The first observation of true leaves occurs in the phylum Angiosperms, also known as flowering plants. True leaves are characterized by a distinct leaf structure that typically includes a petiole (the leaf stem), a lamina (the leaf blade), and often a cuticle that helps prevent water loss.
Angiosperms evolved from earlier plant groups and are unique because they possess flowers and fruit, which aid in reproduction and seed dispersal. The presence of true leaves is an important adaptation that allows these plants to perform photosynthesis more efficiently, maximizing energy absorption from sunlight.
Prior to angiosperms, other plant groups like non-vascular plants (mosses) and vascular plants (ferns) had simpler leaf structures or leaf-like appendages but did not show the full complexity of true leaves. Angiosperms display a wide variety of leaf types and arrangements, illustrating their evolutionary advancement and adaptability in diverse environments.