Kingdom Plantae
This is the kingdom of the multicellular organisms known as plants. Plant species are thought to number about 287,000.
What makes a plant a plant
The cell wall
Plant cells have walls made of cellulose.
Chlorophyll
They appear green. The reason is the pigment chlorophyll inside the chloroplasts of their cells.
Photosynthesis
With it they absorb solar energy and produce food.
They propagate their kind by sexual means of reproduction and by asexual reproduction.
Algae photosynthesise too — and are not plants
Having chlorophyll does not by itself make something a plant. Algae photosynthesise as well, and they belong to Kingdom Protista. The difference is that plants have tissue differentiation — roots, stem and leaves.