CRYPTOGAMS 377 



thallus of the homoiomerous Lichens, particularly of the filamentous 

 forms., is determined by the Algae. 



In other cases the Lichens have stratified or HETEROMEROUS thalli ; 

 their form is then determined essentially by the Fungus. The en- 

 closed Algae are usually termed GONIDIA. They are arranged in a 

 definite GONIDIAL LAYER covered, externally, by a CORTICAL LAYER, 

 devoid of algal cells and consisting of a pseudo-parenchyma of closely- 

 woven hyphse. It is customary to distinguish the three following 

 forms of heteromerous Lichens. 



1. Crustaceous Lichens, in which the thallus has the form of 

 an incrustation adhering closely to a substratum of rocks or to the 

 soil, which the hyphse to a certain extent penetrate. 



2. Foliaceous Lichens (Fig. 303), whose flattened, leaf-like lobed 

 or deeply-cleft thallus is attached more loosely 

 to the substratum by means of rhizoid hyphse, 

 springing either from the middle only or irregu- 

 larly from the whole under surface. 



3. Fruticose Lichens (Fig. 304), with a 

 filamentous or band-like thallus branched in a 

 shrub-like manner and attached only at the 

 base. They are either erect or pendulous, or 

 may sometimes lie on the surface of the sub- 



i . Fig. 303. — Xanthoria parietlna 



swaram. ^ on a piece of barfc (Nat 



The manner in which the Fungus unites size.) 

 with the Alga may be seen from the adjoining 



figure (Fig. 302) showing the mode of formation of the orange-yellow 

 thallus of Xanthoria parietina, a foliaceous Lichen frequently occurring 

 on tree-trunks and walls. The branching germ-tube produced by the 

 germinating ascospore (Fig. 302, 1, sp) of a Fungus belonging to the 

 order Discomijcetes has already formed an intimate union with two 

 algal cells of the Protococcoideous genus Cystococcus, which furnishes 

 the gonidia of this Lichen. By the repeated branching of the hyphse 

 they entwine more completely round the group of Gystococcus cells and 

 form the thalloid rudiment (Fig. 302, 2), from whose continued growth, 

 accompanied by the division of the algal cells, the closely-woven hyphal 

 tissue of the thallus of the mature Lichen is produced. 



In their natural condition the germinating spores of the Lichen 

 Fungi appear to be capable of continuing their further development 

 only when they are enabled to enter into symbiosis with the proper 

 gonidia. For a few genera of Lichens, however, it has been determined 

 that the Fungi sometimes exist in nature without the presence of the 

 Algae ; it has been shown that the tropical Lichen, Cora pavonia, 

 whose Fungus belongs to the order Hymenomycetes, may produce 

 fructifications even when deprived of its Alga, which have a form 

 resembling those of the Fungus genus Thelephora. Small thalli have 

 also been successfully grown from the spores of certain Lichen-forming 



