Introduction to Fungi, Third Edition

92 STRAMINIPILA: OOMYCOTA
Fig 5.12 Thraustotheca clavata.
(a) Zoosporangium showing formation of
auxiliary cysts within the sporangium.
Theauxiliarycystsarebeingreleasedthrough
breakdown of the sporangial wall.
(b) Auxiliary cyst. (c) Auxiliary cyst
germinating to release a principal zoospore,
the first motile stage in this species.
(d) Principal zoospore. (e) Principal cyst.
(f) Principal cyst germinating by means of
a germ tube. (g) Sexual reproduction.
Six-month-old oospore germinating after17 h
in charcoal water.The germ tube is terminated
by a germ sporangium. Bar¼20 mm(a)or
10 mm(b) (g).
encyst (Figs. 5.13b,c). Electron micrographs have
shown that the wall of the secondary cyst of
D. sterile bears a series of long spines looking
somewhat like the fruit of a horse chestnut
(Fig. 5.14; Heath et al., 1970). Following the
formation of the first zoosporangium, a second
may be produced immediately beneath it by the
formation of a septum cutting off a subterminal
segment of the original hypha, or growth may
be renewed laterally to the first sporangium
(Fig. 5.13a).
Because there is only one motile stage in
Thraustotheca and Dictyuchus (i.e. a zoospore of
the principal type), they are said to be monomorphic.
Pythiopsis cymosa (Figs. 5.13e i) is also
monomorphic, but in this species the only
motile stage is of the auxiliary type and principal
zoospores are not formed. After swimming,
the zoospore encysts and then
germinates directly by means of a germ tube
(Figs. 5.13g i).
5.2.4 Aplanetic forms
In certain cultures of Saprolegniaceae the zoosporangia
produce cysts which do not release any
motile stage. Instead, germ tubes are put out
which penetrate the sporangial wall. Forms
without motile spores are said to be aplanetic.
The aplanetic condition is occasionally found
in staling cultures of Saprolegnia, Achlya and