List the structural features of brown algae.

Brown algae exist in a wide range of sizes and shapes. The smallest members of the group grow as tiny, fluffy bundles of filamentous cells no more than a few centimeters long. Some species have a stage in their life cycle that consists of only a few cells. Other groups of brown algae grow in much larger sizes. Stones and leather algae are often the most prominent algae in their habitat. Kelp can range in size from small kelp to a giant algae called macrocystis pear-shaped, which reaches over 45 m in length and is the largest of all algae.

Regardless of their shape, the body of all brown algae is called a thallus, indicating that it lacks the complex xylem and phloem in vascular plants.