ORDER PEZIZALES: SUBORDER INOPERCULATAE 



235 



Fig. 82. Pezizales, Family Phacidiaceae. Rhytisma acerinum (Pers.) Fr. (Courtesy, 



F. C. Strong.) 



Several species of Lophodermium also attack the needles of conifers, caus- 

 ing serious leaf fall. In this genus the stroma contains only a single 

 apothecium, but both stroma and apothecium are elongated and narrow, 

 and the dehiscence is by means of an elongated slit. Rhytisma, the cause 

 of the tar spot of leaves of maple (Acer) and other plants, produces a 

 large more or less isodiametric subcuticular stroma on the upper side 

 of the leaf and usually a smaller sterile stroma on the lower side. After 

 leaf fall the apothecia begin to develop slowly but do not reach maturity 

 until the folloAving spring. In a single stroma are produced numerous 

 elongated apothecia. These do not lie strictly parallel but are more or 

 less sinuately curved or sometimes radiately arranged. At maturity the 

 stroma forms a slit over each apothecium, under proper moisture con- 

 ditions pulhng back at the sides so that the hymenium is fully exposed. 

 At least three species occur on various species of Acer, each showing a 

 narrow specialization to only one host or to a group of host species. The 

 Red Maple {Acer ruhrum L.) is very subject to the disease in some parts 

 of North America. S. G. Jones (1925) has shown that in the stroma there 

 arise ascogonia, with at first one or two cells, which become three- to 

 five-celled. The cross walls become perforated and almost completely 

 absorbed and the nuclei pass into one of the central cells Avhich we must 

 conclude is the oogone. From this arise the ascogenous hyphae with 



