172 THE FUNGI WHICH CAUSE PLANT DISEASE 



formed at once and the mycelium develops to a more or less 

 circular colony, producing new conidia in a few days. Artificial 

 inoculations on susceptible plants, using conidia, usually result 

 within two to five days in typical mildew spots. 



Neger,*" who studied the germination of conidia extensively has 

 shown that light hastens the growth of the germ tubes, which in 

 many cases are negatively phototropic. Con- 

 tact stimulus leads to the growth of appres- 

 soria. 



The perithecia are subspherical, often some- 

 _ what flattened, white to yellow when young, 



showing lobed haus^ dark to black and reticulated when mature; 

 toria. After Salmon. ^^^ ^thout ostiolc but are provided with 

 appendages of various types. Figs. 130, 133-136, which give main 

 characters to mark the genera. The appendages serve by hygro- 

 scopic movements to aid in the distribution of the fungus.*^ The 

 ascospores become free after dissolution of the perithecium by 

 weathering. The asci are either solitary or quite numerous within 

 the perithecium and bear two to eight hyaline spores each. 



The conidia are short-lived summer spores. The perithecia 

 mature more slowly and constitute the hibernating condition. 

 In some instances the ascus-form is unknown; 

 the fungus is then classified solely by its 

 conidial stage and falls under the form genus 

 Oidium (see p. 569.) 



In Sphserotheca *'' an antheridial and an 

 oogonidial branch, each uninucleate, are de- 

 veloped, and cut off by septa. The oogo- 

 nium enlarges; the antheridium lengthens, fig. i24.^AZ^nium- 

 its nucleus divides, and a septum is run in *"'^ '^^ ^™ nuclei, 

 separating the stalk cell from the antheri- ^^ "^"' 



dium. The sperm nucleus enters the oogonium and fuses 

 with the oogonial nucleus. Simultaneous with fertilization oc- 

 curs, from the stalk cell of the oogonium, the development of 

 a sterile system of enveloping threads which surround and pro- 

 tect the fertihzed oogonium and eventually mature into the 

 sporocarp. The fertilized oogonium divides several times trans- 

 versely producing a series of cells, one of which is binucleate. 



