F U N G I 



L83 



::'_"J. Summer-spores of willow 

 mililew. 



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. 321'. 

 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. 

 Tins 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 Pig. 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 oul - 

 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. Pig. 324 

 shows a cross-section of a peri- 

 thecium with the asci arising 

 from t lie bottom. The sport a 



remain securely packed in the 



peril hecia. They do uol ripen in 

 the autumn but fall t<> 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 read\' to attack llie tin - 



folding leaves of the willow and repeal the work 



of the summer before. V ' ' 



Wheat in i. The development ..t some of the "' 



i , like the common wheat rusl I Puccinin grn- 

 minis). is even more interesting and complicated ,; 



thorium "I nil- 

 than that o| the mildew-. Wheat rusl i- alHO a [ow uilldi 



Perithecium of Ulow mildew. 





