1912] Goodspeed: Nicotiana Hybrids 115 



respectively, the first bearing one of the two determiners for 

 outward characters and the second one of the two determiners 

 for seed-weight. Thus with the union of the proper tube 

 nucleus and the fusion nucleus bearing the same "influence" 

 the seed resulting from this fertilization would be heavy. With 

 our present cytological knowledge of the processes involved, it 

 is a matter of speculation whether this mature seed is heavy 

 because of the relatively high specific gravity of the contents 

 of the endosperm cells, or because the fully developed endosperm 

 results from a large number of cell-divisions, or finally, because 

 the endosperm is made up of a smaller number of very large 

 cells (see East and Hayes, 1911, p. 101 ; Harris, 1911, and 

 the literature cited therein). If, in the same fertilization we 

 were describing above, the generative nucleus of the same pollen 

 grain and the egg nucleus of the same ovule on uniting both 

 carry the macropliylla influence, a heavy seed capable of pro- 

 ducing a plant in Fo which resembled macro pliylla would be 

 produced. 



Other com.binations of nuclei might give medium weight 

 seed plants resembling either one parent or the other, or plants 

 to express the composite appearance of which the terms "inter- 

 mediate" and "blend" have above been used. Still other pos- 

 sible combinations might result in the formation of light seed 

 in the mature ovary of an F^ hybrid, a certain portion of which 

 would give rise to plants in F^ resembling one parent, another 

 portion to plants resembling the other parent, and still another 

 and larger portion to plants called "intermediates" and 

 "blends." 



Without attempting to include any schematic representation 

 of the possible combinations of nuclei and the resulting weights 

 of seed and F, generation plants, it will only be said that such 

 a scheme seems to represent fairly closely the numerical propor- 

 tions which we have noted above as occurring in the weights 

 in the F^ hybrid seed and in the appearance of the F2 genera- 

 tion plants produced therefrom. Any such explanation of our 

 experimental results is, as previously mentioned, only tentatively 

 advanced and it is recognized that little can be claimed for it 

 without more evidence than that whicli we are able to offov. 



