248 



STRUCTURAL BOTANY 



sterile tissue at the edge of the apothecium, the whole 

 disc within this rim being covered by the hymenium or 

 thecium. Below the thecium is a dense mass of closely 

 interwoven hyphse forming the hypothecium. The thecium 

 itself consists of elements of two kinds, the asci and the 



paraphyses. The asci, of which 

 a great number are present 

 in each fruit, are stout, club- 

 shaped cells set vertically to the 

 surface of the apothecium, each 

 ascus when ripe containing eight 

 ascospores. The paraphyses are 

 sterile hairs rising to a greater 

 height than the asci, both being 

 closely packed together, so that 

 the thecium has a smooth, con- 

 tinuous surface (see Figs. 98 

 and 101). 



At an earlier stage of de- 

 velopment the apothecium is 

 closed, and consists of a mass 

 of hyphse surrounded by a cor- 

 tical layer. The paraphyses 

 which arise from the hypo- 

 thecium are the first elements 

 of the thecium to be developed. 

 The asci, which in many cases 

 have been observed to arise 

 the branches of a distinct hypha, differing from 



FIG. 101. Physcia parietina ; 

 part of a vertical section 

 through an apothecium. p, 

 paraphyses ; a, asci one 

 immature, the other two 

 containing eight ascospores 

 (sp) each ; c (above), hypo- 

 thecium ; A, A, layers of 

 algal cells ; m, medullary 

 layer ; c (below), the lower 

 cortical layer. Magnified 

 about 250. (After Lauder 

 Lindsay.) 



from 



those which produce the paraphyses, are developed re- 

 latively late. They grow up among the paraphyses, 

 insinuating themselves between them until they attain 

 nearly the same height. At the same time the envelope 



