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 
is the edible morel (Morchella), the surface of the cap 
region of the ascocarp being reticulated with irregular 
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 
FIG. 174. Morchella 
(morel) : the fleshy asco- 
carp arising from the 
mycelium. 
