FUNGI 



145 



as apple, pear, cherry, rose, hop, grape, wheat, gooseberry, 

 cucumber, pea, verbena, sunflower, aster, etc. In fact, very 

 few seed-plants seem to escape their attacks. Being exter- 

 nal parasites, mildews are not 

 necessarily destructive ; but 

 they often cause the death of 

 the host. 



An examination of the my- 

 celium shows that its filaments 

 have partition walls; and hence 

 the body is not ccenocytic, as 

 in Mucor and Peronospora, but 

 made up of a row of cells as in 

 the Conferva forms among the 

 green Algse. Small disk-like 

 outgrowths are sent into the 

 epidermal cells of the host, 

 anchoring the mycelium and 

 absorbing the cell contents. 



During the summer, numer- 

 ous sporophores arise from the 

 mycelium, not bearing sporan- 

 gia, as in Mucor ( 79), but 

 forming spores in a peculiar 



way. The end of the sporophore rounds off, almost separat- 

 ing itself from the part below, and becomes a spore. Below 

 this another organizes in the same way, then another, until 

 a chain of spores is developed (Fig. 135, A), easily broken 

 apart and scattered by the wind. Falling upon other suit- 

 able leaves, these spores germinate and produce new my- 

 celia, enabling the parasite to spread with great rapidity. 



The mycelium produces also minute antheridia and 

 oogonia, which come in contact with one another as do 

 those of Peronospora ( 80), but it is not worth while for the 

 untrained student to try to observe them. As a result of 



FIG. 134. Lilac leaf covered with 

 mildew, the shaded regions repre- 

 senting the mycelium, and the 

 black dots the spore-cases. 



