DIV. I 



THALLOPHYTA 



471 



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 substratum with new rhizines. 

 The majority of the heteromerous 

 Lichens possess in the formation 

 of SOREDIA another means of 

 vegetative 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 thallllS, are FIG. VS.-Parmelia physodes. 



scattered in great numbers by , Formation of soredia ; 6, single soredium. 



the wind and give rise to new 



Lichens. Frequently the soredia arise in circumscribed receptacles 



(Fig. 423). 



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

 by the algae, which are always purely vegetative. 



1. Ascolichenes 



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

 longing to the Pyrenomycetes (Endocarpon, Vei-rucaria). Most genera produce, as 

 the ascus-fruit of their fungus, cupular or discoid apothecia, sessile or somewhat 

 sunk in the thallus. In structure they resemble those of the Discomycetes, and 

 bear on their upper side an hymenium of asci and paraphyses. One of the 

 commonest species of fruticose Lichens belonging" to this group is Usnea barbata, 

 the Beard Lichen, frequently occurring on trees and having large fringed apothecia 

 (Fig. 424). JRamalinaframnea, which has a broad ribbon-shaped branched thallus 

 and grows on trees, and the numerous species of Roccella found on the rocks of 

 warmer coasts, have similar apothecia. Cetraria islandica, Iceland Moss (Fig. 425), 

 occupies an intermediate position between the fruticose and foliaceous Lichens. 

 It has a divided, foliaceous, but partially erect thallus, which is of a light bluish- 

 green or brown colour, whitish on the under side, and bears the apothecia 

 obliquely on its margin. This Lichen is found in mountainous regions and in the 

 northern part of the Northern Hemisphere. The numerous species of Parmelia 

 (Fig. 426) are foliaceous Lichens growing on trees and on rocks. Graphis scripta 

 s a well-known example of the crustaceous Lichens ; its greyish-white thallus 

 occurs on the bark of trees, particularly of the Beech, on whose surface the 

 apothecia are disposed as narrow, black furrows resembling writing. 



A peculiar mode of development is exhibited by the genus Cladonia, whose 

 primary thallus consists of small horizontal scales attached directly to the ground ; 

 from this thallus springs an erect portion, the PODETIUM, of varying form and 



2H3 



