396 DIVISION 111. MODE OF LIFE OF THE FUNGI. 



grows round the cells of the Algae and developes into a thallus. The Alga follows 

 this growth, inclosed in a definite form between the hyphae of the Fungus, and thus 

 forms an integral part of the thallus. As the Alga grows, the chlorophyll in its 

 protoplasm is constantly engaged in assimilating carbon dioxide and forming organic 

 carbon compounds for the use of the Fungus. On the other hand the rhizoid- 

 branches of the Fungus (see page 45) spread in and on the substratum and 

 contribute the needful mineral food. The two processes of vegetation mutually 

 support and supplement each other. The Alga can live alone, as a plant containing 

 chlorophyll, though it may be open to question whether it can do so in all cases; 

 but its vegetation is not hindered by the Fungus, it can often be shown to be 

 permanently promoted by it. The Fungus as a strictly obligate parasite is 

 dependent for its vegetative growth on the Alga, without which it cannot attain 

 to its full development, and in most cases can scarcely get beyond the first stages of 

 germination. 



The power of assimilating carbon dioxide which the Alga possesses in virtue 

 of its chlorophyll makes it unnecessary that organic carbon compounds should be 

 supplied as food to the thallus formed of itself and the Fungus in combination. 

 Very many of the forms in this group do in fact live on places like bare stone or 

 the ground, which do not supply these compounds. The question whether other 

 species, which live on the bark of trees, humus-soils and similar substances, do partly 

 vegetate as saprophytes has not been thoroughly examined ; but Frank's observations 

 on species living upon the bark of trees, which will be further noticed below, 

 are in favour of the existence of a saprophytism of the kind. 



In one point the Fungus is the superior in the common household ; it alone 

 produces spores, the Alga with a few exceptions remaining barren as long as it is 

 combined with the Fungus l . 



The bodies thus composed of Fungus and Alga combined are known by the 

 name of Lichens ; it is equally well known that this name indicates an important 

 phenomenon in nature, and is applied to a very varied series of forms, large enough 

 to fill the pages of many descriptive works and local floras. 



It follows necessarily from the above that two component parts, a Fungus 

 and an Alga, sometimes even several Algae, can be distinguished in every Lichen. 



The general morphological characters of the Fungi which form Lichens have 

 been already stated in previous sections of this book. In the vegetative part they 

 consist of hyphae connected together in the various ways described in section I, 

 and in most cases united into a Fungus-body. According to the course of develop- 

 ment indicated by the sporiferous structure the great majority of Lichen-fungi belong 

 to the Ascomycetes, both Discomycetes and Pyrenomycetes, a few recently become 

 known to us to the Hymenomycetes. 



The number of algal forms which enter into combination with Fungi is 

 according to our present knowledge not inconsiderable. There is one corresponding 

 Alga to each species of Lichen, in other words, each species of Lichen-fungus is 

 confined to a certain species of Alga. It is rare that two are found together in a 

 Lichen, and then one of them always predominates over the other. On the other 



Synalissa symphorea Nyl. See Bornet in Ann. d. sc. nat. ser. 5, XVII, p. 50. 



