134 PLANT MORPHOLOGY 



a height of 15 to 20 cm. A mature ascocarp of Morchella is differentiated 

 into a thick hollow stalk and a conical cap (Fig. 109A). The surface of 

 the cap contains numerous depressions lined with a hymenium consisting 

 of parallel asci and paraphyses. 



Helvella. The mycelium is subterranean and composed of hyphae 

 with multinucleate cells. It gives rise to fleshy ascocarps that push 

 upward to the surface of the ground, there reaching a height of about 5 cm. 

 These are differentiated into a stout stalk and a saddle-shaped cap, the 

 outer surface of which is covered with a hymenium consisting of parallel 

 asci and paraphyses (Fig. 109B). The asci contain eight ascospores and 

 discharge them into the air with considerable force. 



7. Tuberales 



The Tuberales are the well-known truffles, esteemed as a gastronomic 

 delicacy. There are nearly 300 species, the representative genus being 

 Tuber. The mycelium is subterranean, especially in woods, some forming 

 the mycorrhiza of forest trees. Truffles occur in California and in various 

 parts of southern and central Europe. Their life history is incompletely 

 known. The ascocarp is fleshy and matures underground. It is more or 

 less globular, its diameter rarely exceeding 8 cm. It is usually open when 

 young but later nearly or completely encloses the asci. The ascocarp is 

 thus a modified apothecium. The hymenium may surround a large cen- 

 tral cavity or it may form irregular folds that divide the cavity into 

 chambers. 



8. Pyrenomycetales 



The Pyrenomycetales, or black fungi, are a large order of about 450 

 genera and 14,000 species that are generally segregated into three smaller 

 orders, the Hypocreales, Dothideales, and Sphaeriales. They include 

 saprophytes that live on decaying wood, humus, etc., and parasites that 

 attack various seed plants. Some representative genera are Nectria, 

 Claviceps, Plowrightia, Venhiria, Xylaria and Neurospora. 



Nectria. This large genus of about 250 species grows on living or dead 

 wood. It is responsible for several important fungous diseases. One of 

 the most destructive of these, canker of woody plants, is caused by Nectria 

 cinnabarina and Nectria galligena. They attack a great variety of shrubs 

 and trees, but not conifers. The fungus gains entrance through wounds 

 in the stem. The cortex becomes infected and its cells are immediately 

 killed. This results in a wound that gradually enlarges. Sometimes 

 enough cork tissue is developed around the infected area to close the 

 wound, but usually this is not possible and the trunk is finally girdled. 

 During the summer the mycelium produces large, pinkish, disk-like 

 masses, or stromata, that break through the bark and give rise to large 



