In this article we will discuss about the anatomy of pinus with the help of diagrams.
Anatomy of Needle:
The solitary needle of P. monophylla (Fig. 16.3A) is circular. This is semicircular in the two-needle pine (P. merkusii), and triangular in the three-needle pine (P. roxburghii) (Fig. 16.3B). The epidermis is of isodiametric lignified cells covered with cutin. Hypodermis of 2-3 cell layers may be thin or thick-walled.
Mesophyll is chlorenchymatous, with wall infoldings in the cells, and is interspersed with resin ducts. The endodermis is of barrel-shaped cells, containing starch. Pericycle of parenchymatous cells is interspersed with transfusion tracheids. A single vascular bundle is placed medianly (P. wallichiana) and two vascular bundles (P. roxburghii) are placed at an angle (Fig. 16.3B.C). A vascular cambium is present, which cuts of secondary phloem and little or no secondary xylem.
On the basis of one or two vascular bundles of needles, pines have been divided into haploxylon or diploxylon.
Sunken stomata may be present around diploxylon needles of pines and may be absent in some haploxylon pines.
Anatomy of Root:
The long root of Pinus may be diarch as in P. roxuburghii (Fig. 16.4A) or tetrarch as in P. edulis (Fig. 16.4B). The epidermis is followed by starch-filled cortex of two zones outer zone of small parenchymatous cells and inner zone of large ones. The single-layered endodermis of casparain strips is followed by 6-7-celled pericycle.
The vasculature is of 8-16 protoxylem elements, each one associated with a resin duct. Secondary growth occurs very early, due to the differentiation of cambium below the primary phloem. It cuts off secondary xylem towards pith and secondary phloem towards cortex. In the region of resin duct the cambium cuts off parenchymatous cells forming xylem rays. Cork cambium cuts off cork cells, in the outer region of the pericycle, which becomes highly tanniferous. The structure of root in the final stages resembles that of the stem.
Dwarf root is similar to the long root. However, it differs from long root in absence of root cap (Fig. 16.5A) and resin ducts are absent here, and it is without starch in cortical cells and has reduced number of vascular elements. Also the secondary growth is absent. Dwarf roots are characterized by dichotomous branching and are mycorrhizic (Fig. 16.5B, C, D). The fungal hyphae penetrate into the cortical cells forming ‘Hartig-net’ (Fig. 16.5E). The association is symbiotic, the pine plant is benefitted due to increase in the surface area of absorption.
Anatomy of Stem:
A young stem of Pinus is not circular, it shows ridges and furrows due to the surrounding leaves. The epidermis is followed by a broad parenchymatous cortex, and the vascular tissue in the form of provascular strands, is arranged in a ring. These provascular strands mature into discrete collateral and open vascular bundles.
In an old stem, vascular bundles form a ring and are separated from each other by medullary rays. Resin ducts are also visible in the cortex and vascular strands.
Before the occurrence of secondary growth, the fascicular cambium present within the vascular bundle and interfascicular cambium present between the bundles join to form a ring. This cambium cuts off two types of cells, fusiform and ray initials. The former forms the axial system and the latter forms the radial system. The axial system consists of xylem and phloem, whereas the radial system is composed of rays.
The cambium (Cb) cuts off a continuous cylinder of secondary xylem towards the inside and secondary phloem (Sph) towards the outside. The former comprises the tracheids with bordered pits on radial walls (Fig. 16.6A,B). Mature tracheids develop thickenings in between the pits. These are bars of Sanio described in P. sylvestris, as rod-shaped thickening between radial walls of tracheids of secondary wood.
However, electron microscopic study has shown that these are mere refractions. The secondary phloem consists of sieve elements, which arise directly by the transformation of phloem initials. Rarely, phloem initial divides to form two sieve cells. A characteristic feature of sieve cells is the formation of sieve plate.
In the wood, one can demarcate the outer lighter zone (the sap wood) and inner dark region (heart wood). The wood consists of parenchyma cells and tracheids, and lacks vessels. On examination of a TS of wood one comes across concentric circles known as annual or growth rings (Fig. 16.6A). These are the result of alternate formation of vigorously produced thin-walled cells in early or spring wood and thick-walled cells of late or summer wood.
The age of a pine tree can be determined by counting the number of these rings. The resin ducts (rd) lined by epithellial cells are present throughout the wood.
The vascular rays initiated by the cambium and are of two types: uniseriate and multiseriate or fusiform. These are to be examined in sections cut in three planes (Fig. 16.6B). In a transverse section (TS) can be seen the width and length of the ray, whereas in a transverse longitudinal section (TLS) height and width of the ray can be seen. In a radial longitudinal section (RLS) length and height of the ray can be seen.
In the first or second layer of cortex differentiates cork cambium which cuts off cork on the outside and secondary cortex towards inside.
