Introduction to Fungi, Third Edition

548 HOMOBASIDIOMYCETES
growth often occupied by other plants which
have colonized the previously bare ground.
The stimulated growth of grass in the outer
ring zone is associated with more rapid decomposition
of soil organic matter and the concomitant
release of nutrients. The amount of soil
organic carbon may be lowered by about 50% as
compared with a non-invaded turf. The death of
grass in the bare central ring zone is probably
due to a combination of factors, including
parasitic attack by the fungus, drought resulting
from impeded water percolation, and toxins of
fungal origin (e.g. cyanide, HCN) which damage
the grass root tips (see Dix & Webster, 1995).
Other common species of Marasmius include
M. androsaceus with thin, black, horsehair-like
rhizomorphs and small basidiocarps arising
from pine needles, dead heather, etc., and
M. ramealis which forms clusters of basidiocarps
on dead twigs and herbaceous stems. Marasmius
rotula grows in similar situations. Its gills are not
attached directly to the stem but to a cylindrical
collar.
Oudemansiella (10 spp.)
This is a genus containing temperate and
tropical species. Some species, e.g. O. mucida
(porcelain fungus), have a membranous ring
on the stem, but in others, e.g. O. radicata, a
ring is lacking. Oudemansiella mucida is parasitic
on beech (Fagus sylvatica) and forms white
basidiocarps with slimy caps on the branches of
infected trees. Oudemansiella radicata is so called
because the base of the stipe is prolonged into a
tapering pseudorhiza which connects with
buried woody branches extending for several
cm beneath ground level (Fig. 19.18c; see also
Buller, 1931).
Flammulina (10 spp.)
The best known species is F. velutipes, the velvet
shank or winter fungus, which grows on deciduous
tree stumps and branches and fruits in
winter, forming golden yellow basidiocarps
singly or in clusters (Plate 9d). Basidiocarps
can survive being frozen and soon continue to
discharge spores on thawing. In the past, most
collections of Flammulina were named F. velutipes,
but it is now believed that several taxa had
been included under this name (see Hughes et al.,
1999). The Latin and trivial names refer to the
brown velvety hairs at the base of the stipe.
The distribution of the fungus is throughout
temperate regions of the Northern Hemisphere.
An anamorph is produced as chains of dry
arthroconidia which have already been described
(see Fig. 18.14a). The basidiocarps of F. velutipes
are edible and the fungus is cultivated for food in
Japan as enoki-take (Chang & Miles, 2004). Spawn
of the fungus develops on sawdust in plastic bags
or on logs incubated at 21 24°C for 14 18 d and
then subjected to a cold shock (4 10°C) for 3 5d
to induce fruiting, which follows within 5 8dat
a temperature of 10 16°C. Light is required for
fruiting.
Graviperception and gravitropism
in Flammulina
When normally erect basidiocarps of F. velutipes
are displaced into a horizontal position, they
respond within about 3 h by bending upwards to
restore their original orientation (Fig. 19.19).
Experimental studies, some conducted in orbit
in a space laboratory, have helped elucidate the
mechanism (Moore et al., 1996; Kern et al., 1998;
Kern, 1999). Bending occurs by curvature of
the stipe in a transitional zone (2 3 mm long)
immediately beneath the cap, where the hyphae
making up the stipe and cap intertwine.
Although basidiocarps which develop on Earth
are erect, those formed in space were randomly
orientated, growing out in all directions from
their substrate. Despite this, normal caps, basidia
and spores were produced. This shows that
gravitational force is needed for stipe orientation
but not for other aspects of fruit body development.
Two responses operate to control fruit
body development, namely negative hydrotropism
causing basidiocarps to grow away from
their moist substrate into drier air, and negative
gravitropism. Bending is caused by greater
enlargement of the cells making up the lower
flank of the stipe as compared with those of the
upper flank. The cells of the lower flank are more
vacuolate than those of the upper. In green
plants with gravitropic responses, graviperception
is correlated with the sedimentation of
denser cytoplasmic particles, statoliths, which