164 BOTANICAL GAZETTE [SEPTEMBER 
vation of the homothallic forms from the heterothallic group. There 
are seven species known to be homothallic, among which three are 
heterogamic, while sixteen are known to be heterothallic. In all prob- 
ability the large majority of the species which produce zygospores are 
heterothallic, yet the sexual character in but a small proportion of 
the mucors has been definitely determined, and it is unknown whether 
in this group species may not exist in which sexuality is entirely 
lacking. The writer has as yet no theories to offer as to the origin 
of sexuality in the group. 
The fact that zygospores when germinating in a proper nutrient 
medium may give rise directly to a mycelium has led botanists to 
discard the idea of an alternation of generations comparable to that 
in higher plants, which was formerly seen in the succession from 
mycelia bearing sexually formed zygospores to germ tubes producing 
non-sexual sporangiospores which complete the cycle by the forma- 
tion again of sexual mycelia. The cytological history of the forma- 
tion and germination of the zygospores is at present too little known, 
and the writer would not care to be responsible for advocating as 
yet a too close homology between the conditions seen in the mucors 
_ and in the mosses for example, although the branching out of the 
germ tube under special conditions to form a mycelium might be 
considered of no great significance, since paralleled by the capacity 
of the moss sporophyte to give rise directly to a protonema. ie 
gross analogy, however, between the germination of the zygote 
mucors and that in the mosses is much more obvious than betwee? 
the conditions in the mosses and those in the flowering plants oF in 
animals (9), and is sufficiently close to justify one in concluding the 
mucors in a general comparison of the varying grades of sexual 
differentiation in the plant kingdom. In the accompanying dia- 
grams and in the ensuing discussion, therefore, the same terminology 
will be applied to the mycelium and to the germ tube that has been 
found advisable for the gametophyte and sporophyte of forms ’ 
which it is at present orthodox to speak of an alternation ° 
generations. : 
The terms dioecious, monoecious, and hermaphroditic have been 
used to designate varying grades of sexual differentiation, and hav° 
been applied to both gametophyte and sporophyte. Dioecism among 
