Evolutionary trends in secondary phloem anatomy of trees, shrubs and climbers from Africa (mainly Ivory Coast)

Trends were analysed in the occurrence of certain characteristics of secondary phloem in relation to the categories bark structure, habit and habitat of 463 woody plant species (belonging to 261 genera and 31 families) mainly from the Ivory Coast, Africa. Relationships were investigated between these three categories. Three types of bark structure were distinguished, mainly by differences in the arrangement of axial parenchyma and type of mechanical tissue. Presumed primitive features of sieve tubes, axial parenchyma cells and cellular composition of phloem rays, become less prominent from climbers to trees and from rain forest to savannah. Presumed advanced types of axial parenchyma and mechanical tissue, often associated with primitive sieve tubes and companion cells (bark structural type Datura), were more frequently present in species from rain forest than from savannah. The same combination of types of secondary phloem from species from rain forest was more representative of climbers than of trees. Secondary phloem of shrubs was often not intermediate between trees and climbers, especially in rain forest. Secondary phloem anatomy suggested that rain forest shrubs had evolved from trees by developing a more advanced axial parenchyma and mechanical tissue, and from climbers by developing more advanced sieve tubes and companion cells.