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1. Populus Linnaeus, Sp. Pl. 2: 1034. 1753.

杨属 yang shu

Trees; trunk usually straight; bark furrowed or smooth, often gray or tan; pith mostly 5-angled in cross section. Terminal bud terete or angled; bud scales several, unequal. Stipules small, deciduous; petiole terete or laterally compressed; leaf blade usually ovate to deltoid-ovate. Flowering precocious; catkins pendulous. Flowers anemophilous; bracts apically lobed or laciniate, membranous, caducous; flowers with disc obliquely cupular. Male flower: stamens 4-many; filaments short, free; anthers dull red. Female flower: ovary 1-loculed; style 1, short or not; stigmas 2-4. Capsule 2-4(or 5)-valved. Seeds few to numerous, small. Cotyledons elliptic.

About 100 species: N Africa, Asia, Europe, North America; 71 species (47 endemic) in China, including at least nine hybrids.

The taxonomy of Populus in China, at species level, is at a preliminary stage. To reach an adequate understanding, it is necessary to make direct observations in natural populations of all morphological variations between individuals, for all species and at all seasonal developmental stages. Moreover, other methods beyond morphological studies need to be applied, especially anatomic and chemotaxonomic analyses.

1 Leaves grayish blue on both surfaces; disc membranous, caducous (5. Sect. Turanga).   (2)
+ Leaves not grayish blue on both surfaces; disc persistent.   (3)
2 (1) Branchlets rarely pilose; leaves glabrous; capsule glabrous; leaf margin dentate distally   70 Populus euphratica
+ Branchlets, leaves, and capsule shortly tomentose; leaf margin entire or with 2 or 3 teeth apically   71 Populus pruinosa
3 (1) Leaf margin lobed, incised, or with sinuous teeth or, if serrate, then petiole with 2 large glands at apex; bracts with long, straight marginal hairs (1. Sect. Populus).   (4)
+ Leaf margin serrate or nearly entire; bracts without long, marginal hairs.   (16)
4 (3) Leaves of long shoots 3-5-palmately lobed, leaves of short branchlets and petioles abaxially densely white tomentose   1 Populus alba
+ Leaves of long shoots not palmately lobed, leaves of short branchlets and petiole abaxially glabrous or gray tomentose.   (5)
5 (4) Leaf margin incised or with sinuous teeth; buds pubescent.   (6)
+ Leaf margin with sinuous teeth or, if serrate, then leaf blade with 2 glands at base; bud scales usually glabrous or pilose basally and ciliate.   (8)
6 (5) Leaf apex acuminate; leaves of short branchlets usually deltoid-ovate, 7-11(-18) × 6.5-10.5(-15) cm   10 Populus tomentosa
+ Leaf apex acute or obtuse; leaves of short branchlets ovate, ovate-elliptic, ovate-orbicular, or suborbicular, 3-8 × 2-7 cm.   (7)
7 (6) Branchlets grayish; leaf margin obtusely notched, teeth blunt, not incurved; female catkin 5-10 cm   2 Populus canescens
+ Branchlets grayish brown; leaf margin coarsely serrate, teeth acute, incurved, sometimes sinuous; female catkin 3-5 cm   6 Populus X hopeiensis
8 (5) Leaves usually ovate to broadly ovate; petioles on short branchlets with 2 large glands at apex.   (9)
+ Leaves usually suborbicular; petioles on short branchlets eglandular at apex (P. davidiana sometimes small glandular).   (12)
9 (8) Leaf blade long acuminate or caudate at apex; petiole with 2 raised, nearly stipitate glands at apex.   (10)
+ Leaf blade acute or shortly acuminate at apex; petiole with obscure, flattened glands at apex.   (11)
10 (9) Bracts palmatiparted   9 Populus adenopoda
+ Bracts subulate   8 Populus qiongdaoensis
11 (9) Leaves of short branchlets broadly ovate, margin with sinuous teeth; bud scales ciliate   12 Populus ningshanica
+ Leaves of short branchlets ovate-orbicular, margin coarsely serrate; bud scales glabrous   11 Populus X pseudotomentosa
12 (8) Branchlets brown or dull brown; leaf apex shortly acuminate or obtuse   7 Populus rotundifolia
+ Branchlets purplish or reddish brown, shiny; leaf apex obtusely rounded, acute, shortly acuminate, or acuminate.   (13)
13 (12) Leaf margin densely serrulate   5 Populus wulianensis
+ Leaf margin with sparse or dense, sinuous teeth or serrate.   (14)
14 (13) Leaf margin with sparse, sinuous teeth, apex obtusely rounded   4 Populus tremula
+ Leaf margin with dense, sinuous teeth or serrate , apex acute, shortly acuminate, or acuminate.   (15)
15 (14) Leaf margin serrate, apex acuminate   39 Populus alaschanica
+ Leaf margin with dense, sinuous teeth, apex acute or shortly acuminate   3 Populus davidiana
16 (3) Leaves with a well-defined, translucent margin (4. Sect. Aigeiros).   (17)
+ Leaf margin not translucent.   (23)
17 (16) Petiole compressed, at least distally, capsule with a longer, slender stipe.   (18)
+ Petiole terete or subterete; capsule with stipe short.   (20)
18 (17) Leaves of short branchlets deltoid or deltoid-ovate, margin ciliate   69 Populus X canadensis
+ Leaves of short branchlets ovate, rhomboid, or rhombic-ovate, rarely deltoid, margin glabrous (only P. ×beijingensis loosely ciliate).   (19)
19 (18) Branchlets grayish green or red; leaves of long shoots broadly ovate or deltoid-broadly ovate, those of short branchlets ovate   66 Populus X beijingensis
+ Branchlets yellowish; leaves of long shoots and short branchlets uniformly shaped or not, those of short branchlets rhombic-ovate-orbicular, rhombic-deltoid, or rhombic   62 Populus nigra
20 (17) Leaves of short branchlets deltoid-orbicular or compressed orbicular, as wide as or wider than long, 2-7 cm   68 Populus afghanica
+ Leaves of short branchlets ovate, rhombic-ovate, or rhombic-elliptic, apparently narrower than long, 5-10 cm.   (21)
21 (20) Petiole glabrous, leaf margin with loose hairs   63 Populus X xiaohei
+ Petiole pilose or with loose hairs; leaf margin glabrous.   (22)
22 (21) Branchlets pilose; leaf apex acuminate to long acuminate; petiole ca. as long as leaf blade; capsule 2- or 3-valved   65 Populus X jrtyschensis
+ Branchlets glabrous; leaf apex long acuminate to caudate; petiole ca. 1/3 as long as leaf blade; capsule 2-valved   67 Populus X berolinensis
23 (16) Petiole usually glandular at apex or, if eglandular, then ca. 4/5 as long as leaf blade; leaves greenish or grayish green abaxially or, if glaucous, then densely tomentose or downy and capsule densely tomentose; flower disc parted or sinuous (2. Sect. Leucoides).   (24)
+ Petiole usually eglandular (except P. shanxiensis and P.yatungensis); leaves often glaucous, abaxially rarely yellowish green or greenish white (P. iliensis greenish); flower disc entire; capsule usually glabrous or rarely pilose (3. Sect. Tacamahaca).   (28)
24 (23) Buds, petioles, and capsules glabrous or subglabrous; petiole nearly equaling leaf blade; leaf blade broadly ovate, suborbicular, or broadly long ovate-elliptic, abaxially glabrescent   14 Populus wilsonii
+ Buds, petioles, and capsules pilose (if petiole glabrous, then also violet); petiole less than 1/2 to 2/3 as long as leaf blade; leaf blade usually ovate, abaxially often tomentose.   (25)
25 (24) Petiole violet, glabrous   15 Populus violascens
+ Petiole red or green, not violet, pilose.   (26)
26 (25) Leaf blade to 30 cm, base deeply cordate, often auriculate, apex acuminate; capsule ovoid, 1-1.7 cm   13 Populus lasiocarpa
+ Leaf blade to 20 cm, base cordate to rounded, rarely truncate, apex acute or shortly acuminate; capsule broadly ovoid to globose, less than 1 cm.   (27)
27 (26) Leaf blade to 20 cm, base subcordate to truncate; petiole downy; capsule 2-valved   16 Populus glauca
+ Leaf blade to 14 cm, base cordate or subrounded; petiole tomentose; capsule 3- or 4-valved   17 Populus pseudoglauca
28 (23) Petiole compressed throughout or only apically; leaves greenish or slightly greenish white abaxially.   (29)
+ Petiole terete; leaves yellowish green or glaucous abaxially.   (33)
29 (28) Young branchlets pilose; capsule 2- or 3-valved.   (30)
+ Young branchlets glabrous; capsule 2-valved.   (31)
30 (29) Leaves ovate-orbicular or ovate, apex shortly acuminate, base rounded or broadly cuneate   46 Populus iliensis
+ Leaves rhombic-deltoid, rhombic-elliptic, or rhombic-ovate-orbicular, apex acuminate, base cuneate to broadly cuneate   61 Populus X xiaozhuanica
31 (29) Capsule pilose   38 Populus minhoensis
+ Capsule glabrous.   (32)
32 (31) Capsule 2-valved   36 Populus X gansuensis
+ Capsule (2 or)3-valved   37 Populus wenxianica
33 (28) Leaf margin revolute   20 Populus kangdingensis
+ Leaf margin not revolute.   (34)
34 (33) Leaf blade usually broadest at or above middle (especially on leaves of long shoots).   (35)
+ Leaves broadest below middle.   (43)
35 (34) Leaf blade rhombic-elliptic or rhombic-obovate, base cuneate (except P. simonii var. rotundifolia); capsule 2-valved.   (36)
+ Leaf blade suborbicular or elliptic to obovate-elliptic, base rounded or shallowly cordate; capsule 3- or 4-valved.   (38)
36 (35) Petioles, veins on both leaf surfaces, catkin rachis, and capsule pilose   19 Populus przewalskii
+ Petioles, veins on both leaf surfaces, catkin rachis, and capsule glabrous (catkin rachis pilose in P. simonii var. liaotungensis).   (37)
37 (36) Branchlets grayish yellow; leaf margin crenate-serrate, ciliate or glabrous   64 Populus manshurica
+ Branchlets yellowish brown; leaf margin serrulate, glabrous   18 Populus simonii
38 (35) Branchlets glabrous.   (39)
+ Branchlets pilose, at least when young.   (40)
39 (38) Leaf blade of short shoots suborbicular, rarely ovate-orbicular or ovate, not wrinkled adaxially, greenish abaxially, often ± as long as broad; capsule usually 2-valved   22 Populus charbinensis
+ Leaf blade of short shoots elliptic to obovate-elliptic, conspicuously wrinkled adaxially, whitish or pinkish abaxially, longer than broad; capsule usually 4-valved   31 Populus koreana
40 (38) Leaves with veins glabrous, or only proximal veins with sparse hairs adaxially   29 Populus suaveolens
+ Leaves with veins pilose.   (41)
41 (40) Leaf blade of short shoots suborbicular, 5-8 cm; petiole more than 3/4 as long as leaf blade; capsule pilose   45 Populus pamirica
+ Leaf blade of short shoots elliptic or, if suborbicular, then petiole less than 1/2 as long as leaf blade; capsule glabrous.   (42)
42 (41) Branchlets angled; catkin rachis pilose   33 Populus ussuriensis
+ Branchlets terete; catkin rachis glabrous   32 Populus maximowiczii
43 (34) Branchlets and catkin rachis glabrous or, if catkin rachis pilose, then buds or leaf margins glabrous.   (44)
+ Branchlets or shoots and catkin rachis pilose or, if catkin rachis glabrous, then buds, petioles, and leaf margins pilose.   (58)
44 (43) Leaves rhombic-elliptic, rhombic-ovate, or rarely ovate-lanceolate, margin alternating high and low serrate, teeth not arranged on same horizontal plane   21 Populus pseudosimonii
+ Leaves neither rhombic-elliptic nor rhombic-ovate, margin with teeth arranged on same horizontal plane; if leaves ovate-lanceolate, then margin loosely serrate only distally.   (45)
45 (44) Petiole pilose or, if glabrous (P. koreana), then leaves clearly wrinkled.   (46)
+ Petiole glabrous, leaf surface never wrinkled.   (50)
46 (45) Catkin rachis glabrous; leaves 5-14 cm, apex obtuse, acute, or rarely shortly acuminate.   (47)
+ Catkin rachis pilose; leaves 4-7 cm, apex acuminate or shortly acuminate.   (48)
47 (46) Leaves broadly ovate or ovate, both surfaces long pubescent along veins, smooth; capsule (2 or)3-valved, pilose, rarely glabrous   49 Populus pseudomaximowiczii
+ Leaves elliptic, elliptic-oblong, or obovate-elliptic, both surfaces glabrous; capsule (2 or)4-valved, glabrous   31 Populus koreana
48 (46) Capsule glabrous, 3-valved   28 Populus nakaii
+ Capsule pilose, 2- or 4-valved.   (49)
49 (48) Capsule 4-valved   34 Populus amurensis
+ Capsule 2-valved   27 Populus keerqinensis
50 (45) Bud scales ciliate; leaves suborbicular-cordate, rarely ovate-orbicular, apex shortly acuminate, usually twisted   30 Populus hsinganica
+ Bud scales glabrous; leaf neither suborbicular nor orbicular-cordate, apex acuminate, long acuminate, or caudate, rarely mucronate or, if shortly acuminate, then apex not twisted.   (51)
51 (50) Catkin rachis pilose; capsule 2- or 3(or 4)-valved.   (52)
+ Catkin rachis glabrous; capsule 3- or 4-valved.   (53)
52 (51) Stipe pilose   47 Populus talassica
+ Stipe glabrous   48 Populus platyphylla
53 (51) Branchlets terete; leaves often with prominent lateral veins adaxially, to 7 cm.   (54)
+ Branchlets and shoots angled; leaves with obscure lateral veins adaxially, more than 7 cm.   (55)
54 (53) Branchlets reddish brown or yellowish brown; leaf apex caudate or long acuminate   25 Populus trinervis
+ Branchlets yellowish green or grayish yellow; leaf apex mucronate or acuminate   23 Populus cathayana
55 (53) Leaves pilose along veins of both surfaces, at least basally, apex acuminate, ciliate; bark exfoliating (except P. purdomii var. rockii)   24 Populus purdomii
+ Leaves glabrous, or pilose only along veins abaxially, apex long acuminate, not ciliate; bark not exfoliating.   (56)
56 (55) Fruiting catkin to 35 cm; capsule (4 or)5-valved; leaves of short branchlets ovate or elliptic-ovate, 16-23 cm   59 Populus yuana
+ Fruiting catkin less than 20 cm; capsule 3- or 4-valved; leaves of short branchlets broadly ovate, ovate-orbicular, long ovate, or ovate-lanceolate, to 18 cm.   (57)
57 (56) Branchlets purplish or greenish brown; leaf apex shortly acuminate, midvein yellowish green abaxially   51 Populus szechuanica
+ Branchlets yellowish brown, with reddish tinge; leaf apex long acuminate, rarely obtuse, midvein usually reddish abaxially   52 Populus yunnanensis
58 (43) Leaf margin glabrous; petiole with loose hairs   26 Populus girinensis
+ Leaf margin pilose, at least at young leaf apices; petiole densely pubescent (P. suaveolens with loose hairs).   (59)
59 (58) Leaves glabrous or pilose along veins of one surface.   (60)
+ Leaves pilose, at least along veins on both surfaces.   (62)
60 (59) Petiole with 2 glands at apex; branchlets reddish brown   50 Populus shanxiensis
+ Petiole eglandular; branchlets yellowish brown to brown.   (61)
61 (60) Leaves ovate, 10-15 × 8-12 cm, base cordate or rounded, apex acute, downy along veins abaxially; capsule glabrous, 4-valved; stipe 5-10 mm   57 Populus ciliata
+ Leaves ovate, 6-10 × 2.5-7 cm, base rounded, apex acuminate, pilose adaxially along veins; capsule densely downy, 2(or 3)-valved; stipe ca. 5 mm   53 Populus qamdoensis
62 (59) Leaf base rounded or cuneate.   (63)
+ Leaf base shallowly cordate, cordate, or rarely rounded.   (65)
63 (62) Branchlets waxy golden, angled   40 Populus laurifolia
+ Branchlets not waxy golden, terete.   (64)
64 (63) Leaves ovate or narrowly ovate, base rounded; capsule pilose   42 Populus schneideri
+ Leaves ovate-lanceolate to narrowly lanceolate, base cuneate; capsule glabrous   41 Populus lancifolia
65 (62) Leaves of short branchlets mostly less than 10 cm.   (66)
+ Leaves of short branchlets mostly more than 10 cm.   (69)
66 (65) Buds with hairs, or bud scales ciliate.   (67)
+ Buds glabrous.   (68)
67 (66) Capsule pubescent, 2-valved, sessile; leaves ovate or broadly ovate   43 Populus pilosa
+ Capsule glabrous, with short stipe, (2 or)3(or 4)-valved, leaves long elliptic, ovate-elliptic, or rhombic-elliptic   44 Populus intramongolica
68 (66) Catkin rachis pilose; leaves usually elliptic; branchlets angled   33 Populus ussuriensis
+ Catkin rachis glabrous; leaves usually broadly elliptic; branchlets terete   32 Populus maximowiczii
69 (65) Leaf apex acute, shortly acuminate, or acuminate; branchlets terete.   (70)
+ Leaf apex long acuminate or rarely acuminate; branchlets angled, at least when young.   (72)
70 (69) Branchlets pubescent; leaves of short branchlets ovate-orbicular or ovate   55 Populus haoana
+ Branchlets tomentose; leaves of short branchlets broadly deltoid or broadly ovate-cordate.   (71)
71 (70) Leaf broadly deltoid, 12-16 × ca. 10 cm, apex acuminate; capsule 2-valved, long stipitate   35 Populus candicans
+ Leaf broadly ovate-cordate, to 10 × 8 cm, apex acute; capsule 4-valved, subsessile   54 Populus mainlingensis
72 (69) Petiole glandular at apex   60 Populus yatungensis
+ Petiole eglandular.   (73)
73 (72) Leaves of short branchlets leathery, ovate-lanceolate or broadly lanceolate, 8-18 × 3-10 cm; fruiting catkin to 30 cm; capsule glabrous   58 Populus wuana
+ Leaves of short branchlets not leathery, ovate or broadly ovate, 7-14 × 4-10 cm; fruiting catkin 8-16 cm; capsule pilose   56 Populus xiangchengensis

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