Plant reproduction- an overview

 Plant reproduction

Plant reproduction is the production of new offspring in plants, which can be accomplished by sexual or asexual reproduction. Sexual reproduction produces offspring by the fusion of gametes, resulting in offspring genetically different from either parent. Asexual reproduction produces new individuals without the fusion of gametes, resulting in clonal plants that are genetically identical to the parent plant and each other, unless mutations occur.

 Asexual reproduction

Asexual reproduction does not involve the production and fusion of male and female gametes. Asexual reproduction may occur through budding, fragmentation, spore formation, regeneration, and vegetative propagation. Asexual reproduction is a type of reproduction where the offspring comes from one parent only, thus inheriting the characteristics of the parent. Asexual reproduction in plants occurs in two fundamental forms, vegetative reproduction, and agamospermy.

 Vegetative reproduction involves a vegetative piece of the original plant producing new individuals by budding, tillering, etc., and is distinguished from apomixis, which is a replacement of sexual reproduction, and in some cases involves seeds. Apomixis occurs in many plant species such as dandelions and also in some non-plant organisms. For apomixis and similar processes in non-plant organisms, see parthenogenesis.

Natural vegetative reproduction is a process mostly found in perennial plants and typically involves structural modifications of the stem or roots and in a few species leaves. Most plant species that employ vegetative reproduction do so as a means to perennialize the plants, allowing them to survive from one season to the next and often facilitating their expansion in size. A plant that persists in a location through the vegetative reproduction of individuals gives rise to a clonal colony. 

A single ramet, or apparent individual, of a clonal colony, is genetically identical to all others in the same colony. The distance that a plant can move during vegetative reproduction is limited, though some plants can produce ramets from branching rhizomes or stolons that cover a wide area, often in only a few growing seasons.

Sexual reproduction

Sexual reproduction involves two fundamental processes: meiosis, which rearranges the genes and reduces the number of chromosomes, and fertilization, which restores the chromosome to a complete diploid number.

 In between these two processes, different types of plants and algae vary, but many of them, including all land plants, undergo alternation of generations, with two different multicellular structures (phases), a gametophyte and a sporophyte. The evolutionary origin and adaptive significance of sexual reproduction are discussed in the pages Evolution of sexual reproduction and Origin and Function of Meiosis.

The gametophyte is the multicellular structure (plant) that is haploid, containing a single set of chromosomes in each cell. The gametophyte produces male or female gametes (or both), by a process of cell division, called mitosis. 

In vascular plants with separate gametophytes, female gametophytes are known as mega gametophytes, and the male gametophytes are called micro gametophytes (micro=small, they produce small sperm cells). The fusion of male and female gametes (fertilization) produces a diploid zygote, which develops by mitotic cell divisions into a multicellular sporophyte. 

 Pollination

Plants that use insects or other animals to move pollen from one flower to the next have developed greatly modified flower parts to attract pollinators and to facilitate the movement of pollen from one flower to the insect and from the insect to the next flower. Flowers of wind-pollinated plants tend to lack petals and or sepals; typically large amounts of pollen are produced and pollination often occurs early in the growing season before leaves can interfere with the dispersal of the pollen. Many trees and all grasses and sedges are wind-pollinated.

Plants have a number of different means to attract pollinators including color, scent, heat, nectar glands, edible pollen, and flower shape. Along with modifications involving the above structures two other conditions play a very important role in the sexual reproduction of flowering plants, the first is the timing of flowering and the other is the size or number of flowers produced.

 Often plant species have a few large, very showy flowers while others produce many small flowers, often flowers are collected together into large inflorescences to maximize their visual effect, becoming more noticeable to passing pollinators. Flowers are attraction strategies and sexual expressions are functional strategies used to produce the next generation of plants, with pollinators and plants having co-evolved, often to some extraordinary degrees, very often rendering mutual benefit. 

 Ferns

Ferns typically produce large diploids with stems, roots, and leaves. On fertile leaves, sporangia are produced, grouped together in sori and often protected by an indusium. If the spores are deposited onto a suitable moist substrate they germinate to produce short, thin, free-living gametophytes called prothalli that are typically heart-shaped, small, and green in color. 

The gametophytes produce both motile sperm in the antheridia and egg cells in separate archegonia. After rains or when dew deposits a film of water, the motile sperm are splashed away from the antheridia, which are normally produced on the top side of the thallus, and swim in the film of water to the antheridia where they fertilize the egg. 

To promote outcrossing or cross-fertilization the sperm is released before the eggs are receptive to the sperm, making it more likely that the sperm will fertilize the eggs of the different thallus. A zygote is formed after fertilization and grows into a new sporophytic plant. The condition of having separate sporophyte and gametophyte plants is called alternation of generations. 

Bryophytes

The bryophytes, which include liverworts, hornworts, and mosses, reproduce both sexually and vegetatively. The gametophyte is the most commonly known phase of the plant. Bryophytes are typically small plants that grow in moist locations and like ferns, have motile sperm with flagella and need water to facilitate sexual reproduction. These plants start as a haploid spore that grows into the dominant form, which is a multicellular haploid body with leaf-like structures that photosynthesize. 

Haploid gametes are produced in antheridia and archegonia by mitosis. The sperm released from the antheridia respond to chemicals released by ripe archegonia and swim to them in a film of water and fertilize the egg cells, thus producing zygotes that are diploid. The zygote divides by mitotic division and grows into a sporophyte that is diploid.

 The multicellular diploid sporophyte produces structures called spore capsules. The spore capsules produce spores by meiosis, and when ripe, the capsules burst open and the spores are released. Bryophytes show considerable variation in their breeding structures and the above is a basic outline. In some species each gametophyte is one sex while other species may be monoicous, producing both antheridia and archegonia on the same gametophyte which is thus hermaphrodite.

 Dispersal and offspring care

One of the outcomes of plant reproduction is the generation of seeds, spores, gemmae, and other vegetative organs that allow plants to move to new locations or new habitats.

Plants do not have nervous systems or any will for their actions. Even so, scientists are able to observe mechanisms that help their "children" thrive as they grow. All organisms have mechanisms to increase survival in offspring.

Offspring care is observed in the Mammillaria hernandezii, a small cactus found in Mexico. A cactus is a type of succulent, meaning it retains water when it is available for future droughts. M. hernandezii also stores a portion of its seeds in its stem and releases the rest to grow. This can be advantageous for many reasons. By delaying the release of some of its seeds, the cactus can protect itself from potential threats from insects, herbivores, or mold caused by micro-organisms. 

A study found that the presence of adequate water in the environment causes M. Hernandezii to release more seeds to allow for germination. The plant was able to perceive a water potential gradient in the surroundings and act by giving its seeds a better chance in this preferable environment. This evolutionary strategy gives a better potential outcome for seed germination.

There are similar reproductive strategies found in both mammals and plants. A divergence between the two is that in harsh environmental conditions, mammals produce fewer and larger offspring, whereas plants produce more seeds.