VII. LICHENES. 
551 
tally over dead wood, bark of trees or rocks, but in many genera it is terres- 
trial and erect, or pendulous from rocks or branches ; it may be effuse, that is, 
without determinate shape ; or ejfigurate, that is, having a definite shape ; or 
scaly,' formed of small coriaceous scales ; or crustaceous, of a thick crusty 
substance ; or powdery ; or leprous, when formed of minute membranous 
scales ; or granular ; or folia ceoas (also called frondose). The attachment of 
the foliaceous thallus may be by the whole under- surface, or by fibrils (bundles 
of short filaments), and may be by one point or many. When the thallus 
is erect, it is often called a podetium, a name also given to the erect cylin- 
drical portion of a horizontal thallus. Reproductive organs of four kinds. 
I. Apothecia, circular or variously-shaped shields cups slits or prominences, 
formed of closely-packed jointed filaments and closed tubes ( asci ), which 
latter contain simple or septate spores. II. Spermagones, minute open 
cavities in the thallus, containing filaments ( sterigmata ), upon which are most 
minute colourless bodies ( spermatic ). III. Pycnidia, or superficial sper- 
magones. IV. Gonidia; granules often scattered like powder over the 
thallus, which are analogous to buds, and reproduce the species. 
A large Natural Order, found in all climates and latitudes. Upwards of 1300 species are 
described, very many of which have most exteusive ranges in distribution, from the arctic 
circle to the equator and in both hemispheres. Like most other Cryptogamic Orders, they 
chiefly affect damp temperate climates. 
The iuterual substance of the thallus usually consists of three layers : — 
1. The cortical, which is tough, leathery, and formed of densely-packed minute cells, with 
thick walls. 
2. A green gonidic layer, formed of loose bright green or yellow globular cells, which 
either have a proper cellular coat, and are called gonidia, or have none, and are called gonima. 
These cells are almost peculiar to Lichens ; they often burst through the upper layer in 
masses, called soredia, or are scattered like powder over the cortical layer, or fringe the lobes 
of the thallus. In the genus Sticta, they burst through the under surface of the thallus, 
and occupy small circular depressious or cups called cyphella. Many Lichens are exten- 
sively propagated by gonidia, which may be seen forming green or yellow powdery strata on 
bark, stones, etc. ; these strata were formerly supposed to be independent Lichens, and con- 
stituted the genus Lepraria. Sometimes the gonidia invade the apothecium of Lichens, 
which gave rise to another false genus (V ariolaria) . 
3. A medullary, spongy, filamentous or cottony layer, composed of a network of jointed 
delicate threads ; these are sometimes developed downwards, forming rootlets or fibrils on 
the under surface of many horizontal thalli. 
The hypothaltus is a rudimentary horizontal stratum, from which the thallus grows, and 
is only distinguishable in the youngest state of the more highly organized Lichens, and lies 
beneath the crustaceous or granular thallus of the more lowly. 
The apothecia are usually discoid dark-coloured bodies, occupying various portions of the 
thallus, rarely however ( Nephromium ) the under surface. They may be orbicular, linear 
(lirellate), sessile or stalked, superficial or sunk in the thallus, convex concave or subglobose, 
and of all colours. The apothecia consists of the excipte or receptacle and thalamium. 
The receptacle is the enveloping part of the apothecium, and is formed of the substance of 
the thallus. When the apothecium is adnate to the thallus, the receptacle only borders it, and 
when the apothecium is peltate or stalked, the receptacle forms its under surface also ; vvlieu 
the bordering portion is of the same colour as the thallus, it is called a thalline or thallodal 
border ; when of a different colour, or that of the thalamium, it is called a proper border. 
The receptacle may be altogether absent, or almost enclose the thalamium, or form a glo- 
bular capsule, called a perithecium. The thalamium is usually solitary in each receptacle, but 
sometimes there are several, which are separate or confluent. It consists of a series of vertical 
elongate microscopic bodies, rising from a layer of minute cells, called the hypothecium ; and 
they are usually held together hy a transparent gelatine. The vertical bodies are two, para- 
physes and theca or asci. The paraphyses are the most numerous, they are linear, subcla- 
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