CELLULABES, OE CELLULAR PLANTS. 63 



like thallus of Cladonia coccinea, a beautiful lichen found on 

 the decaying stumps of old trees, and which bears its fructifi- 

 cation in rounded red masses at the summit of its podetium. 



72. It not unfrequently happens that the reproductive 

 matter of lichens appears on the surface or margin of the 

 thallus in the form of irregularly-shaped powdered masses of 

 spores, soredia {sorus, a heap), which propagate either on the 

 original thallus, forming foliaceous or squamulous expansions, 

 or external to the original thallus, forming new individuals 

 of the species. 



73. The thallus of lichens, when it exists under the form 

 of a membrane, is composed of three superposed beds or 

 layers of cellular tissue ; the cortical layer, which is formed 

 of spherical utricles, containing interiorly granules of various 

 colors. These cells have received the name of Gonidia, and 

 constitute the soredia or masses of spores, which, when de- 

 tached from the plant, form new individuals of the species. 

 The medullary layer is situated immediately beneath the 

 cortical, and is composed of both rounded and filamentous 

 cells. The hypothallus or under surface of the lichens is 

 composed of cellules elongated and cylindrical, the analogues 

 of roots by means of which the plant attaches itself firmly to 

 the substance on which it grows. 



74. The limits of the three great families of cellular 

 plants, the lichens, the fungi, and the algae, have not yet 

 been distinguished; and some naturalists, who have studied 

 these tribes with great attention, are of opinion that a spore 

 from any one of them will develop into a fungus, alga, or 

 lichen, according to the medium in which it happens to be 



