128 Rayner.—Obligate Symbiosis in Calluna vulgaris. 
mortelle et celui des plantes qui jouissent d’une immunite complete * 
(loc. cit.). 
A similar interpretation may perhaps be suggested by the condition 
in Ericaceae, many of the members of which have solved the problem of 
growth upon the poorest and most unpromising soils, but have solved it at 
the price of their independence. 
A study of these delicately-balanced relations inevitably provokes 
comparison with phagocytosis and the phenomena of immunity as observed 
in animals. 
The recent work of Fellmer (50), and of Wendelstadt and Fellmer (51), 
is suggestive in this connexion as indicating how the specificity shown in 
relations between a vascular plant and a Fungus, such as those described 
above, or possibly in symbiotic associations in general, can be brought into 
line with the facts of immunity and reaction to immune sera in animals. 
These authors have shown (i) that plant extracts (‘ Eiweissstoff’), when 
injected into animals, produce specific precipitin reactions, with anaphylactic 
phenomena quite analogous to those produced by the injection of animal 
sera; and ( 2 ) that similar specific reactions are given by animals to extracts 
of Fungus protoplasm prepared in the same way. 
The attempt to use the latter to produce immunity in the higher plants 
against attacks of Fungi, although theoretically possible, has not yet proved 
successful in practice. 
Only investigation of special cases can demonstrate with certainty the 
exact relation between the degree of development of the Fungus in the 
ericaceous plant and the well-being of the latter for any given conditions 
of growth, or provide a key to the soil preferences of these plants, both in 
the field and under cultivation. 
It is unlikely that observed peculiarities in this respect can always be 
explained as consequences of the xerophytic habit, with the decreased 
transpiration current and retarded absorption which this entails. 
I am indebted to Dr. R. Stenhouse Williams for testing the sterility of 
seedlings used in inoculation cultures, and to Professor Keeble for helpful 
criticism of this paper. 
The expenses of the research have been defrayed by a grant from 
University College, Reading. 
APPENDIX. 
5 . The Endophyte in pure Culture. 
1 . Rice and tap-water. 
Thirty days culture. Vigorous superficial growth ; mycelium snow-white at first, 
becoming greyish brown. The medium slowly changes colour from yellow to dark 
brown. 
Six weeks culture (PI. VI, Fig. 19 ). As last, but many small dark dots on the 
