48 MASSACHUSETTS HORTICULTURAL SOCIETY. 



bear the summer spores. The manner in which these spores are 

 borne differs slightly from that characteristic of the other mildews, 

 but the difference is not important for our present purpose. 



Wherever the fatal fungus threads penetrate, the invaded tissue 

 dies, and under favorable conditions their progress throughout the 

 plant is very rapid. A few days of warm and moist weather at 

 the right time may suffice to convert an apparently flourishing 

 field into a mass of fallen, decaying, blackened stalks. The sum- 

 mer spores not only infest the leaves of fresh plants, but their 

 germinating tubes, produced in the soil, penetrate young tubers 

 which lie near the surface. Old tubers are protected by the cork- 

 layer constituting the so-called "skin." Unlike the mildew of 

 the vine, that of the potato is not known to form winter spores, 

 but is perpetuated b}' the vegetative threads which hibernate in the 

 tubers and resume active growth upon the development of new 

 stems from them. Where the tubers are stored in a warm place, 

 considerable rotting may occur during the winter, from the growth 

 of the fungus in them ; and sound tubers may even become 

 infected from the diseased ones. 



Perhaps next of the mildews in economic importance may be 

 mentioned that which attacks lettuce, especially under glass. This 

 fungus also appears in white downy or felted masses on the under 

 side of the leaf, which becomes first yellow and then dead and 

 brown. It produces both summer and winter spores in essentially 

 the same manner as does the grape mildew, but each summer 

 spore produces a single tube directly, instead of several motile 

 bodies each of which may subsequently produce a tube. With 

 reference to the germination of the summer spores, therefore, the 

 grape mildew and lettuce mildew may stand as types of two 

 different modes. It is evident that those mildews whose spores 

 produce several zoospores from each, which can give rise to as 

 many new plants, have, at least theoretically, an advantage over 

 those whose spores can produce but one new plant each ; for the 

 number of possible new parasites from a given number of spores is 

 about eight times larger in the former than in the latter case, since 

 eight is commonl}' the number of zoospores formed from the con- 

 tents of a single summer spore. 



The lettuce mildew is found also on several plants closely related 

 to the lettuce, including thistles, chicory, and the wild lettuce. 

 This may serve to call attention to the fact that some of our com- 



