NITROGEN FIXATION IN ERICACEAE 
M. CHEVELEY RAYNER 
(WITH FOUR FIGURES) 
Introductory 
Since the middle of the nineteenth century it has been known 
that plants belonging to the Ericaceae form mycorhiza of a char- 
acteristic kind. Further knowledge of the relations between plant 
and endophyte in this group has only recently been forthcoming. 
In 1915 RAYNER’ showed that the relationship in Calluna 
vulgaris is of a remarkable character, involving obligate symbiosis 
between the two organisms and a much more extensive distribution 
of the fungus throughout the green plant than had been suspected. 
As in Orchidaceae, root formation by seedlings is dependent upon 
early infection by the endophyte, failing which, development ceases 
and the plant perishes in the seedling stage. Unlike the condition 
in Orchidaceae, infection at the appropriate moment is provided for 
by the presence of mycelium on the seed coat, a condition ensured 
by the distribution of the endophyte throughout the vegetative 
tissues and eventually within the ovary chamber. These facts 
have been demonstrated with certainty in Calluna, and the evi- 
dence points to a similar condition throughout the family. Thus 
ovarial infection has been reported for many species in all the sub- 
orders of Ericaceae, and the inability of seedlings to complete their 
development without infection has already been proved for a 
number of these. 
In such remarkable associations between flowering plants and 
fungi as are found in the orchids and in Ericaceae, it is of great 
interest to learn the exact nutritive relations between the symbi- 
onts. In orchids there is ocular evidence of digestion of mycelium 
by the cells of the root, and it is clear that by this means the plant 
can draw indirectly upon organic compounds of carbon and nitrogen 
in the soil. In the chlorophyllous orchids the endophyte can utilize 
1 Rayner, M. C., Obligate symbiosis in Calluna vulgaris. Ann. Botany 29:97- 
153. 1915. 
Botanical Gazette, vol. 73] [226 
