ON THE STUDY OF MICRO-FUNGI. 365 



Herb, for instance, they will cover the whole under surface of the 

 leaf. In some cases the fungi appear as black, brown, orange, or 

 reddish patches or spots, and in other cases as a fine black or 

 golden powder peppered over the leaf. Then we have species 

 which adorn the backs of leaves like spores on the fronds of ferns 

 species which give the plants the appearance of having been well 

 sooted, and species which invest the stems and leaves with a 

 covering as though enamelled with a coat of white paint. We 

 shall find fungi which follow the lines of venation, and fungi which 

 spread themselves quite independently. On the Dog's Mercury 

 we shall see the parasite ( Uredo confluents) slowly destroying the 

 leaves on which it is established ; and on the Nettle we shall often 

 notice how the Cluster Cup (Vadium urticce) irritates the 

 tissues and causes the stem to swell and to become much bent 

 and distorted. 



Of course, there are species — such as Puccitiia calthce on the 

 Marsh Marigold — which are rarely met with, and species — such as 

 the jEcidium on the Coltsfoot — which are very common. In one 

 locality a certain fungus may be exceedingly plentiful, whereas in 

 another place, although the host plant is common enough, it may 

 not be possible to find a single specimen of it. These parasites 

 make a host of all manner of trees and plants — the oak tree, the 

 rose bush, and the tender moschatel alike forming a habitation for 

 them. But it does not by any means follow, that because a fungus 

 has been recorded as growing upon the young leaves of the oak in 

 one part of the country, that therefore we are likely to find speci- 

 mens of it wherever we find an oak tree \ nor should it always be 

 taken for granted that because we have found a certain fungus in 

 one locality, that all the host plants of the same species which are 

 growing even comparatively near at hand will also be infected with 

 the fungus. The case of the ^cidium on the Daisy before men- 

 tioned illustrates this, as, although there were plenty of the same 

 plants growing around the spot where the three diseased leaves 

 were found, yet all of them appeared to be perfectly free from the 

 fungus. 



What causes affect the distribution of the fungus and lead to its 

 appearance in one place and not in another, and how it comes to 

 pass that the spores which grow upon, say, the Primrose will not 



