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SEM images of different species of birch pollen grains: a, b B. utilis Doorenbos; c, d B. utilis dahurica; e, f B. maximowicziana; g, h B. pendula; i, j B. dahurica; k, l B. humilis. The right-side image is always an overview of pollen. The scale bars correspond to 10 μm in left-side images and 20 μm in right-side images
