EU-MYCETESi (a) ASCOMYCETES 



439 



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

 areas (Fig. 372). 



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 photo -synthetic. They are distributed variously 

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

 closely invested by the fungal constituent, which is composed of 



FIG. 373- 



Cladonia furcatn. A = vertical section of the thallus showing the inverted gonidial 

 layer below the cortical sheath ( x 330). B=part of the same highly magnified, to 

 show the mode of attachment of the hyphae to the gonidia ( x 950). (After Bornet.) 



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

 establishing intimate physiological relations (Fig. 373). 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 ; and 

 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 Disco-mycetous and Pyreno-mycetous Fungi in form and con- 

 struction (Fig. 372, A}. In the Iceland Moss (Cetraria Islandica), which 



