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 Tabacum 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 
Tabacum 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 m 
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- 
