274 



A TEXTBOOK OF THEORETICAL BOTANY 



Surface of stroma 



Ascus layer 



Leaf mesophyll 



Fig. 264. — RI.Ytisiiui aceiinum. Apothecium with 

 developing asci showing beginning of rift in roof. 



Ost/o/e 



Stroma 



Leaf mesophyll 



^ftj^SIM 



Fig. 265. — Rhytismo acerimim. Mature apothecium 

 showing roof breaking open. 



Since the development of the asci takes place during the winter it must 

 obviously occur when the leaves have been shed, hence the discharge of the 

 ascospores will take place in the early spring at a time when the leaf remains 

 have largely rotted and the bits are blown about. In this way additional dis- 

 tribution of the Fungus is effected quite apart from the fact that the ascospores 

 are forcibly ejected from the ascus. Moreover, as a result of contraction of 

 the epithecium the ascospores may be shot up as much as i mm. above 

 the surface of the substratum. They ultimately reach the young leaves as 

 they unfold in the spring and there germinate to start the life-cycle over again. 



