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Article from Bean's Trees and Shrubs Hardy in the British Isles
'Rhododendron desquamatum' from the website Trees and Shrubs Online (treesandshrubsonline.
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An evergreen shrub or small tree up to 25 ft high; young branchlets scaly, not hairy. Leaves 31⁄4 to 4 in. long, 1 to 13⁄4 in. wide, elliptic, oblong-elliptic, or occasionally lanceolate or oblanceolate, acuminate at the apex, cuneate to rounded at the base, upper surface dull green and varying from densely scaly (type of R. catapastum) to not scaly (type of R. desquamatum), underside densely scaly, the scales unequal in size and the larger ones darker than the others; petioles about 1⁄2 in. long, scaly. Inflorescence terminal, with four to eight flowers on pedicels 1⁄2 to 3⁄4 in. long, scaly. Calyx rim-like, scaly. Corolla funnel-shaped, very often widely so or almost flat, up to about 11⁄2 in. long and 2 in. wide, scaly on the outside, varying in colour from mauve or pinkish mauve to purple, spotted with crimson. Ovary densely scaly; style glabrous. Flowering-time March or April. Bot. Mag., t. 9497. (s. Heliolepis)
Native of N.W. Yunnan and of bordering parts of Szechwan, Tibet, and Burma. Although found earlier by one of the French missionaries, R. desquamatum was described from a specimen collected by Forrest on the Shweli-Salween divide, near the border between Yunnan and Burma in 1917. The flowers were described as ‘saturated purple’ and the leaves as being devoid of scales on the upper surface (whence the specific epithet). The type of R. catapastum was a fruiting specimen collected by Forrest in the mountains north of Yungpeh, east of the Yangtse; in this the leaves are scaly above and the flowers (which Forrest saw on a later visit) rose or purplish rose. It is now included in R. desquamatum, though Hutchinson provisionally kept the two separate in his article accompanying the plate in the Botanical Magazine.
R. desquamatum is closely allied to R. rubiginosum, but the leaves and flowers are larger and, more significantly, the scales on the leaf underside are unequal in size and colour. The purple-flowered forms of this species are very fine. They were mainly raised from seeds collected on the Salween-Nmai Hka divide, not far north of the type locality, e.g., Farrer 875 and Forrest 26482 and 26488. But there is some variation even in plants from a single seed-collection. Sprays from five plants of F.24535, exhibited by Capt. Talbot Fletcher on April 5, 1938, had flowers ranging from clear pale mauve to reddish mauve. The species received an Award of Merit on the basis of this exhibit.
Included in R. rubiginosum, q.v. in this supplement.