Leaf Efficiency

So, why are some leaves — even on the same individual tree or plant — bigger than others?  Biologists describe the size difference simply as “sun leaves” and “shade leaves.” For the most part, leaves growing in the sun are physiologically different from those growing in the shade.  The difference relates to the environment they have to deal with and the light resources they receive.

At the top of a tree, a leaf receives more light and wind.  These sun leaves are thicker and smaller.  They are thicker because of the distribution of chloroplasts within the palisade cells, which are tall cells standing on end just under the leaves’ “skin.”  This arrangement makes the leaves efficient at converting light to make sugars.  It also suggests upper leaves are often darker green.  Because upper leaves gather more light, they get hotter.  Therefore, they have more and smaller stomata, which are a leaf structure designed to exhaust oxygen and water — a cooling mechanism — than are found on shade leaves.  The advantage of having lots of small stomata on a smaller leaf is the ability to adjust moisture loss not only to heat, but to increased wind moving across the leaf’s surface.

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Lower down in the tree, where the leaves are shaded by those higher up, the logic begins to reverse.  The leaf is thinner because the palisade cells are shorter.  This arrangement makes sense because light is less intense and does not reach as deeply into the leaf, and the leaf is often a lighter shade of green.  Because the chloroplasts are closer to the surface, the leaf area increases to make it more efficient resulting in bigger leaves.  The stomata are larger and wider spaced.  In general the shade leaf stays cooler, has less wind moving across its surface, and needs less cooling.

 

 

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