36 



LICHENES 



In the heteromerous lichens a special and characteristic method of 

 reproduction is often met with, by means of small structures known as sored id. 

 Each of these consists of a tew algal cells, about which some fungus hvphae 

 are entwined, and they are set free in large numbers from the thallus, appearing 

 to the naked eye as a powder on the surface (Figs. 34 and 37). 



Of foliaceous forms we have in South Africa a considerable number of 

 species of Parmelia, which possesses a leathery, leaf-like thallus, forming both 

 small apothecia and the much smaller spermogonia. big. 33 shows the 

 general structure of the thallus ami apothecium of a species of Parmelia. It 

 can be seen that the alga consists of isolated cells, and these belong to the 

 genus Pleurococcus. Soredia are frequently formed on the upper surface of the 

 thallus (big. 34). 







Fig. 34. Soredium <>t Parmelia, 



Section. 1000 1 . w. 1. s. 



Fig. 33. Parmelia arizonica (Tuck) Nyl. Section 

 through apothecium. (After Reinke) Enlarged. 



The apothecia have a slightly concave, smooth upper surface, in which 

 a large number of sterile hairs or paraphyses 

 occur as well as the asci. The latter contain 

 (usually) eight two-celled ascospores (Fig. 35). 



The spermogonia or pyenidia are very small 

 and appear as minute black dots on the surface 

 of the thallus. In section they are spherical or 

 egg-shaped and contain large numbers of Sper- 

 matid {pyeno Spores). While it has been 

 shown that in certain lichens the spermatia probably 

 act as male cells, it is very doubtful whether 

 they have such a function in other cases, and prac- 

 tically certain that as a rule they have not. It is 

 likely, however, that the}' hail their origin as male 

 cells, and on this view it is customary to speak of 

 them as spermatia, ami of the cavities within which 

 they are borne as spermogonia, though the alterna- 

 tive names mentioned above are also in use. 



Of the fruticose lichens an example known from 



South Africa is Usnea fiorida, shown in big. 36. Here 



the structure of the thallus is similar to that of 



the external form. The structure of 



Fig. 35. Asci anil para- 

 physes of Parmelia. 



7OO/1. W. T. S. 



Parmelia^ the chief difference being in 



