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. 
“ is 
Sprayed. 
every 
2 Weeks 
Sprayed 
3 Times 
le Not 
| 
| Sprayed 
Fie. 187, Experiments in destroying potato blight 
RETR 
Photograph by the New York Agricultural Experiment Station illustrating the 
results of spraying potatoés 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 3504 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 aérial hyphe 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 zodspores (fig. 186, Z’). 
The conidia, therefore, act like sporangia, since they form 
asexual spores. The zodspore may swim in the dew or rain for 
