2 26 
Jean Dufrenoy. 
remain attached long after fertilisation, but dry up. It may be 
observed also on all drying leaves. In such situations, instead of 
the fine light threads which grow in the living tissues of Arbutus. 
the fungus forms its mycelium on the surface of the dried leaves 
The hypha 2 of such a surface mycelium are large, of a brownish 
colour, and closely resemble those that grow in the earth outside 
the roots. There, however, they form numerous conidia or 
exogenous spores (Fig. 1,5). Such a mycelium is always present on 
the dried sepals which protect the young fruits; and it also exists on 
the surface of pollen-grains. Here, then, are two other ways in 
which the fruit may be inoculated. Ovules may be inoculated from 
the pollen-grains, or later on, when the fruit matures, it may be 
inoculated through its surface by the spores or the parts of mycelium 
which break off from the sepals. 
Relation of Endophyte to Host. 
Roots of Arbutus live in a medium teeming with micro¬ 
organisms ( Nectria). Some fungus, living (as a saprophyte) in the 
ground, penetrates the roots and extends into practically every 
part of the tree, even into the assimilating and reproductive tissues. 
As a rule, the cells of Arbutus do not react, and the presence 
of the fungus is difficult to trace. Infected rootlets, however, very 
often give rise to special symbiotic organs ( root-tubercles): here the 
plant cannot prevent invasion by the fungus, but checks its 
excessive propagation. This power of resistance may be defeated, 
and the fungus may become locally parasitic: it then causes the 
host cell to react and results in pathological tubercles. When the 
host tissues die, either in a normal way by external causes, or as a 
result of the parasitic attacks of the fungus, the fungus, being no 
longer provided with a favourable living medium, becomes a 
saprophyte, and forms spores. 
Whether the original relation is to be considered as sym¬ 
biosis or as parasitism depends upon the pathogenic reaction of 
the host. In most cases the cells of Arbutus do not react, and 
symbiosis may be considered to exist. Local reactions, however, 
prove that the fungus may also become a parasite. 
General, as well as specific infection is, however, responsible 
for some of the observed pathogenic reactions: fungal hyphas and 
bacteria aggregate around the root tubercles, rusts invade the 
shoot tubercles, and moulds grow on the dying leaves or sepals. 
The precise biological rdle of each is not yet known. 
