Reproduction in Angiosperms

Angiosperms are flowering plants. They are the largest group of plants with about 90% of all plant species. They evolved from gymnosperms during the Mesozoic and became widespread during the Cenozoic.

The seeds of angiosperms are covered by a fruit. In many species, the fruit helps with dispersal of the seeds by attracting animals to consume them. Flowers may have contributed to the enormous success of angiosperms.

The flowers of many species attract insect and animal pollinators which carry pollen to other individuals of the same species.

Main Groups of Angiosperms

Angiosperms can be simply classified into two groups. They are the monocotyledonous plants (monocots) and the dicotyledonous plants (dicots).

Table 1: Characteristics of monocotyledonous and dicotyledonous plants

Sexual Reproduction in Angiosperms

The life cycle of flowering plants is similar to that of gymnosperms. It involves alternation of generations. A diploid sporophyte alternates with a haploid gametophyte.

Morphology of Flowers

Flowering plants are heterosporangiate, producing two types of reproductive spores. The pollen (male spores) and ovules (female spores) are produced in different organs, but the typical flower is a bisporangiatestrobilusin that it contains both organs.

A flower is regarded as a modified stem with shortened internodes and bearing, at its nodes, structures that may be highly modified leaves (Eames, 1961). In essence, a flower structure forms on a modified shoot or axiswith an apical meristem that does not grow continuously (growth is determinate).

Flowers may be attached to the plant in a few ways. If the flower has no stem but forms in the axil of a leaf, it is called sessile. When one flower is produced, the stem holding the flower is called a peduncle. If the peduncle ends with groups of flowers, each stem that holds a flower is called a pedicel.

The flowering stem forms a terminal end which is called the torusor receptacle. The parts of a flower are arranged in whorls on the torus. The four main parts or whorls (starting from the base of the flower or lowest node and working upwards) are as follows:

Floral Structures

Calyx: the outer whorl of sepals; typically these are green, but are petal-like in some species.

Corolla: the whorl of petals, which are usually thin, soft and colored to attract animals that help the process of pollination. The coloration may extend into the ultraviolet, which is visible to the compound eyes of insects, but not to the eyes of birds.

Androecium (from Greek and rosoikia: man’s house): one or two whorls of stamens, each a filament topped by an anther where pollen is produced. Pollen contains the male gametes.

Gynoecium (from Greek gynaikosoikia: woman’s house): one or more pistils. The female reproductive organ is the carpel: this contains an ovary with ovules (which contain female gametes). A pistil may consist of a number of carpels merged together, in which case there is only one pistil to each flower, or of a single individual carpel (the flower is then called apocarpous).

The sticky tip of the pistil, the stigma, is the receptor of pollen. The supportive stalk, the style becomes the pathway for pollen tubes to grow from pollen grains adhering to the stigma, to the ovules, carrying the reproductive material.

Although the floral structure described above is considered the “typical” structural plan, plant species show a wide variety of modifications from this plan. These modifications have significance in the evolution of flowering plants and are used extensively by botanists to establish relationships among plant species.

For example, the two subclasses of flowering plants may be distinguished by the number of floral organs in each whorl: dicotyledons typically having 4 or 5 organs (or a multiple of 4 or 5) in each whorl and monocotyledons having three or some multiple of three. The number of carpels in a compound pistil may be only two, or otherwise not related to the above generalization for monocots and dicots.

In the majority of species individual flowers have both pistils and stamens as described above. These flowers are described by botanists as being perfect, bisexual, or hermaphrodite.

However, in some species of plants the flowers are imperfector unisexual: having only either male (stamens) or female (pistil) parts. In the latter case, if an individual plant is either female or male the species is regarded as dioecious.

However, where unisexual male and female flowers appear on the same plant, the species is considered monoecious.

In those species that have more than one flower on an axis – so-called compositeflowers – the collection of flowers is termed an inflorescence; this term can also refer to the specific arrangements of flowers on a stem.

In this regard, care must be exercised in considering what a ‘‘flower’’ is. In botanical terminology, a single daisy or sunflower for example, is not a flower but a flower head – an inflorescence composed of numerous tiny flowers (sometimes called florets).

Each of these flowers may be anatomically as described above. Many flowers have a symmetry, if the perianth is bisected through the central axis from any point, symmetrical halves are produced – the flower is called regular or actinomorphic, e.g. rose or trillium. When flowers are bisected and produce only one line that produces symmetrical halves the flower is said to be irregular or zygomorphic. e.g. snapdragon or most orchids.

Floral formula

A floralformulais a way to represent the structure of a flower using specific letters, numbers, and symbols. Typically, a general formula will be used to represent the flower structure of a plant family rather than a particular species. The following representations are used:

Ca = calyx (sepal whorl; e. g. Ca⁵ = 5 sepals)

Co = corolla (petal whorl; e. g., Co^3(x) = petals some multiple of three)

Z = add if zygomorphic(e. g., CoZ⁶ = zygomorphic with 6 petals)

A = androecium (whorl of stamens; e. g., A^∞ = many stamens)

G = gynoecium (carpel or carpels; e. g., G¹ = monocarpous)

x: to represent a “variable number”

∞: to represent “many”

A floral formula would appear something like this:

_Ca5_Co5_A10 – ∞_G1

Pollination

The primary purpose of a flower is reproduction. Since the flowers are the reproductive organs of plant, they mediate the joining of the sperm, contained within pollen, to the ovules – contained in the ovary.

Pollination is the movement of pollen from the anthers to the stigma. The joining of the sperm to the ovules is called fertilization.

Normally pollen is moved from one plant to another, but many plants are able to self-pollinate. The fertilized ovules produce seeds that are the next generation. Sexual reproduction produces genetically unique offspring, allowing for adaptation.

Flowers have specific designs which encourage the transfer of pollen from one plant to another of the same species.

Many plants are dependent upon external factors for pollination, including: wind and animals, and especially insects. Even large animals such as birds, bats, and pygmy possums can be employed. The period of time during which this process can take place (the flower is fully expanded and functional) is called anthesis.

Pollination Attraction Methods

Plants cannot move from one location to another, thus many flowers have evolved to attract animals to transfer pollen between individuals in dispersed populations. Flowers that are insect-pollinated are called entomophilous; literally “insect-loving” in Latin.

They can be highly modified along with the pollinating insects by co-evolution. Flowers commonly have glands called nectarieson various parts that attract animals looking for nutritious nectar. Birds and bees have color vision, enabling them to seek out “colorful” flowers.

Some flowers have patterns, called nectar guides that show pollinators where to look for nectar; they may be visible only under ultraviolet light, which is visible to bees and some other insects. Flowers also attract pollinators by scent and some of those scents are pleasant to our sense of smell.

Not all flower scents are appealing to humans, a number of flowers are pollinated by insects that are attracted to rotten flesh and have flowers that smell like dead animals, often called Carrion flowers including Rafflesia, the titan arum.

Flowers pollinated by night visitors, including bats and moths, are likely to concentrate on scent to attract pollinators and most such flowers are white.

Pollination Mechanism

The pollination mechanism employed by a plant depends on what method of pollination is utilized. Most flowers can be divided between two broad groups of pollination methods:

Entomophilous: flowers attract and use insects, bats, birds or other animals to transfer pollen from one flower to the next. Often they are specialized in shape and have an arrangement of the stamens that ensures that pollen grains are transferred to the bodies of the pollinator when it lands in search of its attractant (such as nectar, pollen, or a mate).

In pursuing this attractant from many flowers of the same species, the pollinator transfers pollen to the stigmas – arranged with equally pointed precision – of all of the flowers it visits. Many flowers rely on simple proximity between flower parts to ensure pollination.

Anemophilous: flowers use the wind to move pollen from one flower to the next, examples include the grasses, Birch trees, Ragweed and Maples. They have no need to attract pollinators and therefore tend not to be “showy” flowers.

Whereas the pollen of entomophilous flowers tends to be large-grained, sticky, and rich in protein (another “reward” for pollinators), anemophilous flower pollen is usually small-grained, very light, and of little nutritional value to insects, though it may still be gathered in times of dearth.

Honeybees and bumblebees actively gather anemophilous corn (maize) pollen, though it is of little value to them.

Some flowers are self-pollinated and use flowers that never open or are self-pollinated before the flowers open, these flowers are called cleistogamous. Many Viola species and some Salvia have these types of flowers.

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Uses of Flowers

In modern times, people have sought ways to cultivate, buy, wear, or otherwise be around flowers and blooming plants, partly because of their agreeable appearance and smell.

Around the world, people use flowers for a wide range of events and functions that, cumulatively, encompass one’s lifetime. This includes:

For new births or Christenings

As tokens of love or esteem

For wedding flowers for the bridal party, and decorations for the hall

As brightening decorations within the home

As a gift of remembrance for bon voyage parties, welcome home parties, and “thinking of you” gifts

For funeral flowers and expressions of sympathy for the grieving

For worshiping goddesses: in Hindu culture it is very common to bring flowers as a gift to temples.

People therefore grow flowers around their homes, dedicate entire parts of their living space to flower gardens, pick wildflowers, or buy flowers from florists who depend on an entire network of commercial growers and shippers to support their trade.

Hundreds of fresh flowers are edible but few are widely marketed as food. They are often used to add color and flavor to salads. Squash flowers are dipped in breadcrumbs and fried.

Edible flowers include nasturtium, chrysanthemum, carnation, cattail, honeysuckle, chicory, cornflower, Canna, and sunflower. Some edible flowers are sometimes candied such as daisy and rose (you may also come across a candied pansy).

Flowers can also be made into herbal teas. Dried flowers such as chrysanthemum, rose, jasmine, camomile are infused into tea both for their fragrance and medical properties. Sometimes, they are also mixed with tea leaves for the added fragrance.

Fruit Development

A fruit is a ripened ovary. Inside the ovary is one or more ovules where the megagametophyte contains the mega gamete or egg cell. The ovules are fertilized in a process that starts with pollination, which involves the movement of pollen from the stamens to the stigma of flowers.

After pollination, a tube grows from the pollen through the stigma into the ovary to the ovule and sperm are transferred from the pollen to the ovule, within the ovule the sperm unites with the egg, forming a diploid zygote.

Fertilization in flowering plants involves both plasmogamy, the fusing of the sperm and egg protoplasm and karyogamy, the union of the sperm and egg nucleus.(Mauseth,2003)^. When the sperm enters the nucleus of the ovule and joins with the megagamete and the endosperm mother cell, the fertilization process is completed.(Rost et al, 1979)^.

As the developing seeds mature, the ovary begins to ripen. The ovules develop into seeds and the ovary wall, the pericarp, may become fleshy (as in berries or drupes), or form a hard outer covering (as in nuts).

In some cases, the sepals, petals and/or stamens and style of the flower fall off. Fruit development continues until the seeds have matured. In some multiseeded fruits, the extent to which the flesh develops is proportional to the number of fertilized ovules. (Mauseth. Botany. Chapter 9: Flowers and Reproduction).

The wall of the fruit, developed from the ovary wall of the flower, is called the pericarp. The pericarpis often differentiated into two or three distinct layers called the exocarp(outer layer, also called epicarp), mesocarp(middle layer), and endocarp(inner layer). In some fruits, especially simple fruits derived from an inferior ovary, other parts of the flower (such as the floral tube, including the petals, sepals, and stamens), fuse with the ovary and ripen with it.

The plant hormone ethylene causes ripening. When such other floral parts are a significant part of the fruit, it is called an accessoryfruit. Since other parts of the flower may contribute to the structure of the fruit, it is important to study flower structure to understand how a particular fruit forms. (Mauseth, 2003).

Types of Fruits

Fruits are so diverse that it is difficult to devise a classification scheme that includes all known fruits. Many common terms for seeds and fruit are incorrectly applied, a fact that complicates understanding of the terminology. Seeds are ripened ovules; fruits are the ripened ovaries or carpels that contain the seeds.

There are three basic types of fruits: Simple fruit, Aggregate fruit, and Multiple fruit.

1. Simple Fruit

Simple fruits can be either dry or fleshy, and result from the ripening of a simple or compound ovary with only one pistil. Dry fruits may be either dehiscent (opening to discharge seeds), or indehiscent (not opening to discharge seeds). (Schlegel. Encyclopedia Dictionary.pp.123).

Types of dry, simple fruits, with examples of each, are:

Achene – (dandelion seeds, strawberry seeds).

Capsule – (Brazil nut).

Caryopsis – (wheat).

Fibrous drupe – (coconut, walnut).

Follicle – (milkweed, magnolia).

Legume – (pea, bean, peanut).

Loment.

Nut – (hazelnut, beech, oak acorn).

Samara – (elm, ash, maple key).

Schizocarp – (carrot seed).

Silique – (radish seed).

Silicle – (shepherd’s purse).

Utricle – (beet).

Fruits in which part or all of the pericarp(fruit wall) is fleshy at maturity are simplefleshy fruits. Types of fleshy, simple fruits (with examples) are:

Berry – (redcurrant, gooseberry, tomato, avocado).

Stone fruit or drupe (plum, cherry, peach, apricot, olive).

False berry – Epigynous accessory fruits (banana, cranberry, strawberry (edible part).

Pome – accessory fruits (apple, pear, rosehip, saskatoon berry).

2. Aggregate fruit

An aggregate fruit, or etaerio, develops from a flower with numerous simple pistils. An example is the raspberry, whose simple fruits are termed drupelets because each is like a small drupe attached to the receptacle.

In some bramble fruits (such as blackberry) the receptacle is elongated and part of the ripe fruit, making the blackberry an aggregate-accessory fruit. (McGee. On Food and Cooking. pp. 361-362).

The strawberry is also an aggregate-accessory fruit, only one in which the seeds are contained in achenes. (McGee. On Food and Cooking. Pp.364-365). In all these examples, the fruit develops from a single flower with numerous pistils.

3. Multiple fruit

A multiple fruit is one formed from a cluster of flowers (called an inflorescence). Each flower produces a fruit, but these mature into a single mass. (Schlegel, 2003). Examples are the pineapple, edible fig, mulberry, osage-orange, and breadfruit.

There are also many dry multiple fruits, e.g. Tuliptree, multiple of samaras.

Sweet gum, multiple of capsules.

Sycamore and teasel, multiple of achenes.

Magnolia, multiple of follicles.

4. Seedless fruits

Seedlessness is an important feature of some fruits of commerce. Commercial cultivars of bananas and pineapples are examples of seedless fruits. Some cultivars of citrus fruits (especially navel oranges), satsumas, mandarin oranges table grapes, grapefruit, and watermelons are valued for their seedlessness.

In some species, seedlessness is the result of parthenocarpy, where fruits set without fertilization. Parthenocarpic fruit set may or may not require pollination. Most seedless citrus fruits require a pollination stimulus; bananas and pineapples do not.

Seedlessness in table grapes results from the abortion of the embryonic plant that is produced by fertilization, a phenomenon known as stenospermocarpy which requires normal pollination and fertilization (Spiegel et al, 1996).

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Uses of Fruits

Many hundreds of fruits, including fleshy fruits like apple, peach, pear, kiwifruit, watermelon and mango are commercially valuable as human food, eaten both fresh and as jams, marmalade and other preserves.

Fruits are also in manufactured foods like cookies, muffins, yoghurt, ice cream, cakes, and many more. Many fruits are used to make beverages, such as fruit juices (orange juice, apple juice, grape juice, etc) or alcoholic beverages, such as wine or brandy (McGee, 2004).

Apples are often used to make vinegar. Fruits are also used for gift giving, Fruit Basket and Fruit Bouquet are some common forms of fruit gifts.

Seed Dissemination/Dispersal

Variations in fruit structures largely depend on the mode of dispersal of the seeds they contain. This dispersal can be achieved by animals, wind, water, or explosive dehiscence. (Capon, 2005).

Some fruits have coats covered with spikes or hooked burrs, either to prevent themselves from being eaten by animals or to stick to the hairs, feathers or legs of animals, using them as dispersal agents. Examples include cocklebur and unicorn plant. (Heiser, 2003).

The sweet flesh of many fruits is “deliberately” appealing to animals, so that the seeds held within are eaten and “unwittingly” carried away and deposited at a distance from the parent.

Likewise, the nutritious, oily kernels of nuts are appealing to rodents (such as squirrels) who hoard them in the soil in order to avoid starving during the winter, thus giving those seeds that remain uneaten the chance to germinate and grow into a new plant away from their parent. (McGee, 2004).

Other fruits are elongated and flattened out naturally and so become thin, like wings or helicopter blades, e.g. maple, tuliptree and elm. This is an evolutionary mechanism to increase dispersal distance away from the parent via wind. Other wind-dispersed fruit have tiny parachutes, e.g. dandelion and salsify.

Coconut fruits can float thousands of miles in the ocean to spread seeds. Some other fruits that can disperse via water are nipa palm and screw pine (Capon, 2005).

Some fruits fling seeds substantial distances (up to 100 m in sandbox tree) via explosive dehiscence or other mechanisms, e.g. impatiens and squirting cucumber. (Feldkamp, 2002).

Development of Gametophytes

The diagram below shows the parts of the life cycle that is located within the ovary and within the anther. The ovary and the anther represent the female and the male reproductive structures, respectively.

The Megasporangium:

A sporangiumis a structure that produces spores (see the diagram of an ovule below in Fig 4.7). Two protective layers called integumentssurround the megasporangiumof flowering plants (angiosperms).

The entire structure including the integuments is the ovuleand is destined to become the seed. The integuments will become t./ he seed coat.

The diploid cell within the megasporangium will divide by meiosis to produce four megaspores (fig 4.7b).

Three of the megaspores disintegrate.(fig 4.7c)

The remaining megaspore nucleus divides 3 times to produce a cell with 8 nuclei. Cell walls form around them producing a gametophyte that has 7 cells and 8 nuclei. One of the cells contains two nuclei (Fig 4.7d). The micropyle is the opening in the integuments near the egg cell. Eventually, sperm pass through this opening.

Within the microsporangium:

An anther has 4 microsporangia (pollen sacs). Each contains many microsporocytesthat will divide by meiosis to produce 4 microsporeseach.

The diagram below shows a cross-section of an anther at three different stages of development. Initially, microsporangiacontain diploid cells. The sporangia and cells are part of the sporophyte (2N) plant.

The microspore produces a 2-celled microgametophyte and the microsporangia rupture to release pollen.

Pollen

In Fig.4.9, Pollen contains two nuclei, a generative nucleus and a tube nucleus. A membrane surrounds the generative nucleus and so it is technically a cell, but it contains very little cytoplasm. The generativecellis contained within the larger tubecell.

Pollination and Fertilization in Angiosperms

1. Pollination

Pollinationis the transfer of pollen to the stigma .After landing on the stigma of a flower (pollination), the tube cell elongates to produce a pollentube, which grows from the stigma through the style and through the micropyle to the egg.

The generative cell will divide by mitosis to produce two sperm. As in gymnosperms, the sperm of angiosperms are contained within the pollen tube and therefore do not require water.

2. Fertilization

Double Fertilization: One sperm fertilizes the egg the other one combines with the two polar nuclei forming a triploid (3N) cell.

The zygote grows by mitosis to form an embryo.

The 3N cell divides by mitosis and becomes endosperm, a food-containing material for the developing embryo.

The ovary, sometimes with other floral parts, develops into a fruit. It usually contains seeds.

Embryonic Development

During embryo development, the suspensor anchors and transfers nutrients to the developing embryo. In dicots, two heart-shaped cotyledons develop and absorb endosperm, which will be used as food when the seed germinates. Monocot cotyledons do not store endosperm. Instead, when the seed germinates, the cotyledon absorbs and transfers nutrients to the embryo.

The ovary of flowering plants becomes the fruit. Seeds are contained within the fruit. Gymnosperms do not produce fruit.

The wall of the ovary thickens to become the pericarpof the fruit.

Fruits can be either fleshy or dry. Peaches, tomatoes, and oranges are fleshy fruits. Nuts and grains are dry fruits.

Asexual Reproduction in Angiosperms

1. Stems

New plants can grow from horizontal stems.

Aboveground horizontal stems are called stolons (runners).

Underground horizontal stems are called rhizomes.

White potatoes are underground stems. They eyes are buds and can be used to produce new plants.

2. Roots

Sweet potatoes are modified roots and can be used to produce new plants.

The roots of some trees (apple, cherry) produce suckers (small plants) that can produce a new tree.

3. Cuttings

Cut stems can be treated with hormones and to encourage root growth. Cassava and flower stem cuttings produce new plants

Stems can be grafted to plants that have roots while yam and potato stem cuttings also produce new plants

Axillary buds can be grafted to another plant to produce new branches from the grafted bud.

4. Tissue Culture

Plant tissue is grown on special culture and treated with hormones to stimulate the cells to grow into plants. This is a specialized technique that requires special equipment and expertise.

This technique is usually carried out in dedicated laboratories for tissue culture studies. Many plants can be produced from a few cells.

5. Genetic Engineering

Genetic engineering is concerned with modifying the DNA of organisms. Plants have been genetically engineered to produce species that are resistant to freezing, fungi and bacteria infections, insect pests, herbicides, stresses and spoilage.

Transgenic plants contain DNA from a different species. The information above on sexual reproduction were obtained from anonymous 2010.

In conclusion, Angiospermsare plants that produce flowers. The latter are pollinated by different agents which results in fertilization and the production of fruits containing seeds. The seeds are also wildly dispersed contributing to the huge success of the angiosperms on earth.

The angiosperms can also be asexually reproduced by various technique/methods such as stems, roots cuttings, tissue culture and genetic engineering.

Angiosperms are classified into monocot and dicot. The life cycle of flowering plants involve alternation of generations- a diploid sporophyte alternating with a haploid gametophyte

Flower parts are modified leaves attached to a stem tip called the receptacle. Monocots have flowers in multiple of threes; dicot parts are in multiples of fours or fives

Stamens are composed of an anther and a filament which are the male reproductive parts. The anther contain the microsporangium which produces the microspores. Ovules are structures that will become the seeds

All of the female reproductive structures form the pistils. The bottom portion of a pistil is the ovary. Plants can reproduce asexually by stem, tissue culture, cuttings, roots, and genetic engineering.

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