Comparative study on development and structural characteristics of bracts and leaves of Tilia japonica Simonk

Inflorescence bracts are the most typical characteristics of Tilia Linn., but their evolutionary relationship with other organs is not clear. In this study, correlations between the growth and development of bracts and leaves of Tilia japonica Simonk. were analyzed, and their epidermal micromorphological features and internal anatomical structures were compared. Results indicated that the growth and development processes of the bracts and leaves had very similar regularity. Both grew rapidly in the young stage, slowly in the middle stage, and only slightly in the mature stage. Analysis between the morphological characters of the bract and leaf showed significant correlations between bract length, bract width, leaf length, and leaf width. In addition, epidermal micromorphological features were very similar. Anatomically, compared with the leaf, there was no differentiation between palisade tissue and spongy tissue in the bracts. Combined with the fossil record of Tilia bracts, we suggest that the bract is specialized from the leaf and may be regarded as a leaf-like structure. The differences in epidermal micromorphological features and anatomical structures are mainly due to adaptations to their different functions during evolution.