Which leaf shape predominates in rainy regions? Explain?

What determines the shape of a leaf (excluding DNA)?

• I know DNA is what determines the shape of an oak tree leaf or the shape of an elm tree leaf but I want to get deeper into this for a project. I want to know what shape particular leaves serve the tree i.e. I read somewhere that rain forest leaves' purpose is to collect water to drip off their leaves. I'm having a very hard time with finding any info. on this. I've found lots of sources that list different leaf shapes but it doesn't go any deeper into it than that. So if anyone knows anything about this and has any good sources to check out? Thanks!

• Answer:

  Thin, broad leaves are an adaptation that provide maximum surface to collect sunlight with the least volume. They are thin so there is a minimal distance for diffusion between the photosynthetic cells and the stomata. Broad leaves have greater evaporative water losses so plants with the largest leaves are found in the consistently wet soils in humid climates. Leaf area is directly related to having a constant water supply. Leaves with lobes and compound leaves resist tearing by wind damage. Many trees grow leaves with deeper lobes at the canopy surface and shallower lobed leaves sheltered within the canopy. Needle shaped leaves have less surface but still have a short diffusion path from stomata to mesophyll cells with their long narrow shape. They resist dessication. The presence of a toothed leaf edge offers unfolding leaves maximal photosynthesis as trees open their leaf buds when breaking winter dormancy and temperature is a rate limiting factor to photosynthesis. Plants have toothed leaf edges to increase the transpiration/gas exchange rate so the plant is maximally photosynthetic during the brief warmth of a spring day and light penetrates to all the young leaves of the canopy. Leaves have a petiole, a stem, that holds the leaves out into the light while reducing self shading (leaves blocking light from each other). In some the petiole is hinged with a pulvinus motor in some so the leaf can be folded in response to an external trigger: touch, temperature, light intensity.