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Left: anastomoses between hyphal elements to form a 3-D grid in Arthrobotrys.   Right: Septate hyphae of Sordaria fimicola

Hyphae and Stuff      -   see also -   Hyphae Revisited

Fungi are either unicellular or filamentous.  Unicellular fungi such as yeasts reproduce by budding and the parent cell buds off a daughter cell and this process is repeated indefinitely. (click here).  The vast majority of fungi, however are filamentous and the vegetative (=assimilative) stage is a cylindrical thread-like filament called a hypha (plural = hyphae).  Most hyphae are 2-10 um across although there are some that are nearly 300 um across but these latter are usually specialized reproductive hyphae.   For the most part, therefore, individual hyphae are so fine that they cannot be resolved by the eye.  To give you some idea of scale, a human hair is about 100 um = 1/10th of a millimetre (click here).  Under ideal conditions  a fungus can colonize a substrate and produce masses of hyphae visible to the eye.  A hyphal mass is referred to as a mycelium.  The hyphal system used by fungi has proved very successful and is the vegetative base for more than a million species.   It deserves a little more attention.

     In the lower fungi the cells are  elongate, branching, cylindrical filaments (click here) but in more advanced 'higher' fungi the hyphae have cross walls that break them up into cells (above right).  A cross wall in fungi is called a septum.  So, lower fungi are non-septate and 'higher' fungi are septate although, as always, there are exceptions to this general rule.  The hyphal wall is only about 0.2 um thick. But it is quite tough and composed of chitin, chitosan, glucan and mannan.  Plant cell walls are composed of cellulose.   The wall serves to contain the protoplasm and protect it from attack by other organsims that would use it as a food source.  The wall is also permeable to water, dissolved materials, inorganic ions,  and simple breakdown products in aqueous solution e.g. sugars, amino acids etc.  A fungal hyphal system might encompass many kilometres of individual  threads.   It is important to maintain continuity and communication between various parts of the hyphal system.  This is done by protoplasmic flow inside the filaments.  This movement of protoplasm is also referred to as cytoplasmic streaming.   In non-septate fungi such as Rhizopus the cytoplasm can stream more or less unrestricted.   The cystoplasm streams along well defined 'trails, inside the hyphae.  It is propelled by a network of contractile protein (actomyosin).  Under a microscope in a fungus such as Rhizopus with large hyphae the protoplasm can be seen streaming  in both directions simultaneously (= bidirectional).  Streaming transports nutrients and various signals around the hyphal system fairly rapidly.  Cytoplasm streams at a rate of about 12 cm an hour.  This does not seem fast but at the micro scale it is more than  adequate.

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Anastomoses points in the formation of the 3-D hyphal grid in Arthrobotrys

The hyphal system of the fungus forms a 3-D grid to explore the substrate and maintain coninuity within the hyphal system.   Chemical signals can be passed quickly to different regions of the system by cytoplasmic streaming.    The continuity in the hyphal system can be interrupted in various ways.   Just as fungi attack all other organisms, so also are the fungi themselves a desireable food source for a host of  predatory organisms that consume not only fungus spores by the trillion but also fungus hyphae for their rich protoplasmic content.    Stylet feeding nematodes will penetrate through a hyphal wall and suck out the internal content.   Similarly amoebae can envelop a hyphae or fasten to the outside wall (testaceous amoebae) and drill neat litle holes through the hyphae.  Fine tendrils of pseudopodia can then enter this portal and consume the contents.   Larger soil fauna such as springtails will just chew up the hyphae for lunch or whatever.     

Thus the network system must be designed for self repair to stop the 'bleeding' of their cytoplasmic contents through the damaged ends as quickly as possible.   Anastomoses create an energy grid such that damaged areas can be walled off and the living protoplasm shunted along the hyphal system using alternative routes.

SEE ALSO Hyphae Revisited