Why Leaves Evolved

The Glory of Leaves

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Water Lily:  Like snorkels, the mouthlike stomata on water lily leaves point up, where they find the air they need.
From National Geographic Magazine, October 2012, http://ngm.nationalgeographic.com/2012/10/leaves/dunn-text

This article is very interesting, describing some of the evolutionary processes that created the wide diversity of plant leaves that exist over the surface of the Earth.  It also discusses the process of photosynthesis as a unique mechanism for deriving food directly from the sun.  The article, unfortunately, never once mentioned the extremely important temperature regulation mechanism of evapotranspiration, which is the other important function of the stomata (microscopic pores on the surface of leaves).  The reader may find it interesting that only about 1% of the solar energy received by the leaf is used for photosynthesis.  The other 99% is converted into other energy fluxes for maintaining the leaf temperature within a comfortable range.  When sufficient water is present in the soil, virtually all of the solar radiation is converted into the latent energy flux  (i.e., water vapor), which keeps the plant leaf cool,  the same way perspiration is used to cool humans.  Keeping cool is critically important for plants, to avoid heat stress and a reduction in their growth and yield (e.g., fruit, seed, etc.).