288 ADAPTATIONS AMONG. ALPINE PLANTS 



actually eat up the substance of the host plant itself. 

 If the leaves are attacked, the plant loses its power of 

 assimilating (p. 10). If the flowers or fruits are 

 attacked, as in the case of the Smuts on wheat, few 

 or no seeds result. The loss on crops of wheat, rye, 

 oats, and other cereals may amount to many millions 

 sterling in one country alone in a single year, when a 

 particularly bad epidemic of disease has prevailed. 



The commonest signs of disease are the presence 

 of spots, patches, or swellings on the leaves or shoots 

 of the host, often dark in colour, or again, brightly 

 pigmented. Little pustules of various colours also 

 break out on the surface of the stem or leaf. Often 

 the plant loses some of its green colour, and becomes 

 whitish, or shows some other obvious signs of ill 

 health. The number of plant diseases known is 

 almost endless. Each disease has its own symptoms, 

 and these depend on the nature of the life history of 

 the particular parasite in question. 



One special feature connected with many plant 

 diseases, is that the host has the power of accom- 

 modating itself to the disease in various ways. The 

 presence of a parasite, whether plant or animal, may 

 stimulate the host plant to adapt itself against the 

 invader by producing certain modifications of its 

 normal form. The growth of the branch or other 

 organ attacked may be either arrested or greatly 

 increased, or some other abnormality may result. In 

 some cases the host becomes reconciled, as it were, 

 to the parasite, and goes out of its way to provide for 



