Fig. 99.—Ustilago. 1. Formation of brand-spores. 2. Germinating brand-spore of U. perennans. 3. Germinating brand-spore of U. cardui (after Brefeld). 4. U. filiformis. a A brand-spore with developed basidium; b another, with a conidium; c with two conidia; d with two conidia placed diametrically opposite to each other; e, detached conidia which are growing into hyphæ.
Fig. 100.—Tilletia tritici: a an ear of Wheat in which all the grains are attacked by Stinkbrand; b a blighted corn surrounded by the chaff; c a blighted corn grown together with a stamen; d the same cut across; e a brand-spore; f, g, h germinating brand-spores; i germinating conidia; j the mycelium; k-k brand-spore-forming mycelium-threads. (c-h magnified 400 times; i-k 300 times.)
The conidia (also called “sporidia”) of many species unite generally into an H-form (Figs. [97], 4; [100] h; [101], 4). This union in pairs does not, however, take place with a view to germination, there is no fusion of nuclei, and therefore in this “fusion” there is no sexual act.
Order 1. Ustilaginaceæ. Conidiophores with transverse walls and lateral conidia.—Ustilago (Fig. [99]) generally developes its spores in the floral organs of its host-plant, the ovary or anthers, where they arise from hyphæ, and form a slimy mass which when mature becomes a black dust.
To this order belong U. avenæ, parasitic on Oats, U. hordei and U. nuda (U. jenseni), on Barley; these are the usual cause of “Smut” on cereals. U. hypodytes on straw of Elymus and Agropyrum. U. filiformis in the leaves of Glyceria. U. caricis transforms the fruits of various species of Carex into black, dusty balls. U. violacea developes its violet spore-powder in the anthers of the Caryophyllaceæ. U. tragopogonis, transforms entire inflorescences of Tragopogon into a black-violet mass. Among the largest are U. grandis, which causes the large swollen nodes in the stem of Phragmites, and the Maize Blight, U. maydis, which produces outgrowths about the size of a hand on the spadix of the Maize.
Order 2. Tilletiaceæ. Conidiophores undivided, generally several conidia arise at their apices.—Tilletia tritici, the Stinkbrand on Wheat (Fig. [100]). The mycelium lives in Wheat-plants, producing its spores in the ovary after the whole interior of this body has been destroyed by the mycelium, with the exception of the external layer of the wall of the ovary, which remains essentially unaltered and encloses the closely packed, firm mass of spores (Fig. 100 d). The grains of Wheat thus attacked are shorter and thicker than the sound ones, and the ears show the presence of this Fungus by their erect position, and the wide separation of the chaff (Fig. [100] a). The unpleasant odour of the ovary prior to the ripening of the spores, has given the name “Stinkbrand,” and, in like manner, its hardness when it encloses the ripe spores, is the reason of its being also called “Stonebrand.” On account of this hardness, the diseased grains are readily harvested together with the healthy ones, which become infected by the spores at the threshing. T. lævis (T. fœtens) also occurs on Wheat and has smooth brand-spores.
Fig. 101.—Urocystis. 1, U. covalloides. A spore-ball, magnified 450 times. 2–4, U. anemones: 2–3, brand-spores which are about to germinate (magnified 450 times). 4, Conidia, the two in a state of fusion, a third with vacuoles and division-wall, magnified 500 times.
Entyloma (Fig. [96]), a genus with numerous species, which appear in spots on the leaves of the host-plant, and Tuburcinia (Fig. [98]), which makes its appearance on the Primulaceæ, produce white conidia-spots on the surface of the host-plant. The first-named has single spores, the latter has its spores closely massed together.—Urocystis (Fig. [101]) has its spores surrounded by a number of small and lighter coloured barren spores. U. occulta, Rye-stem Blight, forms its spores in long streaks in the stems and leaves of the Rye, and does considerable damage. U. cepulæ on Onions. U. violæ forms large dark-violet swellings in the leaf-stalk and stems of Violets.—Thecaphora (Fig. [102]) appears in seedlings of Convolvulus and Astragalus.