Introduction to Fungi, Third Edition

598 HETEROBASIDIOMYCETES
(Uetake et al., 1992), and the same protocorm,
mature root or even individual cell can be
colonized simultaneously by different fungi.
The unstable nature of the orchid mycorrhiza
is indicated by the quick and repeated cycle of
peloton formation and degradation, and the
several different outcomes of the orchid fungus
interaction observed under laboratory conditions
(Fig. 21.2d). A balanced mycorrhizal symbiosis
will develop only in a proportion of
protocorms, whereas other seeds of the same
orchid species may be parasitized and killed by
the fungus, or simply resist infection and stall in
their development (Hadley, 1970; Smreciu &
Currah, 1989; Beyrle et al., 1995). There is also
vevidence of a succession of mycorrhizal fungi
during the development of an orchid in nature,
and the mycorrhizal fungi isolated from adult
plants may not support protocorm growth and
vice versa (Xu & Mu, 1990; Zelmer et al., 1996).
Mature orchids may be associated with
Rhizoctonia spp. and/or a range of other Basidiomycota,
including saprotrophic (e.g. Mycena),
necrotrophic (e.g. Armillaria; see p. 546) or
ectomycorrhizal species related to Russula and
Thelephora (Rasmussen, 2002). Ectomycorrhizal
fungi transport carbohydrates from their tree
host to the orchid, thus allowing green orchids
to grow even in densely shaded woodland
conditions (Taylor & Bruns, 1997; McKendrick
et al., 2000; Bidartondo et al., 2004). Interestingly,
these fungi form typical ectomycorrhiza with
their tree hosts but pelotons in infected orchid
roots (Zelmer & Currah, 1995). The orchid therefore
calls the shots in its symbiosis with
basidiomycetes, and peloton formation and
degradation is traditionally interpreted as a
balanced defence reaction of the orchid against
invasion attempts by the fungus.
21.3 Dacrymycetales
This order is characterized by forked (furcate)
basidia (Figs. 21.4 and 21.5), which are found in
all species except Dacrymyces unisporus. The fruit
bodies are coloured yellow or orange due to the
presence of a wide range of carotenoids (for
references, see Gill & Steglich, 1987). Fruit bodies
are gelatinous and show a striking diversity of
forms as exemplified by the cushion-like basidiocarps
of Dacrymyces stillatus (Fig. 21.3) and the
clavarioid ones of Calocera viscosa (Plate 11g).
Not much is known about the life cycle of
the Dacrymycetales, but it is presumed that
the usual basidiomycete pattern of alternating
mono- and dikaryotic stages operates. There are
no clamp connections. The dolipore-type septa
Fig 21.3 Fruit bodies of Dacrymycetales. (a) Basidial
cushions of Dacrymyces stillatus on rotting wood.
(b) Basidiocarps of Calocera cornea.