ZYGOMYCETES 351 



Polygonum Hydropiper, and replaces it by a dense plexus of hyphae, 

 •which exhibits a differentiation of a thick outer wall enclosing the 

 "whole, a cylindrical columel, both colourless, and a dark-violet mass 

 of resting-spores filling up the space between. An undifferentiated 

 basal portion discharges the function, as it were, of a meristem, and 

 adds new elements to wall, columel, and spore-mass. The fungus 

 attacks only the ovule ; and as it grows thus in bulk, the unaffected 

 wall of the ovary is burst, and exposes the brittle wall of the sporal 

 mass, which breaks at the slightest touch, and sets the resting-spores 

 free. 



The germination of the resting-spores, which are usually round or 

 many-sided, and possessed of both intine and extine, the latter often 

 covered with characteristic fine granulations, varies according as it takes 

 place in water merely or in a nutritive fluid. In water only the resting- 

 spores of Entyloma, Tilletia, Tuburcinia, and Urocystis produce a 

 short promycele-tube, which bears simultaneously on its blunt apex 

 a whorl, or • crown ' of elongated gametes, varying in number. ]\Iany 

 species of Ustilago (Pers.) and Tolyposporium (Woron.) produce a 

 promycele-tube which divides into a number of cells, from each of 

 which bud off laterally rod-shaped sporids or gametes, somewhat as a 

 yeast-cell buds off. The resting-spores of Thecaphora LatlA-ri (Kuhn) 

 and Ustilago longissima (Tul.) give rise to a short slender promycele- 

 tube, which bears an acrogenous sporid, or several serially ; while in 

 Thecaphora hyalina (Fingerh.), Ustilago carbo (Tul.), and U. destruens 

 (Tul.) it divides mto several cells, each of which produces, if any, 

 only one sporid. 



The conjugation of gametes in pairs takes place either before or 

 after separation from the promycele, by means of short or long cross 

 hyphje, according to the distance between them. The product of this 

 union is the outgrowth from the connected gametes of a mycelial germ- 

 tube, or, as occurs in Tilletia, Entyloma, Tuburcinia, and Urocystis, 

 sporids are produced on the outgrowth from the connected pair, and 

 these then emit mycelial germ-tubes. In a number of forms gametes 

 are constantly produced which do not conjugate, but give rise to 

 mycelial germ-tubes, as sporids do, while in Sorosporium Saponarise 

 (Rudolph.) the resting-spore itself emits the mycelial germ-tube directly, 

 without the intervention of promycele, gametes, and sporids. What 

 occurs regularly in these species may occur exceptionally in the case of 

 individuals the gametes of which ordinarily conjugate ; that is, they 

 may fail to produce gametes or sporids at all, or their gametes may fail 

 to conjugate, either all of them, or, where the number of gametes is an 

 uneven one, the odd gamete. In such exceptional mstances the 



