240 INTRODUCTION TO BOTANY 



by the grape leaf, until the fungus permeates the entire leaf. 

 If the leaf is not able to make a surplus of food material, the 

 amount taken by the parasite may result in the starvation 

 and death of the grape leaf. Possibly, also, the parasite may 

 excrete substances that poison the host plant. 



FIG. 187. Experiments in destroying potato blight 



Photograph by the New York Agricultural Experiment Station illustrating the 

 results of spraying potatoes to prevent disease. Those that were not sprayed 

 yielded at the rate of 161 bushels per acre ; those sprayed three times during the 

 season yielded at the rate of 350i bushels per acre ; those sprayed every two weeks 

 yielded at the rate of 380 bushels per acre. In other experiments the results are 

 even better. In this same station, during the year 1904, the average gain per acre 

 in the yield for three sprayings is 191 bushels, and the gain for spraying every 

 two weeks is 233 bushels 



228. Reproduction of grape mildew. Sometimes the upright 

 branches of aerial hyphse of grape mildew produce rounded, 

 spore-like bodies (conidia) (fig. 186, A). When these conidia 

 come into favorable moisture (rain or dew) and temperature, 

 they divide, each one forming several zoospores (fig. 186, E^). 

 The conidia, therefore, act like sporangia, since they form 

 asexual spores. The zoospore may swim in the dew or rain for 



