210 DISEASES OF PLANTS 



grow to only half or a third their normal length, but are 

 two or three times the normal thickness at the base. These 

 short thick leaves give to Monterey pine twigs a very 

 curious appearance when there is extensive galling. One 

 of the effects of this in the case of Monterey pines is the 

 failure of the branch, and often of the whole tree, to make 

 the usual amount of wood; hence it is seen that the efifect 

 of this disease is not limited to the immediate point of 

 injury. 



Thrips, '' Red Spider. " — Pear and other fruit trees 

 are subject to attack by small insects such as thrips, and 

 by " red spider," etc., which cause true disease and not 

 simple mechanical injury, for the trees which these in- 

 sects attack yield less than the normal amount of fruit. 



PLANT PARASITES 



Fungi. — The fungi form a very large division of the 

 vegetable kingdom. They have in common the one qual- 

 ity of dependence, they cannot make their own food. 

 Hence they must provide themselves with food, either 

 by robbing other organisms or by living on the products 

 or remains of other organisms. The fungi are divided 

 according to their food habits into saprophytic and 

 parasitic, the former living on dead material, the latter on 

 living plants or animals. Certain smaller fungi are the 

 immediate cause of disease in plants, while the larger 

 fungi, known as mushrooms and toadstools, are mainly 

 saprophytic. Fungi causing plant disease are nearly all 

 of minute size. They are commonly spoken of as rusts, 

 mildews, rots, and smuts. 



Nature of Diseases due to Fungi. — Fungi bring 

 about their characteristic disturbances of the normal life 

 of their host plants by abstracting from the host the food 

 which they require, and by the mechanical or chemical 

 injury which they do in addition. Thus they may grow 

 superficially in and upon the tissues of the host, or they 



