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Key to Humulus

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Show caption*© WATANABE Hitoshi 渡辺仁, some rights reserved (CC BY), uploaded by WATANABE Hitoshi 渡辺仁
1 Veins on lower surface of leaves armed with rigid, spinulose hairs; bracts of pistillate flowers spinulose-ciliate; most leaves 5-9 lobed
1 Veins on lower surface of leaves more or less pubescent with lax, weak hairs, but lacking rigid, spinulose hairs; bracts of pistillate flowers smooth-margined; most leaves 1-3 lobed.
  2 Lower surfaces of leaves (measured on middle lobe of 4-6 cm long leaves of flowering or fruiting branches) usually with 15-20 (-25) hairs per cm of length of midrib; glands (measured on leaves as above) < 25 per cm2 of intervein lower leaf surface; [introduced, sometimes showing introgression with native taxa]
  2 Lower surfaces of leaves (measured on middle lobe of 4-6 cm long leaves of flowering or fruiting branches) usually with > 20 hairs per cm of length of midrib; glands (measured on leaves as above) > 25 per cm2 of intervein lower leaf surface; [native varieties, though often weedy and sometimes showing introgression with Eurasian H. lupulus].
    3 Lower surfaces of leaves (measured on middle lobe of 4-6 cm long leaves of flowering or fruiting branches) conspicuously pubescent between the veins and on the veins, with > 100 hairs per cm of length of midrib; smaller leaves unlobed (less commonly 3-lobed)
    3 Lower surfaces of leaves (measured on middle lobe of 4-6 cm long leaves of flowering or fruiting branches) not conspicuously pubescent, the pubescence usually limited to the veins, usually with < 100 hairs per cm of length of midrib; smaller leaves generally 3-lobed