32 
CECIL YAMPOLSKY 
On the assumption that gametes vary or are graded in strength, the 
one-to-one ratio may be explained in dioecious forms, especially in dealing 
with mass populations. As has already been pointed out (I.e.), it is only 
when large numbers are considered that the one-to-one ratio appears. 
To be sure the law of chance comes into effect in such an explanation of 
sex ratios. The explanation of the one-to-one ratio may very well lie in the 
assumption that the gametes of the female have as much chance to dominate 
over the male gametes as the male gametes have to dominate over the female. 
That the gametes of one sex may in cases completely or almost completely 
dominate over those of the other sex is brought out in aberrant sex ratios. 
This advantage may, when large numbers of individuals are considered, be 
offset by a parallel condition resulting in the dominance of gametes of the 
other sex (Doncaster, 1913, 1916; Montgomery, 1908). 
In Mercurialis, though the species is prevailingly dioecious, it is obvious 
that we must assume that the potentialities for the development of both 
sexes are present in practically all the individuals of the species. There is 
nowhere evidence that sex is determined in this plant by the presence or 
absence of a sex-determining factor. Those individuals which remain 
purely male or purely female throughout are not to be conceived as very 
different from those which produce a few flowers of the opposite sex. There 
is no evidence for the localization of the sex difference either in a special 
part of the plant or in a special part of the cell. The appearance of the 
sporadic flowers of the other sex may occur anywhere on the plant and at 
any stage of its development. Their occurrence is comparable to that of 
bud variation, and like the latter they show that the organism may contain 
latent potentialities as well as visibly expressed characters. Nor does the 
production of a few flowers of the other sex alter essentially the sex character 
of the plant as a whole. It is still prevailingly male or female and transmits 
its sex as such. It is highly probable that as a rule at least the pollen from 
sporadic male flowers on a female plant pollinates the nearest female flowers 
on the same branch. The seeds so produced, however, grow into female 
plants like the branch which bore them. It is sometimes questioned whether 
a plant with its potentialities of unlimited growth and with its succesive 
crops of reproductive organs is an individual in the sense that an animal 
is, with its more limited growth and definitely localized reproductive and 
other organs. The behavior of these prevailingly dioecious Mercurialis 
plants with reference to sex transmission certainly shows that they are unit 
individuals male or female in a very strict sense. But it is just as clear that, 
as noted above, the dioecious condition is only an extreme, a climax con- 
dition in the evolution of sex differentiation. As the data at the end of this 
paper show, the transition from the hermaphroditic and monoecious to the 
polygamo-dioecious and dioecious condition is going on at numerous and 
widely distributed points in the orders and families of seed plants. 
