water mold, common name for a group of multinucleated organisms that superficially resemble fungi but are now recognized as having an independent evolutionary lineage and are placed in the kingdom Protista. There are two important phyla (divisions) of water molds, the chytrids (Chytridiomycota) and the oomycetes (Oomycota).

The chytrids live in salt- and freshwater and in moist soil. They live as saprobes, obtaining their metabolic energy from decaying plant and animal material, or as parasites, attacking plants, fungi, and algae. They typically take the form of small coenocytic (multinucleated) masses, called sporangia, from which many hairlike rhizoids protrude. Like roots, the rhizoids absorb nutrients. Reproduction can be by simple division of a sporangium into individual motile, flagellated spores or by more complicated sexual processes that yield flagellated gametes. There are approximately 900 species of chytrids.

The oomycetes resemble fungi, taking the form of coenocytic filaments (hyphae). They differ from fungi, however, in that cellulose is present in their cell walls. The hyphae of oomycetes have specialized regions that can produce distinct male and female gametes. Oomycetes can also reproduce asexually. Many oomycetes are aquatic. Many of the others live in water in certain stages of the life cycle. Most of the 800 species of oomycetes are saprobes, but those that are parasitic are of great significance: they cause downy mildew, a disease often affecting grapes; late blight of potatoes, an outbreak of which led to the Great Potato Famine in Ireland (1845-49); sudden oak death syndrome (also known as ramorum leaf blight or ramorum dieback; redwoods, Douglas firs, and other plants also are harmed by the same water mold); and blue mold of tobacco. Other parasitic water molds cause diseases of fish and fish eggs.

slime mold or slime fungus, a heterotrophic organism once regarded as a fungus but later classified with the Protista. In a recent system of classification based on analysis of nucleic acid (genetic material) sequences, slime molds have been classified in a major group called the eukarya (or eukaryotes), which includes plants and animals. There are two groups of slime molds, the plasmodial slime molds of the phylum (division) Myxomycota and the cellular slime molds of Acrasiomycota.

Slime molds have complex life cycles that may be divided into an animallike motile phase, in which growth and feeding occur, and a plantlike, immotile, reproductive phase. The motile phase is commonly found under rotting logs and damp leaves, where cellulose is abundant. It consists in the cellular slime molds of solitary, amebalike cells, and in the Myxomycota of a coenocytic (multinucleate) mass of protoplasm called a plasmodium, which creeps about by ameboid movement. Plasmodia often grow to a diameter of several inches and are frequently brightly colored. Both types ingest solid food particles using a process called phagocytosis (see endocytosis). They feed on living microorganisms, such as bacteria and yeasts, as well as decaying vegetation. Before entering the reproductive stage, a plasmodium moves to a drier, better-lit place, such as the top of a log. In the amebalike, or cellular, slime molds, up to 125,000 individual cells aggregate and flow together, forming a multicellular mass called a pseudoplasmodium that resembles a slug and crawls about before settling in a location with acceptable warmth and brightness.

In the reproductive stage the plasmodium or pseudoplasmodium is transformed into one or more reproductive structures called fruiting bodies, each consisting of a stalk topped by a spore-producing capsule that resembles the reproductive structures of many fungi. Eventually the cellulose-walled spores are released and dispersed; they germinate in wet places, releasing naked cells. In a typical plasmodial slime mold the germinated spores go through an ameboid or flagellated swimming stage, followed by sexual fusions and cell divisions. The diploid ameboid cell (i.e., the zygote) grows and its nucleus divides repeatedly, resulting in the formation of a new plasmodium. Under adverse conditions a plasmodium may be transformed into a hard, dry, inactive mass called a sclerotium. Resistant to desiccation, it becomes a plasmodium again when favorable conditions return.

In the case of the cellular slime molds, each spore released becomes a single ameba, which feeds individually until starving cells release a chemical signal that causes them to aggregate into a new pseudoplasmodium, and the process is repeated. In sexual reproduction two haploid amebas fuse, then engulf surrounding amebas, forming a single organism called a macrocyst. The macrocyst then undergoes meiosis and mitosis and releases haploid individuals.

There are about 65 cellular and 500 known plasmodial slime mold species, found in forests and sometimes lawns throughout the world. In a few species the plasmodium, under favorable conditions, may cover an area of several square feet. A slime mold is the cause of clubroot, a disease of cabbage and related plants.

mold, name for certain multicellular organisms of the various classes of the kingdom Fungi, characteristically having bodies composed of a cottony mycelium. The colors of molds are caused by the spores, which are borne on the mycelium. Most molds are saprobes and can obtain moisture and nutriment from fruits, vegetables, jelly, cheese, butter, bread, silage, and almost any dead organic matter. Among the commonest forms is the black bread mold (Rhizopus nigricans), which grows on decaying vegetables and fruits as well as on bread. Some molds, e.g., species of Penicillium, are useful in the preparation of Camembert, Roquefort, and other cheeses. Penicillin and other antibiotic substances are also obtained from molds. A few molds are pathogenic, e.g., those which cause ringworm and other skin diseases and several which cause diseases of plants. Some molds produce toxic chemicals called mycotoxins that can cause serious diseases (see ergot). Some organisms traditionally thought to be mold (e.g., slime molds) have now been placed in the kingdom Protista.

leaf mold, crumbly brown humus typical of forest floors. It is composed of decayed leaves and other plant material mixed with soil.

Any of about 500 species of primitive organisms that contain true nuclei and resemble both protists and fungi (see fungus). Originally grouped within the kingdom Fungi, some classification systems consider slime molds to be in the kingdom Protista. They typically thrive in dark, cool, moist conditions such as on forest floors. Bacteria, yeast, molds, and fungi provide the main source of slime-mold nutrition. The complex life cycle of slime molds, exhibiting complete alternation of generations, may clarify the early evolution of both plant and animal cells. In the presence of water a tiny spore releases a mass of cytoplasm called a swarm cell, which later develops into an amoebalike creeping cell called a myxamoeba. Both swarm cells and myxamoebas can fuse in sexual union; the resulting fertilized cell, or plasmodium, grows through nuclear division and forms a spore case, which, when it dries, disintegrates and releases spores to begin the cycle again.

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In manufacturing, a cavity or surface in which a fluid or plastic substance is shaped into a desired finished product. A molten substance, such as metal or plastic, is poured or forced into a mold and allowed to harden. Molds are made of various materials, depending on the application; sand is frequently used for metal casting, hardened steel for molds for plastic materials, and plaster for various purposes. Seealso ingot, patternmaking, tool and die making.

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