The picture shows a stoma, an opening on the underside of a leaf regulating the flow of gases and decreasing water loss. Leaves are tiny machines adapted to clean air, store water and withstand strong sunlight. They use a lot of principles that are potentially adaptable to building envelopes. Following are a look at these principles and some responding ideas of architectural solutions.
The above diagram shows the section through a leaf. Outermost on each side is a waxy layer called the cuticle, that protects the plant and stops moisture from escaping. Inside, toward the top surface, are tightly packed cells that captures sunlight. Below is a spongy layer of more loosely packed cells that holds water. On the bottom surface are the stomata that control the airflow into, and out of, the leaf.
Adaptation to the desert: reduced surface area, reduced air flow, increased reflectivity
Plants in hot climates tend to have large leaves to more maximize heat loss, but this only works in climates with a high humidity. In arid climates they would quickly loose water, dry up and die. Because of this desert plants have adapted in a way as to reduce the surface area, creating shorter, thicker leaves.
Plants in the desert often have leaves grouped in a way that creates a localized climate with higher moisture around the base of the leaves. The way they are placed creates an air pocket within the plant reducing air flow and avoiding moisture being blown away. Some plants also have tiny hairs to reduce the air flow in a similar manner.
Another of the uses for a coating of hair is to reflect sunlight. Shielding against too much sun is very important in the desert. Other methods are self-shading, like applied by the cactus, as well as a reflective cuticle, like the one of Dudleya brittonii, which has the highest ultraviolet light (UV) reflectivity of any known naturally occurring biological substance. (Mulroy, Thomas W. (1979))
A simple theoretical building
When looking at adapting these principles to architecture, I chose to focus mainly on the water holding capabilities and the regulated flow of air, using it to cool and add moisture to the air at the peak hours of the day but be closed the rest of time. To narrow the thought process down, I have chosen clay as material for this theoretical building, because it is widely available, and 3D-priting as the method of construction, to be able to (roughly) emulate some of the structures found in a leaf.
In the picture below a dome shape was used to visualize the idea, an optimal shape for 3d-printing, as well as being easy to read, and adhering to the principle in nature of using as little material as possible.
The layers of the envelope
The outermost layer of the leaf, the Cuticle, protects the surface and stops water from escaping the interior. This could simply be translated to a covering of sun reflective tiles. Other potential solutions could be incorporating solar power or plants. The occasional rain being caught on the surface could be led into a pool, or into the envelope (in a controlled manner).
The Palisade Mesophyll is a layer of tightly packed cells that collect sunlight as well as take up the major heat load. As I have only focused on the heat transfer, I interpreted this layer as a 3D-printed layer of insulation protecting the layer beneath from extreme heat, printed in a way to slow down the movement of air but maintain a high structural capability.
The Spongy Mesophyll consists of loosely packed cells that allows for a flow of air and exchange of gases. This layer could be printed in a way to create voids to allow air to pass, for example in the way shown below. The clay lining the voids is porous, enabling it to store a lot of water. A layer of waterproofing should be added above this layer to stop moisture from vaporizing.
The stomata consist of circular shaped guard cells that open when it is humid and/or critically hot, allowing absorption or evaporative cooling to take place. At other times it is closed to reduce the loss of water. Incorporating this principle into desert architecture, this could be a series of valves placed mainly in areas where people gather at daytime, which open and close at a certain temperature. There are many examples in nature of materials reacting to temperature change that potentially could be used. Another simpler solution is to use bimetallic strips, made of two laminated strips of metal with a different heat expansion coefficient, for example in the way illustrated below. It consists of a bimetallic coil like the one found in some thermometers that unfolds and in turn opens or closes a hatch. The coil shape makes it very sensitive to changes in temperature.
Breaking it down
There are of course a lot of questions left like for example how well the interior responds to water being trapped inside, and how water is to be dispersed in a good way. And there is also a lot more to delve into, like for example the air cleaning principles and the different highly advanced coatings that can be found on plants. But it is evident that there potentially could be a lot of ways to incorporate more functions into the envelope of a building, and with the prospects of 3D-printed architecture this could be available soon at no added production cost.
(Mulroy, Thomas W. (1979)). “Spectral properties of heavily glaucous and non-glaucous leaves of a succulent rosette-plant”. Oecologia. 38 (3): 349–357.)
Palisade cells. https://en.wikipedia.org/wiki/Palisade_cell
Desert plant ecology. https://www.desertmuseum.org/books/nhsd_plant_ecology.php
Bimetallic strips. https://en.wikipedia.org/wiki/Bimetallic_strip
Plant cuticles. https://en.wikipedia.org/wiki/Plant_cuticle