The primary function of a leaf’s waxy coating, or cuticle, is to control water loss, says Phil Gates. Some water needs to evaporate from the leaf’s surface to keep water moving from the roots up through the plant’s internal plumbing (the xylem vessels).
The cuticle layer of the plant
The thickness of the cuticle varies; it is barely discernible in some aquatic plants, where water loss is never a problem, but very thick in species that need to retain water during drought. Gases pass through the cuticle via small pores called stomata, which open when water is plentiful.
The cuticle is also the plant’s first line of defence against fungal attack. In some species, such as holly, it extends into spines that deter herbivores. Its hydrophobic properties also prevent the accumulation of surface moisture that would otherwise hamper the uptake of carbon dioxide, so rain and mist form droplets that simply roll off.
This is particularly important in rainforests, where the water from frequent showers is often shed from pointed ‘drip-tips’ at the apex of each leaf.
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