11th Class Biology Biological Classification And System Of Classification / जैविक वर्गीकरण और वर्गीकरण की प्रणाली Consumer / Decomposer protists

Slime moulds include very interesting and peculiar organisms which share the characters of both animals and fungi. Due to this peculiarity they are commonly called fungus animals. Modern biologist include slime moulds under the kingdom-protista and called them protistan fungi.

General characters

(1) They do not have chlorophyll.

(2) They are surrounded by the plasma membrane only (somatic parts are without cell walls).

(3) At one stage of the life cycle they have amoeboid structure.

(4) The slime moulds live usually amongst decaying vegetation. They are quite common on lawns and moist fields.

(5) They exhibit wide range of colouration.

(6) They have phagotrophic or saprotrophic nutrition. Parasitic forms are not known

(7) The sporangia produce spores. Each spores possesses a cellulose cell wall.

(8) The slime moulds resemble both protozoa and the true fungi. They are like protozoa in their amoeboid plasmodial stage and similar to true fungi in spore formation. Slime moulds are of two types :

Acellular (Plasmodial) Slime moulds

General characters

(1) Acellular slime moulds commonly grow as slimy masses on damp places rich in dead and decaying organic matter.

(2) The somatic phase is diploid and consists of a free living organic matter multinucleated protoplasm called plasmodium.

(3) The plasmodium slowly streams or glides over decaying organic matter putting out blunt finger like pseudopodia showing amoeboid movement.

(4) They also absorb dissolved organic substances from the substratum showing saprotrophic nutrition.

(5) Under unfavourable conditions the plasmodium contracts and gets surrounded by thick horny wall. It is called sclerotium.

(6) Each plasmodium reproduces asexually by the formation of several, small, sessile or stalked, brightly coloured sporangia.

(7) The multinucleated protoplasm of sporangium is cleaved to produce a large number of small uninucleate spores.

(8) When fully mature, the wall of the sporangium bursts to release the spores. The spores are dispersed by air.

(9) On germination, a spore generally releases one biflagellate, spindle-shaped swarm cell or a non-flagellate myxamoeba. The myxamoebae feed on bacteria and yeasts and multiply in number. The swarm cells swim about actively and finally fuse in pairs at the posterior nonflagellate ends to form zygotes.

(10) The diploid nucleus of zygote undergoes repeated mitotic divisions. As a result, the zygote gradually changes into a multinucleate amoeboid structure, the plasmodium. The plasmodium repeats the life cycle.

Examples : Physarum, Physarella, Fuligo, Dictydium, Lucogala, Tubifera.

Cellular Slime moulds

General characters

(1) The cellular slime moulds occurs in the form of haploid uninucleated, naked (without cell wall) cell covered by plasma membrane. These cells are called myxamoebae.

(2) The myxamoebae move freely with the help of amoeboid movement and phagotrophic or holozoic nutrition.

(3) They grow and divide to form a large population of individuals.

(4) Under unfavourable condition a myxamoeba secrete a rigid cellulose wall to form the microcyst. Microcyst formation is a means of perennation.

The microcysts can be dispersed. On the return of favourable conditions, the microcyst wall ruptures to release a myxamoeba. The latter resumes its function of feeding, growth and multiplication forming amoeboid cells.

(5) When the food supply is exhausted, the amoeboid cells get aggregated without any fusion. The stimulus for the aggergation process is due to release of cyclic \[3',5'\] adenosine monophosphate (cyclic AMP) from the amoeboid cells. This aggregated mass of cells is called pseudoplasmodium. It is a sort of community association. Because of this reason, cellular slime moulds are called the communal slime moulds.

(6) The pseudoplasmodium crawls about until it settles at one point and from a stalked, branched or unbranched fruiting structure called sporocarp. The multicellular stalk of sporocarp is formed from the front part of the pseudopodium while the rear end forms the naked sporangium.

(7) The stalk of sporocarp may remain upright or bend. Finally the spores are released and disseminated.

(8) Some time the myxamoebae form clusters. The central myxamoeba of the cluster engulfs a surrounding myxamoeba to become larger structure which forms a thick wall to form the zygote. This zygote is called macrocyst. Karyogamy occurs inside the macrocyst which is followed by meiotic and several mitotic divisions. Ultimately the macrocyst wall ruptures to release a number of haploid myxamoebae.

(9) The most important character of cellular slime moulds is the complete absence of flagellated cells in their life cycle.

(10) The cellular slime moulds resemble plant in having cellulose cell wall in spores and resemble animals in having amoeba-like myxamoebae.

Examples : Dictyostelium, Polysphondylium.