DIFFERENT FORMS OF MYCELLIUM

Different forms of mycelium refer to as the various shapes and the functions of mycelium which
modified according to the circumstances. These includes the following forms

Plectenchyma (fungal tissue): In a fungal mycelium, hyphae organized loosely or compactly

woven to form a tissue called plectenchyma. It is two types:tissue hyphae are loosely interwoven
lying more or less parallel to each other.

Pseudoparenchyma or paraplectenchyma: In these fungal tissue hyphae are compactly

interwoven looking like a parenchyma in cross-section.

Prosenchyma or Prosoplectenchyma: In these fungal tissue hyphae are loosely interwoven

lying more or less parallel to each other

Sclerotia (Gr. Skleros=haid): These are hard dormant bodies consist of compact hyphae
protected by external thickened hyphae. Each Sclerotium germinates into a mycelium, on return
of favourable condition, e.g., Penicillium

Rhizomorhs: They are root-like compactly interwoven hyphae with distinct growing tip. They
help in absorption and perennation (to tide over the unfavourable periods), e.g., Armillaria
mellea.

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Rhizoids: A rhizoid is a short, root like filamentous branch of the thallus, generally formed in
tufts at the base of the thallus. These also function as anchoring and absorbing, e.g.,
Rhizophydium, Rhizopus.

Rhizoids

Appressoria (Sing. appressorium): It is a terminal simple or lobed swollen mucilaginous

structure of infecting hyphae which adheres to the surface of the host or other substratum and
helps in the penetration of the infection hyphae. These are formed by some parasitic fungi such
as powdery mildews and rust

Haustoria (Sing. haustorium): A haustorium is an organ that is developed from a hypha usually
performing the function of absorption. They are characteristic of obligate parasites. They vary in
shape and may be knob like or button shaped, elongated, finger-like or branched. They secrete
some specific enzymes which hydrolyse the protein and carbohydrates of the host plant.

Hyphal traps (Snares): The predacious fungi develop sticky hyphae or network of hyphal loops
known as hyphal traps or Snares. They help in capturing nematodes

Hyphal traps

Stromata: These are compact somatic structures much like mattresses.Fructifications are
generally formed on or in them

Thus, it is very difficult to say, how much fungus contributes to the world economy. Fungus,
however, is not a widely accepted food. Vegetarians always had a negative view of considering
mushrooms as a vegetarian food. However, in many parts of the world, mushrooms are eaten
widely and considered as a good source of proteins and essential amino acids

Structural-functional relationships
The fungi are more evolutionarily advanced forms of microorganisms, as compared to the
prokaryotes (prions, viruses, bacteria). They are classified as eukaryotes, i.e., they have a
diploid number of chromosomes and a nuclear membrane and have sterols in their plasma
membrane. Genetic complexity allows morphologic complexity and thus these organisms have
complex structural features that are used in speciation.

Fungi can be divided into two basic morphological forms, yeasts and hyphae. Yeastsare
unicellular fungi which reproduce asexually by blastoconidia formation (budding) or fission.

Hyphae are multi-cellular

fungi which reproduce asexually and/or sexually

. Dimorphism is the condition where by a fungus can exhibit either the yeast form or the hyphal
form, depending on growth conditions. Very few fungi exhibit dimorphism. Most fungi occur in
the hyphae form as branching, threadlike tubular filaments. These filamentous structures either
lack cross walls (coenocytic) or have cross walls (septate) depending on the species. In some
cases septate hyphae develop clamp connections at the septa which connect the hyphal
elements.

Ø Body of fungi consists of thallus make up of hyphae which together constitute the mycelium

A mass of hyphal elements is termed the mycelium (synonymous with mold). Aerial hyphae
often produce asexual reproduction propagules termed conidia(synonymous with spores).
Relatively large and complex conidia are termed macroconidia while the smaller and more
simple conidia are termed microconidia. When the conidia are enclosed in a sac (the
sporangium), they are called endospores. The presence/absence of conidia and their size, shape
and location are major features used in the laboratory to identify the species of fungus in clinical
specimens.

Ø Fungi show much diversity in form, structure of plant body and method of reproduction
Ø Fungi are cosmopolitan in distribution, occurs in any habitat where life is possible
Ø Some fungi are aquatic, which may be fresh water or marine
Ø Most of the fungi are terrestrial forms
Ø Many species of fungi are parasite to plants, animals and human
Ø Majority of the diseases of cultivated plants are caused by fungi and thereby they have
immense economic importance
What is mycelium and hyphae?
 Ø Plant body of fungi typically consists of
branched and filamentous hyphae
Ø Hyphae forms a net like structure called mycelium
Ø Hyphae may be aseptate (without septa) or septate (with septa)
Ø In aseptate forms, the hyphae will be coenocytic (multinucleate condition)
Ø In septate forms, the hyphae may be uninucleate or binucleate or rarely multinucleate
Ø Septa usually have simple pore (hole) at the centre for the cell to cell communication

What is dolipore septum?

Ø In Basidiomycetes (a division of fungi) the septa is highly advanced, here dolipore septa
occurs
Ø On both sides of dolipore septum a double membrane structure called septal pore cap or
parenthosome occurs
Ø Parenthosomes of dolipore septa act as valves which can regulate the passage of cell contents
between cells
How fungal cell wall is different from the cell wall of other plants?
Ø Protoplast of fungi is surrounded by distinct cell wall
Ø In slime molds (lower fungi, closely related to Protistis) the cell wall is absent
Ø Main component of cell wall is chitin (a major difference of fungal cell wall from plant cell
wall)

Chitin is a polymer of an N-acetylglucosamine, a derivative of glucose

Ø In some lower fungi (Oomycetes) cell wall is composed of cellulose and glucan
How mitosis in fungi is different from other eukaryotes?
Ø The nucleus of fungi is very small
Ø Fungi have special type of mitosis called nuclear mitosis
Ø Mitosis in fungi is different from that in other eukaryotes
Ø In fungi, during mitotic cell division, the nuclear envelope does not break down and re-form.
Instead, mitosis takes place within the nucleus
Ø Spindle apparatus is formed within the nucleus, which drag the chromosomes to two opposite
poles of nucleus (not the cell as in most other eukaryotes)
What is meant by dikaryotization?
Ø Fungal hyphae may be homo-karyotic (only one strain of nuclei) or hetero karyotic (different
strains of nuclei)
Ø Hyphae may be haploid, diploid or dikaryotic
Ø Dikaryotic condition is seed in higher fungi only (members of Ascomycetes and
Basidiomycetes)
Ø In dikaryotic mycelium, two different strains of nuclei (+ and -) stay separately in the cell and
they simultaneously divide when the cell divides
Ø The process of formation of dikaryotic mycelium is called dikaryotization
What are the hyphal modifications in fungi?
Ø In majority of fungi, hyphae are simple
Ø In some advanced fungi, hyphae may undergo certain modification in response to functional
needs
Ø Important hyphal modifications in fungi are: Prosenchyma, Pseudoparenchyma,
Rhizomorphs, Sclerotia, Appressorium, Haustoria, Stroma and Hyphal traps (snares)
What is the type of nutrition in fungi?
Ø Fungi are heterotrophic in nutrition (they do not manufacture food)
Ø Fungi are entirely devoid of chlorophyll, however carotenoids and other colour pigments are
present
Ø On the basis of made of nutrition, fungi are classified into three groups:
1. Parasites: grow and feed on other living plants or animals (hosts)
2. Saprophytes: grown and feed on dead organic matters
3. Symbionts: mutual association between algae or roots of higher plants (lichens and
mycorrhiza are examples)

Ø Fungi digest food first and then ingest (absorb) the food into cells, to accomplish this the
fungi produce exoenzymes (enzymes which acts outside the cell)
Ø The food reserve of fungi is glycogen (similar to animals cells)

Ø Fungi are heterotrophic in nutrition

Ø They are chlorophyll deficient plants and hence they cannot manufacture carbohydrates using
carbon dioxide, water and sunlight

Ø Fungi are with simple structural organization, thus they always depends on dead or living
organic matter for their energy requirements

Ø On the basis of mode of nutrition, fungi are classified into four groups:

(1). Saprophytes

(2). Parasites

(3). Symbionts

(4). Predacious fungi

Rhizopus (source wikipedia)

(1). Saprophytes

Ø Saprophytic fungi obtain nutrition from dead organic matter

Ø These fungi lives on dead organic matter or excreta of both plant and animal origin

Ø Examples: Mucor, Rhizopus, Penicillium and Aspergillus

Ø Vegetative hyphae of these fungi directly absorb food materials from organic matter

Ø Saprophytic fungi may be of two types:-

(1). Ectophytic saprophytes: grown on the surface of organic matter

(2). Endophytic saprophytes: grown inside the organic matter

Ø In some ectophytic fungi such as Rhizopus, special absorptive structures such as rhizoids are
developed for the easy absorption of food materials

Ø Saprophytic fungi produce exo-enzymes (enzymes which acts outside the cell)

Ø These enzymes digest the complex organic matter in the substratum into simpler compounds
to facilitate easy absorption by the hyphae

(2). Parasites

Ø Parasitic fungi take food from other living plants or animals

Ø The living organism on which the fungi parasitize are called host

Ø Parasitic fungi are harmful to the host and they produce disease condition in host organisms

Ø The relationship of host and parasite in pathology is known as parasitism

Ø Parasitic fungi are of three types:-

(1). Obligate parasites: these fungi can live only as parasite on a living host

Obligate parasites cannot live on dead organic matter

Example: Puccinia which cause rust disease in several crop plants including wheat

(2). Facultative saprophytes: They are parasites, but they can also survive on dead organic
matter in the absence of living host

Example: Taphrina

Rhizopus infection in Jack-fruits

(3). Facultative parasites: these fungi usually follow saprophytic mode of nutrition

Under certain conditions, they parasitize suitable host plants

Example: Fusarium and Pythium which cause soft rot disease in crop plants

Ø On the basis of location of parasite in host organism, the parasites may be:

Endoparasite: parasite live inside the host tissue

Ectoparasite: parasite fungi which live on the outside surface of host

Ø Parasitic fungi possess specialized absorptive structures called haustoria for the absorption of
nutrients from the host cells

Ø Haustoria are specialized hyphal modifications

Ø Haustoria may be inter-cellular (occupy between two cells) in intra-cellular (occupy within
the cell)

Ø Size and shape of haustoria varies in different fungal groups

Ø Haustoria may be round, knob like, club like or branched

Haustoria of Parasitic Fungus

(3). Symbionts

Ø These fungi grow on or with other living organism but both of them are mutually benefited
Foliose Lichen (source wikipedia)

Ø Lichens and mycorrhiza are examples

Ø Lichens are the symbiotic association between algae and fungi

Ø Here both fungi and algae are mutually benefited

Ø Algae synthesize carbohydrates where as the fungi provides shelter for algae

Ø Mycorrhizae are the symbiotic association between fungi and roots of some higher plants

Ø Mycorrhizae helps in the absorption of nutrients by the host plant

Amanita Mycorrhiza

Ø Mycorrhiza may be ectophytic or endophytic

Ø Ectophytic mycorrhiza are external mycorrhiza and they are confined to the outer region of
the roots
Ø Endophytic mycorrhiza are internal mycorrhiza and they are found deeply in the root cells

(4). Predacious fungi

Ø They are animal capturing fungi (predators)

Ø These types of fungi possess special hyphal traps called snares to tap and capture small
animals such as nematodes and protozoa

Ø They usually inhabit in the soil

Ø They possess rapidly constricting hyphal traps which hold the captive for long time

Ø They also have haustoria which directly inserted into the tissue of the prey

Ø Some predacious fungi also produce sticky secretions for capturing nematodes

Examples of predator fungi: Ditylenchus dipsaci, Aphelenchoides besseyi; Dactylaria

candida, Dactylella sp, Arthrobotrys sp, Candelabrella musiformis