OP ee Re Te, 
1906] BLAKESLEY—DIFFERENTIATION OF SEX 175 
are homothallic, while the majority of the mucors investigated are 
heterothallic. Both conditions, therefore, may be expected a priori 
in any group under investigation, whatever may be its phylogenetic 
rank. 
In groups in which sexuality is present, in both fungi and algae, 
there are many forms for which the sexual spores have been but 
rarely found or are entirely unknown. The absence of-sexual repro- 
duction may be due (1) to constitutional sterility, (2) to conditions 
of growth unfavorable to the production of sexual organs, or (3) to 
the fact that the form is heterothallic and thalli of both sexes have 
not been found together. In the last case the apparent sterility 
would not be due to a lack but rather to an excess of sexuality which 
separates the male and female individuals. Even in heterothallic 
species, neutral races have been found to exist, and the conditions 
within which sexual reproduction is possible are frequently very 
limited. 
A morphological investigation may suffice to show that the male 
and female organs are borne on the same thallus, and the form in 
question can then be at once classified as homothallic. A hetero- 
thallic condition, on the other hand, can never be recognized by a 
morphological investigation alone. The appearance of but one set 
of sexual Organs on an individual form studied under the microscope 
may be due either to dichogamy or to suppression of the other sex 
brought about by conditions of growth, as well as to a unisexual 
character of the thalli. Carefully conducted cultures are therefore 
essential to a determination of the sexual character of forms inves- 
tigated. A few examples may be briefly given to illustrate the neces- 
sity of employing the cultural method in a study of even well-known 
orms. Many other examples equally as appropriate will suggest 
themselves to the reader. 
In the mosses the leafy shoots arise from an inconspicuous pro- 
tonema, and if certain shoots bear only antheridia and others only 
archegonia, a cursory investigation would lead one to consider the 
forms heterothallic, especially if the antheridial and archegonial 
“plants” differ in appearance. Funaria hygrometrica, for example, 
'S classified as monoecious by LIMPRICHT (14) and CorRRENS (10), 
yet CAMPBELL (8, p. 187) says “‘Funaria is strictly dioecious.” The 
