The different parts of an alga

Marine algae are quite different from land plants. They are made up of three parts:

• a holdfast to grip the rock, but no roots;
• a stipe, more or less long and rigid depending on the species, but no stem;
• a blade (or frond), sometimes divided into strips, but no leaves.

Together these three parts form the seaweed’s vegetative body, known as the thallus.

Not every marine plant is a seaweed

Marine plants such as eelgrass or Neptune grass do have roots and flowers: they are not seaweeds. Nor is glasswort.