STUDIES IN CRYPTOGAMS 



191 



Fig. 276. — Colonies of Willow Mildew. 



(Fig. 276). These patches consist of numerous interwoven 

 threads that may be recognized under the microscope as the 

 mycehum of the fungus. 

 The mycehum in this 

 case hves on the surface 

 of the leaf and nour- 

 ishes itself by sending 

 short branches into the 

 cells of the leaf to ab- 

 sorb food materials from 

 them. 



Numerous summer-spores are formed of short, erect branches all 

 over the white surface. One of these branches is shown in Fig. 



277. When it has grown to a cer- 

 tain length, the upper part begins 

 to segment or divide into spores 

 which fall and are scattered by the 

 wind. Those faUing on other wil- 

 lows reproduce the fungus there. 

 This process continues all summer, 

 but in the later part of the season 

 provision is made to maintain the 

 mildew through the winter. If some 

 of the white patches are closely ex- 

 amined in July or August, a number 

 of Httle black bodies will be seen among the threads. These little 

 bodies are csHA&A. perithecia, shown in Fig. 278. To the naked eye 



they appear as minute specks, 

 but when seen under a magnifi- 

 cation of 200 diameters they 

 present a very interesting appear- 

 ance. They are hollow spheri- 

 cal bodies decorated around 

 the outside 

 with a fringe 

 of crook-like 

 hairs. The 

 resting-spores 

 of the willow 

 mildew are 

 produced in 

 sacs or (jij'cnn- 

 closed witli- 

 in the leath- 

 ery perithecia. Figure 279 shows a cross-section of a perithecium 

 with the asci arising from the bottom. The spores remain securely 



Fig. 



277. — Summer-spores of 

 Willow Mildew. 



Fig. 278. 



— Perithecium of Wii^ 

 LOW Mildew. 



Fig. 279. — Section 

 THROUGH Peri- 

 thecium of Wil- 

 low Mildew. 



