434 Wilson.—Sex Determination in Mnium hornum. 
Organs of mixed sex had, however, been noted in two dioecious 
mosses previously to 1909. Lindberg in 1879 (8) described and figured such 
organs in Brachythecium erythrowhizon. This moss is described as monoe¬ 
cious in the £ Bryologia Europaea ’ ( 4 , 14 ), but Lindberg, after the examina¬ 
tion of numerous specimens, concluded that this species is really dioecious. 
The mixed organs were discovered in heads ’situated on stems which also 
bore normal female inflorescences ; some of the organs resembled antheridia 
and others archegonia, while a complete series of intermediate forms were 
also present. Bergevin in 1902(1) discovered and figured similar structures 
in Plagiothecium sylvaticum , an undoubtedly dioecious species ; among the 
examples described, some are monoecious, some synoecious, and organs 
showing all stages of transition between antheridia and archegonia are 
found; no sections of these organs were made. A similar instance has 
been discovered in Mnium hornum by the present writer. 
The axis which bore the organs of mixed sex had the appearance 
of a male individual and was collected in Kent in the spring of 1911, 
with a number of others which, as far as they were examined, all bore 
normal antheridia. The specimen was preserved in Flemmings weak 
fluid, and was examined by means of longitudinal sections. A considerable 
number of normal antheridia are borne in the head, and the majority 
of these contain almost mature spermatozoids ; no normal archegonia 
are present. 
Unfortunately the whole of the sections were not retained, but in those 
kept fourteen organs of mixed sex were discovered. These show almost 
all transitions in structure between archegonia and antheridia. The organs 
represented in PI. XX, Figs. 1, 2, 3, 4 resemble the normal female organ in 
form, consisting of a venter and elongated neck. The walls of the venter are, 
in most cases, only one cell in thickness, and thus differ from those of the 
normal archegonium ; traces of a double wall are however seen in the organ 
represented in Fig. 4. The neck canal-cells have in many cases divided by 
walls parallel to the axis of the organ, and the resulting cells resemble 
spermatogenic cells; this is particularly the case in Fig. 1. It is, however, 
difficult to definitely ascertain the nature of these cells as, in most cases, 
they appear to have undergone partial degeneration ; it seems evident that 
the organs under consideration possess archegonial characters, in so far that 
a ferment is produced by the neck-cells which acts upon the cells present in 
the canal and brings about their partial conversion into mucilaginous 
material. It is improbable that the condition of the cells in question is due 
to imperfect fixation as spermatogenic cells in neighbouring normal anthe¬ 
ridia are well preserved. In the organ shown in Fig. 3 two cells are present 
in the venter, each containing a deeply-staining nucleus of medium size and 
somewhat scanty cytoplasm. It is probable that the upper of these repre¬ 
sents the ventral canal-cell, and the lower the ovum. Similar cells are 
