Plants are composed of one or more cells. Plants are composed of specialized cells and tissues. In plants, the formation of new cells, tissues and organs is restricted almost entirely to regions known as meristems.
Depending on who you ask, plants can be both unicellular and multicellular, or they can be JUST multicellular. In the second group, all unicellular eukaryotic (meaning they have a nucleus) organisms are all Protists, but can exhibit plant-, animal-, or fungus-like properties.