Short answers, pulled from the story.
The leaf functions as a biological engine that converts sunlight into chemical energy through photosynthesis. This process allows green plants to be autotrophic by creating their own food from carbon dioxide and water.
True leaves known as euphylls became widespread during the Devonian period. The earliest forms of simple vascularized leaves called microphylls appeared in early Devonian lycopsids like Baragwanathia.
Leaves follow precise mathematical patterns known as phyllotaxis that optimize light exposure while minimizing shading. The divergence angle often relates to Fibonacci numbers and the golden angle of approximately 137.5 degrees to form a helix.
Plants produce tannins which are chemicals that hinder the digestion of proteins and have an unpleasant taste to deter animals. Other protective strategies include the growth of thorns and the production of phytoliths and lignins.
During abscission leaves are shed to survive inclement seasons and leave a leaf scar on the twig. In cold autumns leaves sometimes change color to yellow bright-orange or red as chlorophyll production is curtailed.
Leaf veins contain xylem cells that bring water and minerals from the roots into the leaf and phloem cells that move sap with dissolved sucrose out of the leaf. These cells are embedded in a dense parenchyma tissue called the sheath.