196 



SPORE DISSEMINATION 



in volume, and when it reaches a definite size, the spore is shot 

 away, carrying the drop with it. Drop excretion mav be^in at 

 the hilum of one or more of the other members of the quartet 

 before the first spore is discharged, and only a few seconds or 

 minutes elapse between successive discharge of each member. 

 The sterigmata are turgid after discharge, and apparently the tips 



li TTrmniMiii ijj-. 





l—"V/s :••"». 



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Fig. 34. Two tvpes of evidence of spore discharge among Hvmenomvcetes. 

 A. Diagram of a spore print of an agaric, made bv placing the ventral 

 surface of the pileus horizontally on white paper in a moist chamber. B. 

 Diagram to indicate how spore discharge mav be observed in a beam of 

 light. A fragment of pileus is attached to cork, fastened to cover of vessel. 

 Basidiospores are circulated bv convection currents. 



are sealed. Afterward the basidia mav slowly lose turgor, but 

 they collapse only when death occurs. 



This description of the structures involved and the sequence 

 of events does not offer any explanation to account for the asym- 

 metrical position of the spore on its sterigma, as Ingold (1939) 

 points out, nor does it explain how the drops are excreted. Con- 

 cerning the force employed to discharge each spore, Bullcr (1922) 

 believes that it is caused by surface tension energy. From ingeni- 

 ous experiments and from calculations he found that the surface 

 energy on a drop of exudate on the spore of Psalliota cavipestris 

 is 0.000012 erg. To derive this figure the value of surface tension 

 is considered as 72 on a drop 2.3 \x in diameter with a surface area 

 of 0.000000166 sq cm. Not all this energy is available for dis- 



