392 A. B. STOUT 



application of this conception to the almost complete impotence of 

 the Fi hybrids of Nicotiana TabacumXN. sylvestris, they are dealing 

 with the well-known cases of degeneration so often observed during 

 sporogenesis in interspecific hybrids. They believe that the very few 

 perfect spores formed represent the Tahacum and sylvestris extremes of 

 a combination series. In other words, these few spores represent the 

 cases where the parental germ plasms segregated without mutual 

 influence. The greater number of recombinations, however, were 

 incompatible combinations of various elements derived from the two 

 germ plasms. There are very few of the two original combinations 

 that survive reduction and sporogenesis. In somatogeneses the in- 

 compatibility is seen, they believe, in a complete dominance of the 

 Tahacum characters (1717a, 191 76). Whether involving chemical or 

 mechanical reactions or involving differences in developmental ten- 

 dencies in the sense used by Tischler (1907), (Stout, 1916, p. 423-427) 

 such intra-cellular incompatibilities arise especially in the reorganiza- 

 tion of cells during or immediately following reduction as has long 

 been known. 



In the case of physiological incompatibility, as in chicory, there 

 appears to be no impotence except of a purely accidental sort. Any 

 recombination system may survive, and in chicory sporogenesis in the 

 offspring of crosses between the red-leaved Treviso variety and a wild 

 ■white-flowered plant must, it would seem, give many new recombina- 

 tions. The range of these recombinations must be quite the same in 

 the various sister plants both of the Fi generation hybrids and of the 

 various series of red-leaved Treviso. Yet for the self-sterile plants, 

 and these are here in greater number, all the pollen grains fail to func- 

 tion irrespective of the character of the particular germ plasm organ- 

 ization from which they came and of which they may be variously 

 composed. On the other hand in the self-fertile plants that are sister 

 plants of such self-sterile plants, germ cells of much the same hereditary 

 constitutions (as judged by the characters of the plants that bear them) 

 are compatible. 



Furthermore, in the cases of self-fertility of any degree (or cross- 

 fertility as well), the evidence thus far obtained from hybrid genera- 

 tions does not indicate that the fertilizations involved selective or 

 preferential mating which favored fusion between particular recom- 

 binations of germ plasm with respect to hereditary characters. 



The determination of whether physiological self- and cross-incom- 



