126 



Mycologia 



scope. The ruptured outer wall contracts both in length and 

 width. Soon the outer wall contracts at a point about one third 

 of the distance up from the base of the ascus, thus forming a 

 ring, while the ascus continues to swell by taking in more water. 

 The pressure, partly released after the rupture of the outer wall^ 



to 



abed 



Fig. I. Camera lucida drawing of an ascus of Pyrenophora in different 

 stages of ascospore discharge, showing: a, normal ascus immediately after 

 leaving the perithecium ; h, outer wall ruptured and contracted in form of 

 ring; c, ascus with all ascospores above the ring, pressure in ascus nearing 

 critical point ; d, discharged ascus with ascospores lying as thrown from ascus. 



continues to increase until it reaches the critical point once more ; 

 the inner wall then ruptures, not at the apex, but just above the 

 ring formed by the contracted outer wall, and the spores are 

 thrown out with great force. Because of the resistance of the 



Fig. 2. Camera lucida drawing of ascus under high power, showing the 

 two ascus walls, the outer ruptured and partially contracted, the inner much 

 thickened and pressing upon the ascospores especially in the lower part. 



water, they usually remain near the point of rupture of the inner 

 wall, but that they are thrown out with great force is shown by 

 the rapidity of their motion which is so great that it is very diffi- 

 cult to follow their exit from the ascus. The rapid shrinking of 

 the empty ascus after the discharge of spores corroborates this 

 point. 



