242 ASCOMYCETES. 



Schizothyrium. 



The roundish or oblong apothecia dehisce by lobes. The club- 

 shaped asci contain oblong, hyaline, two-celled spores. 



Sch. ptarmicae Desm. (Britain). This occurs as a parasite 

 on living green leaves and stems of Achillea Ptanni<-<i. The 

 apothecia form little black points, which on rupturing break 



FIG. 127. Schizothyrium ptarmicae on Achillea Ptarmica. (v. Tubeuf del.) 



up the epidermis into lobes. The thick asci contain two to 

 four large two-celled spores. Paraphyses are present. A pycnidial 

 form is known as Leptothyrium ptarmicae (Sacc.). 



Rhytisma. 



The fungi of this genus live in the tissues of living plants and 

 form sclerotial cushions as isolated black spots. In these places 

 the pycnidia are developed, and are followed by apothecia after 

 the death of the leaves. The apothecia open by a fissure, and 

 contain thread-like paraphyses and club-shaped asci with eight 

 needle-shaped spores, which are septate when mature. 



Rhytisma acerinum (Pers.) (Britain and U.S. America). 

 Towards the close of summer, the large black spots caused 

 by this fungus on leaves of various species of Acer (sycamore 

 and maple) are by no means uncommon. Pycnidia (Melasmia 

 acerinum Lev.), containing little unicellular conidia, are first 

 produced under the cuticle, while the epidermis and under- 

 lying cells become filled with mycelium till a black sclerotium 



