Introduction to Fungi, Third Edition

494 BASIDIOMYCOTA
dikaryotic ballistospore directly. A discharged
primary ballistospore may germinate by a germ
tube or by repetition to form a secondary
ballistospore (Fig. 18.6b). Yeast-like growth can
also occur, especially on rich media.
Working with Itersonilia, several observations
were made which provided clues to the mechanism
of discharge. (1) Ballistospores can be
detached from their sterigmata with a micromanipulator
needle and spores so detached still
develop Buller’s drop. This shows that the liquid
in the drops does not originate from liquid
transported through sterigmata. (2) During
normal discharge, although the volume of
Buller’s drop may attain 60% of that of the
spore, there is no decrease in the dimensions of
the ballistospore, indicating that Buller’s drop
does not come from within the spore. The same
observation has been made on other basidiomycetes
and has led to the suggestion that the
liquid in Buller’s drop and also in the adaxial
drop is formed by condensation of water vapour
around a hygroscopic substance extruded from
the hilar appendix and through the spore wall
(Webster et al., 1984a,b, 1989). Washings from the
spores of basidiomycetes belonging to several
different taxonomic groups were analysed by
gas liquid chromatography, and all gave a positive
result for the presence of mannitol. Glucose
was also sometimes detected. The presence of
mannitol and hexose in liquid drawn off by
a micropipette from Buller’s drops in Itersonilia
was confirmed by microscope fluorimetry, and
measurements of the solute concentrations in
Buller’s drop corresponded closely to the calculated
concentrations which would be necessary
to drive the uptake of water from a saturated
atmosphere at the rates observed (Webster et al.,
1995).
The surface tension catapult mechanism
postulates that, as Buller’s drop develops, the
centre of mass of the spore plus drop moves
towards the hilar appendix (Fig. 18.8b). The
coalescence of Buller’s drop with the adaxial
drop causes a rapid redistribution of mass away
from the hilar appendix, resulting in a momentum
which carries the spore plus drop away from
the sterigma (Fig. 18.8c; Webster et al., 1988).
Pringle et al. (2005) have suggested that an even
Fig18.6 Itersonilia perplexans. (a) Basidium (sporogenous cell)
bearing a single ballistospore. Note the clamp connection at
the base of the basidium (arrow). (b) A primary ballistospore
has germinated by repetition to form a secondary
ballistospore.
greater momentum may be generated by the
fusion drop moving towards the basidiospore
apex and coming to an abrupt halt upon reaching
it. For this mechanism to be effective, a rigid
sterigma is required, and it is likely that the
turgor pressure of the vacuolated basidium
contributes to the required rigidity (Money, 1998).
The requirement of high humidity for effective
operation of the mechanism is a likely
explanation for observations that basidiospore
concentrations in the air peak at night (Kramer,
1982). High humidity develops in the space
between agaric gills or inside the hymenial
tubes of toadstools and bracket fungi. The
presence of free water would, of course, prevent
operation of the surface tension catapult, and
this may be the reason why agaric basidiocarps
are often umbrella-shaped. The impossibility of
the surface tension catapult mechanism operating
under water also explains why, although
some basidiomycetes grow vegetatively in fresh