Somatic Phase of Myxomycota (With Diagram)

In this article we will discuss about the somatic phase of myxomycota, explained with the help of suitable diagrams.

The somatic phase consists of coenocytic mass of highly granular protoplasm con­taining hundreds or thousands of nuclei and vacuoles without cell walls, but delimited only by a thin plasma membrane—a plasmodium (pl. plasmodia) (Fig. 321A). The Plasmodium is a flat amoeboid sheet-like structure that usually spreads over a large area in the form of a very thin network with irregular and slowly shifting contours.

It commonly assumes the form of a fan-shaped network with vein-like branches conti­nues along the outer margin. The granular protoplasm streams rapidly through network of veins which ultimately join the larger veins through which also protoplasm flows, comes to rest for a short while and then immediately reverses its course.

The flow is then continued from the larger to the smaller veins which branch again and again till they are lost in the broad margin of the advancing plasmodium. A rhythmic flow is thus maintained backwards and forwards at nearly equal intervals, but always of somewhat longer duration in the direction in which the plasmodium is creeping.

The ever-flowing and ever-changing of plasmodial protoplasm result the plasmodium to creep over the surface of the substratum in an amoeboid fashion and on its way feed like amoeba by engulfing living or dead microscopic organisms or other small bits of organic matter.

This food is digested within food vacuoles, and waste products or undigested particles are left behind in the form of a trail (Fig. 321B), as the plasmodium moves ahead.

In the laboratory, plasmodia have been maintained by feeding them bacteria or yeast cells or other organic matter in small bits. They have also been successfully grown in a chemically defined liquid medium. The absence of cell walls in the plasmodium, together with the method of obtaining food (holozoic) are features characteristic of animals, rather than of plants.

The colour of the Plas­modium is usually either white, yellow, or pink; in some cases it is purple or green and often brightly coloured; it is generally constant in each species.

The advancing margin is consistently succeeded by a great many strands which have all the properties of the former region; farther away, the strands become more and more separated.

At first each strand is profusely branched and its branches are connected by anastomosis into a frequently delicate net; the thickness of the strands increases with reduction in branching. The advancing region of the plasmodium, particularly the margin, is homogeneous, viscous, and in steady motion.

The general appearance of the plasmodial protoplasm is comparable with that of Amoeba and similar forms, as is the manner of movement, although the form of the strand is different and its rate of movement may be slower or faster, depending on the age of the Plas­modium and the temperature of the surrounding medium.

The plasmodium consists of two interpenetrating substances, a border layer of homogeneous ground substance devoid of granules known as hyaloplasm and an interior substance of granular protoplasm containing numerous nuclei and vacuoles. This interior mass of granular material flows with regularly-timed pauses and reversals.

The plasmodial strands possess a wide circumscribing layer of hyaloplasm and highly granular flowing fluid of central core. The thickness of the hyaloplasm gradually diminishes in the net­work region and finally both the layers merge near the margin of the advancing plas­modium. The hyaloplasm of the advancing margin exhibits amoeboid movement, projecting and withdrawing pseudopodia.

The nuclei are the same in the plasmodium with a diameter of 3 to 4.5µm. Each nucleus is surrounded by a membrane and contains a nucleolus. As a plasmodium grows, the nuclei within it increase in number keeping pace with its growth by repeated division. The nuclei of the plasmodium are diploid. The plasmodium thus repre­sents the diplophase in the life history of a myxomycete.

Three different types of plasmodia are recognised:

I. Protoplasmodium:

This is a type of plasmodium which is microscopic in size. It is composed of homogeneous mass of protoplasm which exhibits very slow, irregular streaming movement, e.g., Echinostelium minutum. During fruiting the entire plasmodium gives rise to a single sporangium.

II. Aphanoplasmodium:

This type of plasmodium has great resemblance with the protoplasmodium at its initial stages, but with maturity elongates, becomes branch­ed producing a network of very fine transparent strands and is often not even visible.

Such a plasmodium does not possess very granular protoplasm which is not conspi­cuously differentiated into gelified and fluid portions even in the veins though it exhibits rapid and rhythmically reversible streaming movement., e.g., Stemonitis fusca.

III. Phaneroplasmodium:

Such a plasmodium is large, massive and well-spread out. The protoplasm is very granular, the gelified and fluid regions are easily distin­guishable particularly in the veins. It exhibits very conspicuous rhythmic and rever­sible streaming movement, e.g., Physarumpolycephalum.

The Sclerotium:

The plasmodium withstands unfavourable conditions like dry or cold weather, by passing into a hardened mass, the sclerotium or resting stage. The change of plas­modium into sclerotium may also occur even when water and food material are present. The sclerotium can remain dormant for a long time and can resume the plasmodium state again with the return of favourable conditions.

The Fructification:

In many cases, as long as the supply of food is sufficient, the plasmodium continues to feed and grow. When the nourishment is exhausted it prepares to pass into the fruiting stage. But cases of fructification development in presence of abundant supply of food are not infrequent.

Preparatory to the development of fructifications the plasmodium usually leaves the moist surroundings where it has been feeding and creeps to some drier place. Even­tually the plasmodium heaps itself up somewhat into fructification initials which develop into a number of fructifications.

The plasmodial protoplasm may also con­centrate around some of the main veins of the plasmodium and develop into a fructi­fication whose shape is the same as that of the plasmodial vein.

As the fructification development continues, a thin wall, called the peridium, is secreted by the protoplasm at the surface. The peridium is often calcified and may also contain cellulose. A network of slender vacuoles is often formed within the developing fructification and this network is usually augmented by slender invagina­tions of the peridium.

The vacuolar network is ultimately transformed into a network of fibrous, often calcified strand, called the capillitium (pl. capillitia) which is formed by deposition of capillitial material from the surrounding protoplasm within or on the surface of the vacuolar network (Fig. 322A & B). The capillitium may also be absent in certain fructifications.

In many Myxomycetes, sometimes irregular threads, plates or other structures resembling capillitium known as pseudocapillitium (p1. pseudocapillitia) may be present among the spores within the fructifications. A central column, the columella is often developed in the fructificaton (Fig. 322A).

The formation of spores is preceded by the division of the nuclei. The nuclei within the developing fructification undergo reduction division, and a very large number of spores, each with a haploid nucleus, is formed by cleavage of the protoplasm.

The process of cleavage-is not quite simultaneous throughout the whole fructification but begins at the periphery and passes inwards. Each spore is surrounded by a definite wall, which in various species has been reported to contain cellulose, chitin, or neither of these substances.

There are three types of fructifications among the Myxomycetes, the sporangium, aethalium and plasmodiocarp.

The erect fructifications of definite form usually stalked are the sporangia (Fig. 323B & G). They are either scattered or clustered in groups. At the base of the sporangia there may remain a membrane or membranous strand not used up in sporan­gium development, this is termed the hypothallus (pl. hypothalli).

The sporangia are of many patterns, some being marvels of delicacy and intricacy. The capillitium and columella are usually present in the sporangia. A sporangium represents only a small part of the plasmodium and is of definite form and structure for a particular species.

An aethalium (pl. aethalia) is a cushion-like structure resembling a sessile sporangium (Figs. 322B & 323D). Here all or considerable part of a given plasmodium is involved. Numerous aethalia may be closely combined without much clear delimi­tation of peridium of the individual aethalium.

The plasmodiocarp is a sessile and vein-like fructification that is spread on the substratum with an irregular outline (Fig. 323E). The plasmodiocarp is developed by the concentration of the plasmodial protoplasm along the veins, as such its outline is like the veins of the plasmodium. But it has the interior structure of a sporangium.

There are few Myxomycetes where the fructification has no peridium. The spores are developed externally being borne on slender stalks arising from the surface of the fructification (Fig. 323A & A’). Some consider these structures as sporangia. The mature spore contains four nuclei.

The Spore:

The spores are generally globose with smooth, spiny, warded, recticulate or ridged- reticulate wall (Fig. 324). Spore wall is of cellulose and not of chitin. The colour of spores are usually yellow, rosy, deep-violet, brown to black, or often shades of various other colours. The spores are uninucleate. They are exceptionally resistant to un­favourable conditions, particularly to prolonged periods of desiccation.

The capillitia when developed are of different types (Fig. 325). They regulate the release of the spores, preventing all of them from being discharged at once. In certain species granules of carbonate of lime are in plenty remaining incorporated in various parts of the fructification either in the peridium, in the capillitium or in expan­sions of capillitial threads as lime-knots, or in all these parts.

Swarm Cells and Myxamoebae:

The different genera and species of the Myxomycetes are mainly distinguished by the form and colour of the fructification and capillitium; by the colour, size and mark­ings of the spores; and presence or absence of granules of lime.

The spores while remaining in a dry state retail- their vitality for several years. The length of time elapses before the germination of the spore after it has been placed in water may be from a few hours to several days. The time required for germination of spores, and the percentage of germination vary not only with conditions and with the age of the spores but also with the species.

Spores germinate either by cracking of the spore wall or by means of a tiny pore through which the protoplast of the spore .emerges. The protoplast then either starts exhibiting amoeboid movement and develops into a myxamoeba (pl. myxamoebae) or after remaining quiescent for a few minutes develops two unequal flagella giving rise to a swarm cell (Fig. 326).

The swarm cell development may also take place within the spore before the emergence of the proto­plast. Uniflagellate swarm cells have also been reported in some species of the Myxo­mycetes. But irrespective of bi- or uniflagellate swarm cells, the flagella are of whiplash type. Division of swarm cell begins a few hours after it leaves the spore wall.

Myxa­moeba also divides repeatedly producing a large number of myxamoebae.

Although the myxamoebae and the plasmodia of many Myxomycetes have been grown in pure culture, none has been induced to complete its entire life cycle in media of defined chemical composition in the absence of living or killed bacterial cells.