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probasidium is short and usually continuous—although it sometimes becomes two-celled—and on this structure is produced a terminal conidium which on germination (in nutrient solution) again produces a probasidium, which in turn gives rise to a conidium; in U. Tritici and U. striaeformis the probasidium gives rise directly to an infection hypha, conidia being absent. Key to Species of Ustilago. Spores smooth.   Spores under 6 mmm.* In this article the contraction “mmm.” is used for micromillimetres. long U. comburens.   Spores over 6 mmm.    Sori compact, covered with a membrane 3. U. Jensenii.    Sori pulverulent, without a membrane 2. U. levis. Spores rough; granular, veruculose, or verrucose.   Spores somewhat coarsely verrucose.    Sori principally in the leaves 5. U. striaeformis.    Sori principally in inflorescences.     Sori semi-compact 7. U. bullata.     Sori pulverulent 6. U. bromivora.   Spores minutely verruculose.    Spores over 10 mmm. 9. U. Readeri.    Spores under 10 mmm.     Sori destroying entire inflorescences 4. U Tritici.     Sori semi-compact, only partially destroying spikelets 1. U. Avenae. 1. Ustilago Avenae Jensen. (Text-fig. 2, and Plate 41, fig. 2.) Jens., Charb. Cereales, p. 4, 1889. Gramineae. U. perennans Rostr., Overs. K. Danske Vid. Selsk. Forh. 1890, p. 15, 1890. Cintractia Avenae Ell. et Tr., Jour. Myc., vol. 6, p. 77, 1890. Sori in spikelets, seldom in the leaves, usually completely destroying the floral parts, forming a pulverulent, olivaceous spore-mass, which eventually falls away, leaving only the bare axis of the inflorescence. Spores globose or subglobose, 5–9 mmm. diam.; epispore distinctly but finely verruculose, pallid olive, more lightly coloured on one side, 0.5-0.75 mmm. thick. Host: Arrhenatherum elatius (L.) Beauv. In inflorescences. Herb. No. 1247. Plimmerton (Wellington), sea-coast, E. Bruce Levy! 22 Dec., 1920. E. H. Atkinson! 1 Dec., 1921. Ashburton (Canterbury), 70 m., H. H. Allan! 15 Dec., 1921. Crookston; Dunrobin (Otago), J. C. Neill! G. H. C. 7 Feb., 1924.† A few localities obtained between the dates of reading the paper and publication have been inserted. Distribution: World-wide. The form on Arrhenatherum elatius was separated as a distinct species on account of the presence of perennating mycelium in the perennial parts of the host; it is identical morphologically with the form on the oat, so that it cannot be considered as more than a biologic race. Germination.—In water the spores commence to germinate in from twelve hours (fresh material) to three days (old material). A long and slender probasidium (occasionally two) is produced; into this the protoplasm of the spore penetrates, and after the probasidium has attained a length several times that of the diameter of the spore several (3–5) transverse septa appear. On or near these septa several elliptical, hyaline, minute, continuous conidia appear. When detached these may in turn germinate and produce long and narrow infection hyphae. In nutrient solution, according to Brefeld (1883), the conidia do not produce hyphae, but produce secondary or tertiary conidia by a process of budding.

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