Red, brown, and green algae may be considered words apart from blue-green algae with respect to their classification.
These algae may be single-celled like blue-green algae, or they may be as complex as some of the largest ocean dwelling plants None of them have leaves, flowers, roots, stems, or seeds like true plants, but many scientists assume that today's flora have evolved gradually from these algae.
The debate then revolves around the the classification of red, brown, and green algae with higher plants in the kingdom Plantae, or in the mixed kingdom Protista.
The kingdom Protista is a sort of "catchall" kingdom, which includes both single- Celled and multi- Celled organisms.