43S 



BOTANY OF THE LIVING PLANT 



susceptible to impurities in the air, and are therefore absent from 

 urban areas (Fig. 326). 



Their structure shows two distinct constituents. Certain cells 

 have algal characters, and often closely resemble Algae known in 

 the free state ; they contain chlorophyll or some related colouring 

 matter, and are photosynthetic. They are distributed variously 



B 



Fig. 326. 



A =Xanthoria (Parmelia) parietina, the common foliaceous yellow Lichen. 

 B =Cladonia rangiferina, a fruticose Lichen. Both bear ascus-fruits, and are shown 

 natural size. (After Strasburger.) 



in the thallus, often in a definite gonidial layer. These cells are 

 closely invested by the fungal constituent, which is composed of 

 septate and branched hyphae, twigs of which enwrap the algal cells, 

 establishing intimate physiological relations (Fig. 327). Not only 

 does this dual organism flourish, but it may also propagate as such. 

 In wet weather many Lichens are covered by a mealy powder, extruded 

 from within. Examination shows that it is composed of soredia, 

 which are bodies containing both constituents of the thallus ; one or 

 more algal cells are enveloped in a weft of fungal hyphae. Each 

 soredium is thus able to grow directly into a new Lichen. 



The fruiting bodies of the Lichens are, however, produced from 

 the fungal constituent only, and most of them closely resemble those 

 of Discomycetous and Pyrenomycetous Fungi in form and con- 

 struction (Fig. 326, A). In the Iceland Moss (Cetraria Islandica), 

 which is officinal, the fruits appear as marginal discs. When cut 

 vertically they show a superficial hymenium, with numerous asci 

 arranged as in Peziza (compare Fig. 322, B, p. 42 5) . Male sexual organs 



