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(a) Needle cross-sections for selected North American Picea species, including A, P. glauca; B, P. rubens; C, P. mariana; D, P. chihuahuana; E–G, P. critchfieldii from Tunica Hills sediments. (b) Box plots showing resin-duct diameter of selected species. (c) Box plots showing resin-duct position index for selected species. The position index is the ratio of the distance of the resin duct from the lateral angle of the needle and the distance between adjacent needle angles (32).
