388 
MABEL MARY BROWN 
(or heterothallism) in the mosses. This situation is partly due to the fact 
that investigators have depended upon the morphological examinaton of 
specimens for the determination of their sexual conditions rather than upon 
cultural studies. Separate axes, one bearing archegonia, the other bearing 
antheridia exclusively, may have arisen from the same protonema, but this 
situation is not ordinarily revealed by observation. Some writers have 
uncritically classed a moss having such separate gametophores as dioecious, 
regardless of the origin of the gametophores. Thus, Miiller (1853, pp. 
47-48) says: "On a common stem the inflorescences are monoecious; on 
different stems they are dioecious." Limpricht (1890, p. 37) agrees with 
this idea of dioecism and remarks: "So far dioecism of the protonema has 
not been observed." Ruhland (1909, p. 210) is more emphatic, declaring 
that in all cases of dioecism both sexes are borne beside one another on the 
same protonema. Lotsy (1909, pp. 261-262) observes that it is an unsolved 
question as to whether genuine dioecism occurs in the mosses, since both 
male and female plants arise from the same protonema, and since in certain 
so-called instances of dioecism it has been found that sexual organs of one 
kind may be suppressed for a time by unfavorable environment, but that 
upon the return of suitable conditions the organs in question appear. 
It is evident that the question of genuine dioecism or monoecism in 
mosses can not be settled by the usual methods of observation. It is 
necessary to follow the history of single protonemata and their branches 
from the time of the germination of the spore until the formation of the sex 
organs. 
This method of determining the sexual conditions of certain mosses was 
first used by E. and E. Marchal (1906). They showed that in Barhula 
unguiculata, Br yum argenteum, and Ceratodon purpureus, spores produced 
in individu-^al capsules are heterogeneous and unisexual, some producing 
protonemata bearing male axes exclusively, others giving rise to protonemata 
which bear female axes only. Protonemata produced by regeneration 
from various parts of the gametophyte transmit the sexual characters of 
the parent plant to the gametophores which they themselves later produce. 
The sexuality of protonemata thus produced by regeneration, as well as 
that of protonemata resulting from the germination of spores, can not 
in any way be influenced by external conditions. 
Later the same writers (Marchal and Marchal, 1907) report that pro- 
tonemata arising aposporously from young sporophytes of such dioecious 
mosses as Bryum caespiticium, B. argenteum, B. fallax, and Mnium hornum 
bear hermaphroditic axes, a large proportion of male axes, and a very small 
number of female axes; in other words, bisexual or monoecious gametophytes 
have been produced in these normally dioecious species. They conclude 
that the separation of sex potentialities is bound up with the reduction 
division and that some of the chromosomes carry the sex-determining factors. 
M. Wilson (191 5) describes organs combining the characters of both 
