Page:Encyclopædia Britannica, Ninth Edition, v. 9.djvu/867

 FUNGUS 831 suborder is effected by means of the conjugation of two apparently similar sexual cells, which as a result of their union form a zygospore. It will be sufficient to describe one typical case, viz., that of lihizopus nigricans. The con jugating cells of this fungus are elongated, stout tubes. Where two meet, each pushes against the other a protuber ance, at first cylindrical and of equal thickness. They re main closely adhering, and soon grow considerably, chiefly in thickness. At the end of each a separate cell is formed by the growth of a partition. These two cells are usually of unequal size, one as long as it is broad, the other only half as long as its breadth. The membrane which originally separated them is now perforated in the middle, and soon vanishes altogether ; the two conjugating cells thus unite and form the zygospore, which increases rapidly, usually attaining a diameter of over -*- millim. It is, as a rule, drum-shaped, the ends smooth, and the free surface clothed with wart-like protuberances. The contents are coarsely granular protoplasm, in which float drops of oil. On germination the zygospore produces a hypha (as observed in Syzygites), which by repeated dichotomous branching soon forms a new mycelium. &quot;&quot; Asexual reproduction is accomplished by sporangia similar in function, and in some degree in structure, to the corre sponding organs of the two preceding suborders. They are borne terminally on special branches of the mycelial hypkoe, which, like the mycelium, are aseptate up to the time of fructification. The spores produced in the sporangia ger minate by the emission of a filament, which, like that pro duced by the zygospore, forms by dichotomous branching a new mycelium. This was the only mode of reproduction known in the Mucorini, until the researches of Professor De Bary and others brought to light the sexual form, which, however, has not yet been observed in all the species. The Mucorini are usually to be found growing on excre ment and on decaying substances ; none of the species are known to cause any important disease to plants or animals. Order 1L HYPODERMIC. No mode of sexual repro duction is known to belong to the plants of this order. They are (so far as is known) reproduced wholly by asexual spores arising by cell-division. All the species inhabit the parenchyma of living plants, through the epidermis of which the receptacles burst. Suborder I. Urcdineos. The life-history of the Uredinece consists of a cycle of three generations, the transition in each case being effected (so far as is at present known) by asexual spores. The cycle begins in spring with the ger mination of thick-walled spores, called teleutosporcs, borne usually in pairs at the end of sterigmata. Germination takes the form of the emission of a hypha, which rapidly (usually in two or three hours) forms a pro-mycelium, bear ing three or four sporidia. These sporidia become detached, and on finding a suitable host-plant penetrate the epidermis cells by means of a germinating filament, and form a new mycelium within the parenchyma. After a few days this mycelium begins to form receptacles under the epidermis of the host, on which are borne cccidia and spermogonia. The latter appear before the former. They are small, narrow- necked sacs, in which are found minute spore-like bodies, called spermatia, by Tulasne, who believed them to be male organs, partly on comparative grounds and partly because he found them incapable of germinating. No female organs have been found ; though it was suggested that the spermatia stood in sexual relations to the recidia beside which they are constantly found. Professor De Bary, however, found that the spores of ajcidia, on a plant on which no trace of spermo gonia or spermatia could be found, germinated exactly as under other circumstances. The aecidia are at first round or oval but on bursting basin-shaped receptacles of pseudo-parenchyma, at the base of each of which a circular, flat hymenium is formed. From this hymcnium there rise erect basidia bearing in vertical series a number of spores of a round polyhedral form, and filled with protoplasm coloured red or yellow by oil. These spores on being liber- A FIG. V.Puccinia tjraminis. A. Mass of telcutosporcs (/) on a leaf of conch-prass; e, epidermis ruptured ; 6. sub-epidermal fibres. (After De Hnry.) B. part of vertical section through leaf of Herberts rvlgaris, with a, acidiiim fruits, p, peridium, and fp, spermogonia. (After Sachs.) C. Mass of uredospores (urJi with one tcleutospore (0 ; th, sub-hymenial hyphsc. (After De Bary.) ated produce germ-tubes which penetrate the stomata of the next host-plant (one of the same species as that on which the teleutospores were found), and soon form another mycelium in the intercellular spaces. Again, in a few days, this mycelium forms a new kind of receptacle, the uredo. This receptacle is flat and circular, and is situated immedi ately under the epidermis. On it basidia bearing round or oval uredospores arise and break through the epidermis. The uredospores constantly reproduce the uredo, which is the cause of the extensive ravages of this form of tha disease ; on the same mycelium which produces the uredo, teleutospores are ultimately formed. These hibernate, and when spring arrives again germinate, and start the same cycle of generations. Teleutospores oocidium-spores (accompanied by spermatia) uredospores, and later on the same mycelium teleutospores again. This cycle of generations is carried out on two different host plants. The uredospores and teleutospores affect the Graminece chiefly, but the ravages of accidia, though not so severe in their nature, extend over many species of Com posite, Ranunculacece, Leguminosw, and Labiatce. Suborder II. Ustilaginece. The life-history of this sub order, so far as yet known, consists in the formation of asexual spores, either singly, in series, or in masses, on very slender sporophores ; these germinate, and form a promy- celium bearing sporidia (as in the case of the teleutospores of the Uredinece), which in turn germinate and form a mycelium bearing the same kind of spores as those with which we started. This life-history, it will be seen, is, so far as it goes, similar to that of the Uredinece. The present order resembles the preceding one also in the habit