magnoliid -- Any member of the basal assemblage of flowering plants.
marine terrace -- n. A platform of marine deposits (typically sand, silt, gravel) sloping gently seaward. Such a platform may be exposed along the coast, forming cliffs, due to uplift and/or the lowering of sea level, e.g., marine terraces of coastal Southern California.
marsupial -- n. A mammal that gives live birth to young that have gestated for only a short period of time. The young usually crawl into a pouch (the marsupium) or protected area and attach to their motherís teat to finish developing. Examples of marsupials include kangaroos, opossums, and koalas.
mastigoneme -- Small hair-like filaments found on the "hairy" flagellum of the Chromista.
Meguma -- n. - A terrane or microcontinent that formed in the Cambrian as part of the continental shelf of Gondwana, rifted apart in the Ordovician, and collided with the Laurentia in the Devonian; it forms the southern part of mainland Nova Scotia (Meguma Zone), separated from the Avalon Zone by a large fault that runs from Cobequid Bay to Chedabucto Bay.
meiosis -- A two-stage type of cell division in sexually reproducing organisms. In meiosis, a diploid cell divides to produce four haploid cells, each with half the original chromosome content. For this reason, meiosis is often called a "reduction division". In organisms with a diploid life cycles, the products of meiosis are usually called gametes. In organisms with an alternation of generations, the products of meiosis are caled spores.
meristem -- Group of undifferentiated cells from which new tissues are produced. Most plants have apical meristems which give rise to the primary tissues of plants, and some have secondary meristems which add wood or bark.
mesoderm -- In animals with three tissue layers (i.e. all except sponges and cnidarians), the middle layer of tissue, between the ectoderm and the endoderm. In vertebrates, for instance, the mesoderm forms the skeleton, muscles, heart, spleen, and many other internal organs.
mesogloea -- Jellylike material between the outer ectoderm and the inner endoderm of cnidarians. May be very thin or may form a thick layer (as in many jellyfish).
mesokaryotic -- Nuclear condition unique to the dinoflagellates in which the chromosomes remain permanently condensed.
metabolism -- n. The chemical processes within an organic body that supply the energy necessary for life. The rate of metabolic processes is sometimes used as a way to differentiate organisms. For example, mammals generally have a higher metabolism than reptiles and can thus sustain higher levels of activity for longer periods of time.
metamorphism -- n. The process of altering the chemical or mineralogical composition of a rock through different amounts of heat and pressure below the surface of the Earth; metamorphose- v; metamorphic - adj.
metamorphosis -- n. A process of developmental change whereby a larva reaches adulthood only after a drastic change in morphology; occurs in most amphibians and insects, for some insects, this change may include another stage (pupa) before the adult stage; metamorphose- v.
microphyll -- A kind of leaf, specifically one which has a single, unbranched vein in it. Microphylls are only found in the lycophytes.
microvilli -- Thin fingerlike protrusions from the surface of a cell, often used to increase absorptive capacity or to trap food particles. The "collar" of choanoflagellates is actually composed of closely spaced microvilli.
mitochondrion -- Complex organelle found in most eukaryotes; believed to be descended from free-living bacteria that established a symbiotic relationship with a primitive eukaryote. Mitochondria are the site of most of the energy production in most eukaryotes; they require oxygen to function. See: double membrane.
mitosis -- The process of nuclear division in eukaryotes. It is one step in cytokinesis, or cellular division. MORE ?.
monophyletic -- Term applied to a group of organisms which includes the most recent common ancestor of all of its members and all of the descendants of that most recent common ancestor. A monophyletic group is called a clade. More?
monotreme -- n. A mammal that lays eggs rather than giving live birth. Though laying eggs is a primitive reptilian trait, monotremes share many morphological, physiological, and reproductive characteristics with other mammals, making them true mammals. Extant monotremes include the duck-billed platypus and echidna.
monsoon -- n. - A seasonal weather pattern where winds, and often rain, come consistently from one direction for many months. It is caused by the temperature differences between land and ocean, and in general, a larger landmass makes a greater difference, which makes a more extreme monsoon.
moraine -- n. A mound or ridge of sediment deposited by a glacier; lateral moraine- n. deposited to the side of a glacier; terminal moraine- n. deposited to the front of a glacier; ground moraine- n. deposited on the land surface.
mouth -- Front opening of the digestive tract, into which food is taken for digestion. In flatworms, the mouth is the only opening into the digestive cavity, and is located on the "belly" of the worm.
MTOC -- (microtubule organizing center) MTOCs are bundles of protein tubes which may be found at the base of a eukaryotic flagellum. In animals, they also function in creating the arrays of microtubules that pull the chromosomes apart during mitosis.
muscle -- Bundle of contractile cells which allow animals to move. Muscles must act against a skeleton to effect movement.
myotome -- Segment of the body formed by a region of muscle. The myotomes are an important feature for recognizing early chordates.