The Fungi: Archiycetes and Phycomycetes
THE Fungi are generally included as the second group of the phylum Thallophyta. Some authorities, as we have already indicated, regard the Fungi as a group of organisms of independent origin which cannot be correctly included in a natural classification of plants, since, they argue, Fungi are not plants at all. To follow such an argument to its limit it would be necessary to postulate not two but three kingdoms, Plants, Animals and Fungi. It is outside the scope of this book to discuss the relative merits of these two views, and though there is much to be said in favour of the latter we shall, for the sake of simplicity of treatment, regard the Fungi as members of the Thallophyta.
Whatever may be their correct systematic position the Fungi are a very large and important group. There are something like 37,500 species definitely known, and probably about three times that number in existence. About 9,000 species are known from Britain. Two of the largest orders are the U rediniales and the Agaricales which include more than 4,000 species apiece.
Fungi may be readily separated from green plants by the absence of chlorophyll, and, since they do not feed holozoically, they must of necessity live either as parasites on other organisms or as saprophytes on organic compounds. The mode of nutrition exhibited by the Fungi varies to some extent, but, in general, they make use of carbohydrates, amino-acids and other organic substances. They also absorb nitrates, ammonia, phosphates and sulphates. Such metals as Sodium, Calcium and Magnesium do not appear necessary for their metabolism, though they may be essential to higher plants. Some Fungi, however, appear to require traces of some of the heavy metals such as Zinc, which are not generally necessary.
With few exceptions all Fungi can live as saprophytes on nutrient media composed of soluble organic substances, and it is in this way that many hundreds of cultures are kept in laboratories for study. In fact many Fungi which in nature appear to live exclusively as parasites are able to grow saprophytically under cultural conditions. This plasticity of Fungi is one of their most important features, coupled with a faculty for changing their appearance to a considerable extent according to the composition of their food. Many Fungi exhibit pleomorphism, that is, the power of assuming various shapes not only in culture but also under natural conditions, a fact which has greatly complicated their systematic study.
In all but the simplest groups the vegetative thallus consists of a web of filaments termed hyphae which together make up the body or mycelium. This mycelium may either develop on or in the substrate, in the case of saprophytes, or inside or on the surface of the host in the case of parasites.
The hyphae may be either coenocytic or may consist of cells, each possessing one or sometimes two nuclei. In the higher groups the wall of the hyphae is composed of fungus cellulose, \yhich differs from ordinary cellulose in the fact that it does not stain violet with Iodine and sulphuric acid. True cellulose walls occur in certain of the lower groups.
The reproductive bodies in the lower groups may be small and inconspicuous, but among the higher members it is the fruiting bodies which are the only obvious part of the fungus, as, for example, in the l\Iushroom, where the mycelium is underground and passes unnoticed. In almost all groups asexual reproduction by spores occurs, and in many it represents the more general and prolific method of propagation, though in any case the ultimate result of sexual reproduction is also the formation of an abundant crop of spores. In the higher groups the sex organs become more and more reduced until often all that remains as an expression of sex is the union of two nuclei which may be of vegetative origin.
The Fungi are classified according to the way in which the spores are
produced :-
- Archimycetes (e.g., Club Root Fungus).
- Phycomycetes (e.g., Pin Moulds and Water Yloulds).
- Ascomycetes (e.g., Morels, Truffles and Blue Moulds).
- Basidiomycetes (e.g., Mushrooms, Toadstools and Puff Balls).
- Fungi Imperfecti
In the Archimycetes and Phycomycetes the spores, and frequently the zoospores, are borne in large numbers in sporangia. In the Ascomycetes the spores are produced in an ascus or sac, each ascus containing usually eight ascospores. In the Basidiomycetes the basidiospores are produced externally in fours on a club-shaped basidium. The Fungi Imperfecti produce only asexual spores.
