GROUP I. THALLOPHYTA: FUNGI: LICHENS. 



321 



by gemmae, termed soredia, which consist of one or more algal cells invested 

 by hyphae ; they are budded off from the surface of the thallus, and grow into 

 new plants (Fig. 227). 



The Lichen-Algae belong either to the Cyanophyceae or to the Chlorophyceae. 

 In the following list the principal algal forms with their corresponding Lichens 

 are enumerated. 



A. Cyanophyceae. 



Scytonemaceae (Scyfconema, Sirosiphon) . 



Kivulariaceae 

 Nostocaceae . 



Chroococcaceae 



B. Chlorophyceae. 



Protococcaceae (Pleurococcus, Cystococcus, 

 etc.) 



Confervoidese (Trentepohlia) 

 Coleochastaceae 



Ephebe, Spilonema, Polychidiuni, 



Khipidonema, Dictyonema, 



Heppia, Porocyphus. 

 Lichina, Eacoblenna. 

 Collema, Lempholemma, Lepto- 



gium, Peltigera, Paunaria. 

 Omphalaria, Euchylium, Phyllis- 



cium, Cora. 



Sticta, Physcia, Evernia, Cla- 

 donia, Usuea, Parmelia, An- 

 aptychia, Endocarpon, etc. 



Graphideae, Verrucarieaa. 



Opegrapha. 



The algal cells or filaments may be distributed throughout the thallus, when 

 it is said to be liomoiomerous (Fig. 208) ; this is usually the case in gelatinous 

 Lichens (such as the Collemaceae), in which the Alga belongs to the Cyanophyceae, 

 but also in some non-gelatinous forms in which the Alga belongs to the Chloro- 

 phyceas (such as Coenogonium, Kacodium, and others, in which the Alga is 

 Trentepohlia) : or they may be arranged in a 

 definite layer near the surface of the tballus, 

 when it is said to be heteromerous (Fig. 229), 

 as in the case of nearly all these Lichens of 

 which the Algae belong to the Chlorophycere, 

 and some in which the Algas belong to the 

 Cyanophyceae (e.g. Peltigera, Pannaria). In 

 some heteromerous forms (e.g. Thelidium), 

 the Algae are quite on the surface. Occasion- 

 ally (e.g. Endocarpon) algal cells are present 

 in tbe hymenium. 



It may be generally stated that the form 

 of the thallus is determined in the homoio- 



merous Lichens by the Alga, in the heteromerous Lichens by the Fungus, 

 the latter case three main forms are distinguished : 



(a) fruticose Lichens, in which the thallus grows erect, branching in a shrub- 

 like manner. Of this form are the various species of Usnea (Fig. 230 A), and 

 allied genera with a cylindrical thallus, which grow on trees : Eoccella tine- 

 toria grows on rocks in regions bordering on the Mediterranean ; from it and 

 V. S. B. Y 



FIG. 228. A gelatinous Lichen, 

 Collema pulposum, slightly magni- 

 fied. It is homoiomerous, and the 

 Alga is Nostoc. (After Sachs.) 



In 



