68 
W. B. McDOUGALL 
Frank (4) first advanced the hypothesis that the ectotrophic 
mycorhizas represent symbiotic associations in which the fungus 
serves as a conveyor of food from the humus to the root. He really 
had no direct evidence of this, but conceived the idea principally 
because of the apparent similarity to' the lichens. This comparison 
of the tissues of the mycorhizas to those of a lichen is very apt, pro- 
vided that we recognize the vital differences between the two organ- 
isms. If the individual cells of the root were all separate plants^ 
the similarity of the mycorhiza to a lichen would be very striking, 
but those cells are merely minute parts of a complex organism. As 
soon as a single cell is separated from a root, it becomes a helpless, 
functionless body, and soon dies. Yet that is exactly what the 
mycorhiza forming fungus does to the cortical cells of the root. We 
have shown that in all well developed ectotrophic mycorhizas the 
external cells of the cortex are entirely surrounded and separated 
from each other by the fungus hyphae, while, in some cases, nearly all 
of the cortical cells are similarly involved. These cells, which are thus 
separated from each other and from the root, can certainly never 
function again as root cells. The root, therefore, cannot get anything 
from the fungus through these cells. The only way it could get 
anything from the fungus, in such cases, would be by the direct contact 
of the fungus with the central cylinder. 
Stahl (24) accepted Frank's hypothesis and attempted to prove it. 
After a large number of observations on plants of very diverse nature, 
he stated that a well-developed root system, and active transpiration, 
accompanied by the excretion of much water, and the presence in 
the leaves of an abundance of starch, calcium oxalate, and nitrates, 
characterize mycorhiza free plants. Plants with mycorhizas, on the 
other hand, he found to have a less active transpiration current, and 
the carbohydrates in the leaves are in soluble form. Stahl imagined 
a great struggle to be going on among plants growing in a humus soil 
to obtain the requisite amount of nutrient salts. Plants with a 
strong transpiration current can get along very well by themselves, 
but those with a weak transpiration current can get the necessary 
amount of food only by making use of mycorhizal fungi. On a similar 
basis he explains the usual absence of mycorhizas in habitats which are 
lacking in humus but rich in nutrient salts. The struggle there is not 
so great and the plants do not need mycorhizas. Also, according to 
this hypothesis, in a habitat which is abundantly supplied with water, 
