Tremuloides (Flowering Specimens): Answers to key questions in Conifers & Catkin-Bearing Trees and Shrubs of Saskatchewan, Fascicle 5, Flora of Saskatchewan by George Argus, Vernon Harms, Anna Leighton and Mary Vetter leading to this species. 

Floral bracts ciliate with long, white, silky hairs; stamens 6-12; floral discs narrowly cup-shaped, obviously oblique; catkin rachises sparsely hairy with fine, white hairs; stigmas slenderly cylindrical; pedicels 0.5-1.5 mm; branchlets glabrous; winter buds only slightly resinous, glabrous, proximally shiny; NOT [Floral bracts not ciliate; stamens 10-60 (70), or 0 in P. x jackii; floral discs broadly cup-shaped or saucer-shaped, not obviously oblique; catkin rachises glabrous or variously hairy; stigmas broad and plate-like; pedicel lengths usually over 1.5 mm; branchlets glabrous or sparsely to densely hairy; winter buds usually distinctly resinous, glabrous or with short stiff hairs]

Populus: Answers to key questions in Conifers & Catkin-Bearing Trees and Shrubs of Saskatchewan, Fascicle 5, Flora of Saskatchewan by George Argus, Vernon Harms, Anna Leighton and Mary Vetter leading to this genus.

trees; buds covered by 3-10 scales, usually resinous;  catkins pendulous, sessile; floral bracts deeply cut; perianth reduced to a non-nectariferous, cup-shaped or shallow saucer-shaped floral disc; stamens 6-60 (70); capsules 2-4 valved, narrowly ovoid to spherical, with the floral disc persistent at base; NOT [shrubs or trees; buds covered by 1 scale, not resinous; catkins erect, spreading, or +/- pendulous, sessile or terminating flowering branchlets; floral bracts entire, erose, bifid, or irregularly toothed; perianth reduced to an adaxial nectary (rarely also an abaxial nectary, then the 2 nectaries distinct or connate into a shallow cup); stamens 1, 2, or 3-10; capsules 2-valved, obclavate to ovoid or ellipsoid]