Introduction to Fungi, Third Edition

PYTHIALES
99
Fig 5.17 Pythium middletonii. Stages in zoospore discharge. (a) Sporangium shortly before discharge. Note the thickened tip of
the papilla which consists of a cap of cell wall material. (b) Inflation of the vesicle begins. (c,d) Protoplasm is retreating from the
sporangium.Note the shrinkage in sporangium diameter as compared with (a). (e) Zoospores have differentiated within the vesicle,
with flagella visible between the vesicle wall and the zoospores. (f) Zoospores escape following the rupture of the vesicle wall.
The whole process of discharge takes about 20 min.
vesicle is probably the result of several forces
including the elastic contraction of the sporangium
wall and possibly surface energy (Webster
& Dennis, 1967). Lunney and Bland (1976) have
also suggested that the fibrillar material
extruded from the cleavage vesicles at the
zoosporangium periphery may imbibe water,
resulting in a build up of turgor pressure. The
vesicle enlarges as cytoplasm from the sporangium
is transferred to it, and during the next few
minutes the cytoplasm cleaves into 8 20 uninucleate
zoospores which jostle about inside the
sporangium, causing the thin vesicle wall to
bulge irregularly (Fig. 5.17). Finally, about 20 min
after the inflation of the vesicle, its wall breaks
down and the zoospores swim away. Internal
sporangial proliferation, i.e. the formation of a
new sporangium inside an old discharged one,
occurs in certain species, e.g. P. middletonii and
P. undulatum.