However, there are a few exceptions:

* Algae: While algae belong to the kingdom Plantae, they can be both single-celled (like diatoms) and multicellular (like seaweed).

* Green algae: Some green algae, which are closely related to land plants, are single-celled.

So, while most plants are multicellular, the kingdom Plantae also contains some single-celled organisms.