342 Mottier . — The Development of the Heterotypic 
to sex or some other distinct quality, may be present in one, or distributed 
among several chromosomes. It does not necessarily follow that any given 
chromosome should always contain these pangens ; that is to say, there is 
among plants no ground for the conclusion that any one chromosome con- 
tains certain qualities, and only those, throughout the history of the species. 
It is worthy of note here that in Marchantia polymorpha , whose spores 
produce both male and female gametophytes, these thalli retain their 
unisexual character when propagated vegetatively by gemmae. Noll 1 
is reported to have cultivated both thalli by means of gemmae for over 
thirty generations, and under varying conditions of growth, without being 
able to change the sexual character of either strain. In a homosporous 
fern, which the writer now has under investigation, the spores produce 
small prothallia that are strictly male, and large ones that bear archegonia. 
The gametophyte of this fern is reported in recent literature to be strictly 
dioecious, the statement being made that the large prothallia never bear 
antheridia. Out of a number of the larger prothallia bearing archegonia, 
selected at random from a pure culture, about 85 per cent, were pure 
females and 15 per cent, hermaphrodite. Investigation is in progress 
to determine the percentage of spores that produce strictly male prothallia, 
and also the ratio of pure females to hermaphrodite forms among the large 
so-called female gametophytes. At present my study has not proceeded 
far enough to justify any statement beyond that made in the foregoing. 
The commonly accepted view is that by sowing the spores thickly, male 
prothallia are produced in greater abundance or almost exclusively, while 
in the less crowded cultures affording better nutrition a larger number 
of spores will develop into female gametophytes. Efforts will be made 
in the experiments to determine more accurately, if possible, whether the 
sex of the prothallia is influenced or determined by conditions of growth. 
If the studies under consideration bear out the conclusion obtained 
from an examination of a very limited quantity of material, it would seem 
that in the fern in question some spores are strictly male, some purely 
female, while others contain male and female characters with the female 
greatly predominating. This condition can be explained in the light 
of the theory advanced in this paper by assuming that some spores possess 
a predominance of strictly male pangens, others female, and still others 
pangens of both sexes with the female predominating. Neither in the 
spore-bearing cells of Marchantia nor in those of the fern in question 
has anything comparable to an accessory chromosome, or special sex- 
differential, been found, and, as in the case of the higher plants mentioned 
in the preceding pages, it seems not impossible that the smaller granules 
observed in the spirem, which are not included in any of the chromomeres 
of a larger and more uniform size, may represent almost exclusively male 
1 Reported by Blakeslee (’06). 
