164 M. Chevely Rayner. 
cells of the root, forming a system continuous with hyphae ramifying 
in the soil around the roots. Invasion of the root-cells is tolerated by 
the plant, which yields up a certain amount of nutritive material to 
the intruding hyphae. Some degree of parasitism on the part of 
the fungus in this phase may doubtless be inferred in the case of 
green plants like Listera and the majority of orchids, in which there 
is evidence that the root-cells yield a supply of carbohydrate to the 
mycelium. In the case of non-chlorophyllous plants, e.g., Neottia, 
which are equally dependent with the fungus upon organic carbon 
compounds in the soil, it is not clear that the fungal partner can 
benefit to any extent at the expense of the plant. 
The second phase is initiated by increased resistance on the 
part of the root-cell. The protoplast assumes the role of phagocyte, 
attacks the hyphae which have invaded and vegetated in it, and 
ultimately destroys and digests them. 
So much is clear from the researches named above and from 
those of later workers, who inferred a ‘ symbiosis ’ analogous to that 
which exists between the leguminous plant and its tubercle organism. 
The exact nature of this symbiosis has given rise to much speculation 
and can be more profitably discussed in the light of recent 
researches. 
It has usually been assumed that ability of the fungus to utilize 
organic carbon and nitrogen compounds is an important factor in the 
nutrition of the plant, especially in the case of non-chlorophyllous 
saprophytes like Neottia and Monotropa. Up to the beginning of 
the present century, however, there was no experimental evidence 
of the degree of interdependence of plant and fungus, nor was 
anything known of the life-histories or systematic positions of the 
fungi concerned. There are cases of endotrophic mycorhiza, 
moreover, in which the root-cells are invaded without such changes 
in the cell-structure as allow deductions to be safely drawn as to 
exchange of metabolic material. It is clear that interpretations of 
the physiology of the association based on cytological observations 
of plants in which such changes do occur cannot validly be extended 
to cover all the cases described. 
Speculations as to the evolutionary history of such relationships 
were also unprofitable until more knowledge was available as to 
the behaviour of infected as compared with uninfected plants; the 
time and source of infection ; and the growth of the endophytic 
fungus on different nutrient media outside the plant. Only when 
such evidence became available could a theory of symbiosis be 
formulated which would include all the observed cases and link 
