Phloem in Pinophyta

The gymnospermous phloem shows major differences from those of the cryptogams on one hand and the angiosperms on the other. These include both the type of conducting cells as well as the cellular composition. Even more important is the degree and nature of functional inter-relationship between the conducting and parenchyma cells. The following evolutionary trends have been suggested. Increase in the amount of axial parenchyma; Decrease in the number of albuminous cells in the rays; Increase in the axial albuminous cells; Increase in the fibres; Increase in the regular arrangement of cells. The work on the fossil taxa does provide variable support to these suggestions. It is generally believed that the cryptogamic sieve element arose from a parenchyma cell and all the phloem produced in the fossil lycopods, Sphenopsida and ferns is primary in origin. Here the phloem consists of either sieve elements only or the sieve elements with scattered parenchyma cells. There is no definite relationship between the conducting elements and the parenchyma cells as seen in the seed plants. Additional parenchyma is often present in the form of a sheath separating the xylem from the narrow phloem tissue. The typical cryptogamic sieve elements are identical to the elongate parenchyma cells. These are relatively small in diameter, longer than the parenchyma cells but shorter than the gymnospermous sieve cells. In some taxa, the sieve elements are of two sizes: large (up to 600µm) and small (between 100-1500 µm). The end walls of the cryptogamous sieve elements are horizontal or slightly oblique and the sieve areas are small and vary markedly in their outline. The sieve pores occur in the sieve areas as well as scattered on the vertical walls. The callose deposition has been found in Psilotum, lycopods and the ferns.