24-i 



LICHENS. 



form a single organism and thenceforward so co-operate in their functions that 

 ultimately both derive advantage from the arrangement. The one takes food-stufis 

 from the substratum and from the air and transmits them to the other; whilst, in 

 the green cells of the other, the raw material is worked up, under the influence of 

 sunlight, into organic compounds. The organic compounds thus created are used 

 by both for the further production of organs, and therefore a connection such as 

 this must be looked upon as a true case of symbiosis, i.e. associated existence for 

 purposes of nutrition. 



The first place amongst social communities of the kind must be assigned to 

 Lichens, a section of Cryptogams possessing an extraordinarily large number of 

 species and differentiated into thousands of forms, representatives of which are 



Fig. 57. — Gelatinous Lichens. 

 ^ Ei/hebe Eenieri ; x460. ' Collema imlposmn ; natural size. » Section through CoHcmo pMiposum; x450 



everywhere distributed, from the sea-shore to the highest mountain peaks j'et 

 scaled by man, and from the tropics to the arctic and antarctic zones. 



The partners in the Lichen communities appear to be, on the one hand, groups 

 and filaments of round, ellipsoidal, or discoid green cells belonging to plant species 

 included under the general name of Algse; and, on the other hand, pale, tubular 

 cells or hyph^, which are destitute of chlorophyll, and pertain to species of plants 

 comprised under the general name of Fungi (see fig. 58). 



The form assumed by a large proportion of these lichens is that of incrustations 

 on stones, earth, bark, or old wood-work ; the entire structure of the lichen is either 

 ensconced and imbedded in the depressions of weathered surfaces of stone, or else 

 between the cell-walls of dead fragments of wood and bark, so that it often happens 

 that attention is only drawn to its presence by the altered colour of the substratum, 

 or by the fructifications which lift their heads above the substratum. 



Lichens of the kind are termed Crustaceous Lichens, and the wide -spread 

 Graphic Lichen {Lecidea geographica) may serve as an example. A second great 

 group nearly allied to the first is that of Foliaceous Lichens. The form of the 



