Class: Coniferopsida
Order: coniferales
Family :Pinaceae
Genus: Pinus
Pinus is the most important genus of the family Pinaceae and is represented by 90 to 100 species in the world. They usually grow on the slopes of hills and form dense and extensive forest of evergreen trees in North temperate regions of the world.
In India the genus is represented by 6 species of Pinus.They are P. longifolia, P. roxburghii, P. wallichiana, P. gerardiana.
Pinus species are distributed at 1200 to 3,500 metres height above sea level particularly in Himachal Pradesh, Punjab, Sikkim, Nepal and hilly areas of Kashmir valley. In India some are introduced species and are cultivated in Araku valley in Andhra Pradesh and hilly areas of Nilgiris in Tamil Nadu.
Habit: The Pinus trees vary in height from 70 feet to 200 feet and reach a diameter of about 12 feet or even more.
Pinus plant is sporophyte. It is a tall evergreen, perennial, xerophytic tree. It is conical in appearance. Typically the stem displays the excurrent habit. It is straight and is covered with rugged scaly bark.
The main stem is branched and the branches are dimorphic, that is the branches are of two types - namely long branches and short or dwarf branches.
Long branches: These branches grows indefinitely by means of an apical bud. They are also called shoots of unlimited growth. They are covered with brown bud scales. They occur at regular intervals on the main trunk and are horizontal. They become gradually shorter towards the apex. Hence the pine tree presents a tapering or pyramidal appearance.
Dwarf or short branches: They arise on the i axils of scale leaves on the long branches. They are branches of limited growth, they are called dwarf shoots or shoots of limited growth. The dwarf shoots consists of a short axis terminating in a cluster of 3-5 green needles. It has an ephemeral terminal bud that stops growing and becomes inactive. It is covered with scales.
Leaves: The leaves are also of two kinds
Foliage leaves: These leaves are long, narrow, acircular, tough and green and known as pine needles. They are not born on the long shoots but they appear only on the dwarf shoots. The dwarf shoots bearing foliage leaves are known as spurs.
Long branches: These branches grows indefinitely by means of an apical bud. They are also called shoots of unlimited growth. They are covered with brown bud scales. They occur at regular intervals on the main trunk and are horizontal. They become gradually shorter towards the apex. Hence the pine tree presents a tapering or pyramidal appearance.
Dwarf or short branches: They arise on the i axils of scale leaves on the long branches. They are branches of limited growth, they are called dwarf shoots or shoots of limited growth. The dwarf shoots consists of a short axis terminating in a cluster of 3-5 green needles. It has an ephemeral terminal bud that stops growing and becomes inactive. It is covered with scales.
Leaves: The leaves are also of two kinds
Foliage leaves: These leaves are long, narrow, acircular, tough and green and known as pine needles. They are not born on the long shoots but they appear only on the dwarf shoots. The dwarf shoots bearing foliage leaves are known as spurs.
Different species of pinus have different number of foliage leaves on the dwarf shoots. For example P. monafolia has only one needle on the dwarf shoots, it is called as monofoliar spurs, P. merkusii has bipolar spurs, P. roxburghii has trifoliar spurs and P. wallichiana has pentafoliar spurs.
The needles are persistent. They fall only when the spur is shed as a whole. Hence the pine tree is evergreen.
Scale leaves: These leaves are brown, membranous and are protective in function. They are born both on the dwarf shoots as well as on the long shoots. They fall off as the branches mature. Male cones also arise in the axils of scale leaves on the longshoots. The scaly leaves on the dwarf shoots are called cataphylls and possess a distinct midrib.
Reduce size of leaves have been associated with xerophytic habitat of the species. It grows on slopes where rainwater is easily drained off. Extreme cold in winter also interferes with efficient root absorption by lowering temperature and thus causing physiological drought.
Roots: The plant has a taproot system. As the plant grows on shallow soil it is impossible for the tap root to penetrate, it remains short and develops strong lateral roots forming massive root system spread out over an extensive area.Tip of the root has a protective root cap. The root hairs are not well developed. The young roots are covered with ectotrophic fungus, the mycorrhiza. The mycorrhizal roots increases the water absorption by roots and the states of the function of root hair.
Mycorrhizic roots differ from the normal roots in the following aspects:
1. They are short and thick.
2. They lack root hairs.
3. They lack root caps.
4.They are more extensively branched.
5. They are covered with fungal hyphae.
Internal Morphology:
Anatomy of Root:
Transverse section of root is circular in outline. It consists of piliferous layer, cortex and stele. The arrangement of tissues is in general similar to those of the roots of dicotyledons.
Piliferous Layer: It is also called epiblema. It is outermost single layer. Gives out many, unicellular root hairs.
Cortex: It is multilayered, parenchymatous and forms the bulk of the root. It is bounded on the inside by endodermis.
Endodermis: It is single layered. The cells are of brownish orange colour. They are suberized and impregnated with tannin.
The needles are persistent. They fall only when the spur is shed as a whole. Hence the pine tree is evergreen.
Scale leaves: These leaves are brown, membranous and are protective in function. They are born both on the dwarf shoots as well as on the long shoots. They fall off as the branches mature. Male cones also arise in the axils of scale leaves on the longshoots. The scaly leaves on the dwarf shoots are called cataphylls and possess a distinct midrib.
Reduce size of leaves have been associated with xerophytic habitat of the species. It grows on slopes where rainwater is easily drained off. Extreme cold in winter also interferes with efficient root absorption by lowering temperature and thus causing physiological drought.
Roots: The plant has a taproot system. As the plant grows on shallow soil it is impossible for the tap root to penetrate, it remains short and develops strong lateral roots forming massive root system spread out over an extensive area.Tip of the root has a protective root cap. The root hairs are not well developed. The young roots are covered with ectotrophic fungus, the mycorrhiza. The mycorrhizal roots increases the water absorption by roots and the states of the function of root hair.
Mycorrhizic roots differ from the normal roots in the following aspects:
1. They are short and thick.
2. They lack root hairs.
3. They lack root caps.
4.They are more extensively branched.
5. They are covered with fungal hyphae.
Internal Morphology:
Anatomy of Root:
Transverse section of root is circular in outline. It consists of piliferous layer, cortex and stele. The arrangement of tissues is in general similar to those of the roots of dicotyledons.
Piliferous Layer: It is also called epiblema. It is outermost single layer. Gives out many, unicellular root hairs.
Cortex: It is multilayered, parenchymatous and forms the bulk of the root. It is bounded on the inside by endodermis.
Endodermis: It is single layered. The cells are of brownish orange colour. They are suberized and impregnated with tannin.
Stele: Pericycle is multilayered. The pericycle cells contain starch and are usually impregnated with tannin. Lateral roots arise from the pericycle.
Stele consists of 2-6 vascular bundles. The vascular bundles are radial i.e., the phloem alternates with xylem.
The xylem is diarch to hexarch and exarch in condition.
Each protoxylem group is forked so that the xylem bundle appears Y- shaped. A resin passage lies between the arrows of the Y.
Pith: The pith is very small.
Stele consists of 2-6 vascular bundles. The vascular bundles are radial i.e., the phloem alternates with xylem.
The xylem is diarch to hexarch and exarch in condition.
Each protoxylem group is forked so that the xylem bundle appears Y- shaped. A resin passage lies between the arrows of the Y.
Pith: The pith is very small.
Anatomy of stem:
Young pinus stem shows epidermis cortex and stele.
Epidermis:
Epidermis layer and irregular in outline due to the presence of scale leaves. The epidermal cells are highly thickened with wall strongly cutinized.
Cortex:
The cortex is multilayered and lies below the epidermis. The outermost layers of the cortex are sclerenchymatous and forms the hypodermis.
The inner cortex consists of thin walled parenchymatous cells. The inner cortex shows presence of large resin ducks or canals. Each duct or canal is lined by a layer of thin walled parenchymatous glandular secretory cells constituting the epithelium. External to the epithelial layer is the sclerotic sheath consisting of the thick walled cells. The epithelial cells secret resin into the canal. The resin flows out when an injury breaks the tissue open. The resin serves as a valuable protection against the invasion of wounded tissue by fungi or bacteria. Resin is the chief source of natural turpentine.
Neither endodermis nor pericycle is recognisable.
Stele
It consists of a ring of five to six separate vascular bundles. The vascular bundles are collateral, open and endarch. Each vascular bundle has phloem on the outside and xylem on the inner side with primary cambium in between the two. Xylem consists of tracheids and xylem parenchyma only, xylem vessels are absent. The phloem is made up of sieve tubes and phloem parenchyma companion cells are absent
Primary medullary rays:
they lie between the primary vascular bundles and connect the pic with the cortex they are parenchymatous and narrow.
Pith:
Present in the centre of the stem. It forms the core of the woody stem of Pinus.
Young pinus stem shows epidermis cortex and stele.
Epidermis:
Epidermis layer and irregular in outline due to the presence of scale leaves. The epidermal cells are highly thickened with wall strongly cutinized.
Cortex:
The cortex is multilayered and lies below the epidermis. The outermost layers of the cortex are sclerenchymatous and forms the hypodermis.
The inner cortex consists of thin walled parenchymatous cells. The inner cortex shows presence of large resin ducks or canals. Each duct or canal is lined by a layer of thin walled parenchymatous glandular secretory cells constituting the epithelium. External to the epithelial layer is the sclerotic sheath consisting of the thick walled cells. The epithelial cells secret resin into the canal. The resin flows out when an injury breaks the tissue open. The resin serves as a valuable protection against the invasion of wounded tissue by fungi or bacteria. Resin is the chief source of natural turpentine.
Neither endodermis nor pericycle is recognisable.
Stele
It consists of a ring of five to six separate vascular bundles. The vascular bundles are collateral, open and endarch. Each vascular bundle has phloem on the outside and xylem on the inner side with primary cambium in between the two. Xylem consists of tracheids and xylem parenchyma only, xylem vessels are absent. The phloem is made up of sieve tubes and phloem parenchyma companion cells are absent
Primary medullary rays:
they lie between the primary vascular bundles and connect the pic with the cortex they are parenchymatous and narrow.
Pith:
Present in the centre of the stem. It forms the core of the woody stem of Pinus.
Anatomy of leaf or Pinus Needle:
The transverse section of Pinus needle is shaped like a tri- sector of a circle with curved surface facing outwards and the vertex inwards.
It shows epidermis, mesophyll and vascular bundle.
Epidermis:
The outer boundary of the leaf is a single layered epidermis. The epidermal cells are thick walled and covered on the outside by a thick cuticle. The deeply sunken stomata are present on all the sides of the leaf. So the needle is amphistomatic. Their guard cells are situated well below the level of the epidermis.
Hypodermis:
Epidermis is followed by hypodermis. It consists of 1 or 2 layers of sclerenchyma cells. Thehe hypodermis is interrupted by air cavities beneath the stomata.
Mesophyll:
Below the hypodermis is parenchymatous mesophyll. It is compact and shows no differentiation into pallisade and spongy tissue. It consists of thin walled cells which contain numerous chloroplast.
The cells shows numerous infoldings of cellulose walls into their cavities, they are called arms. These arms serve to increase the assimilatory, absorptive, aerating surface in each cell and thus compensate for reduced leafy surface.
The mesophyll contains a number of resin ducts. They are similar in structure to those of the stem.
Endodermis:
The vascular bundle is enclosed by a conspicuous single layered endodermis. The cells are large oval or barrel shaped and possess casparian strips.
Pericycle:
vitamin d endodermis lies the mini layered pericycle it consists of a number of different types of cells of the cells constitute the pericycle or ordinary parenchymatous cells containing starch
embedded in this parenchymatous cells are two types of other cells
The first type of cells are parenchymatous and rich in proteins. They are called the albuminous cells. The albuminous cells lie along the phloem of the vascular bundles. The albuminous cells serve to translocate food from mesophyll to the phloem.
The second type of the cells resemble the trachieds. They are known as tracheidial cells. They are longitudinally elongated, bordered pitted and dead. Tracheidial cells are contiguous to the xylem of the vascular tissue and thus helps in conduction of water from xylem to mesophyll.
This special kinds of cells collectively constitute the transfusion tissue. Transfusion tissue makes up for the poor development of the vascular tissue.
Vascular Bundles:
Embedded in the pericycle are two vascular bundles. They are separated by a band of sclerenchymatous fibres. The vascular bundles run parallel from base to the apex of leaf.
Each vascular bundles is collateral and open. The xylem is towards pointed side and phloem is towards the convex side of the leaf.
Xerophytic characters of Pinus Needle:
1. Acircular form of the leaves, which reduces the transpiring leaf surface.
2. Thick walled and heavily cutinized epidermis.
3. Sclerenchymatous hypodermis which renders the leaf tough in texture.
4. Sunken and amphistomatic stomata
5. The inner walls of mesophyll cells have peg like infoldings.
6. Many layered pericycle with transfusion tissue
7. Poor development of vascular tissues
Pollination:
Pollination is by wind, therefore, anemophilous. It takes place towards the end of May. The microsporangium or pollen sac opens by a medium longitudinal slit along its under side. On slight shaking the tree will emit clouds of yellow dust which rise into the air and colour the whole landscape in the pine forests.
The pollen grains are blown by the wind and reach female cones and are trapped between the scales. From there they roll down to obliquely tilted scales and finally come to rest close to the micropyle of the ovule.
Gametophytes:
Pinus is a heterosporous plant which bears pollen grains (microspore) and megaspore. Both microspore mother cell and megaspore mother cell which serve as a last stages of sporophyte generation (2n) undergo reduction division while still present in the sporangia to produce a large number of functional microspores and four megaspores ( only one functional megaspore).
Male Gametophyte
The microspore or pollen grain is the first cell of the male gametophyte.
Microspore or pollen grain germinates into a male gametophyte. It starts germination before it is liberated from the pollen sac.
The pollen grain nucleus undergoes a periclinal division to form a small lens shaped cell called prothallus cell and a large central or antheridial cell.
The antheridial cell again divides and forms a second prothalial cell on the top of the first. Both the prothalial cells soon degenerate and disappear.
The antheridial cell again divides into two cells, a generative cell and a tube cell. At this 4-celled stage the pollen grain is liberated. Further development of male gametophyte takes place on the ovule after pollination.
Development of male gametophyte after pollination:
When the liberated pollen is lodged in the ovule and makes contact with the tip of the nucellus, it undergoes further development.
The exine of the pollen grain bursts and the tube cell protrudes and grows out to form a pollen tube. The tube nucleus moves down to its advancing tips whereas the generative cell remains in the pollen grain end.
The pollen tube grows down into the nucellar tissue upon which it is now dependent for its nourishment and protection. The pollen tube rests for about a year in this condition. The ovuliferous scales, during this period, have thickened and closed the cone completely.
Pollen tube becomes active again in the winter. It penetrates the nucellar tissue. The generative cell divides to give rise to a barren stalk cell or sterile cell and a fertile body cell or spermatogenous cell. The former develops no further. The body cell along with stalk cell passes down the pollen tube.
On its way the body cell divides into two unequal cells - the male cells or the gametes. The gametes are formed only a week before fertilisation.
Female Gametophyte
The haploid megaspore is the first cell of the female gametophyte. It is never shed and is retained permanently within the nucellus. By the end of first year i.e., after pollination, it starts to germinate.
The megaspore increases in size and its nucleus undergoes many free nuclear divisions producing about 2,000 - 2,500 free daughter nuclei. These daughter nuclei, become arranged in a peripheral layer surrounding a large central vacuole. The walls are then laid between these nuclei. There is also a gradual accumulation of reserve food materials n the gametophyte.
Nuclear division followed by cross walls result in an increase in the number of cells. The megaspore is now filled with solid mass of thin-walled parenchymatous tissue. It is the female gametophyte or endosperm. The megaspore containing the female prothallus is enclosed in the nucellus (diploid tissue).
From the superficial cells of the female prothallus are developed 2-4 archegonia. The archegonial initials are superficial cells larger in size, prominent nuclei and vacuolated contents. The initial divides by a periclinal wall into an upper primary neck cell and a lower central cell.
The primary neck cell divides by two vertical walls into a tier of four cells that constitute the neck of archegonium. The central cell undergoes rapid enlargement and its cytoplasm shows conspicuous vacuoles, which later disappear. By this time a distinct jacket layer surrounds the central cell. The nucleus of the central cell divides into two daughter nuclei and a wall is laid down between them to form a small lenticular ventral canal cell and a large egg cell.
Archegonium
The mature female gametophyte is a small ovoid structure. Each archegonium consists of a short neck and a swollen venter. The neck consists of four cells arranged in a single tier. There are no neck canal cells. The venter contains the oosphere or the egg and the ventral canal cells. Venter is embedded in the tissue of the prothallus. The cells of the prothallus adjacent to the neck, grow faster than the neck cells so that the neck eventually lies in a depression called the archegonial chamber.
Fertilization:
It takes place after about a year of pollination. The pollen tube containing a tube nucleus, one stalk nucleus and two male gametes gradually elongates and reaches the archegonium.
Finally the tip of the pollen tube enters the neck of the archegonium after rupturing it an swells at its tip. The pollen tube ruptures in the venter and the two male gametes are discharged into the venter. Out of two, one male gamete disintegrate. The nucleus of the surviving male cell fertilizes the egg and zygote is formed.
The transverse section of Pinus needle is shaped like a tri- sector of a circle with curved surface facing outwards and the vertex inwards.
It shows epidermis, mesophyll and vascular bundle.
Epidermis:
The outer boundary of the leaf is a single layered epidermis. The epidermal cells are thick walled and covered on the outside by a thick cuticle. The deeply sunken stomata are present on all the sides of the leaf. So the needle is amphistomatic. Their guard cells are situated well below the level of the epidermis.
Hypodermis:
Epidermis is followed by hypodermis. It consists of 1 or 2 layers of sclerenchyma cells. Thehe hypodermis is interrupted by air cavities beneath the stomata.
Mesophyll:
Below the hypodermis is parenchymatous mesophyll. It is compact and shows no differentiation into pallisade and spongy tissue. It consists of thin walled cells which contain numerous chloroplast.
The cells shows numerous infoldings of cellulose walls into their cavities, they are called arms. These arms serve to increase the assimilatory, absorptive, aerating surface in each cell and thus compensate for reduced leafy surface.
The mesophyll contains a number of resin ducts. They are similar in structure to those of the stem.
Endodermis:
The vascular bundle is enclosed by a conspicuous single layered endodermis. The cells are large oval or barrel shaped and possess casparian strips.
Pericycle:
vitamin d endodermis lies the mini layered pericycle it consists of a number of different types of cells of the cells constitute the pericycle or ordinary parenchymatous cells containing starch
embedded in this parenchymatous cells are two types of other cells
The first type of cells are parenchymatous and rich in proteins. They are called the albuminous cells. The albuminous cells lie along the phloem of the vascular bundles. The albuminous cells serve to translocate food from mesophyll to the phloem.
The second type of the cells resemble the trachieds. They are known as tracheidial cells. They are longitudinally elongated, bordered pitted and dead. Tracheidial cells are contiguous to the xylem of the vascular tissue and thus helps in conduction of water from xylem to mesophyll.
This special kinds of cells collectively constitute the transfusion tissue. Transfusion tissue makes up for the poor development of the vascular tissue.
Vascular Bundles:
Embedded in the pericycle are two vascular bundles. They are separated by a band of sclerenchymatous fibres. The vascular bundles run parallel from base to the apex of leaf.
Each vascular bundles is collateral and open. The xylem is towards pointed side and phloem is towards the convex side of the leaf.
Xerophytic characters of Pinus Needle:
1. Acircular form of the leaves, which reduces the transpiring leaf surface.
2. Thick walled and heavily cutinized epidermis.
3. Sclerenchymatous hypodermis which renders the leaf tough in texture.
4. Sunken and amphistomatic stomata
5. The inner walls of mesophyll cells have peg like infoldings.
6. Many layered pericycle with transfusion tissue
7. Poor development of vascular tissues
Reproduction
Vegetative reproduction is unknown in pines. Reproduction involves both reproduction by spores and reproduction by gametes.
Reproduction by spores or asexual reproduction
All seed plants are heterosporous which means that the reproductive organs called the spores are of two kinds - microspores or pollen grains and Megaspores or embryo sac. They are produced in specialised structure called microsporangia or Pollen sacs and megasporangia ( ovules) respectively. The sporangia are born on sporophylls which are organised to form cones are strobili. The cones are of two types one containing only microsporangia called the male cones or microsporangiate strobili and other containing megasporangia called female cones or megasporangiate strobili.
Pinus is monoecious as it bears both types of cones on the same tree on separate branches.
The microsporangiate or staminate or male cones
Vegetative reproduction is unknown in pines. Reproduction involves both reproduction by spores and reproduction by gametes.
Reproduction by spores or asexual reproduction
All seed plants are heterosporous which means that the reproductive organs called the spores are of two kinds - microspores or pollen grains and Megaspores or embryo sac. They are produced in specialised structure called microsporangia or Pollen sacs and megasporangia ( ovules) respectively. The sporangia are born on sporophylls which are organised to form cones are strobili. The cones are of two types one containing only microsporangia called the male cones or microsporangiate strobili and other containing megasporangia called female cones or megasporangiate strobili.
Pinus is monoecious as it bears both types of cones on the same tree on separate branches.
The microsporangiate or staminate or male cones
The male cones occur in clusters of 15-35 in the terminal region of dwarf branches in the axil if a scale leaf. Male cones develop during March to April months in the species growing in hilly areas and during the January in plain areas
Male cones are modified dwarf branches.t
They consist of an elongated central axis bearing a number of small spirally arranged and closely fitting scale like microsporophylls. The microsporophylls are equivalent to stamens of angiosperms.
Each microsporophyll originate from the central axis runs out horizontally to end in a sterile flattened head, the tip of which is turned back side. This apical sterile part is called the apophysis. Two microsporangia or pollen sacs develop on the lower or abaxial surface of the microsporophylls. The microsporophyll is attached to the axis by a short stalk. This are filled with numerous winged microspores. A number of scales at the base of male cone remain small and sterile. Young cones are green or reddish purple but become yellow or dark brown as they mature
Development of Microsporangia:
Development of Microsporangia is eusporangiate type. Group of meristematic cells divide and form the epidermis and hypodermis cells.
The hypodermal cells divide and form the outer parietal cells and inner archesporial cells.
The parietal cells divide anticlinally and periclinally to form a 4 layered wall of the sporangium. The outer wall is called the epidermis. The innermost wall layer differentiates into tapetum whose cells have prominent nuclei and dense cytoplasmic contents. At the tetrad stage the tapetal cells become binucleate.
The archesporial cells divide to form sporogenous cells which ultimately differentiate into microspore mother cells. These cells are polygonal in outline and contain prominent nuclei and vacuolated cytoplasm.
The microspore mother cells undergo meiosis and form tetrads of microspores. Later the spores seperate from the tetrads. They are haploid and two winged.
Microspore:
It is unicellular. The spore wall is three layered. The outer exine, the middle exointine and the innermost intine. The exine is cuticularized. It covers the spore on one side. The rest of the spore is covered by the exointine.
The exointine becomes inflated into two balloon like expansions or wings. These are not cells but air sacs or bladders. They render the grains extremely light and thus facilitate the dispersal by wind.
Male cones are modified dwarf branches.t
They consist of an elongated central axis bearing a number of small spirally arranged and closely fitting scale like microsporophylls. The microsporophylls are equivalent to stamens of angiosperms.
Each microsporophyll originate from the central axis runs out horizontally to end in a sterile flattened head, the tip of which is turned back side. This apical sterile part is called the apophysis. Two microsporangia or pollen sacs develop on the lower or abaxial surface of the microsporophylls. The microsporophyll is attached to the axis by a short stalk. This are filled with numerous winged microspores. A number of scales at the base of male cone remain small and sterile. Young cones are green or reddish purple but become yellow or dark brown as they mature
Development of Microsporangia:
Development of Microsporangia is eusporangiate type. Group of meristematic cells divide and form the epidermis and hypodermis cells.
The hypodermal cells divide and form the outer parietal cells and inner archesporial cells.
The parietal cells divide anticlinally and periclinally to form a 4 layered wall of the sporangium. The outer wall is called the epidermis. The innermost wall layer differentiates into tapetum whose cells have prominent nuclei and dense cytoplasmic contents. At the tetrad stage the tapetal cells become binucleate.
The archesporial cells divide to form sporogenous cells which ultimately differentiate into microspore mother cells. These cells are polygonal in outline and contain prominent nuclei and vacuolated cytoplasm.
The microspore mother cells undergo meiosis and form tetrads of microspores. Later the spores seperate from the tetrads. They are haploid and two winged.
Microspore:
It is unicellular. The spore wall is three layered. The outer exine, the middle exointine and the innermost intine. The exine is cuticularized. It covers the spore on one side. The rest of the spore is covered by the exointine.
The exointine becomes inflated into two balloon like expansions or wings. These are not cells but air sacs or bladders. They render the grains extremely light and thus facilitate the dispersal by wind.
The Megasporangiate or Ovulate Cones:
They arise single or in a small cluster of two or four on the axial region of scaly leaves of the shoots of unlimited growth. When young the Ovulate Cones are small, erect, reddish to pinkish structure.
It consists of a central axis which bears paired scales in a close spiral. The lower scale of the pair is small and is called bract scale. It is leathery and directly attached to the cone axis. The upper scale of the pair is larger, thicker and stouter and is called ovuliferous scale. It develops from the upper surface of the bract scale.
Ovuliferous scales bears two ovules side by side on its upper side. The bract scale is larger than the ovuliferous scale before pollination but later it grows bigger than the bract scale. The ovuliferous scale is woody and wedge shaped with its broader sterile end, the apophysis directed upwards.
Development of the Ovule:
On the upper side near the base of the ovuliferous scale the Ovule arise as a small white swelling consisting of a group of parenchymatous cells, the nucellus. The nucellus is covered by a layer called the integument. The integument encloses the nucellus completely leaving only a narrow passage, the micropyle at the top. The micropyle faces the base of the scale and is directed downwards towards the axis of the cone.
A hypodermal cell with denser contents appears near the apex of the nucellus. It enlarges to form a single large archesporial cell which divides to form an outer primary parietal cell and the inner sporogenous cell which functions as the megaspore mother cell.
The megaspore mother cell divides by meiosis into a tetrad of four small haploid megaspores arranged in a row. Of these three degenerate and only one, the innermost enlarges into a female gametophyte. Development of embryosac or female gametophyte is monosporic.
They arise single or in a small cluster of two or four on the axial region of scaly leaves of the shoots of unlimited growth. When young the Ovulate Cones are small, erect, reddish to pinkish structure.
It consists of a central axis which bears paired scales in a close spiral. The lower scale of the pair is small and is called bract scale. It is leathery and directly attached to the cone axis. The upper scale of the pair is larger, thicker and stouter and is called ovuliferous scale. It develops from the upper surface of the bract scale.
Ovuliferous scales bears two ovules side by side on its upper side. The bract scale is larger than the ovuliferous scale before pollination but later it grows bigger than the bract scale. The ovuliferous scale is woody and wedge shaped with its broader sterile end, the apophysis directed upwards.
Development of the Ovule:
On the upper side near the base of the ovuliferous scale the Ovule arise as a small white swelling consisting of a group of parenchymatous cells, the nucellus. The nucellus is covered by a layer called the integument. The integument encloses the nucellus completely leaving only a narrow passage, the micropyle at the top. The micropyle faces the base of the scale and is directed downwards towards the axis of the cone.
A hypodermal cell with denser contents appears near the apex of the nucellus. It enlarges to form a single large archesporial cell which divides to form an outer primary parietal cell and the inner sporogenous cell which functions as the megaspore mother cell.
The megaspore mother cell divides by meiosis into a tetrad of four small haploid megaspores arranged in a row. Of these three degenerate and only one, the innermost enlarges into a female gametophyte. Development of embryosac or female gametophyte is monosporic.
Pollination:
Pollination is by wind, therefore, anemophilous. It takes place towards the end of May. The microsporangium or pollen sac opens by a medium longitudinal slit along its under side. On slight shaking the tree will emit clouds of yellow dust which rise into the air and colour the whole landscape in the pine forests.
The pollen grains are blown by the wind and reach female cones and are trapped between the scales. From there they roll down to obliquely tilted scales and finally come to rest close to the micropyle of the ovule.
Gametophytes:
Pinus is a heterosporous plant which bears pollen grains (microspore) and megaspore. Both microspore mother cell and megaspore mother cell which serve as a last stages of sporophyte generation (2n) undergo reduction division while still present in the sporangia to produce a large number of functional microspores and four megaspores ( only one functional megaspore).
Male Gametophyte
The microspore or pollen grain is the first cell of the male gametophyte.
Microspore or pollen grain germinates into a male gametophyte. It starts germination before it is liberated from the pollen sac.
The pollen grain nucleus undergoes a periclinal division to form a small lens shaped cell called prothallus cell and a large central or antheridial cell.
The antheridial cell again divides and forms a second prothalial cell on the top of the first. Both the prothalial cells soon degenerate and disappear.
The antheridial cell again divides into two cells, a generative cell and a tube cell. At this 4-celled stage the pollen grain is liberated. Further development of male gametophyte takes place on the ovule after pollination.
Development of male gametophyte after pollination:
When the liberated pollen is lodged in the ovule and makes contact with the tip of the nucellus, it undergoes further development.
The exine of the pollen grain bursts and the tube cell protrudes and grows out to form a pollen tube. The tube nucleus moves down to its advancing tips whereas the generative cell remains in the pollen grain end.
The pollen tube grows down into the nucellar tissue upon which it is now dependent for its nourishment and protection. The pollen tube rests for about a year in this condition. The ovuliferous scales, during this period, have thickened and closed the cone completely.
Pollen tube becomes active again in the winter. It penetrates the nucellar tissue. The generative cell divides to give rise to a barren stalk cell or sterile cell and a fertile body cell or spermatogenous cell. The former develops no further. The body cell along with stalk cell passes down the pollen tube.
On its way the body cell divides into two unequal cells - the male cells or the gametes. The gametes are formed only a week before fertilisation.
Female Gametophyte
The haploid megaspore is the first cell of the female gametophyte. It is never shed and is retained permanently within the nucellus. By the end of first year i.e., after pollination, it starts to germinate.
The megaspore increases in size and its nucleus undergoes many free nuclear divisions producing about 2,000 - 2,500 free daughter nuclei. These daughter nuclei, become arranged in a peripheral layer surrounding a large central vacuole. The walls are then laid between these nuclei. There is also a gradual accumulation of reserve food materials n the gametophyte.
Nuclear division followed by cross walls result in an increase in the number of cells. The megaspore is now filled with solid mass of thin-walled parenchymatous tissue. It is the female gametophyte or endosperm. The megaspore containing the female prothallus is enclosed in the nucellus (diploid tissue).
From the superficial cells of the female prothallus are developed 2-4 archegonia. The archegonial initials are superficial cells larger in size, prominent nuclei and vacuolated contents. The initial divides by a periclinal wall into an upper primary neck cell and a lower central cell.
The primary neck cell divides by two vertical walls into a tier of four cells that constitute the neck of archegonium. The central cell undergoes rapid enlargement and its cytoplasm shows conspicuous vacuoles, which later disappear. By this time a distinct jacket layer surrounds the central cell. The nucleus of the central cell divides into two daughter nuclei and a wall is laid down between them to form a small lenticular ventral canal cell and a large egg cell.
Archegonium
The mature female gametophyte is a small ovoid structure. Each archegonium consists of a short neck and a swollen venter. The neck consists of four cells arranged in a single tier. There are no neck canal cells. The venter contains the oosphere or the egg and the ventral canal cells. Venter is embedded in the tissue of the prothallus. The cells of the prothallus adjacent to the neck, grow faster than the neck cells so that the neck eventually lies in a depression called the archegonial chamber.
Fertilization:
It takes place after about a year of pollination. The pollen tube containing a tube nucleus, one stalk nucleus and two male gametes gradually elongates and reaches the archegonium.
Finally the tip of the pollen tube enters the neck of the archegonium after rupturing it an swells at its tip. The pollen tube ruptures in the venter and the two male gametes are discharged into the venter. Out of two, one male gamete disintegrate. The nucleus of the surviving male cell fertilizes the egg and zygote is formed.
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