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FUNGAL GROWTH 79

A similar duplication cycle has been shown to occur
in many fungi, even those with normal, perforated septa
(Trinci 1984). This was done by allowing the spores of
various fungi to germinate on agar and the develop-
ing colonies were photographed at intervals (for an
example of this, see Fig. 3.5). From the photographs,
the number of hyphal tips and the total hyphal length
were recorded at different times and used to calculate
a hyphal growth unit(G) where:

After initial fluctuations in the very early stages of
growth, the value of Gbecame constant and charac-
teristic of each fungal species or strain. For example, a
Gvalue of 48μm (equivalent to a hyphal volume of
217 μm^3 ) was calculated for hyphae of Candida albicans,
and values of 32μm (629μm^3 ), 130μm (4504μm^3 ), and
402 μm (11,986μm^3 ) were found for a wild-type strain
and two “spreading” mutants of Neurospora crassa.
The constancy of these values for individual strains
demonstrates that, as a colony grows, the number of
hyphal tips is directly related to the cytoplasmic
volume. For example, when a colony of C. albicanshad
produced an additional 48μm length of hypha (or a
hyphal volume of 217μm^3 ) it had synthesized enough
protoplasm to produce a new tip. We can therefore con-
sider a fungal colony as being composed of a number
of “units” (the hyphal growth units), each of which
represents a hyphal tip plus an averagelength of
hypha (or volume of cytoplasm) associated with it. They
are not seen as separate units because they are joined
together, but in some respects they are equivalent to
the separate cells produced in the yeast cell cycle. In
fact, the duplication cycle of a typical mycelial fungus,
Emericella nidulans, has been shown to be closely asso-
ciated with a nuclear division cycle. The apical com-
partment grows to about twice its original length,
then the several nuclei in this compartment divide more
or less synchronously and a septum is laid down near
the middle of the apical compartment. After this, a series
of septa are formed in the new subapical compartment
to divide it into smaller compartments, each with just
one or two nuclei, while the multinucleate tip grows
on and will repeat the process in due course.
Earlier in this chapter we mentioned the peripheral
growth zoneof a fungal colony – the length of hypha
needed to support the normal extension rate of tips
at a colony margin. It can be estimated by cutting the
hyphae at different distances behind the tips, and
it can be as large as 5–7 mm. Clearly, this is quite
different from the hyphal growth unit which ranges
from about 30 to 400μm. The difference is explained
by the fact that the hyphal growth unit is measured
in nutrient-rich conditions and is a true reflection

G =

Total length of mycelium

Number of hyphal tips

of growth(increase in biomass, or numbers of tips)

whereas the peripheral growth zone is a reflection

of the rate of extensionof a colony margin, and it

applies to older colonies, where some of the hyphae

become leading hyphae, which are much wider and

have much faster extension rates than the rest.

The fungal mycelium as a nutrient-

capturing system

The fungal mycelium is a highly efficient and adaptable

device for capturing nutrients. Over a wide range of

nutrient concentrations, a fungal colony will extend

across an agar plate at the same rate – whether on water

agar or a standard nutrient-rich medium. But on water

agar the colony is very sparsely branched, whereas on

nutrient-rich agar the branching pattern is dense. This

high degree of adaptability is a key feature of fungi,

especially in soil and bodies of water where nutrients

are likely to occur in localized pockets.

Figure 4.14 is a classic demonstration of this beha-

vior. A young larch seedling was inoculated with a

Fig. 4.14The fungal nutrient-capturing system. (Courtesy

of D.J. Read; from Read 1991.)