Comparative Studies On Secondary Phloem In Coniferous Species

The secondary phloem of35species of31genera in9families were studied. Through thestudy of secondary phloem of different species, species without xylem can be identified in someways and some reference can be provided for the study of evolution and relationships in therespect of secondary phloem. The results are as follows:(1) Druses are the most typical characteristic in the secondary phloem of Ginkgoacea. It’seasy to recognize Ginkgo among coniferous species. The shape of phloem fiber in Ginkoacea isdifferent with which of Taxodiaceae and Cupressaceae, and phloem fibers distributing in severallayers is not much in species of conifers.(2) Phloem fibers diffuse is the most typical characteristic in the secondary phloem ofAraucariaceae. Phloem fibers are less and disorder compared with Ginkgoaceae,.(3) Lack of phloem fiber is the most typical characteristic in the secondary phloem ofPinaceae (Expect Larix and Pseudotsuga). It doesn’t have a high level of evolution. Some speciesof Pinaceae have fibers, stone cells and secretory cells.(4) As a family of single species, Sciadopityaceae has different characteristics from otherspecies in secondary phloem. Only Gingkoaceae and Sciadopityaceae have phloem fibersdistributing in several layers in conifers, and the arrangement of phloem fiber in Gingkoacea isless regular than which in Sciadopityaceae. Besides, the shape of phloem fiber inSciadopityaceae is more regular, and the cell wall is more thicker.(5) The structure of secondary phloem in Taxodiaceae, Cupressaceae and part ofPodocarpaceae is similar. In cross section sieve cells, parenchyma cells, and phloem fibers formtangential bands respectively. There must be some genetical relationships among them. Mostspecies in Taxodiaceae and Podocarpacea don’t have axial resin canals, while axial resin canalsexist in most species of Cupressaceae. The shape, number and arrangement of cells also havedifferences.(6) In the secondary phloem of Cephalotaxaceae, phloem fibers distribute in single anddiffuse patterns. The shape is circular or rectangular. It usually forms a row in the outer part ofsecondary phloem. Among the samples only species of Araucariaceae is similar with which ofCephlotaxaceae, but it doesn’t have tangential rows of phloem fibers.(7) The structure of secondary phloem in Taxaceae is similar with which in Taxodiaceae,Cupressaceae and part of Podocarpaceae. The phloem fiber with crystals is the mostdistinguishing feature.