THALLOPHYTES 



which reproduce by rapid budding, forming sprout chains (figs. 168- 

 172). When cultivated under certain conditions, four internal spores 

 are formed, and this is the only ascomycete connection (fig. 173). If 

 the connection is a true one, the yeast cell under certain conditions 

 becomes an ascus. The great economic importance of the group in 

 alcoholic fermentation is well known (see Part II, p. 409). 



The group Protoascales, however, is based upon a few forms with a 

 true mycelium, which are otherwise about as simple as yeasts. 



(b) Protodiscales 



This is a small group parasitic on seed plants, especially trees. A character- 

 istic genus is Exoascus, E. deformans causing the disease known as peach curl, 

 which results in a characteristic crinkling and deformity of the leaf. The mycelium 

 sends to the surface patches of asci, each ascus discharging eight ascospores. The 

 form is simple in the absence of ascocarp formation, the layer of asci, called the 

 hymenium or hymenial layer, arising from the mycelium 

 with no accompanying sterile structure. E. pruni 

 forms the so-called plum pockets, in which the young 

 fruit becomes of abnormal size and shrivels, the asci 

 appearing in the wrinkles. Other species of Exoascus 

 form brushlike deformities on certain trees, as wild 

 cherry, hornbeam, etc., known as witch brooms. The 

 best-known witch brooms, however, are formed by a 

 very different group of fungi. 



(c) Helvellales 



The mycelium of these forms is usually subterranean, 

 being saprophytic on decaying organic matter, and is 

 common in the humous soil of forests. The ascocarp is 

 a remarkable fleshy structure, rising above the surface 

 like a mushroom, the hymenium occurring as a super- 

 ficial layer variously distributed. The best-known form 



FIG. 174. Morchella 

 (morel) : the fleshy asco- 

 from the 



is the edible morel (Morchella), the surface of the cap carp arising 



region of the ascocarp being reticulated with irregular mycelium. 



pitlike depressions lined with hymenium (fig. 174). A 



section shows that the hymenium is a mixture of paraphyses (sterile filaments) 



and asci. 



(d) Pezizales 



General character. The cup fungi form a very large group of sapro- 

 phytes, characterized by a broadly open ascocarp lined with the hy- 

 menium. The ascocarp may take the form of a flat disk, a bowl, a cup, 

 a funnel, and is usually called an apothecium, to distinguish it .from 



