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BOARD OF AGRICULTURE. 



leaf. The second occurs only on the under surface, and 

 appears when fresh like frost-work. The leaf-stalks are 

 somethnes covered by the fungus ; but it does not attack the 

 grapes themselves. An examination of the frost-like spots 

 on the under surface shows that they consist of branching 

 threads, on whose tips are borne oval bodies (Fig. 7), which 

 are the conidial spores. The threads do not, however, grow all 

 over the surface of the leaf, but make their way from the in- 

 terior into the air, through the holes 

 which abound on the under surface of 

 most leaves, and are known by the 

 name of "breathing-pores." If we 

 follow the threads still farther, we 

 shall find that they penetrate through 

 all parts of the leaf and stems, mak- 

 ing their way between the leaf- 

 cells. The threads are also fur- 

 nished with small suckers, which 

 push their way directly through the 

 walls of the plant-cells into their 

 interior. The bodies which we may 

 call the winter spores of this blight 

 are not found on the surface, but 

 embedded in the leaf. They are 

 round, and have thick walls. The 

 name of the second form of grape- 

 blight is Peronospora viticola. 



A comparative study of the two 

 blights which we have just de- 

 scribed is instructive. They both 

 first appear on the leaves about the 

 first of August, and both cause 

 them to shrivel and drop off. One 

 sometimes attacks the grapes, while 

 the other does not. The two blights may be distinguished 

 with the naked eye by an ordinary observer, as one forms 

 a sort of dusty-looking web on any part; the other, frost- 

 like spots on the under surface of the leaves. Both have 

 conidial spores, which grow on stalks in the air. Both 

 have what we may for convenience call winter spores, which 

 ripen late in the autumn. Those of the Uncinula are in the 



Fig. 7. 



