Abstract: The developmental morphology of the inflorescence and floret of Alnus mandshurica and Alnus sibirica was investigated using scanning electron microscopy. Their female inflorescences all appeared multiple aments that consist of multiple spirally arranged cymelets. Each cymelet had one primary bract, two secondary bracts and two tertiary bracts. Each floret had a bicarpellate pistil with a dichotomous columnar style. Two pistils of a cymelet were orientated perpendicularly. During the winter, the female inflorescences of Alnus mandshurica were protected by bud scales, while those of Alnus sibirica naked. Their male inflorescences also appeared multi-ple aments. Cymelets were arranged spirally and each had one primary bract, two secondary bracts and two terti-ary bracts. The two secondary bracts developed earlier than the two tertiary ones but all were originated from cymelet primordia. Of the three floret primordia, the middle one was differentiated earlier than the lateral ones. A floret had four tepals and four stamens, and each stamen had four pollen sacs. The tepals were developed earlier than the stamens, but both were from the floret primordia. During the winter, the male inflorescences of Alnus mandshurica and Alnus sibirica were naked.

Key words: Alnus mandshurica, Alnus sibirica, floret, inflorescence, morphogenesis, SEM