BIRCH TREES IN EPPING FOREST AND ELSEWHERE. 283
The lenticular tubercules consist of black pseudo-parenchyma situated
beneath the periderm. Owing to their rupturing the periderm at the highest
point of their concave surface, a small depressed aperture is formed."
The diagnosis of the disease of the birch is almost precisely
that of Alnus viridis. In each case leafy branches are infected ;
the fungus is with difficulty discerned on the living wood ; the
progress of the diseases is indicated by colour ; its highest
development is only on the dead wood. Valsa and Melanconis are
very closely allied, as the synonyms quoted above go to prove,
and as everyone knows, Betula and Alnus are of the same natural
order.
Granted that Melanconis stilbostoma causes the death of the
trees, we can account for the rapid spread of the disease in so
many districts several miles apart. The conidiospores carried
by the wind, are ready to attack any suitable trees with which
they may come in contact, and such a disease as this will be
most difficult to cope with on account of the great numbers of
trees already attacked.
Other causes have been suggested to account for the death
of the trees, as drought and overcrowding ; but neither of these
bear the test of investigation : for instance, isolated trees like the
one in the churchyard at Buckhurst Hill have suffered, and
those growing by streams and on marshy localities have died in
great numbers.
There are many questions that suggest themselves to which
no answer is as yet forthcoming. For instance, Melanconis
stilbostoma has long been known as a saprophyte ; has it gradually
assumed the habit of a parasite, or was it always a parasite,
which has at length become a very formidable tree destroyer ?
It is possible that its parasitic nature has up to the present been
unobserved. We have in Cladosporium epiphyllum a fungus widely
known as saprophytic, but quite recently it has been shown to
be the cause of disease and death of Pyrus japonica (G. Massee,
Kew Bulletin, Jan., 1899). This fungus occurs on the living
leaves of Betula alba, but the fact is not mentioned in Sarcardo's
list of over two hundred species of fungus, known to attack
various parts of this tree.