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 /'; some will possess eight wheat 
chromosomes, others seven wheat and one rye, and soon. 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 NV. tabacum varieties with N. sylvestris and have 
found that in all cases the /'; hybrid duplicates very closely the total set 
of characters of the particular tabacwm 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 
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