2.Plant Physiology

2.6 Reproduction (including ferns and mosses)

Plant Reproduction

Dr V Malathi

 Plant Reproduction
Different reproductive techniques have evolved in plants to ensure the survival of their species. Unlike animal species, which nearly always reproduce sexually, certain plants reproduce asexually while others do it sexually.
Asexual reproduction in plants occurs without the need for pollination agents, whereas sexual reproduction typically depends on them.
Often, the most visually striking or fragrant parts of plants are their flowers. In order to fulfill their need for pollination, flowers draw insects, birds, and animals with their vivid colors, intriguing forms, and sizes. Some plants self-pollinate, while others are pollinated by the wind or water.
Asexual Reproduction
Asexual reproduction is a common method used by plants to reproduce. This mode of reproduction does not need  resources to create a flower, draw pollinators, and figure out a way to distribute seeds.
As there is no mixing of male and female gametes during asexual reproduction, the resultant plants are genetically identical to the parent plant which has enabled these plants to fare better in stable environments than plants resulting from sexual reproduction.
Asexual reproduction is seen in a wide variety of roots . For example Garlic and gladiolus both use the corm. Other examples are bulbs, as the tunicate bulb in daffodils and the scaly bulb in lilies.
 Stem tubers such as potatoes also propagate asexually .
Rhizomes  produced by ginger , runners, or stolons of chrysanthemum are other examples for asexual propagation .
Shown are photos of various roots. Part A shows bulbous garlic roots. Part B shows a tulip bulb that has sprouted a leaf. Part C shows ginger root, which has many branches. Part D shows three potato tubers. Part E shows a strawberry plant.

(a) & (b) Bulbs   ; (c) Rhizome ; (d) Tuber ; (e) Runner or Stolons

“Asexual Reproduction” by OpenStaxCollege is licensed under CC BY 4.0

Apomixis

Plants that produce seeds without fertilization are said to reproduce by a method called apomixis. In this method either the diploid ovule or part of the ovary  gives rise to a new seed.

Natural methods of Asexual/Vegetative Propagation

  • This is a method of propagation by which  plants may emerge from the roots, stem and leaves of the parent plant.and grow naturally in to a new plant without any human intervention.
  • Plant structures allowing natural vegetative propagation include bulbs, rhizomes, stolons or runners and tubers.
  • The plants propagated vegetatively are
  •  Stem- The vegetative plant structures arising from the stem are known as rhizomes, bulbs, runners, tubers, etc.
    • Rhizomes are horizontal underground plant stem capable of producing the shoot and root systems of a new plant. Rhizomes are used to store starches and proteins and enable plants to perennate i.e.,survive an annual unfavourable season, underground.
    •  A stolon, or runner, is a stem that runs along the ground. At the nodes, it forms adventitious roots and buds that grow into a new plant..
    • Bulbs can branch below the ground,The branches appear at first as miniature bulbs called bulbils or bulblets. Bulblets take their energy from the parent bulb and produce aerial stems or leaves which can be separated from the parent.

Artificial Methods of Asexual Reproduction

These methods include grafting, cutting, layering, and micropropagation.

Grafting 

  • Plants such as citrus species and new types of roses have long been created through grafting.
  • Two plant species are utilized in grafting;
  • The desired plant’s stem is partially grafted onto the stock, a rooted plant.
  • The term “scion” refers to the portion that is joined or grafted. Both are retained together after being cut at an oblique angle—any angle other than a straight angle—and brought into close proximity to one another .
  • Since these two surfaces will be holding the plant together, it is crucial that they match as closely as possible.
  • The two plants’ vascular systems develop and unite to produce a graft.
  • The scion gradually begins to produce shoots and eventually bears fruit after a certain period of time.

Cutting

  • Stem cuttings are a method of propagating certain plants, like coleus and money plant,
  • Here a section of the stem with nodes and internodes is buried in damp soil and allowed to take root.
  • The stems of certain species have the ability to produce roots even while submerged in water alone. For instance, if African violet leaves are left in water unattended for a few weeks, they will eventually root.

Layering

  • In this method the  stem attached to the plant is bent and covered with soil.
  • It is best to use young stems that bend easily without breaking . Example include  Jasmine and bougainvillea (paper flower)
Micropropagation
  • Micropropagation, also known as plant tissue culture, is a technique used in laboratories to quickly multiply a large number of plants from a single plant
  • This technique makes it possible to propagate uncommon and endangered species that would be challenging to produce in the wild, have significant economic value, or are in high demand as disease-free plants.
  • Micropropagation requires sterile conditions  to propagate plants

Sexual Reproduction in Angiosperms 

Involving gametes

  • All flowering plants show sexual reproduction
  • The end products of sexual reproduction, in plants are the fruits and seeds.
  • Before flowering several hormonal and structural changes are initiated in the plant
  • This lead to the differentiation and further development of the floral primordium.
  • Inflorescences are formed which bear the floral buds and then the flowers.
  • In the flower the male and female reproductive structures, the androecium and the gynoecium differentiate and develop.
  •  The androecium consists of a whorl of stamens and represents  the male reproductive organ
  •  The gynoecium represents the female reproductive organ.

File:Parts-of-a-flower.png

“Parts of a flower” by Anjubaba is licensed under CC BY 4.0

Stamens

  • Stamens are the male reproductive organs of flowering plants.
  • They consist of an anther, the site of pollen development,
  • In most species a stalk-like filament transmits water and nutrients to the anther and positions it to aid pollen dispersal.
  • The stamens comprise of two parts
  •  The long and slender stalk called the filament,
  • and the terminal generally bilobed structure called the anther.
  • The proximal end of the filament is attached to the thalamus or the petal of the flower.
  • The number and length of stamens are variable in flowers of different species

Anthers

  • An anther is the top, bulbous portion of the stamen
  •   The male gametophytes, or male reproductive cells, are produced in the anther in a flower.
  • A typical angiosperm anther is bilobed
  • Each lobe have two theca, i.e., they are dithecous .
  • Often a longitudinal groove runs lengthwise separating the theca.
  • The anther has a four-sided (tetragonal) structure
  • It consist  of four microsporangia located at the corners, two in each lobe.
  • The microsporangia develop further and become pollen sacs.
  • They extend longitudinally all through the length of an anther and are packed with pollen grains.

Microsporangium

  • A microsporangium ( pl. microsporangia) is a sporangium produces microspores that give rise to male gametophytes when they germinate.
  • Microsporangia occur in all vascular plants that have heterosporic life cycles, such as seed plants, spike mosses and the aquatic fern genus Azolla.
  •  A microsporangium appears  circular in outline in the transverse section.
  • It is generally surrounded by four wall layers namely the epidermis, endothecium, middle layers and the tapetum.
  • The outer three wall layers protect and help in dehiscence of anther to release the pollen.
  • The innermost wall layer is called the tapetum.
  • It nourishes the developing pollen grains.
  • Cells of the tapetum possess dense cytoplasm and generally have more than one nucleus.

Illustration A shows cross section of an anther, which has four lobes each containing a pollen sac, or microsporangium. Inside the pollen sac is a layer called the tapetum, and within this ring are the microspore mother cells. As the microsporangium matures, two pollen sacs merge and an opening forms between them so that the pollen can be released. Micrographs in part B show pollen sacs with a visible opening between them.

“Reproductive Development and Structure “ by Open Stax is licensed under CC BY 4.0

Microsporogenesis 

  • It is the process of formation of microspores
  • They are formed from the cells of sporangenous tissue through the process of meiosis
  • Each cell of the sporogenous tissue is also called the pollen or microspore mother cell.
  • The microspores, as they are formed, are arranged in a cluster of four cells–the microspore tetrad
  • As the anthers mature and dehydrate, the microspores dissociate from each other and develop into pollen grains
  •   Each microsporangium contains several thousands of microspores or pollen grains
  • They are released with the dehiscence of anther

Illustration shows the formation of pollen from a microspore mother cell. The mother cell undergoes meiosis to form a tetrad of cells, which separate to form the pollen grains. The pollen grains undergo mitosis without cytokinesis, resulting in four mature pollen grains with two nuclei each. One is called the generative nucleus, and the other is called the pollen tube nucleus. Two projective layers form around the mature pollen grain, the inner intine and the outer exine. Micrograph shows a pollen grain, which looks like puffed wheat.

“Reproductive Development and Structure “ by Open Stax is licensed under CC BY 4.0

Watch the Video on Microsporogenesis by Khadijatul Qubra from Khan Academy

Female Gametophyte (The Embryo Sac)

  • The development of the female gametophyte has two distinct phases.
  • Megasporogenesis,
  • A single cell in the diploid megasporangium—an area of tissue in the ovules—undergoes meiosis
  • This  produces four megaspores, only one of which survives.
  •  The second phase is called megagametogenesis,
  • During the megagametogenesis  the surviving haploid megaspore undergoes mitosis
  • This produces an eight-nucleate, seven-cell female gametophyte, also known as the megagametophyte or embryo sac.
  • Of the eight  two of the nuclei called the polar nuclei move to the equator and fuse, forming a single, diploid central cell.
  • This central cell later fuses with a sperm to form the triploid endosperm.
  • Three nuclei position themselves on the end of the embryo sac opposite the micropyle . These develop into the antipodal cells
  •  These  later degenerate.
  • While the nucleus closest to the micropyle becomes the female gamete, or egg cell, and the two adjacent nuclei develop into synergid cells .
  • The synergids guide the pollen tube for successful fertilization, after which they disintegrate.
  • Once fertilization is complete, the resulting diploid zygote develops into the embryo, and the fertilized ovule forms the other tissues of the seed.
  • A double-layered integument protects the megasporangium and, later, the embryo sac.
  • After fertilization the integument will develop into the seed coat and protect the entire seed.
  • The ovule wall will become part of the fruit.
  • The integuments, while protecting the megasporangium, do not enclose it completely, but leave an opening called the micropyle.
  • The micropyle allows the pollen tube to enter the female gametophyte for fertilization.

Illustration depicts the embryo sac of an angiosperm, which is egg-shaped. The narrow end, called the micropylar end, has an opening that allows pollen to enter. The other end is called the chalazal end. Three cells called antipodals are at the chalazal end. The egg cell and two other cells called synergids are at the micropylar end. Two polar nuclei are inside the central cell in the middle of the embryo sac.

“Reproductive Development and Structure “ by Open Stax is licensed under CC BY 4.0

Sexual Reproduction in Gymnosperms

      • Like  angiosperms, the lifecycle of a gymnosperm is also characterized by alternation of generations ( i.e., consist of sporophytic and gametophytic generation) 
      • In conifers such as pines, the green leafy part of the plant is the sporophyte,
      •  The cones contain the male and female gametophytes . T
      • The female cones which are positioned towards the top of the tree, are larger than the male cones 
      • The male cones are smaller and are  located in the lower region of the tree.
      • Because the pollen is shed and blown by the wind
      • This requirement makes it difficult for a gymnosperm to self-pollinate.
      • Pollen from male cones blows up into upper branches, where it fertilizes female cones.

The conifer life cycle begins with a mature tree, which is called a sporophyte and is diploid (2n). The tree produces male cones in the lower branches, and female cones in the upper branches. The male cones produce pollen grains that contain two generative (sperm) nuclei and a tube nucleus. When the pollen lands on a female scale, a pollen tube grows toward the female gametophyte, which consists of an ovule containing the megaspore. Upon fertilization, a diploid zygote forms. The resulting seeds are dispersed, and grow into a mature tree, ending the cycle. Both the male and female cone are made up of rows of scales, but the male the female cone is round and wide, and the male cone is long and thin with thinner scales.Reproductive Development and Structure “ by Open Stax is licensed under CC BY 4.0

Reproduction in Mosses:

  •  Mosses, liverworts and hornworts are the three groups of plants lacking vascular tissue
  • Mosses are  the gametophyte plant that lives the longest
  • Life Cycle:
    • Dominant Gametophyte Generation: In mosses, the gametophyte is the dominant, visible phase of the life cycle. It consists of small, green, leaf-like structures.
    • Sexual Reproduction:

Gametangia Formation:

      • The Male gametophyte is  antheridia (singular = antheridium) and produces sperms
      • The female gametophyte is called archegonia (singular = archegonium) and produces eggs
      • These structures are typically produced in the midst of modified ‘leaves’ at ends of stems/branches in mosses 

Fertilization:

  • The sperm swims through water to reach the egg in the archegonium,
  • This leads to fertilization and the formation of a diploid zygote.

Sporophyte Formation:

  • The zygote develops into a sporophyte, which remains attached to the gametophyte.
  • The sporophyte produces spores in a capsule through meiosis.

Spore Dispersal:

Spores are released from the capsule, dispersed by wind or water, and germinate into new gametophytes, completing the cycle.

Asexual Reproduction:

    • Fragmentation: Mosses can reproduce asexually through fragmentation, where a piece of the gametophyte breaks off and grows into a new plant.

Reproduction in Ferns:

Dominant Sporophyte Generation: The sporophyte stage of a fern’s life cycle is the most noticeable. It is made up of stems, roots, and fronds, or leaves.
Sexual Reproduction:
Production of Spores: On the undersides of their fronds, fern sporophytes create spores known as sporangia, which are frequently arranged in groups known as sori.
Germination: After being discharged, spores develop into prothalli, which are tiny, heart-shaped gametophytes that are typically only a few millimeters in size.
Formation of Gametangia: The prothallus, or gametophyte, gives rise to both female archegonia and male antheridia. Sperm are released from the antheridia and swim to the archegonia to fertilize the eggs.
Fertilization: A diploid zygote is formed through fertilization, which takes place inside the archegonium.
Development of the Sporophyte: The zygote gives rise to a new Sporophyte that finally separates from the Gametophyte.

Asexual Reproduction:

    • Rhizome Propagation: Ferns can reproduce asexually through the spread of rhizomes, horizontal underground stems that produce new fronds and roots.

Comparison of Mosses and Ferns:

Mosses Ferns
Generally small and simple, with no true roots, stems, or leaves.

 

 Larger and more complex, with true roots, stems, and leaves (fronds).
The gametophyte is the dominant and most visible stage The sporophyte is the dominant and most visible stage.
Require water for sperm to swim to the egg, making them dependent on moist environments for sexual reproduction.

 

 Require water for sexual reproduction.

 

Both ferns and mosses demonstrate unique adaptations to their environments and life cycles, showcasing the diversity and complexity of plant reproduction.

For Further reading visit the link to the Chapter on Plant Reproduction from Organismal Biology by Georgia Tech Biological Sciences

Test your understanding about Plant Reproduction by attempting the practice problems with videos from Numerade 

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2.6 Reproduction (including ferns and mosses) by Dr V Malathi is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License, except where otherwise noted.

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