Introduction to Fungi, Third Edition

ZYGOMYCETES: MUCORALES
171
Fig 7.6 Diagram of developmental stages of
the sporangiophore of Phycomyces. Regions
in which growth is taking place are stippled.
The rotary component of growth is indicated.
During stage I the axis of growth is directed
sinistrally, in stages II and III growth is
unoriented. In Stage IVA dextral spiralling
occurs and in Stage IVB sinistral spiralling again
takes place.
above). During this stage, which lasts about an
hour, markers attached to the growth zone may
make up to two complete revolutions around the
axis. During stage IVB the direction of rotation
reverses once more. The reasons for the spiral
growth are far from clear (see Ortega et al., 2003).
It is known that the chitin microfibrils which
make up the wall of the sporangiophore show
a right-handed or Z-spiral orientation. One
possible explanation is that the laying down of
the fibrils in this way is responsible for the
rotation. A second is that the extension due to
turgor pressure of a cylinder whose walls are
composed of spirally arranged fibrils would
naturally result in a passive rotation. The
phenomenon of spiral growth is not peculiar to
Phycomyces, occurring also during elongation of
the sporangiophores of other members of the
Mucorales such as Thamnidium and Pilobolus, and
in various cylindrical plant cells.
The mechanical properties of the sporangiophore
of Phycomyces change during development.
During stage II, when no elongation of the
sporangiophore is taking place, the sporangiophore
shows elastic deformation when small
loads are applied to it, i.e. the fractional change
in length is directly proportional to the applied
load, and on removal of the load, the
sporangiophore returns to its original length.
During stage IV, although the sporangiophore
changes in length in response to applied loads,
upon unloading the sporangiophore does not
return to its original length.
There is evidence that the spores secrete
one or more unknown substances which control
elongation of the sporangiophore. If mature
sporangia are removed, growth of the sporangiophore
ceases. Replacement of the detached
sporangium with a substitute sporangium,
with a suspension of spores, or with a drop
of supernatant liquid from a centrifuged spore
suspension, results in resumption of growth.
Another effect of the removal of a ripe sporangium
is that branching is induced in the
sporangiophore, and this phenomenon has been
likened to the breaking of apical dominance
upon removal of the terminal bud in shoots of
angiosperms.
7.2.4 Sporangium development
The tip of the sporangiophore expands to form
the sporangium initial containing numerous
nuclei which continue to divide. A dome-shaped
septum is laid down and cuts off a distal portion
which will contain the spores, from a cylindrical
or subglobose spore-free core, the columella.