IV J PHACIDIALES I33 



of calcium oxalate; when the fruit opens it forms a white border around the 

 hymemum. The pale colour, and the ragged or toothed dehiscence of the 

 sheath are very characteristic. 



Phacidiaceae 



The Phacidiaceae are distinguished by their black, thick-walled apothecia, 

 usually scattered, sometimes, as in Rhytisma, grouped on a black stroma. 

 Where the fertile disc is circular the sheath splits in a stellate manner, but 

 where it is elongated, dehiscence takes place by means of a slit running 

 along its entire length. The species occur chiefly on dead herbaceous stems 

 or leaves, but a few are parasitic. 



Rhytisma Acerinum (fig. 93) infects the leaves of various species of Acer 

 (maple and sycamore). The mycelium ramifies in the living tissues of the 



Fig. 93. Rhytisma Acerinum (Pers.) Fr.; apothecium, x 160. 



host and causes yellow spots on the leaves about three weeks after infection. 

 Some five weeks later pycnidia develop under the cuticle and produce small 

 unicellular conidia. The epidermis and underlying tissues of the host become 

 filled with hyphae and a dense, black sclerotium is completed. In this state 

 the leaf falls and next spring the sclerotia thicken and become wrinkled; 

 finally they burst by elongated fissures and expose the discs of the apothecia. 

 The ascospores are filiform and septate; they are ejected with some force 

 and reach the living leaves to which they are probably carried by the wind. 



HYSTERIALES 



The Hysteriales are characterized by the black, elongated ascocarp, 

 dehiscing by a longitudinal slit, so narrow that the disc.is almost permanently 

 concealed. 



The species are all minute; in some the disc is narrowly elliptical, in 



