FUNGI, THE LICHENS 303 



and disintegrated rock which lodges in crevices. Here many 

 ferns, grasses, and other plants can find a foothold and obtain 

 nourishment. The algae can also grow on moist rocks, but are 

 killed when long exposed to dry air in these situations. But when 

 the alga is surrounded by the close mat of fungus elements in the 

 lichen body it is prevented from injurious desiccation and is held 

 on even to steep slopes or perpendicular rock faces. 



453. The relation of the algae and fungus in the lichen 

 thallus. In the early hisory of the study of lichens it was thought 

 by some that the lichen was an individual plant (autonomous 

 plant), and that the algal cells were special spores (gonidia) 

 which were cut off from the colorless threads of the mycelium. 

 Opposed to this theory another one came to be accepted by some 

 students, i.e., that the alga and the fungus are distinct plants, and 

 this theory has finally prevailed. The most important arguments 

 presented for this theory were furnished by some investigators 

 who separated the spores of the lichen and grew fungi from them 

 free from the algae, and also separated the algal cells and grew 

 them as separate algae. In this way it has been possible to iden- 

 tify some of the algas with those which are found in a free state. 

 Pleurococcus vulgaris, so common in cool shady places, is one of 

 these. The blue-green alga Nostoc, with cells like a chain of 

 beads, has been found to be a component of many of the gelatin- 

 ous lichens. When the spores of these lichens are germinated 

 in the presence of these algae, the mycelial threads coil around the 

 algae, envelop them, and finally a complete lichen thallus is 

 developed which ultimately bears the fruit bodies and asci. 



454. Uses of lichens. As mentioned above, the " reindeer 

 moss" in the arctic regions is eaten by the reindeer. In lower 

 latitudes where it is abundant it is often used as packing material 

 to protect furniture or frangible wares. Some arctic lichens 

 (Cetraria), called "Iceland moss," are used for food by grinding 

 them and mixing them with ground wheat. Others which con- 

 tain purple or blue or crimson pigments are used for making dyes 

 called "archil," also orchil or cudbear. This lichen (Roccella 

 tinctoria) grows on rocks in the Canary and Cape Verde Islands. 



