Phylum: Ochrophyta   Family: Sargassaceae

Halidrys siliquosa  (Linnaeus) Lyngbye, 1819

Sea Oak

A fairly common seaweed of the middle to lower shore. A tough plant, up to a metre in length, with a flattened main stem and alternate branches. The tips bear elongated air-bladders which are septate

References: 
Bunker, Brodie, Maggs & Bunker (2017) - Seaweeds of Britain and Ireland