The Lichens are found growing upon the bark of trees, old palings, &c. Those most easily seen with the naked eye form grey or coloured dryish patches or pendulous tufts; while the smaller ones are singly easily overlooked, from their minute size and close adhesion to the _mátrix_ or body upon which they grow, forming, by their aggregation, the grey or otherwise-coloured dry and brittle coatings of almost every tree or decaying branch. The Lichens derive their nourishment from the air, and not from the matrix--in this respect differing from the Fungi, with some of which, as we shall presently see, they agree in the structure of the fruit. The structure of the Lichens is simple, no distinction of root, stem, and leaves existing in them, although certain dry root-like fibres exist in many of them, by which the plant is fixed to the matrix. The whole consists mainly of a frond or _thal´lus_ (θαλλὀς, a leaf). This is either raised above the surface of the matrix in a shrubby form, or spread upon the surface as a flexible lobed layer (Pl. II. fig. 2), or it is dry and brittle (crustaceous) and closely adherent (Pl. II. fig. 26). The fructification consists of little saucers, disks, or streak-like furrows, often of a different colour from the thallus, the structure of which will be best illustrated by reference to a few common species. PARMELIA´CEÆ.--_Parmélia parietína_ (Pl. II. fig. 2) is a very common Lichen, found on the bark of trees, on old palings, &c. It is of an orange-yellow colour, the thallus being flat, lobed, and scalloped (crenate) at the margins. The structure of the thallus may serve to represent that of most of the Lichens. It consists of three layers,--an upper cortical or epidermic layer, which is continued over the margins to the under surface of the thallus, to form the under layer; and between these is the middle or medullary layer. The medullary layer consists of interwoven fibres, which are more closely packed towards the upper and under surfaces, so as to give them a cellular appearance. Near the upper part of the medullary layer, a number of minute rounded green cells exist, lying loosely in its meshes (Pl. II. fig. 4 _a_). These green cells (gonidia) appear to correspond to the buds of the higher plants, and, when detached from the plants, they are capable of growing into new individuals. On the upper surface of the thallus of _Parmelia_ (fig. 2) the fructification may be observed. This consists of saucers or shields (_apothécia_, ἁποθἡκη, a repository), formed of a raised and expanded portion of the thallus (fig. 3), and containing the spores. The spores are enclosed in closely set upright cells, or spore-sacs (figs. 4 _b_ and 5 _b_), called _as´ci_ (ἁσκὀς, a bottle) or thecæ; and intermingled with the asci are filaments, enlarged and coloured at the ends (_paraphyses_), which are probably abortive asci.