Introduction to Fungi, Third Edition

IMPORTANCE OF HOMOBASIDIOMYCETES
525
19.2.5 The hymenium
The ends of the tramal hyphae turn outwards
to form a distinct layer of shorter cells, the
subhymenium, which lies immediately beneath
the hymenium of a palisade-like layer of ripe
basidia, developing basidia (basidioles) and sometimes
other structures such as cystidioles,
cystidia and paraphyses. All these represent the
terminal cells of hyphae making up the fruit
body and are therefore homologous with basidia.
Cystidioles are thin-walled, sterile elements of
the hymenium, about the same diameter as the
basidia, and usually protruding only slightly
from the hymenial surface.
Cystidia are more varied. They are often
enlarged conical or cylindrical cells which may
arise in the hymenium along with the basidia
(hymenial cystidia) or sometimes deeper, for
example in the trama (tramal cystidia). Fusion
of paired nuclei within cystidia has been
reported and in some cases 2 4 basidiospores
have been observed to develop on cystidia. In
Coprinus and its allies, the cystidia on the face of
the gill may stretch across the space between the
gills (Fig. 19.7). The tip of a cystidium makes
contact with a cell on the opposing hymenium,
and this cell, termed a cystesium (from cystidium
þ Lat. haerare ¼ to adhere), then differentiates
by developing a granular, vacuolated
cytoplasm and becomes cemented to the tip of
the cystidium, so that the cystidium bridging the
gill cavity becomes firmly attached to both
hymenia (Horner & Moore, 1987; Moore, 1998).
The role of the hymenial cystidia in Coprinus
was initially thought to be that of buttresses
helping to space the inaequi-hymenial gills
apart. However, it is now believed that the
cystidium cystesium pairs function as tension
elements, balancing the stretching forces generated
by expansion growth of the cap and helping
to straighten out the thin lamellae which are
folded during early development (Chiu & Moore,
1990a). In Pluteus the tramal cystidia bear hooklike
tips (Fig. 19.9d) whose function is not
understood. The suggestion that they might
deter animals such as slugs from eating the gill
tissue is not supported by feeding experiments
(Buller, 1924). In Volvariella bombycina, cystidia on
the face of the gill have aqueous drops adhering
to them and are thus believed to be secretory in
function, releasing water vapour into the space
surrounding the basidia (Chiu & Moore, 1990b).
Various terms have been used to describe cystidia
from different parts of the basidiocarp. Those on
the gill face are termed facial cystidia or
pleurocystidia, those on the gill margin cheilocystidia.
Cystidia are not confined to the hymenium.
Similar structures have been found on the
surface of the pileus (pileocystidia) and the stipe
(caulocystidia). We are still ignorant of the
function of many of these structures. For a
fuller discussion see Singer (1986) and
Clémençon (2004).
Paraphyses are present in the hymenium of
Coprinus. They develop on branches from the
same hyphae which produce basidia (Fig. 19.7),
arising after the young basidia are committed to
meiosis. The tips of the paraphyses force themselves
into the hymenium between developing
basidia, grow to about two-thirds of the height of
a basidium, become spherical and enlarge, thus
playing an important role in the expansion of
the maturing basidiocarp (Rosin & Moore, 1985;
Moore, 1998).
19.3 Importance of
homobasidiomycetes
Homobasidiomycetes have a significant impact
on our lives. The fruit bodies of thousands
of species are potentially edible, although
only some 40 species have been cultivated.
According to worldwide production data for
the year 1997 (Chang & Miles, 2004), the most
popular cultivated mushrooms are Agaricus
bisporus (2 000 000 t per annum), Lentinula edodes
(1 500 000 t), Pleurotus spp. (875 600 t), Flammulina
velutipes (284 000 t) and Volvariella volvacea
(180 000 t). Mushroom production is expanding
rapidly, especially in the Far East. However,
some of the most prized edible mushrooms are
mycorrhizal associates of trees which cannot yet
be cultivated away from their host. Mycorrhizal
associations involving Homobasidiomycetes are
prominent in forest situations and are dealt
with in more detail below and also on p. 581.