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Water droplets on leaves

Look at the two leaves below. The oak leaf has flat, rather dull drops. The redbud leaf has round, shiny drops.  If you look around at leaves after a rain, you will see some leaves with round, shiny raindrops like the redbud, while other leaves will have flat drops like the oak, or will shed all the drops and appear dry.  Scroll down past the pictures to learn more.

Water droplets on a Shumard oak leaf.
Water drops on Shumard oak leaves
Water droplets on a redbud leaf.
Water droplets on redbud leaves

The diagram below shows the difference between the leaves. Leaves are covered in a waxy cuticle. The structure and chemistry of the cuticle on the redbud leaves repels water more strongly. As the water is repelled, it pulls away from the leaves and forms an almost spherical ball, which reflects light. The angle between the leaf and the water is the contact angle – the more obtuse the angle, the higher the water stands from the leaf. The oak repels water with less force, leading to a more acute contact angle – more of the water is in contact with the leaf, and the flattened pool of water reflects less light. Scroll down past the pictures for more.

Water droplets and contact angles. The oak leaf on the left (A, C) is less waxy than the redbud leaf on the right (B,D).
A. Oak leaf with raindrops; B. Redbud leaf with raindrops; C. Contact angle of water on oak leaf. The less waxy oak leaf allows more of the water drop to be in contact with the leaf surface; D. Contact angle of water on a redbud leaf. The waxy redbud leaf repels water, pushing the water into a tighter ball with less contact with the leaf.

Why the difference between leaves? A very water-repellent cuticle  reduces water loss from a leaf.  The difference in the ability of the cuticles to repel water is mostly due to chemical differences in the wax, but also in the structure and thickness of the cuticle.

Waxy leaves are often more drought resistant than less waxy leaves, and redbud is certainly very drought tolerant. But that is too simple an explanation. Other factors, such as leaf thickness and density of hairs, are important in drought tolerance.  And leaves may be waxy for reasons other than drought tolerance. A heavy coating of wax can protect the leaf from UV radiation, insects, and pathogens, such as fungi, bacteria and viruses.  Most plant structures have evolved to play multiple roles, so we can’t say that the redbud leaves are more water repellent only for drought tolerance.

Whatever the reason, leaves with waxy cuticles like redbud produce very attractive patterns on the leaf surface after a rain, making them more interesting to photograph.