Family Description:
This order contains large brown seaweeds with a distinctive pattern of growth. The apex of the sporophyte is a filament with an intercalary meristem, which produces the new cells that form the main axis of the thallus. Similar meristems occur along the length of the thallus so that the entire thallus can be fringed with pigmented hairs terminating the intercalary lateral meristems. These pigmented hairs are most evident during the spring growing season. At maturity, the plant appears parenchymatous, but it is actually pseudoparenchymatous in construction. Members of the Desmarestiales probably originated in the southern hemisphere where the greatest diversity of species occur—some rivaling northern hemisphere kelps in their size and morphological complexity.

Gametophytes are microscopic. They produce eggs and sperm on the same or different thalli. A sperm-releasing and -attracting factor called desmarestene has been identified in this order of seaweeds. Other pheromones, with slight differences in chemical structure, have been found in other orders of brown algae.

Species description:
Coarse Acid Kelp is an annual species that can attain a length of 8 m (26 feet). The holdfast can be quite variable, but is not branched; it grows to a diameter of about 4 cm (1.6 in). A single, flattened stipe arises from this holdfast. The blade supported by this stipe is 4 to 10 cm (1.6 to 4 in) wide and has flattened branches at least as wide as the blade bearing them. In mature individuals, these branches have distinct midribs, and slender veins from these midribs lead into side branches.

In Desmarestia the characters of different species can overlap somewhat.

Source:  North Pacific Seaweeds