The title of this week’s leaf is not intended to downplay the importance of epidermis, quite the contrary. Superficial is an anatomical term meaning topmost and the epidermis is vitally important as the front line of defence.

Most people know that the epidermis is the top layer of our skin, but for those of us who studied botany, epidermis is also the top layer of cells on leaves, flower petals and other plant tissues.

In both animals and plants, the epidermis serves to keep pathogens out water in. But epidermis also has some very different functions and challenges in plants and animals.

Beneath the top epidermal layer, human and animal skin is important too for thermal insulation, with a fatty layer below, and with hair growing from follicles (pores) that also produce sebum to grease the skin and hair. Additional pores secrete sweat, for cooling, for skin conditioning and sometimes carrying scented signalling molecules (pheromones – we’ll leave that to the zoologists).

Plants have a special problem because the leaves need CO₂ for photosynthesis, and it enters through pores called stomata so it can get absorbed in the moist, spongy tissue inside. The problem is that the same path operates in reverse for water to evaporate and diffuse out. Stomata, therefore, open and close as needed, closing at night and sometimes during the day when the uptake of water cannot keep up with losses. All this is controlled by intricate mechanisms, but we’ll leave the details to the plant physiologists. With stomata closed, leaf epidermis is watertight, helped in part by waxy biopolymers called cutin and suberin[1]. Suberin also accumulates in tree bark and is a main component of the water-impermeable cork we use in wine bottles.

So let’s now get back to human skin. There is a huge cosmetic industry based on the perceived inability of our epidermides to perform properly without help. Many skincareproducts contain plant-derived materials for this purpose; oils, waxes and suberin are among them.

[1]       C. Nawrath, The Biopolymers Cutin and Suberin, The Arabidopsis book/American Society of Plant Biologists1 (2002). https://doi.org/10.1199/tab.0021