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156 illCEOSCOPIC FUNGI.
oiiion_,
but appears on otlier allied species of AUlinii
(to wliicli the onion belongs). The threads form
large patches or blotches on the leaves, and sometimes
cover them entirely. It very much resembles
the turnip mould, from which the form of the spores
considerably differs. This is the same species as
that described by Caspary, and afterwards by
under the name of P. destructor.
Berkele}',
Lettuce Mould.— A very similar mould {Ptrono^
spora gangJiformis) is sometimes very common in
spring on the under surface of the leaves of the cultivated
lettuce, appearing in definite white mouldy
spots. By reference to the figure of a portion of a
thread magnified (fig. 265),
it will be seen that the
peculiar form of the tips of the branchlets evidences
the distinctness of this species.
The oospores are small, globose, and of a yellowish
This mould is bv no means confined
tawnv colour.
to lettuces, but has also been found on species of
ragwort, sow-thistle, nipplewort, endive, and other
composite plants and has from time to time
;
received numerous names, which it is unnecessary
to enumerate.
Taee Mould.— The under surface of the leaves of
tares, and sometimes also of peas, is liable to attack
from an allied species of mould [Peronospora
Vicice). In the spring of 1846 it appeared amongst
vetches in some districts to such an extent as at
one time to threaten the destruction of the crops
;
but a succession of dry weather at once abridged