94 



PHYSIOLOG Y. 



plant fits so closely to the rock on which it grows that it seems merely U. 

 paint the rock a slightly different color, and in the case of many which occur on 

 trees there appears to be to the eye only a very slight discoloration of the bark 

 of the trunk, with here and there the darker colored points where fruit bodies 



Fig. 84. 

 Frond of lichen (peltigera), showing rhizoids. 



are formed. The most curious thing about them is, however, that while they 

 form plant bodies of various form, these bodies are of a "dual nature" as 

 regards the organisms composing them. The plant bodies, in other words, are 

 formed of two different organisms which, woven together, exist apparently 

 as one. A fungus on the one hand grows around and encloses in the 

 meshes of its mycelium the cells or threads of an alga, as the case may be. 



If we take one of the leaf-like forms known as peltigera, which grows on 

 damp soil or on the surfaces of badly decayed logs, we see that the plant 

 body is flattened, thin, crumpled, and irregularly lobed. The color is dull 

 greenish on the upper side, while the under 'side is white or light gray, and 

 mottled with brown, especially the older portions. Here and there on the 

 under surface are quite long slender blackish strands. These are composed 

 entirely of fungus threads, and serve as organs of attachment or holdfasts, 

 and for the purpose of supplying the plant body with mineral substances 

 which are in solution in the water of the soil. If we make a thin section of 

 the leaf-like portion of a lichen as shown in fig. 85, we shall see that it is 

 composed of a mesh of colorless threads which in certain definite portions 

 contain entangled green cells. The colorless threads are those of the fungus, 

 while the green cells are those of the alga. These green cells of the alga per- 

 form the function of chlorophyll bodies for the dual organism, while the threads 

 of the fungus provide the mineral constituents of plant food. The alga, 



