They are of two kinds, fertile, composed of a vascularized stalk and its terminal sporangium, and vegetative, or sterile, termed "phylloids " (this word is used in a different sense from that of Lignier who applied it to enations, or emergences).
In the simplest cases the phylloids became united by the development of parenchymatous tissues betwen them, thus forming a foliar appendage with open dichotomous venation.
Each ovule, in turn, was derived from three telomes, a central fertile one (megasporangium) and two lateral phylloids, which became modified into an integument, free from the sporangium except at the base.
The vegetative portion terminated in branch systems composed of phylloids; these systems originally branched dichotomously in three planes, but later, after such processes as overtopping, webbing and planation, they became converted into expanded foliar appendages.
The 3 ft (1 m) long phylloids (leaves) are distributed along the stipes and have an air bladder, or pneumatophore, that helps them to float.
Winter growth occurs thanks to the translocation of stored reserves in the stipe and old phylloids during the previous summer.
The increase in length of the phylloids during the maximum growth period is between 0.08-1.2 in (0.2-3 cm) a day in species of Laminaria that grow in the infralittoral, while Macrocystis pyrifera can grow up to 20 in (50 cm) a day in optimal conditions on the coastline of California.
The photo shows one surfacing among the phylloids of Macrocystis in Monterey Bay, California.
Especially in the spring, filamentous algae, hydroids, and bryozoans are common on the fronds, or phylloids.