JARS v52n4 - Tips for Beginners: Leaf Scales, Papillae, and Indumentum

Tips for Beginners: Leaf Scales, Papillae, and Indumentum
Eleanor Philp
Fort Bragg, California

Early in the botanical classification of rhododendrons, two fundamental distinctions became known: lepidote (scaly) and elepidote (non scaly - an e preceding a Latin word means without). Besides scales, immature leaves in the foliage buds of the lepidote species roll inward; in the elepidote species, they roll outward. All rhododendron species belong to one of these divisions: elepidote (without scales) or lepidote (with scales).

leaf of R. maddenii ssp. maddenii
A leaf of R. maddenii ssp. maddenii (also known as
R. polyandrum
) shows the characteristic varying size
and almost overlapping scales on the underside.
Photo by Eleanor Philp

Besides scales, some rhododendron leaves contain small, waxy pegs called papillae, which cannot be wetted. This keeps the undersurface of the leaves dry, even in the rainy season. During the cold, dry season when the plants struggle to conserve all the water they can, the waxy pegs form miniature windbreaks to retard the loss of water.
A type of water-regulating device on some rhododendron leaves is a coating of hairs. Some hairs form a fringe around the edge of the leaves, a coating on the leaves, or on parts of the flower. Other hair, called indumentum, forms a protective, woolly layer that sheds water or lies next to the surface of the leaf, providing shelter. During the cold, dry weather, the hairy indumentum that covers the leaf's underside becomes an insulating shield. Some of these hairs catch water and absorb it to provide the plant with the moisture needed in times of drought. During times of heavy rain, the hairs are used by the plant to transpire excess water from the surface of the leaf. While plants in both the lepidote and elepidote groups may have hair, rarely does indumentum show on a plant with scales. An exception to this, the species R. edgeworthii , named for botanist and plant collector Michael P. Edgeworth (1812-1881), contains not only scales but also beautiful heavy and long hair.
Some remarkable examples of plants with indumentum belong in subsection Falconera , recognized by its large non-scaly leaves and heavy indumentum on the underside of the leaf. The species R. falconeri , named for Hugh Falconer, M.D. (1808-1865), grows to an extremely handsome tree of 40 to 50 feet with large leaves and large trusses of creamy white to pale yellow flowers with a purple blotch.