Family Description:
In most members of this family, the intercalary meristem at the junction of the stipe and blade is split so that the stipe itself is branched, and there are numerous terminal blades. Midribs are lacking. Sori usually develop on blades or on sporophylls that develop from the blade side of the meristem rather than the stipe part as in the Alariaceae. This family contains the giant kelps, and many members of the family have pneumatocysts (gas-filled floats). This family contains Lessonia, Macrocystis, Nereocystis, Pelagophycus, Postelsia and possibly Ecklonia, Egregia and Eisenia.

Species description:
The sporophytes of Sea Palm are annual and disappear by late November each year in Washington State. They do, indeed, look like miniature palm trees ("palmaeformis" means "having the shape of a palm tree"). They are up to 60 cm (24 in) tall, and have a hollow, cylindrical stipe that is often curved and tapers upwards. At the base of the stipe is a rather small but densely branched holdfast, and at the other end the stipe has numerous branches, each of which carries a single, drooping blade. The blades are straplike and have toothed margins. A single individual can have up to one hundred or more blades, each up to 25 cm (10 in) long, and additional ones can be produced through the splitting of pre-existing blades. Pneumatocysts (air-filled bladders) are lacking.

This seaweed grows in dense patches on rocks in areas where it is exposed to extreme wave shock. It usually grows in clumps in beds of the California Mussel (Mytilus californianus), where it colonizes patches of rock bared by the removal of mussels by predators, storms or drift logs. It can also overgrow barnacles or other algae, causing them eventually to be ripped off the rock and providing new sites for Sea Palm settlement.

Spores develop in grooves on both sides of the blades in late spring. They are released during low tide, and so drip down onto nearby rocks where they adhere quickly, limiting most dispersal to distances of 1 to 5 meters (3 to 16 feet). Long-distance dispersal is probably effected by floating individuals that have been ripped off the substratum. Within a clump, the individuals are siblings.

A species bearing some similarities to Postelsia is Eisenia arborea (Double Pompom Kelp). Its stipe, up to a meter or more in length, is flattened above the base and forks at the apex into two flattened branches that bear numerous stipitate blades. This species occurs sporadically in the low intertidal and subtidal zones along the outer coast of British Columbia, California, and Baja California, Mexico.

Source:  North Pacific Seaweeds