284 PHYLOGENY 



a. GLOEOLICHENS. Among crustaceous forms the thallus is generally 

 elementary, more especially in the Gloeolichens (Pyrenopsidaceae). The 

 algae of that family, Gloeocapsa, Xanthocapsa or Chroococcus, are furnished 

 with broad gelatinous sheaths which, in the lichenoid state, are penetrated 

 and traversed by the fungal filaments, a branch hypha generally touching 

 with its tip the algal cell-wall. Under the influence of symbiosis, the algal 

 masses become firmer and more compact, without much alteration in form; 

 algae entirely free from hyphae are often intermingled with the others. Even 

 among Gloeolichens there are signs of advancing development both in the 

 internal structure and in outward form. Lobes free from the substratum, 

 though very minute, appear in the genus Paulia, the single species of which 

 comes from Polynesia. Much larger lobes are characteristic of Thyrea, a 

 Mediterranean and American genus. The fruticose type, with upright fronds 

 of minute size, also appears in our native genus Synalissa, It is still more 

 marked in the coralloid thalli of Peccania and Phleopeccania. In most of 

 these genera there is also a distinct tendency to differentiation of tissues, 

 with the gonidia congregating towards the better lighted surfaces. The only 

 cortex formation occurs in the crustaceous genus Forssellia in which, according 

 to Zahlbruckner 1 , it is plectenchymatous above, the thallus being attached 

 below by hyphae penetrating the substratum. In another genus, Anema 2 , 

 which is minutely lobate-crustaceous, the internal hyphae form a cellular 

 network in which the algae are immeshed. As regards algal symbionts, 

 the members of this family are polyphyletic in origin. 



b. EPHEBACEAE AND COLLEMACEAE. In Ephebaceae the algae are 

 tufted and filamentous, Scytonema, Stigonema or Rivularia, the trichomes of 

 which are surrounded by a common gelatinous sheath. The hyphae travel 

 in the sheath alongside the cell-rows, and the symbiotic plant retains the 

 tufted form of the alga as in Licliina with Rivularia, Leptogidium with Scyto- 

 nema, and Ephebe with Stigonema. The last named lichen forms a tangle of 

 intricate branching filaments about an inch or more in length. The fruticose 

 habit in these plants is an algal characteristic ; it has not been acquired as a 

 result of symbiosis, and does not signify any advance in evolution. 



A plectenchymatous cortex marks some progress here also in Lepto- 

 dendriscum, Leptogidium and Polychidium, all of which are associated with 

 Scytonema. These genera may well be derived from an elementary form 

 such as Thermutis. They differ from each other in spore characters, etc., 

 Polychidium being the most highly developed with its cortex of two cell- 

 rows and with two-celled spores. 



Nostoc forms the gonidium of Collemaceae. In its free state it is extremely 

 gelatinous and transmits that character more or less to the lichen. In the 

 crustaceous genus Physma, which forms the base of the Collema group or 



1 Zahlbruckner 1907. 2 Reinke 1895. 



