The official definition of Tomentum by the American Rhododendron Society is:..Matted wooly hairs on stems, leaves or seeds. The silver to white to brown fuzz on the top of leaves will usually wipe off. This substance has often been found to prevent insect predation, and can help leaves hold in moisture in times of drought. To the seasoned rhododendron collector, this is "as good as it gets!".
Indumentum (pl. indumenta) is a general term denoting the covering of hairs found on the underside of the leaves of many rhododendrons. On rhododendrons, indumentum is commonly a woolly covering on the underside of mature leaves, while a similar surface on the upper side of leaves is called tomentum. The hairs that make up this wooly covering are much finer than human hair, with the thickest still being only 1/10th the thickness of a human hair. There are many types of hairs, resulting in a variety of types of indumentum. The rhododendron below is R. pachysanthum from Taiwan. The leaves come out this interesting orange/tan color, but turn green by mid summer.