Division in Mnium hornum . 
143 
reagents. The acetic alcohol was allowed to act from 15-20 minutes, 
and the material was then washed in several changes of absolute alcohol. 
When Flemming’s Mixtures were used, the air present in the tissues was 
extracted by means of an air-pump. They were allowed to act from 18-24 
hours, after which the material was washed in running water for 24 hours, 
and then placed in a 10 per cent, solution of glycerine in large watch 
glasses. These were kept in a warm place, so that after about 24 hours 
the whole of the water had evaporated, leaving the pure glycerine. The 
material was then transferred directly to methylated spirit and ultimately 
to absolute alcohol. Much better results were obtained by this method 
than by dehydration by using gradually increasing strengths of alcohol. 
The sporogonia were preserved from the beginning of January until 
about the end of April, when spore formation was completed. Clumps 
of the plants were dug up in the afternoon and taken into the laboratory or 
to a greenhouse, and the sporogonia were preserved during the next and 
following days at hours varying from 6 a.m. to 8 p.m. In the early part 
of the year the soil in which the plants were growing was often frozen when 
collected, and the higher temperature of the laboratory probably caused 
increased cell division. Material was also preserved in the field. The 
younger capsules were preserved entire, but when further developed the 
hypophysis and operculum were cut off immediately before fixing. In 
some cases the capsular wall was also removed, thus laying bare the 
sporogenous cells, but it was found that the preservation of the cells was 
not improved by the treatment. Transverse sections of the sporogonium 
varying from 3 [i -6 n in thickness were generally used, but longitudinal 
sections were also cut. These were stained either with Flemming’s Triple 
Stain (safranin-gentian violet-orange G) or with Heidenhain’s Iron Haema- 
toxylin. 
Premeiotic Development. 
Fertilization in Mnium hornum usually takes place about the beginning 
of May. The development of the sporogonium is slow, and in the following 
January the archesporium becomes differentiated. At this stage the sporo- 
gonium consists of a long seta and capsule, but the latter is yet only little 
developed, and can only be distinguished with difficulty from the stalk. In 
a transverse section of the capsule taken at this stage the single-layered 
archesporium is very obvious. Its cells are filled with dense finely alveolar 
protoplasm, and contain a large centrally placed nucleus (Fig. 1, PI. X). No 
vacuoles are present and the protoplasm at this and at all later stages com- 
pletely fills the cell ; with a high magnification alveoli can be distinguished 
in it, but with moderate magnification the appearance is granular, and usually, 
in well-fixed cells, very regular in texture. In a considerable number of 
cases the protoplasm towards the centre of the cell appears to be less dense, 
