372 
POPULAR SCIENCE REVIEW. 
modifications of the peristome are advantageously employed 
by Mr. Mitten for divisions of the order. 
The inner peristome is quite different from the outer, and 
consists of a thin membrane arising from the outer wall of the 
spore sac, and divided into processes which stand opposite the 
interspaces of the outer teeth ; in its most perfect form these 
processes are sixteen in number, each two cells wide at base, 
projecting outwardly along the middle line as a keel, or perfo- 
rated and cohering only by the transverse articulations of the 
cells ; between each pair of processes are three very fine cilia 
each one cell wide, so that the whole circuit of the inner 
peristome takes up eighty cells. 
This fully-developed, internal peristome is well seen in the 
genus Mnium, and in Plagiothecium undulatum, Hypnum 
riparium, commutatum, &c. ; but in many mosses with a 
double peristome the cilia are abortive, and the processes also 
become quite rudimentary. The columella is a continuation 
of the central part of the pedicel, extending to the operculum, 
with which it often comes away ; it is commonly united to the 
wall of the spore sac, but is wanting in Archidium and Ephem- 
erum ; in the Polytrichacese its summit is dilated into a 
membrane like a drum-head, which closes the mouth of the 
capsule. 
The spores are globose or kidney-shaped, smooth on the 
surface or rough with papillae, and in colour yellow, red, or 
ochreous ; in Archidium very large and few in number, but in 
the majority of mosses they are extremely minute. 
Propagation. 
Having thus briefly glanced at the various organs found in 
mosses, we may notice the mode by which their growth and 
continuance is provided for. 
The spore does not (like the seed of a flowering plant) pro- 
duce an individual like its parent, but on rupturing its outer 
coat, the primordial utricle is protruded as a proembryo, and 
commences a process of cell division, resulting in a confervoid 
filament, which gives off lateral branches and branchlets, form- 
ing the delicate green film we may often notice on damp earth. 
From various cells of this prothallium young plants are deve- 
loped, fine radicles push downward, and true leaves are formed 
characteristic of the species, numerous plants thus resulting 
from a single spore ; and when this rising generation is able to 
take care of itself, the prothallium disappears, except in a few 
annual species constituting the genus Ephemerum, where it 
continues throughout the life of the plant. Many mosses, 
however, are met with which do not produce fruit, yet the 
