40 MacDougaL — Symbiotic Saprophytism. 
between the two, is not yet above the horizon. Presumably 
the immersion of the free portion of the mycelium in a sub- 
stratum rich in available food is a pre-requisite in symbiosis 
of this character. Given then a fungal body with one portion 
immersed in a nutritive substratum and another in or on 
the tissues of a higher plant, it seems reasonable to suppose 
that certain substances would be most easily obtainable from 
the soil and others from the body of the higher plant. The 
same reasoning may apply to the higher plant, and it may 
be supposed to withdraw from the fungal protoplasm sub- 
stances which may not be so easily obtained elsewhere by 
the laws of absorption. The deprivation of the Fungus of 
a nutritive substratum would certainly result in parasitism. 
As may be seen by reference to the anatomical details 
given by previous investigators, the protoplasm of the two 
symbionts is generally very closely united, thus avoiding the 
necessity for the special adaptations mentioned above. 
The chemical interchange between the symbionts must be 
that which would result from the capacity of the Fungus 
to assimilate compounds of nitrogen poor or lacking in 
oxygen, and the superior ability of the higher plant to 
carry on metabolism of the carbohydrates, in accordance 
with Groom’s well-founded conclusions (13). 
The Fungi entering into the formation of mycorhizal 
structures are comprised in the Oomycetes, Gasteromycetes, 
Hymenomycetes, and Pyrenomycetes, an accident of habitat 
rather than morphological constitution so far as present in- 
formation goes. These Fungi are capable of an indepen- 
dent existence outside the body of the higher plant. The 
period of existence of the portion of the mycelium in contact 
with the other symbiont may be identical with that of the 
absorbing organ ; may exceed it as the writer found in 
Aplectrum and Pogonia, and Groom in Thismia (13) ; or 
the mycelium may perish early, thus hastening the death of 
the absorbing organs, as Mangin concludes in reference to the 
ectotropic mycelia of forest-trees. (26). 
The hypliae of Fungi forming ectotropic mycelia show 
