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trunks and branches of our forest trees, giving them a bearded and venerable 
appearance. Sometimes it is fruticulose or shrub-like, growing on trees, or 
upon the face of high rocks. Another form is that of a horizontal laciniated 
frond, which grows sometimes to a large diameter on the bark of old trees, 
upon the ground, or upon mossy stones. But the predominant form of 
Lichen-thallus is what is called the crustaceous form. This is a light, or 
yellow, or dark-coloured crust, usually circular or oblong in shape, smooth 
or rough upon the surface, found profusely scattered upon rocks, rails, and 
old walls in country districts. As well as the form, the tissue of the lichen- 
thallus also varies in consistence and colour; but whether frondose or 
crustaceous, its character is the same. It is entirely cellular, and, in the 
lichen proper, it consists of three distinctly stratified layers. The first or 
cortical layer, is an elastic or transparent mass of originally globular cells, 
three or four deep, deformed by lateral pressure. They generally become 
filled with the colouring-matter of the plant, Adjoining this, and lying 
close beneath it, is the gonidial layer. This isa stratum of green globular 
cells, usually in a free condition. The third, or medullary layer, consists 
of a spongy mass of cells which are long and cylindrical in shape. These 
form the base of the plant, and they are the first development of the spore 
in the production of anew growth. The torn frond of some of the foliaceous 
species, such as Peltigera, will show these separate layers of cells to the 
naked eye. An important element which also enters into the constitution 
of the Lichen-thallus, is a gummy or gelatinous substance called lichenine, 
which undoubtedly plays an important part in nourishing the several organs 
of the plant. 
Like the Algz, the Lichens are endowed with a double system of repro- 
duction. The primary and direct method is by sporidia. These are 
produced in asci, or flask-shaped vesicles, developed in the apothecium. 
This apothecium is the female organ, and it consists of a flattened or wart- 
like disc, growing upon the surface or the margin of the thallus. In most 
species it is easily recognised by the naked eye. There are other primary 
organs of fructification called spermogones and pyenides, the exact function 
of which is not so clearly understood ; but it is supposed that, in some way 
or other, they are useful in either directly propagating the plant or in 
fecundating the sporidia of the apothecium. The secondary system of 
reproduction is by the gonidia. I have already referred to these green 
globular cells, lodged in the tissue of the Lichen-thallus, and called the 
gonidial layer. These isolated cellules are the most active members of the 
plant, and, once they are produced from the hyphal-tissue, they take the 
leading part in its development. Gonidia are both vegetative and repro- 
ductive, and are peculiar to the Lichen-thallus. They frequently burst 
through the outer coating of the plant and appear as soredia upon the 
