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crustaceous, or leprous substance, called a thallus. This thallus is formed of a cortical and 
medullary layer, of which the former is simply cellular, the latter both cellular and fila- 
mentous ; in the crustaceous species the cortical and medullary layer differ chiefly in tex- 
ture, and in the former being coloured, the latter colourless ; but in the fruticulose or 
foliaceous species, the medulla is distinctly floccose, in the latter occupying the lower half 
of the thallus, in the former enclosed all round by the cortical layer. Reproductive matter 
of two kinds ; 1, sjoorwies lying in membranous tubes (thecce) immersed in nuclei of the 
medullary substance, which burst through the cortical layer, and colour and harden by 
exposure to the air in the form of little disks called shields ; 2, the separated cellules of the 
medullary layer of the thallus. 
Affinities. According to Fries, Lichens are types of Algse born in the 
air, interrupted in their developement by the deficiency of water, and stimu- 
lated into forming a nucleus (or receptacle of sporules) by light. No Lichen 
is ever submersed ; there is none of which the vegetation is not interrupted by 
the variable hygrometrical state of the atmosphere ; and, finally, none that 
ever develope in mines, caverns, or places deprived of light. On this ac- 
count, their shields are more rare in the fissures of mountains, or in shady 
groves, than in places fully exposed to light. In wet places, also, their shields 
are not produced ; for so long as they are under the influence of water 
they are hardly distinguishable from Hydrophycae (forms of Algse) ; as, for 
instance, Collema, &c. But these plants, when exposed to the sun, do per- 
fect their shields, as is found by Nostoc lichenoides, foliaceum, &c., which, 
when dry, are ascertained to be Collema limosum, flaccidum, &c., surcharged 
with water. By being acquainted with this rule, the same author says, he 
has succeeded in discovering many Swedish Lichens with shields, which have 
for many years been constantly found sterile ; as Parmelia conoplea, lanugi- 
nosa, gelida, &c. ; and he even asserts that he has succeeded artificially in in- 
ducing sterile Lichens to become fruitful, as Usnea jubata, and others. Plant. 
Horn. 224. Lichens consist, according to Eschweiler, of a medullary and a 
cortical layer of tissue, of which the former is imperfectly cellular or filamen- 
tous, and bursts through the latter in the form of shields (apothecia), which 
contain a nucleus, consisting of a flocculose-gelatinous substance, among which 
lie the cases of sporules. These cases (thecse) are transparent membranous 
tubes, either simple or composed of several placed end to end, which either 
lie free in the nucleus, or are themselves contained in other membranous cases 
(asci). In the beginning Lichens are stated to be in all cases developed in 
humidity, and to be, in fact, at that time, mere Phycese or Confervae ; but as 
soon as the humidity diminishes, the under part dies, and an inert leprous 
crust is formed, which ultimately becomes the basis of the plant. Hence Li- 
chens consist of two distinct sorts of tissue, — living cellules forming the ve- 
getating part, and dead cellules the cohesion of which is lost ; when separate, 
the former is Palmella botryoides, and the latter Lepraria. Of these two sorts 
of matter, the leprous is incapable of perpetuating the Lichen, while every 
part of the living stratum has been ascertained to become reproductive matter. 
See Fries, as above quoted, and Meyer Ueher die Entwickelung , S^c., der Flech- 
ten. The investigations of the latter are exceedingly interesting. By sowing 
Lichens, he arrived at some curious conclusions, the chief of which are, that, 
like other imperfect plants, they may owe their origin either to an original 
elementary, or to a reproductive generation — the latter by the creation of parts 
capable of developement in conformity to the plant by which they are borne : 
that decomposed vegetable, and some inorganic, matter, are equally capable of 
assuming organisation under the influence of water and light ; and that the 
pulverulent matter of Lichens is that which is subject to this kind of indefi- 
nite propagation, while the sporules lying in the shields are the only part that 
will really multiply the species. He further says, that he has ascertained, by 
means of experiments from seed, that supposed species and even some genera 
