Pinus

Occurrence

The genus Pinus has about 90 species. It has world wide in distribution. They are mostly present in the temperate regions. Four species of pinus are found in Pakistan: Pinus wallichiana; Pinus halepensis; Pinus roxburghil; Pinus gerardiana.

GENERAL STRUCTURE

The plant body is sporophyte. It is an evergreen tall tree. Plant body is divided into root, stem and leaves Stem: The main trunk is unbranched. It has branchesconfined to the apical region. These branches form a characteristic canopy. It is covered with a scaly bark. Its branches are dimorphic.

(a)Branches of unlimited growth: The main branches have an unlimited growth. They bear only scale leaves.

(b)Branches of limited growth: Branches of limited growth or dwarf branches are produced in the axil of the scale leaves on the main branches. These are about 1 -2cm long. These are covered by one or two scale leaves. These branches also bear foliage leaves. A dwarf shoot with its foliage leaves is called spur.

Leaf:Pinus has two types of leaves: scale leaves and foliage leaves.

(a) Scale leaves: The scale leaves are small, membranous and brownish in colour. These are protective in function. They are present on the main and dwarf shoots.

(b)Foliage leaves: The foliage leaves are green and needle-like. They are found only on the dwarf shoots forming the spur.

Roots: Pinus has a well developed tap root. It remains short and grows on hard ground or rocks. The lateral roots are well developed. These roots spread over a large area. Young roots are infested with a fungus to form mycorrhizae.

Fig: TS of stem of pinus

Internal Structure of the Stem

Stem is differentiated internally into epidermis, cortex, vascular tissue and central pith.

1.  Epidermis:
   The surface is covered by an epidermis. It consists of a single layer of cells. Outer walls of these cells are highly cutinized. Below the epidermis is a hypodermis which is formed of layers of lignified cells.
2. Cortex:The cortex is formed of parenchymatous cells. Cortex has a large number of resin canals.Each resin canal is surrounded by a layer of resin secreting glandular epithelial cells.
3. Vascular bundler:The vascular bundles are conjoint, collateral and open. They form a ring around the pith. In each vascular bundle the xylem is towards the innerside and phloem towards the outerside.
   A narrow strip of cambium is present between xylem and phloem. Pericycle is present outer to the ring of vascular bundles. A single layered thin walled endodermis is present outside the pericycle. Secondary growth takes place by the activity of cambium ring. There are distinct annual growth rings in the wood.
4. Bark:
   Phellogen originates in the deeper layers of the cortex. It is present in the form of strips. It produces characteristic scaly bark.

INTERNAL STRUCTURE OF LEAF

A transverse section shows following internal parts of the leaf:

1. Epidermis:
   Leaf is covered by thick walled epidermis. Epidermis is covered by a thick layer of cuticle. Sunken stomata are present below the general surface. Two or three layered hypodermis is present underneath the epidermis. Hypodermis is composed of sclerenchymatous tissues. This hypodermis is the main strengthening tissue of the leaf.
2. Mesophyll:The mesophyll of the leaf is parenchymatous. It is not differentiated into palisade and spongy parenchyma. Resin canals are present below the hypodermis. Each resin canal is lined by a layer of small epithelial cells. Each leaf is supplied by two unbranched veins.
3.  Endodermis and vascular tissues:Endodermis is present outside the pericycle. Pericycle surrounds the xylem. Pericycle is formed of parenchymatous cells. Its cells adjacent to the phloem are called aluminous cells. The cells adjacent to the xylem are called tracheidal cells. These specialized cells form the transfusion tissue. They help in the lateral flow of nutrients.

INTERNAL STRUCTURE OF ROOT

young root is di- to pentarch. Xylem bundles are Y-shaped. Siwtil parenchymatous pith may be present or absent in the centre. The ring of vascular bundles is surrounded by two to three layered pericycles. Pericycle is surrounded by endodermis. The cortex consists of a few layers of thin walled parenchymatous cells. Root hairs may be present or absent. Roots may be covered by mycorrhizal fungus.

REPRODUCTION

Pinus is monoecious. Plant develops both male and female strobili on the same plant. The strobili are monosporous. There is no vegetative reproduction in Pinus.

Male Cone

Fig: Pinus male cones

The male cones are much smaller. They are produced in clusters near the tip of the long shoots. The male cones are produced in the spring. Each male cone has a central axis. It bears a number of spirally arranged microsporophylls or stamens. Each microsporophyll has sac like microsporangia (pollen sacs) on the ventral side. Each microsporangium produces a large number of microspores(pollen grains). The wall of each microspore (pollen grain) consists of an inner intine and an outer exine. It has balloon like wings. The wings help in the dispersal of spores by wind.

DEVELOPMENT OF MICROSPORANGIUM (STAMEN)

1. A number of hypodermal cells act sporangial initials. The sporangial initials divides to form outer wall initials and the inner archesporial initial
2. The wall initials divide to form a many layered wall of the sporangium. The archesporial initials also increase in number by the repeated divisions. The peripheral cells of the archesporium form the tapetum.

3. Some of the archesporial cells are transformed into microspore mother cells. The remaining archesporial cells and the tapetal layer provide nourishment to the developing microspore mother cells.
4. The microspore mother cell divides by meiosis to form four microsporesor pollen grains. The exine of spore forms wings.
   The pollen grain divides in to smallerand larger cells.
   The smaller cell again divides to form two small prothalial cells.The larger cell becomes antheridial cell. The sporangium splits and microspores are released from the microsporangia at this stage.

MALE PROTHALLUS

1. The pollen grain has two prothalial cells and an antheridial cell. These cells soon disintegrate. Further development of the pollen grain takes place at the surface of the nucellus.
2. The antheridial cell cut off second prothalial cell.
3. It also cut off generative cell adjacent to the prothalial cells.
4. The remaining large cell is known as the tube cell. Its nucleus is called tube nucleus. Tube nucleus controls the growth of the pollen tube.
5. Exine of the pollen grain ruptures.
6. Intine grows out to form the pollen tube. It grows through the nucellus. But its activity stops till spring.
7. Female cone enlarges very much in size after pollination. Outer ends of the ovuliferous scales increase very much. They meet each other to close the gaps in between them. The cone is covered with a lot of resinous secretions.

FEMALE CONE

Fig: Female cone of Pinus

Female cones are produced in the axils of the scale leaves. The production of female cones is initiated in the winter.These become ready for pollination during the following spring. Each young female cone has a central axis. It bears spirally arranged scales. The scales are of two types. Some are thin membranousand are directly attached to the central axis. They are called bract scales. Woody ovuliferous scales are present on the ventral surface of each bract scale. The broader end of the ovuliferous scale has projection called the umbo. Each ovuliferous scale bears two ovules. They are situated side by side on upper side. Each ovule (megasporangium) has a mass of nucellar tissue. They are surrounded by a single integument. The micropylar end of the ovule is directed towards the central axis. A single megaspore mother cell is differentiated in the nucellus near the micropylar end. This megaspore mother cell undergoes meiosis to form four megaspores. Only the lower mostmegaspore remains functional. The others disintegrate. Functional megaspore (embryo sac) increases in size. It occupies the major part of the nucellus. Pollination takes place at this stage.

FEMALE PROTHALLUS

The megaspore divides many times to form female prothallus. Megaspore wall encloses the female prothallus. Three archegonia are produced towards the micropylar end.
Each archegonium develops from a single prothalial cell.
Archegonia consist of a large venter and a short neck.The oosphere or egg is very large. It is bounded by the prothalial cells.
There is a small ventral canal cell below the neck.
The neck is without any neck canal cells.
The prothalial tissue enlarges very much in size. It crushes all nucellar tissue except a small amount near the micropylar end.

POLLINATION

Each ovule secretes a mucilaginous drop at the micropylar end. A gap is produced between the ends of the ovuliferous scales. It forms a passage for the entry of pollen grains. Wind carried pollen grains. The mucilage drop entangles the pollen grain. Pollen grain is carried through the micropyle to the surface of the nucellus.

Formation of Embryo and Seed

Diploid nucleus divides thrice to form eight cells. The lower four cells becomes proembryonal cells. The upper four nuclei are separated by incomplete cell walls. Four proembryonal divides to produce three tiers of cells:

1. Embryonal cell: The cells of the lower tier become embryonal cells. The four embryonal cells separate from each other. Each develops into a separate embryo independently. Each embryonal cell forms secondary suspensor cells. The formation of more than one embryo from a single fertilized oosphere is called polyembryony. Only one embryo reaches maturity. The rest are aborted.

1. Suspensor cells: The cells of the middle tier become suspensor cells. Suspensor cells elongate very much. It pushes the developing embryos into the prothalial tissue for nutrition.
2. Rosette cells: The cells of the upper most tiers are called the rosette cells. These cells do not take part in the development of the embryo.

A fully developed embryo is in the form a short straight axis. Its radicle is present towards the micropylar end. Plumule is present towards the inner side. Plumule is surrounded by ten cotyledons. The unutilized prothalial tissue forms the endosperm. The persistant nucellus tissues near the micropylar end form the perisperm. The integument becomes hard testa. Some part of the ovuliferous scale fuses with the developing seed. It makes a large wing for dispersal of seed. The axis of the female cone rapidly increases. It produces gaps in ovuliferous scales. The cone becomes woody for the dispersal of seeds.

GERMINATION OF SEED

The radicle grows out. It splits the testa at the micropylar end. This radicle grows down into the soil and forms the primary root. The hypocotyl elongates to form a loop. Then it becomes straight. It carries with it the plumule and the cotyledons. The testa is also carried up with the cotyledons.

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Cycas

Cycas

Systematic Position

Division: Gymnospermae
Class      : Cycadophyta
Order     : Cycadales
Family   : Cycadaceae
Genus    : Cycas

Occurrence

Cycas has a limited distribution. It is found in south East Asia, South China, Southren India, South Japan and Australia. The plants grow under xerophytic conditions. Cycas is also cultivated as an ornamental plant in the gardens and parks. Cycas plants live upto hundred years or more. Cycas is called a living fossil.

PLANT BODY

The plant body is a sporophyte. It is differentiated into root, stem and leaves.

1. Stem: Stem is unbranched. It is covered by thick, woody, persistent leaf bases. It makes the stem rough. The apex of the stem is ensheathed by a group of brown scales. The lower of stem is covered by pinnate compound leaves. The growth of the stem is very slow. It produces a cluster of leaves each year. Older leaves fall of after two years.
2. Leaves and scales: The leaves are produced in the axils of the scalesnear the apex. Each leaf is composed of a petiole, rachis and lateral pinnae.The young leaves show circinnate vernation. Scales are also produced each year. Therefore, the clusters of green leaves and scales alternate with each other. Scales are also persistent. Scales and leaf bases cover the surface of the old stem.
   
   

   Fig: Circinate Vernation of Leaf
3. Roots: The primary root persists in Cycas. It becomes tuberous. Cycas produces coralloid roots.Coralloid roots are short tufts and dichotomously branched roots. These roots contain an endophytic alga in the inner part of their cortex. Sometimes, bacteria are also present in the cortex. Bacteria fix nitrogen.

INTERNAL STRUCTURE OF STEM

Stem is internally composed of outer epidermis, cortex and vascular bundles.

1. The cortex is very wide. It is composed of parenchymatous cells. Parenchymatrs tissue of the cortex contains numerous mucilaginous canals.
2. Primary vascular bundles are present inside the cortex. They surround the central pith. These bundles are collateral. The primary medullary rays present between the adjacent bundles.
3. Cambium is present in the form of a narrow strip in each vascular bundle. The xylem is endarch. The strength of the stem is mainly due to the presence of woody leaf bases on the surface.
4. Phellogen is produced in the periphery of the cortex. It produces cork (peridium) under the persistent leaf bases.

Fig: T.S of Stem of cycas

Fig: T.S of Stem

INTERNAL STRUCTURE OF THE LEAF

Each pinna of leaf has typical bifacialstructure in transverse section.

1. Surface of leaf is covered by a single layered epidermis. The cells of the upper epidermis are slightly thick walled. But those of the lower epidermis are thin walled. Sunken stomata are present in the lower epidermis. A layer of hypodermis is present beneath the upper epidermis.
2. The mesophyll is differentiated into an upper palisade layer a lower spongy parenchyma. The cells of the mesophyll are rich in chloroplasts.
3. Each pinna is supplied by a single undivided vein. Transfusion tissuesare present around mid ribs. They cause lateral conduction in the leaf. The vascular bundles are surrounded by pericycle and endodermis.

Fig A: TS of leaf of Cycas

Fig B: T.S of leaf of Cycas

INTERNAL STRUCTURE OF ROOT

1. Epidermis: the surface of root is covered by a single layered epidermis
2. Cortex: It is many layered thick and made up of thin walled parenchymatous cells.
3. Endodermis: The inner most layer of cortex is called as endodermis
4. Pericycle: Thin walled cells around the vascular bundle
5. Stele: The young primary root is Diarch (vascular bundles are arranged in two groups) but in older roots it become Polyarch (vascular bundles are arranged in many groups).
   The xylem is exarch (Protoxylem is present towards teh outside and metaxylem towards the centre. Phloem alternates with protoxylem elements.
6. Pith: is present in the centre of stele. It is composed thin walled parenchymatous cells. Root cap is present at the tip of root.

CORALLOID ROOTS

The structure is similar to that of primary root. In addition there is a conspicuous broad blue green zone in middle cortex. This is the algal zone. It lies midway between the vascular bundle and epiblemma. The cortical cells in this region get disorganized and are inhibited by blue green algae such as Nostoc and Anabaena.

In coralloid root the cortex is divided into three zones;Outer cortex, Middle cortex and Inner cortex.

The stele is diarch, triarch or tetrarch and surrounded by an endodermis which is followed by pericycle. 

REPRODUCTION

Cycas is dioecious. The male and female plants are separate. Sometimes, Cycas plants also reproduce vegetatively. It produces buds on the stem. These buds grow to form new plant.

Fig: Cycas male cones

MALE CONES

Male cones are produced on the male plants. Number of cones produced each year varies from one to many. Each male cone is fusiform in shape. Each cone has a central axis. It bears a number of spirallyarranged microsporophyll. The microsporophylls are woody in texture. They are wedge-shaped. The microsporangia (Pollen sacs) cover the lower surface of the microsporophylls. The sporangia form sori. Each sorus has groups of two to six sporangia. Each sporophyll has several hundred sporangia. A large number of spores are produced in Cycas.

DEVELOPMENT OF THE SPORANGIUM

1. Each sporangium develops from a single sporangial initial. This cell arise form the hypodermis of the sporophyll. This sporangial initial divides into an outer primary wall cell and an inner primary sporogenous cell.
2. The primary wall cell divides to produce a several layered wall. The number of layers in the wall varies from four to eight. The primary sporogenous divides to produce a mass of cell called sporogenous tissue or archesporium.
3. The outer most layer of the sporogenous tissue forms the tapetum. Some of the sporogenous cells increase in size and become the spore mother cells. But other cells disintegrate with the tapetum and nourish the spore mother cells.
4. The spore mother cells divide meiotically to form four microspores (pollen grain).
5. The sporangia become boat-shaped. Each mature microspore has two layered wall, the exine and intine. Exine is thickest at one end and becomes thinner towards the opposite end. The spores start germination before its liberation from the microsporangium.

GERMINATION OF MICROSPORE (POLLEN GRAIN)

The microspore cut off lateral prothalial cell towards one side of the spore. The larger cell then cuts off a small generative cell adjacent to the prothalialcell. It itself becomes tube cell. The microspore is liberated at this stage. Spores are dispersed by wind.

FEMALE CONE

The female cones are produced on the female plants. Female cones of Cycas are very large. It is formed of megasporophylls.
Megasporophylls are loosely arranged to form crowns.
Each megasporophyll is leaf like in form.
The upper portion of the sporophyll is pinnate.
Ovules (megasporangium) are arranged in two rows in the basal half of the sporophyll.
The whole sporophyll and young ovules are covered by a dense mat of hairs.
The ovules loose this hairy covering on maturity.
Each ovule is covered by a single massive integument. It has a narrow micropyle at the tip. Integument projects around the micropyle to form a small beak. Nucellus projects into the micropyle. But later the nucellar cells in this region disorganize to form a small cavity called pollen chamber. It plays an important part in early stages of fertilization. One of the nucellar cells increases in size and becomes megaspore mother cell. It undergoes meiosis to form four megaspores. Three megaspores degenerate. Only one becomes functional megaspore.

FEMALE GAMETOPHYTE

The megaspore (embryo sac) enlarges in size. Its nucleus undergoes many nuclear divisions. Thus several nuclei are formed in the megaspore. These nuclei occupy the peripheral region of the cytoplasm in the megaspore. Then the nuclei are surrounded by cell wall. Thus the megaspore becomes multicellular. It gives rise to the female gametophyte. The cells of the gametophyte develop numerous starch grains. The original megaspore wall persists around the prothalial tissue.

1. Three cells enlarge and function as archegonial initials at the micropylar end of the prothallus. Each archegonial initial divides into tipper neck cell and a lower central cell.
2. Neck cell divides to produce neck of the archegonium. The central cell enlarges. It forms the oosphere cell. The whole interior of oosphere cell is filled with dense granular protoplasm. The wall of the oosphere cell is thick. The cells of the prothallus which surround the oosphere cell form the jacket layer.
3. The nucleus of the oosphere cell divides in two. The small ventral canal nucleus disintegrates. The other large nucleus becomes oosphere nucleus. It increases very much in size. The archegonium is now ready for fertilization.
4. The archegonial necks open in archegonial chamber. Original megaspore wall ruptures above the archegonial chamber. The nacellar tissue below the pollen chamber disintegrates. A passage is formed between the archegonial chamber and the pollen chamber.
5. A drop of mucilaginous fluid oozes out of the micropyle. The pollen grains are lodged on the micropyle. Pollen grain is trapped this mucilage and the pollen grain moves into the pollen chamber.

MALE GAMETOPHYTE

The pollen grain resumes its development after pollination. Generative cell divides into a stalk cell and a body cell. Both these cells represent an antheridium. A pollen tube grows out of the pollen grain. It penetrates the nucellus. The pollen grain becomes dormant at this stage. This period lasts for about four months. After that the pollen grain resumes its activity. Two blepharoplasts appear on the two sides of the nucleus. These blepharoplasts develop cilia. The body cell then divides into two antherozoids. The antherozoid develops thousands of cilia. The nucleus of each antherozoid enlarged and completely fills the whole of the antherozoid. Antherozoids remain motile for several hours.

Pollen grain reaches the archegonial chamber by pollination. The wall of the pollen grain protrudes towards the archegonial chamber. The pollen grain bursts and release antherozoids into the archegonial chamber. Antherozoid enters the oosphere. Male nucleus unites with the oosphere nucleus. Fertilized oosphere secretes a thick wall and becomes the oospore. Oospore develops embryo.

Development of Embryo and Seed

Oospore divides into 200-300 cells. The cells of the central region disorganize to produce a cavity. This cavity is surrounded by two to three layers of cells. The cells in the lower region elongate very much to form suspensor. The cells at the tip of the suspensor develop into the embryo. The elongating suspensor pushes the developing embryo deep into the prothalial tissue. These tissues provide nutrition to the developing embryo. Whole of the nucellus is consumed during the development of the embryo. Some prothalial tissue persists in the seed and form endosperm. A thick pad of tissue develops near the micropylar end. It functions as coleorhiza. Coleorhiza protects the radical of the embryo. Ovule is transformed into the seed. Embryo has two cotyledons. They occupy the whole seed. The seeds germinate immediately. Cotyledons remain within the seed on germination. It .absorbs nutrients for the developing embryo

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