Plant Reproduction: Asexual and Sexual Methods
Plant Reproduction: Asexual and Sexual Methods
The Diplohaplontic Cycle
The diplohaplontic cycle is the cycle that characterizes plants with alternating generations: a diploid sporophyte and a haploid gametophyte.
The sporophyte undergoes meiosis to form haploid spores. These spores germinate and give rise to haploid gametophytes, which produce gametes by mitosis. The gametes fuse, forming a diploid zygote that divides to form a diploid sporophyte.
Types of Reproduction in Plants
Plants have two basic ways to reproduce:
- Asexual or Vegetative Reproduction: Involves only one parent, meaning there is no fusion of gametes. There are several types of asexual reproduction in plants:
- By rhizomes: Underground stems with buds, nodes, and fleshy leaves.
- By tubers: Rhizomes with thickened ends that store food.
- By bulbs: Buds formed by a modified underground stem joined by short, fleshy storage leaves.
- By stolons: Stems that lie flat on the ground, with long internodes, in which buds develop, each giving rise to a new system.
- Sexual or Generative Reproduction: Involves two individuals, each contributing a gamete, which are stored in the gametangia (oogonia and spermatogonia).
Plant Life Cycles
In plants, the existence of generations and the dominance of one over another is a characteristic feature of each of the major groups:
- In bryophytes: The main stage is the haploid gametophyte. The diploid sporophyte lives as a parasite on the gametophyte.
- In pteridophytes: The two generations are independent. The main, well-developed stage is the diploid sporophyte.
- In spermatophytes: The relative importance of the sporophyte increases further. The plant we see is the diploid sporophyte. The gametophyte has been reduced to a set of few cells forming flower structures.
Reproduction in Bryophytes
Bryophytes are nonvascular, seedless plants that need a moist environment for growth and reproduction. They exhibit alternation of generations in which the gametophyte predominates over the sporophyte, originating asexually and sexually.
The Life Cycle of a Moss
The antheridium (male gametophyte) produces antherozoids that fertilize the oosphere, formed in the archegonium. This fertilization occurs only in the presence of water.
Once fertilized, the diploid zygote undergoes mitosis and develops into an embryo that forms the multicellular sporophyte (diploid). This sporophyte grows on the gametophyte (haploid) and consists of a foot and a capsule containing the spore mother cells. These cells divide by meiosis, giving rise to haploid spores. When mature, the spores are released from the capsule and, under the right conditions, form a filamentous network of green cells (protonema) from which the new gametophyte is formed.
Reproduction in Pteridophytes
Pteridophytes are seedless plants that have roots, stems, leaves, and a vascular system to transport water. Their life cycle presents an alternation of generations in which the predominant phase is the sporophyte.
The Life Cycle of a Fern
The sporophyte, which is the typical fern, is formed by a horizontal underground stem (rhizome), roots, and true leaves called fronds. On the underside of the fronds, sporangia develop, where meiosis occurs, leading to the formation of spores. When these spores are released and find the right conditions, they germinate and give rise to the gametophyte (haploid) called a prothallus. The prothallus is very small and fixed to the ground by rhizoids. The prothallus produces a large number of antheridia and archegonia, where gametes are formed. Fertilization occurs in the presence of water, giving rise to an embryo that grows and becomes a sporophyte, which later becomes independent of the gametophyte.
Reproduction in Seed Plants
Seed plants do not require water for fertilization, and their life cycle shows a clear dominance of the sporophyte. The most representative groups of seed plants are gymnosperms and angiosperms, in which the flower is the reproductive organ.
The Flower
In angiosperms, the reproductive function is located in a special organ called the flower, which contains the sexual organs and whose main function is to carry out the reproductive process.
A flower consists of:
- Floral stalk
- Floral receptacle, where the whorls are inserted (leaves coming out of the same node)
- Sepals: Form the calyx, which, together with the corolla, constitutes the perianth. They are green.
- Petals: Form the corolla. They often have bright colors to attract animals.
- Androecium: The male sexual organ of the flower. It consists of the stamens (modified leaves consisting of a filament and an anther, a widened part divided into two thecae, each with two pollen sacs containing diploid spore mother cells, which undergo meiosis and develop into pollen grains with two haploid nuclei: the vegetative and generative).
- Gynoecium: The female sexual organ. It consists of one or more carpels (modified leaves shaped like a bottle, made up of the ovary at the base, where the ovules are located; the style, which connects the stigma and the base; and the stigma, where pollen grains are deposited).
Pollination
Pollination is the process by which pollen grains are transferred from the pollen sacs to the stigmas, which contain the oospheres. It can be self-pollination (between flowers of the same plant) or cross-pollination (between different plants of the same species). Cross-pollination is the most common and produces greater genetic variability. The transport of pollen grains can be performed by different means:
- Anemophilous pollination: Pollen grains are transported by wind. Anemophilous plants have light pollen, are not very showy, and lack nectar or sweet aroma. Flowers are often dioecious to avoid self-pollination.
- Zoophilic pollination: Carried out by insects, birds, or bats. Zoophilic plants have large pollen equipped with structures to stick to animals, and angiosperms have specialized structures to attract animals, producing food, for example.
Fertilization
Once the pollen grain has reached the stigma, it begins to germinate, emitting a cytoplasmic projection called the pollen tube (male gametophyte), which reaches the oosphere, where fertilization occurs.
The germination of the megaspore consists of three successive mitoses, resulting in eight cells arranged as follows: three near the micropyle, with the central one being the oosphere and the other two synergids; three others on the opposite side called antipodes; and two secondary nuclei in the middle. The entire set is the embryo sac (female gametophyte).
In angiosperms, double fertilization occurs: one of the sperm nuclei fuses with the egg cell, while the other fuses with the two polar nuclei of the embryo sac, resulting in a triploid nucleus that gives rise to the endosperm, which nourishes the seed.
Seed Formation
After fertilization, seed formation begins with embryo development, the albumen (reserve tissue), and external covers.
The zygote produces the embryo by mitosis.
The fertilized ovule becomes a seed, and simultaneously, the ovary undergoes a series of changes that result in the fruit.
Seed Germination
The seed germinates when it falls to the ground. It can remain dormant until environmental conditions are suitable for further development.
Initially, the seed must absorb water. Then, the embryo, using the nutrient reserves of the albumen, continues its growth, and the radicle emerges, followed by the stem with the gemmules. The radicle becomes the main root.
Fruits
After fertilization, the ovary undergoes a series of transformations that result in the fruit (plant organs that, after various processes of development and transformation, contain seeds that are dispersed either in isolation or associated with the fruit).
Fruit dispersal can be:
- Autochory: Self-dispersal. When the fruit ripens, the pressure it exerts on the envelope increases until it bursts, and the seeds are spread in several directions.
- Hydrochory: Dispersal by water. Occurs in fruits that can float.
- Anemochory: Dispersal by wind. Occurs in fruits with feathery organs that act as parachutes.
- Zoochory: Dispersal by animals. It can be exozoochory (outside animals) or endozoochory (inside animals).