red algae Common name for the algae placed in the division Rhodophyta.

red blood cell Component of the blood that transports oxygen with the hemoglobin molecule. See also erythrocyte

red tides Phenomenon associated with population explosions (blooms) of certain types of dinoflagellates; red structures inside the dinoflagellates cause the water to have a reddish color. PICTURE

reduction  The gain of an electron or a hydrogen atom. The gain of electrons or hydrogens in a chemical reaction.

reductional division  The Þrst division in meiosis; results in each daughter cell receiving one member of each pair of chromosomes. PICTURE

reflex  A response to a stimulus that occurs without conscious effort; one of the simplest forms of behavior.

reflex arc Pathway of neurons, effector(s) and sensory receptors that participate in a reflex.

region of division  The area of cell division in the tip of a plant root. PICTURE

region of elongation  The area in the tip of a plant root where cells grow by elongating, thereby increasing the length of the root. PICTURE

region of maturation (differentiation)  The area where primary tissues and root hairs develop in the tip of a plant root. PICTURE

relative time Type of geologic time (absolute time being the other) that places events in a sequence relative to each other.

renal tubule  The portion of the nephron where urine is produced. PICTURE

renin  An enzyme secreted by the kidneys that converts angiotensinogen into angiotensin II.

replication Process by which DNA is duplicated prior to cell division. PICTURE

reproductive isolating mechanism  Biological or behavioral characteristics that reduce or prevent interbreeding with other populations; e.g., the production of sterile hybrids. Establishment of reproductive isolation is considered essential for development of a new species.

reproductive system  One of eleven major body organ systems in animals; is responsible for reproduction and thus the survival of the species. PICTURE

reptiles Taxonomic class of vertebrates characterized by scales and amniotic eggs; the first truly terrestrial vertebrate group.

resolution  In relatio