I 62 
M. Clievely Rayner. 
the physiology of plants and need not here be further elaborated. 
They mark the first stage on a path of enquiry since traversed by 
many investigators. 
The object of this review is to consider certain recent researches 
on endotrophic mycorhiza, to note their bearing on current con¬ 
ceptions of the physiology of the relationship, and to take stock 
more especially of their significance in the region of experimental 
ecology. The subject has hitherto received inadequate treatment 
in botanical text-books. Observations have accumulated and have 
been recorded as isolated details in the life-histories of certain 
plants rather than as expressions of a fundamental biological 
phenomenon common to plants and animals. It is felt that the 
advance in knowledge resulting from a study of recent researches 
into the physiology of the subject must materially alter this con¬ 
ception and will focus the attention of botanists on the physiological 
similarity of the phenomena underlying these remarkable associ¬ 
ations with those already revealed by a study of infectious disease 
in the realm of animal pathology. Full historical reviews of the 
earlier literature on mycorhiza with very complete bibliographies 
have been published in the papers here reviewed; and these render 
it unnecessary to do more than briefly indicate the general history of 
research on the subject from the time of Frank up to the beginning 
of the present century. 
The value and importance of Frank’s work can be judged by 
the position still assigned to it in botanical literature and by 
appreciation of the fact that his general interpretation of the 
physiology of the association has been a starting-point for many 
subsequent researches. 
Frank clearly recognized the possibility of specialized relations 
between plant and fungus in certain groups. He drew attention 
to the necessity for comparison of infected with non-infected plants 
in order to determine the exact nature of the relations in any given 
case, and he pointed out that material was not then available for 
the purpose of such comparisons. 
It is significant of the difficulties that beset such researches 
that this region of the mycorhiza problem remained so long 
unexplored, and that the exact relations between plant and fungus 
in two of the families mentioned by Frank as likely to prove ‘ special 
cases,’ namely, Orchidaceae and Ericaceae, have only recently been 
elucidated. The demonstration of such specialized relations in 
these two groups and the recognition of their true nature are an 
important contribution to our understanding of endotrophic myco- 
