RHODODENDRON, a large genus of shrubs and trees be longing to the heath family (Ericaceae). No adequate distinc tion can be drawn between this genus and Azalea (q.v.). The rhododendrons are trees or shrubs, never herbs, with simple, ever green or deciduous leaves, and flowers in terminal clusters sur rounded in the bud by bud-scales but not as a rule by true leaves. The flowers are remarkable for the frequent absence or reduced condition of the calyx. The funnel- or bell-shaped corolla, on the other hand, with its five or more lobes, is usually conspicu ous, and in some species so much so as to render these plants greatly prized in gardens. The free stamens are usually ten, with slender filaments and anthers opening by pores at the top. The ovary is five- or many-celled, ripening into a long woody pod which splits from top to bottom into a number of valves, liberating a large number of small bran-like seeds.
The species, which exceed 30o in number, are for the most part natives of the mountainous regions of the northern hemisphere, extending as far south as the Malay archipelago and New Guinea, but not hitherto found in South America or Australia ; none are natives of Great Britain. They vary greatly in stature, some of the alpine species being mere pygmies with minute leaves and tiny blossoms, while some of the Himalayan species are moderate sized trees with huge flowers. Some are epiphytal, growing on the branches of other trees, but not deriving their sustenance from them. Several notably handsome species are natives of North America. Among these are the great rhododendron (R. maxi mum), called also great laurel and rose bay, found from Nova Scotia and Ontario and southward to Georgia ; the Catawba or Carolina rhododendron or mountain rose bay (R. catawbiense),
of the high Appalachian mountains; the western rhododendron (R. californicum), found from British Columbia to California, and adopted as the state flower of Washington; and the delicate rhodora (q.v.), of the north-eastern States. The great laurel and the mountain rose are widely planted ; less frequently R. minus and R. carolinianus, both small evergreen shrubs with showy flow ers, native to the mountains of the south-eastern States, are grown as ornamentals.
The varieties grown in gardens are mostly grafted on the Pontic species (R. ponticum) and the North American R. cataw biense. The common Pontic variety is excellent for game-covert, from its hardiness, the shelter it affords, and the fact that hares and rabbits rarely eat it. Variety of colour has been infused by crossing or hybridizing the species first named, or their deriva tives, with some of the more gorgeously coloured Himalayan American varieties.
The hardy evergreen kinds are readily propagated by seed, by layers and by grafting: Grafting is resorted to only for the propagation of the rarer and more tender kinds. Loamy soil containing a large quantity of peat or vegetable humus is essen tial, the roots of all the species investigated being associated with a fungus partner or mycorrhiza (q.v.). An excess of lime or chalk in the soil proves fatal to rhododendrons and their allies, sooner or later. The hardy deciduous kinds are valuable for forcing, and withstand cold-storage treatment well.