ago. "s tructural ae d Physiol ical Boi : on 
i spores on the gonidia-forming Alga, Nostoc Tichempee Al eae ae 
opposite view is entertained by others, that Lichens are a Wai: 
distinct class of plants by themselves, the gonidia of which ~ ay ae 
sometimes become detached from the parent-plant, and. s i 
then carry on an algoid existence, in which state they have te s 
often been erroneously described as Algee. | ? 1 A 
Lichens are classified as under, viz. :— | rey i 
I. Lichens with a stratified thallus :— HETEROMEROUS LICHENS. — 
I. FRUTICOSE Lichens; thallus more or less arborescent in ie) 
form (Figs. 409, p. 285; 414, 415). ; 
2. FoLiaceous Lichens ; thallus leaf-like (Figs. 416, 417). | 
3. CRUSTACEOUS Lichens ; thallus crustaceous. 
II. Lichens with unstratified thallus :—HOMOOMEROUS LICHENS. 
1. GELATINOUS Lichens ; thallus gelatinous (Fig. 412, p. 287). 
2. PICTORIAL Lichens ; thallus forming a very thin irregular 
incrustation (Fig. 410, p. 286). | re 
The gelatinous lichenin, characteristic of many Lichens, renders 
them useful as articles of food to northern nations and their domestic ~~ 
animals, or as medicinal products. = 
Of this character are the so-called sa ‘ 
‘Iceland moss,’ Cetrarza islandica, Z | 
(Fig. 415),° the’ ‘®eim-deer moss, “ohe 
Cladonia rangiferina, and Sticta — 
pulmonacea. Larmelia parietinais 
also used as a febrifuge, from its ” 
containing a bitter principle which » ‘ 
is present also in other kinds, | 
Several other Lichens, as occella iz 
Fic. 416.—Parmelia tiliacea, a Foli- tinctoria, Lecanora tartarea, Varto- ~~ 
ee oem, yt epottiocia. laria dealbata, Gyrophora pustulata, 
and Lecanora atra, are also important articles of commerce, in conse- 
é quence of a pigment which is blue or red according to the mode in which eae 
it has been prepared, and which is known as orchil, litmus, and camel 
cudbear. Parmelia esculenta of Asia Minor, the Sahara, Persia, &c., is 
believed to be the ‘manna’ of Scripture. ; 
_ Lichens are most abundant in the colder regions of the globe, and 
occur on the utmost limits of vegetation. From this circumstance, and 
from the ease with which they grow where the means of living are ex- 
| tremely scanty—some of them flourishing on bare hard rocks, and others” 
even thriving on glass and iron—they are of consiaee able importance in- 
_ the economy of Nature. 4. 
