FUNGI 



183 



322. Summer- spores of willow 

 mildew. 



leaf and nourishes itself by sending short branches into the cells of 

 the leaf to absorb food -materials from them. 



Numerous summer -spores are formed on short erect branches all 

 over the white surface. One of these branches is shown in Fig. 322. 

 When it has grown to a certain length, 

 the upper part begins to segment or di- 

 vide into spores which fall and are scat- 

 tered by the wind. Those falling on 

 other willows reproduce the fungus there. 

 This process continues all summer, 

 but in the later part of the season pro- 

 vision is made to maintain the mildew 

 through the winter. If some of the white 

 patches are closely examined in July or 

 August, a number of little black bodies 

 will be seen among the threads. These little bodies are called peri- 

 thecia, shown in Fig. 323. To the naked eye they appear as minute 

 specks, but when seen under a magnification of 200 diameters they 



present a very interesting appear- 

 ance. They are hollow spherical 

 bodies decorated around the out: 

 side with a fringe of crook- like 

 hairs. The resting- spores of the 

 willow mildew are produced in 

 sacs or asci inclosed within the 

 leathery perithecia. Fig. 324 

 shows a cross -section of a peri- 

 thecium with the asci arising 

 from the bottom. The spores 

 remain securely packed in the 

 perithecia. They do not ripen in 

 the autumn but fall to the ground 

 with the leaf and there remain 

 securely protected among the dead foliage. The following spring 

 they mature and are liberated by the decay of the 

 perithecia. They are then ready to attack the un- 

 folding leaves of the willow and repeat the vrork 

 of the summer before. 



Wheat rust. The development of some of the a 

 rusts, like the common wheat rust (Puccinia gra- 



minis), is even more interesting and complicated 324. Section through 



perithecium of wii- 

 than that of the mildews. Wheat rust is also a low mildew. 



323. Perithecium of willow mildew. 



