162 BOTANICAL GAZETTE [SEPTEMBER 
they are grown separately in pure cultures. Thus the opposite 
strains of Phycomyces and Mucor Mucedo have been cultivated by 
means of sporangiospores to respectively 107 and 106 non-sexual 
generations without apparent change in their sexual behavior. This 
differentiation into (+) and (—) mycelia, which are capable of 
retaining their respective characters apparently for an indefinite num- 
ber of vegetative generations, renders the heterothallic mucors as 
striking an example of dioecism as is to be found in the plant kingdom. 
In those heterothallic species investigated in which a difference 
in vegetative growth is apparent, the (+) strain is the more luxuriant. 
In higher forms when a difference in size exists between the two 
sexes, the female is usually the larger. In such heterothallic forms 
the zygote develops entirely from the female thallus, and it would not 
seem unnatural that the thallus which supplies nourishment for the 
formation of the reproductive bodies should have a greater develop- 
ment than the thallus which produces only the comparatively small 
male gametes. The zygote of the heterothallic mucors, on the other 
hand, is formed by the union of morphologically equal gametes cut 
off from similar branches of the sexually opposite thalli. The zygo- 
spore is suspended midway between the (+) and (—) thalli which 
take equal share in supplying the nutriment for its development. 
The difference which sometimes exists in vegetative luxuriance 
between the two strains is independent therefore of the demands of 
the reproductive bodies, and is to be connected in some way with 
the primary sexual differentiation into the two opposite strains. 
There are no heterothallic species as yet known in which a con 
stant difference between the size of the two gametes has been observed. 
Two genera from the homothallic group are heterogamic, and in 
these forms the smaller gamete may be assumed to be male and the 
larger female. If it were found that a (+) test strain would show 
a reaction with the male, while the (—) strain showed a reaction 
with the female branch, one would have evidence for considering the 
(+) strain female and the (—) strain male. Unfortunately, attemp's 
to hybridize test (+) and (—) heterothallic strains with these hetero 
gamic forms have been as yet entirely unsuccessful. It is to be 
‘hoped that other heterogamic forms may be discovered which will 
lend themselves more readily to experiments in hybridization. That 
{Se Aa ee ere Oe Ca eis be 
