FERTILITY IN CICHORIUM INTYBUS 
393 
patibilities giving sterility involve similarity or dissimilarity of con- 
stitutional organization is, of course, very fundamental to the under- 
standing of the nature of fertilization. Although rather widely differ- 
ing in particular applications, the conceptions advanced as to the 
causes of physiological sexual self-incompatibility in such hermaphro- 
dite plants as Eschscholtzia (Darwin, 1877), Cardamine (Correns,. 
1912, 1913), Reseda (Compton, 1912, 1913), Nicotiana (East, 1915);. 
and in such hermaphrodite animals as Ciona (Morgan, 1904, 1910) 
have in general agreed in considering that a similarity or lack of dif- 
ferentiation is responsible for the sterility. The writer has already 
(1916) discussed these conceptions and has presented for consideration 
the view that the evidence is more readily to be interpreted on the 
basis of the principle that in general a marked degree of similarity in 
constitution is necessary for sexual fertility. In this relation it is to^ 
be noted that inbreeding in the variety ''red-leaved Treviso" has led: 
to a somewhat greater similarity in general characteristics than existed] 
in the original stock grown from commercial seed. In this sense the 
continued inbreeding of sister plants has led to a greater homozygosity. 
It is in the 1916 cultures of the offspring of inbred plants that self- 
fertile plants appeared as noted above. As far as the results in chicory 
extend, and it may be said that there are no more comprehensive data 
to be had for any other species, the general results are not in disagree- 
ment with the view expressed above. 
The sporadic variability of the sex relations and their fluctuating 
inheritance is very obvious in chicory. Self-fertile plants appear 
irregularly among the offspring of wide crosses and among plants of 
inbred strains which are prevailingly self-sterile. In both types of 
offspring the number of self-fertile plants that appear varies con- 
siderably. The manner of their appearance is not to be correlated 
closely with similarities or dissimilarities as these are ordinarily 
judged by the expression of characters. The condition of complete 
functional sex vigor is in many hermaphrodites so comiplete that it 
appears to be very definitely fixed in heredity. In chicory, however, 
we see that highly individual and epigenetic developments may arise, 
evidently in differentiation and in the transition to the gametophytic 
stage, which lead to wide and sporadic variations in the functional sex 
vigor. 
The various phenomena of self- and cross-compatibility and incom- 
patibility raise many questions that are fundamental to an under- 
