ALGAL AND FUNGAL PROTLSTS 33 



often have thick walls. In AMya prolifera de Bary saw 

 them form acrogenously. They germinate either in clean 

 oxygenated water or in suitable nutrient fluids, producing 

 either new hyphae or sporangia. 



In the sexual process antheridia and oogonia are formed 

 and shut off by septa. The contents of the oogonium 

 divide into several oospores, which require a variable time 

 to ripen. They germinate by forming a sporangium, which 

 produces a bunch of conidia, which become zoospores, 

 Fig. 4, C. 



Biologic forms of Fungi. Before passing to the con- 

 sideration of some parasitic fungi the subject of what is 

 known as biologic variation should be mentioned. The 

 facts have been well stated by Massee as follows : "In the 

 case of many parasitic fungi certain members of a given 

 species have become so modified and specialised in their 

 parasitism, that they can only infect a given species of host- 

 plant, or, at most a few closely allied species. 



" Such are termed biologic forms, on account of their 

 speciality in this direction being of a purely physiological 

 nature. ... No morphological differences are presented by 

 biologic forms belonging to the same species. . . . The 

 morphological species called Erysiphe graminis is parasitic 

 on barley, oats, wheat, and many wild grasses. Culture 

 experiments have proved, however, that the particular 

 form parasitic upon any one of the plants enumerated above 

 cannot infect any of the other plants." 



3 



