MYCORHIZA 107 
farming operations, and the presence of Clover in 
pasture is a source of increasing wealth. 
Again, in the roots of most of our forest trees, both 
hardwoods and conifers, and of many other plants 
such as the Ericacee and Orchidacee, the root-hairs 
are replaced by minute fungi known as mycorhiza, 
whose branches take on the function of absorption, 
while the roots in turn absorb the material which the 
fungus collects. The fungus obtains from the roots a 
direct and convenient supply of carbohydrates; the 
host obtains from the fungus a ready supply of salts 
and of nitrogenous compounds. In the case of the 
forest trees and some other plants, the fungus forms 
a close felt around the roots; but in the Heaths, etc., 
it penetrates the roots, living in the cells and in some 
instances, as in the Ling (Calluna vulgaris), permeating 
the whole plant, even to the seed-coat, so that seed 
and fungus are sown together. Since the higher 
partner of the symbiosis cannot mature without the 
lower, this is an obvious advantage to the former, as 
the two develop together from the commencement of 
growth. Where the fungus is not present in the seed, 
the seedling has to rely on its presence in the soil. 
And so, if we wish to raise any of our common 
terrestrial Orchids from seed, we try to ensure the 
presence of the fungus by using soil in which the 
species has been growing already. 
The state of mutual dependence existing between 
seed plants and mycorhizic fungi sometimes ends in 
the higher organism ceasing to manufacture its food 
by means of green leaves, and depending wholly on 
the lower for its sustenance. This is the condition to 
which some of our Orchids have come, such as the 
