240 STRUCTURAL BOTANY 



Sphcerotheca represents the course of development of 

 an Ascomycete in almost its simplest form. It is quite 

 exceptional for the ascogonium to produce merely a 

 single ascus ; in the great majority of the Ascomycetes 

 a very large number are formed ; in many cases, how- 

 ever, it has not been found possible to refer the origin 

 of the asci to any definite ascogonium, and there are 

 only very few species in which there is at present 

 any clear evidence for the occurrence of an act of 

 fertilisation. 



The formation of a completely closed fruit around the 

 ascus or asci is characteristic of the Family (Erysipheae) 

 to which Sphcerotheca immediately belongs. In the 

 majority of the Order Ascomycetes, the fruit is more or 

 less open, either having a small pore at the apex, or 

 taking the form of a widely open cup or disc. Our 

 next type will afford an example of the form of fruit last 

 mentioned. 



TYPE XXI, PHYSCIA PARIETINA 



A very large group of Ascomycetes have their 

 ascus - fruits in the form of an open cup, or even 

 a flat, shield - like disc. These constitute the Sub- 

 order Discomycetes. The inside of the cup or the free 

 surface of the disc is coated by the hymenium, a name 

 applied in descriptions of the higher Fungi to the 

 layer of spore-producing cells. In the case of Dis- 

 comycetes the hymenium is made up of a large number 

 of vertical asci, with sterile hairs the paraphyses 

 between them. 



The particular plant we have chosen to represent this 



