124 



FIRST GROUP. THALLGPHFTES. 



which, when wrapt in hyphal tissue and set free from the thallus, are able at once to 

 grow into a new Lichen-thallus ; they issue from the thallus of non-gelatinous Lichens 

 as a fine powder, sometimes forming thick cushion-like masses on them, as in Usnea, 

 Ramalina, Evernia, Physcia, Parmelia, Pertusaria, and others. In the thallus of a he- 

 teromerous lichen the soredia are formed in the gonidial layer, where single gonidia, or 

 a number of them together, become surrounded by hyphae which cling closely to them 

 and form a fibrous envelope for them ; the gonidia divide repeatedly, and each 

 daughter-cell is again invested with hyphal threads, and by the frequent repetition of 

 this process the soredia collect in large numbers in the gonidial zone, and at length 

 rupture the thallus. When thus released the soredia may go on multiplying outside 

 the thallus, or under favourable circumstances each soredium or a number of 

 them may develope into a new thallus (Fig. 83). Schwendener says that this can 

 take place in Usnea barbala when the soredium is still in the mother-thallus, and 

 that in this way the branches are produced, which have been termed 'soredial 

 branches.' 



It has been already intimated that the gonidia, the other element which unites with 

 the Fungus to form the thallus of a Lichen, are simply Algae which the ascomycetous 

 Fungus has attacked and grown round, and from which it obtains food, being inca- 

 pable itself of carbon-assimilation. Attention has also been called in connection 



FIG. 83. A Z?.soredia of Usnea barbata. A a simple soredium, consisting'of a gonidium with hyphae woven round 

 it. B a soredium in which the gonidium has multiplied by division. C a group of simple soredia formed by hyphae 

 forcing their way between the gonidia. D and E germinating soredia; the hyphae form an apex, the gonidia are 

 multiplying, a c soredia of Physcia parutina. a with an envelope of pseudo-parenchyma b the envelope producing 

 attaching filaments, c a young thallus, developed from a soredium, magn. 500 times. After Schwendener. 



with the hymenial gonidia to the changes produced in the gonidia by the Fungus. If 

 the hyphal tissue in Physcia, Evernia, and Cladonia is decomposed in water, and the 

 gonidia thus enabled to vegetate at liberty, they will produce zoospores. 



The following is Schwendener's summary of the Algae which are employed as 

 Lichen-gonidia : 



I. Algae with bluish-green cell-contents (Phycochromaceae). 



Name of Alga-group. Name of the Lichens in which the Algae occur as gonidia. 



1. Sirosiphoneae Ephebe, Spilonema, Polychidium. 



2. Rivularieae Thamnidium, Lichina, Racoblenna. 



3. Scytonemeae Heppia, Porocyphus. 



4. Nostocaceae Collema, Lempholemma, Leptogittm, Pannaria, Pel- 



tigera. 



5. Chroococcaceae Omphalaria, Euchylium, Phylliscum. 



II. Algae with chlorophyll-green cell-contents (Chlorophyceae). 



6. Confervaceae Cystocoleus. 



7. Chroolepideae Graphideae, Verrucarieae, Roccella, Lecanora sp. 



Coenogoniu f n. 



