Well, now, I guess I need to talk about the morphology of Phaeophyta, huh? I'm sure everyone is just dying to know about it! All righty, here we go!
As was discussed on the phyologeny and evolution page, many phaeophyta have the following structures: the blades, the stipe, and the holdfast. All of these together are called the thallus (plural thalli). Some also have floatation bladders to help keep their blades near the water surface in order to better photosynthesize. These adaptations arose as a result of the environment in which brown algae are found.
All brown algae are multicellular; none are unicellular or colonial. They have a large surface area in order to acquier dissolved nutrients form the surrounding water. Some kelp blades get up to 100 meters in length!
Now for a discussion on the insides of a brown alga. Inside of phaeophyta, the following pigments are found: chlorophyll a, chlorophyll c, carotenoids, and xanthophylls. Chlorophyll c is only found in the photosynthetic members of Chromista as well as Dinoflagellata. Cool, huh? This is what separates them from "plants." Phaeophyta reserve their food as Laminarin, which is a beta 1-3 glucose polymer. The cell wall is made of cellulose, as well as some other polysaccharides.