Medical Officers of the Army of India.
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form of an impalpable powder which readily adheres to the healthy grains and
to the leaves and stems of the same and surrounding plants.
These thus come to be covered with spores which are capable of retaining
vitality for periods of years, so long as they are kept dry, and then of wakening
up to germinate and serve as a source of renewed infection where their products
come into relations with young seedlings of the host-plant. This being the case, it
is evident that one most important factor in securing a recurrence of the disease
must be the employment of grain which is polluted by adherent spores as seed-
corn, unless some special measures are adopted which are calculated to destroy
the vitality of the spores without affecting that of the grain. If, therefore, grain
derived from a crop affected by the disease must be employed as seed-corn, the
practice in Europe is to immerse it for a period of about twelve hours in a solu
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tion of sulphate of copper as a reagent which effectually kills the spores without
affecting the life of the grain. After having been treated in this fashion, the
grain is simply well washed in water and then carefully dried, and practical
experience has demonstrated that the procedure does much to limit the preval
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ence of disease. But as the spores do not adhere merely to the grain, but also
to the leaves and stalks of the plants, all straw coming from an infected area
must be dangerous, more especially where it enters into the constitution of
manure in which the spores will encounter all the conditions for germination and
subsequent luxuriant production of the conidia which, when they in their turn
germinate, provide the direct means of infection. The straw from any infected
crop should, therefore, not be employed in manure, and the residual stubble
remaining in the fields ought to be, as far as possible, destroyed. By the adop
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tion of measures of this nature the occurrence and extension of such blights
may to a great extent be controlled, but where either crops of like nature are
cultivated year after year in the same sites, or where the parasite is not limited to
cultivated hosts, but occurs among certain individuals of the natural vegetation
of the area, it must be impossible to obviate completely the possibility of
renewed infection. Under such circumstances the soil must remain constantly
liable to contain varying numbers of spores, and the only remedial measure which
can be attempted is to endeavour to promote the rapid growth of the seedling
host-plants as much as possible, for we know, thanks to Professor Oscar
Brefeld's brilliant series of observations, that, even after the mycelium of the
parasite has gained access to the tissues of the host, no harm will follow unless
it gains access to the special site in which its further development normally occurs,
and that it is incapable of travelling far in search of this. Where, therefore, as
in the case of the rice-plant, the site for the ultimate evolution of the parasite
lies in the floral organs, it is only at a very early stage in the life of the host that
the intrusive mycelium, with its very limited capacity for extension, will be able to
reach its aim,— i.e., the embryonic tissue contained in the very apex of the shoot
O