GROUPS OF PLANTS. 



39 



LICHENS. 



General Characters. — The Lichens are a peculiar group of 

 plants in that an individual lichen consists of both an alga called 

 a GONiDiuM and a fungus. These are so intimately associated that 

 they appear to be mutually beneficial, and such a relation is known 

 as SYMBIOSIS (Fig. 25). The Algas which may be thus asso- 

 ciated in the Lichens are those members of the Blue and Green 

 Algae which grow in damp places, as Pleurococcus, Nostoc, Lyng- 



FiG. 25. Lichens showing manner of union of algae or gonidia (g) and hyphae (h) 

 of Fungi. A, Pleurococcus, showing the manner in which hyphae penetrate the cell and in- 

 fluence cell division; B, Scytonema, an alga surrounded by richly branching hyphs; C, 

 chain of Nostoc showing hypha of fungus penetrating a large cell known as a heterocyst; 



D, fungal hyphae have penetrated the cells of Glmocapsa a blue-green, unicellular alga; 



E, Chlorococcum, a reddish or yellowish alga found in Cladonia furcata, the cells of which 

 are surrounded by the short hyphae of the fungus. — A.afterHedlund; B-E, after Bomet. 



bya, etc. (Fig. 25). The Fungi which occur in this relation 

 belong both to the Ascomycetes and Basidiomycetes and it is on 

 the characters of the fruit bodies of these particular Fungi that 

 the main divisions of Lichens are based. The Fungi, however, are 

 not known to exist independently of the Algae with which they 

 are associated, that is, the mycelia of the fungi will not live for 

 any length of time unless they come in contact with a suitable 



