Introduction to Fungi, Third Edition

BLASTOCLADIALES
157
Fig 6.20 (a h) Allomyces arbuscula. (a) Zoospores (haploid meiospores). (b) Young gametothalli, 24 h old. (c) Young sporothalli,18 h
old. (d) Sporothallus, 30 h old. Perforations are visible in some of the septa. (e) Gametangia at the tips of the branches of the
gametothallus. Note the disparity in the size of the gametes (anisogamy).The smaller male gametes are orange in colour whilst
the larger female gametes are colourless.Compare the hypogynous arrangement of the male gametangia with the epigynous
arrangement in A. macrogynus shown at (i). (f) Meiosporangia (resting sporangia, R.S.) and mitosporangia (zoosporangia, Z.S.) on a
sporothallus. (g) Release of mitospores from zoosporangia (¼ mitosporangia) on sporothallus. (h) Rupture of meiosporangium
(¼ resting sporangium). (i) Allomyces macrogynus. Branch tip from gametothallus showing the arrangement of gametangia with
terminal, epigynous male gametangia and anisogamous gametes.
before it encysts and casts off the flagella.
Nuclear fusion then follows (Pommerville &
Fuller, 1976). The zygote develops immediately
into a diploid asexual thallus which differs
from gametothalli in bearing two types of
zoosporangia instead of gametangia. The first
formed are thin-walled papillate zoosporangia
formed singly or in rows at the tips of the