668 LICHENS UNDER THE MICROSCOPE. 
face, sl, and from which proceed the slender fibres by which 
the plant is attached to the matrix on which it grows. These 
four layers make up the thallus of lichens. In some genera, 
Fig 141. as Collema (Fig. 140), the upper cel- 
lular layer is wanting, and the gonidia 
lie close to the surface; in others, as 
Peltigera, the lower is deficient, and 
bundles of long fibres proceed imme- 
diately from the medullary layer. 
These are very conspicuous and cu- 
Parmelia colpodes; c, cortical rious in Parmelia colpodes (Fig. 141). 
t 
er; I, ‘conidia ml, ine ae 
ary layers A, hypothallus. They constitute the hypothallus, which 
forms the substratum on which the other parts of the thallus 
are built up. 
In the fruticulose lichens, which bear some resemblance 
to the stem of a plant, the thallus is Fig. 142. 
more or less rounded, and the gonidia 
are arranged around the medullary layer 
as an axis. In Usnea (Fig. 142) the 
thallus is solid, and the centre is com- 
posed of a mass of compact filaments 
lying parallel to the axis. In other 
genera it is hollow, or composed of 
loose filaments. In some genera, as 
Lichena, the medullary filaments, in- 
stead of running parallel to the axis, 
diverge from the centre to the circum- 
ference. In many crustaceous lichens 
the thallus consists of hardly more than 
a collection of gonidia, sometime buried 
beneath the bark, and of few filamen- 
Usnea barbata; a, longitu- 
dinal seetion of th allus; Gi 
tary elements. In these the hypothallus ^ eross-section of the same. 
often forms a black border around the margin of the thallus. 
The gonidia constitute the peculiar characteristic of the 
lichen thallus, and are present in all true lichens, their 
presence being almost the only mark by which some can be 
