Introduction to Fungi, Third Edition

SPORES OF FUNGI
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Fig1.23 Diagrams to illustrate different kinds of conidial development. (a) Thallic development.There is no enlargement of the
conidium initial. (b) Holoblastic development. All the wall layers of the conidiogenous cell balloon out to form a conidium initial
recognizably larger than the conidiogenous cell. (c) Enteroblastic tretic development: only the inner wall layers of the conidiogenous
cell are involved in conidium formation.The inner wall layers balloon out through a narrow channel in the outer wall. (d) Phialidic
development: the conidiogenous cell is a phialide.The wall of the phialide is not continuous with the wall surrounding the conidium.
The conidial wall arises de novo from newly synthesized material in the neck of the phialide. Diagrams based on Ellis (1971a).
formed by dissolution of septa along a hypha
(Fig. 10.10). In most conidia, development is
blastic, i.e. there is enlargement of the conidium
initial before it is delimited by a septum.
Two main kinds of blastic development have
been distinguished:
1. Holoblastic, in which both the inner and
outer wall layers of the conidiogenous cell contribute
to conidium formation (Fig. 1.23b). An
example of this kind of development is shown
by the conidia of Sclerotinia fructigena (Fig. 15.3).
2. Enteroblastic, in which only the inner wall
layers of the conidiogenous cell are involved in
conidium formation. Where the inner wall layer
balloons out through a narrow pore or channel
in the outer wall layer, development is described
as tretic (Fig. 1.23c). Examples of enteroblastic
tretic development are found in Helminthosporium
velutinum (Fig. 17.12) and Pleospora herbarum
(Fig. 17.9d). Another important method of enteroblastic
development is termed phialidic
development. Here the conidiogenous cell is
a specialized cell termed the phialide. During
the expansion of the first-formed conidium, the
tip of the phialide is ruptured. Further conidia
develop by the extension of cytoplasm enclosed
by a new wall layer which is laid down in the
neck of the phialide and is distinct from the
phialide wall. The protoplast of the conidium is
pinched off by the formation of an inwardly
growing flange which closes to form a septum
(Fig. 1.23d). New conidia develop beneath the
earlier ones, so that a chain may develop with
the oldest conidium at its apex and the youngest
at its base. Details of phialidic development are
discussed more fully in relation to Aspergillus and
Penicillium (p. 299), which reproduce by means of
chains of dry phialoconidia dispersed by wind.
Sticky phialospores which accumulate in slimy
droplets at the tips of the phialides are common
in many genera; they are usually dispersed by
insects, rain splash or other agencies.