Chapter 8FLOWER, FRUITS AND SEEDS
The most obvious characteristic of an angiosperm is the flower.
The flower contains the reproductive parts of the plant.
The name angiosperm is derived from the Greek angeion, means vessel, and sperma means seed.
- Annual plants complete their life cycle in one growing season; grow, bloom, produce seeds and die in a single season.
- Biennial plants complete their life cycle in two years; they grow during the first growing season, and spent the second growing season blooming and producing seeds, then die.
- Perennial plants live several to many years. They may bloom many times or only once in their life time.
DIFFERENCES BETWEEN MONOCOTS AND DICOTS
Angiosperms are divided into two classes, the Dicots (Magnoliopsida) and the Monocots (Liliopsida).
MONOCOTS DICOTS
Flower parts usually in multiples of three. Flower parts usually in multiples of
four or five.
Pollen with single furrow or pore, monoaperturate. Pollen with three furrows or pores,
triaperturate.
Embryo with single cotyledon. Embryo with two cotyledons.
Major leaf veins parallel Major leaf veins reticulated
Stem vascular bundles scattered Stem vascular bundles in a ring
Roots are adventitious Roots develop from radicle.
Vascular and cork cambium absent Vascular and cork cambium
present
There are about 65,000 species of monocots and about 165,000 species of dicots.
STRUCTURE OF THE FLOWER
Flower formation involves a phase change from vegetative growth to reproductive growth.
Meristem identity genes control the transition from a vegetative shoot to a floral shoot.
These genes confer floral identity on the peripheral zone cells that give rise to primordia, so that they produce flowers rather than vegetative structures.
Some of these genes are under the influence of photoperiod (length of day light) and the plant hormone gibberillin.
Once a shoot meristem is induced to flower, position information commits each primordium that arises on the flanks of the shoot tip to develop into an organ of specific structure and function.
Organ identity genes are regulated by positional information and produce transcription factors.
They function in the development of the floral pattern, e. g. sepals form the first whorl, petals the next whorl, then anthers, etc.
We know that there are three classes of organ identity genes that control formation of sepals, petals, stamens, and carpels.
In many angiosperms, the flowers are arranged in clusters called inflorescence.
The stalk of an inflorescence is called the peduncle.
The stalk of an individual flower is the pedicel.
The flower parts are attached to the receptacle.
The receptacle bears sterile and fertile (reproductive) parts.
The perianth, usually consists of an outer whorl of sepals and an inner whorl of petals.
In monocots and "primitive" dicots, the sepals and petals may be indistinguishable. In this case the perianth parts are called tepals.
Bracts are specialized leaves that may be as colorful as petals and can attract pollinators, e. g. red bracts of poinsettias.
Reproductive parts
- The stamens or male parts are collectively called the androecium.
- The stamens consist of a filament, which supports the two-lobed anther.
- The anther contains four microsporangia in two pairs within which microspores (pollen) are produced by meiosis.
- The microsporangia are also known as pollen sacs
The carpels or female part of the flower is collectively called the gynoecium.
- A flower may have one or several carpels, which may be separate or fused.
- The individual carpel or the group of fused carpels is often called the pistil.
- The pistil of most flowers consists of the stigma, style, and ovary containing one or more ovules.
- Fused carpels may have a common style and stigma or may maintain separate ones.
- The ovary of fused carpels is often divided into locules.
- The number of locules is related to the number of fused carpels.
The ovules are attached to the ovary wall at the placenta.
- The arrangement of placentas is known as placentation.
- Placentation varies in different groups of flowering plants.
- Parietal placentation: ovules are born on the ovary wall or extensions from it.
- Axil placentation: ovules are borne on a central column in a partitioned ovary. E.g. tomatoes.
- Free central placentation: ovules are borne on a central column in a non-partitioned ovary, e. g. bell pepper.
- Apical or basal placentation: one ovule is found at the apex or base of a unilocular ovary.
Attachment of perianth:
- Hypogynous: perianth and stamens are attached below the ovary; this ovary is also called a superior ovary.
- Epigynous: perianth and stamens are attached to the top of the ovary; this ovary is also called an inferior ovary.
- Perigynous: perianth and stamens adnate and forming a short tube (hypanthium) free from the ovary.
FRUITS
Fruits have evolved for dispersal.
A fruit is mature ovary, which may or may not include some additional flower parts, and usually contains seeds.
Fertilization often determines if an ovary is going to develop into a fruit.
Pollen grains contain specific stimulants called hormones that may initiate fruit development, and sometimes a little dead pollen is all that is needed to stimulate an ovary into becoming a fruit.
Fruits that develop without fertilization are called parthenocarpic.
Fruit regions
Most mature fruits have three regions, sometimes difficult to distinguish.
- Pericarp is the collective name of the three regions: Exocarp, Mesocarp, endocarp.
- Exocarp is the skin of the fruit.
- Mesocarp is the middle portion, often juicy and edible.
- Endocarp is the area around the seed often hard.
CLASSIFICATION OF FRUITS
Based on composition:
- True fruits are made of the ovary; accessory fruits include other parts of the flower.
Based on texture:
- Fruits can be fleshy and juicy or dry.
Based on the flower type or origin of the fruit:
- Developed from a single flower with many ovaries, e.g. blackberries
- Developed from many flowers whose parts fuse together to make one fruit, e.g. pineapples, Osage orange.
Some fruits consist of the ovary and seeds; others have adjacent flower parts incorporated into the mature fruit.
Fruits may fleshy or dried at maturity; may split or not; may be made from one or several ovaries.
- Accessory fruits include other parts in addition to the ovaries, e.g. strawberry, apple, and pear.
- Parthenocarpic fruits develop without seed formation, e.g. bananas, seedless grapefruits.
- Simple fruits develop from a single carpel or several united carpels.
Simple fruits:
A. Fleshy fruits
Fleshy fruits can be classified according to a combination of several characteristic:
Drupe: one with one seed, the inner layer of the fruit is stony and adherent to the seed. The exocarp is a thin skin. Characteristic of a drupe is the hard, lignified stone (or pit) derived from the ovary wall of the flower, e. g. apricots, plums, cherries, peaches, olives and almonds.
Berry: derived from one or many carpels with one or many seeded, the inner layer of the fruit wall is fleshy and juicy, and the skin is thin, e.g. grapes, tomatoes, peppers, eggplant.
- Pepo (an accessory fruit), a berry with a hard rind, the receptacle partially or
completely enclosing the ovary, e.g. pumpkin, cucumbers, squash.
- Hesperidium, a specialized berry with a leathery rind made of the exocarp and mesocarp; the fleshy area is the endocarp, e.g. oranges, lemons.
Pome: characteristic of the rose family, a juicy accessory fruit derived from the receptacle, e. g. apples, pears, quinces.
B. Dry fruits can be either dehiscent or indehiscent.
- Dehiscent dry fruits:
Follicle: derived from a single carpel that splits down one side at maturity, e.g. milkweed, columbine.
Legume: derived from a single carpel that splits down both sides at maturity, e.g. string beans, peas, kudzu. It is the fruit type produced by the Bean family, Leguminosae.
Silique: derived from two fused carpels, the seed remain attached to the central portion and the two enclosing valves fall away, e.g. mustard family: cabbage, radish, watercress.
Capsule: several fused carpels, dehiscence in several slits or pores, e.g. orchids, irises, lilies, poppies, violets.
- Indehiscent dry fruits.
Achene: derived from one superior ovary, small, single seeded fruit in which the seed is attached to the fruit wall at one point only, e.g. sunflower seed, buttercup, buckwheat.
Nut: hard, dry, one-seed fruit derived from a compound ovary, e.g. acorn, hickory nuts.
Caryopsis or grains: achenes with the fruit wall tightly attached to the fruit, e.g. grass seeds: corn, wheat, rice.
Samaras: winged achenes, e.g. ash, maples.
Schizocarp: the fruit splits at maturity into two or more one-seeded portions, e. g. unique to the Parsley family, Apiaceae; includes carrots, parsley, anise and dill.
Fruits produced by the fusion of several carpels
Aggregate fruits are produced from separate carpels in one gynoecium, e.g. blackberries, magnolia.
- The individual parts of the aggregate fruit are known as fruitlets.
- aggregate of capsules — sweetgum
aggregate of follicles — magnolia
aggregate of samaras — yellow-poplar, ash
aggregate of nutlets — sycamore, birch
aggregate of drupes — blackberry
Multiple fruits consist of several gynoecia and accessory flower parts that become fused into one fruit, e.g. pineapple, Osage orange, mulberry, breadfruit.
FRUIT DISPERSAL
Wind:
- Adaptations for wind dispersal include dust-like seeds, seeds with wings, pappus, wooly hairs around the seeds, forceful discharge of the seeds, and in tumbleweeds, and the entire plant breaks off and the seeds are scattered as the plant is blown over the fields.
A pappus is a tuft of hairs or feather like processes in the place of a calyx.
Water:
- These fruits have air spaces that keep them afloat to be dispersed by water currents.
- Rain and floods help scatter the seeds of fruits that drop to the ground and do not have any special adaptation for water buoyancy.
Animals:
- Vertebrates usually eat fleshy fruits. The seeds pass unharmed through the digestive track of the animal. Sometimes digestion helps germination by weakening the seed coat.
- Smell and color of ripe fruits attract animals.
- Many fruits and seeds have hooks, barbs, spines, sticky covering, etc. that allow them to become attached to the fur and feathers of animals and be transported over long distances.
- Ants bring seeds with elaiosome, a fleshy pigmented appendage rich in lipids, proteins and sugars. The ants bring the seeds to their nest and consume the elaiosome. The seeds germinate in the nest. Squirrels burry acorns some of which are eaten and germinate at a distance from the mother tree.
Some plants eject their seed forcefully.
SEEDS
The hilum is the scar left on the seed by the attachment to the ovary wall.
A tiny pore called the micropyle is located right next to the hilum.
Dicot embryos.
In some embryos, only the apical meristem occurs above the cotyledons.
In others, there is a stem-like axis, the epicotyl, with one or two leaves. This shoot is called the plumule.
The region of the axis below the cotyledons is called the hypocotyl.
Below the hypocotyl is the embryonic root or radicle with distinctive root characteristics.
In many dicots most or all of the endosperm is absorbed by the cotyledons and embryo.
In other dicots, the cotyledons are thin and the endosperm large.
Monocot embryos.
The single cotyledon absorbs food from the endosperm by enzymatic activity.
In grasses, the cotyledon is large and called the scutellun.
The scutellum is attached to one side of the axis of the embryo.
The embryo has a plumule and a radicle both surrounded by a protective sheath: the coleoptile surrounds the plumule and the coleorhiza surrounds the radicle.
GERMINATION
The growth of the embryo is arrested during seed maturation and dispersal.
Germination depends on external and internal factors.
Dormancy is of great survival value to the plant because it is a method of ensuring conditions will be favorable for growth on the seedling when germination occurs.
Dormant seeds will not germinate under favorable conditions due to some physiological requirement that has to be met.
The most common causes of dormancy are physiological immaturity of the seed and impermeability of the seed coat to water and oxygen.
Physiological immature seeds must undergo a complex series of enzymatic and biochemical changes called after-ripening before they germinate, e.g. American holly.
In temperate regions low winter temperatures trigger after-ripening. This prevents germination during the inclement winter months.
Some seeds must pass through the digestive track of birds thus insuring good dispersal.
Some desert seeds germinate only after inhibitors in the seed coat are washed away by rain insuring that the seedling will have sufficient water to mature.
Other seeds have to be cracked mechanically.
Some seeds are dormant until released when the cones or fruits are heated by fire.
External factors.
- Water, oxygen and temperature are the most important factors.
- Mature seeds are very dry normally containing 5 to 20% water.
- Imbibition of water is needed to start enzymatic activity.
- Cell enlargement and division starts.
- Food digestion begins to provide energy for growth and division.
- Cell respiration is anaerobic at the beginning but once the seed coat ruptures, it becomes aerobic.
- Seeds normally will not germinate below (0oC - 5 oC) and above (45 oC - 48 oC) certain temperatures. The optimum range is 20oC to 30oC.
- Light is the requirement for the germination of some seeds. The seeds will not germinate in the dark, e.g. lettuce.
LONGEVITY
Some seeds remain viable for days, others for more than 100 years. Lotus seeds have been known to germinate after 300, 600 and 1200 years of dormancy in archeological sites.
Seeds remain viable for different periods of time depending on the species and conditions of storage, usually dry and cool temperatures contribute to dormancy and viability.
Some weed species have their viability extended by storage under humid, cool conditions that include little oxygen.
Some plants produce seeds that do not have any dormant period. E.g. mangrove seeds begin to germinate while the fruit is still attached to the mother plant. This condition is called vivipary.
Chapter 8FLOWER, FRUITS AND SEEDS
The most obvious characteristic of an angiosperm is the flower.
The flower contains the reproductive parts of the plant.
The name angiosperm is derived from the Greek angeion, means vessel, and sperma means seed.
- Annual plants complete their life cycle in one growing season; grow, bloom, produce seeds and die in a single season.
- Biennial plants complete their life cycle in two years; they grow during the first growing season, and spent the second growing season blooming and producing seeds, then die.
- Perennial plants live several to many years. They may bloom many times or only once in their life time.
DIFFERENCES BETWEEN MONOCOTS AND DICOTS
Angiosperms are divided into two classes, the Dicots (Magnoliopsida) and the Monocots (Liliopsida).
MONOCOTS DICOTS
Flower parts usually in multiples of three. Flower parts usually in multiples of
four or five.
Pollen with single furrow or pore, monoaperturate. Pollen with three furrows or pores,
triaperturate.
Embryo with single cotyledon. Embryo with two cotyledons.
Major leaf veins parallel Major leaf veins reticulated
Stem vascular bundles scattered Stem vascular bundles in a ring
Roots are adventitious Roots develop from radicle.
Vascular and cork cambium absent Vascular and cork cambium
present
There are about 65,000 species of monocots and about 165,000 species of dicots.
STRUCTURE OF THE FLOWER
Flower formation involves a phase change from vegetative growth to reproductive growth.
Meristem identity genes control the transition from a vegetative shoot to a floral shoot.
These genes confer floral identity on the peripheral zone cells that give rise to primordia, so that they produce flowers rather than vegetative structures.
Some of these genes are under the influence of photoperiod (length of day light) and the plant hormone gibberillin.
Once a shoot meristem is induced to flower, position information commits each primordium tat arises on the flanks of the shoot tip to develop into an organ of specific structure and function.
Organ identity genes are regulated by positional information and produce transcription factors.
They function in the development of the floral pattern, e. g. sepals form the first whorl, petals the next whorl, then anthers, etc.
We know that there are three classes of organ identity genes that control formation of sepals, petals, stamens, and carpels.
In many angiosperms, the flowers are arranged in clusters called inflorescence.
The stalk of an inflorescence is called the peduncle.
The stalk of an individual flower is the pedicel.
The flower parts are attached to the receptacle.
The receptacle bears sterile and fertile (reproductive) parts.
The perianth, usually consists of an outer whorl of sepals and an inner whorl of petals.
In monocots and "primitive" dicots, the sepals and petals may be indistinguishable. In this case the perianth parts are called tepals.
Bracts are specialized leaves that may be as colorful as petals and can attract pollinators, e. g. red bracts of poinsettias.
Reproductive parts
- The stamens or male parts are collectively called the androecium.
- The stamens consist of a filament, which supports the two-lobed anther.
- The anther contains four microsporangia in two pairs within which microspores (pollen) are produced by meiosis.
- The microsporangia are also known as pollen sacs
The carpels or female part of the flower is collectively called the gynoecium.