Plants are eukaryotic autotrophs that belong to the kingdom Plantae.
Unicellular organisms possess a single cell.
All plants are not unicellular. They are multicellular organisms comprising more than one cell. Example: Pinus, maize.
Algae are a group of unicellular, autotrophic organisms that lack complex plant organs. They are called plant-like protists. Example: Chlamydomonas, Volvox.