Why are algae considered plants?

Algae are considered protists in the old classification of eukaryotes. Algae are able to photosynthesize, which is a key characteristic of plants, but do not have true roots, stems or leaves to be considered complete plants. They also have a cellulosic cell wall. The two main groups of algae, prokaryotes (without a nucleus), include blue-green algae; and eukaryotes (contain a nucleus). Thus, having all the characteristics of plants, algae can be considered as plants.