1906] BLAKESLEE—DIFFERENTIATION OF SEX 167 
groups represented in the accompanying diagrams. In all the figures 
the gametophyte has been shaded with parallel lines, the antheridia 
and zygotes with cross-hatching; while the sporophyte and the spor- 
angia have been left unshaded. The drawings are entirely diagram- 
matic, and no attempt has been made, therefore, to preserve the 
relative proportions of the parts figured. As has been already 
explained, the Mucorineae have been included in this scheme for the 
purpose of comparison, and the germ tube has been thus homologized 
with the sporophyte. The mucors then as represented in the first 
column in the diagram are the only group outlined in which all the 
three main types of sexual differentiation are as yet known. 
In Sporodinia grandis, which may be taken as representative of 
the homothallic group, the mycelium (gametophyte), the germ tube 
(sporophyte), and the germ sporangia are all alike bisexual. The 
two opposed gametes, and perhaps the branches from which they 
are cut off, may not unreasonably be considered unisexual and of 
Opposite sex. It has not been found possible as yet, however, to 
confirm this assumption experimentally. In the terminology adopted 
the species is to be considered homothallic, homophytic, homosporic, 
and homosporangic. The same condition is found in the “monoe- 
cious”? mosses represented by Physcomitrium pyriforme, and in the 
homosporous ferns represented by Polypodium. The sporangium 
of the latter is represented as a side branch, since in the ferns, as 
also in the flowering plants, the sporangia are not simple terminations 
of unbranched sporophytes of limited growth, as in the bryophytes, 
but are borne on the sporophylls of a sporophyte more or less highly 
developed. 
If the sexual character of the thallus be preserved, the spores and 
the sporophyte producing them must be also bisexual. There can 
only one type therefore of homothallic forms. Of heterothallic 
forms, on the contrary, two types are possible—namely, those with 
bisexual sporophytes, i, e., homophytic, and those with unisexual 
Sporophytes, 7. ¢., heterophytic. These two types are represented 
by Phycomyces nitens and Mucor Mucedo respectively. 
In the heterothallic species Phycomyces it will be convenient for 
the purposes of the present paper to neglect those instances in 
which the germination follows the Mucor Mucedo type, as well as the 
