Strange thermal conductivity of leaves?

One way to tell if a plant is a plastic fake or not is to touch a leaf. If it feels cool, the plant is a real one. Have you ever wondered why a leaf feels cool? (A leaf of an indoor plant always rests at about the room temperature, plastic or real. It is not really cooler before you touch it. You can confirm this by measuring its temperature using a sensitive temperature sensor.)

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.

Source: Wikipedia.

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.

Updates on 8/14/2012: 

See these YouTube videos of IR imaging:

Fresh leaf vs. dry leaf: http://www.youtube.com/watch?v=5I2eAU6AZ3Y
Wet sponge vs. dry sponge: http://www.youtube.com/watch?v=2LGfriM3O0Y

The thermogenesis of a moth under an IR camera

Is a moth warm-blooded or cold-blooded? If you google this, some would tell you it is cold-blooded. They are not completely right. This infrared study shows how a moth warms up before it can fly. So at least a moth is warm-blooded when it moves.

The moth (is this a winter moth -- operophtera brumata?) was kept in a glass jar. The first IR image shows that when it was idle, its body temperature is the same as the ambient temperature. This means that it does not lose energy to the environment -- a clever way for saving energy and probably protecting itself from predators that hunt by detecting thermal radiation.

However, before making a move, it needs to warm up its flying muscles (near its head where the wings are attached, called the thorax) to above 30 degrees Celsius. In this observation, the warming process took 1-2 minutes for the subject, as shown by the sequence of the IR images to the right. (Note: You may only observe this effect when the moth is energetic. A moth on the verge of death does not have enough energy to warm up.)

Click to view a larger image

Note that we used the automatic color remapping, i.e., the heat map is rescaled based on the lowest and highest temperatures detected in the view. As a result, while the moth warmed up and appeared more reddish in the IR view, the background -- in contrast -- became bluer in the IR view. This, however, does not mean that the temperature of the background has decreased. This automatic remapping could create some confusion, but it is necessary in many cases, especially when you don't know what to expect. It maximizes the difference by increasing the contrast and, therefore, allows the observer to pick up subtle changes like this one.

The last image shows that, after the temperature was high enough, the moth started to move. In this particular experiment, the moth responded slowly because it could have been exhausted as it had struggled quite a bit in the jar before it was imaged.

What interests me in this experiment is thermogenesis: the process of heat production in organisms. What biochemical reactions are responsible for the thermogenesis in moths and bees? Can we learn from them to find a green way to heat our homes?

An infrared view of bees

A bumble bee.

I have been wanting to see what I can do with IR imaging in my backyard. Folks at the Discovery and Animal Planet channels use IR imaging regularly to show thermal patterns of animals and plants. So I guess I could do something with it. I cannot afford a high-definition IR camera. But I think my low-grade IR camera should be able to catch something. Here is an interesting story about bees.

Bees are warm-blooded insects. In order to fly, bees must heat up their flight muscles to above 30^oC. So let's check this using an IR camera.

Indeed, a bee looks warm through the IR camera. To be more specific, the thorax of a bee appears to be warmer than the rest of its body (see the IR image to the right). I observed both a honey bee and a bumble bee. Both types have a warmer thorax, where the flight muscles are located. Exactly why the muscles can operate only at a warm temperature is an interesting question.

Bees are known to form societies that depend on successful division of work. Researchers have been using high-definition IR imaging to study bee behavior. With the assistance of IR imaging, German researchers led by Prof. Dr. Jürgen Tautz at Würzburg University found a new type of role known as the heater bees. The heater bees are responsible for maintaining the temperature in the hive where young bees (pupae) grow in sealed wax cells. The bees purposely leave some empty cells among those pupa cells so that the heater bees can crawl into them to warm up the pupae. By varying the temperature of each pupa they can determine what kind of bee it will become. As a result, the heater bees are vital in determining what job a young bee will perform once it matures. In the IR video, heater bees' thoraxes also appeared to be warmer, agreeing with what I observed using my IR camera for a worker bee.

Another article published in Optics Express discussed using IR imaging to evaluate beehive population. The idea is based on the assumption that the more bees in a hive, the warmer it would be. An unhealthy colony that has lost population would appear colder in the IR view, as the number of heater bees might have died down because of the lack of worker bees and hence the food they bring back. And if there are not enough heater bees, the pupae would not grow up normally, worsening the situation.

If you don't want to disturb bees and get stung by them, the non-touch, non-invasive IR imaging is probably the best way to go. :-)

PS on 8/3/2011:

Other flying insects like flies, dragonflies, cicadas, and wasps have a similar thermogram (i.e., warm thorax while active). See these two additional images. Or see this blog post about a moth.

I didn't observe warming in ants. They probably don't produce heat. Or they could be just too small to emit any appreciable IR radiation.