RHYTISMA. 



245 



eight needle-shaped unicellular spores ; pycnidia (spermogonia) 

 with little unicellular conidia are also formed. 



The fungus attacks leaves of sycamore {Acer Pseudoplatanus), 

 the black spots making their appearance in September. The 

 apothecia ripen on the ground during the following summer. 



Fig. 130. — Sections of Maple leaves 

 showing the upper epidermis ruptured 

 by 1, RKytisma acerinwm ; 2, Rhytisma 

 2mnctatum. 



Fig. 131. — Rhytisma symvietricum Miill. Two 

 leaves of Salix purpurea vrith stromata. A, The 

 upper side. B, The lower side. C, Longitudinal 

 section through the same leaf, showing numerous 

 apothecia on the upper side, fewer on the lower ; 

 the shaded middle part represents leaf-tissue, 

 the remainder is the light fungal stroma in 

 which the darker apothecia are embedded, 

 (v. Tubeuf del.) 



Rh. salicinum Pers. (Britain and U.S. America). Thickened 

 black wrinkled spots appear frequently on living leaves of various 

 species of willow, e.g. Salix Caprea, S. cinerea, etc., also on some 

 alpine willows, e.g. S. reticulata. These contain apothecia of this 

 fungus, which reach their full maturity during the second 

 summer. 



1 Dyscomycopsis rhytismoides Abull. Black spots similar to those of Rhytisma. 

 appear on the leaves of sycamore. The black crusts are here only subcuticular 

 and enclose a transparent tissue from which large spherical spores are produced. 

 The systematic position of this fungus is unknown. 



