io PLANT DISEASES 



The function of the rapidly grown, quickly germinating 

 conidia, produced throughout the summer season, while the 

 host-plant is in the full vigour of growth, is to enable the 

 fungus to extend its geographical area of distribution. 



When the vine leaves begin to fade, the production of 

 summer fruit ceases ; and after the leaves have fallen, the 

 same mycelium, which during the summer has given origin 

 to conidia, now produces a second kind of reproductive 

 body called winter fruit, or technically oospores. These 

 bodies are also sometimes called resting-spores, from the 

 fact that, unlike conidia, they will not germinate the moment 

 they are mature, but only after a period of rest. Oospores 

 are spherical bodies of a dark brown colour, furnished with 

 a very thick, protective cell-wall, and are produced, often in 

 immense numbers, in the tissues of dead leaves (Fig. i, 5). 

 Although such diseased leaves often decay and disappeai 

 during the winter or early spring, the oospores they con- 

 tained are not destroyed, but retain their vitality, lying on 

 the ground, and germinate about the month of May, pro- 

 ducing two or three germ-tubes much branched at the tip 

 and bearing conidia similar to the summer form of fruit 

 (Fig. i, 6). These conidia are dispersed by wind, and 

 those that happen to alight on the surface of a young, damp 

 vine leaf, germinate, enter the tissues of the leaf, form a 

 mycelium, which soon gives origin to the summer form 

 of fruit. 



^ 



The function of oospores or the winter form of fruit is 

 that of continuing the species in time, or from one season 

 to another ; enabling it to tide over that period of the year 

 when its host-plant is also in a resting condition. 



