Short answers, pulled from the story.
A leaf is a principal appendage of the stem of a vascular plant, usually borne above ground and specialized for photosynthesis. It captures sunlight to make simple sugars such as glucose and sucrose from carbon dioxide and water. The leaf is also the principal site of transpiration, the process that draws water up from the roots.
Leaves turn red in autumn because trees respond to cold and reduced sunlight by curtailing chlorophyll production, which reveals carotenoids and xanthophylls and produces red anthocyanin pigments as the leaf dies. The anthocyanins are thought to mask the yellow hue, which appears to attract herbivores such as aphids, and to reduce photo-oxidative damage during senescence.
Leaf arrangement, called phyllotaxis, often follows divergence angles tied to the Fibonacci sequence of 1, 1, 2, 3, 5, 8, 13. Oak and apricot rotate by two fifths, sunflowers and pear by three eighths, and willow and almond by five thirteenths. These rotation fractions are typically a Fibonacci number divided by the number two places later in the sequence.
In any square centimeter of a plant leaf there may be from 1,000 to 100,000 stomata. These are pores that intake and output gases, and their opening and closing is controlled by the turgor pressure in a pair of guard cells. Stomata are usually more numerous on the lower abaxial surface and in plants from cooler climates.
In parallel venation the primary veins run side by side and equidistant for most of the leaf's length before converging at the apex, and it is typical of monocots. In reticulate venation a central midrib, the costa, sends secondary veins toward the margins that branch into a dense net, which is more typical of broad-leaved plants. The patches of tissue between the finest reticulate veins are called areoles.
Welwitschia retains its two main leaves throughout a lifetime that may exceed a thousand years. The longest leaves belong to the Raffia palm, Raphia regalis, which may be up to 25 meters long and 3 meters wide.