PHYCOMYCETES 



157 



same zoospore may undergo considerable change. The period of 

 activity is, as a rule, from fifteen to twenty minutes, at the end of 

 which time the resting condition is assumed by the loss of cilia, by 

 becoming spherical, and by the development of a cell wall ; then 

 follows germination by means of a germ tube. Germination is 

 effected practically irrespective of ordinary conditions of light and 

 temperature. 



The oospores of this species are not so commonly found as 

 the conidiophores. They are more common, apparently, in the 

 northeastern states and are generally found upon Vitis ccstivalis. 

 They are commonly present during late September and always 

 " in the discolored, shriveled parts of the leaves, and are most 

 abundant just inside what are called the palisade cells of the 

 upper surface." The formation of these oospores is characteristic 

 of the family; that is, large terminal or intercalary swellings of 

 the mycelium are cut off by septa, and there results an oogonium. 

 In the neighborhood -of this oogonium there may be produced 

 one or more smaller antheridia. The subsequent development 

 of these two structures, the fusion phenomena, and the develop- 

 ment of the oospores (Fig. 53, d) take place approximately as 

 described for Pytkium de Baryanum and Cystopus candidus. 

 At maturity the oospore almost completely fills the original 

 oogonium wall, and the wall of the oospore itself is comparatively 

 smooth, thick, and yellowish. The oospores measure about 30 \x 

 in diameter. For the study of the oospores dried material may 

 be teased out in potassium hydrate solution, or it may be neces- 

 sary to boil the material in this solution, afterward neutralizing 

 with hydrochloric acid. The oospores are set free by the disin- 

 tegration of the tissues of the leaf, and they are probably impor- 

 tant in carrying the fungus over winter. Nevertheless, much work 

 needs to be done in the way of determining to what extent the 

 oospores are necessary in the annual propagation of this species. 



Control measures. In the control of this fungus Bordeaux 

 mixture is most effective, and it is only during very moist 

 seasons, that is, where it is difficult to keep the surfaces of 

 the leaves covered with the preparation, that the fungus has 

 been able to gain headway in spite of spraying operations. In 

 this connection it is interesting to note that copper fungicidal 



