COMPOUND ORGANISMS 193 
attracted to the root by substances which 
have a food value for its hyphe, just as para- 
sitic fungi are induced to enter the bodies 
of their victims. But in a _ mycorhizal 
association the cells of the root control the 
degree of invasiveness which the fungus can 
manifest, and not only so, but they often 
proceed to actually digest the fungus itself 
after it has flourished within them, and at 
their expense for a while. 
We have here, then, a beautiful example 
of two-sided parasitism, in which the final 
balance of profit very clearly lies with the 
flowering plant. It is practically certain that 
the fungus obtains some carbohydrate food, 
at first at any rate, but in return for this the 
plant acquires mineral substances in solution, 
which the fungus absorbs from the soil. 
A considerable number of flowering plants 
are unable to thrive unless their roots become 
infected in this way. This is especially true 
of orchids. Indeed, one of the great diffi- 
culties experienced in raising these plants 
from seed has been solved by supplying the 
young seedling, during its germination, with 
the fungus appropriate to it. And so close 
has the degree of association between fungus 
and flowering plant become, both in orchids 
and in many other plants, that neither 
can grow properly in the absence of the 
other. 
Now this intimate mycorhizal relationship 
is found to exist in all the flowering sapro- 
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