Plant Reproduction

• Reproduction is a process by which an organism produces young ones similar to itself, involving either one or two compatible organisms.

Types of Reproduction

• Vegetative reproduction involves fragmentation.
• Asexual reproduction involves zoospore formation.
• Sexual reproduction involves gamete formation.

Sexual Reproduction

• Sexual reproduction is a process involving gametogenesis, fertilization, and meiosis to produce young ones.
• Fertilization is considered the most crucial event in sexual reproduction.

Flower

• A flower originates from a floral bud, which is of the same origin as the primordia.
• The decision for flower making occurs before the actual flower appears on the plant.

Stamen

• The stamen, also known as microsporophyll, is a leaf-like structure that produces microspores.
• The number of stamens may vary in different plants.
• The whorl of stamens forms the male part of the flower, known as the androecium.

Stamen Structure

• The stamen consists of an anther and a filament.
• The anther has a distal end, while the filament has a proximal end.
• Anthers have lobed, fertile parts, while filaments are long, slender, and sterile.

Corolla and Androecium

• In the context of floral structures, C stands for corolla and A for androecium.
• Thalamus is where the Stamen is attached.
• Anther is attached to the filament
• Monolobed anthers are single-lobed, while bilobed anthers are double-lobed.
• Tepal is a single unit of perianth found as the outer whorl when sepals and petals are the same color.
• Each anther lobe carries two spore sacs.
• Malvaceae and cotton families are examples of angiospermic families.
• Bisporangiate: Malvaceae family has only two spores.

Angiosperms

• Most angiosperms have four spores, making them tetrasporangiate.
• Typical angiosperms have bilobed, dithecous, and tetrasporangiate anthers.
• Anther lobes are connected via connective tissue.

Anther Development

• Development of the anther starts from the stem, followed by the node and floral bud.
• Anther initials develop into archesporial cells, which then form the epidermis and meristematic cells.
• Anther has an other layer called sporogenous.
• Archesporial cells divide and form sporogenous tissue and other layers.

Anther Structure

• The structure of an anther includes middle layers, the epidermis, endothecium with fibrous thickenings, tapetum, and sporogenous tissue.
• Spore sacs in the anther are circular in outline.

Anther Layers

• A typical anther is tetragonal and carries four spore sacs, covered with four layers.
• The epidermis protects the anther
• The endothecium carries cellulosic fibers and helps in dehiscence.
• The middle layer provides food, and the tapetum performs multiple functions.
• Tapetum has dense cytoplasm and more than one nucleus per cell (binucleate), resulting from cryptokinesis/endomitosis.

Tapetum Function

• The tapetum provides nutrition.
• Tapetum secretes callose around the spore mother cell for protection and secretes callase to dissolve the callose.
• Tapetum forms the exine layer over the microspores, secreting sporopollenin in the form of Ubisch bodies.

Microsporogenesis

• Microsporogenesis is the formation of microspores.
• Microspore mother cells divide by meiosis to produce microspore tetrads inside the spore sac.

Microspore Arrangement

• Microspores can be arranged in a tetrad in tetrahedral or isobilateral arrangements.
• Tetrahedral arrangement is most common in dicots
• Isobilateral arrangement is most common in monocots
• Microspores can be arranged in decussate, T-shape, or linear arrangements.

Microspore

• Microspores have a ploidy of N in the tetrad stage

Microspore Structure

• The microspore structure has characteristics that allow it to withstand high temperatures, acids, alkali, and enzymes due to sporopollenin.
• It has a central, haploid nucleus surrounded by a plasma membrane.
• The exine layer is made of sporopollenin, is discontinuous layer.
• Germpores are areas where intine is close to the surface and play a role in pollen tube generation.

Pollen Grains

• Vegetative cells are larger, have abundant food reserves, and have large, irregularly shaped nuclei.
• Palynology is the study of pollen grains and it is used in taxonomy.

Microgametogenesis

• Microgametogenesis is the germination of the microspore or the formation of the male gamete.
• Vacuolation begins with new synthesis, forming vacuoles, and the nucleus migrates to the periphery.
• Mitosis of the microspore occurs with asymmetrical spindles.
• Two cells are formed: a large vegetative cell and a small generative cell.
• The generative cell divides by mitosis to produce two male gametes.
• A mature male gametophyte typically carries three cells only.

Angiosperms Pollen Grain

• In 60% of angiosperms, pollen grains are shed at the 2-celled stage after endosporic development.
• In 40% of angiosperms, pollen grains are shed at the 3-celled stage after endosporic development.

Anther Dehiscence

• In a mature anther, the tapetum and middle layer degenerate.
• Fibers in the endothecium absorb moisture from the atmosphere, increasing the fiber's size.
• This generates a force on the anther wall, causing it to rupture and release pollen.

Pollen Facts

• Pollen grains are typically 25-50 micrometers in diameter.
• Pollen grains are reduced male gametophytes typically restricted to three cells: one vegetative and two male gametes.
• Pollen grains form in each spore sac or microsporangium.
• Highly reduced male gametophytes are present in plants like Pinus.
• Pollen grains from some plants are highly nutritious and enhance the performance of athletes and racehorses
• These pollens are available in western countries.
• Some pollens can cause severe allergies and respiratory disorders such as asthma and bronchitis.
• Pollen from Parthenium (carrot or congress grass) can cause allergies.
• Parthenium was a contamination along with wheat when wheat was imported from the USA.