=398.= In sexual reproduction oogonia and antheridia are developed on the mycelium within the tissues. Fig. 204 represents the antheridium entering the oogonium, and the male nucleus fusing with the female nucleus in fertilization. The sexual organs of Phytophthora infestans are not sufficiently known. =399.= Mucor, saprolegnia, peronospora, and their relatives have few or no septa in the mycelium. In this respect they resemble certain of the algæ like vaucheria, but they lack chlorophyll. They are sometimes called the alga-like fungi and belong to a large group called _Phycomycetes_. CHAPTER XX. FUNGI CONTINUED. “Rusts” (Uredineæ). =400.= The fungi known as “rusts” are very important ones to study, since all the species are parasitic, and many produce serious injuries to crops. [Illustration: Fig. 206. Wheat leaf with red-rust, natural size.] [Illustration: Fig. 207. Portion of leaf enlarged to show sori.] [Illustration: Fig. 208. Natural size.] [Illustration: Fig. 209. Enlarged.] [Illustration: Fig. 210. Single sorus. Figs. 206, 207.—Puccinia graminis, red-rust stage (uredo stage). Figs. 208-210.—Black rust of wheat, showing sori of teleutospores.] =401. Wheat rust (Puccinia graminis).=—The wheat rust is one of the best known of these fungi, since a great deal of study has been given to it. One form of the plant occurs in long reddish-brown or reddish pustules, and is known as the “red-rust” (figs. 206, 207). Another form occurs in elongated black pustules, and this form is the one known as the “black rust” (figs. 208-211). These two forms occur on the stems, blades, etc., of the wheat, also on oats, rye, and some of the grasses. [Illustration: Fig. 211. Head of wheat showing black rust spots on the chaff and awns.] [Illustration: Fig. 212. Teleutospores of wheat rust, showing two cells and the pedicel.] [Illustration: Fig. 213. Uredospores of wheat rust, one showing remnants of the pedicel.] =402. Teleutospores of the black rust form.=—If we scrape off some portion of one of the black pustules (sori), tease it out in water on a slide, and examine with a microscope, we see numerous gonidia, composed of two cells, and having thick, brownish walls as shown in fig. 212. Usually there is a slender brownish stalk on one end. These gonidia are called _teleutospores_. They are somewhat oblong or elliptical, a little constricted where the septum separates the two cells, and the end cell varies from ovate to rounded. The mycelium of the fungus courses between the cells, just as is found in the case of the carnation rust, which belongs to the same family (see Parag. 186). [Illustration: Fig. 214. Barberry leaf with two diseased spots, natural size.] [Illustration: Fig. 215. Single spot showing cluster-cups enlarged.] [Illustration: Fig. 216. Two cluster-cups more enlarged, showing split margin. Figs. 214-216.—Cluster-cup stage of wheat rust.]