94 



PHYSIOLOGY. 



plant fits so clnsely to the rock on winch it grow^ that it seems men^ly tc, 

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

 trees there appears to be to the eye < )iily a Ycr\' flight discoloration of the bark 

 of the trunk, with here tnd (Iicrr tin- A trl -t colon-'l points where fruit bodies 



Fig. 84. 

 Fmnd i.>f UcIilmi (pultigera), slmwing rliizoids, 



are formed. The most curious thing about them is. h'_>wever, that while they 

 form plant boibcs 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 -wliich, 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 tlie leafdike forms known as peltigera, which grows on 

 damp soil or on the surfaces of ba<lly decayed logs, we see that the plant 

 body is flattened, thin, crumjiled, and irregularl}' lobed. The Color is dull 

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

 mottled with bniwn. es])ecially the oldi-r jxirtions. Here and tliere on the 

 under surface are quite long slender blackish strands. Idiese are conij^nsed 

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

 and for tlie purpose of supi)lying the plant body with mineral substances 

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

 the leafdike portion ..[ a lichen as slmwu in hg. 85, we shall see that it is 

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

 contain entangled green cells. The colorle--s threads arc tln.ise 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 consl itnenls of pkmt food. The alga, 



