Why do different plants have different shapes of leaves although the meristem tissue is present in all plants?

Leaves are adapted to support basic plant functions.The shape of the leaves of a tree are a response to the tree species' long term ecological and evolutionary histories. An ecosystem's limiting factors may also modify the finished form and shape of the leaves.
Leaves high in the tree canopy receive a great deal of sunlight. These leaves tend to be smaller in size and therefore, have reduced light absorptive surface area, and also tend to have complex edges and lobes (which enables them to disperse absorbed heat very rapidly).
Leaves in the lower tree canopy are more shaded. These lower canopy leaves tend to be larger (more light absorptive surface area) and tend to have reduced expressions of lobes and edges.
Needle-shaped leaves have a very low light absorptive surface area. Needles also have a very thick, outer cuticle coating and special "pit-like" stomatae designed to prevent excessive water loss.Trees with needle-shaped leaves are especially well suited to site's that have drier soils and to climates in which the careful conservation of water is an important survival strategy.