ORGANIC RESPONSE 291 



and an alga, in the structure known as lichens. 

 Lichens are found incrusting rocks and tree-trunks 

 and are frequently beautifully colored. They consist 

 of a fungus mycelium, surrounding various uni- 

 cellular or filamentous algae. The alga, by virtue 

 of its chlorophyll, synthesizes carbohydrates, which 



FIG. 105. The building up of a lichen (Physcia paratina) out of the 

 alga and fungus; A, germinating ascospore (sp) ; the filaments of the 

 fungus have seized upon two cells (a) of Cystococcus humicola; B, more 

 advanced stage ; sp, ascospores which have produced a web of filaments 

 (hyphae), enveloping the algal cells in every direction. Magnified about 

 400 times. (From Scott, after Bonnier.) 



the fungus appropriates. On this account the 

 condition is sometimes referred to as helotism, 

 since the relation is that of master and slave. 



Parasitism in Plants. As in animals, plants 

 are, in some cases, parasitic upon other plants. 



