LICHENES 29 



LICHENES 



The immense number of plants belonging to this family 

 differ from fungi in possessing chlorophyll, and conse- 

 quently can assimilate inorganic matter. A lichen in 

 reality consists of a body formed of two distinct elements, 

 a fungus and an alga, the two living together and mutually 

 assisting each other in the procuration and assimilation of 

 food, a condition known as mutualism ; differing widely 

 from those cases where a parasitic fungus lives on another 

 plant, in which case the parasite has all the advantage, 

 and the host, or plant preyed upon, all the disadvantage. 



Lichens often form leafy expansions covering the trunks 

 of trees, and such are not parasites, although their presence 

 does not benefit the tree, especially when young. On the 

 other hand, many lichens vegetate in the living bark or on 

 coriaceous evergreen leaves, especially in the tropics, and 

 may fairly be considered as parasites. 



ALGAE 



Seaweeds are the best-known representatives of this 

 family, although numerous representatives live in fresh- 

 water streams, ditches, and ponds. All the members 

 possess chlorophyll ; nevertheless, some of the simplest 

 forms are parasitic on higher plants, while others live in 

 a condition of mutualism. 



MYXOGASTRES 



The Myxogastres or ' Slime-fungi ' were at one time 

 considered as belonging to the fungi, but are now separated 

 on account of important structural differences, and by some 

 are considered as belonging to the animal rather than 

 the vegetable kingdom. 



