Histological investigations of the secondary phloem of gymnosperms

An anatomical study of secondary phloem of the different species of Gymnosperms showed that three categories could be distinguished, which may represent three evolutionary stages. These three categories were:Pseudotsuga taxifolia type, to which belong many Pinaceae (while the other Pinaceae species belong to a subtype, e.g. the Tsuga canadensis subtype);Gingko biloba type, to which belong the Cycadaceae, Araucariaceae and many Podocarpaceae and Taxaceae;Chamaecyparis pisifera type, to which belong the Cupressaceae, Taxodiaceae and the rest of the Taxaceae and Podocarpaceae.The phylogenetic sequence of the axial system and the reduction of the phloem rays starts in the Ps. taxifolia type with an almost uniform axial system and ray- albuminous cells, and a complex heterogeneous phloem ray, differentiating through the G. biloba type to the C. pisifera type, which has a highly specialized axial system with phloem-albuminous cells and a simpler, reduced homogeneous phloem ray. The reduction of the phloem rays parallels the differentiation of the axial system