In this article we will discuss about the classification of myxomycota.
The Myxomycetes may be classified into the following orders depending on the nature of sporangia, presence or absence of lime, and its location; presence or absence of columella and capillitium; and colour of spores in a mass.
A. Sporangia monosporous, borne on delicate structure
AA. Sporangia multisporous, simple or compound
B. Spores rusty to deep violet in mass
C. Without lime, except in the sporangiophore Stemoni tales
CC. With lime in the peridium or capillitium
BB. Spores pale or bright coloured, not rusty or violaceous
C. Columella and capillitium absent
CC. Columella and capillitium present
CCC. Columella usually present, capillitium present, sporangia very small
Since naming of the Myxomycetes as Mycetozoa or Fungus animals by De Bary (1887), slime molds were grouped by the Zoologists in the animal kingdom under the name Mycetozoa.
True, these organisms possess some animal characters like: somatic body resembling a giant amoeba, absence of cell wall, exhibit amoeboid movement, and have holozoic mode of nutrition.
But in course of time, Macbride (1899), Martin (1932, 1960-61), Alexopoulos (1962), and others have demonstrated that the Myxomycetes have characters very much like plants and as such they should be placed in the plant kingdom and that too close to the fungi with whom they have closer affinity.
The major plant-like characters they possess are: can be grown spore-to-spore in artificial cultures; and life cycle pattern, habit and habitat; existence of sclerotial stage; nature and behaviour of swarm cells; development of fructification and spore development by meiosis in the fructification.
This order is characterized by the absence of lime from the peridium and from the capillitium. But lime may be present on the stalk of the fructification. The capillitium is usually abundant and dark-coloured. The spores are also dark-coloured. The order Stemonitales includes three families: Collodermaceae, Stemonitaceae, and Amaurochaetaceae.
The sporangia of the Stemonitaceae are stalked. The upper part of the stalk extends within the sporangium as a columella from which the branching threads of the capillitium originate. The sporangial wall is composed of a delicate membrane which often may be evanescent.
This is one of the most common Myxomycetes growing on dead wood and dead leaves (Fig. 328A). The plasmodium is of aphanoplasmcdium type. The colour of the plasmodium is usually watery to creamy-white, often may be pale-yellow.
The Plasmodium does not possess the typical reticulated fans but is composed of network of fine strands. The young plasmodium consists of long main veins with branches here and there which bear secondary branches with some anastomosis. In the young, actively growing stage of the plasmodium, the; veins are almost completely devoid of an outer layer.
Streaming of protoplasm is almost imperceptible in the smaller veins.
Protoplasmic streaming, however, is very rapid and rhythmically reversible in the larger veins. The protoplasm is not very granular. As the plasmodium grows, it often spreads in thin, sheet-like, highly vacuolated areas. The plasmodium eventually becomes concentrated into thick, blob-like masses which are transformed into sporangia.
The sporangia are attached in small clusters on a common hypothallus. They are cylindrical, stalked, and fasciculate (Fig. 328B). The solid stalk extends within the sporangium usually to near the apex as a columella. The capillitium is formed of numerous threads radiating from all parts of the columella and is combined into a loose network, the ultimate branches of which are united into a surface net (Fig. 328G).
The surface net of capillitium may be complete or incomplete above. But the surface net may also be absent. Spores remain embedded in the capillitium.
The spores are abundant in a sporangium, and vary in colour, wall marking and size. They are grey, violet-grey, rufous-violet to pale ferruginous. The spore wall may be smooth, warty or reticulated. The dimension of spores usually ranges from 4 to 12µm in diameter. The mature spore on germination produces one or two swarm cells or myxamoebae.
The fusion between the swarm cells or myxamoebae as the case may be, results in the formation of a zygote.
The zygote grows and undergoes a series of mitotic nuclear divisions developing into a plasmodium. Eventually the Plasmodium heaps itself up somewhat developing into sporangia. The nuclei within the developing sporangium undergo meiosis and a very large number of spores, each with a nucleus bearing haploid number of chromosomes is formed by cleavage of the protoplast.
Simultaneously with the development of spores, the capillitium and columella also develop. Life cycle of Stemonitis sp. is presented in Figure 329.