Resting cell. Any cell that is not undergoing active division.

Resythesis. The production of a substance that has previously been broken down.

Retina. The light-sensitive membrane that lines the interior of the eye. The retina consists of two layers. The inner layer contains nerve cells, blood vessels, and two types of light-sensitive cells (rods and cones).
The outer layer is pigmented, which prevents the back reflection of light and consequent decrease in visual activity. Light passing through the lens stimulates individual rods and cones, which generates nerve impulses that are transmitted through the optic nerve to the brain, where the visual image is formed.

Retuse. Blunt or truncate and slightly indented.

R. genes. The genes present of the R factor concerned with resistance to antibiotics.

They can acquire such antibodies, as by blood transfusion; or in the case of a woman by bearing Rh-positive children, whose antigen crosses the placenta. The antibody thus formed in a Rh-negative mother may cross the placenta again and if in sufficient concentration, which usually requires at least one preceding pregnancy with Rh-positive child, it may damage a Rh-positive foetus.

Rhizoplane. The part of the Rhizosphere made up of the root surfaces.

Rhodophyta (red algae). A division of algae that are often pink or red in colour due to the presence of the pigments Phycocyanin and phycoerythrin.

Members of the Rhodophyta may be unicellular or multicellular, forming branched flattened thalli or filaments and are commonly found along the coasts of tropical areas.

Rhythm. The regular occurrence of strong and weak impulses of a particular phenomenon.