THALLOPHYTA: FUNGI 



139 



Xylaria. Xylaria is a large genus of about 200 species. It is a common 

 saprophyte, the myeehum Uving in decaying wood. It produces sclerotia 

 from which black, club-shaped, often branched stromata arise. At first 

 these are covered with a mass of white conidiophores from which small 

 oval conidia are abstricted. Later the stromata produce numerous 

 embedded, flask-shaped perithecia lined with a hymenium (Fig. 115). 



Neurospora. This is the pink bread mold, a form much used experi- 

 mentally in genetics. The mycelium produces conidia in branched chains. 

 Perithecia are rarely formed. They are 

 dark-colored, pear-shaped, and without 

 paraphyses. Like Rhizopus, Neurospora 

 is heterothallic and sexual reproduction 

 occurs only when a plus and a minus 

 strain come together. The young peri- 

 thecium contains a coiled ascogonium 

 from which trichogynal hyphae grow out. 

 If these come in contact with spermatia, 

 conidia, or hyphae of the opposite strain, 

 the perithecia mature and asci are pro- 

 duced. Two nuclei of opposite sex fuse 

 in the young ascus, the fusion nucleus 

 undergoes three divisions of which the 

 first two are meiotic, and eight ascospor^ 

 are formed in the usual way. Experi- 

 ments have shown that sexual differen- 

 tiation occurs in connection with asco- 

 spore formation, usually during the first 

 meiotic division but sometimes during 



the second. As a result, four ascospores in each ascus will produce plus 

 mycelia and four minus mycelia. Other genetic characters behave 

 similarly. 



Summary. The Pyrenomycetales include both saprophytes and para- 

 sites. They are characterized by a flask-shaped ascocarp (a perithecium) 

 with a small opening at the top. It is Uned with a hymenium composed 

 of parallel asci and paraphyses. The perithecia may arise singly on the 

 mycelium, in small groups, or may be embedded in a compact mycelial 

 mass, the stroma. Sex organs are present in some members. 



Differences in the character of the perithecia and stromata provide a 

 basis for splitting up this large order into three smaller orders. 



1. Hypocreales. These forms have soft, bright-colored perithecia with 

 a definite wall. The perithecia may occur singly or in a stroma, which is 

 also bright-colored. They include Nectria and Claviceps. 



2. Dothideales. Members of this group have black stromata in which 



Fig. 115. Longitudinal section 

 through a perithecium of Xylaria, 

 showing asci arising from the 

 hymenium, X 100. 



