244 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-stutfs 
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. 
1 Ephebe Kerneri; x450. 2% Collema pulposum; natural size. 3 Section through Collema pulposum; x 450. 
everywhere distributed, from the sea-shore to the highest mountain peaks yet 
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 Algx; and, on the other hand, pale, tubular 
cells or hyphz, 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 inerustations 
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 fructifieations 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 
