a colorless or brightly colored vegetative body of myxomycetes that consists of multinucleate protoplasm lacking a membrane. It varies in size from several sq mm to 1 sq m (sometimes 1.5 sq m). One distinguishes protoplasmodium, which consists of undifferentiated protoplasm (Echinostelium minutum); aphanoplasmodium, which consists of a network of undifferentiated strands of nongranular protoplasm (species of Stemonites); and phaneroplasmodium, which consists of well-differentiated protoplasm made up of strands and lobes with clearly visible granular contents (species of Physarum).
Internal movement of protoplasmic currents capable of changing the direction of their motion is characteristic of plasmodium. The motile mass feeds saprophytically, absorbing nutrient matter through its entire surface. It moves by means of protoplasmic processes known as pseudopodia. Plasmodium lives in the dark under tree bark, inside rotten and damp wood, and under fallen leaves. At the time of spore formation, it moves onto the surface of a substrate and is transformed into a spore-bearing organ, which varies in shape and coloration in different species. In plasmodiophoraceous myxomycetes, the plasmodium parasitizes the tissues of algae, fungi, and higher plants. It causes diseases in higher plants, such as clubroot of cabbage and other crucifers.
V. A. MEL’NIK
a genus of parasitic protozoans of the order Hemosporidia. More than 60 species parasitize vertebrates, including man, and cause malaria. The carriers of plasmodia are insects, mainly mosquitoes of the family Culicidae. Plasmodia, in the form of spindle-shaped sporozoites, enter the vertebrate along with saliva from the mosquito and embed themselves in the endothelium of the blood vessels or in liver cells. The protozoans reproduce asexually (schizogony), giving rise to numerous tiny mononuclear cells, or merozoites. The merozoites either asexually reproduce in the tissue or enter the blood and penetrate the red blood cells. They undergo a series of schizogonies in the blood cells, as a result of which the number of parasites in the blood sharply increases. The release of the merozoites from the destroyed red blood cells is accompanied by entry into the plasma of harmful products of the parasite’s metabolism.
At a certain stage in the life cycle, some of the merozoites formed in the red blood cells, having become embedded in new red blood cells, are transformed into gametocytes. The female gametocytes, or macrogametocytes, are transformed into macrogametes in the body of the vertebrate; development of the male gametocytes, or microgametocytes, is possible only in the body of the mosquito. After the plasmodia enter the mosquito’s stomach along with the blood of the vertebrate, each microgametocyte gives rise to several flagelliform microgametes, which fuse in pairs with macrogametes, forming motile zygotes, or ookinetes. After penetrating the epithelium of the mosquito’s stomach, the ookinetes surround themselves with sturdy membranes under the stomach’s muscular layer and form oocysts. After repeated division of the nucleus of the oocyst, the contents break up into numerous (up to 10,000) tiny mononuclear sporozoites. The membrane of the oocyst ruptures, and the sporozoites emerge into the body cavity of the insect. Moving actively in the hemolymph, the sporozoites enter the salivary glands of the mosquito. When the insect sucks blood, they enter the body of the vertebrate.
Four species of Plasmodium parasitize humans: P. vivax (the causative agent of tertian malaria), P. malariae (quartan malaria), P. falciparum (falciparum malaria), and P. ovale (ovale malaria). Mosquitoes of the genus Anopheles are the carriers of these species of Plasmodium. Primates are parasitized by P. rei-chenowi and P. knowlesi; rodents by P. berghei; birds by P. relicturn, P. gallinaceum, P. durum, P. lophurum, and P. catemerium; reptiles by P. agamae and P. lacertiliae; and amphibians by P. bufonis and P. catesbiana.
O. I. CHIBTSOVA