THE DISEASES OF PLANTS 287 



are, as a rule, the more numerous, as well as the 

 more disastrous. The subject of plant disease is too 

 difficult and conn)licated to be discussed here, except 

 in the barest outline. We may, however, learn how 

 to recognise the conspicuous signs of disease so 

 commonly to be met with in the Alpine zone. 



We will first notice the effects of certain diseases 

 due to the presence of parasitic plants, living on or 

 wuthin the host, as the plant attacked by them is 

 termed. These parasites nearly all belong to the 

 group of Fungi, a race of plants consisting of threads 

 of cells, destitute of green colouring matter or chloro- 

 phyll. The Mushroom is one of the higher and more 

 advanced of the Fungi, as regards organisation. The 

 Moulds, Mildews, and Rusts are other examples, 

 lower in the scale. Some of them are saprophytes, 

 living on dead and decayed vegetable or animal 

 matter. Others are parasitic on or in living tissues. 



Some fungal parasites may only infect certain 

 portions of the host plants, such as the leaves, at any 

 rate during some stages of the disease. They are 

 thus local in their effect. They injure certain portions 

 of the leaves or the stems of the host plants, but 

 they do not, as a rule, imperil their existence. But, 

 more commonly, the injuries are more widely spread 

 throughout the plant. The very small threads of 

 colourless cells — often quite invisible to the naked 

 eye unless many thousands of them are matted 

 together into a felt, as in the Moulds and Mildews — 

 not only rob the host of its own nourishment, but 



