You can have a look see at this phylogeny chart until I can get some more info up here, all righty?
As you can see, this branch (Chromista) is separated from the red and green algae. They are separated because of a certain chlorophyll that they contain. It is chlorophyll c, which is not only found in Phaeophyta, but also the following phyla: Dinoflagellata, Bacillariophyta, and Chrysophyta. More about chlorophyll c can be found on the classisfication and diversity page:
Now, we'll learn about some of the evolutionary adaptations of seaweed found in this picture. The first thing is the thallus. It is used for seaweeds that have a plantlike body, but lack true plant parts. The root sort of things on the bottom are called holdfasts. These help keep the seaweed in place when rough waves splash about. There is also a stipe, which is the stemlike structure. The blades (leaflike structures) are supported by the stipe. Like plants, the blades are the part wherein photosynthesis takes place. In some brown algae, floats enable the blades to stay near the surface of the water. The floats, (not pictured above) are like little balls near the end of the blades. Another adaptation is found in the cell wall. In addition to cellulose, brown algae also have a gel-forming polysaccharide which helps to cushion the thalli (the plural of thallus) against the powerful waves. This explains the sliminess of a lot of seaweeds.
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