This species is distinguishable by its huge complicated paniculiform inflorescences. Premna grandipaniculata shares the same primary inflorescence structure with P. bracteata and P. interrupta, but is distinct from the latter two in its spike-like thyrses forming a panicle with tertiary branches (vs. with secondary branches in P. bracteata, while without branches in P. interrupta) and in having nearly glabrous branchlets, petioles, leaves and inflorescences (vs. densely pubescent throughout in the latter two species) (Table 1).

MYANMAR. Kachin State, Putao District, ca. 2–3 miles from Wasandum village, 27°29'00.29"N, 97°12'01.48"E, Alt. 1050 m, 29 April 2016, Y.H. Tan & S.S. Zhou 20160031 (holotype: HITBC!; isotypes: HITBC!,RAF!, JXAU!).

Woody climbers. Branches grey, terete, robust, nutant, without an interpetiolar ridge. Branchlets purplish brown, with densely small white elliptic lenticel stomentose, without bracts at the base. Leaves simple, opposite-decussate, glabrous, broadly ovate to suborbicular, leathery, 14–23 × 10.5–17.5 cm, apex acute, base subrounded, rounded to slightly cordate, margin entire; veins 4–7 pairs, abaxially raised and adaxially slightly compressed, secondary veins curved and jointed near margin; petiole 2.4–3.5 cm long, purplish dark green, slightly inflated, purplish furrowed on upper part. Inflorescences terminal, a large complicated panicle with tertiary branches, 18–30 cm long, 12–20 cm wide, peduncle nearly glabrous, terminal branches spike-like thyrses, 10–20 cm long, formed by sessile cymes laxly arranged on axis; bracts ovate, 3.0–10 mm long, easily deciduous; bracteoles subulate, tiny. Calyx campanulate, 2.0–2.5 mm long, outside minutely brownish pubescent, 2-lipped; lips entire, or upper lip emarginate and ciliate, apex subrounded. Corolla green to greenish white, subglabrous, outside glandular, inside densely white villose around throat, 4-lobed; lobes broadly obovate, apex subrounded. Stamens 4, equal, exserted; anther purple. Ovary oblong, 1.0–1.5 mm long, glabrous, glandular; style white, slender, 3.5–4.5 mm long. Fruits drupaceous, narrowly obovate, 4.0–5.0 × 2.5–3.5 mm, yellowish brown.

Figure 1. 

Morphological comparisons amongst Premna grandipaniculata (A–C), P. bracteata (D–F) and P. interrupta (G–I). A, D, G habit B, E, H branchlets with leaves C, F, I inflorescences.

Figure 2. 

Line drawing of Premna grandipaniculata Y. H. Tan & Bo Li, sp. nov. A a branchlet with leaves and inflorescence B abaxial surface of leaf blade C flowers D. dissected corolla and stamens in a bud E calyx and style.

Flowering was observed from early March to April and fruiting from late May to late June.

The species is currently known only from the type locality of Putao, Kachin State, northern Myanmar, grows in tropical montane forests, at an elevation 700–1200 m a.s.l.

The specific epithet “grandipaniculata” indicates that the species bear large complicated paniculiform inflorescences.

This species is only known from a single locality in Myanmar and as the habitat, in which it is found, is threatened by deforestation (author’s personal observation), it is categorised as critically endangered under criteria B and D following IUCN Red List Categories (IUCN 2012).

The most noticeable trait of the new species is its huge complicated paniculiform inflorescence. After examination, it was found to be formed by tertiary branches of spike-like thyrses. Such a spike-like thyrse is a rare type of inflorescence in Premna, currently found in only two species, P. bracteata and P. interrupta. In P. interrupta, sessile cymes form a single spike-like thyrse without branches, while in P. bracteata the lower parts of inflorescence bear 2–4 pairs of secondary branches. Besides its inflorescence structure, P. grandipaniculata also differs from P. bracteata and P. interrupta in having larger leaves (Figure 3), nearly glabrous branchlets, petioles and inflorescences (Table 1).

Figure 3. 

Box plots of two quantitative characters, leaf length (A) and leaf width (B), of Premna grandipaniculata, P. bracteata and P. interrupta. The boxes (rectangle region) represent the interquartile range and the whiskers (vertical line) represent the range excluding the outliers (circles). The three upper, middle and lower lines on the boxes represent the 75%, 50% and 25% of the variables, respectively. The upper and lower ends of the whiskers represent the maximum and minimum values of the variables, respectively. The circles represent the single value, where the variable value exceeds 1.5 times the difference between the 75% and 25%.

Geographically, P. bracteata is mainly recorded from the southern and eastern slopes of the Himalayas with several collections from Bangladesh, Bhutan, Myanmar, north eastern India and southeast Tibet of China and has also been collected from a rare and isolated population in Xishuangbanna, south Yunnan of China (Chen and Gilbert 1994, Govaerts et al. 2008, unpublished data). Premna interrupta frequently occurs from southwest China (Guangxi, Guizhou, Sichuan, Xizang and Yunnan provinces) to southern and south east Asia (Bangladesh, Bhutan, Cambodia, Laos, Myanmar, north eastern India, Nepal, Peninsular Malaysia, Thailand and Vietnam) (Chen and Gilbert 1994, Govaerts et al. 2008). In Myanmar, P. bracteata is recorded from Chin, Mandalay and Sagaing, while P. interrupta from Kachin, Rakhine and Sagaing (Kress et al. 2003). In Kachin States, the distribution of P. interrupta overlaps that of P. grandipaniculata and their habitat and habit are also similar, but P. grandipaniculata significantly differs from P. interrupta in morphology as mentioned above.