chapter 12 - reprod and growth in plants

8/2/2019 Chapter 12 - Reprod and Growth in Plants

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BIO 250/ Diploma Science System and Maintenance (Animals and Plants)

Ainun Jariah Manaf/UiTM Pahang1

CHAPTER 12: Angiosperm Reproduction

Sexual and Asexual Reproduction of Angiosperm

Sexual:

Progeny are different genetically. Some are less adapted than the parents but others are moreadapted. Offspring cannot colonize a new site as rapidly because not all progeny are adaptedfor it, but some can colonize different sites with characeristic not suitable for parents. Changesin habitat may adversely affect some progeny, but others may be adapted to the new conditions.Isolated individuals cannot reproduce

Asexual (vegetative reproduction)

All progeny are identical genetically to parent and to each other. All are as adapted as parent is,but none is more adapted. Rapid colonization of anew site is possible. All may be adverselyaffected by even minor changes in the habitat. Even isolated individuals can reproduce.

Mechanisms of Asexual Reproduction

Many plants can clone themselves by asexual reproduction. The offspring are mature vegetativefragments from the parent plants, and that is why asexual reproduction in plants is also knownas vegetative reproduction. Asexual reproduction in plants involves the production of offspringfrom single parents and occurs without genetic recombination and resulting in a clone.

There are two major natural mechanisms of vegetative reproduction:

1) Apomixis:

Seed develop without meiosis and fertilization. A diploid cell in the ovule gives rise to anembryo. The ovules mature into seeds, which are dispersed. An example is dandelion.

2) Fragmentation

Involve the separation of a parent plant into parts that develop into whole plants.Fragmentation is the most common form of vegetative reproduction. In some species theroot system of a single parent gives rise to many adventitious shoots that becomeseparate shoot systems.

There are several types of asexual reproduction in plants. Following are the examples offragmentation in vegetative reproduction:

a. Stolon or runners are horizontal aboveground stems that grow along the surface andare distinguished by long internodes. Buds develop along the stolon, and each bud givesrise to a new shoot that roots in the ground. When the stolon dies, the daughter plantslive separately and grow to start off a new plant. Strawberry plants and sweet potatoconnected by stolons.


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Fig. 12.1 a. several types of asexual reproduction in plants

b. Rhizomes are horizontal modified underground stems that may or may not be fleshywhere it can give rise to new plants. Fleshy rhizome indicates that it is used for storingfood materials such as starch. Although rhizome looks like root, they are, actually stembecause of the presence of scalelike leaves, buds, nodes, and internodes. Ginger andmany grasses are good examples of plants that reproduce asexually by formingrhizome.

c. Tubers are fleshy underground stems enlarged for food storage. When the attachmentbetween a tuber and its parent plant breaks, the parent plant will die and the tuber growsinto new separate plants. Potato is a good example of plant that produces tubers. Theeyes of potato are axillary buds, this is a proof that the tuber is underground stem rather

than storage root like carrot and sweet potato.

d. Bulb is a modified underground bud in which fleshy storage leaves are attached to ashort stem. A bulb is round and covered by paper-like bulb scales that are modifiedleaves. Bulb normally forms axillary buds, which develop into bulblets or small daughterbulbs that attached to the bulb. Each bulblets will develop into new separate plants whenthe parent bulb dies and rots away. Onions, lilies and tulips are some of the plants thatproduce bulbs in their asexual reproduction.


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e. Budding plantlets perform by several plants they bud off tiny plantlets along the leafedges that develop into a miniature version of the parent plant. These may even developroots while still attached to the parent plant. When these plantlets reach certain size,they drop to the ground, forming roots and grow.

.

Fig. 12.1b. Bryophyllumor Mexican Hat plant produces plantlets along leaves.

f. Suckers are above-ground shoots that develop from the roots of the parent plants.Each sucker grows additional roots and become an independent plant when the parentdies. Example of plants that form suckers is Banana

Vegetative Propagation and Agriculture:

Humans have devised various methods for asexual propagation of angiosperms.

a. Clones from CuttingsClones may be obtained from either shoot or stem cuttings or plant fragments. Cuttings

may come from stems, leaves, or specialized storage stems (potatoes).

b. GraftingA twig or bud from one plant can be grafted onto a plant of a closely related species ora different variety of the same species. The plants providing the root system is thestock. The twig grafted onto the stock is the scion.

c. Test-tube cloning and related techniques

Plant biotechnologists have adopted in vitro methods to create and clone novel plantvarieties. Test-tube cloning makes it possible to culture small explants (pieces ofparental tissue) or single parenchyma cells on an artificial medium containing nutrientsand hormones. Tissue culture is often used to regenerate genetically engineered plants.

Another approach combines protoplast fusion with tissue culture methods to invent newplant varieties that can be cloned. Protoplast are plant cells which have had their cellwalls removed. In protoplast fusion, researchers fuse protoplasts to from hybridprotoplasts. Protoplasts regenerate cell walls and become hybrid plants


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Reproduction: To Seed or Not to Seed

The parasitic plant Rafflesia arnoldii produces enormous flowers that can produce up to 4million seeds.

Pollination enables gametes to come together within a flower

In angiosperms, the dominant sporophyte produces spores that develop within flowers into malegametophytes (pollen grains) and produces female gametophytes (embryo sacs).

Fig. 12.2An overview of angiosperm reproduction

Sexual Reproduction

Sporophyte and gametophyte generations alternate in the life cycles of plants.

The life cycles of angiosperms and other plants are characterized by an alternation ofgenerations, in which haploid (n) and diploid (2n) generations take turns producing eachother.The diploid plant, the sporophyte, produces haploid spores by meiosis. These sporesdivide by mitosis, giving rise to multicellular male and female haploid plants; the gametophytes.


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The gametophytes produce gametes; sperm and eggs. Fertilization of sperm and eggs results indiploid zygotes, which divide by mitosis to form new sporophytes.

In angiosperms, the plant we see is the sporophyte and it is the dominant generation. Over thecourse of seed plant evolution, gametophytes became reduced in size and dependent on theirsporophyte parents. Angiosperm gametophytes are the most reduced of all plants, consisting of

only a few cells.

In angiosperms, the sporophyte produces a unique reproductive structure, the flower. Male andfemale gametophytes develop within the anthers and ovules, respectively, of a sporophyteflower.

Pollination by wind, water, or animals brings a male gametophyte (pollen grain) to a femalegametophyte contained in an ovule embedded in the ovary of a flower. Union of gametes orfertilization takes place within the ovary. Ovules develop into seeds, while the ovary itselfdevelops into the fruit around the seed.

Flower Structure

Fig. 12.3 Floral variations


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Flowers are specialized shoots bearing the reproductive organs of the angiosperm sporophyte.Flowers, the reproductive shoots of the angiosperm sporophyte, are typically composed of fourwhorls of highly modified leaves called floral organs, which are separated by very shortinternodes. Unlike the indeterminate growth of vegetative shoots, flowers are determinateshoots in that they cease growing once the flower and fruit are formed.

The four kinds of floral organs are the sepals, petals, stamens, and carpels. Their site ofattachment to the stem is the receptacle. Sepals and petals are sterile. Sepals, which encloseand protect the floral bud before it opens, are usually green and more leaflike in appearancethan the other floral organs. In many angiosperms, the petals are brightly colored and advertisethe flower to insects and other pollinators.

Stamens and carpels are the male and female reproductive organs, respectively. A stamenconsists of a stalk or the filament and a terminal anther containing chambers called pollen sacs.The pollen sacs produce pollen. A carpel has an ovary at the base and a slender neck, thestyle.

At the top of the style is a sticky structure called the stigma that serves as a landing platform for

pollen. Within the ovary are one or more ovules. Some flowers have a single carpel. In others,several carpels are fused into a single structure, producing an ovary with two or morechambers, each containing one or more ovules. The anthers and the ovules bear sporangia,where spores are produced by meiosis and where gametophytes later develop.

The male gametophytes are sperm-producing structures called pollen grains, which form withinthe pollen sacs of anthers. The female gametophytes are egg-producing structures calledembryo sacs, which form within the ovules in ovaries.

Gametophyte Development and Pollination

In angiosperms, pollination is the transfer of pollen from an anther to a stigma. If pollination is

successful, a pollen grain produces a structure called a pollen tube, which grows down into theovary and discharges sperm near the embryo sac. Pollen develops from microspores within thesporangia of anthers.


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Fig. 12.4 a. The development of angiosperm gametophyte (pollen grain)b. The development of angiosperm gametophyte (embryo sac)

1. Each one of themicrosporangiacontains diploidmicrosporocytes(microspore mothercells).

2. Each microsporocytedivides by meiosis toproduce four haploidmicrospores, each ofwhich develops into apollen grain.

3. A pollen grain

becomes a maturemale gametophytewhen its generativenucleus divides andforms two sperms. Thisusually occurs after apollen grain lands onthe stigma of a carpeland the pollen tubebegins to grow.

1. Within the ovulesmegasporangium is a largediploid cell called themegasporocyte (megasporemother cell)

2. The megasporocyte divideby meiosis and gives rise tofour haploid cells, but in mosspecies only one of thesesurvives as the megaspore.

3. Three mitotic divisions of tmegaspore form the embryosac, a multicellular femalegametophyte. The ovule nowconsists of the embryo sacalong with the surroundinginteguments (protectivetissue).


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The male gametophyte begins its development within the sporangia (pollen sacs) of the anther.Within the sporangia are microsporocytes, each of which will form four haploid microsporesthrough meiosis. Each microspore can give rise to a haploid male gametophyte.

A microspore divides once by mitosis and produces a generative cell and a tube cell. Thegenerative cell will eventually form sperm. During maturation of the male gametophyte, the

generative cell passes into the tube cell. The tube cell encloses the generative cell. Tube celllater on develop to form pollen tube, which delivers sperm to the egg. The microspore is nowknown as a pollen grain, an immature male gametophyte. This two-celled structure is encasedin a thick, ornate, distinctive, and resistant wall.

A pollen grain becomes a mature gametophyte when the generative cell divides by mitosis toform two sperm cells. In most species, this occurs after the pollen grain lands on the stigma ofthe carpel and the pollen tube begins to form. The pollen tube grows through the long style ofthe carpel and into the ovary, where it releases the sperm cells near the embryo sac.

Embryo sacs develop from megaspores within ovules. Ovules, each containing a singlesporangium, form within the chambers of the ovary. One cell in the sporangium of each ovule,

the megasporocyte, grows and then goes through meiosis, producing four haploidmegaspores. In many angiosperms, only one megaspore survives.

This megaspore divides by mitosis three times without cytokinesis, forming in one cell with eighthaploid nuclei. Membranes partition this mass into a multicellular female gametophyte,theembryo sac.

Three cells sit at one end of the embryo sac: two synergid cells flanking the egg cell. Thesynergids function in the attraction and guidance of the pollen tube. At the other end of theembryo sac are three antipodal cells of unknown function. The ovule now consists of theembryo sac and the surrounding integuments, layers of protective tissue from the sporophytethat will eventually develop into the seed coat.

Pollination

Pollination is the transfer of pollen from an anther to a stigma. It begins the process by whichthe male and female gametophytes are brought together so their gametes can unite. Pollinationoccurs when wind, water, or animals carry pollen released from anthers to land on stigma.

Each pollen grain produces a pollen tube, which grows down into the ovary via the style anddischarges sperm into the embryo sac, fertilizing the egg. The zygote gives rise to an embryo.The ovule develops into a seed, and the entire ovary develops into a fruit containing one or

more seeds. Fruits carried by wind, water, or animals disperse seeds away from the sourceplant where the seed germinates.


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There are two types of pollination:

a. Self-pollinationPollination that occurs within the same flower or between different flowers on the same plant

b. Cross pollination

Pollination that occurs between flowers of different individual plants of the same species

Table 1: Differences in insect-pollinated flowers and wind-pollinated flowers

Mechanism That Prevent Self-Fertilization

Many angiosperms have mechanisms that make it difficult or impossible for a flower to fertilizeitself.

a. Self-incompatibility:

Recognition of self pollen is based on genes for self-incompatibility, called S-genes,with dozens of different alleles in a population. It is anti-selfing mechanism in floweringplants. It involve the ability of a plant to reject its own pollen and sometimes the pollen ofclosely related individuals.

b. Dichogamy:Some species that have stamens and carpels mature at different times.

Insect-pollinated flowers Wind-pollinated flowers

1. Large petals with colors that isvisible to the pollinating insects,and often lines or other marks toguide the insect in pollinating.

2. Sturdy filaments hold the anthers ina precise position inside the flowerwhere insects brush past and

collect pollen.

3. Relatively small numbers of large,spiky pollen grains are produced,which stick firmly to the insects

4. Sturdy style holds the stigma in aprecise position inside the flowerwhere it can collect pollen as insectsbrush past

5. Petals often release scent andnectaries secrete nectar, to attract

and reward insects

1. Small petals or equivalent structureswith green or dull coloration onlyneeded to protect the flower beforeit opens.

2. Long thin filaments hold the anthersloosely outside the rest of the flower,where pollen can be shaken off by

the wind.

3. Larger amounts of small, smoothpollen grains, with low density,easily carried by wind

4. Large feathery stigmas protrude fromthe rest of the flower, to increase thechance of catching pollen from thewind

5. No scent or nectar


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c. Relative positions of stamens and stigmas.Alternatively, they may be arranged in such a way that it is mechanically unlikely that ananimal pollinator could transfer pollen from the anthers to the stigma of the same flower.

d. Dioecious plants:Male and Female flowers are born on different plants, such as papaya.

Diagram of a pin-eyed flower cut from top to

bottom (longitudinal cross section)

Diagram of a thrum-eyed flower cut from top to

bottom (longitudinal cross section)

Fig. 12. 5 Pin andthrum flower types reduceself-fertilization


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Double Fertilization

After landing on a receptive stigma, a pollen grain germinates and produces a pollen tube thatextends down between the cells of the style toward the ovary. The pollen tube then dischargestwo sperm into the embryo sac, which then undergo double fertilization: One sperm fertilizes theegg and the other sperm combines with the polar nuclei, giving rise to the food-storing

endosperm.

Fig. 12.6 Growth of the pollen tube and double fertilization


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After landing on a receptive stigma, the pollen grain absorbs moisture and germinates,producing a pollen tube that extends down the style toward the ovary. The nucleus of thegenerative cell divides by mitosis to produce two sperms, the male gametes. The germinatedpollen grain thus contains the mature male gametophyte.

Directed by a chemical attractant, possibly calcium, the tip of the pollen tube enters the ovary,

probes through the micropyle (a gap in the integuments of the ovule), and discharges twosperms within the embryo sac.

Both sperm fuse with nuclei in the embryo sac. One sperm fertilizes the egg to form the zygote.The other sperm combines with the two polar nuclei to form a triploid nucleus in the central cell.This large cell will give rise to the endosperm, a food-storing tissue of the seed.

The union of two sperm cells with different nuclei of the embryo sac is termed doublefertilization. Double fertilization ensures that the endosperm will develop only in ovules wherethe egg has been fertilized. This prevents angiosperms from wasting the nutrients.

Normally nonreproductive tissues surrounding the embryo have prevented researchers from

visualizing fertilization in plants, but recently, scientists have been able to isolate sperm cellsand eggs and observe fertilization in vitro. The first cellular event after gamete fusion is anincrease in cytoplasmic Ca2+ levels, which also occurs during animal gamete fusion.

In another similarity to animals, plants establish a block to polyspermy, the fertilization of an eggby more than one sperm cell. In plants, this may be through evidence of cell wall material thatmechanically block the sperm. In maize, this barrier is established within 45 seconds after theinitial sperm fusion with the egg.

After fertilization, ovules develop into seeds and ovaries into fruits

From Ovule to Seed

After double fertilization each ovule develops into a seed. The ovary develops into a fruitenclosing the seed(s). Endosperm development usually occur before embryo development. Inmost monocots and some eudicots, the endosperm stores nutrients that can be used by theseedling after germination

In other eudicot the food reserves of the endosperm are completely exported to the cotyledons

Embryo Development

The first mitotic division of the zygote is transverse, splitting the fertilized egg into a basal celland a terminal cell


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Fig. 12.7 The development of eudicot plant embryo


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Structure of the Mature Seed

The embryo and its food supply are enclosed by a hard, protective seed coat. In a commongarden bean, a eudicot, the embryo consists of the hypocotyl, radicle, and thick cotyledons. Theseeds of other eudicots, such as castor beans have similar structures, but with thin cotyledons

Fig. 12.8a Seed structure

Fig. 12.8 b Seed structure

(a) Common garden bean, a eudicot with thickcotyledons. The fleshy cotyledons store food absorbedfrom the endosperm before the seed germinates.

(b) Castor bean, a eudicot with thin cotyledons. Thenarrow, membranous cotyledons (shown in edge andflat views) absorb food from the endosperm when the

seed germinates.


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The embryo of a monocot has a single cotyledon, a coleoptile, and a coleorhiza.

Fig. 12.8 c Seed structure

Germination Process Differ Among Plants

The radicle iIs the first organ to emerge from the germinating seed. There are types of seedgermination:

a. Epigeal:

A hook forms in the hypocotyl, elongation of the hypocotyl and growth pushes the hook(shoot apex and cotyledon) above ground. The seed has small cotyledon. Stimulated by

light, the hypocotyl straightens, raising the cotyledons and epicotyl, thus allow delicateshoot and bulky cotyledon to be pulled upward through the abrasive soil. Epigealgermination occur in many eudicots, for example, beans: Phaseolus vulgaris

Fig. 12.9 Epigeal germination incommon garden bean

(c)Maize, a monocot. Like all monocots, maize has only onecotyledon. Maize and other grasses have a large cotyledon called ascutellum. The rudimentary shoot is sheathed in a structure called thecoleoptile, and the coleorhiza covers the young root.


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b. Hypogeal

Hypogeal germination involves elongation of the epicotyl and straightening of the hook,cotyledon which is big remains behind underground. Maize and other grasses, which aremonocots, use a different method for breaking ground when they germinate. In the caseof maize, the coleoptile, the sheath enclosing and protecting the embryonic shoot,pushes upward through the soil and into the air. The shoot tip then grows straightup through the tunnel provided by the tubular coleoptile.

Fig. 12.10 Hypogeal germination in maize

Evolutionary adaptations of seed germination contribute to seedling survival.

Seed germination represents the continuation of growth and development, which wasinterrupted when the embryo become inactive or dormant at seed maturation. Some seedgerminate as soon as they reach a suitable environment. Others need a specific environmentalsignal before they will break dormancy.

a. Seed dormancy

As a seed matures, it dehydrates and enters a dormancy phase, a condition of extremely lowmetabolic rate and a suspension of growth and development. Conditions required to breakdormancy and resume growth and development vary between species.

Seed dormancy increases the chances that germination will occur at a time and place mostadvantageous to the seedling. For example, seeds of many desert plants germinate only after asubstantial rainfall, ensuring enough water to complete development. Where natural fires arecommon, many seeds require intense heat to break dormancy, allowing them to take advantage


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of new opportunities and open space. Where winters are harsh, seeds may require extendedexposure to cold. Small seeds such as lettuce require light for germination and break dormancyonly if they are buried near the surface.

Other seeds require a chemical attack or physical abrasion as they pass through an animalsdigestive tract before they can germinate.

The length of time that a dormant seed remains viable and capable of germinating varies from afew days to decades or longer. It depends on the species and on environmental conditions.Most seeds are durable enough to last for a year or two until conditions are favorable forgermination. Thus, the soil has a pool of nongerminated seeds that may have accumulated forseveral years. This is one reason vegetation reappears so rapidly after a fire, drought, flood, orsome other environmental disruption.

b. From seed to seedling

Germination of seeds depends on imbibition, the uptake of water due to the low water potential

of the dry seed. This causes the expanding seed to rupture its seed coat and triggers metabolicchanges in the embryo that enable it to resume growth. Enzymes begin digesting the storagematerials of endosperm or cotyledons, and the nutrients are transferred to the growing regionsof the embryo.

The first organ to emerge from the germinating seed is the radicle, the embryonic root. Next, theshoot tip must break through the soil surface. In garden beans and many other dicots, a hookforms in the hypocotyl, and growth pushes it aboveground. Stimulated by light, the hypocotylstraightens, raising the cotyledons and epicotyl.

As it rises into the air, the epicotyl spreads its first foliage leaves (true leaves). This foliageleaves expand, become green, and begin making food by photosynthesis. After the cotyledons

have transferred all their nutrients to the developing plant, they shrivel and fall off the seedling.

Monocots use a different method for breaking ground when they germinate. The coleoptile, thesheath enclosing and protecting the embryonic shoot, pushes upward through the soil and intothe air. The shoot tip then grows straight up through the tunnel provided by the tubularcoleoptile.

The tough seed gives rise to a fragile seedling that will be exposed to predators, parasites,wind, and other hazards. Because only a small fraction of seedlings endures long enough tobecome parents, plants must produce large numbers of seeds to compensate for low individualsurvival. However, flowering and fruiting in sexual reproduction is an expensive way of plantpropagation in terms of the resources consumed.

From Ovary to Fruit

A fruit develops from the ovary to protect the enclosed seeds and aids in the dispersal of seedsby wind or animals. Fruits are classified into several types depending on their developmentalorigin.


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Fig. 12.11 Developmental origin of fruits

a). Simple fruit. A simple fruit develops from a single carpel (or several fusedcarpels) of one flower (examples: pea, lemon, peanut).

b). Aggregate fruit. An aggregate fruit develops from many separate carpels of

one flower (examples: raspberry, blackberry, strawberry).

c). Multiple fruit. A multiple fruit develops from many carpels of many flowers(examples: pineapple,)

chapter 12 - reprod and growth in plants

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