DIV. I 



THALLOPHYTA 



453 



transformed into a dark brown or black mass of spores. These smut-spores, brand - 

 spores, or resting-spores are scattered by the wind, and germinate only after an 

 interval of rest, producing the basidia in the succeeding spring ; the formation of 

 these is characteristically different in the two families of the Ustilaginaceae and 

 the Tilletiaceae. 



The most important genus of the Ustilaginaceae is Ustilago. Ust. Avenae, 

 U. Hordei, and U. Tritici segetum, which were formerly united as U. Carbo, cause 

 the "smut" or "brand" of Oats, Barley, and Wheat. The mycelium penetrates 

 the ovary, and forms dark brown, dust-like masses of escaping resting-spores. 

 V. Maydis produces on the stalks, leaves, and inflorescences of the Maize tumour- 

 like swellings filled with bjand-spores in the form of a black powder. Other 



FIG. 399. Ustilago. A, Germinating smut-spore (cl), cultivated FIG. 400. UstUago Scabiosae. A, 



in nutrient solution ; t, transversely septate basidium 

 with lateral and terminal basidiospores (conidia) (c) (x 

 450). B, Germinating conidia, which are multiplying by 

 budding (x 200). C, An aggregation of budding conidia 

 (x 350). (After BREFELD, from v. TAVEL, Pilze.) 



Young basidium with four nuclei 

 formed on germination of the 

 resting spore. B, Spore-forma- 

 tion on the 4-celled basidium. 

 (After HARPER.) 



species live on the leaves of different grasses ; while U. antherarum, occurs in the 

 anthers of various Carophyllaceae (e.g. Lychnis, Saponaria}. In the case of female 

 flowers of Lychnis the presence of the fungus causes the development of stamens, 

 the anthers of which are filled with brand-spores. 



The brand -spores of Ustilago fall to the ground, and after a period of rest give 

 rise, on germinating, to a short tube (promycelium) which becomes divided by 

 three or four transverse walls (Fig. 400 B}, and, functioning as a basidium, 

 produces ovate basidiospores (sporidia), both laterally from the upper ends of 

 the intermediate cells and also from the tip of the terminal cell. When abundantly 

 supplied with nourishment, as when cultivated in a nutrient solution, conidia are 

 continuously abstricted in large numbers (Fig. 399), and then multiply further by 

 budding. If the supply of nutriment in the substratum is insufficient, fusions 

 between conidia or between cells of the promycelium take place in many Smut Fungi 

 (Fig. 402). After the food-supply of the substratum is exhausted, the conidia grow 

 out into mycelial hyphae. The formation of the conidia in the damp manured soil 

 of the grain fields is accomplished during a saprophytic mode of existence, but the 

 hyphal filaments which are eventually produced become parasitic, and penetrate 

 the young seedlings as far as the apical cone where the inflorescence takes its 



