Question

Explain why green algae are now included in the same kingdom as land plants and why they previously were not. Be sure to give a thorough explanation of why kingdom Plantae was taxonomically restructured into kingdom Viridiplantae. Mention relevant cladistic terminology and plant groups in your answer.

Answer 1

The old concept explained Kingdom Plantae as a group of multicellular photosynthetic organisms that developed from an embryo. This definition did not include the groups of green algae in the plant kingdom.  They were classified as Protists with Chloroplasts as they are structurally simple compared to true plants. Plants are multi-cellular while algae are unicellular and multi-cellular. The green algae bear many of the structural and biochemical traits of plants but lack many other structures of true plants which make their classification challenging. The charophytes species of green algae are considered the most related to the embryophytes (land plants) as they share many characteristics.

Viridiplantae are a monophyletic group of eukaryotic organisms that includes aquatic green algae and the embryophytes. The brown, red, and gold algae still belong to the Protista kingdom. Green algae and the embryophytes possess same carotenoids and chlorophyll a and b while different accessory pigments and chlorophyll molecules are found in other algae. Green algae store carbohydrates in the form of starch and their chloroplasts are enclosed in a double membrane as seen in land plants. They both have cellulosic cell wall. The kingdom Viridiplantae is divided into two clades as Chlorophyta and Streptophyta. Land plants and closely related charophytes are grouped as Streptophytes whereas the remaining green algae are called chlorophytes.