Family Description: Members of this family have a single, terminal blade that is divided in some species. The stipe is rarely branched. Blades are smooth or undulate, entire or with holes, of uniform thickness or with a central, thickened region (midrib). Sori develop directly on the blades.
Species description: Fringed Sieve Kelp anchors itself to rocks, wood, or sometimes other algae via a branched holdfast. The unbranched stipe is single, flattened, quite short (2 to 6 cm or 0.8 to 2.4 in long) and 4 to 7 mm (0.2 to 0.3 in) in diameter. The distinctive feature of the stipe is the numerous and often branched "fimbriations" or projections that occur along the lateral margins, making this species easy to separate from Sieve Kelp, which lacks such processes.
The massive blade is circular to elliptical in shape, rather thin, and measures 20 to 80 cm (8 to 31 in) in length and 15 to 26 cm (6 to 10 in) in width. The broad, flattened midrib is smooth and lacks holes, but the surface of the blade is bullate (carries large circular depressions) and has a few scattered, irregular holes. The base of the blade is either rounded or somewhat heart-shaped, and small teeth can occur along the blade margins. Usually, the distal end of the blade is rather tattered and torn.
Spores are formed in broad, dark patches termed sori that occur on both surfaces of the blade.
When wounded, Fringed Sieve Kelp responds by elevating the levels of phlorotannins (= polyphenolics) within one to three days of injury to blades, stipes or holdfasts. If no further injury occurs, phlorotannin production decreases by the seventh day. Elevated phlorotannin production is thought to help protect this species from herbivores. Similar observations have been made on Split Kelp (Laminaria bongardiana) and Sea Spatula (Pleurophycus gardneri).
Fringed Sieve Kelp is often abundant; for example it is very common at depths down to 20 meters (66 feet) in the San Juan Islands, Washington.
