Handout (1)

PLANT TISSUE SYSTEM I. Meristematic tissues: composing of actively dividing, embryonic cells responsible for the growth of the plant

A. Apical meristems: located at the shoot and root tips of the plant; responsible for the elongation of the roots and the stems (primary growth)

B. Lateral meristems: cylinders of meristematic tissues located along the stems and roots; functions for the increase in plant’s diameter/ girth (secondary growth)

II. Permanent tissues: composing of cells that do not have the ability to divide

A. Simple Tissues: composing of only one (1) cell type

§ Fundamental tissues (Ground tissues): makes up the bulk of a young plant; fills up the spaces between the epidermis and the vascular tissues.

• Parenchyma: cells having relatively thin cell walls; most abundant cell type in most plants; functions for food storage and photosynthesis.

• Collenchyma: have thicker cell walls; main function is to provide support in parts of the plant that are still growing; found along the stem and petiole of the leaves

• Sclerenchyma: have the thickest cell wall; mature cells cannot elongate and occur only in regions of the plant that have stopped growing in length; functions for support.

- Fibers: long, thin, and slender; occurs in bundles - Sclereids: shorter, irregularly- shaped

§ Lining tissues (Epidermis): ‘skin’ of the plant; covers and protects the plant organs; a number of

types of cells occur in the epidermis: • Guard cells: dumbbell- shaped cells surrounding the stoma (mouth- shaped epidermal

opening in the leaves and other parts of the plant); regulates the closing and opening of the stoma.

• Trichomes: hair- like outgrowths of the epidermis; occurs frequently on leaves, stems, and reproductive organs; functions for protecting the plants against diseases as well as herbivores by secreting toxic substances.

• Root hairs: extensions of the epidermal cells in the roots; maximize water and mineral absorption.

B. Complex tissues: composing of two (2) or more cell types § Vascular tissues: tissues that functions for transport

§ Xylem: water- conducting tissues of plants.

• Tracheids: long cells with tapered ends • Xylem vessels: wider, shorter, and less tapered

*both cells are interrupted with pits and are hollow because once mature, both tracheids and xylems vessels are dead, and only their cell walls remain

§ Phloem: food- conducting tissue of plants.

• Sieve- tube elements: lacks nucleus, ribosomes, and vacuoles (lost during development) • Companion cells: connected to the sieve- tube elements; functions in loading sugars and

in making certain proteins for the sieve- tube elements.

PLANT ORGANS § A plant can be divided into two basic systems, a subterranean root system and an aerial shoot system.

§ This two system arrangement reflects the evolutionary history of plants as terrestrial organisms. § Each system depends on the other for survival of the whole plant.

• Roots depend on shoots for sugar and organic nutrients. • Shoots depend on roots for mineral, water, and support.

I. Root System: located below the ground A. Root: non- photosynthetic; functions in anchoring the plant body, absorption of water and minerals and storage of food (carbohydrates)

Types of Root Systems 1. Taproot: one large vertical root (taproot) produces many smaller secondary roots (lateral roots);

provides firm anchorage

Modifications in taproot (most of these roots are modifies for food storage)

o Fusiform: roots that are shaped like a spindle; broad in the middle and tapering on both ends (Ex. parsnip, ginseng)

o Napiform: spherical in shape (Ex. turnip, beet root, potato) o Conical: cone- shaped (Ex. carrots) o Tuberous: thick and fleshy with no definite shape (Ex. 4 o’ clock plant)

2. Fibrous roots: a mat of thread- like roots spreads out below the soil surface; provides extensive exposure to soil water and minerals.

Modifications in fibrous roots

o Pneumatophores or prop roots: also called as the respiratory roots; resemble conical spikes and are commonly found in marshy and salty water system (Ex. mangrove trees)

3. Adventitious roots: roots that grows in an unusual location (stems or leaves)

Modifications in adventitious roots (grouped according to function)

• For storage

o Nodulose: slender roots become swollen at the apex (Ex. mango- ginger) o Moniliform: roots alternately swollen and constricted presenting a bead- like

formation (Ex. grasses, sedges) o Annulated roots: when the roots has ring- like swellings as in arrow root (Ex.

ipecac, skunk cabbage)

• For mechanical support

o Stilt or brace roots: produced from the main stem and grow vertically downwards to the ground (Ex. sugarcane, screw pine)

o Climbing roots: roots that climb and are often attached to a support- like fence, a stake, or to another plant (Ex. money plant)

• Other functions

o Haustoria (-ium) or sucking roots: present in parasitic plants; these penetrate the host plant and absorb nutrients from it (ex. cuscuta, hydnora)

o Aerial roots or epiphytic roots: roots that undergo contraction at the uppermost part (Ex. orchids)

II. Shoot System: located above the ground A. Vegetative part: consists of a stem and attached leaves

1. Stem: main axis of the plant that bears all structures which is located above the ground; consisting of an alternating system of nodes and internodes.

v In the upper angle (axil) formed by each leaf and the stem is an axillary bud, a structure that can form a lateral shoot, commonly called as a branch. The elongation of the shoot is usually concentrated near the shoot tip, which consists of a terminal bud.

§ Most axillary buds of a shoot are dormant (not growing). The proximity of the axillary bud to the terminal bud is partly responsible for their dormancy. The inhibition of axillary bud by a terminal bud is called the apical dominance.

§ Removing the terminal bud stimulates the growth of the axillary bud.

§ This evolutionary adaptation of apical dominance increases the plant’s exposure to light.

Node: point at which leaves are attached

Internode: the stem segments between nodes

Types of stems

a. Underground stems: these are stems that grow underground. Most are used as food storage organs.

o Bulbs: are underground, condensed shoots, usually flattened and consist of short, disc- like stems enclosed in overlapping layers of fleshy leaves

o Corms: are vertical, thick stems that have thin, papery leaves. o Rhizomes: stems that grow horizontally below ground level. o Stem tubers: swollen ends of the underground lateral stems or the thickened portion of a

rootstock.

b. Aerial stems: stems that grow above the ground. They exhibit variations and can be classified in many ways:

• According to texture

o Herbaceous: plants that are soft- stemmed; they usually die at the end of one growing season

o Suffrutescent: stems that are woody at the base, while the more distal parts are herbaceous; plants that are half- woody and half- herbaceous.

o Woody or arborescent: when the stem becomes tough due to the formation of secondary tissues and usually with a main trunk that lasts for several years.

• According to direction of growth

o Erect: when the stem grows perpendicular to the ground o Ascending or assurgent: when the stems rises obliquely o Decumbent: when the stem more or less reclines on the ground or the stem lies flattened

immediately above the ground. o Prostate or procumbent: stem lies on the ground but does not bear roots at the nodes o Creeping or repent: when the stem lies closely on the ground and bears roots at nodes o Climbing: stem ascends by means of special support offered by other plants.

Special kinds/ modifications in stems

o Boles: unbranched stems of forest trees o Caudex: unbranched woody stems of some monocotyledonous plants such a palms o Culm: hollow or solid stems of grasses o Stolons or runners: stems that lie flat on the ground, bears roots at the nodes and produce new plants o Scape: leafless flowering stem that arise from the ground o Cladophyll: modified stem resembling a leaf in form and appearance

2. Leaves: lateral outgrowths that develops from the node of the stem; main photosynthetic organ of the plant

§ A leaf usually exists in the shape of a flattened blade which is joined to the node of a stem by a petiole

§ Most monocots lack petioles; instead, the leaf base forms a sheath surrounding the stem. Its leaves

have parallel major vein running the length of the blade § Dicot leaves have a multi- branched network of major veins.

§ Almost all leaves are specialized foadaptations that enable them to perfor

o Bracts: leaf-like structure fro Spines: the leaves of man

deter predators o Window leaves: succulent,

in sand blown by the wisurface.

o Reproductive leaves: prodin the soil

o Storage leaves: leaves thao Tendrils: typically helps theo Insectivorous leaves: leave

found in acid swamps whe

§ Phyllotaxy is the pattern or arrangephyllotaxis:

o Alternate: when there is on

leaf on one node does not

o Opposite: when there are the stem

o Whorled or verticillate: wharranged regularly on the s

§ Petiole: stalk joining the leaf to the stem at a node

§ Blade/ Lamina: thin and flattened part of the plant

§ Stipules: pairs of small leaf-like structures located at the base of a petiole.

§ Midrib: strengthened vein along the midline of a leaf

§ Veins: term used for the vascular bundles in leaves

r photosynthesis. However, some species have leaves with m additional function:

om the axil of which the stalk of a flower arises y cacti are modifies into spines to reduce water loss and to

cone- shaped leaves with transparent tips that are often buried nd; allows photosynthesis to take place beneath the ground

uces adventitious plantlets which fall off the leaf and take root

t are modified to store water plant clings to a support s that trap insects in order to digest and extract minerals; often re needed minerals are not readily available

ment of the leaves along the stem. There are three basic

ly one leaf at a node and are arranged in such a way that one shade the one below it

two (2) leaves borne on the same node from opposite sides of

en three or more leaves are attached to ne node and are tem

§ Leaves have a considerable value in plant identification. Plants varied in leaf forms such as that, some plant taxa are distinguishable by their leaf characteristics. They vary in leaf base, leaf tip, leaf margin as well as terms of the attachment.

§

B. Reproductive part: comprising of flowers, fruits, and seeds

A. Flower: most private part of the plant since the reproductive structures are hidden in it

§ The flower is the main basis in plant identification and classification.

§ Inflorescence is the arrangement of flowers on a stalk along the stem.

General types of inflorescence:

o Determinate inflorescence: an inflorescence in which the oldest flower is at the terminal part of the main axis and the general progression of blooming is outward.

o Indeterminate inflorescence: an inflorescence in which the younger flower is terminal on the floral axis and the progression of blooming is inward.

Variation of inflorescence according to location:

o Axillary: the individual flowers are born at the axil of the leaves o Cauline: flowers are attached on the stem below the leaves o Extra- axillary: flowers are attached at the internodes o Leaf- opposed: flowers are opposite a leaf o Radical: flowers are attached to the roots by a long stalk called the scape o Terminal: inflorescence terminates a branch o Fascicle or fasciculate: flowers are clustered in the axil of leaves

Variation in inflorescence according to the arrangement of the flowers:

o Catkin or ament: a short or long, usually dense, scaly spike o Corymb or corymbose: indeterminate inflorescence where the branches and *pedicels all start from different points and attain the same level. The lower pedicels are longer than the upper ones. The inflorescence has a flat top appearance; the outer flowers open first before the central flowers.

o Cyathium: inflorescence consist of a cup- like *involucres that contain a single pistil and male flowers with a single stamen.

o Cyme or cymose: very similar to corymb, but the inner flower open first o Head, capitates, capitulum, or disc: flowers are sessile and are crowded in a globose mass o Panicle or paniculate: a branched racemose inflorescence o Raceme or racemose: similar to a spike, but the flowers are pedicelled o Spadix or spikelet: an unbranched, elongated inflorescence with *sessile flowers o Thyrse: similar to a panicle but more congested and more or less cylindrical o Umble or umbellate: inflorescence with several pedicelled flowers attached at the same point on top of the stem

*pedicels: stalk of a single flower *involucres: collective term for the bracts

*sessile flower: filament of the stamen is absent

PLANT GROWTH

§ Plant growth begins with germination of the seed and continues for the lifespan of the plants.

§ Indeterminate growth: continued growth as long as the plant lives.

o In contrast, most animals cease growing after reaching a certain size (determinate growth) o Certain plant organs such as flower parts show determinate growth

§ Plants do not live indefinitely; they do have finite life spans.

o Most have genetically determined life span. o Some are environmentally determined:

Annuals complete their life cycle in one year or growing season Biennials typically have life span of two years Perennials, such as trees and some grasses, live many years.

§ Indeterminate growth is made possible by meristems (perpetually embryonic tissues)

§ Meristematic cells are unspecialized and divide to generate new cells near the growing point.

§ New meristematic cells formed by division may remain in the region and produce new cells (initials) or be displaced and become incorporated and specialized into tissues (derivatives)

§ Apical meristems, located in root tips and shoot buds supply cells for plants to grow in length

o Primary growth (elongation) is initiated by apical meristems and forms primary tissues organized into three (3) systems.

o Secondary growth (increases girth) is the thickening of roots and shoots which occur in woody plants due to the development of the lateral meristems.

• Cell division in the lateral meristems produces secondary dermal tissues which are thicker and tougher than the epidermis it replaces. It also adds new layers of vascular tissues.

Primary Growth of Roots

• Root growth is concentrated near its tips and results in roots extending through the soil. o The root tip is covered by a root cap, which protects the meristems and secretes a polysaccharide coating that lubricates the soil ahead of the growing root.

o The root tip contains three (3) zones of cells in successive stages of primary growth. Although described separately, these zones blend into a continuum.

1. Zone of Cell Division

• Located near the tip of the root; includes the apical meristems and its derivatives, the

primary meristems • The apical meristem is centrally located; it produces the primary meristems and

replacement of cells in the root cap. • A quiescent center is located near the center of the apical meristems. It is composed of

resistant, slowly dividing cells which may serve as reserve replacement cells in case od damage to the meristem

• The primary meristems form as three concentric cylinders of cells (the protoderm, procambium, ground meristem) which will produce the tissues system of the roots.

2. Zone of Cell Elongation

• In this region, cells elongate to at least 10 times their original length • The elongation of cells in this region pushes the root tip through the soil • Continued growth is sustained by the meristem’s constant addition of new cells to the

youngest end of the elongation zone 3. Zone of Maturation

• Farthest from the root tip • Region where newly produced cells become specialized in structure and function and

where the three (3) tissues systems completer their differentiation.

• The apical meristem produces three (3) primary meristems, which in turn give rise to the three primary tissues of roots:

1. The protoderm is the outermost primary meristem

• Gives rise to the epidermis over which water and minerals must cross • Root hairs are epidermal extensions which increase surface area, thus enhancing uptake

2. The procambium forms a stele (central cylinder) where xylem and phloem develop.

3. The ground meristesm is located between the protoderm and procambium; it gives rise to the

ground tissue. The ground tissue is:

• Is mostly parenchyma and fills the cortex (root area between the stele and the epidermis) • Stores food; the cell membranes are active in mineral uptake • Has endodermis, the single- cell thick, innermost layer of the cortex that forms the

boundary between the cortex and the stele. o It selectively regulates the passage of substances from soil to the vascular tissue of

the stele.

• Lateral roots may sprout from the outermost layer of the stele

• The pericycle, just inside the endodermis, is a layer ofdivide to form the lateral root.

• A lateral root forms a clump of cells in the pericyclecortex until it emerges from the primary root.

• The lateral root maintains its vascular connection to th Primary Growth of Shoots

§ Shoot apical meristem is a dome- shaped mass of dividing cell

• Forms the primary meristems that differentiate into the• On the flanks of the apical meristem dome are leaf prim• Meristematic cells left by the apical meristem at the

axillary buds

§ Most shoot elongation occurs actually due to the growth of apex.

• Growth is a result of both cell division and elongation w• Intercalary meristems are present at the base of ea

plants. o These tissues permit prolonged internode e

§ Axillary buds may form branches later in life of the plant.

• Branches originate at the surface of the shoot and are • This is a direct contrast to development of lateral shoo

root. Secondary Growth

§ Secondary growth results from two lateral meristems: the vasco Vascular cambium produces xylem and phloem o Cork cambium produces a tough, thick covering for roo

Secondary Growth of Stems

§ Vascular cambium forms when meristematic parenchyma cellprimary phloem of each vascular bundle and in the rays of gro

§ within t

§ in the r

of a root.

cells that may become meristematic and

, then elongates and pushes trough the

e stele of the main root.

s at the tip of the terminal bud.

three (3) tissues systems ordia which form leaves. base of the leaf primordial develop into

slightly older internodes below the shoot

ithin the internode ch internode in grasses and some other

longation along the length of the shoot

connected to the vascular system ts which form deep in the pericycle of the

ular cambium and the cork cambium.

ts and stems that replaces the epidermis.

s develop between the primary xylem and und tissue between the bundles

Fascicular cambium: cambium he vascular bundle

Interfascicular cambium: cambium ays between vascular bundles

o Together, meristematic bands in the fascicular and interfascicular regions form a continuous cylinder of dividing cells around the xylem and pith of the stem

o Ray initials (meristematic cells of interfascicular cambium) produce radial files of parenchyma cells (xylem and phloem rays) which permit lateral transport of water and nutrients as well as storage of starch and other reserves.

o Fusiform initials (cells of fascicular cambium) produce new vascular tissues; secondary xylem to the inside of the vascular cambium and secondary phloem to the outside.

• Accumulate layers of secondary xylem produces wood which consist mostly of tracheids, vessels

elements, and fiber.

o The hardness and strength of wood results from these cells which, while dead at maturity, have thick walls

o Forms annual growth rings due to yearly activity: cambium dormancy, spring wood production and summer wood production.

• The secondary phloem does not accumulate extensively. The secondary phloem, and all tissues

external to it, develops into bark which sloughs off the tree trunk.

§ Cork cambium is a cylinder of meristematic tissues that forms protective layers of the secondary plant body. • First forms in the outer cortex of the stem • Phelloderm (parenchyma cells) forms to the inside of cork cambium initials as they divide • Cork cells form to the outside of cork cambium initials as they divide • As cork cells mature, they deposit suberin (a waxy material) in their walls and die • These dead cork tissues protect the stem from damage and pathogens while reducing water loss • The combination of cork cambium, layers of cork, and phelloderm form the periderm (the protective

coat of the secondary plant body that replaces primary epidermis) • The term bark, refers to all tissues external to the vascular cambium (phloem, phelloderm, cork

cambium, and cork)

§ Cork cambium is a cylinder of fixed size and does not grow in diameter. • As continued secondary growth splits this, it is replaced by new cork cambium formed deeper in the

cortex. • When no cortex is left, it develops from parenchyma cells in the secondary phloem (only the

youngest secondary phloem that is internal to cork cambium functions in sugar transport) • Lenticels are present in the bark

o Lenticel is a spongy region in the bark which permits gas exchange by living cells within the

trunk Secondary Growth of Roots

§ Vascular cambium and cork cambium also function in secondary growth of roots • The vascular cambium produces secondary xylem to its inside and secondary phloem to its outside

o It is first located between xylem and phloem of the stele o The cortex and epidermis split and are shed as the stele grows in diameter

§ Cork cambium forms from the pericycle of the stele and produces the periderm, which becomes

secondary dermal tissue

• Periderm is impermeable to water, consequently, the roots with secondary growth function to anchor the plant and transport water and solutes between the younger roots and shoot system

§ Older roots become woody, and annual rings appear in the secondary xylem • Old roots and old stems may look very similar

NUTRITION AND TRANSPORT IN PLANTS

§ Plants require a variety of nutrients in addition to the direct products of photosynthesis

§ Plants necessitate a number of inorganic nutrients. Some of these are macronutrients, which the plant need in relatively large amounts, and others are micronutrients, which are required in trace amount. Essential Nutrients in Plants

Elements Principal Form in which the Element is Absorbed Examples of Important Functions

Macronutrients Carbon CO2 Major component of organic molecules Oxygen O2, H2O Major component of organic molecules Hydrogen H2O Major component of organic molecules

Nitrogen NO3-, NH4

+ Component of amino acids, proteins, nucleotides, nucleic acids, chlorophyll, coenzymes, enzymes

Potassium K+ Protein synthesis, operation of stomata

Calcium Ca ++ Component of cell walls, maintenance of membrane structure and permeability, activates some enzymes

Magnesium Mg++ Component of chlorophyll molecule, activates many enzymes

Phosphorus H2PO4-, HPO4

= Components of ADP and ATP, nucleic acids, phospholipids, several coenzymes

Sulfur SO4= Components of some amino acids and proteins,

coenzyme A Micronutrients Chlorine Cl- Osmosis and ionic balance Iron Fe++, Fe +++ Chlorophyll synthesis, cytochromes, nitrogenase Manganese Mn++ Activator of certain enzymes

Zinc Zn++ Activator of many enzymes, active in formation of chlorophyll

Boron BO3-, B4O7

= Possibly involved in carbohydrate transport, nucleic acid synthesis

Copper Cu++ Activator or component of certain enzymes Molybdenum MoO4

= Nitrogen fixation, nitrate reduction

§ Some plants have novel strategies for obtaining nutrients.

• Venus flytraps and other carnivorous plants lure and capture insects and then digest them to obtain energy and nutrients.

• Some plants entice bacteria to produce organic nitrogen for them. These bacteria may be free- living or form a symbiotic relationship with a host plant

• Most vascular plants (~90%) rely on fungal associations to gather essential nutrients, especially phosphorus

§ Water and minerals move upward through the xylem.

§ Water moves from the soil into a plant’s roots, and then throughout the plant through a combination of several processes:

o Osmosis: The method plants use to draw water from the soil into the xylem cells in their roots is

called osmosis. Root cells have higher concentration of minerals than the soil they’re in, so during osmosis, water flows toward the higher concentration of dissolved substances found in the root cells. This intake of water increases pressure in root cells and pushes water into the plant’s xylem.

o Capillary action: This causes liquid to rise up through the tubes in the xylem of plants. This action results from adhesion (the property of water to stick with other molecules), which is caused by the attraction between water molecules and the walls of the narrow tubes. The adhesion forces water to be pulled up the column of vessel elements in the xylem and in the tubules in the cell wall.

o Transpiration and Cohesion: Transpiration is the technical term for the evaporation of water from

plants. As water evaporates through the stomates in the leaves (or any part of the plant exposed to air), it creates a negative pressure (also called as tension or suction) in the leaves and tissues of the xylem. The negative pressure in the leaves and xylem exerts a pulling force on the water in the plant’s xylem and draws the water upward. When water molecules stick to each other through cohesion (property of water to stick together), they fill the column in the xylem and act as a huge single molecule of water. As water evaporates from the plant through transpiration, the rest of the water gets pulled up, causing the need for more water to be pulled into the plant. The back and forth of transpiration and cohesion is known as the cohesion- tension theory.

§ Dissolved sugars and other materials are transported in the phloem.

§ Phloem moves these sugars throughout the plant via translocation (the transport of dissolved materials

in a plant)

§ Unlike xylem, which can only carry water upward, phloem carries dissolved materials upward and downward (bidirectional) from sugar sources to sugar sinks.

o Sugar sources are plant organs such as leaves that produce sugars. o Sugar sinks are plant organs, such as roots, tubers, or bulbs that consume or store sugars.

§ The specific way translocation works in a plant’s phloem is explained by the pressure- flow theory:

1. Sugars are loaded into phloem cells called sieve tube elements within the sugar sources,

creating a high concentration of sugar at the source

o The concentration of sugars in sink organs is much lower. 2. Water enters the sieve tube elements by osmosis.

o During osmosis, water moves into the areas with the highest concentration of solutes (in this case, sugars)

3. The inflow of water increases pressure at the source, causing the movement of water and carbohydrates toward the sieve tube elements at a sugar sink.

4. Sugars are removed from cells at the sugar sink, keeping the concentration of sugars low.

o As a sugar sink receives water and carbohydrates, pressure builds. But before the sugar sink can turn into a sugar source, carbohydrates in a sink are actively transported out into

needy plant cells. As the carbohydrates are removed, the water then follows the solutes and diffuses out of the cell, relieving the pressure

§ Sugar sinks that store carbohydrates can become sugar sources for plants when sugars are needed.

Whenever a plant needs sugar, like at night when photosynthesis doesn’t occur as well, the plant can break down its sugars, allowing a tissue that would normally be a sugar sink to become a sugar source.

PLANT REPRODUCTION

§ Reproductive success depends on uniting the gametes (egg and sperm) found in the embryo sacs and pollen grain of flowers.

§ The male gamete of flowering plants is the pollen grains whereas the female gamete is the embryo sac.

§ Pollen grains and the embryo sac both are produces in separate, specialized structure of the angiosperm flower.

Pollen Formation

§ Pollen grains form in the two pollen sacs located in the anther.

§ Each pollen sac contains specialized chambers in which the microspore mother cells are enclosed and protected.

§ The microspore mother cells undergo meiosis to form four (4) haploid microspores.

§ Mitotic division follows producing four (4) pollen grains each having a tube cell and a generative cell

that will later divide into two (2) sperm cells.

Embryo Sac Formation

§ Eggs develop in the ovules of the angiosperm flower. Within each ovule is a megaspore mother cell.

§ Each megaspore mother cell undergoes meiosis to produce four haploid megaspores. But only one of these megaspores will survive, the rest are absorbed by the ovule.

§ The lone remaining megaspore undergoes repeated mitotic division to produces eight (8) haploid nuclei

that are enclosed within a seven- celled embryo sac.

§ Within the embryo sac, the eight (8) nuclei are arranged in precise positions.

§ One (1) nucleus is located near the opening of the embryo sac in the egg cell.

§ Two (2) are located in a single cell in the middle of the embryo sac and are called polar nuclei.

§ Two (2) nuclei are contained in cells called synergids which surround the egg cell.

§ The remaining three (3) nuclei reside in cells called antipodals, located at the end of the sac, opposite the egg cell.

Pollination

§ Pollination is the process of transfer of pollen grains from the anther to the stigma of a flower. It is of two types:

o Self-pollination: If the pollen grains from the anther of a flower are transferred to the stigma of the same flower; also termed as autogamy (auto: self; gamy: marriage) .

o Cross pollination: If the pollen grains from anther of one plant reach the stigma of a flower on another plant of the same species; also called as allogamy (allos: other; gamy: marriage). Cross pollination has the advantage of increasing the chances of variations.

§ After pollination, the pollen grains germinate on the stigma to produce a pollen tube.

§ This tube grows down through the style and finally reaches the ovule.

§ The ovule contains the egg cell inside the embryo sac.

§ The tip of the pollen tube ruptures in the ovule and discharges two male gametes into it.

§ One of the male gametes fuses with the egg to form the zygote. This fusion is called fertilization.

§ The other male gamete fuses with the diploid secondary nucleus and forms the endosperm nucleus.

§ The zygote that is formed as a result of fertilization divides several times and gives rise to an embryo.

The endosperm nucleus grows to form the endosperm of the seed.

§ Following fertilization, the sepals, petals, style and stigma degenerate and usually fall off. The ovary wall ripens and forms the pericarp of the fruit. Each ovule develops into a seed. The seed contains a potential plant or embryo. The whole ovary after fertilization changes into a fruit.