r t 8 Ray tier.—Obligate Symbiosis in Calluna vulgaris. 
> 
Whether this association of hyphae and crystals is accidental, or 
whether it is one of the links in a chain of events which connects the 
metabolism of the plant with that of the Fungus, remains an open ques¬ 
tion ; but the presence of such relatively enormous accumulations of 
a calcium salt in the tissues of a plant believed to be unusually intolerant of 
lime salts in the soil raises many points of interest. 
The oxalic acid may be produced by the plant cells or by the Fungus; 
if by the former, it may be an indication of interference with normal 
cell metabolism, induced possibly by the increased demand upon the 
available oxygen supply. 
If the calcium absorbed by the plant from the soil, in the form of 
cajcium salts, is constantly in requisition for the neutralization of oxalic 
acid, the edaphic preferences of the plant for acid soils and those deficient 
in lime salts are puzzling. 
If, on the other hand, the calcium salt is derived immediately from the 
Fungus, it must be obtained either from the soil (which raises the same 
difficulty), or from some part of the plant tissues, e. g. the middle lamella of 
the cell-walls. 
The establishment of the fact that the relation existing between 
the plant and the Fungus is an obligate one permits the problem of the 
origin of the calcifuge habit to be stated in a new form, and hence 
provides a fresh point of departure for experimental research into this 
important phenomenon. 
Assuming that the Fungus forms and excretes oxalic acid—and 
possibly makes its primary attack upon the plant by this means—it 
may well be that the amount of calcium salts present in the plant at 
the moment of infection is a decisive factor in determining whether in¬ 
fection shall or shall not occur. 
If the amount is considerable, the plant will resist the attack of the 
Fungus, and, by its very resistance, cuts off all chance of growth on 
normal calcareous soils, the presence of the Fungus having become one 
of the first essentials to development on the part of the seedling. 
If, on the other hand, the plant cells are deficient in lime salts, the 
excess of oxalic acid can act, directly or indirectly, by preparing the 
way for the entrance of the parasite, symbiosis is established, and, so 
long as equilibrium is maintained, the plant flourishes. 
This working model of the conditions underlying the origin of the 
‘ lime-shy ? habit makes no claim to completeness, but, as already stated, it 
immediately suggests fresh methods by which to attack the problem 
experimentally. 
The excretion of calcium oxalate by Fungi growing in plant tissues or 
on artificial media is not uncommon ( 42 ), and an alteration of the calcium 
oxalate content of the cells of the host has recently been noted as one of 
