Sex Determination in Mnium hornum. 
BY 
MALCOLM WILSON, D.Sc., A.R.C.Sc., F.L.S., 
Lecturer in Mycology in the University of Edinburgh. 
With Plate XX. 
T HE method of sex determination in dioecious plants has recently been 
the subject of a number of investigations, and, amongst these, those 
published by 6l. and 6m. Marchal on the Apospory and Sexuality of the 
Mosses have created very considerable interest. 
In a paper which appeared in 1906 ( 9 ), these investigators showed 
experimentally that, in the case of dioecious mosses, the spores produced in 
a single sporogonium are heterogeneous as regards their sexual characters ; 
the spores are unisexual, the male giving rise to protonemata which bear 
exclusively male axes while the female produce protonemata which develop 
female axes only. They also proved that the protonema obtained vege- 
tatively from the gametophyte always retains the sex of the parent plant. 
In protonemata produced either from spores or vegetatively from the 
gametophyte the sex is constant, and is not affected by external conditions. 
In 1907 ( 10 ), a further paper was published dealing with the sexual 
characters of protonemata produced aposporously from the sporophyte 
of dioecious mosses. It was discovered that such protonemata always gave 
rise to a certain proportion of hermaphrodite axes, and it was presumed that 
no meiotic phase had taken place prior to the production of these individuals. 
The writer therefore concluded that, in the normal life-history of these 
mosses, the sex of the spores is determined at the reduction division 
immediately preceding their formation. 
A third contribution appeared in 1909 ( 11 ), and in this the writers 
showed that, although the plants produced aposporously develop sexual 
organs, these are sterile and no sporogonia are produced. In this paper 
the discovery of organs of mixed sex is recorded in plants of Bryum 
caespiticium and Mnium hornum which had been produced aposporously. 
The authors emphasized the fact that up to that time no mixed organs had 
ever been described in dioecious mosses ; Hy ( 7 ) had previously observed 
similar organs in A trichum undulatum and Holferty (6) in Mnium cuspi - 
datum> but both these species are monoecious. 
Annals of Botany, Vol. XXIX. No. CXV. July, 1915.]: 
