The sea cabbage is also known as the 'stipeless kelp' because the holdfast, or haptera, is connected directly to the thickened lower margin of the blade and there is no stipe present.

It can grow to 150 cm (5 feet) in length and 80 cm (32 inches) wide (5), but it is more commonly 30-60 cm (1-2 ft) across in the San Juan Islands.

There is much variation in morphology among individuals from a broad, ruffled or bullate blade to a smooth and deeply divided blade:

• In sheltered locations (such as Mar Vista and Deadman's Bay on San Juan Island) the blades are broad and ruffled, which according to Armstrong (1989), is an adaptation to increase water flow across the thallus for nutrient absorption and gas exchange in low flow areas.
• In exposed areas (on the outer coast of WA and Vancouver Island) the blades are smooth and deeply divided allowing for a more compact shape at high current speeds (6). Exposed sites also have individuals with stiffer and stronger tissues. For example, individuals at Cattle Point, a more exposed location on San Juan Island, had larger mean breaking strains than those found at the more sheltered Mar Vista location (7).

The intercalary meristem is near the base of the blade (6) and the thallus consists of one large blade with inrolling at the basal margins (8) and has a thickened, smooth area at the base usually looping into a U- shape and the holdfasts conform to rock contours.

Like all Laminariales, a cross-section of S. sessile reveals the elongated cells of the inner medulla surrounded by the large round cortex cells and an outer epidermis, usually covered in mucus (8).

A cross-section through a blade showing the tissue layers (1 mm wide)