BALFOUR—NEW SPECIES OF RHODODENDRON. 145 
The under-leaf indumentum here as in so many species possess- 
ing such a covering formed of hairs is penetrated in all directions 
by the mycelium of a fungus. In species which have the hairs 
of the indumentum agglutinated for instance Rh. Clementinae, 
G. Forrest, Rh. aganniphum, Balf. f. et Ward, the fungus-threads 
are so many they conceal the form of the hairs through which 
they spread. In members of the Sanguineum series the fungus- 
threads are dark brown in colour and become so numerous as 
to change the colour of the indumentum layer from a bright grey 
to almost black. The mycelium may be traced passing into 
the interior of the cells of the leaf without deforming them in 
cases which I have examined and the whole manner of occurrence 
of it raises the question for what purpose is it there? Is this 
a case of ordinary parasitism of fungus upon host or is there 
commensalism? Of all families of Dicotylous plants the 
Ericaceae is one of the most commensal. Not only is there the 
endotrophic relation of fungus in the root but as has been so 
happily shown by Miss Rayner * the fungus may penetrate 
the whole body of the plant and entering the seed so ensure 
its presence as an adjuvant to the young plantlet from the out- 
set of its extra-seminal life. The elements of construction in 
the leaves of so many of these rhododendrons suggest that an 
investigation from the standpoint of a possible commensalism 
may give interesting results. What the fungus is I do not know. 
The form of the mycelium is different in the leaves of different 
species of Rhododendron. There are fungi on the leaves and 
elsewhere in rhododendrons which are clearly simple parasites 
deforming the tissues and producing conidia and spore-fructifi- 
cations on the surface of the organ attacked, and they seem dif- 
ferent from those to which I refer as traversing by their mycelium 
the indumentum, wefting together its hairs, and sometimes 
through the abundance of their threads making difficult the 
recognition of the exact hair-form of the indumentum. If the 
mycelium is not present on all leaves, as seems to be the case, 
that is no valid objection to the idea of commensalism where 
it is present. We know that mycorhiza may develop in any 
plant if the conditions call for it, we know also that in individual 
plants some roots may become mycorhiza others adjacent 
showing no relation to the fungus, and therefore should there 
be here a mycophyllon its occurrence may be as sporadic as is 
that of mycorhiza. . 
* Miss Rayner in Annals of Botany, xxix (1915). 
