INTRODUCTION 207 



containing pairs of nuclei and forming asci at their tips by means of hooks 

 or croziers in the manner described below in Pyronema. As the asci en- 

 large, the thin-walled hyaline pseudoparenchymatous cells of the peri- 

 thecium are destroyed except for a number of slender hyphae at the top 

 (periphyses) which seem to play a part in the formation of an apical 

 opening (ostiole) in the spore fruit, through which the mature asci pro- 

 trude as they discharge their ascospores. (Fig. 69.) 



An appreciation of the characteristic sexual reproductive processes 

 in the more complex Ascomyceteae can perhaps best be obtained by a 

 study of the phenomena in Pyronema omphalodes (Bull, ex Fr.) Fckl. 

 This is by no means a very primitive form nor is it a simple structure. It 

 illustrates, however, most of the features that occur in this class. This 

 fungus is found in nature most frequently on patches of soil where there 

 has recently been a fire, such as the site of a camp fire. It also appears 

 frequently in greenhouses on flower pots that have been steamed to 

 sterilize the soil. At first there appears a thin whitish, moldy growth on 

 which arise groups of orange-colored apothecia which give the whole sur- 

 face of the soil an orange color lasting only a few days, after which the 

 fungus disappears, to be followed by other fungi. The mycelium is color- 

 less and septate, with its cells mostly multinucleate. Tufted branches are 

 produced, each bearing terminally a more or less spherical, multinucleate 

 oogone, from whose apex there grows out a curved hypha, also multi- 

 nucleate, the trichogyne. From one of the basal cells supporting the 

 oogone arises an obovoid or clavate multinucleate antherid. The tricho- 

 gyne grows to the antherid and coils upon or around its apex. An opening 

 is then formed from one to the other. The majority of the hundred or 

 more antheridial nuclei pass into the trichogyne, whose nuclei have al- 

 ready begun to degenerate, and then through an opening in the septum at 

 the base of the trichogyne into the oogone which itself contains 100 to 

 200 nuclei. Here they pair with the oogone nuclei. According to Harper 

 (1900) and to Gwynne-Vaughan and Williamson (1931) the paired 

 nuclei fuse, forming about half as many diploid zygote nuclei. According 

 to Claussen (1912) they do not fuse but merely pair closely. According 

 to Dangeard (1907) and his followers no opening is formed between the 



Fig. 69 — {Continued) 



(B) Spermogonium with sperm cells mostly discharged. (C) Very young perithecium 

 showing oogone with one large nucleus and short trichogyne. (D) Apical portion of 

 fully developed trichogyne with attached sperm (a) and the small trichogyne nucleus 

 (b). (E) Oogone and lower half of trichogyne showing sperm nucleus (a) and tri- 

 chogyne nucleus (b). (F) Oogone containing two nuclei (male and female). (G) Oogone 

 with eight pairs of nuclei, each in a mass of denser cytoplasm. (H) Ascogenous hyphae, 

 branching out from oogone. (I) Early stages of ascus formation on branching asco- 

 genous hypha. (J, K) Nearly mature and mature perithecia. (Courtesy, Higgins: Am. 

 J. Botany, 23(9):598-602.) 



