Diversity of Thallus Structure in
Fungi
The thallus is the vegetative body of a fungus. It may consist of a
single cell, a network of delicate filaments, or a compact tissue-like mass of
hyphae. Fungi show remarkable diversity in thallus organization, and this
diversity reflects differences in their mode of nutrition, habitat, growth, and
reproduction. The organization of the thallus is also an important character
used in the study and classification of fungi.
1. Unicellular thallus
In the simplest fungi, the thallus consists of a single cell rather
than a system of hyphae. Such fungi are generally microscopic and may reproduce
by budding or fission. Yeasts are the best-known examples of unicellular fungi.
A common example is Saccharomyces
cerevisiae, the baker’s and brewer’s yeast. Each individual is usually a
single oval or spherical cell, although daughter cells produced by budding may
temporarily remain attached. Thus, Saccharomyces
represents a typical unicellular fungal thallus.
2. Filamentous or mycelial thallus
In most fungi, the thallus is filamentous and is composed of long,
tubular, branched structures called hyphae. The collective mass of hyphae is
known as the mycelium. A filamentous thallus provides a large surface area for
absorption of nutrients from the substrate. Filamentous fungi may possess
either septate or coenocytic hyphae, depending on whether cross walls are
present. Aspergillus niger is a
typical example of a filamentous fungus because it develops an extensive
branched mycelium.
3. Aseptate or coenocytic thallus
In a coenocytic thallus, the hyphae lack regular cross walls or
septa, so the cytoplasm remains continuous and contains many nuclei. Septa may
occasionally appear in old or injured regions or to separate reproductive
structures, but the actively growing hypha is essentially non-septate. Rhizopus stolonifer, the common black
bread mold, is a standard example. Its broad, multinucleate hyphae are
coenocytic. Mucor species also show
this type of thallus organization.
4. Septate thallus
In a septate thallus, transverse walls called septa divide the
hyphae into successive compartments or cells. The septa usually contain pores
through which cytoplasm and, in some fungi, nuclei can move from one
compartment to another. Septate hyphae are characteristic of many Ascomycota
and Basidiomycota. Aspergillus niger
is a familiar example of a fungus with septate hyphae. Penicillium species and mushrooms such as Agaricus also possess septate mycelia.
5. Pseudomycelial thallus
Some yeasts can produce an apparent filamentous body even though
they are fundamentally unicellular. During repeated budding, the daughter cells
may remain attached and become elongated, forming chains that resemble true
hyphae. These chains are called pseudohyphae, and the resulting growth is
called a pseudomycelium. Candida albicans
is a well-known example. It can grow as budding yeast cells and can also form
pseudohyphae under suitable environmental conditions.
6. Holocarpic thallus
In a holocarpic fungus, the entire vegetative thallus is converted
into a reproductive structure. Consequently, no separate vegetative portion
remains after reproduction begins. This condition occurs in some simple fungi. Synchytrium endobioticum, the fungus
responsible for potato wart disease, is a classical example because its entire
thallus is transformed into a reproductive body during the appropriate stage of
its life cycle.
7. Eucarpic thallus
In a eucarpic fungus, only a portion of the thallus becomes
reproductive, while the remaining portion continues to function vegetatively in
absorption and growth. This represents a greater degree of structural
differentiation than the holocarpic condition. Rhizopus stolonifer is a clear example: most of its mycelium
remains vegetative while specialized aerial sporangiophores develop sporangia
for reproduction. Most advanced filamentous fungi are eucarpic.
8. Monocentric thallus
A monocentric thallus has a single center of growth and generally
produces one main reproductive structure, usually associated with a rhizoidal
system that absorbs nutrients from the substrate. This type is found among some
chytrid fungi. Rhizophydium is a
representative example. Its thallus commonly develops one sporangium from which
a system of rhizoids extends into the substrate.
9. Polycentric or rhizomycelial thallus
In a polycentric thallus, several reproductive centers develop and
are connected by a branching system of rhizoid-like filaments called a
rhizomycelium. This arrangement allows the fungus to spread through a larger
portion of the substrate and to produce several reproductive structures. Nowakowskiella is a classical example of
a polycentric chytrid. It develops an extensive rhizomycelial system bearing
more than one sporangium.
10. Plectenchymatous thallus
In many higher fungi, the hyphae become closely interwoven and form
compact tissue-like masses known as plectenchyma. Although fungi do not possess
true tissues comparable with those of higher plants, densely packed hyphae may
appear tissue-like. Plectenchyma commonly occurs in fruiting bodies, stromata,
and sclerotia. Agaricus, the common
mushroom, provides a familiar example because the tissues of its fruiting body
are formed by tightly organized hyphae.
11. Prosenchymatous organization
Prosenchyma is a type of plectenchyma in which the component hyphae
remain elongated, lie more or less parallel to one another, and can still be
individually recognized. This organization commonly occurs in parts of fungal
fruiting bodies where hyphae retain their filamentous appearance. The trama of
many mushrooms, including Agaricus,
may show prosenchymatous organization, with elongated hyphae arranged in an
orderly manner.
12. Pseudoparenchymatous organization
Pseudoparenchyma is a compact type of plectenchyma in which the
hyphae are so closely packed that individual filaments are difficult to
distinguish. The cells appear nearly isodiametric and resemble the
parenchymatous tissue of higher plants. A good example is the sclerotium of Sclerotinia sclerotiorum, in which
densely packed fungal cells form a firm, resistant structure. Similar
pseudoparenchymatous tissues occur in many stromata and fungal fruiting bodies.
13. Dimorphic thallus
Some fungi can occur in two distinct morphological forms depending
on environmental conditions such as temperature, nutrients, or host
environment. This ability is called dimorphism. A classic example is Histoplasma capsulatum. It grows
predominantly as a filamentous mold at about 25 °C in the environment, but in
mammalian tissues at about 37 °C it occurs mainly as a yeast-like form.
Dimorphism is especially important in several medically significant fungi
because the change in thallus form is associated with adaptation to different
environments.
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