188 BOTANY 



really independent organisms, and these researches were followed 

 by many others which soon placed the dual nature of the Lichen- 

 thallus beyond any question. It has been conclusively shown that 

 the Algae can live quite as well, or better, when removed from their 

 association with the Fungus, which, on its side, dies, if deprived of 

 its algal associates, or if not artificially supplied with the necessary 

 food constituents. Careful experiment has also demonstrated the 

 possibility of producing a Lichen-thallus by associating the germi- 

 nating spores of the Lichen with Algae which were growing free, 

 and Mb'ller even succeeded in producing small Lichens upon steril- 

 ized glass plates, by supplying them with artificial nutriment, but 

 eliminating the Algee from which the food ordinarily is derived. 



Germination. Under normal conditions, the Lichen-spores, on 

 germinating, produce a mycelium of limited growth, which on com- 

 ing in contact with the proper algal cells (Fig. 154, A) attaches 

 itself to them and ultimately produces the complete Lichen. In 

 case the green cells are not available, the mycelium dies as soon as 

 it has exhausted the food-materials within the spore. 



The Algae which occur within the body of various Lichens are 

 identical with species which also live quite independently. They 

 represent most of the families of the Schizophyceae and several of 

 the lower families of Chlorophyceae", most of the latter being Pro- 

 tococcaceae, although a few Confervaceae have also been found as the 

 gonidia of Lichens. These Algae, when associated with the Lichen, 

 multiply only by fission ; but in some cases, at least, when removed 

 from their association with the Fungus, they develop zoospores. 



Parasitism and Symbiosis. The amount of injury caused by the 

 Fungus to the algal cells varies in different cases. Sometimes haus- 

 toria are sent into the cells, which are finally killed. Sometimes 

 the haustorium penetrates the wall of the algal cell, but does not 

 injure the protoplast. In still other instances, there is no penetra- 

 tion of the algal cells, and the substances taken from them must 

 diffuse through their walls. It is clear, however, that the association 

 of the two Lichen elements must be considered as a case of parasit- 

 ism. It is true that the Algae may derive certain advantages in 

 being protected by the enveloping Fungus filaments, which also 

 retain water somewhat tenaciously, and thus enable the Algae to 

 grow where otherwise they could not. This mutual association has 

 been termed Symbiosis. A further remarkable phenomenon is the 

 occasional parasitism of one complete Lichen upon another. 



Semi-lichens. A small number of Ascornycetes have been de- 

 scribed which live as saprophytes during their earlier stages, but 

 later become associated with Algae, which as a rule are injured 

 by the attacks of the Fungus. Sphceria lemanece and TJiermutis 

 velutina are examples of the Half-lichens. 



