200 REPRODUCTION OF DOWNY MILDEWS 



contents breaks up into several zoospores (Fig. 132, B-E) which 

 finally come to rest and develop the characteristic tubular hyphae 

 of the fungi. This behavior of the sporangium is doubtless a 

 survival of the zoospore stage seen in the algae. It is equally 

 suggestive that definite changes in the environment, as the dry 

 air, causes the sporangium to germinate as a non-motile spore, 

 whereas the presence of water causes the sporangium to produce 

 several zoospores. 



FIG. 132. 



FIG. 132. Asexual reproduction of the mildew : A, hyphae of Plasino- 

 para emerging from a stoma and bearing numerous sporangia.. B, en- 

 larged view of sporangium of Peronospora germinating on a dry leaf. In 

 this case the sporangium behaves as a spore sending out a hypha that will 

 penetrate the tissues of the leaf. C, sporangium of Phytophthora germina- 

 ting in the water and forming zoospores. D, zoospore enlarged. E, 

 zoospore has come to rest and is forming a tube that will penetrate the 

 tissues of the leaf as in the case of B. 



FIG. 133. Reproduction of the white rust, Albugo: A, asexual stage, 

 showing several erect hyphae forming spor/^ B, sexual stage which is 

 characteristic of the Peronosporales in ge/eral o, gametangium contain- 

 ing a single female gamete which is being penetrated by tube from male 

 gametangium, an. After Wager. 



