CLASS ASCOMYCETEAE 265 



trichogyne, often aided perhaps by insects or mites crawling over the 

 growing mycehum, it adheres to it and soon there occur growth and 

 multipHcation of the cells and in about four days the perithecium is 

 mature. Similar structures have been studied in a number of other fungi. 

 Higgins (1936) studied the development of Mycosphaerella tulipiferae 

 (Schw.) Higgins which forms its conidial stage (Cercospora liriodendri 

 E. and H.) on the living leaves oi Liriodendron tulipijera L. and its asciger- 

 ous stage in the fallen leaves. In the small dark spots formed by the 

 conidial stage ascogonia may be found, each consisting of an ovoid, uni- 

 nucleate oogone with a slender uninucleate trichogyne several times its 

 length. This trichogyne projects from the surface of the leaf. Ostiolate 

 spermogonia are also produced and within these are many sperm mother 

 cells in each of which are formed four non-motile, thin-walled sperm cells. 

 Under the influence of moisture the gum contained in the spermogonia 

 swells and the sperm cells are extruded and carried passively in the 

 surface film of water to the trichogynes to which they adhere. Usually 

 near the tip of the trichogyne a sperm grows fast to a small papilla and 

 the nucleus enters through the latter and passes down to the oogone, 

 enlarging as it progresses. The sperm and egg nuclei do not unite in the 

 oogone but divide conjugately until eight or more pairs are formed, each 

 pair enclosed in a mass of more deeply staining cytoplasm. Short ascogen- 

 ous hyphae then arise and the paired nuclei pass into them. Asci are 

 formed by the crozier method and in these asci the nuclear fusion occurs, 

 followed by three divisions which thus give rise to the nuclei of the eight 

 ascospores. When the latter are formed another nuclear division occurs 

 and each ascospore becomes two-celled. The ascocarp walls and internal 

 pseudoparenchymatous contents arise as more or less parallel hyphae 

 growing upward beneath the epidermis before the ascogonium is initiated. 

 By their growth and branching a pseudoparenchymatous stroma is formed 

 with a firm black external wall around the developing ascogonium and 

 ascogenous hyphae. The developing asci press aside or dissolve the color- 

 less thin-walled cells of the interior of the stroma and eventually an 

 ostiole is produced through which the mature asci one by one stretch to 

 the exterior and discharge their ascospores. (Figs. 68, 87.) 



In two other species of Mycosphaerella, one with Septoria as its conid- 

 ial stage and the other with a Cercospora stage Higgins has observed the 

 formation of trichogyne and sperm cells. According to Wolf (1943) only 

 about 12 species out of the 500 species of Cercospora listed by Miss 

 Lieneman (1929) as occurring in North America have their perfect stages 

 known, all belonging to the genus Mycosphaerella. 



The formation of sperm cells is done away with in Venturia inaequalis 

 (Cke.) Wint., fertilization being effected by the direct passage of several 

 male nuclei from the antherid through a pore into the tip of the tricho- 



