Life Cycle of Angiosperm (flowering plants)

The life cycle of angiosperms, or flowering plants, involves a complex alternation of generations known as the sporophytic and gametophytic phases. The sporophytic phase is the dominant phase in angiosperms. Alternation is a change resulting in something different from the original.

Alternation of Generations Definition :

Alternation of generations is a life cycle in which subsequent generations of plants alternate between diploid and haploid organisms.

Angiosperms – Taxonomy

The taxonomy of angiosperms is based on several primitive and advanced characteristics proposed by many systematists like Linnaeus and Takhtajan. Their ideas established different principles in taxonomy which are now helping us to understand the concept of angiosperms’ phylogeny.

Kingdom	Plantae
Division	Spermatophyta
Infradivision	Angiosperm

Angiosperms were formerly termed Magnoliophyta. This division of flowering plants is further divided into 2 classes:

(Monocotyledons) – Banana

 (Dicotyledons) – Sunflower

Types of Generations In Life Cycle of Angiosperm:

The life cycle of an angiosperm comprises two generations:

1. Sporophyte: Spore-producing generations

2. Gametophyte: Gamete-producing generation.

Both generations alternate with each other in regular sequence. The Reduction Division occurs at the time of the formation of spores, and the diploidy is restored at the time of the zygote formation. The zygote is, therefore, the first cell of the sporophyte.

Another characteristic of angiosperms is the formation of endosperm that provides Nourishment to the developing embryo.

Angiosperm Plant Body :

Angiosperm, also called flowering plant, is any of about 300,000 species of flowering plants, the largest and most diverse group within the kingdom Plantae. Angiosperms represent approximately 80 percent of all the known green plants now living.

An angiosperm plant consists of the following:

 Vegetative organs:

• Root
• Stem
• Branches
• leaves

Reproductive organs:

• Flowers

Flowers vary in shape and color from spp. to spp. Vegetative organs are meant for the Absorption of water and minerals (raw materials), the manufacture of food, and its Utilization

For :

• Growth
• Development.
• Some Vegetative organs are used for Reproduction.

The angiosperms are vascular seed plants in which the ovule (egg) is fertilized and develops into a seed in an enclosed hollow ovary. The ovary is usually enclosed in a flower, that part of the angiospermous plant that contains the male or female reproductive organs or both.

Fruits are derived from the maturing floral organs of the angiospermous plant and are, therefore, characteristic of angiosperms.

The Sporophyte

A diploid phase (2n) in the life cycle of a plant and some algae is known as the sporophyte. Sporophyte develops from the zygote and produces spores by meiosis. The spores then develop into haploid gametophytes. It produces two morphologically and physiologically different kinds of spores :

• Microspore.
• Megaspore.

Structure of Typical Flower :

A typical flower has four main parts or whorls known as the calyx, corolla, androecium, and gynoecium.

The outermost whorl of the flower has green, leafy structures known as sepals. The sepals, collectively called the calyx, help to protect the unopened bud. The second whorl comprises petals usually brightly colored collectively called the corolla. The number of sepals and petals varies depending on whether the plant is a monocot or dicot.

In monocots, petals usually number three or multiples of three; in dicots, the number of petals is four or five, or multiples of four and five. Together, the calyx and corolla are known as the perianth. The third whorl contains the male reproductive structures known as the androecium.

The androecium has stamens with anthers that contain the microsporangia. The innermost group of structures in flowers is the gynoecium, or the female reproductive component(s). The carpel is the individual unit of the gynoecium and has a stigma, style, and ovary. A flower may have one or multiple carpels.

Complete Flower :

If all four whorls (the calyx, corolla, androecium, and gynoecium) are present, the flower is described as complete.

Incomplete Flower :

The flower is incomplete if any of the four parts is missing.

There are two types of incomplete flowers:

1. Staminate flowers contain only an androecium.

2. carpellate flowers have only a gynoecium.

Perfect Flower :

Flowers that contain both an androecium and a gynoecium are called perfect, androgynous or hermaphrodites.

Monoecious Flower :

If both male and female flowers are borne on the same plant, the species is called monoecious (meaning “one home”): examples are corn and pea.

Dioecious Flower :

Species with male and female flowers borne on separate plants are called dioecious, or “two homes,” for example, papaya and Cannabis.

The ovary, which may contain one or multiple ovules, may be placed above other flower parts, referred to as superior, or it may be placed below the other flower parts, referred to as inferior.

Microsporophyll ( Stamen ) :

Stamens are the male reproductive part of the flower. Technically they are called microsporophyll because these are the organs that bear the microsporangium or the anther, within which are the millions of microsporocytes that develop into the male gametophyte or pollen grains.

Microsporangium :

A microsporangium appears to have a circular outline when viewed transversely. Four layers surround it:

• Epidermis
• Endothecium
• Middle layers
• Tapetum

The outer three layers protect the pollen and help split the anther to release the pollen. The tapetum nourishes the pollen grain. The cells of the tapetum are multinucleated and have dense cytoplasm. A young anther comprises a group of compactly arranged homogenous cells called sporogenous tissue.

 Microspore :

Each microspore is a unicellular, uninucleate structure possessing two layered walls. The outer layer is thick, cuticularized, and called exine, and the inner is thin and Known as intine. The exine contains one or more thin places, the germ pores. The In some cases, the exine is provided with spinous outgrowths, warts, or reticulations.

Megasporphyll (Carpel) :

Each megasporophyll or carpel consists of a basal swollen part, the ovary; a Stalk is usually present at the top of the ovary, the style; and a stigma, the variously Modified tip of the style. The stigma is the receptive disc and receives pollen grains.

OVULE OR INTEGUMENTED MEGASPORANGIUM :

The ovary contains one or more rounded or oval bodies, the ovules. It consists of following

PLACENTA :

Ovules are borne on the ovary wall upon a specialized tissue, the placenta (singular placenta).

Funicle:

Each ovule is attached to the placenta through a short stalk, the funicle.

Hilum :

The place of attachment of the funicle with the main body of the ovule is the hilum.

Integuments :

The body of the ovule consists of sporogenous tissue, the nucellus, surrounded by One or two protective coverings, the integuments.

Micropyle :

The integuments do not surround the nucellus. Instead, a tiny opening called a micropyle is left at the top.

Chalaza:

The junction of the nucellus and integument is known as the chalaza Nucellus represents megasporangium; therefore, the ovule is also termed integumented Megasporangium. A nucellus cell enlarges in size and acts as a megaspore mother cell. It divides meiotically to produce a linear tetrad of megaspores.

The upper three usually disintegrate, and the remaining enlarges to form an embryo sac or female gametophyte.

The Gametophyte :

The gametophyte phase of angiosperms is haploid, much reduced, and dependent upon sporophyte.

Male Gametophyte :

The microspores develop into male gametophytes respectively. The first gametophytic structure modifies the microspore mother cell into the pollen grain. Male gametophyte development can be classified into two stages:

1. Pre-pollination development.
2. Post-pollination development.

PRE – POLLINATION DEVELOPMENT :

The pollen grain grows into the male gametophyte on germination, which is initiated before pollination takes place.

Inside the microsporangium, the Pollen mother cell (PMC) undergoes meiosis, resulting in four microspores that eventually mature into pollen grains. The inner layer called the tapetum, nourishes the developing microspores.

The pollen grains consist of two cells :

• Vegetative cell.
• Generative cell.

Once the microsporangium is matured, it bursts and releases the pollen from the anther. The pollen grain consists of two layers.

• The outer thick layer is called the exine.
• The inner thin layer is called the intine, which protects the pollen from damage.

Pollination :

The intine grows out through the germ pore to form a pollen tube. In this semi-germinated condition, the microspore, now called pollen grain, moves out of the microsporangium and is carried to the stigma of the carpel by wind, water, insects or animals. This transference is called pollination.

Post–Pollination:

The pollen grain completes the rest of the germination there. The generative nucleus divides into two to form Sperm nuclei, and the pollen tube elongates and carries the sperm nuclei to the egg cell.

Female Gametophyte :

The megaspore is the first cell of the female gametophyte. An eight-nucleate embryo Sac represents the female Gametophyte. A typical embryo sac (Polygonum type) contains an egg apparatus comprising an egg cell and two.

Synergids or help cells at the micropylar end and three antipodal cells at the chalazal end. And a diploid secondary nucleus (fusion nucleus) in the center.

Double Fertilization:-

In angiosperms, during fertilization, one male gamete fuses with the egg cell and forms a diploid zygote in a process called syngamy.

The other male gamete fuses with the two polar nuclei to form a triploid nucleus called the primary endosperm nucleus. This process is called triple fusion. Since fertilization occurs twice here, this process is called double fertilization.

• Significance – Due to double fertilization, the triploid nucleus develops into an endosperm, which serves as nutrition for the embryo.
• Endosperms :
• The initiation of endosperm is a definitive characteristic of the double fertilization of angiosperms.

• Its development requires the fusion of at least one polar nucleus in the embryo sac with one of the two sperm nuclei from the pollen grain.
• In gymnosperms, the nutritive material of the seed is present before fertilization.
• The endosperm is the tissue that surrounds and nourishes the embryo in the seeds of angiosperms (flowering plants).
• In some seeds, the endosperm is completely absorbed in maturity (e.g., pea and bean), and the fleshy food-storing cotyledons nourish the embryo as it germinates.
• In others, some endosperms are present until germination (e.g., wheat, castor bean). The cotyledons are typically thin and membranous and serve to absorb the stored food from the endosperm upon germination.
• In the coconut, the liquid endosperm contains important growth substances. Endosperm accounts for the economic importance of cereal grains and oilseeds.

THE EMBRYO :

The embryo in dicots is octant type and comprises an embryonal axis too. The cotyledons are attached.

Epicotyl :

The part of the axis above the point of Attachment of cotyledons is epicotyl. It bears a bud, the plumule, which develops into the shoot.

Hypocotyl :

The part of the embryonal axis below the point of attachment of Cotyledons is hypocotyl. It possesses a meristem at its tip, the radical which develops into the primary root.

In monocots, the embryo is filamentous type. The cotyledon is terminal And encloses the plumule at the tip of the embryonal axis, whereas the Radical is at the lower end.

SEED :

A seed is a ripened or mature ovule protected by seed coats enclosing a resting embryo provided with ample stored food, Either in endosperm or cotyledons and sometimes in perisperm.

In dicot seeds, the embryo consists of a short axis to which the embryonal Leaves, the cotyledons, are attached. The part of the axis above the point of the attachment of cotyledons is epicotyl, while the part below the point of the Attachment of cotyledons is hypocotyl.

At the upper end of the epicotyl, a minute Bud, the plumule, is present between the two cotyledons. It gives rise to embryonal shoots. The hypocotyl’s lower end is radical, giving rise to the primary root.

In monocot seeds, the cotyledon is terminal, and the upper part of the Embryonal axis bears plumule while the lower part contains radicals.

Number of Cotyledons: Based on the number of cotyledons, the seed may be dicotyledonous (in Which two cotyledons are present, e.g., pea seed ), monocotyledonous (in which a Single cotyledon is a present grain of maize) Similarly, a seed may be :

Endospermic or albuminous :

When endosperm is present (grain of maize)

Non – endospermic  or ex-albuminous :

In which the food in the endosperm is digested and Stored in cotyledons (pea seeds).

FRUIT :

In angiosperms, the seed is enclosed within fruits which protect and disperse seeds. The ovary wall changes to form the fruit wall or pericarp, which may be dry or fleshy. The seeds germinate into a sporophyte upon reaching a suitable place.

Key Points (Life Cycle of Angiosperm)

• 🌱 Alternation of generations is a life cycle in which subsequent generations of plants alternate between diploid and haploid organisms.
• 🌱 The taxonomy of angiosperms is based on several primitive and advanced characteristics.
• 🌱 Angiosperms consist of two generations: Sporophyte, which produces spores, and Gametophyte, which produces gametes.
• 🌱 The angiosperm plant body includes vegetative organs like roots, stems, branches, and leaves, as well as reproductive organs like flowers.
• 🌱 The sporophyte phase produces two morphologically and physiologically different kinds of spores: microspores and megaspores.
• 🌱 A typical flower has four main parts, or whorls: calyx, corolla, androecium, and gynoecium.
• 🌱 The male gametophyte of angiosperms forms two sperm nuclei, which are carried to the egg cell by the pollen tube.
• 🌱 The female gametophyte of angiosperms includes an eight-nucleate embryo sac containing an egg cell, two synergids, three antipodal cells, and a diploid secondary nucleus.
• 🌱 Double fertilization in angiosperms involves the fusion of one male gamete with the egg cell to form a diploid zygote and another male gamete with the two polar nuclei to form a triploid nucleus called the primary endosperm nucleus. This process is called triple fusion and leads to the development of endosperm, which serves as nutrition for the embryo.
• 🌱 Endosperm is the tissue that surrounds and nourishes the embryo in the seeds of angiosperms. It is completely absorbed in some seeds (e.g. peas and beans) and present until germination in others (e.g., wheat, castor bean). Endosperm accounts for the economic importance of cereal grains and oilseeds.
• 🌱 The embryo of dicots has an embryonal axis to which the cotyledons are attached. The epicotyl bears the plumule, which develops into the shoot, while the hypocotyl possesses a meristem at its tip, the radical, which develops into the primary root. In monocots, the embryo is filamentous type, and the cotyledon is terminal and encloses the plumule at the tip of the embryonal axis, whereas the radical is at the lower end.
• 🌱 Seeds can be dicotyledonous (with two cotyledons) or monocotyledonous (with a single cotyledon). They can be endospermic (with endosperm present) or non-endospermic (with the food stored in cotyledons).
• 🌱 In angiosperms, the seed is enclosed within fruits, which protect and disperse seeds. The ovary wall changes to form the fruit wall or pericarp, which may be dry or fleshy.

Frequently Asked Questions:

References:

• https://www.studysmarter.co.uk/explanations/biology/plant-biology/
• https://www.britannica.com/plant/angiosperm/Reproduction
• https://en.wikipedia.org/wiki/Flowering_plant
• Book : The Biology of Reproduction By Giuseppe Fusco, Alessandro Minelli