ORDER LECANORALES (tHE DISK LICHENS) 217 



they were truly algae. He and others following him were able to synthe- 

 size lichens from cultures of algae and lichen ascospores (Bonnier, 1889). 

 In spite of this evidence some lichenologists were loath to give up the old 

 idea and as late as 1913 Elfving, observing green bodies within the hyphae 

 of some lichens, maintained that they were integral parts of the lichen. 

 Liro (1914) confirmed the occasional presence of these green bodies within 

 the hyphae but beheved them to be portions of cells or whole small cells 

 of the host algae that had entered the hyphae through openings. Tobler 

 (1925) summarized the results of his own work and that of others on the 

 biology of the lichens, confirming the belief that they consist of fungi 

 parasitizing upon algae with a mutual interaction of the two organisms 

 to produce the characteristic structures. 



The algal hosts are usually Blue-green Algae (Myxophyceae) or Green 

 Algae (Chlorophyceae) whose habitat consists of moist situations on land, 

 such as on the ground, rocks, trees, etc. More often the one-celled algae 

 are preferred (Chroococcus, Chlorococcum, Protococcus, etc.), but some of 

 the filamentous forms are also captured and made prisoners {Nostoc, 

 Trentepohlia, etc.). Most genera of lichens are confined to a single species 

 or to closely related species of algae but a few lichens are able to use as 

 algal hosts forms from widely diverse groups. It is noteworthy that the 

 thalli formed by the same species of fungus with these widely separated 

 algal hosts are very different, according to the host present. 



Lichens may be grouped morphologically in accordance with the type 

 of thallus into crustose, foliose, fruticose, and pendent lichens. The first 

 form closely adhering crusts on the substratum to which they are held 

 fast by hyphal strands; the foliose lichens are flat and thin ("leaf-like") 

 but adhere to the substratum only at definite points; the fruticose lichens 

 are upright in habit and more or less branched, resembling little shrubs 

 {frutex means shrub). The pendent forms are attached only at localized 

 spots and are long and slender and branching, hanging from the twigs 

 or branches of the trees to which they are attached. They are often con- 

 sidered only a special type of fruticose lichen. It must be noted that 

 these distinctions are not absolute for there are gradations between these 

 types. Most species of Cladonia are at first crustose or foliose (but with 

 small "leaves"), later forming upright podetia upon which the apothecia 

 are developed, at which time the prostrate foliar growth may disappear. 

 The distinction between crustose and foliose lichens is not always clear. 

 (Fig. 72.) _ 



In tropical and subtropical regions many lichens grow on the algae 

 attached to the surface of leaves. The fungus hyphae may then enter 

 through the stomatal openings into the interior of the leaf where it 

 seems probable that they are somewhat parasitic upon the leaf tissues. 

 McWhorter (1921) showed that some lichens of the genera Cladonia and 



