86 DISEASES OF CULTIVATED PLANTS 



spores enable the fungus to extend rapidly from one host- 

 plant to another, and thus extend its area of distribution ; 

 winter or resting-spores, that tide the fungus over the winter 

 and set up a new infection on the host-plant the following 

 season. In addition to these there are various modes of 

 vegetable reproduction which are quite as effective, and more 

 difficult to combat than true spores, in starting a disease, as 

 they are usually formed in the diseased parts of plants, and 

 find their way back to the land, or are produced at first-hand 

 by mycelium present in the soil. In many instances such 

 reproductive bodies consist of dense masses of mycelium, 

 usually black externally, and varying in size in different kinds 

 of fungi, from a pin's head to that of a cricket ball. These 

 sclerotia remain for some time in a resting condition and then 

 produce either spore-bearing bodies or give off mycelium, 

 capable of infecting a host-plant. 



A general account of the fungi will be found in the follow- 

 ing books : 



Massee, Geo., A Text-Book of Fungi. Duckworth and Co., 

 London (1906). 



Tavel, Dr. F. von, Vergleichende Morphologic der Pilze. 

 Fischer, Jena (1892). 



BIOLOGIC FORMS OF FUNGI 



In the case of many parasitic fungi, certain members of a 

 given species have become so modified and specialised in 

 their parasitism, that they can only infect a given species of 

 host-plant, or, at most, a few closely allied species. Such 

 are termed biologic forms, on account of their speciality in 

 this direction being of a purely physiological nature, depend- 

 ing on possessing distinct and sharply-defined powers of 

 infection. No morphological differences are presented by 

 biologic forms belonging to the same species. As an ex- 

 ample, the morphological species called Erysiphe graminis 

 (D. C.) is parasitic upon barley, oats, wheat, and many wild 

 grasses. Culture experiments have proved, however, that 

 the particular form parasitic upon any one of the plants 

 enumerated above cannot infect any of the other plants. 

 Thus the form parasitic upon wheat cannot infect rye, etc. 

 Salmon has shown that this specialised and restricted power 



