126 Rayner.—Obligate Symbiosis in Calluna vulgaris. 
As a general statement affecting all members of the Natural Order 
Ericaceae, the conclusions expressed in the passage quoted above can no 
longer be accepted. 
It is true that an obligate relationship with the mycorrhizal Fungus 
cannot, at present, be predicted with certainty for every member of the 
group, but, in view of the fact that some species of Vaccinium (p. 124) show 
ovarial infection of the kind described for Calluna , a repetition of Stahl’s 
experiments, under conditions in which negative evidence regarding infection 
of the roots can be more satisfactorily tested than by microscopic examination 
only, is required. 
Recent work by Russell ( 45 ), on the sterilization of soil by heat and 
antiseptics, appears also to have a bearing on the interpretation of the 
results in these and similar experiments carried out by Stahl. 
The condition described for Calluna does not seem to have an exact 
counterpart among mycorrhizal plants. 
The dependence of the plant on the fungal symbiont is paralleled, 
if not exceeded, among the Orchids, in some species of which development 
of the embryo ceases at an early stage, unless infection occurs. In the 
Orchids, however, the endophyte is strictly confined to the non-chlorophyllous 
tissues, and this restricted distribution is not accidental, for Bernard has 
shown that portions of the stem of some Orchids have a poisonous effect 
upon the mycorrhizal Fungus of the same plant. In cultures, the fluid 
diffusing from these tissues killed the hyphae ; heated to 55 0 C., the toxic 
properties disappeared, from which it was inferred that the poisonous 
substance was probably of the nature of an enzyme ( 46 ). 
While also exhibiting complete dependence upon the Fungus at an 
early stage in the life-history, Calluna has an advantage over the Orchids, 
since the seeds are insured against the risk of non-infection—no small 
advantage in the case of small, light seeds, distributed by wind. This 
advantage would seem to be counterbalanced by the presence of mycelium 
of a facultatively parasitic nature in the tissues of the shoot. 
There is no evidence for nitrogen fixation from the air by the Orchid 
Fungi, but it is usually believed that the Orchid plant obtains nitrogenous 
food material from the Fungus by the digestion of mycelium in specialized 
cells (‘ Verdauungszellen ’). 
It is now suggested that Calluna and its allies have solved the nitrogen 
problem in a different way. 
The toleration by the plant of mycelium in the intercellular spaces 
of the leaves continuous with that on the outside of the shoot, and the 
ultimate relations of the hyphae with the mesophyll cells, point to the possi¬ 
bility of the fixation of atmospheric nitrogen in some degree by the 
Fungus. 
Indirect evidence favouring the same view has already been cited 
