290 GENERAL BIOLOGY 



parasite draws its sustenance from the tissues of the 

 crab by the penetrating, root-like ingrowth men- 

 tioned. The latter eventually pervades every part 

 of the crab's body, and of course soon causes its death, 

 but not before the bag-like Sacculina has matured 

 its eggs, and thus insured the possibility of another 

 generation. 



Association among Plants. Nearly all the de- 

 grees of association that exist among animals are 

 also paralleled in the plant world, modified, of course, 

 by the very different conditions that obtain among 

 plants. Plants of the same species are often found 

 associated together, but this is usually the result of 

 accident or of similar favorable conditions, and is, 

 of course, not comparable to the gregariousness of 

 many animals. On the other hand, plants of dif- 

 ferent species are sometimes associated together in a 

 true symbiosis. One of the most remarkable of 

 these is the association between the roots of legu- 

 minous plants and the nitrogen-fixing bacteria, 

 which has already been described in a previous 

 chapter. (See p. 76.) A somewhat similar sym- 

 biosis is found between the roots of trees^ such as 

 oak, walnut, apple, maple, etc., and an investing 

 fungus. The latter covers the root as a mantle or 

 sheath or else penetrates the cells of the root cortex. 

 The delicate branches (hyphae) of the fungus function 

 as root-hairs for the host-plant. 



Lichens. An equally remarkable association be- 

 tween two diverse forms of plants is that of a fungus 



