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DIFFERENTIATION AND DEVELOPMENT 93

even on flat surfaces. Group 4included many cereal
rusts (Puccinia graminis, P. recondita, etc.) that did not
respond to single ridges of any dimension. However,
the cereal rusts have since been shown to differentiate
in response to multiple, closely spaced ridges of opti-
mal 2.0μm height and 1.5μm spacing. The finding
that either ridges or grooves elicit the same response
indicates a minimum requirement for two consecutive
right angles as the topographical signal. It is suggested
that the different requirements for ridge heights or
spacings by different rust species could reflect adapta-

tion to the stomatal topography of the host, consistent

with the high degree of host specificity of these

biotrophic parasites (Chapter 14). Further details of con-

tact sensing and the possible underlying mechanisms

can be found in Read et al. (1992).

Sclerotia

Sclerotia (singular: sclerotium) are specialized hyphal

bodies involved in dormant survival (Fig. 5.9).

(a)

(c)

Fig. 5.9 Sclerotia:specialized multicellular dormant survival bodies. (a) Colony of Sclerotinia sclerotiorumthat has
produced a ring of sclerotia on an agar plate. (b) Diagram of part of a cross-section of a sclerotium of Athelia rolfsii,
showing a crushed, melanized rind, a tissue-like cortex of thick-walled cells, and a central medulla of normal hyphae.
(c) Scanning electron micrograph of a mature sclerotium of the mycorrhizal fungus Paxillus involutus. This sclerotium
developed from nutrients that were translocated along a mycelial cord (an aggregated mass of hyphae) shown at the
bottom of the image. (d) Scanning electron micrograph of a cut sclerotium of Cenococcum geophilum(a mycorrhizal
fungus), showing the internal zonation of the tissues. ( (c,d) Courtesy of F.M. Fox; see Fox 1986.)

(d)