190 



STUDIES IN CRYPTOGAMS 



349. Summer-spores of willow 

 mildew. 



Numerous summer-spores are formed on short erect branches all 

 over the white surface. One of these branches is shown in Fig. 349. 

 When it has grown to a certain length, the upper part begins to segment 

 or divide into spores which fall and are scattered 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, called perithecia, are shown in 

 Fig. 350. To the naked eye they appear 

 as minute specks, but when seen under a magnification of 200 diameters 

 they present a very interesting appearance. 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 asd inclosed within the 

 leathery perithecia. Fig. 351 

 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 lib- 

 erated by the decay of the peri- 

 thecia. They are then ready to attack the unfolding leaves of the 

 willow and repeat the work of the summer before. 



Wheat rust. The development of some of the 

 rusts, like the common wheat rust (Puccinia gram- 

 inis), is even more interesting and complicated 

 than that of the mildews. Wheat rust is also a true 

 parasite, affecting wheat and a few other grasses. 

 The m y celium here cannot be seen by the unaided 

 low mildew. eye, for it consists of threads which are present 



350. Perithecium of willow mildew. 



