INTERNAL STRUCTURE OF PINUS

Anatomy of Root: The primary root of Pinus resembles a dicot root in the arrangement of tissues. In consists of piliferous layer, cortex, and stele.
(i) Piliferous layer: It is the outermost layer, which is composed of tightly packed cells. The cells give rise to unicellular root hairs. Root hairs are poorly developed, as the young root forms mycorrhizal association.
(ii) Cortex: Just beneath the piliferous layer, there lies a broad cortex. It consists of 4-5 layers of thin parenchymatous cells. The innermost layer or cortex is differentiated into endodermis. The endodermis consists of a single layer of brown suberised cells containing tannin in them.
(iii) stele: The stele is surrounded by many layered pericycle. The cells contain resins and tannin. The second layer of the pericycle gives rise to the lateral roots. The roots are diarch to tetrarch with radial and exarch vascular bundles. The protoxylem bifurcates to form a y-shaped structure and a resin duct lies in between the two arms of the protoxylem. The xylem consists of only tracheids. Vessels are absent. Phloem strands alternate with the xylem strands. Phloem consists of sieve tubes and phloem parenchyma. Companion cells are absent. The pith cells are rich in starch. The primary medullary rays arise opposite to each protoxylem group. Secondary growth takes place in the manner similar to dicot root.
Anatomy of Stem:
Primary Structure: A young stem of Pinus is more or less identical to the dicot stem in the arrangement of tissues. The stem shows wavy outline due to the presence of leaf bases and dwarf shoots.
Epidermis: It is the outermost layer composed of compactly arranged and heavily cutinised cells. The epidermis bears, scattered stomata.
Cortex: The cortex is multilayered, but not extensive. The hypodermis is formed by a few layers of sclerenchyma cells. The inner cortex consists of thin-walled parenchyma cells. Presence of resin canals is a characteristic feature of the cortex. Each resin canal is surrounded by a layer of glandular epithelial cells, which secrete resin. Each resin canal lies outside the vascular bundle. The innermost layer of the cortex forms the endodermis, however it is not clearly demarcated. The pericycle is parenchymatous and inconspicuous.
Vascular Tissues: The vascular cylinder is of eustelic type. It consists of a ring of five to eight closely placed vascular bundles. The bundles are conjoint, collateral, endarch and open. The bundles are separated from one another by narrow medullary rays. Each bundle has primary phloem on the outside and primary xylem on the innerside with primary cambium in between the two. The primary phloem consists of sieve tubes and phloem parenchyma. There are no companion cells. Some albuminous cells are also present in the phloem which are associated with sieve tubes like companion cells. The primary xylem contains tracheids. Vessels are absent. The protoxylem is endarch. The protoxylem elements have a loose spiral thickening with a few small bordered pits on its radial walls. The metaxylem element are reticulate and pitted. The bordered pits on their radial walls are large and uniseriately arranged.
Pith: The centre of the stem is occupied by thin walled parenchymatous pith. Some of the pith cells are filled with resinous substances.
Secondary growth in Stem: The stem of Pinus grows in thickness by secondary growth, in the same way as in dicot stems. Interfascicular strips of cambium develop in the primary medullary rays in between the Vascular bundles. The strips of interfascicular cambium join with the intrafascicular cambium in the vascular bundles and form a complete ring of cambium. The cambial cells cut off secondary xylem towards the inner side and secondary phloem to the outside. The secondary vascular tissues are added continuously due to the activity of the vascular cambium and as a result the stem increases in girth. The primary phloem and xylem are crushed due to the presence of secondary Vascular tissues. Secondary wood consists of tracheids, traversed by rays. The tracheids are about 4 mm long and pointed at both ends. Bordered pits occur on their radial and tangential walls. Bordered pits are uniseriate and are separated by crescent shaped structures called bars of sanio. If bordered pits are in two rows they are placed opposite to each Other. Such pits are called Abietinian pits. Resin canals are also present in the secondary wood. Vessels and xylem fibers are absent. The secondary phloem consists of sieve cells and phloem parenchyma. The sieve cells are elongated, taper at both their ends and possess sieve plates on their radial walls. The companion cells are absent, but phloem parenchyma in albuminous cells are present.
Annual rings: Every year (winter - spring summer - autumn) the cambium forms a band of secondary xylem and secondary phloem. The successive bands of secondary xylem are quite distinct from each other. In a cross section it appears as a series of concentric layers. Each layer or band represents a year's growth and is called as an annual ring. The annual rings are formed due to the difference in the size of tracheids formed in the spring as compared with those formed in the autumn. In spring season (warm weather) new leaves and branches are formed and thus there is active translocation of water and nutrients. Hence the tracheids formed in this season are broad, polygonal, thin - walled with large bordered pits. In autumn (cool weather) when there is leaf - fall, active transportation of water and nutrients is not required. Hence, the tracheids formed in this season are squarish with narrow lumen ,thick-walled ,with small bordered pits. Autumn wood and spring wood formed in a year constitute an annual ring. The age of a plant can be calculated with help of these annual rings.
Periderm or cork: Concurrently with the secondary growth in the vascular region, a lateral meristem known as phellogen or cork cambium develops in the outer region of the cortex. The phellogen divides periclinally to cut off cork cells (phellem) towards the outerside and secondary cortex (phelloderm) towards the innerside. As the stem increases in girth, the epidermis ruptures and the cork cells form a protective covering. The cork cells are impervious to water and check transpiration from the stem surface. The phellem, phellogen and phelloderm together form the periderm or bark.
Secondary Medullary Rays: During secondary growth, the primary medullary rays are replaced by secondary medullary rays. They are formed from the fascicular cambial cells. They traverse the secondary xylem and phloem zones at certain places. They vary in size from 2-12 cells in height and only one cell broad(uniseriate). However the rays associated with the resin canals are multiseriate. New medullary rays are formed each year, alternating with those previously laid down. The secondary wood consists of exclusively dead tracheids. It lacks wood parenchyma. So the radial diffusion of food materials become a matter of difficulty. The Secondary medullary rays have a significant role in this connection. Structurally the medullary rays are much more complex than those of the dicotyledons. The rays in the secondary wood consists of thin walled rectangular parenchymatous cells containing cytoplasm, a nucleus and starch grains. They possess simple pits. The starchy layer is surrounded on either side with 2-3 layers of dead tracheidal cells. They are elongated, horizontally situated in one or two rows and are called marginal ray tracheids. The marginal ray tracheids have bordered pits on their lateral and end walls. Thus, they allow radial diffusion of fluids and compensate the lack of wood parenchyma. The areas of mutual contact between the medullary ray tracheids and the tracheids of the wood communicate with each other by simple pits. They are known as the pits in the field. The medullary rays in the secondary phloem region also consists of living starch - filled parenchymatous cells. But in this region, marginal ray tracheids are absent. Their place is taken up by large thin walled cells which are elongated vertically. They are commonly called the albuminous cells. They contain protein. In a radial longitudinal Section (R.L.S) of the wood, the medullary rays are cut length wise and their height and length can be observed. The ray is seen crossing the tracheids in a horizontal direction. The bordered pits are seen in surface view on the radial walls of the tracheids. The pits are uniseriate, A tangential longitudinal section (T.L.S) of the wood cuts across the medullary ray. So this section shows the height and width of the ray. The bordered pits are seen in section on the radial walls of the tracheids which are cut through in this case. The rays with a single row of cells are called linear vascular rays. The rays which include a resin canal become multiseriate and fusiform. Such rays are called fusiform vascular rays. The rays vary in size from 2-12 cells in height.

Anatomy of Foliage Leaf: The foliage leaf (needle) of Pinus reveals a structure that makes it adaptable to with stand the low temperature and scarcity of water supply. It is therefore, spoken of as xeromorphic. The outline of the needle in a transverse section depends on the number of needles present in a spur. For example it is circular in p. monophylla (with one needle), semi - circular in P. sylvestris (with two needles) and triangular in P. roxburghii and P. wallichiana (three needles in a spur).
The transverse section of the needle shows the following structure.
1. Epidermis: The epidermis consists of a single - layer of thick - walled cells, covered by a thick cuticle. Sunken stomata are present on all sides of the leaf (diploxylon pines) or they may be absent on the outer surface of the needle as in some haploxylon pines. The stoma are developed in longitudinal rows. Each stoma consists of two guard cells, situated well below the level of the epidermis. It opens internally into a sub-stomatal cavity,
2. Hypodermis: The epidermis is followed by 2 - 3 layers of sclerenchymatous hypodermis. It is well developed at the corners. The hypodermis is frequently interrupted by air spaces beneath the stomata.
3. Mesophyll: The mesophyll is parenchymatous and is not differentiated into palisade and spongy tissues. It consists of thin walled polygonal cells with abundant chloroplasts and starch grains. The walls of the mesophyll cells give rise to many peg - like or plate - like inflodings which increase the photosynthetic area of these cells. These are known as arms, flanges or simply folds. They increase the internal photosynthetic and respiratory area and in a way compensate for the reduction of the leaf surface. Beneath the hypodermis, 2 - 3 resin canals are present. Each resin canal has a layer of secretory epithelial cells, which is surrounded on outside by a sclerenchymatous sheath.
4. Endodermis: Inner to mesophyll, there is a layer of barrel shaped cells, forming the endodermis. These cells are without chloroplasts but starch may be present. The endodermal cells are devoid of casparian strips.
5. Pericycle: The pericycle is multilayered and consists of four types of cells.
(i) Parenchymatous cells: Most of the pericycle is made up of ordinary Parenchyma cells, containing starch grains. They form the transfusion tissue.
(ii) Albuminous cells: These cells occur above the phloem of vascular bundles. They are also parenchyma cells but are filled with proteins and starch grains.
(iii) Tracheidal cells: These are tracheid - like cells which occur close to the xylem of vascular bundles. The cells possess bordered pits and help in conduction of water and nutrients to mesophyll.
(iv) Sclerenchymatous cells: These cells form a T- shaped girdle above the two vascular bundles.
6. Vascular bundles: The number of vascular bundles vary in different species of Pinus. As early as 1893, Koehne divided the genus Pinus into haploxylon (with one bundle) and diploxylon (with two bundles) by studing the anatomy of the needle. When there is only one bundle (P. wallichiana) it is medianly placed. In those species, where there are two vascular bundles (P. roxburghii) they are placed at right angles to each other. The vascular bundles are collateral, open and endarch. Xylem consists of radial rows of tracheids, alternating with rows of parenchyma cells and albuminous cells. Phloem consists of sieve cells and parenchyma. A cambium is present in the vascular bundle. It cuts off secondary phloem and little or no secondary xylem. One or two layers of secondary phloem is formed every year.
Xerophytic characters of Pinus needle:
1.The needles are needle - like and
2.Epidermal cells are thick - walled and covered by thick cuticle.
3.Sclerenchymatous hypodermis.
4.Sunken stomata.
5.Presence of resin ducts.
6.Peg -like infoldings of the walls in mesophyll cells.
7.Presence of sclerenchymatous tissue over the vascular bundles.
8.Presence of peculiar transfusion tissue.
9.Presence of simple vascular system.

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