418 



BOTANY 



I'AKT II 



be capable of continuing their further development only when they 



are 



few 



enabled to enter into symbiosis with the proper gonidia. For a 

 of Lichens, however, it has been determined that the 



genera 



fungi sometimes exist in nature without the presence of the algae ; it 

 has been shown that the tropical Lichen Cora pavon.ia (Fig. 362), 

 whose fungus belongs to the order Hymenomycetes, may produce 

 fructifications even when deprived of its alga ; these have a form 

 resembling those of the fungal genus Thelephora. Small thalli have 

 also been successfully grown from the spores of certain Lichen- 

 forming Ascomycetes, cultivated without algae and supplied with a 

 proper nutrient solution. 



Many Lichens are able to multiply in a purely vegetative manner, 



by means of loosened 

 pieces of the thallus, 



QO 



°d 





no c 



'Oo " ' 





5"^ «as?,?o\SK 



which continue their 

 growth and attach 

 themselves to the sub- 





,0o Co o"o„ 



Fig. 354. — Cetraria islandica. Ti-ansverse section 

 througli the thallus ; or, cortical layer of upper 

 .suil'ace ; ur, of tlie lower surface ; m, medullary 

 layer containing the green cells of the Alga, CJiloro- 

 coccwn hwnirola. ( x 272. ) 



Fig. 355. — Xanthoria parietina 

 on a piece of bark. (Xat. 

 size.) 



stratum with new rhizines. The majority of the heteromerous 

 Lichens possess in the formation of SOREDIA another means of vegeta- 

 tive multiplication. In this process, small groups of dividing gonidia 

 become closely entwined with mycelial hyphae and form small isolated 

 bodies which, on the rupture of the thallus, are scattered in great 

 numbers l)y the wind and give rise to new Lichens. 



The fructifications of the Lichens are produced by the fungi, not 

 by the algae, which are purely vegetative. 



Tlie numerous Lichen acids which are wanting only in the gehitinous Lichens 

 are products of metabolism peculiar to the group. Their production is due to the 

 mutual chemical influence of the alga and fungus. They are deposited on the 

 surface of the hyphae in the form of crystals or granules. Their sui)i)osed use as 

 a protection against snails appears, according to Zoi'F, not to hold generally C"). 



1. Aseoliehenes 



Only a few genera of Lichens have flask-shaped perithecia, the fungus be- 

 longing to Pyrenomycetes {Endocarpon, Verrucaria). Most genera produce, as 



