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Water beads on the waxy cuticle of kale leaves (video).

Plant cuticles are a protective waxy covering produced only by the epidermal cells (Kolattukudy, 1996) of leaves, young shoots and all other aerial plant organs without periderm. The cuticle tends to be thicker on the top of the leaf, but is not always thicker in plants living in dry climates than in those from wet climates, despite the persistent myth.

The cuticle is composed of an insoluble cuticular membrane impregnated by and covered with soluble waxes. Cutin, a polyester polymer composed of inter-esterified straight-chain hydroxy acids which are cross-linked by ester and epoxide bonds, is the best-known structural component of the cuticular membrane (Holloway, 1982; Stark & Tian 2006). The cuticle can also contain a non-saponifiable hydrocarbon polymer known as Cutan (Tegelaar et al., 1989). The cuticular membrane is impregnated with cuticular waxes (Jetter, Kunst & Samuels 2006) and covered with epicuticular waxes, which are mixtures of hydrophobic aliphatic compounds, hydrocarbons with chain lengths typically in the range C16 to C36 (Baker, 1982).

The plant cuticle is one of a series of innovations, together with stomata, xylem and phloem and intercellular spaces in stem and later leaf mesophyll tissue, that plants evolved more than 450 Million years ago during the transition between life in water and life on land (Raven, 1977). Together, these features enabled plant shoots exploring aerial environments to conserve water by internalising the gas exchange surfaces, enclosing them in a waterproof membrane and providing a variable-aperture control mechanism, the stomatal guard cells, which could regulate the rates of H₂O evaporation and CO₂ exchange.

In addition to its function as a permeability barrier for water and other molecules, the micro and nano-structure of the cuticle confer specialised surface properties that prevent contamination of plant tissues with external water, dirt and microorganisms. Many plants, such as the leaves of the sacred lotus (Nelumbo nucifera) exhibit ultra-hydrophobic and self-cleaning properties that have been described by Barthlott and Neinhuis (1997). The lotus effect has potential uses in biomimetic technical materials.

"The waxy sheet of cuticle also functions in defense, forming a physical barrier that resists penetration by virus particles, bacterial cells, and the spores or growing filaments of fungi". (Freeman, 2002).

• Riederer, M. & Müller, C., eds (2006) Biology of the Plant Cuticle. Blackwell Publishing. [1]
• Jetter, R., Kunst, L & Samuels, A.L. (2006) Composition of plant cuticular waxes. Chapter 4 in Riederer, M. & Müller, C. (2006)Biology of the Plant Cuticle. Blackwell Publishing, pp145–181.
• Barthlott, W. and Neinhuis, C. (1997) Purity of the sacred lotus, or escape from contamination in biological surfaces. Planta 202, 1–8.
• Baker, E.A. (1982) Chemistry and morphology of plant epicuticular waxes. In Cutler, D.F., Alvin, K.L. and Price, C.E. The Plant Cuticle. Academic Press, pp. 139–165.
• Raven, J.A. (1977) The evolution of vascular land plants in relation to supracellular transport processes. Advances in Botanical Research, 5, 153–219.
• Holloway, P.J. (1982) The chemical constitution of plant cutins. Cutler, D.F., Alvin, K.L. and Price, C.E. The Plant Cuticle. Academic Press, pp. 45–85.
• Kolattukudy, P.E. (1996) Biosynthetic pathways of cutin and waxes, and their sensitivity to environmental stresses. In Plant Cuticles. Edited by Kerstiens, G. BIOS Scientific publishers Ltd., Oxford, pp 83–108.
• Freeman, Scott (2002) Biological Science. Prentice-Hall, Inc., New Jersey