Introduction to Fungi, Third Edition

HYPOCREALES
337
Fig12.13
serpens.
The conidial (Geniculosporium)stateofHypoxylon
Fig12.12 Hypoxylon. (a) Perithecial stromata of H. fragiforme
on beech (Fagus sylvatica).One stroma has been broken open
to show the perithecia embedded in the outer layer.
(b) Perithecial stromata of H. multiforme on birch (Betula
pendula).
12.4 Hypocreales
The Hypocreales are a large group of fungi,
although estimates vary as to the number of taxa
contained in it. Kirk et al. (2001) included 117
genera and 654 species, whilst Rossman (1996)
stated that there may be 2000 5000 holomorphic
taxa. Some are known only as anamorphs.
Hypocreales are characterized by pale or brightly
coloured perithecia (or rarely cleistothecia)
which may be single or borne on or embedded
in a fleshy stroma. The asci are unitunicate, with
or without a well-defined apical apparatus.
Perithecial development conforms to the
‘Nectria’ type of Luttrell (1951). The ascogonia,
which are formed within a stroma, become
surrounded by concentric layers of vegetative
hyphae which form a true perithecial wall. The
cells of the inner wall layer in the apical region
of the young perithecium produce a palisade of
inward-growing hyphal branches. These hyphal
branches grow downward to form a vertically
arranged mass of hyphae with free ends termed
apical paraphyses (Luttrell, 1965). Pressure
exerted by the elongation of the apical paraphyses,
accompanied by expansion of the wall,
creates a central cavity within the perithecium.
The free tips of the apical paraphyses ultimately
push into the lower portion of the wall so that
they become attached at both the top and
bottom of the perithecial cavity. Ascogenous
hyphae arising from the ascogonium spread out
across the floor and sides of the cavity and
produce asci by means of croziers. The asci grow
upward among the apical paraphyses and form