150 



PLANT MORPHOLOGY 



basidium. These fuse, two successive divisions take place, and the four 

 resulting nuclei pass through the sterigmata into the basidiospores (Fig. 

 V2iiE-G). The reduction of chromosomes occurs when the fusion nucleus 

 divides. The cultivated variety of Agaricus cmnpestris is exceptional in 

 that only two basidiospores are borne on a basidium, each of which 







U.'ry. 



'■■■&X-* 



Fig. 123. Clamp formation and development of the basidium in Armillaria mucida. 

 A, beginning of damp formation in binucleate terminal cell; B, one nucleus passing into the 

 clamp; C, conjugate division of the two nuclei; D, appearance of walls cutting off uni- 

 nucleate clamp and basal cells from young binucleate basidium; E, fusion of clamp and 

 basal cells, the latter sending out another branch; F, basidium with diploid fusion nucleus; 

 G, basidium with four haploid nuclei and the developing sterigmata. {Ajter Kniep.) 



receives two of the four haploid nuclei. The mycelium of both the wild 

 and cultivated form is multinucleate and probably unisexual (homo- 

 thallic). 



In most mushrooms the basidiospores, upon germination, give rise to 

 mycelia of two different sexes. These have uninucleate cells. When two 

 mycelia of opposite sex come together, fusions take place between vegeta- 

 tive cells, resulting in the formation of a binucleate mycelium. Upon 

 this the basidiocarps are produced. 



