The seaweeds range from large multicellular algae to single-celled organisms and are classified into three groups, the brown, red and green algae.
All living things were traditionally placed into one of two groups, plants and animals.
The green plants or Viridiplantae were traditionally divided into the green algae (including the stoneworts) and the land plants.
However, it is now known that the land plants evolved from within a group of green algae, so that the green algae by themselves are a paraphyletic group, i.e.
It is this clade that is mainly the subject of this article (e.g., Plantae Plants in a broad sense comprise the green plants listed above plus the red algae (Rhodophyta) and the glaucophyte algae (Glaucophyta that store Floridean starch outside the plastids (in the cytoplasm).
This clade includes all of the organisms that eons ago acquired their chloroplasts directly by engulfing cyanobacteria (e.g., Plantae Another way of looking at the relationships between the different groups that have been called "plants" is through a cladogram, which shows their evolutionary relationships.