BOTANY. 61 
perpendicularly to the outer layer. The fructification gradually projects 
more and more from the surface, and either remains covered with the outer 
layer, or bursts through it. When it remains closed, there is a nucleus in 
the centre. When the fructification bursts through the cortical or outer 
layer, it expands in the form of shield-like disks, called apothecia or patella, 
or linear expansions called lirelle. Sometimes the cortical matter forms 
a border round the fructification, at other times it grows up in the form of 
a stalk, so as to give rise to the podetium. The young thece (asci) contain 
spores, varying from four to eight, or from twelve to sixteen. Occasion- 
ally, the spores are in sets of two. Separated cells of the medullary layer, 
of a green color, called gonidia, or gongyli, are considered as another kind 
of reproductive organ. ‘There is much uncertainty as to the real character 
of the spherical or sub-spherical green bodies called gonidia, which are 
characteristic of true lichens. When separated from the parent structure, 
they are capable of forming new plants. Lichens are found in all quarters 
of the globe, adhering to stones, rocks, trees, &c. During their entire 
growth, they appear to be capable of deriving most of their nourishment from 
the atmosphere. They have the power of acting on hard rocks, so as to dis- 
integrate them in process of time, and many of them contain much inorganic 
matter in their composition. They all grow in the air; none are found sub- 
mersed. 
Sub-order 1. Coniothalamee : pulverulent lichens ; shields open, without a 
nucleus, cavity filled with free spores. 
Sub-order 2. Idiothalamee : shields closed at first, opening afterwards, con- 
tainmg free spores in a nucleus composed of the gelatinous remains of the 
paraphytes and sporangia. 
Sub-order 3. G'asterothalamee : shields either closed always, or opening 
by bursting through the cortical layer of the thallus, the nucleus containing 
the deliquescing or shrivelled sporangia. 
Sub-order 4. Hymenothalamee : shields open, discoid permanent, nucleus 
bearing the sporangia on its surface. 
The economical value of some lichens is considerable. Cetraria islandica 
(pl. 54, fig. 31), or Igeland Moss, contains a nutritious substance called lich- 
enin. Cladonia rangiferina furnishes the principal winter food of the Reindeer. 
Fig. 29 represents Cladonia pyxidata and verticillata. Parmelia parietina 
(pl. 54, fig. 30) contains a yellow coloring matter called parietin. Rocella 
-tinctoria (fig. 32) furnishes part of the archil of commerce. Fig. 33 repre- 
sents Usnea florida. The tripe de roche, a nutritious lichen found in the 
Arctic regions of America, belongs to the genus Gyrophora. 
Sub-class 2. Aitheogame or Cormogene. 
Orver 4. Hepatica, the Liverwort Family. Plants having an axis 
which either bears cellular leaves or is leafless, and is bordered by a mem- 
branous expansion or thallus. Stomata are found in the epidermis of 
some. The reproductive organs are: 1. Antheridia, which are either 
61 
