as when a probasidium is produced which develops directly into an infection hypha, conidia being absent. In such a case the species is placed in one or other family on account of the possession of certain other characters. (See under “Taxonomy.”) In water the conidia produce short infection hyphae, seldom secondary conidia, but in nutrient solution they often give rise to secondary or tertiary conidia by a process of budding. Cytology. The vegetative mycelium (prior to the formation of spores) is invariably binucleate until shortly after gelatinization of the walls of the sporogenous hyphae, when the two nuclei fuse, the mature spore being uninucleate. When the spore germinates the protoplasm passes into the probasidium, but the nucleus usually remains within the spore and there divides, the daughter nucleus passing into the probasidium. In Ustilago this probasidial nucleus and the one within the spore again divide, and all then migrate into the probasidium, where each takes up such a position that when the probasidium becomes septate each cell contains one nucleus. As each conidium is formed, one of the probasidial nuclei divides and the daughter nucleus migrates into the conidium. In those members of this family in which conidia are produced the conidia often conjugate, a short conjugation-tube passing from a conidium to one adjacent, with which it fuses, or both may produce tubes which meet and fuse. Through the tube the nucleus of one conidium passes to the other, where it remains, but does not fuse with its fellow. When this binucleate conidium germinates it produces an infection hypha, the cells of which are binucleate owing to simultaneous division of the two nuclei. This binucleate condition persists until spore-formation. In Tilletia the probasidium is at first non-septate, and the spore nucleus divides until the number of daughter nuclei corresponds with the number of terminal conidia produced, into which they pass. In Tilletia conjugation occurs whilst the conidia are still attached to the probasidium. Here a short conjugation-tube is produced; this fuses with a contiguous conidium (or conidial tube), and the nucleus of the one migrates to the other. As in nutrient solution these conidia may produce an aerial mycelium from which secondary conidia arise, it follows that this mycelium, together with the secondary conidia, is binucleate. In those species of either genus in which no conidia are produced conjugation is effected between neighbouring hyphae derived from the probasidium. Short lateral outgrowths are produced; these fuse, and the nucleus of one migrates into the cell of the other. In this manner the mycelium becomes binucleate. Exceptions occur, however, for Rawitscher (1912) has shown that with Ustilago Maydis Cda. (= U. Zeae Ung.) the conidia do not conjugate, the mycelium remaining uninucleate throughout its vegetative existence until the period of spore-formation, when during the formation of the sporiferous hyphae the ends of adjacent cells come in contact, their walls break down, and two nuclei come together in the swollen terminal region so produced. These nuclei fuse almost immediately, so that the developing spores are uninucleate as in normal plants. This matter cannot here be discussed at greater length; further particulars may be obtained from the papers of Dangeard (1894), Harper (1899), Lutman (1911), Rawitscher (1914), Kniep (1921).