SYMBIOSIS 133 



the alga always moist. The mutual advantage of this 

 association is evident from the almost universal distribu- 

 tion of lichens. The lichen lives in conditions where 

 the fungus or the alga alone could never exist. It 

 flourishes at the limits of vegetation. The dominant 

 plant near the polar snows is the Iceland moss (Cetraria 

 islandica), a lichen, which is also found on lofty moun- 

 tains. Encrusting lichens are found on dry rocks and 

 walls where no other vegetation can grow. 



We have come across two other cases of symbiosis in 

 previous chapters : 



(1) Mycorhizas (p. 124). 



(2) Root-nodules in the Leguminosae (p. 95). 



It is possible also that many of the chemical changes 

 taking place in the soil, and hitherto referred to the 

 activities of soil-fungi (p. 96), may in reality be the 

 result of symbiotic activity, algae co-operating with fungi 

 to bring about the observed phenomena. 



Symbiosis occurs not only between plant and plant, 

 but also between plants and animals. The most familiar 

 instances, taking the term " symbiosis " in its widest sense, 

 are to be found in the relations which exist between insects 

 and flowers in pollination, and in a minor degree between 

 birds and fruit in seed- dispersal. Insect-pollinated flowers 

 attract insects with pollen and honey. In some cases the 

 flower provides shelter for their eggs and nutriment for 

 the subsequent broods e.g., Yucca and certain species 

 of Ficus (fig). But in pollination the relations between 

 the insects and the plant are temporary and often acci- 

 dental. They have arisen through the association of two 

 needs, one ever reacting upon the other namely, the 

 need of the insect for food, and the need of the plant for 

 pollination. A much closer association is that which has 

 been found to exist between certain plants and ants 

 myrmeeophily (Gr. myrmex, ant). Attention was first 

 drawn to this by Belt (The Naturalist in Nicaragua) in 

 his description of the marvellous bull's -horn acacia 

 (Acacia cornigera). The stipules of this plant take the 

 form of large hollow thorns, which harbour a species of 

 warlike ants. These protect the tree against the ravages 

 of leaf-cutting ants, which, in the absence of its martial 

 defenders, would very quickly strip the tree bare, of 

 leaves and permanently injure it. In return for this 



