242 



STRUCTURAL BOTANY 



Physcia parietina is described as a foliaceous Lichen, 

 because its thallus has a flat, somewhat leaf -like form. 

 It is attached to the substratum by its under surface, 

 except at the edges of the thallus, where it is free (see 

 Fig. 98). The upper exposed surface of the thallus is 

 of a deep orange colour, while the lower side is much 

 paler. It is attached to the wall, rock, tree, or whatever 

 else it may be growing on, by fibrous rhizoids, which 

 perform in all respects the functions of roots. 



The anatomical 

 structure of the thallus 

 is shown in transverse 

 section in Fig. 99. 

 Towards the upper 

 surface is a dense layer 

 of rather thick-walled 

 tissue, which appears 

 to be parenchymatous. 

 In reality, however, it 

 is built up, like all 

 fungal tissues, of fila- 

 mentous hyphae, which 

 in this case are so 

 densely packed and 

 closely interwoven that their limits cannot be traced, and 

 the whole structure appears to represent an ordinary 

 cellular tissue. The orange colour is due to crystalline 

 granules of chrysophanic acid deposited outside the cells, 

 both on the free surface of the thallus and between the 

 hyphae of which it is composed. Such acids (which 

 belong chemically to the Benzole series) are common in 

 Lichens, and from some of them Litmus (so much used in 

 chemical testing for acids and alkalis) is prepared. 



FIG. 98. Physcia parietina; thallus 

 seen from above. a, apothecia, of 

 which the ripest are near the middle. 

 Natural size, h, section of apothecium, 

 showing the hymenium. Magnified 

 about 5. (After Lauder Lindsay.) 



