CRYPTOGAMS 



417 



n n tT*V V? ^O C^***"- *^~* ^ ** t^*~^- 



FIG. 366. Cetraria islandica. Transverse section 

 through the thallns ; or, cortical layer of upper 

 surface; ur, of the lower surface; m, medullary 

 layer containing the green cells of the Alga, Chloro- 

 corcumhumieolc. (x 272.) 



substratum by means of rhizoid-like hyphae (rhizines), springing either 

 from the middle only or irregularly from the whole under surface. 

 FRUTICOSE LICHENS (Fig.368) have a filamentous or ribbon-like thallus 

 branched in a shrub-like manner and attached at the base. They are 

 either erect or pendulous, or 

 may sometimes lie free on the 

 surface of the substratum. 



In nature the germinating 

 spores of 'the Lichen Fungi 

 appear to be capable of con- 

 tinuing their further develop- 

 ment only when they are en- 

 abled to enter into symbiosis 

 with the proper gonidia. For 

 a few genera of Lichens, how- 

 ever, it has been determined 

 that the fungi sometimes exist 

 in nature without the pres- 

 ence of the algae ; it has been 

 shown that the tropical Lichen, 

 Cwa pavonia (Fig. 374), whose 

 fungus belongs to the order Hymenomycetes, may produce fructi- 

 fications even when deprived of its alga ; these have a form re- 

 sembling 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, which continue their 

 growth and attach themselves to the sub- 

 stratum with new rhizines. The majority of 

 the heteromerous Lichens possess in the forma- 

 tion of SOUEDIA another means of vegetative 

 multiplication. In this process, small groups 

 of dividing gonidia become closely entwined 

 with mycelial hyphse, and form small isolated 

 bodies which, on the rupture of the thallus, 



are scattered in great numbers by 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. 



Most Lichens secrete special substances, especially in their cortical layers, among 

 which numerous acids must lie mentioned ( 85 ) ; according to STAHL these are useful 

 as a protection against attack by snails ( 88 ). 



2 E 



Fio. 307.- -Xontharia / 



on u piece of bark. 

 si/f. ) 



(Xat. 



