194 SPORE DISSEMINATION 



SPORE DISCHARGE AMONG BASIDIOMYCETES 



Most of our knowledge of spore discharge among the Basidio- 

 mycetes comes from the painstaking researches of Buller (1924, 

 1933). Throughout this entire group with its numerous species, 

 except for the Gastromycetes, which is constituted of relatively 

 few species, essentially the same mechanism of discharge prevails. 

 This generalized mechanism has been termed the "drop-excretion 

 mechanism." As a matter of fact, no satisfactory explanation of 

 how this mechanism causes the spores to be forcibly expelled is 

 as yet forthcoming, but the problem can be properly appreciated 

 if the structure of the basidium is first learned and is kept clearly 

 in mind. The hymenium, whether plane or having pores, gills, 

 teeth, or other modification to increase the spore-bearing surface, 

 is composed of a palisade of basidia. In some species sterile cells 

 (paraphyses or cystidia) are interspersed among the basidia. Each 

 basidium is a turgid clavate to saccate cell. Apically on this cell 

 are formed typically four conical projections, the sterigmata. 

 The tip of each sterigma soon becomes slightly bulbous, and the 

 inflated portion increases, simulating the appearance of a soap 

 bubble being blown. These portions are the basidiospores, which 

 vary in shape and surface markings among the different species. 

 Mature basidiospores are always inequilateral, with the more plane 

 surfaces of the quartet of spores directed toward each other. The 

 hilum of each points inward and is thus asymmetrically placed. 

 Presumably a wall eventually forms to separate the basidiospore 

 from the tip of the sterigma. 



As far as the structural features just recounted are concerned, 

 all investigators are in accord. In connection with discharge it- 

 self and the forces involved, however, there remain unexplored 

 possibilities. For a long time it was generally believed that a 

 water-squirting mechanism somewhat comparable to that in Pilo- 

 bolus causes discharge. This would be expected to operate most 

 effectively if all 4 spores of a basidium were discharged simul- 

 taneously. As a matter of fact, the basidiospores are discharged 

 successively one at a time. Of course this type of mechanism 

 might still be capable of operating to discharge the spores succes- 

 sively if the tip of the sterigma were to be sealed before appreci- 

 able loss of turgor within the basidium. That there is actually no 



