PUFFING IN THE DISCOMYCETES 247 



the ascus-wall, when an ascus explodes, contracts circumferentially 

 to the extent of 25 per cent, of its original measure. As we have 

 seen, an ascus on exploding contracts in length only about 6 • 3 per 

 cent. It is clear therefore that, 'per linear unit, an ascus-wall con- 

 tracts about four times as much in the circumferential direction as 

 in the longitudinal. 



From the fact that the ascus-wall is four times as elastic in the 

 circumferential direction as in the longitudinal we may infer that 

 the physical structure of the ascus-wall is not homogeneous, but 

 is different in the one direction from what it is in the other. This 

 difference in structure makes for efficiency in the working of the 

 ascus when the ascus is considered as a mechanism for squirting 

 away ascospores ; for it is important : (1) for the ascus to contract 

 longitudinally as little as possible, so that before discharge it will 

 not be obhged to project much beyond the paraphyses ; and (2) for 

 the ascus to contract as much as possible circumferentially (and 

 therefore radially), so that the eight spores and the sap-drops shall 

 be shot out of the ascus-mouth with as great a velocity as possible. 



Owing to the contraction of the cell-wall, an ascus, on exploding, 

 reduces its volume to about one-half. Therefore an exploding ascus 

 must discharge about one-half its contents through its apical aper- 

 ture. The half of the contents which becomes discharged must be 

 the apical half, and this includes all the eight spores and a mass of 

 cell-sap having a volume perhaps equal to that of the spores. 



The ascus, as a gun, is a squirt from which a jet of watery fluid 

 containing eight spores is ejected by the pressure of a contracting 

 elastic cell- wall. The hydi'ostatic pressure of the cell-sap within Is 

 transmitted equally in all directions through the fluid and thus acts 

 upon the cell- wall equally at all points. So long therefore as the 

 ascus-gun during discharge is held firmly fixed in one position by 

 the paraphyses which surround it, it makes little or no difference, so 

 far as the range of the gun is concerned, whether the ascus- jet is 

 shot through a terminal opening in the ascus-wall, as in most Dis- 

 comycetes, or through an obhque subterminal opening, such as that 

 which is present in Sarcoscypha protracta. Thus we obtain a physical 

 explanation for the success of a gun which has the strange pecuharity 

 of not firing its projectiles in the dii'ection of its long axis but obhquely 



