332 FOUNDATIONS OF BIOLOGY 



for each other's existence, are termed SYMBIOTIC. A familiar 

 illustration is the common green Hydra (Hydra viridis) 

 which owes its characteristic color to the presence of a large 

 number of unicellular green plants which live in its endoderm 

 cells. The products of the photosynthetic activity of the 

 plant cells are at the disposal of the Hydra and the latter 



Fia. 179. The formation of a Lichen, Physcia paratina, by the combination of an 

 Alga and a Fungus. A, germination of a Fungus spore (sp), whose filaments are sur- 

 rounding two cells (a) of the unicellular Alga, Cystococcus humicola. B, later stage in 

 which spores have formed a web of filaments (mycelium), enveloping many algal cells. 

 Magnified about 400 times. (From Abbott, after Bonnier.) 



in return affords a favorable abode and the material neces- 

 sary for the life of the plant. 



A far more striking example of symbiosis is afforded by 

 Lichens which represent intimate combinations of various 

 species of Fungi and Algae. (Fig. 179.) In each case the 

 Fungus supplies attachment, protection, and the raw mate- 

 rials of food, while the green Alga performs photosynthesis. 

 Each can live independently under favorable conditions, but 



