close ap- 

 proximation to a Mendelian analysis, so that it can scarcely be doubted 

 that in the sorting out of the factors to establish the constant races of 

 further generations, the phenomena displayed were such as to indicate 

 clearly the operation of a Mendelian mechanism. 



But when we consider the phenomena in the light of the characters 

 involved, then it may be seen that the results obtained are truly remark- 

 able. Wheat and rye differ strikingly in their characters and the recovery 

 of approximately the parental form so often in these back-crosses is out 

 of the question from a strict Mendelian viewpoint, if all combinations 

 are assumed to survive. 



As an explanation of these phenomena, Jesenko calls attention to the 

 fact that there are eight chromosomes in the germ cells of rye and wheat, 

 so that in the formation of gametes in the F\ some will possess eight wheat 

 chromosomes, others seven wheat and one rye, and so on. When back- 

 crossed to wheat, therefore, union with those gametes which contain only 

 wheat chromosomes or at most two or three rye chromosomes results in 

 wheat-like plants which are fertile, whereas greater proportions of rye 

 chromosomes results in plants which are less like wheat and sterile. 

 Similarly, as Jesenko in fact found, pollination with rye results in plants 

 resembling rye, because of the union of the rye pollen with gametes 

 which contain all or nearly all rye chromosomes. The sterility in these 

 hybrids, therefore, Jesenko regards as the consequence of the inharmoni- 

 ous action of a "plasma" built up of large proportions of both rye and 

 wheat elements. 



Partially Sterile Hybrids in Nicotiana. A similar state of affairs 

 has been found to exist in hybrids between various varieties of Nicotiana 

 tabacum, the commercial tobacco, and N. sylvestris, a very different 

 species. N. tabacum occurs in a very large number of distinct varieties 

 some of which are so different that they could justly lay claim to recogni- 

 tion as distinct species. Goodspeed and Clausen have studied the 

 hybrids of a number of N. tabacum varieties with N. sylvestris and have 

 found that in all cases the FI hybrid duplicates very closely the total set 

 of characters of the particular tabacum variety used in the hybrid save 

 on a very much enlarged scale, for these hybrids are conspicuous for the 

 increased vigor due to hybrid stimulation. In Fig. 102 is illustrated a 

 typical plant of N. sylvestris. N. sylvestris is a monotypic species and 

 has been grown under cultivation for over thirty years without producing 

 any distinct varieties. It is a strikingly beautiful plant with its stout, 

 erect growth; stiff, broad ascending leaves; and its star cluster of long pure 

 white flowers. Nothing even approximating its flower characters 

 occurs in the numerous varieties of N. tabacum, in fact it belongs to a 

 totally distinct section of the genus Nicotiana. In spite of its distinct 



