OF THE FUNGI. 121 



Rome 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 ' Peronospora clevastatrix ' 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 

 ? mildew,' belonging to a genus Erysiphe, the members 

 of which work fad 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 Pucciniag (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 ' smut,' 

 (Ustiiago segetum), a dust-like agglomeration of minute 



