Introduction to Fungi, Third Edition

98 STRAMINIPILA: OOMYCOTA
Fig 5.16 Sporangia and zoospores of Pythium.(a)Pythium debaryanum. Spherical sporangium with short tube and a vesicle
containing zoospores. (b k) Pythiumaphanidermatum. (b) Lobed sporangium showing a long tube and the vesicle, which is beginning
to expand. (c g) Further stages in the enlargement of the vesicle, and differentiation of zoospores.Note the transfer of protoplasm
from the sporangium to the vesicle in (c).The stages illustrated in (b g) took place in 25 min. (h) Enlarged vesicle showing the
zoospores. Flagella are also visible. (i) Zoospores. (j) Encystment of zoospore showing a shed flagellum. (k) Germination of
azoosporecyst.(b g) to same scale; (a) and (h k) to same scale.
the sporangia are filamentous and are scarcely
distinguishable from vegetative hyphae.
In P. aphanidermatum, the sporangia are formed
from inflated lobed hyphae (Fig. 5.16b). In many
species, however, e.g. P. debaryanum, the sporangia
are globose (Fig. 5.16a). A terminal or
intercalary portion of a hypha enlarges and
assumes a spherical shape, then becomes cut
off from the mycelium by a cross wall. The
sporangia contain numerous nuclei. Cleavage of
the cytoplasm to form zoospores begins in the
sporangium, but is completed within a thinwalled
vesicle which is extruded from the
sporangium. This is a homohylic vesicle because
its glucan wall is continuous with one layer of
the sporangial wall (Dick, 2001a). Within the
sporangium, cleavage vesicles begin to coalesce
to separate the cytoplasm into uninucleate
portions; membrane-bound packets of TTHs
are already present within the cytoplasm of the
sporangium. In P. middletonii (Fig. 5.17), the
fascinating process of differentiation from amorphous
cytoplasm to motile zoospores takes about
30 45 min (Webster, 2006a) and is readily
demonstrated in the laboratory (Weber et al.,
1999). The sporangium is extended into an apical
papilla capped by a mass of fibrillar material
which is lamellate in ultrastructure (Lunney &
Bland, 1976). Shortly before sporangial discharge,
there is an accumulation of cleavage vesicles
behind the apical cap and at the periphery of
the cytoplasm close to the sporangium wall. The
cleavage vesicles around the sporangial cytoplasm
discharge their contents to form a loose,
fibrous interface between the cytoplasm and the
sporangial wall.
Discharge of the sporangium occurs by the
formation of a thin-walled vesicle at the tip of
the papilla from the fibrillar material of the
apical cap, and the partially differentiated
zoospore mass is extruded into it. The movement
of the cytoplasm from the sporangium into the