PYREXOMYCETES. 



131 



Order 18. Xylariaceae. This order is the most highly de- 

 veloped of the Sphferiales. The stroma arises on the surface of 

 the substratum, which is generally dead or decorticated wood ; 

 it is well-developed, crustaceous, hemispherical or upright. In 

 the younger conditions it is covered with a layer of conidia, and 

 later on it bears the perithecia, arranged in a layer immediately 

 beneath its siirface. The ascospores are of a dark colour. Often 

 also there are free conidiophores. 



Fia. 122. Xylaria Uypoxyton (nat. size) on a tree stump : a younger, b an older stroma, 

 both of which, with the exception of the black lower portion, are covered with white 

 conidia ; n, spot where the perithecia are developed ; c an old stroina with upper part 

 fallen off ; d, c large branched stromata ; fe conidia. 



Hypoxylon and Ustulina have a cushion-like or crustaceous stroma. Xylaria 

 has a club-shaped or branched stroma, often several centimetres high. X. 

 hijpoxylon (Fig. 122) and X. polymorplia occur on old tree stumps. Poronia 

 grows on old horse dung, and has a conical stroma. 



Sub-Family 3. Dothideales. 



The perithecia are always embedded in a black stroma, and are 

 not distinctly separated from it. The accessory forms of reproduc- 

 tion are : conidiophores, conidiocarps, and yeast-like conidia. The 

 majority are parasites. One order. 



Order Dothideaceae. Phyllachora graminis produces scab-like patches on 

 the leaves of the Grasses. Scirrhia rimosa grows on the leaf-sheathes of 

 Phragmites. Rhopographus pteridis on Pteridium aquilinum. 



