THE FUNGI : ASCOMYCETES AND BASIDIOMYCETES 



309 



twice, during which meiosis occurs, resulting in a row of four nuclei. This 

 hypha functions as a basidium, and it becomes transversely septate into four 

 uninucleate cells (Fig. 303). From the upper end of each cell a short sterigma 

 is formed, from the outer tip of which a single basidiospore is abstricted. 



A 



Fig. 303. — Ustilago nudo. \ and B, Germination 

 of chlamydospore to produce a four-celled 

 basidium. C, Fusion of basidia from two 

 chlamydospores. {After Rmcitscher.) 



The original nucleus of each cell divides into two, and one of the daughter 

 nuclei then migrates through the sterigma into the basidiospore. These 

 basidiospores are oval in shape and thin walled. 



The segregation into ( + ) and ( - ) strains takes place during the meiosis 

 before the formation of the basidiospores. As a result of cultural experiments 

 in growing separately all four spores from a single basidium, it has been shown 

 that segregation of the sexes may occur either in the first or in the second 

 division. In an allied species U. longissima, for example, high temperature 

 favours reduction at the first division. In another set of experiments it has 

 been found that not only the temperature but also the Hydrogen ion con- 

 centration of the medium, as well as the concentration and relative pro- 

 portions of Carbohydrate and Nitrogen in the medium are factors which 

 determine whether the reduction shall be in the first or second nuclear 

 division in the basidium. 



These basidiospores may not immediately produce a hypha, but may 

 develop by a process of yeast-like budding, whereby secondary' spores are 

 produced. This may occur particularly in manure heaps after smutted grain 

 has been fed to stock. 



These secondary spores are then capable of infecting fresh grain plants. 

 It should be noted that the practice of spreading manure on fields shortly 



