ON THE MYCORHIZAS OF FOREST TREES 
67 
Cannon (3) found that certain desert perennials annually produce 
a crop of fine roots, which he calls deciduous roots, at the beginning 
of the rainy season. During the dry season these roots all die. The 
annual mycorhizas really represent a second kind of deciduous roots. 
It must be pointed out, however, that, in the case of the mycorhizas, it 
is the infection by the fungus which determines whether a particular 
root shall become deciduous or shall develop into a perennial root, 
while the roots described by Cannon are all deciduous. 
5. Physiological Relations 
The question as to whether mycorhizas represent symbiotic associa- 
tions or parasitic associations has been much debated, and more or 
less evidence, applying to particular cases, has been presented on both 
sides. It is worth while in the first place to consider just what the 
difference is between symbiosis and parasitism. By parasitism is 
usually understood a condition in which one organism obtains nourish- 
ment at the expense of another living organism. Symbiosis has been 
defined as a condition in which two organisms live in intimate relation- 
ship with each other in such a way that both are mutually benefited 
by the association. In the case of plants it is usually taken for granted 
that this benefit has to do with obtaining food. In a case of perfect 
symbiosis, therefore, the two symbionts must get equivalent amounts 
of food from each other, and the interchange must be brought about 
in such a way that neither plant is harmed, by a loss of a particular 
kind of food, more than the other. It is probable that such an ideal 
condition never exists in nature. We cannot, of course, suspect any 
symbiont of altruistic motives in supplying another plant with nourish- 
ment. Plants do not give food to each other: they take food from 
each other. In other words, each sym.biont is a parasite on the other, 
and the difference between parasitism and symbiosis is only relative. 
Symbiosis is a double parasitism; a constant struggle for supremacy 
between two organisms. It is obvious, then, that a sort of association 
may exist which in some cases would rightly be called parasitism, and 
in other cases would be called symbiosis, depending on the relative 
potency of the individuals concerned. 
There are several hypotheses which might apply to the mycorhizas : 
the higher plant may be parasitic on the fungus; the fungus may be 
parasitic on the higher plant; the association may represent a true 
symbiosis; or the individual cases may vary from symbiosis to 
pa rasitism. 
