Igt2] : STEVENS—HETEROSTYLOUS PLANTS 285 
in which Ustilago violacea has never appeared. He reverses (p. 119) 
STRASBURGER’S interpretation of the origin of the diseased hermaph- 
rodites, and suggests that the infected plants were males in which 
the disease allowed the pistils to develop. SHULL’s discovery that 
hermaphrodite plants arise occasionally in normal races and his 
criticism of STRASBURGER’s interpretations do not alter the impor- 
tance of the fact that in a normally dioecious plant the bisexual 
condition may sometimes occur, perhaps because of some patho- 
logical stimulation. Another instance of the same condition is 
cited by STRASBURGER (38, p. 471). He reports, in the normally 
dioecious Mercurialis annua, male plants bearing a few female 
flowers, some of which when pollinated produced good seed. 
The condition just described would seem to indicate that dioe- 
cious plants arose from the hermaphrodite condition. If such is 
the case, dimorphic heterostylous plants, since they already exist 
in two classes, which differ considerably and are adapted for 
reciprocal fertilization, might be more likely to become dioecious 
than would homostylous plants. 
In this connection it is tempting to extend LILLIz’s view of the 
origin of sex to the origin of dioeciousness in the higher plants. He 
assumes (23, p. 375) that fertilization may be always selective, 
even when there is no morphological gametic differentiation. 
According to his idea, gametes may be physiologically different 
even when they are morphologically alike. Morphological differ- 
entiation would then follow naturally, as the expression of these 
physiological differences, and sex differentiation as a further stage 
in the same process of evolution. 
Is it not entirely probable that different “strains” may exist 
in some species of hermaphrodite plants which differ in their rela- 
tions of fertility somewhat as do the different ‘‘forms” of hetero- 
stylous plants? Panmixia has always been assumed to be the 
natural condition of hermaphrodite species. That is, it has been 
assumed that any individual can fertilize or be fertilized by any 
other individual in the species with equal ease, but that such is 
~ actually the case has never been proven. The existence of differ- 
ent “strains” having such relations as suggested above would not 
be easily demonstrated under natural conditions, as each stigma 
