DISEASES OF PLANTS. 



85 



If a portion of this delicate grey mildew is examined under a 

 microscope, it will be seen to consist of a delicate felt of inter- 

 woven mycelium resting on the surface of the leaf, and sending 

 numerous short suckers into the cells of the leaf on which 

 it is parasitic, for the purpose of absorbing food. When the 

 mycelium has been growing for some time it presents a delicately 

 frosted appearance, due to the presence of numerous upright 

 strings of exceedingly minute conidia or summer-spores. These 

 spores are scattered by wind as soon as they are ripe, and 

 germinate at once ; and as these bodies are produced in rapid 

 succession throughout the summer months, it is easy to under- 

 stand how the disease spreads so rapidly after it once appears. 



Fig. 3. — Kose Leaf Mildew (Sphcerotheca pannosa). 



Winter form of the disease. (1) A rose fruit showing the winter-spore condition in the 

 form of minute black dots on the white mould; natural size. (2) A winter-fruit; highly 

 magnified. (3) A cell containing eight winter-spores ; several of these cells are produced in 

 a fruit ; highly magnified. (4) A single winter-spore germinating ; magnified 3U0 times. 



If the white patches of mould, especially those present on 

 the more persistent parts of the rose tree, as young fruits and 

 shoots, are examined in August and throughout the autumn, 

 minute points not so large as a pin's head will be seen in con- 

 siderable quantity; these are at first yellow, and finally blackish, 

 and are in reality fruits of a complex structure, containing 

 in their interior numerous winter-spores. If taken in hand 



