Introduction to Fungi, Third Edition

186 ZYGOMYCOTA
Fig 7.18 Phycomyces blakesleeanus.Stagesin
zygospore formation.The fungus is
heterothallic. (a) Zygophores consisting of
knobbly hyphal branch tips which become
closely appressed. (b) Paired club-shaped
progametangia which develop from the
appressed zygophores. (c) Septation of the
progametangia to form terminal gametangia
and subterminal suspensors. Appendages
are developing on the suspensor to the right.
(d) Young zygospore overarched by
dichotomous suspensor appendages.
the sensitive region. Despite the bright yellow
carotenoid deposits in the trophocysts and young
sporangiophores, studies of the phototropic
response to light of various wavelengths suggest
that the photoreceptor in the sporangiophore is
more likely to be a flavonoid than a carotenoid
(Page & Curry, 1966). Fully developed sporangiophores
are also highly phototropic. Light
projected along the axis of the sporangiophore
is brought to a focus at a point beneath the
swollen vesicle termed the ocellus. In this region,
there is an accumulation of carotenoid-rich
cytoplasm which glows orange when illuminated
(Plate 3f). When light falls asymmetrically onto
the sporangiophore, it is focused onto the back of
the subsporangial vesicle near its base, and some
stimulus is probably transmitted to the cylindrical
part of the sporangiophore, resulting in
more rapid growth of the wall facing away from
the light. Curvature of the whole sporangiophore
thus occurs until it is again orientated parallel
to the incident light (see Fig. 7.21).
The structure of the sporangium differs in a
number of ways from that of the Mucoraceae.
The sporangium is hemispherical, and its wall is
dark black, shiny, tough and unwettable. At the
base of the sporangium is a conical columella,
which is separated from the spores by a pad of
mucilage. During late morning the sporangium
cracks open by a suture running around the
base, just above the columella. The spores are
prevented from escaping by the mucilaginous
pad which protrudes through the crack in
the sporangium wall as a ring of mucilage
(Figs. 7.20e,f). The subsporangial vesicle is
turgid, and the osmotic pressure of the liquid
has been estimated to be around 5.5 bars (Buller,
1934). Drops of liquid decorate the outside of