Atypical leaf is flat and thin. why is it useful for each leaf cell to be relatively near the surface of the leaf?

The outer layer of the skin covering the exterior body surface of vertebrates ... the primary tissue of higher plants composed of thin-walled cells that remain capable of ... a swelling on a plant stem consisting of overlapping immature leaves or petals ... Older xylem near the center of a woody stem that no longer conducts water.

By water evaporation from the leaves is created a negative pressure at the top of the tree that promotes the ascension  of the water through channels that are called xylem. there are also two molecule forces that are involved: the intra-water molecules cohesion (water molecules stick to each other) and the adhesion when the water molecules stick to the channels they are running in. this is what enables water to go from the roots to the top of a tall tree.