Final answer:
Green algae, once grouped with algae in general, are now often placed under kingdom Plantae or are not classified at the kingdom level due to their close relationship with land plants and the artificial nature of their separation in the five-kingdom system.
Step-by-step explanation:
Formerly classified under algae, green algae are a group of organisms that biologists now place under kingdom Plantae or consider part of a more inclusive group, often not at the kingdom level. The green algae, or Chlorophyta, exhibit several features in common with land plants, such as photosynthetic capabilities, possession of chlorophylls a and b, the storage of carbohydrates as starch, and cellulose cell walls. However, they also differ from plants in not being primarily multicellular, lacking embryos retained within the parent organism, and with only a few species adapting to terrestrial environments.
The taxonomic approach to green algae has evolved, considering the artificiality of separating them from land plants in the traditional five-kingdom classification system. While some scientists include green algae in the Plant Kingdom, others propose abandoning the kingdom level of taxonomy or splitting the group into two phyla: Streptophyta, which includes land plants and closely related algae, and Chlorophyta, comprised of the remaining green algae species.