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Bie 0 ae Structural and | Physiological Botany. 
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proved by a ‘number of observations and careful ee 
ments, that these diseases are contagious, and that healthy 
plants become subject to a particular disease when the — 
spores of that Fungus are sown upon them which always 
occurs in connection with that disease ; and that, finally, the 
best means for preserving or restoring the health of the 
plant are those by which the spores of the Fungus are- 
destroyed. This does not, however, prevent other Fungi 
occurring as accompaniments of the particular disease be- 
sides the one to whose attacks it is due. . 
Among the most important of the diseases of this nature 
to which cultivated crops are liable is the rust or mildew 
of cereals. ‘This term is appropriately given to a disease in 
which, at least at first, reddish masses of a brown rust-like 
' dust break out from the internal tissue of the plant. It is 
most common on barley, wheat, and oats, less so on rye 
and wild grasses. Since it chiefly attacks the vegetative 
parts, the leaves and succulent stems, and but rarely the 
seeds, the injury caused by it consists mainly in a deteriora- 
tion of the straw ; but the grains are also of inferior quality, 
when the parts which contain chlorophyll, and which con- 
sequently assimilate, are attacked at an early stage. ‘The 
species of mildew which infest cereal crops are mainly two, 
Puccinia graminis and P. coronata. ‘The very remarkable 
course of development of the latter will be afterwards more 
exactly described when speaking of Fungi. 
Smut shows itself first on the organs of fructification, the — 
epidermis of which is irregularly ruptured in a great number. 
of places, a black dust then appearing through the slits. It 
attacks the different parts of the flower in a very unequal 
degree. The whole of the parenchymatous tissue is often 
destroyed ; and this occurs to so great an extent in winter-— 
barley, that of the whole ear the common rachis of the in- 
“florescence alone remains, while frequently, as in oats, the 
seed only is destroyed, the pales remaining unaffected. 
Especially in the last stage of the disease, the masses of | 
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