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Let us learn about Secondary Xylem and Phloem in Conifers.
Secondary Xylem (Wood):
The xylem of gymnosperms is generally simpler and more homogeneous than that of angiosperms. The chief distinction between the two kinds of wood is the absence of vessels in the gymnosperms (except in Gnetales) and their presence in most angiosperms. The gymnosperm wood possesses a small amount of parenchyma, particularly vertical parenchyma.
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The Vertical System:
In the secondary xylem of gymnosperms, the vertical system consists mostly of tracheids. The late wood tracheids possess relatively thick walls and pits with reduced borders, and therefore, they are known as fibre-tracheids, but libriform fibres do not occur. The tracheids are long cells (0.5 to 1.1 mm) with their ends overlapping those of other tracheids.
The tracheids of existing gymnosperms are interconnected by circular or oval bordered pit-pairs in single, opposite or alternate arrangement. The number of pits on each tracheid may vary from 50 to 300 (Stamm, 1946). The pit-pairs are abundantly present on the ends where the tracheids overlap each other.
Generally the pits are confined to the radial facets of the cells. Tori are present on the pit membranes in Ginkgo, Gnetales and most Coniferales. The tracheids possess thickenings of intercellular material and primary walls along the upper and lower margins of the pit-pairs. These thickenings are called crassulae.
Another wall sculpture is represented by the trabeculae. They are found in the form of small bars extending across the lumina of the tracheids from one tangential wall to the other. Helical thickenings on pitted walls have been recorded in the tracheids of some conifers.
Wherever present, the vertical xylem parenchyma of the Coniferales is found to be distributed throughout the growth ring and occurs in long strands derived from transverse divisions on the mostly long fusiform cambial cells. Some conifers (Taxus, Torreya and Araucaria) do not have parenchyma in the vertical system.
Structure of Rays:
The rays of gymnosperms are composed either of parenchyma cells alone, or of parenchyma cells and tracheids. Ray tracheids are distinguished from ray parenchyma cells chiefly by their bordered pits and lack of protoplasts. The ray tracheids possess lignified secondary walls. In some conifers these walls are thick and sculptured, with projections in the form of bands extending across the lumen of the cell.
The ray parenchyma cells possess living protoplasts in the sap wood and often dark coloured resinous deposits in the heartwood. The rays of conifers are for the most part only one cell wide and from 1 to 20 or sometimes upto 50 cells high. Ray tracheids may occur singly or in series, at the margins of a ray.
The rays serve to transport the assimilation products formed in the leaves and flowing downwards in the phloem in a radial direction into the wood of the stem and roots. They conduct water away from the wood in the opposite direction. The rays penetrate equally into the xylem and the phloem and thus suited for these functions.
Resin Ducts:
In certain gymnosperms the resin ducts are developed in the vertical system or in both the vertical and horizontal systems. The resin ducts arise as schizogenous intercellular spaces by separation of resin producing parenchyma cells from each other.
These cells make the lining, the epithelium, of the resin duct and excrete the resin. A resin duct may become closed by the enlarging epithelial cells. These tylosis like extensions are known as tylosoids (Record, 1947).
Secondary Phloem:
The structure of phloem of conifers is quite simple. The vertical system contains sieve cells, parenchyma cells and frequently fibres. Companion cells are absent. The sieve plates are present on the lateral walls. The rays are mostly uniseriate and contain parenchyma only or parenchyma and albuminous cells.
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The sieve cells are slender, elongated elements appear like the fusiform initials from which they are derived. They overlap each other at their ends and each sieve cell remains in contact with several rays. The sieve areas are abundantly found on the ends which overlap those of other sieve cells.
The connecting strands in the sieve areas are aggregated into the groups, and the callose associated with the strands in one group fuses into one structure. The phloem parenchyma cells occur in longitudinal strands. They store reserve food material and also contain resins, crystals and tannins. The phloem rays of the trees of advanced age are characteristic in having albuminous cells.
The albuminous cells may also occur among the phloem parenchyma cells (in the vertical system) and in still other plants only among the phloem parenchyma cells. The albuminous cells contain dense cytoplasm and distinct nuclei. They are irregular in shape and store food materials. The secondary phloem also contains resin canals. In Picea canadensis, they occur in rays and possess cyst-like bulbous expansions.
