ii.] CLOVER SICKNESS CLOVER MILDEW. 9 



ing state till the following summer, when they germinate, 

 and produce threads carrying secondary spores or conidia; 

 these conidia drop off from the parent plant, sail through 

 the air, and are carried in different directions by currents 

 of wind. Such spores as light upon clover plants cause 

 the production of the mildew ; such as fall on unsuitable 

 places perish. A description of how resting-spores are 

 produced, and their nature, is given farther on in this 

 work. 



The illustration of this fungus (Fig. 1, A) shows the 

 parasite enlarged 400 diameters, growing from the under- 

 surface of the foliage of Medicago saliva L., whilst one of 

 the very pale gray spores or conidia is enlarged to 1000 

 diameters at B. In the illustration the fungus is really in- 

 verted so that its characters may be more easily understood. 

 All species of Peronospora usually grow from the under 

 surface of leaves, where they may be seen by the unaided 

 eye as small white cottony masses. They commonly 

 burst through the organs of transpiration s tomata ; 

 sometimes, however, the fungi push the leaf cells aside 

 and so get access to the air from the interior of the leaf. 

 Two stomata are seen in section at C, D. The fertile 

 threads which carry the spores are termed by botanists 

 conidiophores or conidia-bearers. The spawn of the fungus 

 causes putrefaction of the tissues by mere contact. The 

 spores, on falling on the foliage and bursting, also cause 

 putrefaction of the leaf. The destruction of invaded 

 clover is further aided by the conidiophores or steins of the 

 fungi obliterating the organs of transpiration. 



Imperfect drainage and thick planting favours the 

 growth of all the Peronosporece, whereas a free circulation 

 of dry air is often fatal to them. When once they make 

 their attack, it must be remembered that they establish 

 themselves within the tissues of the invaded plants, in a 

 position where it is impossible to reach them with any 

 curative material. Attention, therefore, to the mode of 

 cultivation may tend to stop the spread, if not to prevent 



