396 DIVISION IIL.—MODE OF LIFE OF THE FUNGI. 
grows roung 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 '. 
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 

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