Recent Work on Endotrophic Mycorhiza. 171 
The mode of penetration of the tuber and the effect on the 
plant-cell is quite similar to that produced by the haustoria of 
parasitic flowering plants such as Cuscuta or Orobanche. Infection 
is limited in a radial direction and spreads tangentially only around 
the point of infection. In this infected region three zones may be 
distinguished. In the outer zone there is evidence of resistance 
on the part of the cell, but the general appearance of these cells 
suggests a ‘balanced mutualism.’ In the zone below, the fungus 
forms ‘clumps’ as in typical orchid mycorhiza; the nucleus and 
cytoplasm degenerate and are ultimately absorbed by the fungus. 
There is no evidence here of a beneficial symbiosis; the action of 
the fungus is entirely destructive, although the hyphae themselves 
ultimately disappear and apparently undergo auto-digestion. 
In the innermost zone of cells ‘the balance of power’ is 
readjusted, the cell cytoplasm increases in amount and the mycelium 
undergoes digestion by the plant. Kusano describes this inner 
region as “ the chief metabolic centre ” for the plant; food-materials, 
derived from the fungus, accumulate in the cells, serving ultimately 
for the development of the inflorescence and for the nutrition of 
the daughter-tubers. The rhizomorphs attached to the tuber are 
continuous with those in the soil, not uncommonly with branches 
bearing the typical Annilleria fructifications. The antagonism 
between the two components of this curious “ mycorhiza” is shown 
by the reciprocally destructive actions described above and also by 
the interesting fact that occasionally the resistance of the plant is 
inadequate and the fungus parasitizes the tuber in precisely the 
same way as it does in the case of potato. 
The more striking features of the association may be summarized 
as follows:— 
1. Young tubers are uninfected and continue to grow only so 
long as they are attached to the parent-tuber. Failing infection by 
rhizomorphs of Annilleria mellea, they never reach the flowering 
stage. The majority of these daughter-tubers and their offsets 
undergo a gradual diminution in size from lack of food-material, 
and die without flowering. 
2. Infection takes place from a branch of the rhizomorph, 
which infects the tuber locally. The infecting strand is continuous 
with the mycelium outside, and is the only means by which the 
tuber can take up food-material from the soil. 
3. The plant shows a remarkable degree of resistance to 
invasion by the fungus, the rhizomorph of which is parasitic in its 
mode of entry and in its behaviour in certain zones of the infected 
