Abstract: The seasonal development of secondary phloem of 23 species of deciduous trees dis-tributed naturally or introduced on Hainan Island in tropical area of China was studied using microscopy. The functional phloem thickness in main trunk and branchlet of 6 species of the trees wen determined systematically and for the rest trees a comparision of functional phloem thickness in main trunk was made between leaf-absent and leaf-bearing period. According to the characteristics of phloem development in main trunk of the trees studied, they can be di-vided into three groups. The trees in the first group have a functional phloem in leaf-absent pe-riod with a thickness over 70 as that in leaf-bearing period.

Key words: Tropical trees