1906] BLAKESLEE—DIFFERENTIATION OF SEX 169 
among the homosporous ferns which investigators have shown to 
be strictly heterothallic. 
In the monoecious and hermaphroditic phanerogams, illustrated 
by Lilium, the condition is essentially the same as in Selaginella, 
with a differentiation into macrospores and microspores, and like 
the latter species the type may be described as heterothallic, homo- 
phytic, heterosporic, and heterosporangic. 
The homophytic division of the heterothallic group illustrated by 
Phycomyces is the only one of the three types et has representatives 
in all the orders outlined. 
Mucor Mucedo represents the heterophytic division of the hetero- 
thallic group. In contrast to Phycomyces, the zygospores of this 
heterothallic species furnish pure germinations, but the spores are 
unisexual; and while the germ tube and the sporangiospores produced 
from one zygospore are male, those produced from another may be 
female. There are, therefore, two different kinds of germ tubes, 
of sporangiospores, and of sporangia, as well as two different kinds 
of mycelia. These elements in this species show no more recog- 
nizable morphological differences than its mycelia, although the 
sexual differentiation seems to be as marked as in forms in which 
such a morphological differentiation exists throughout the whole 
plant. Mucor Mucedo is heterothallic, heterophytic, heterosporic, 
and heterosporangic. 
Since Marchantia is the only heterothallic form among the bryo- 
phytes the sexual character of whose sporophyte has been investi- 
gated, it is as yet unknown whether any forms of the mosses and 
liverworts exist corresponding to the Mucor Mucedo type. 
No heterophytic forms are known at present among the hetero- 
thallic pteridophytes, and it will be impossible to say whether they ever 
existed in geologic times. The non-appearance of one reproductive 
form on a given sporophyte cannot be taken at once as proof that the 
species is heterophytic. It not infrequently happens, for example, that 
one finds only microsporangia on a single individual of Selaginella. 
Such instances may be compared to the suppression of the organs 
of one sex on the prothalli of homothallic ferns, and may equally be 
explained by assuming that the conditions necessary for the forma- 
tion of the two reproductive bodies do not always coincide. 
