Davis. — Nuclear Sludies on Pdlia. 153 
the earlier nuclear divisions of the gametophyte phase occur 
in the spore before winter, so that the spore in the following 
spring is multinucleate. Therefore material collected during 
October is almost sure to furnish a considerable variety of 
stages. There is a period of several days when the spore- 
mother-cells are, one may say, ripe for division, and such 
plants brought from the cold rocks to a warm room will 
immediately develop spores. Thus in a relatively short time 
complete sets of stages may easily be obtained. 
A number of fixing agents were employed, but 1 °/ o chrom- 
acetic acid and the weak formula of Flemming proved to be 
the most satisfactory. In this case there seems to be little 
choice between the two killing fluids when one understands 
certain peculiarities of the safranin and gentian violet stains. 
These stains hold more readily in Flemming-fixed material, 
but the manipulation may be regulated to give very clear 
results after chrom-acetic acid. Material fixed in corrosive 
sublimate and picro-sulphuric acid proved thoroughly un- 
satisfactory, that in absolute alcohol, sublimate-acetic and 
Carnoy’s fluid was somewhat better, and Hermann’s and von 
Rath’s formulae approached most nearly the excellence of 
weak Flemming and chrom-acetic acid. The sections were 
stained on the slide with safranin and gentian violet or with 
haematoxylin after the method of Heidenhain. 
Mitosis was studied at three periods of ontogeny, viz., in 
the seta of young sporophytes, in the spore-mother-cell and 
at the beginning of the gametophyte period in the germinating 
spores. It is most convenient to begin our account with the 
events of sporogenesis, as the spore-mother-cell so generally 
presents cytological phenomena most favourably. 
The spore-mother-cells of Pellia are conspicuously four- 
lobed, a fact that seems to be generally true of the Junger- 
manniales. This character appears several weeks before the 
spores are formed. The spherical cells of the archesporium 
come to lie freely in a mucilaginous matrix, and develop these 
lobes simultaneously and symmetrically in four divergent 
directions. The spherical nucleus lies in one of the lobes 
