Rhododendron subsect. Irrorata in Flora of China @ efloras.org

10. Rhododendron subsect. Irrorata (Tagg) Sleumer, Bot. Jahrb. Syst. 74: 548. 1949.

露珠杜鹃亚组 lu zhu du juan ya zu

Fang Mingyuan (方明渊); David F. Chamberlain

Rhododendron subser. Irrorata Tagg in J. B. Stevenson, Sp. Rhodod. 332. 1930 ["Irroratum"].

Shrubs or small trees; young shoots mostly glabrous or glabrescent, occasionally glandular or tomentose. Leaf blade leathery, lanceolate or elliptic-lanceolate; apex acute; abaxial surface usually glabrous when mature, more rarely with a veil of persistent thin indumentum at maturity (R. agastum, R. leptopeplum, and R. tanastylum var. pennivenium). Inflorescence 2–21-flowered; rachis 5–40 mm. Calyx usually reduced to a rim; corolla campanulate or tubular- to open-campanulate, 5-lobed, (7-lobed in R. anthosphaerum), pink, white, yellow or deep crimson, with darker flecks and also sometimes with nectar pouches at the base; stamens 10–14; ovary glabrous, tomentose or glandular-hairy; style glabrous or glandular to tip. Capsule oblong-cylindric, 1–2.5 cm.

About 24 species: Bhutan, China, India, Indonesia (Sumatra), Malaysia, Myanmar, Vietnam; 21 species (14 endemic) in China.

Some species are important horticultural plants in China, e.g., Rhododendron annae and R. irroratum.

1 Abaxial leaf surface with persistent thin indumentum, or at least with hairs on midrib. (2)

+ Abaxial leaf surface glabrous, though sometimes with sessile punctate glands on midribs and lateral veins. (9)

2 (1) Leaf blade 13–17(–20.5) cm, adaxial surface with strongly impressed lateral veins, abaxial surface with scattered setose glands. 300 Rhododendron gongshanense

+ Leaf blade never exceeding 15 cm, adaxial surface with at most lightly impressed lateral veins, abaxial surface without setose glands. (3)

3 (2) Abaxial leaf surface with persistent thin indumentum; ovary tomentose and stipitate-glandular. (4)

+ Abaxial leaf surface with only midrib hairy, otherwise glabrous. (5)

4 (3) Leaf blade elliptic or elliptic-lanceolate; style glandular to tip. 299 Rhododendron agastum

+ Leaf blade lanceolate or long-elliptic; style glabrous or glandular only at base. 295 Rhododendron leptopeplum

5 (3) Leaf blade subleathery, lanceolate or oblanceolate, base cuneate; corolla rose. (6)

+ Leaf blade leathery, lanceolate or long-elliptic, base rounded; corolla yellowish or deep crimson. (7)

6 (5) Pedicels, ovary, and style glabrous; leaf blade narrowly lanceolate, abaxial surface of midrib with a thin woolly tomentum. 303 Rhododendron araiophyllum

+ Pedicels, ovary, and lower part of style with dense hairs; leaf blade obovate, abaxial surface of midrib with dense brown hairs. 307 Rhododendron pingbianense

7 (5) Pedicel and ovary glabrous; leaf blade 3–4.5 cm wide, abaxial surface of midrib with brown hairs; corolla deep crimson. 301 Rhododendron ramsdenianum

+ Pedicel and ovary densely hairy; leaf blade 1.5–3.5 cm wide, abaxial surface of midrib thinly rufous tomentose or stellate-tomentose; corolla yellowish or purplish. (8)

8 (7) Leaf blade 1.5–2.5 cm wide, abaxial surface of midrib thinly tomentose, base symmetrical; corolla yellowish. 308 Rhododendron laojunshanense

+ Leaf blade 2.5–3.5 cm wide, abaxial surface of midrib stellate-tomentose, base asymmetrical; corolla purplish with purple blotch and flecks. 310 Rhododendron sikangense

9 (1) Ovary eglandular, tomentose or glabrous; style usually glabrous, rarely tomentose at base. (10)

+ Ovary glandular, or glandular-setose and tomentose; style glandular or glabrous. (15)

10 (9) Calyx usually ca. 10 mm; ovary and style base tomentose. 312 Rhododendron dimitrum

+ Calyx 1–3 mm; ovary and style usually glabrous or puberulent. (11)

11 (10) Leaves subleathery, lanceolate or oblanceolate, apex obtuse or acute; corolla 6–7-lobed. 292 Rhododendron anthosphaerum

+ Leaf blade leathery; apex acute; corolla 5-lobed. (12)

12 (11) Rachis stout, 5–20 mm, glabrous; inflorescence 5–10-flowered. (13)

+ Rachis slender, 15–35 mm, tomentose; inflorescence usually 8–15-flowered. (14)

13 (12) Rachis 0.5–1 cm; corolla fleshy, deep crimson; leaves elliptic-lanceolate. 293 Rhododendron tanastylum

+ Rachis 1.5–2 cm; corolla not fleshy, purplish crimson; leaves oblanceolate. 297 Rhododendron spanotrichum

14 (12) Ovary strigose; leaves lanceolate or elliptic-lanceolate, adaxial surface not wax-coated. 302 Rhododendron kendrickii

+ Ovary glabrous; leaves oblanceolate, adaxial surface usually wax-coated. 294 Rhododendron lukiangense

15 (9) Ovary glandular or glandular-setose; style glabrous or glandular only in lower half. (16)

+ Ovary glandular; style glandular to tip. (18)

16 (15) Leaf blade long-elliptic or obovate-elliptic, 8–13 cm; apex obtuse or rounded; corolla 4.5–5 cm; pedicel and ovary with dense dendritic hairs; calyx lobes rounded, ca. 3 × 4 mm. 296 Rhododendron papillatum

+ Leaf blade elliptic-lanceolate or ovate-lanceolate, 5–10 cm, apex cuspidate; corolla 2.5–4 cm; pedicel and ovary without dendritic hairs; calyx 1–3.5 mm. (17)

17 (16) Ovary and lower part of style glandular-hairy; pedicel glandular-setose. 309 Rhododendron brevinerve

+ Ovary glandular-setose; style glabrous; pedicel sparsely stipitate-glandular. 306 Rhododendron guizhouense

18 (15) Corolla tubular-campanulate, reddish purple; leaves narrowly lanceolate, 8–17 × 2–3 cm; petiole and pedicel densely glandular-setose. 311 Rhododendron mengtszense

+ Corolla campanulate or open-campanulate; leaves lanceolate to elliptic-lanceolate, not more than 12 cm; petiole and pedicel glabrous. (19)

19 (18) Corolla campanulate, 3.5–5 cm, white to yellow. 298 Rhododendron irroratum

+ Corolla open-campanulate or cup-shaped, 2–3(–4) cm, white or rose. (20)

20 (19) Leaf blade thick, leathery, 2.5–5 cm. 304 Rhododendron aberconwayi

+ Leaf blade subleathery, 7–10 cm. 305 Rhododendron annae

List of lower taxa