No. 4.] FUNGOUS DISEASES. 89 



often produce peculiar swellings or distortions, within wliich 

 the threads remain concealed and protected during the winter, 

 ready to spring into pernicious activity as soon as conditions 

 are favorable to growth. Such perennial vegetative threads 

 are found in the swellings on the twigs of plum and cherry 

 trees, known as " black-knot," and in the mummified fruits 

 of the peach affected with fruit mold. 



To recapitulate, fungi are minute plants, which, owing to 

 a lack of chlorophyl, are obliged to depend upon organic 

 food-material of vegetable or animal origin. They may 

 derive their nutriment from the living tissues of the higher 

 plants, in which case they are called parasites. Their vege- 

 tative portion consists of delicate tubes, which, in the case 

 of the parasitic fungi, traverse the living tissues of the host- 

 plant, and l)y absorbing the contents of the latter, produce 

 symptoms of disease. During tiic summer they are propa- 

 gated by means of multitudes of very small, delicate spores, 

 borne on aerial threads. They maintain their vitality during 

 the winter by means of spores similar to the summer spores, 

 but enclosed in dense receptacles ; by larger, thick-walled 

 winter spores ; or by the vegetative threads themselves, 

 which either exist perennially within the tissues of the host- 

 plant or else form small, solid knots, with thick external walls. 

 From any of these forms the summer spores may be repro- 

 duced Avith the return of warm weather. 



I have dealt thus fully with the nature of parasitic fungi, 

 because an understanding of this matter is at the foundation 

 of all methods of preventive treatment of fungous diseases. 

 One further point, however, is deserving of note in this 

 connection. It is, of course, the vegetative portion of a 

 fungus which docs the damage to the plant, and this damage 

 is wrought in a variety of ways, largely dependent upon the 

 location of the threads within the portion of the plant 

 attacked and their effect upon the tissues. They may be 

 deeply buried, and produce the decay or complete destruc- 

 tion of the tissues, as in the "rot" of potatoes and the 

 "smut" of cereals and onions; or they may infest only the 

 surface tissues, and, without destroying them or causing 

 decay, prevent their full development, as is seen in the scab 

 of apples and in most so-called "leaf-spot" diseases; or, 



