DEVELOPMENT OF THE SPOROGONIUM IN THE MOSSES. 265 
Mniaces, Dicranaces, &c., there is an apophysis, but itis obviously _ 
a part of the theca; but in Polytrichum the apophysis is quite 
distinct, being separated from the sporangium by an annular 
constriction (Pl. IX. figs. 1, 3, 4, Pl. X. fig. 13). This constric- 
tion is often seen better if a longitudinal section of the theca is 
made, as it is not always evident from superficial examination. 
In Polytrichum commune, Linn., and P. piliferum, Schreb., it is 
more distinct, while in P. formosum, Hedw., and P. juniperinum, 
Hedw., it is less so *. 
2. Anatomy and Histology. 
ATRICHUM UNDULATUM, Beauv. 
This form is covered by a distinct epidermis easily distin- 
guished from the adjacent tissues, especially on the theca. Atri- 
chum has no stomata; and, so far as I have been able to ascertain, 
stomata are only found on or in close connexion with the 
apophysis of those forms which possess that structure. Imme- 
diately beneath the epidermis of the seta there is a layer of 
thick-walled sclerotic cells, containing protoplasm, whose walls 
are of a brown colour, due, at any rate partially, to the presence 
of tannin, as is shown by the action of chromic acid. In the 
sterome of Atrichum there are no intercellular spaces (fig. 9). 
Passing inwards the cells become thinner-walled, and are sepa- 
rated by intercellular passages (Pl. XI. fig. 28, cp., i.), as the 
cells of the cortical parenchyma of a typical Vascular Plant are ; 
but these intercellular spaces are only found in the Polytrichacez, 
in other Orders they are absent (Pl. IX. fig. 10). This tissue, 
including the outer layer of sterome, may be conveniently called 
the cortex. Longitudinal sections show that the sterome is some- 
what prosenchymatous, although some of the cells have nearly, 
and occasionally quite, horizontal transverse walls (Pl. X. fig. 14). 
The cells of the inner thinner-walled layer are rather peculiar 
in that they are arranged in filamentous strands; each strand is 
attached to the one above and the one below by a cell with one 
prosenchymatous end, but the other ends and the ends of the 
intermediate cells are horizontal (Pl. X. fig. 14). The cortical 
cells contain protoplasm and a well-defined nucleus with num- 
bers of plastids (chloroplastids). 
* From drawings of those species of Pogonatwm which I have not seen in 
which a rudimentary apophysis is present, the constriction appears to be present. 
In P. alpinum, Linn., this constriction, although present, is very slight. 
