194 THE STUDY OF PLANT COMMUNITIES ■ Chapter VIII 



includes only the relationship of intimately associated, dissimilar 

 organisms that live together to their mutual advantage. By append- 

 ing descriptive adjectives, the concept has been expanded by some 

 to include almost any relationship between organisms whether ac- 

 tually in contact or merely in competition with each other (e.g., 

 cattle grazing in a meadow would illustrate antagonistic nutritive 

 disjunctive symbioses 167 ). But the conservative interpretation rec- 

 ognizes only a few plant symbionts as significant in community 

 life. The intimate association of unicellular blue-green algae with 

 a fungus mycelium, termed a lichen, is an example of plant sym- 

 bionts that is familiar to all who have any botanical interest. 

 Lichens, however, can hardly be considered of general importance 

 in community relationships. Although they often play a part in 

 the establishment of communities on bare rock, they probably in- 

 fluence mature, stable communities very little. Fungi and bacteria 

 living symbiotically on plant roots are less noticeable but of far 

 more importance. 



Mycorhiza — When a root and the mycelium of a fungus grow 

 together, the fungus may form a feltlike layer around the root 



FlG. 91. Transverse sections of mvcorhizal roots of forest trees: (1) en- 

 dotrophic, (3) ecto-endotrophic, others all ectotrophic. (1 and 4) Psendo- 

 tsuga imicronata, (2 and 3) Pinus vmrrayana, (5) Popuhis tremuloides, (6) 

 Picea rubens.— After McDougall and Jacobs. 



1G8 



