Introduction to Fungi, Third Edition

SPIZELLOMYCETALES
149
zoospores (Fig. 6.16a). One to several discharge
tubes are formed, and the tip of each tube
contains a clear mucilaginous plug which, prior
to discharge, is exuded in a mass from the tip of
the tube (Fig. 6.16c). While the plug of mucilage
dissolves, the zoospores within the sporangium
show active movement and then escape by
swimming through the tube. In some specimens
of R. rosea it has been found that a membrane
may form over the cytoplasm at the base of
the discharge tubes. If the sporangia do not
discharge their spores immediately, the membrane
may thicken. When spore discharge
occurs, these thickened membranes can be seen
floating free within the sporangia, and the term
endo-operculum has been applied to them. The
genus Karlingia was erected for forms possessing
such endo-opercula, including R. rosea, which
is therefore sometimes referred to as Karlingia
rosea, but the validity of this separation is
questionable because the presence or absence
of endo-opercula is a variable character
(Blackwell & Powell, 1999).
Zoospores of R. rosea are capable of swimming
for several hours. The head of the zoospore is
often globose, but can become pear-shaped or
show amoeboid changes in shape. It contains a
prominent lipid body, several bright refringent
globules, and bears a single trailing flagellum.
Ultrastructural details resemble those of
Olpidium brassicae in the presence of a striated
rhizoplast connecting kinetosome and nucleus
(Barr & Hartmann, 1977). On coming to rest on
a suitable substratum, the flagellum is withdrawn
and the body of the zoospore enlarges
to form the rudiment of the sporangium, whilst
rhizoids appear at various points on its surface.
Within the sporangium, the flagella are tightly
wrapped around the zoospores (Chambers
& Willoughby, 1964).
Resting sporangia are also found. They are
brown, globose or angular and have a thickened
Fig 6.16 Rhizophlyctisrosea. (a) Zoospores. (b) Young thallus formed on germination of zoospore.The zoospore cyst has enlarged
and will form the sporangium. (c) Older sporangium showing three discharge tubes. (d) Sporangium showing mucilage plugs at the
tips of the discharge tubes and thickenings of the cell membrane at the bases of the tubes. Such thickenings are termed
endo-opercula. (e) Globose sporangium and seven visible papillae. (f) Resting sporangium formed inside an empty zoosporangium.
(a,b) to same scale; (c f) to same scale.