PAPAVER
somniferum.
CULTIVATION
Diseases.
Peronospora.
56
Extracted from the Dictionary of
Diseases of the Poppy Plant.
striking form as exemplified in those of 1874.75. The latter, its develop
-ment and effects, we will now briefly notice."
Mr. Scott next deals with the manner in which the mycelial disease
progresses. Starting from the first leaf on the seedling, he says, it ascends
(never descends) attacking in succession each cycle of leaves until the
whole plant is involved, the onward progress being temporarily arrested
between each new cycle.
"From the above summary of a series of careful observations, there
can, I think, be no question as to the correlation of periodicity in the
development of the mould and its foster-parent, each periodic invasion of
the former being strictly limited to a single leaf cycle of the latter. In its
limitation to the fully mature leaves, and in presenting well-marked
periods of rest and activity, the poppy-mould appears to differ altogether
from its ally. the potato-mould, in which, so far as I am aware, no such
phenomena have been observed. It is also important to note that the
poppy-mould does not necessarily extend wherever a green leaf matrix
presents itself. This, as it appears, is a clear proof that atmosphe
-ric influences have really little to do with the extension of the mould, but
that, on the other hand, it is largely, if not wholly, dependent on the degree
of maturation of the matrix; that is to say, on certain peculiar chemical
changes which occur in the juices of the nurse plant at that period, and
that these are alone favourable to any high degree of vital activity in
the mould. This is a matter of high importance. Hitherto, it has
been customary to correlate the sudden appearance and rapid extension
of various fungoid-blights to certain peculiar atmospheric conditions.
Now, there may be some such relation in the spread and extension of the
truly terrestrial or even epiphytal species; but I do think that careful
observations will show that this scarcely, if at all, holds in those of a truly
parasitic character, and especially such as the poppy and potato-moulds
,which are wholly dependent for their development on a supply of the juices
afforded by these plants in the period of their vegetation. As regards the
poppy-mould especially, I am convinced that its development and exten
-sion are largely dependent on certain changes in the juices of the maturing
leaves though I admit that there are grounds for the belief that the parti
-cular mode of development—that is, the dominance of the reproductive or
vegetative function—may be partly due to certain obscure atmospheric
conditions. This, of course, is an important influence, inducing, as I have
shown, in the one case a comparatively harmless, in the other a most mi
s-chievous, parasitism."
The above passages from Mr. Scott ' s account have been reproduced
since, though his conclusions may be subject to modification, his practical
observations of the progression of the disease are doubtless of much prac
-tical value. Subsequent to the publication of M r. Scott's Opium Manual,
Surgeon-Major D. D. Cunningham took up the subject of the Opium Blight
( Peronospora arborescens ), and through his careful microscopic studies
greatly advanced our knowledge of the structural peculiarities of the fungus
and its methods of penetration and destruction of the poppy tissues. Dr.
Cunningham, however, admits of his studies that "They refer solely to
the appearances presented by the parasite and the lesions resulting from
its presence as they occur in the leaves, as, partly owing to the difficulty of
procuring perfectly fresh plants for study and partly to want of time, the
examinations were almost entirely devoted to those points; and, even as
regards them, are confessedly imperfect. experiments on the infection of
previously healthy tissues not having been carried out for want of growing
plants."
D r . C u n n i n g h a m
f o u n d , h o w e v e r , t h a t a l l t h e s p e c i m e n s e x a m i n e d b y