WHITE SPOTS ON LEAVES. 231 



swim about for only a short time, and then the cilia drop off, 

 and the zoospores come to rest. They then give off tlireads 

 like the conidial spores of Uncinula, and the threads penetrate 

 into the interior of the grape-plants on which they may be. 

 Once inside, the threads constitute a mycelium^ which extends 

 through the plant at a rate corresponding with the external 

 moisture; and finally the tlireads make their way through 

 the breathing-pores into the air, and produce new spores. It 

 will be seen that the conidial spores of the Peronospora have an 

 advantage over those of the Uncinula, because they produce 

 a number of zoospores, generally from five to fifteen, each<of 

 which is capable, under favorable hygrometric conditions, of 

 producing a new mycelium and spores. Like the correspond- 

 ing bodies in Uncinula, the conidial spores are destroyed by 

 cold. 



The winter spores of the Peronospora are thick and tough, 

 and are produced in the interior of the grape-leaves by a 

 rather complicated process, which, although interesting from 

 a botanical point of view, need not be described here. They 

 fall to the ground with the leaves in the autumn, and are set 

 free by the rotting away, during the winter and spring, of the 

 leaves in which they are contained ; and, as the season ad- 

 vances, they germinate, and enter the nearest vines, but the 

 details of the germination have not yet been made out. The 

 remarks already made about the burning of grape-leaves apply 

 also in this connection. The mycelium of the Peronospora in 

 the interior of the vines affected during the summer may 

 remain dormant during the winter, and start up again when the 

 warm weather returns. Just how much harm the Peronosp)ora 

 does to the grape-vines is not easy to decide. I have, on a pre- 

 vious occasion, expressed the view that it is not unlikely that 

 the harm done has been exaggerated, because the fungus never 

 attacks the berries, and it does not cause the leaves to shrivel 

 and dry up until comparatively late in the summer, when, as 

 some say, their room is better than their company ; for in Mas- 

 sachusetts the thing to be desired is, that there shall be plenty 

 of sunlight to ripen the grapes, wliich is not the case when 

 the foliage is luxuriant, and covers up the branches. Whether 

 the shrivelling of the leaves in the latter part of August per- 

 manently injures the vines and injures the crop is a point to be 

 settled, not by the botanist, but empirically by the grape- 



