through the septa into the hyphal compartments
further back, providing a transport system for the
movement of metabolites, including phosphates
which often can be in short supply.
The distribution of nucleivaries between and within
fungal groups. The aseptate fungi (e.g. Zygomycota) and
fungus-like organisms (e.g. Oomycota) contain many
nuclei within a common cytoplasm, so these fungi
are coenocytic. Many septate fungi (e.g. Ascomycota
and mitosporic fungi) also have several nuclei in the
apical compartment, but sometimes only one or two
nuclei in compartments behind the apex. However,
nuclei can squeeze through the septal pores, so the
concept of a single nucleus determining the functions
of a fixed volume of cytoplasm does not really apply
to most fungi. Instead, we should regard many fungi
as having several nuclei sharing a common pool of cyto-
plasm. This has important implications for the genetic
fluidity of fungi, discussed in Chapter 9.
Many members of the Basidiomycota (mushrooms,
toadstools, etc.) have a regular arrangement of one
nucleus in each compartment of the monokaryon, but
two nuclei (of different mating compatibility groups)
in each compartment of the dikaryon (see Fig. 2.20).
This arrangement is ensured by a special type of sep-
tum, termed the dolipore septum(see later), which
has only a narrow pore, too small to allow nuclei to
pass through.
The hypha as part of a colonyFungal colonies typically develop from a single germin-
ating spore, which produces a germ tube(a young
hypha) that grows and branches behind the tip. As the
original hypha and the first-order branches grow, they
produce further branches behind their tips.These
branches diverge from one another until, eventually,
the colony develops a characteristic circular outline
(Fig. 3.5). Then, in the older parts of a colony where
nutrients have been depleted, many fungi produce
narrow hyphal branches that grow towards one another
instead of diverging, and fuse by tip-to-tip contact,
involving localized breakdown of their walls (Fig. 3.6).
This process of hyphal anastomosiscreates a network
for the pooling and remobilization of protoplasm to
produce chlamydospores or other, larger differentiated
structures (Chapter 5).
The phenomena seen in Figs 3.5 and 3.6 are remark-
able for several reasons:- In nutrient-rich conditions, the hyphae and
branches at the colony margin always diverge from
one another, positioning themselves so that they grow
in the spaces between existing hyphae. These posi-
tioning mechanisms are very precise, but we have no
firm evidence of their causes – possibly local gradi-
ents of carbon dioxide, or other growth metabolites?
FUNGAL STRUCTURE AND ULTRASTRUCTURE 51
Fig. 3.3Part of a subapical region of
a hypha of Pythium aphanidermatum
showing: endoplasmic reticulum (ER);
Golgi bodies (G) consisting of a stack
of pancake-like cisternae, typical of an-
imals and Oomycota, but distinct from
those of fungi (see Fig. 3.4); mitochon-
drion (M); microtubules (MT); nucleus
(N); nuclear envelope (NE); nucleolus
(Nu); ribosomes (R); large and small
vesicles ( V and v). (Courtesy of C.
Bracker; from Grove & Bracker 1970.)