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 5 cm (2 in).

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.

Inside the gametangium, the nucleus undergoes further repeated mitotic division, so that by the end of summer, when the cap disintegrates, thousands of "tertiary" nuclei are released into the sea as gametes from each gametangial ray.

This effect can be demonstrated by shining a ray of blue light on a stem in the dark and extinguishing it some time later, forming a transient green band.