GROUP I. THALLOPHYTA: FUNGI: LICHENS, 321 



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

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

 new plants (Fig. 227). 



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

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

 are enumerated. 



A. Cyanophyceas. 



Scytonemaceoe (Scytonema, Sirosiphon) . Ephebe, Spilonema, Polychidium, 



Ehipidonema, Dictyonema, 

 Heppia, Porocyphus. 



Eivulariaceaa .......... Lichina, Eacoblenna. 



Nostocaceae Collema, Lempholemma, Lepto- 



gium, Peltigera, Pannaria. 



Chroococcacete Omphalaria, Euchylium, Phyllis- 



cium, Cora. 



B. Chlorophyceae. 



Protococcacese (Pleurococcus, Cystococcus, 



etc.) Sticta, Physcia, Evernia, Cla- 



donia, Usnea, Parmelia, An- 

 aptychia, Endocarpon, etc. 



Confervoideas (Trentepohlia) ..... Graphideaa, Verrucarieae. 



ColeochaBtaceae Opegrapha. 



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

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

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

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

 phyceae (such as Ccenogonium, Eacodium, and others, in which the Alga is 

 Trentepohlia) : or they may be arranged in a 

 definite layer near the surface of the thallus, 

 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 Chlorophyceaa, 

 and some in which the Algae 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 FIG. 228. A gelatinous Lichen, 

 in the hymenium Collema pulposum, slightly magni- 



It may be genera,* stated that the fonn 

 of the thallus is determined in the homoio- 

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

 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 : Roccella tine- 

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



V. s. B. Y 



