coexistence The condition in which two species can live in the same habitat, dividing up resources in a way that allows both to survive.

collecting duct A tube in the nephron that receives urine from several nephron tubules. Some additional reabsorption, especially of water, takes place in the collecting duct, resulting in more concentrated urine.

collenchyma cell The type of plant cell that provides support to allow stretching and elongating the growing parts of the plant.

colon The longest region of the large intestine, between the cecum and the rectum.

commensalism An interaction between two species in which one benefits while the other is neither harmed nor helped.

commitment In development, a cell that is unable to alter its developmental fate has undergone commitment.

common descent with modification The process by which species of living things undergo modification in successive generations, with such modification sometimes resulting in the formation of new, separate species. The lineage of all species traces back to a single common ancestor.

community All the populations of living things that inhabit a given area. The term also is used to mean a collection of populations in a given area that potentially interact with each other.

companion cell In plants, cells that are closely associated with sieve elements in phloem. The companion cells provide housekeeping needs of the sieve elements, which have lost their nuclei to provide room for faster conduction of phloem sap.

competitive exclusion principle When two populations compete for the same limited, vital resource, one will always outcompete the other and thus bring about the local extinction of the latter species.

complement A group of proteins in the blood that supplements the action of antibodies by enhancing phagocytosis, destroying the membranes of invading cells, and promoting inflammation.

complex carbohydrate A carbohydrate built of many monosaccharides. Also known as polysaccharides, these compounds include starch, glycogen, cellulose, and chitin.

concentration gradient A gradient within a given medium defined by the difference between the highest and lowest concentration of a solute. The solute will have a natural tendency to move from the areas of higher concentration to lower, thus diffusing.

cones Photoreceptors that function in bright light situations and that provide color vision.

consumer Any organism that eats other organisms rather than producing its own food.

continental drift The lateral movement of continental plates over the globe, allowing continents to divide and rejoin in different patterns. This process can separate populations of organisms, providing the geographic barriers that can result in speciation.

control A comparative condition in an experiment. A control is typically a treatment in an experiment in which no parts are varied, so that other treatments can be compared to this standard and the effect of the variables can be determined.

convergent evolution Evolution that occurs when nature shapes two separate evolutionary lines in analogous ways, so they appear more similar than would be expected from their evolutionary relationship.

coral reef The piled up limestone deposits that are the remains of many generations of coral animals. These reefs occur in tropical oceans and provide habitat for a rich diversity of organisms.

cork Cells, dead in their mature state, that form the outermost covering of woody plants. These cells are infused with a waxy substance that protects the plant from drying out and from invaders.

cork cambium Secondary meristematic tissue in woody plants that forms the outer living covering of woody plants. This cambium produces the cork cells that, in their dead, mature state, protect the outside of the trunk and branches.

cornea A curved membrane through which light first passes upon entering the eye.

coronary artery An artery that delivers oxygenated blood to the muscles of the heart. Blockage of coronary arteries causes heart attack.

corpus luteum The structure that develops in the mammalian ovary from the ruptured follicle after ovulation. The corpus luteum secretes hormones that help prepare the reproductive tract for pregnancy. If pregnancy does not occur, this body degenerates.

cortex In dicotyledonous plants, the area of ground tissue in the stem that is outside the ring of vascular bundles.