Plant
Algae comprise several different groups of organisms which produce food by photosynthesis and thus have traditionally been included in the plant kingdom. The seaweeds range from large multicellular algae to single-celled organisms and are classified into three groups, the brown, red and green algae. There is good evidence that the brown algae evolved independently from the others, from non-photosynthetic ancestors that formed endosymbiotic relationships with red algae rather than from cyanobacteria, and they are no longer classified as plants as defined here.
Stipe (botany)
A stipe is also a structure found in organisms that are studied by botanists but that are no longer classified as plants. It may be the stem-like part of the thallus of a mushroom or a seaweed, and is particularly common among brown algae such as kelp. The stipe of a kelp often contains a central region of cells that, like the phloem of vascular plants, serves to transport nutrients within the alga.