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 Leguminose (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, alge 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— 
myrmecophily (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 corngera). 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 
