HEMISPHAERIALES 



305 



leaf, a thin, light-brown fruit disc, of radiating prosenchyma, in one layer 

 at the periphery, with pseudoparenchyma in the middle. While this 

 develops to its definite limits, as in Stigmatea, it begins to rise from the 

 center outward, being pulled from the developing core, and to arch up 

 like a flat spherical cap or hemisphere. Thus the generative hyphae are 

 torn off, with increasing tension the cover is soon ruptured, forming an 

 opening which may be widened by histolysis. The asci arise singly in the 

 loculi and are generally eight spored. 



The development of the fructifications of the Microthyriaceae, 

 often called thyriothecia has puzzled the systematist for a long time. As 



rrrrn-T-rri 



Fig. 203. — Dimcrosporium Veronicae. 1. Group of fructifications on lower side of a 

 leaf. The older has ruptured at the top. 2. Section of fructifications. Asterina Usterii. 

 3. Hyphae and epidermal haustoria on lower surface of a leaf. (1 X 7, 2 X 250; 3 X 620; 

 after Arnaud, 1918, and Maire, 1908.) 



the thyriothecia are not oriented by the mycelium but by the host, their 

 generative hyphae hang downward. Hence they are inverted, i.e., their 

 morphological base is at the top. Since the Microthyriaceae have been 

 considered connected, through a series of intermediate forms, with 

 Meliola of the Perisporiaceae, the thyriothecia must be regarded as 

 halved perithecia, i.e., as perithecia in which the tip (lying beneath) has 

 not developed further for lack of space. According to this conception, 

 since the radial cover layer remained unexplained, the thyriothecia were 

 considered complex structures, i.e., as upright perithecia, lying free from 

 the substrate, protected by their radial shield and no longer needing a 



