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