Like all seaweeds, the sporophyte stage of Postelsia consists of a thallus, which is made up of a stem-like stipe topped with possibly over 100 leaf-like blades, and rests on a root-like holdfast. The holdfast anchors the organism to the rocks it lives on.
Like other seaweeds, the sporophyte phase of Postelsia consists of a thallus with a stalk and leaf-like structures, which is secured to its environment by a root-like attachment.