Introduction to the Plant Kingdom Key Concept Plants are multicellular eukaryotes with chloroplasts and cell walls made of cellulose. All plants have specialized reproductive organs and a life cycle with alternation of generations, and almost all plants produce food by photosynthesis. The earliest plants are thought have evolved in the ocean from green alga. Plants were among the first organisms to colonize land, and they evolved important, adaptations to terrestrial life, including vascular tissues, seeds, and flowers. Today, plants are divided into nonvascular and vascular plants, and vascular plants are further divided into seedless and seed plants.