Fungi and their classification pt 1

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Fungi and their classification pt 1

John Evans, Tredegar, Gwent

The general characteristic of all fungi is that they are without the green pigment called chlorophyll which is present (with a very few exceptions) in all true plants. With the aid of a complex chemical process green plants are able to use the energy from light, usually sunlight, to build up the carbohydrates essential for life and growth from water in the soil and carbon dioxide from the air

This process is known as photosynthesis or carbon assimilation. And it is important to remember that only organisms with chlorophyll are able to carry out this important process - the building up of organic material such as sugars, starch and cellulose from inorganic substances.

Fungi have no chlorophyll and must, therefore, obtain organic materials already prepared. In this they resemble animals but they are much more catholic in their choice of food source. Those fungi that take their food from living plants and animals are parasites and, usually cause disease. Only very rarely do fungi attack warm blooded animals, generally the body temperature of the animal is too high to allow fungal growth. Other fungi take their food from dead material, these are saprophytes and are essential to the process of decay in the soil and in our compost heaps. Without them we would be surrounded by countless generations of dead, undecayed animals and plants - but they can also destroy wood in buildings and elsewhere. In their general search for food fungi may cause disease, decay and destruction. However, they also bring benefits to mankind, the most notable is, perhaps, the synthesis of penicillin, by the fungus Penicillium notatum.

So, the fungi are colourless plants, living either on dead plant and animal remains, the saprophytes, or on healthy plant tissues, the parasites. The "body" of a fungus consists of numerous fine threads or hyphae, collectively forming the mycelium. In parasitic fungi the hyphae may remain between the cells of the host, and are then called intercellular. Or they may freely penetrate the cells, this is called intracellular.

Reproduction of fungi can be by means of sexual or asexual spores - or both. The stage in the life cycle when the sexual spores are produced is referred to as the "perfect" stage or phase, while that stage when reproduction is by asexual spores is known as the "imperfect" stage. Toadstools or bracket fungi are examples of the perfect phase of fungi, while the conidia of potato blight are an imperfect phase. Asexual spores are often thick walled, enabling the fungus to survive the winter months. Thick walled asexual spores are known as chlamydospores and are formed by many fungi, sclerotia are thick walled masses of mycelium. They are formed regularly by some fungi, e.g. Botrytis, and are usually black in colour and may remain dormant in the soil for many years.

Fungi are grouped into classes based on two main characters:

The nature of the mycelium
The type of the perfect stage.

Class one - Myxomycetes

These are structurally very simple fungi which have no hyphae. The body of the fungus consists of a naked (not walled) multinicleate plasmodium. Both saprophytic and parasitic species occur.

In the former the plasmodium can creep over dead leaves, twigs etc. ingesting small particles of dead plant and animal material for food as it goes. The reproduction is by means of thick walled spores which produce mobile zoospores on germination.

Examples: Plasmodiophora brassicae - club root of brassicas and other crucifers
Spongiospora subterranea - powdery scab of potatoes

Class two - Phycomycetes

In the fungi belonging to this class, the mycelium consists of aseptate hypae. Asexual reproduction is by means of zoospores produced in sporangia. In a few cases, however, the sporangia may produce a germ tube directly. The phycomycetes are divided into three main groups of importance to gardeners-

1. Arcimycetes - fungi without true hyphae. Sexual reproduction is by means of the fusion of two mature equal gametes.

Example: Synchytium enobiaticum - wart disease of potatoes

2. Oomycetes - hyphae present, sexual reproduction is by the fusion of two unequal uninucleate gametes and results in the production of thick walled zoospores.

Examples: Phytopthora infestans - potato blight

Pythium debaryanum - damping off fungus of seedlings Bremia lactucae - downy mildew of lettuce Peronospora parasitica - downy mildew of crucifers

3. Zygomycetes - hyphae present, asexual reproduction never by zoospores. Sexual reproduction consists of the fusion of two multinucleate gametangia and results in the production of a zygospore.

Example: Mucorssp, - pin moulds of bread and fruit

Continued In Fungi and their classification part 2

This article originally appeared in the Members Bulletin, the journal of the National Vegetable Society, which is sent quarterly to members. You can Join the National Vegetable Society here

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