21S THE BASIDIOMYCETES 



enlarged tip is the uninucleate basidiospore. Sometimes only 2 

 sterigmata are produced; then each of the 2 spores may receive 

 2 nuclei, or 2 nuclei may remain unused within the basidium. 



On germination the basidiospores, which are almost universally 

 uninucleate (monocaryotic), give rise to hyphae with uninucleate 

 cells, which constitute the primary mycelium. Soon the primary 

 mycelium gives rise to binucleate (dicaryotic) cells that com- 

 pose the secondary mycelium. The process of mating in which 

 haploid myceha become diploid is called diploidization. It is 

 very commonly accomplished by clamp connections. These are 

 buckle-like devices, formed at the septa, that permit a pair of 

 nuclei to be walled off in each cell. 



All subsequent nuclear divisions of this pair of nuclei or of 

 their progeny are conjugate; that is, they divide coincidentally. 

 As a result the vesfetative mycelium remains dicaryotic through- 

 out. Eventually fruit bodies are produced from fundaments laid 

 down by the dicaryotic mycelium. When the hymenial layer is 

 differentiated in the fruit body, the terminal cells that are to be- 

 come basidia are dicaryotic. The cycle of development may 

 then be initiated anew by fusion (final stage of diploidization) of 

 the pair of nuclei within each young basidium. 



Types of basidia. Basidia, as has been indicated, vary widely 

 in structure and development. In the Uredinales and Ustila- 

 ginales the teliospore, a uninucleate but diploid thick-walled 

 structure, is the probasidium. On germination it gives rise to 

 a tubular basidium, sometimes called a promycelium. Meanwhile 

 meiotic nuclear division has taken place, after which, by trans- 

 verse septation, the basidium becomes a row of cells, usually 4, 

 each containing a haploid nucleus. Each nucleus then migrates 

 into a laterally or terminally formed bud, which becomes ab- 

 stricted and is the basidiospore. Among the Ustilaginaceae, the 

 primary haploid nuclei within the basidium may continue to 

 divide, and as a result many basidiospores may be produced. 



Quite the same type of basidial structure occurs among all 

 Auriculariales. The cell from which the so-called epibasidium 

 (promycelium) arises is commonly designated the hypobasidium 

 (probasidium). After the first nuclear division in the hypo- 

 basidium, a cross wall is formed in the elongating epibasidium; 

 after the next division other cross walls are laid down to form 

 a row of superimposed cells. From each epibasidial cell in 



