12 



STRUCTURE AND CLASSIFICATION OF FUNGI 



formed (Fig. 4a). Progeny of the original paired nuclei, many- 

 nuclear generations removed, fuse in the basidia. 



Conjugate nuclear division of many species of Basidiomycetes is 

 accomplished by the formation of clamp connections. From the 

 terminal binucleate cell, a hook bends back (Fig. 45), each nucleus 

 divides (Fig. 4c), and two septa are formed, one cutting off a single 

 nucleus in the hook and the other cutting off another nucleus of op- 

 posite mating type in the penultimate cell, but leaving one of each 

 mating type in the terminal cell (Fig. 4(i) . The hook and the penul- 

 timate cell then anastamose and each cell now has two nuclei of 



AT« 



Fig. 3. Section through the margin of a gill of a mushroom (Coprinus sp.) 

 showing: a, basidia; b, sterigmata; c, basidiospores. 



opposite mating type (Fig. 4e). This continues throughout the bi- 

 nucleate stage of the organism. Eventually nuclei in the terminal 

 cells fuse and so form a diploid nucleus (Fig. 4^). This nucleus 

 undergoes a reduction division and divides again (Fig. 4/i). The 

 four nuclei, now again haploid, migrate through projections (sterig- 

 mata) of the now swollen tip cell (basidium) and are cut off to be- 

 come the four haploid basidiospores (Fig. 4i). These basidiospores 

 usually are mononucleate and haploid, of different mating types and, 

 when mature, are forcibly discharged and are capable of germinating 

 to form the haploid mycelium again. Only haploid mycelium is 

 produced unless mycelia of opposite mating type come in contact, in 

 which case cells may fuse to produce the binucleate mycelium again 

 (Fig. 4a). 



The presence of clamp connections is thus an outward sign that 

 the mycelium is binucleate and has arisen from the fused mycelium 

 derived from two different spores (Fig. 4a). Thus the sexual act in 

 such fungi is divided into two phases, a fusion of cells from mycelium 

 arising from different spores early in the growth of the mushroom, 

 and a fusion of the nuclei at a much later stage, when the plant is 

 mature. 



