368 
POPULAB SCIENCE EEVIEW. 
higher plants, but rather as a means of fixing the plants to the 
substratum on which they grow, for we find them on rocks and 
trunks of trees, where their chief support must be derived from 
the atmosphere ; in very many species they occur not only at 
the base but also along the whole stem ; sometimes these ad- 
ventitious radicles are so dense that they mat the plants to- 
gether into a spongy mass, and thus effectually retain a supply 
of moisture ; the microscope shows us that each radicular fibril 
consists of a single series of cells, the transverse partitions of 
which are oblique. Then we see a short stem bearing leaves, 
with which also all mosses are supplied, and in this instance the 
fruit-stalk is a continuation of the stem-axis or is acrocarpic, 
any continued growth being by innovations or lateral repetitions 
of the stem ; but in many mosses the fruit is lateral or pleuro- 
carpic, and lateral branches are produced through a succession 
of years. 
The leaves are attached horizontally, and are always sessile 
and persistent, their true arrangement being spiral ; in Fissidens 
distichous or in two opposite rows (i) i.e. one spiral turn con- 
taining two leaves, in some tristichous (-§-) two spiral turns 
cutting through three leaves, and often five or eight rowed (J- -|). 
If we now tear off a few leaves from our Funaria, and place 
them in water between two slides and transfer to the microscope, 
we may learn a great deal about moss structure. First, as to 
the form of the leaf ; this we observe is ovate, with the apex 
pointed, and the ovate or lanceolate form is the most common 
among the leaves of mosses, though we find every degree of ex- 
pansion between orbicular and awl-shaped. Traversing the 
centre from base to apex is a midrib or nerve, composed of 
several layers of narrow cells ; this in some species is altogether 
absent, in very many it vanishes about the middle or two-thirds 
the length of the leaf, while in others it is excurrent or extends 
beyond the leaf in a point, or it may be prolonged into a bristle 
or hair, and these by their number give the tufts of plants a 
woolly or hoary aspect. The margin is entire, but in many 
species it is variously toothed or serrated, and sometimes it has 
a thickened border. The lamina, or expanded plane of the 
leaf, is we observe composed of cells, by which a network is 
produced termed the areolation, and so constant are the indi- 
vidual cells in form and size, that a careful study of them is of 
the utmost importance in the identification of species, and in- 
deed our only means of determining them when in a barren 
state. 
In Funaria we find the surface is quite smooth, but in many 
species it is covered with papillae ; in none perhaps are these 
more evident than in the leaf of Thuidium tamariscinum , 
