Wilson.—Sex Determination in Mnium hornum. 439 
which are spread over a considerable number of cell generations, and which, 
as a general rule, are unaffected by external conditions. It is possible that 
these processes are initiated at a certain stage in the life-history, but it 
is unlikely that they depend on the separation of actual protoplasmic 
masses at any particular cell-division. If this view is accepted, variations 
from the normal sexual condition of a species may be explained by assuming 
the presence of some unusual factor which has interrupted the normal 
course of metabolism. No adequate explanation of such sexual abnormali¬ 
ties has been given by the upholders of the theory of sexual determination 
by the separation of determinants at some particular division. If, however, 
the view outlined above is accepted, the explanation of such cases becomes 
very considerably simplified. 
In conclusion, it may be pointed out that the majority of investigations 
on the subject of sex determination have been carried out on animals. 
The conclusions so arrived at cannot be justly extended to plants, which 
differ fundamentally from animals in the possession of a definite alternation of 
generations in their life-history. The fact that in animals meiosis always 
corresponds with the gametogenic divisions, while this is rarely or perhaps 
never the case in plants, renders comparison of the two groups very difficult. 
Similar investigations on plants have almost all been confined to 
Angiosperms, in which the alternation does not result in sharply distin¬ 
guished generations. It is probable that further research of a similar nature 
carried out on the Bryophyta and Pteridophyta, in which the generations 
are always distinct and, in the latter group, usually lead an independent 
existence, would give valuable results. 
Summary. 
1. An axis of Mnium hornum is described, bearing normal antheridia, 
bisexual organs, and modified archegonia. 
2. The spermatogenic cells of the normal antheridia possess six 
chromosomes and, since this is the normal gametophytic number, the plant 
in question cannot have been produced aposporously. 
3. The results obtained by £l. and Em. Marchal are discussed, and it is 
suggested that sex determination is not bound up with meiosis, but is 
brought about by metabolic processes which operate in the organism over 
a considerable part of its life-history. 
Bibliography. 
1. Bergevin, E. de : Interversion dans la croissance des organes sexuels du Plagiothecium 
sylvaticum , L. Revue bryologique, 1902, pp. 115-19. Ref. Bot. Centralbl., vol. xcii, 
I 9 ° 3 > p. 57 - 
2 . Blakeslee, A. F.: Sexual Condition in Fegatella. Bot. Gaz., vol. xlvi, 1908, pp. 384-5. 
3 . Brizi, U.: Appunti di teratologia briologica. Ann. d. R. 1 st. Bot. Roma, vol. v, 1893, p. 53. 
4. Bruch, Ph., Schimper, W. Ph., et Gumbel, Th. : Bryologia Europaea. Stuttgart, 1836-55. 
