Q.2. Describe the internal structure of stem of Pinus, with a special reference to structure of Pinus as seen in transverse radial and longitudinal sections. (1994)
Ans. The transverse section of young Pinus stem is more or less identical to the dicotyledonous stem. It shows a wavy outline due to the presence of leaf-bases and the bases of the dwarf shoots. It shows the following arrangment of tissues begining from peribgerg.
(i) Epidermis: - Single layered and thick-walled epidermis, with a thick layer of cuticle on its outer side, bounds the stem. Epidermis is followed by a broad zone of cortex.
(ii) Cortex: - The cortex in its outermost part consists of one or a few layers of sclerenchymatous cell which constitute the hypodermis. Below hypodermis the inner layer of cortex consist of thin walled parenchyma. A number of large resin canal are present in the inner parenchymatous cortex, a conspicuous characters of Pinus stem. Each resin duct is surrounded by a layer of the epithelial cells.
(iii) Endodermis: -
It is innermost layer of the parenchymatous cortex but not marked off.
(iv) Pericycle: -
It is parenchymatous and inconspicuous.
(v) Vascular bundles: -
It consists of a ring of 5 to 8 or more closely arranged conjoint, endarch and open vascular bundles that are separated from one another by board medullary rays.
Each vascular bundle consists of xylem on the inner side and phloem on the outer side, with a strip of cambium between xylem and phloem. Phloem consists of seive tubes and phloem parenchyma but no companion cells. Xylem consists of tracheids; there being no vessels. Resin ducts are found in xylem. The xylem is endarch, i.e., the protoxylem consisting of a few annular and spiral tracheids lies towards the centre which the metaxylem consisting of tracheids with bordered pits on their radial walls towards the periphery cambium consists of a few layer of thin walled rectangular cell lying in between xylem and phloem.
(vi) Medullary Ray: -
Narrow medullary ray run outward from the pith between the vascular
bundles.
(vii) Pith: -
In the centre there is well developed pith, consisting of a mass of parenchymatous cells. A few resin ducts are also present in the pith.
In the young Pinus stem with the help of fascicular and intrafascicular
cambium secondary growth takes place, in exactly the same way as in a dicotyledonous stem. The secondary growth results in the formation of secondary xylem and secondary phloem. Medullary rays are very abundant in the secondary vascular tissues so that the wood is soft. Secondary xylem is zonated into spring and autumn wood. The resin canals are also formed in secondary xylem. There is cork formation also by the cork cambium or phellogen. It originates in the cortical tissue throughout in the outermost layer of cortex. The bark is, therefore scaly.
The xylem (both primary and secondary) are composed of tracheids and the radial walls of xylem tracheids have complex type of bordered pits. The vascular rays of medullary rays are, also quite complex in structure and each medullary ray is a plate like structure which is one cell in thickness but several cell in length as well as is height.
In radial longitudinal section (R.L.S.) we see the xylem cambium, phloem cortex and pith regions. The xylem is made up of tracheids and each tracheid consists of several bordered pits. The phloem consists of seive tubes and phloem parenchyma. The medullary rays in R.L.S. may be observed in the length and height and appears as a sheet like structure. The medullary ray runs frompith to cortex through xylem, cambium and phloem. In the xylem region the medullary has two types of cells.
(i) Ray tracheid: -
These are tracheid like they adjoin the tracheid cells of xylem.
(ii) Ray Cells: -
These are with thin walls which develop in between two rows of rays tracheid and several cells in height.
In the phloem region medullary rays also of two types:
(i) Albuminous cells: -
These cells contain albumin and adjoin the seive tube elements of phloem.
(ii) Starch cells: -
In between the two rows of albuminous cells the starch cells develop.
In a tangential longitudinal section, (T.L.S.) the bordered pits in xylem trachedial cells and the medullary rays are cut across, which are mostly uniseriate, i.e., they are only one cell broad, and are usually less than a dozen in eight. The bordered pits also shows the torus. There will be no pith in the T.L.S. of Pinus wood. The medullary rays may be two types - (i) Linear medullary rasy: These are seen to be extending in finger like projections both in xylem and phloem. These rays have no resin canals, (ii) fusiform vascular ray or medullary rays-are those which have a resin canal.