genus of one-celled, umbrella-like green algae found in subtropical seas, called the “mermaid's wine glass.” At the top of the tall, slender stalk, 0.5 to 10 cm (0.2 to 3.9 inches) long, is a ring of branches that may be separate or fused to form a cap. Near the base of its stalk, Acetabularia has a large nucleus that divides many times when the alga matures and reproductive structures form; streaming cytoplasm carries the daughter nuclei to the saclike sporangium of each umbrella lobe. Because part of one species can be grafted onto another, Acetabularia has been used to study the relative role of nucleus and cytoplasm in the genetic control of growth and development.