Originally published on August 3, 2011 at Blogger, revised on December 5, 2018
One way to tell if an indoor plant is a real one or a plastic fake is to touch a leaf. If it feels cool, the plant is a real one. Have you ever wondered why a real leaf feels cool?
Common sense tells us that the temperature of a leaf of an indoor plant should be about the same as the room temperature, plastic or real. Although plants do cool their leaves through transpiration that ends with water evaporation through the stomata, the effect is not as significant as evaporative cooling in a glass of water (which lowers the temperature of water by 1–2 °C in a dry room), because the area of the stomata of a leaf is small relative to the total surface (a waxy cuticle, if present, may reduce evaporation even further). Therefore, despite transpiration, the temperature of a leaf of an indoor plant not under direct sunlight should not be significantly different from the room temperature. You may confirm this by measuring its temperature using a sensitive temperature sensor or observing it with an infrared thermal camera if you have one. The infrared image of Figure 1 shows that the leaves of a waxy plant were at about the same temperature as the surface of a table under them (both of which were in thermal equilibrium with the air I supposed).
We know a piece of metal feels colder than a piece of wood because they conduct heat faster. Within a given amount of time, our fingers lose more thermal energy to the metal than to the wood. So do leaves also conduct heat faster? Let’s do an infrared imaging experiment to find it out.
I rested a fresh leaf with a waxy cuticle on a piece of dry paper. The sequence of thermal images in Figure 2 shows what happened after two of my fingers pressed on the leaf (left) and the paper (right). Then I moved away my fingers and kept observing the changes of the temperature patterns over time. The result suggests that thermal energy seemed to spread more slowly on the leaf than on the paper. The video above shows the process. So the theory of conductivity from the case of metal vs. wood cannot explain what I saw and felt here.
So why do leaves feel cooler than paper, if they do not conduct heat more quickly 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 leaves, if it doesn’t spread quickly like in metals?
My hypothesis is that the thermal energy goes to warm up the water in the spongy layer of a leaf (Figure 3). The spongy layer lies beneath the palisade layer of the leaf. Its cells are irregular in shape and loosely packed — hence the name “the spongy layer.” For a living leaf, the spongy layer is full of water and there are only some small stomata for water molecules to escape. As the specific heat of water is considerably high — 4.18 J/(g*K), the spongy layer acts like a thermal reservoir to store the heat from the finger such that the temperature of the leaf does not rise as fast as the paper that does not have such a thermal reservoir. A temperature gradient that lasts longer causes more thermal energy to be transferred from the finger to the leaf.
So how did I know if my theory is correct? I repeated the experiment with a dry leaf and discovered that it conducted heat just as fast as a piece of dry paper (which was also made of plant materials).
The next question is why the water in the spongy layer didn’t dissipate thermal energy quickly as water in a cup usually does (I confirmed the energy dissipation in a cup of water by thermal imaging, which is not shown here). I asked this question because 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. With a thermal conductivity more than 10 times greater, the water in the leaf should have conducted heat faster than paper. But it seems that the water trapped in the spongy layer somehow could not conduct heat like water in a cup.
To simulate the situation, I got a wet (approximately 20% of full water absorption capacity) dish sponge and a dry one, both wrapped with plastic film to prevent evaporative cooling and mimic the enclosure of the sponge layer in a leaf, and compared their thermal conduction under an infrared camera, as shown in Figure 4. Again, I used my fingers to as identical heaters to warm them up and I did feel that the wet sponge was significantly cooler than the dry one. I observed that the wet sponge appeared to conduct heat more slowly than the dry one, as shown in the video below. The wet and dry dish sponges exhibited similar patterns as the fresh and dry leaves.
In conclusion, the reason that we feel the leaves of a living indoor plant are cool when we touch them is because the water contained in their sponge layers gives them greater capacity to store thermal energy transferred from our fingers. Case closed!