OF THE FUNGI. 
121 
some destructive Fungus are developing themselves. All 
such leaves he will gather and place in his book for 
future examination ; although, perhaps, there is nothing 
beyond this visible to the eye, to show what mischief is 
at work. By way of illustration, I will refer to the 
disease which so frequently seizes on the leaf of the potato 
during the summer. Its first attacks escape the vigilance 
of even the most experienced eye. Soon, however, a 
chemical change begins to take place in the Chlorophyll : 
the hitherto green leaf assumes a yellowish-brown tint, 
and is finally enveloped in a layer of white cobweb-like 
threads ; all of which is due to the presence of a mould, 
known to botanists as the 4 Peronospora devastatrix ’ of 
De Bary. Another minute Fungus may be observed in 
the form of a pale spot, which gradually envelops the 
leaf on which it is seated with a delicate web ; and a very 
close inspection will detect tiny black dots scattered among 
the meshes. These are the conceptacles, or capsules, en- 
closing the spores ; and the whole mass is the well-known 
4 mildew,’ belonging to a genus Erysiphe, the members 
of which work sad destruction among roses, hops, peas, 
and numerous other plants. 
Wherever, in a word, the eye of the collector detects 
an unnatural colour in a leaf, or a diseased appearance in 
a stem, it is worth his while to examine the sickly part, 
as the chances are greatly in favour of the evil being due 
to the baneful action of some Fungus. Thus the pedicel 
of the thistle, the leaf of the hawthorn, the ripening stem 
of the wheat plant, &c., are frequently swollen and dis- 
coloured by iEcidia and Puccini® (Plate xxn. 106), which 
have worked their way into the cellular tissue, and are 
rapidly destroying it. 
But it is not the stems and leaves alone that are subject 
to the attacks of these active assailants. It is rare to stroll 
through a field of standing corn late in the summer, 
without finding traces of that terrible pest, the 4 smut,’ 
{Ustilago segetum), a dust-like agglomeration of minute 
