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among plants, the author shows that these fungi must have a somewhat 
acid medium to grow upon; yet they require a sugar of some kind, pref¬ 
erably glucose, and asparagin or peptone may be advantageously offered 
as soluble nitrogenous foods. It is also true of these fungi that their 
optimum temperature for oxygen respiration is considerably lower than 
for higher plants, and unlike them, they require no light for their 
healthy growth. Dull, damp weather and a saturated atmosphere, so 
injurious to higher plants if long continued, decidedly favor the growth 
of fungi. 
u Consequently,” he says, “the very set of external circumstances 
which make the host-plant least able to withstand the entry and devas¬ 
tation of a parasitic fungus like Botrytis , at the same time favor the 
development of the fungus itself.’ 7 
A number of examples are given of epidemic diseases caused by Bo¬ 
trytis both artificially induced and occurring in nature. Of the latter 
the lily disease so destructive in England during the very wet, cold, 
and dull summer of 1888 is given as an example. Prof. Ward has al¬ 
ready published a full account of this epidemic in the Annals of Botany, 
Yol. 2, 1888, pp. 319-376. 
The peculiar fact that the conidia of Botrytis on germinating produce 
germ tubes unable to penetrate living plant tissues is noted as well as 
the remarkable discovery that successive generations of parasitically 
or semi-parasitically nourished Botrytis acquire different powers of in¬ 
fection, becoming each time more powerful in the cases studied. 
The last section of the lecture contains a summary of the factors of 
an epidemic, and this is of such general interest that it is quoted here 
in full. 
It will be clear from the foregoing that in the case of an epidemic fungus disease, 
such as we have been considering, there are several classes of factors to be regarded, 
and I may sum up the chief points somewhat as follows: First, we have the normal 
healthy host-plant, with all its hereditary (internal) and adaptive peculiarities; sec¬ 
ondly, we have the parasitic fungus, also with its disposition. Then we find, thirdly, 
that, apart from its inherent powers of variation, the host is subject to variable ex¬ 
ternal influences during its life, which may produce such changes in the cell-walls 
and contents, &c., that the plant approaches nearer and nearer the limits of health, 
wide as we may regard these. On the other hand, we have, as a fourth consideration, 
the parasite also varying under the influence of changes in the factors of the en¬ 
vironment, and its variations may, of course, be also dangerous to its welfare, but 
they may, on the contrary, be in such directions that it is enabled to profit by the 
counter-variations of the host. When the combined efforts of the physical environ¬ 
ment are unfavorable to the host, but not so or are even favorable to the parasite, 
we find the disease assuming a more.or less pronounced epidemic character. 
It is not pretended that we have here a totally new idea, because it has long been 
known that some organisms which bring about parasitic diseases do vary in the in¬ 
tensity of their effects, and can be made to do so artificially, and we know that some of 
the most brilliant results in biology have been obtained in connection with certain 
lower organisms; but I have simply sought to show some of the links in the chain of 
causes and effects in the definite case of certain epidemic diseases of plants produced 
by the parasitism of some of the more highly developed fungi, and this, I think, has not 
