We know metals feel cold because they conduct heat fast. Within a given amount of time, our fingers lose more thermal energy to a piece of metal than to a piece of wood.
Do leaves also conduct heat fast? On the contrary.
Let’s put a fresh leaf on top of a piece of dry paper. The first set of IR images in this post shows what happened after I used two fingers to touch the leaf (on the left) and the paper to warm them up. The result tells that the leaf actually conducted heat more slowly than the paper, which has much lower thermal conductivity than metals.
Now, we have a problem. We know leaves feel cooler than paper. But leaves conduct heat more slowly than paper! Our sense of touch honestly tells us that our fingers lose more thermal energy to leaves than to paper. So where does the thermal energy go on a leaf, if it doesn’t diffuse to other parts?
My theory is that the thermal energy goes to heat up the water in the spongy layer of the leaf. The spongy layer lies beneath the palisade layer–the waxy surface layer of the leaf. Its cells are irregular in shape and loosely packed–hence the name “the spongy layer.” During transpiration, the spongy layer is full of water in the spaces before they exit stoma. The specific heat of water is considerably high–4.18 J/(g*K) and the spongy layer is filled with water.
My theory is backed by the fact that a dry leaf conducts heat as fast as paper (IR images not shown here). This should not surprise you as paper is made of dehydrated wood fibers.
Now, the question is why the water in the spongy layer doesn’t dissipate thermal energy quickly as water in a cup does (I confirmed the energy dissipation in water by IR imaging, which is not shown here). The thermal conductivity of liquid water is about 0.58 W/(m*K), compared with 0.024 W/(m*K) for air, 0.016 W/(m*K) for water vapor, and 0.05 W/(m*K) for paper. Somehow, the water trapped in the spongy layer cannot conduct heat like free water does.
Let’s get get a wet (20% of full water absorption capacity) sponge (left) and a dry one (right) and look at their thermal conductivities under an IR camera. Again, I used my fingers to leave a heat mark on each. The second set of IR images shows a surprising result: the wet sponge appeared to conduct heat more slowly than the dry one!
Does this thermal conductivity protect plants’ leaves? Have you wondered why some plants are anti-freezing and some are not? Leaves may have very complicated thermal regulation that we don’t quite understand.