ON THE MYCORHIZAS OF FOREST TREES 
69 
the transpiration current is usually rapid enough so that plenty of 
mineral food can be obtained without the aid of mycorhizas. On the 
other hand, obligate mycorhiza forming plants are to be found, says 
Stahl, in habitats which are rich in humus or poor in mineral salts, 
since, with either of these conditions present, it is difficult for the 
plants to get sufficient mineral food. 
Schatz (21) carried out a number of cultural experiments in an 
attempt to prove Stahl's hypothesis with respect to the absorption 
of nutrient salts, but his first cultures gave negative results, and his 
later cultures, though giving positive results, are mostly too few in 
number to be convincing, and, judging from the photographs, were 
not very decisive. 
Yet, granting that such competition among plants exists, it is not 
clear that it has anything to do with the formation of mycorhizas. 
The natural habitat of most mycorhizal fungi is in humus soil. Very 
few grow in the habitats that are characterized by mycorhiza free 
plants, and it is obvious that, if the fungi do not occur there, mycor- 
hizas could not be formed. Moreover, the fact that healthy trees of 
mycorhiza forming species are sometimes found entirely free from 
mycorhizas, even when growing in humus soil, shows that the trees 
can get along very well without mycorhizas. Again, the time of year 
at which the mycorhizas are developed is against the hypothesis that 
they aid the higher plant in the absorption of mineral salts from the 
soil. The mycorhizas are developed and the fungi concerned are most 
active during late summer and autumn. At this time of the year the 
trees are relatively inactive and do not need so much food as earlier 
in the season, when the mycorhizas are absent. On the other hand, 
it has been shown by Preston and Phillips (19) that there is more food 
stored in roots, and so available for the fungus, at this time of year 
than in spring and early summer; and this is just the time when the 
fungus needs an abundance of food, for it is preparing to produce its 
fruiting bodies. 
There seems to be no good evidence, then, that the root gets any 
food through, or from, the fungus, the evidence indicating, rather, that 
it does not. On the other hand, there is no question but that the 
fungus gets some food from the root. Just how it does this is not 
known, since no haustoria penetrating the cells have ever been found, 
but it undoubtedly gets something of value from the middle lamellae 
of the cell walls, which it dissolves, and, since the nutrient solutions 
