Introduction to Fungi, Third Edition

188 ZYGOMYCOTA
the sporangiophore of Pilobolus, as they do in
many zygomycetes. Eventually, usually about
midday, the sporangial vesicle explodes at a line
of weakness just beneath the columella. Due to
the elasticity of the vesicle wall the liquid
contents are squirted out, projecting the entire
sporangium forward in the direction of the light.
Photographs of the jet show that it is at first
cylindrical but eventually breaks up into fine
droplets (Fig. 7.22c; Page, 1964). In P. kleinii, the
velocity of projection varies between wide limits
of 4.7 27.5 m s 1 with a mean of 10.8 m s 1
(Page & Kennedy, 1964). The sporangia can be
projected vertically upwards for as much as
2 m and horizontally for up to 2.5 m. On striking
a grass blade or other herbage, the sporangium
becomes attached in such a way that the
mucilaginous ring adheres to it, with the black
sporangium wall facing outwards. Buller (1934)
has suggested that the projectile contains a
drop of liquid attached to the sporangium
(Fig. 7.22a). When the projectile strikes an
object the liquid flows around the sporangium,
but because the sporangium wall is hydrophobic
and the base of the sporangium is surrounded
by the wettable mucilaginous ring, the sporangium
turns round in the liquid so that its wall
faces outwards (Fig. 7.22b). The non-wettable
nature of the sporangial wall may be related to
the presence on its surface of hollow, blunttipped
spines and crystals as seen by electron
microscopy (Bland & Charles, 1972). As the
mucilage dries, the sporangium becomes
cemented onto the surface which it struck. The
spores of Pilobolus are released only after the
sporangium has been ingested by an animal.
They survive gut passage and are voided with
the faeces. A film featuring the life cycle of
Pilobolus has been made (Webster & Hard, 1999).
An unexpected consequence of the attachment
of Pilobolus sporangia to herbage is that the
sporangia may act as vectors for parasitic
nematodes such as Dictyocaulus spp., which multiply
on dung and, when ingested, cause lungworm
disease in sheep, cattle and some wild
mammals.
The physiology of Pilobolus shows a number
of interesting features possibly related to its
coprophilous habit. Spores germinate best above
Fig 7.21 Pilobolus kleinii. Diagrammatic L.S. of sporangiophore
showing the path of light rays falling parallel to the axis of the
sporangiophore which are brought to a focus beneath the
subsporangial vesicle.The sporangiophore illustrated is
orientated symmetrically with respect to the incident light.
Note the mucilaginous ring extruded through the sporangial
wall at its base (after Buller,1934).
pH 6.5, and can be induced to germinate by
treatment with alkaline pancreatin. Germination
can also be triggered by hexoses such as
glucose and mannose. Mycelial growth occurs
over a wide range of temperatures, with optimum
temperatures at 25 35°C. Growth on
synthetic media with asparagine and acetic acid