gametophyte -- The haploid stage in the life cycle of an organism undergoing alternations of generations. The gametophyte is multicellular and mitotically produces gametes. In plants, the gametophyte nourishes the zygote and young sporophyte.
gastrodermis -- In cnidarians, the endodermis which lines the gut cavity. The term is often used instead of endodermis since cnidarians only have two tissue layers instead of three.
geologic province -- n. A group of rocks that share a common history or event, for example, rocks that were formed in the same environment at the same time or that were folded and faulted by the same event.
gill -- In aquatic animals, highly vascularized tissues with large surface area; these are extended out of the body and into the surrounding water for gas exchange.
gill arches -- Stiffenings which support the flesh between the gill slits of chordates. In most vertebrates, the first gill arches have been modified to form the jaw, and in tetrapods, the inner ear bones.
gill slit -- A slitlike or porelike opening connecting the pharynx of a chordate with the outside of the body. Gill slits may contain the gills and be used for gas exchange, as in most fish, but may also be used for filter-feeding, or may be highly modified in land-dwelling vertebrates.
glycoprotein -- a membrane-bound protein which has attached branching carbohydrates. These may function in cell-cell recognition, such as in human blood groups and immune system response, as well as in resisting compression of cells.
gradualism -- A model of evolution that assumes slow, steady rates of change. Charles Darwin's original concept of evolution by natural selection assumed gradualism. Contrast with punctuated equilibrium.
grain -- (1) The texture of wood, produced by the kinds of xylem cells present. (2) The fruit of a member of the grasses.
granite -- Highly felsic igneous plutonic rock, typically light in color; rough plutonic equivalent of rhyolite. Granite is actually quite rare in the U.S.; often the term is applied to any quartz-bearing plutonic rock.
grassland -- Region in which the climate is dry for long periods of the summer, and freezes in the winter. Grasslands are characterized by grasses and other erect herbs, usually without trees or shrubs. Grasslands occur in the dry temperate interiors of continents, and first appeared in the Miocene.
Groundwater -- Water found underground as a result of rainfall, ice and snow melt, submerged rivers, lakes, and springs. This water often carries minerals. These minerals can accumulate in the remains of buried organisms and eventually cause fossilization.
gut (enteron) -- Body cavity formed between the mouth and anus in which food is digested and nutrients absorbed; it consists of the mouth, pharynx, esophagus, stomach, intestine, and anus, though some animals do not have all these regions.
gymnosperm -- n. A plant that produces seeds, which are not enclosed; includes any seed plant that does not produce flowers.
gynostemium -- The central reproductive stalk of an orchid, which consists of a stamen and pistil fused together.