Acetabularia acetabulum
Acetabularia acetabulum is a species of green alga in the family Polyphysaceae. It is found in the Mediterranean Sea at a depth of one to two metres. Description This alga adheres to the substrate with rhizoids (root-like processes), and these are the only part of the alga present in the winter. The thallus consists of a single cell, and in the spring a slender stem develops from the holdfast, growing vertically to a length of about . Growth is interrupted at intervals while a whorl of hairs develop which encircle the stem, branching dichotomously. As the stem lengthens and more whorls grow, the lower hairs drop off leaving behind a circular scar. When the stem is fully developed, a disc-shaped cap up to wide grows at the tip, the whole frond resembling a pale green parasol; further whorls of hairs grow from the upper surface of the cap. Biology Before cap expansion, the organism consists of a single cell with a single nucleus, located at the base of the stem. As the cap expands, the nucleus divides once by meiosis after which it divides multiple times by mitosis, producing thousands of haploid "secondary" nuclei. These migrate up the stem into