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 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 provision 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 _perithecia_, shown in Fig. 278. 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 spherical bodies decorated around the outside 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. Figure 279 shows a cross-section of a perithecium 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 unfolding leaves of the willow and repeat the work of the summer before. * * * * * _The wheat rust._—The development of some of the rusts, as the common _wheat rust_ (_Puccinia graminis_), 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 mycelium here cannot be seen by the unaided eye, for it consists of threads which are present within the host plant, mostly in the intercellular spaces. These threads also send short branches, or _haustoria_ (Fig. 132), into the neighbouring cells to absorb nutriment. [Illustration: FIG. 280.—SORI CONTAINING TELEUTOSPORES OF WHEAT RUST. ] [Illustration: FIG. 281.—TELEUTOSPORE OF WHEAT RUST. ]