39* 



LECTURE XXIV. 



fungus-tissue, in accordance with the growth of the whole, continually increase 

 in number by division, and, according to the nature of each Lichen, constitute with 

 the fungus-tissue a more or less homogeneous mixture (homoiomerous Lichens), or 

 form a layer within the tissue of the Fungus, beneath the surface of the latter 

 (heteromerous Lichens). 



It is obvious that the green Algas in the body of the Lichen act as organs of 

 assimilation, exactly as the chlorophyll-cells in the cortex of a green stem or in a 

 leaf. Their products of assimilation serve as nutritive material for the Lichen-fungus; 

 while, on the other hand, the ash constituents necessary for assimilation are conveyed 

 to them by means of the Fungus. By means of this commensalism, however, the 



Fig. 250.— Various Algse attacked by Lichen-fungi ; the Fungus hyphae are everywhere denoted by A, the Alga-cells 

 (gonidia) by ^. ^ germinating spore of Physica parietina, the germinal tube h having fixed itself upon Protococcus ■viridis. 

 B a filament of Scytonema invested by the hyphas of the Fungus Stereocaulon ramutosum. C from the tissue of the 

 Lichen Pkysyna ckalagannfn ; the end of a hypha is penetrating into a cell of the Nostoc. D from the tissue of the Lichen 

 Synnlysitx symphorea; the Alga ^ is a Glococapra. E from the tissue of the Lichen Cladonia /iircata, the host Alga ^ 

 belonging to the genus Prolococcus (highly magnified— after Bornet). 



Lichens are now independent of an organic substratum ; while all other Fungi are para- 

 sites or humus plants, Lichens are able to establish themselves on purely mineral sub- 

 strata, or even on the surface of crystalline rocks, since the enclosed Alga makes them 

 independent; and when Lichens exhibit a predilection for the bark of trees, this 

 certainly does not happen in order to extract their organic nutritive material from 

 the bark, but for other reasons. While other Fungi decompose organic substrata, we 

 find numerous Lichens capable of decomposing the inorganic substance of stones, 

 e. g. granite, in order, like the roots of the higher plants, to obtain those mineral 

 matters which their chlorophyll-cells (the Algse in their tissue) require for assimi- 

 lation. Inasmuch as these Fungi thus come into connection with certain Algse, in 



