Plant Reproduction

Types of reproduction that plants have evolved


Asexual Reproduction

Plants have two main types of asexual reproduction in which new plants are produced that are genetically identical clones of the parent individual. Vegetative reproduction involves a vegetative piece of the original plant and is distinguished from apomixis which is a replacement for sexual reproduction, and in some cases involves seeds. Apomixis occurs in many plant species and also in some non-plant organisms. For apomixis and similar processes in non-plant organisms.

Natural vegetative reproduction is mostly a process found in herbaceous and woody 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 perennialise 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 vegetative reproduction of individuals constitutes 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. In a sense, this process is not one of reproduction but one of survival and expansion of biomass of the individual. When an individual organism increases in size via cell multiplication and remains intact, the process is called "vegetative growth". However, in vegetative reproduction, the new plants that result are new individuals in almost every respect except genetic. A major disadvantage to vegetative reproduction, is the transmission of pathogens from parent to daughter plants; it is uncommon for pathogens to be transmitted from the plant to its seeds, though there are occasions when it occurs.

Seeds generated by apomixis are a means of asexual reproduction, involving the formation and dispersal of seeds that do not originate from the fertilisation of the embryos. Hieracium, Taraxacum, some Citrus and Poa pratensis all use this form of asexual reproduction. Pseudogamy occurs in some plants that have apomictic seeds, where pollination is often needed to initiate embryo growth, though the pollen contributes no genetic material to the developing offspring. Other forms of apomixis occur in plants also, including the generation of a plantlet in replacement of a seed or the generation of bulbils instead of flowers, where new cloned individuals are produced.

Natural Vegetative Structures

The rhizome is a modified underground stem serving as an organ of vegetative reproduction for example: Polypody, Iris, Couch Grass and Nettles. Prostrate aerial stems, called runners or stolons are important vegetative reproduction organs in some species such as the strawberry, numerous grasses, and some ferns. Adventitious buds form on roots near the ground surface, on damaged stems (as on the stumps of cut trees), or on old roots. These develop into above-ground stems and leaves. A form of budding called suckering is the reproduction or regeneration of a plant by shoots that arise from an existing root system. Species that characteristically produce suckers include: Ulmus, Taraxacum), and members of the Rose Family (Rosa). Another type of a vegetative reproduction is the production of bulbs. Plants like Allium cepa, Hyacinth, Narcissus and Tulipa reproduce by forming bulbs. Other plants like Solanum tuberosum and Dahlia reproduce by a method similar to bulbs: they produce tubers. Gladioli and Crocus reproduce by forming a bulb-like structure called a corm.


Human Uses of Asexual Reproduction

The most common form of plant reproduction utilised by people is seeds, but a number of asexual methods are utilised which are usually enhancements of natural processes, including: cutting, grafting, budding, layering, division, sectioning of rhizomes or roots, stolons, tillers and artificial propagation by laboratory tissue cloning. Asexual methods are most often used to propagate cultivars with individual desirable characteristics that do not come true from seed. Fruit tree propagation is frequently performed by budding or grafting desirable cultivars onto rootstocks that are also clones, propagated by layering.

In horticulture, a cutting is a branch that has been cut off from a mother plant below an internodes and then rooted, often with the help of a rooting liquid or powder containing hormones. When a full root has formed and leaves begin to sprout anew, the clone is a self-sufficient plant, genetically identical to the mother plant. Examples include cuttings from the stems of Rubus occidentalis, Saintpaulia, Verbena to produce new plants. A related use of cuttings is grafting, where a stem or bud is joined onto a different stem. Nurseries offer for sale trees with grafted stems that can produce four or more varieties of related fruits, including Malus. The most common usage of grafting is the propagation of cultivars onto already rooted plants, sometimes the rootstock is used to dwarf the plants or protect them from root damaging pathogens. Since vegetative propagated plants are clones, they are important tools in plant research. When a clone is grown in various conditions, differences in growth can be ascribes to environmental effects instead of genetic differences.

Techniques for vegetative propagation include:

Air or ground layering

Division

Grafting and bud grafting, widely used in fruit tree propagation

Micro-propagation

Stolons or runners

Storage organs such as bulbs, corms, tubers and rhizomes

Striking or cuttings

Twin scaling

Sexual Reproduction

Sexual reproduction involves two fundamental processes, meiosis which rearranges the genes and reduces the number of chromosomes, and fusion of gametes which restores the chromosome to a complete diploid number. In between these two processes, different types of plants vary. In plants and algae that undergo alternation of generations, a gametophyte is the muilticelluar structure, or phase, that is haploid, containing a single set of chromosomes.

The gametophyte produces male or female gametes, by a process of cell division called mitosis. The fusion of male and female gametes produces a diploid zygote, which develops by repeated mitotic cell divisions into a muilticelluar sporophyte. Because the sporophyte is the product of the fusion of two haploid gametes, its cells are diploid, containing two sets of chromosomes. The mature sporophyte produces spores by a process called meiosis, sometimes referred to as reduction division because the chromosome pairs are separated once again to form single sets. The spores are therefore once again haploid and develop into a haploid gametophyte. In land plants such as ferns, mosses and liverworts the gametophyte is very small, as in ferns and their relatives. In flowering plants (angiosperms) it is reduced to only a few cells, where the female gametophyte (embryo sac) is known as a mega-gametophyte and the male gametophyte (pollen) is called a micro-gametophyte.

Adaptations of flowering plants and their advantages

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; as such they have no need for large flowers. In plants that use insects or other animals to move pollen from one flower to the next, plants 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 back to the next flower. Plants have a number of different means to attract pollinators including colour, scent, heat, nectar glands, eatable 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 timing of flowering and the other is the size or number of flowers produced. Often plant species have a few large, very showy flower 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.

The largest family of flowering plants is the Orchidaceae, estimated by some specialists to include up to 35,000 species which often have highly specialised flowers used to attract insects and facilitate pollination. The stamens are modified to produce pollen in clusters called pollinium which are attached to insects when crawling into the flower. The flower shapes are modified to force insects to pass by the pollen, which is glued to the insect. Some orchids are even more highly specialized, with flower shapes that mimic the shape of insects to attract them to 'mate' with the flowers, a few even have scents that mimic insect pheromones.

Another large group of flowering plants is the Asteraceae or sunflower family with close to 22,000 species which also have highly modified inflorescences that are flowers collected together in heads composed of a composite of individual flowers called florets. Heads with florets of one sex, when the flowers are pistillate or functionally staminate, or made up of all bisexual florets are called homogamous and can include discoid and liguliflorous type heads. Some radiate heads may be homogamous too. Plants with heads that have florets of two or more sexual forms are called heterogamous and include radiate and disciform head forms, though some radiate heads may be heterogamous too.

Pollen grains, which contain the male gametes (sperm) to where the female gamete(s) are contained within the carpel; in gymnosperms the pollen is directly applied to the ovule itself. The receptive part of the carpel is called a stigma in the flowers of angiosperms. The receptive part of the gymnosperm ovule is called the micropyle. Pollination is a necessary step in the reproduction of flowering plants, resulting in the production of offspring that are genetically diverse.

The study of pollination brings together many disciplines such as: botany, horticulture, entomology, and ecology. The pollination process as an interaction between flower and vector was first addressed in the 18th century by Christian Konrad Sprengel. It is important in horticulture and agriculture, because fruiting is dependent on fertilization, which is the end result of pollination.

Abiotic Pollination

Abiotic pollination refers to situations where pollination is mediated without the involvement of other organisms. Only 10% of flowering plants are pollinated without animal assistance. The most common form of abiotic pollination, anemophily, is pollination by wind. This form of pollination is predominant in grasses most conifers, and many deciduous trees. Hydrophily is pollination by water and occurs in aquatic plants which release their pollen directly into the surrounding water. About 80% of all plant pollination is biotic. Of the 20% of abiotically pollinated species, 98% is by wind and 2% by water.

Biotic pollination

More commonly, the process of pollination requires pollinators: organisms that carry or move the pollen grains from the anther to the receptive part of the carpel or pistil. This is biotic pollination. The various flower traits that differentially attract one type of pollinator or another are known as pollination syndromes.

There are roughly 200,000 varieties of animal pollinators in the wild, most of which are insects. Entomophily, pollination by insects, often occurs on plants that have developed colored petals and a strong scent to attract insects such as, bees, wasps and occasionally ants (Hymenoptera), beetles (Coleoptera), moths and butterflies (Lepidoptera), and flies (Diptera). In zoophily, pollination is performed by vertebrates such as birds and bats, particularly, hummingbirds, sunbirds, spider hunters, honeyeaters, and fruit bats. Plants adapted to using bats or moths as pollinators typically have white petals and a strong scent; while plants that use birds as pollinators tend to develop red petals and rarely develop a scent (few birds have a sense of smell).

Mechanics

Pollination can be accomplished by cross-pollination or by self-pollination:

    • Cross-pollination, also called allogamy occurs when pollen is delivered to a flower from a different plant. Plants adapted to out cross or cross-pollinate often have taller stamens than carpels or use other mechanisms to better ensure the spread of pollen to other plants' flowers.

A European honey bee collects nectar, while pollen collects on its body.

    • Self-pollination occurs when pollen from one flower pollinates the same flower or other flowers of the same individual. It is thought to have evolved under conditions when pollinators were not reliable vectors for pollen transport, and is most often seen in short-lived annual species and plants that colonise new locations. Self pollination may include autogamy, where pollen moves to the female part of the same flower; or geitonogamy, when pollen is transferred to another flower on the same plant. Plants adapted to self-fertilise often have similar stamen and carpel lengths. Plants that can pollinate themselves and produce viable offspring are called self-fertile. Plants that can not fertilise themselves are called self-sterile, a condition which mandates cross pollination for the production of offspring.

    • Cleistogamy: is self-pollination that occurs before the flower opens. The pollen is released from the anther within the flower or the pollen on the anther grows a tube down the style to the ovules. It is a type of sexual breeding, in contrast to asexual systems such as apomixis. Some cleistogamous flowers never open, in contrast to chasmogamous flowers that open and are then pollinated. Cleistogamous flowers by necessity are self-compatible or self-fertile plants. Many plants are self-incompatible, and these two conditions are end points on a continuum.

Pollination also requires consideration of pollenisers. The terms pollinator and polleniser are often confused: a pollinator is the agent that moves the pollen, whether it be bees, flies, bats, moths, or birds; a polleniser is the plant that serves as the pollen source for other plants. Some plants are self-fertile or self-compatible and can pollinate themselves. Other plants have chemical or physical barriers to self-pollination.

In agriculture and horticulture pollination management, a good polleniser is a plant that provides compatible, viable and plentiful pollen and blooms at the same time as the plant that is to be pollinated or has pollen that can be stored and used when needed to pollinate the desired flowers. Hybridisation is effective pollination between flowers of different species, or between different breeding lines or populations.

Peaches are considered self-fertile because a commercial crop can be produced without cross-pollination, though cross-pollination usually gives a better crop. Apples are considered self-incompatible, because a commercial crop must be cross-pollinated. Many commercial fruit tree varieties are grafted clones, genetically identical. An orchard block of apples of one variety is genetically a single plant. Many growers now consider this a mistake. One means of correcting this mistake is to graft a limb of an appropriate polleniser every six trees or so.

Typical seed structure and anatomy

The embryo is an immature plant from which a new plant will grow under proper conditions. The embryo has one cotyledon or seed leaf in monocotyledons, two cotyledons in almost all dicotyledons and two or more in gymnosperms. The radicle is the embryonic root. The plumule is the embryonic shoot. The embryonic stem above the point of attachment of the cotyledon is the epicotyl. The embryonic stem below the point of attachment is the hypocotyl.

Within the seed, there usually is a store of nutrients for the seedling that will grow from the embryo. The form of the stored nutrition varies depending on the kind of plant. In angiosperms, the stored food begins as a tissue called the endosperm, which is derived from the parent plant via double fertilization. The usually triploid endosperm is rich in oil or starch and protein. In gymnosperms, such as conifers, the food storage tissue is part of the female gametophyte, a haploid tissue. In some species, the embryo is embedded in the endosperm or female gametophyte, which the seedling will use upon germination. In others, the endosperm is absorbed by the embryo as the latter grows within the developing seed, and the cotyledons of the embryo become filled with this stored food. At maturity, seeds of these species have no endosperm and are termed exalbuminous seeds. Some exalbuminous seeds are bean, pea, oak, walnut, squash, sunflower, and radish. Seeds with an endosperm at maturity are termed albuminous seeds. Most monocots and many dicots castor bean have albuminous seeds. All gymnosperm seeds are albuminous.

The seed coat or testa develops from the tissue, the integument, originally surrounding the ovule. The seed coat in the mature seed can be a paper-thin layer or something more substantial. The seed coat helps protect the embryo from mechanical injury and from drying out.

In addition to the three basic seed parts, some seeds have an appendage on the seed coat such an aril or an elaiosome or hairs as in cotton. There may also be a scar on the seed coat, called the hilum; it is where the seed was attached to the ovary wall by the funiculus.

Seed Production

Seeds are produced in several related groups of plants, and their manner of production distinguishes the angiosperms from the gymnosperms. Angiosperm seeds are produced in a hard or fleshy structure called a fruit that encloses the seeds, hence the name. Some fruits have layers of both hard and fleshy material. In gymnosperms no special structure develops to enclose the seeds, which begin their development naked on the bracts of cones. However, the seeds do become covered by the cone scales as they develop in some species of conifer.

Seed production in natural plant populations vary widely from year-to-year in response to weather variables, insects and diseases, and internal cycles within the plants themselves. Over a 20-year period, for example, forests composed of loblolly pine and short leaf pine produced from 0 to nearly 5 million sound pine seeds per hectare. Over this period, there were six bumper seeds, five poor seeds crops, and nine good seed crops, when evaluated in regard to producing adequate seedlings for natural forest reproduction.

Kinds of seeds

Many structures commonly referred to as seeds are actually dry fruits. Sunflower seeds are sold commercially while still enclosed within the hard wall of the fruit, which must be split open to reach the seed. Different groups of plants have other modifications; the so-called stone fruits such as the Prunus have a hardened fruit layer fused to and surrounding the actual seed. Nuts are the one-seeded, hard shelled fruit, of some plants, with an indehiscent seed, such as an acorn or hazelnut.

Seed development

The seed, which is an embryo with two points of growth (one of which forms the stems the other the roots) is enclosed in a seed coat with some food reserves. Angiosperm seeds consist of three genetically distinct constituents:

(1) The embryo formed from the zygote

(2) The endosperm, which is normally triploid

(3) The seed coat from tissue derived from the maternal tissue of the ovule. In angiosperms, the process of seed development begins with double fertilisation and involves the fusion of the egg and sperm nuclei into a zygote.

The second part of this process is the fusion of the polar nuclei with a second sperm cell nucleus, thus forming a primary endosperm. Right after fertilisation, the zygote is mostly inactive but the primary endosperm divides rapidly to form the endosperm tissue. This tissue becomes the food that the young plant will consume until the roots have developed after germination or it develops into a hard seed coat. The seed coat forms from the two integuments or outer layers of cells of the ovule, which derive from tissue from the mother plant, the inner integument forms the tegmen and the outer forms the testa. When the seed coat forms from only one layer it is also called the testa, though not all such testa are homologous from one species to the next.

In gymnosperms, the two sperm cells transferred from the pollen do not develop seed by double fertilization but one sperm nucleus unites with the egg nucleus and the other sperm is not used. Sometimes each sperm fertilises an egg cell and one zygote is then aborted or absorbed during early development. The seed is composed of the embryo and tissue from the mother plant, which also form a cone around the seed in coniferous plants like Pine and Spruce.

The ovules after fertilisation develop into the seeds; the main parts of the ovule are the funicle; which attaches the ovule to the placenta, the nucellus; the main region of the ovule were the embryo sac develops, the micropyle; A small pore or opening in the ovule where the pollen tube usually enters during the process of fertilisation, and the chalaza; the base of the ovule opposite the micropyle, where integument and nucellus are joined together.

The shape of the ovules as they develop often affects the finale shape of the seeds. Plants generally produce ovules of four shapes: the most common shape is called anatropous, with a curved shape. Orthotropous ovules are straight with all the parts of the ovule lined up in a long row producing an non-curved seed. Campylotropous ovules have a curved embryo sac often giving the seed a tight “c” shape. The last ovule shape is called amphitropous, where the ovule is partly inverted and turned back 90 degrees on its stalk or funicle.

In the majority of flowering plants, the zygote's first division is transversely oriented in regards to the long axis, and this establishes the polarity of the embryo. The upper or chalazal pole becomes the main area of growth of the embryo, while the lower or micropylar pole produces the stalk-like suspensor that attaches to the micropyle. The suspensor absorbs and manufacturers nutrients from the endosperm that are utilised during the embryos growth.

The embryo is composed of different parts; the epicotyle will grow into the shoot, the radicle grows into the primary root, the hypocotyl connects the epicotyle and the radicle, the cotyledons form the seed leaves, the testa or seed coat forms the outer covering of the seed. Monocotyledonous plants like corn have other structures; instead of the hypocotyl-epicotyle, it has a coleoptile that forms the first leaf and connects to the coleorhiza that connects to the primary root and adventitious roots form from the sides. The seeds of corn are constructed with these structures; pericarp, scutellum (single large cotyledon) that absorbs nutrients from the endosperm, endosperm, plumule, radicle, coleoptile and coleorhiza - these last two structures are sheath-like and enclose the plumule and radicle, acting as a protective covering. The testa or seed coats of both monocots and dicots are often marked with patterns and textured markings or have wings or tufts of hair.

Seed dispersal methods of plants

Unlike animals, plants are limited in their ability to seek out favourable conditions for life and growth. As a result, plants have evolved many ways to disperse their offspring by dispersing their seeds. A seed must somehow arrive at a location and be there at a time favourable for germination and growth. When the fruits open and release their seeds in a regular way, it is called dehiscent, which is often distinctive for related groups of plants, these fruits include; Capsules, follicles, legumes, silicles and silique. When fruits do not open and release their seeds in a regular fashion they are called indehiscent, which include the fruits achenes, caryopsis, nuts, samaras, and utricles.

Seed dispersal is seen most obviously in fruits; however many seeds aid in their own dispersal. Some kinds of seeds are dispersed while still inside a fruit or cone, which later opens or disintegrates to release the seeds. Other seeds are expelled or released from the fruit prior to dispersal. For example milkweeds produce a fruit type, known as a follicle that splits open along one side to release the seeds. Iris capsules split into three 'valves' to release their seeds.

Anemochory (By wind)

    • Many seeds (e.g. Acer and/or Pinus) have a wing that aids in wind dispersal.

    • The dusts like seeds of orchids are carried efficiently by the wind.

    • Some seeds, (e.g. dandelion, milkweed, poplar) have hairs that aid in wind dispersal.

Hydrochory (By water)

    • Some plants, such as Mucuna and Dioclea, produce buoyant seeds termed sea-beans or drift seeds because they float in rivers to the oceans and wash up on beaches.

Zoochory (By animal)

    • Seeds or burrs with barbs or hooks (for example: acaena, burdock and dock) which attach to animal fur or feathers, and then drop off later.

    • Seeds with a fleshy covering (such as: apple, cherry and juniper) are eaten by animals (birds, mammals, reptiles, fish) which then disperse these seeds in their droppings.

    • Seeds or nuts which are an attractive long-term storable food resource for animals (in the case of: acorns, hazelnut and walnut); the seeds are stored some distance from the parent plant, and some escape being eaten if the animal forgets them.

Types of seed germination

Seed germination is a process by which a seed embryo develops into a seedling. It involves the reactivation of the metabolic pathways that lead to growth and the emergence of the radicle or seed root and plumule or shoot. The emergence of the seedling above the soil surface is the next phase of the plants growth and is called seedling establishment.

Three fundamental conditions must exist before germination can occur.

(1) The embryo must be alive, called seed viability.

(2) Any dormancy requirements that prevent germination must be overcome.

(3) The proper environmental conditions must exist for germination.

Seed viability is the ability of the embryo to germinate and is affected by a number of different conditions. Some plants do not produce seeds that have functional complete embryos or the seed may have no embryo at all, often called empty seeds. Predators and pathogens can damage or kill the seed while it is still in the fruit or after it is dispersed. Environmental conditions like flooding or heat can kill the seed before or during germination. The age of the seed affects its health and germination ability: since the seed has a living embryo, over time cells die and cannot be replaced. Some seeds can live for a long time before germination, while others can only survive for a short period after dispersal before they die.

Seed vigour is a measure of the quality of seed, and involves the viability of the seed, the germination percentage, germination rate and the strength of the seedlings produced.

The germination percentage is simply the proportion of seeds that germinate from all seeds subject to the right conditions for growth. The germination rate is the length of time it takes for the seeds to germinate. Germination percentages and rates are affected by seed viability, dormancy and environmental effects that impact on the seed and seedling. In agriculture and horticulture quality seeds have high viability, measured by germination percentage plus the rate of germination. This is given as a percent of germination over a certain amount of time, 90% germination in 20 days, for example. 'Dormancy' is covered above; many plants produce seeds with varying degrees of dormancy, and different seeds from the same fruit can have different degrees of dormancy. It's possible to have seeds with no dormancy if they are dispersed right away and do not dry. There is great variation amongst plants and a dormant seed is still a viable seed even though the germination rate might be very low. Environmental conditions effecting seed germination include; water, oxygen, temperature and light. Three distinct phases of seed germination occur: water imbibition; lag phase; and radicle emergence.

In order for the seed coat to split, the embryo must imbibe, which causes it to swell, splitting the seed coat. However, the nature of the seed coat determines how rapidly water can penetrate and subsequently initiate germination. The rate of imbibition is dependent on the permeability of the seed coat, amount of water in the environment and the area of contact the seed has to the source of water. For some seeds, imbibing too much water too quickly can kill the seed. For some seeds, once water is imbibed the germination process cannot be stopped, and drying then becomes fatal. Other seeds can imbibe and lose water a few times without causing ill effects, but drying can cause secondary dormancy

F1 stands for Filial 1, the first filial generation seeds/plants or animal offspring resulting from a cross mating of distinctly different parental types. The term is sometimes written with a subscript, as F1 hybrid. The offspring of distinctly different parental types produce a new, uniform variety with specific characteristics from either or both parents. In fish breeding, those parents frequently are two closely related fish species, while in plant and animal genetics those parents usually are two inbred lines. Gregory Mendel's groundbreaking work in the 19th century focused on patterns of inheritance and the genetic basis for variation. In his cross-pollination experiments involving two true-breeding or homozygous, parents, Mendel found that the resulting F1 generations were heterozygous and consistent. The offspring showed a combination of those phenotypes from each of the parents that were genetically dominant. Mendel’s discoveries involving the F1 and F2 generation laid the foundation for modern genetics.

Controlled pollination methods

Crossing two genetically different plants produces a hybrid seed by means of controlled pollination. To produce consistent F1 hybrids, the original cross must be repeated each season. As in the original cross in plants this is usually done through controlled hand-pollination, and explains why F1 seeds can often be expensive. F1 hybrids can also occur naturally, a prime example being peppermint which is not a species evolved by cladogenesis or gradual change from a single ancestor, but a sterile stereotyped hybrid of water mint and spearmint. Unable to produce seeds, it propagates through the vining spread of its own root system.

In agronomy the term F1 hybrid is usually reserved for agricultural cultivars derived from two different parent cultivars, each of which are inbred for a number of generations to the extent that they are almost homozygous. The divergence between the parent lines promotes improved growth and yield characteristics through the phenomenon of heterosis ("hybrid vigour"), whilst the homozygosity of the parent lines ensures a phenotypically uniform F1 generation. Each year, for example, specific tomato "hybrids" are specifically recreated by crossing the two parent heirloom cultivars over again.

Two populations of breeding stock with desired characteristics are subject to inbreeding until the homozygosity of the population exceeds a certain level, usually 90% or more. Typically this requires more than ten generations. After this happens, both populations must be crossed while avoiding self-fertilisation. Normally this happens in plants by deactivating or removing male flowers from one population, taking advantage of time differences between male and female flowering or hand-pollinating.

In 1960, 99% of all corn planted in the United States, 95 percent of sugar beet, 80 percent of spinach, 80 percent of sunflowers, 62 percent of broccoli, and 60 percent of onions were hybrid. Such figures are probably higher today. Beans and peas are not commercially hybridised because they are automatic pollinators, and hand-pollination is prohibitively expensive.

Hybrids (F1, F2, F3, F4 & F5)

While an F2 hybrid, the result of self or cross pollination of an F1, does not have the consistency of the F1 hybrid, it may retain some desirable traits and can be produced more cheaply as no intervention in the pollination is required. Some seed companies offer F2 seed, particularly in bedding plants where consistency is not as critical, and of course you can repeat this until you get to F5 hybrids.


Advantages:

    • Homogeneity and predictability - If the parents are homozygous pure lines, there is limited genetic variation between individual F1 plants or animals. This makes their phenotype extremely uniform and thus attractive for mechanical operations and makes it easier to fine-tune the management of the population. Once the characteristics of the cross are known, repeating this cross will yield exactly the same result.

    • Higher performance - As most non-junk DNA alleles code for different versions of a protein or enzyme, having two different versions of this allele amounts to having two different versions of the enzyme. This will increase the likelihood of having an optimal version of the enzyme present and reduce the likelihood of a genetic defect. This effect is referred to in genetics as the genetic heterosis effect.

    • F1 hybrids can give higher yields than traditional varieties.


Disadvantages:

    • The main advantage of F1 hybrids in agriculture is also their drawback. When F1 cultivars are used for the breeding of a new generation, their offspring (F2 generation) will vary greatly from one another. Some of the F2 generation will be high in homozygous genes, as found in the weaker parental generation, and these will have a depression in yield and lack the hybrid vigour. From the point of view of a commercial seed producer which does not wish its customers to produce their own seed, this genetic assortment is a desired characteristic.

    • Both inbreeding and crossing the lines require a lot of work, which translates into a much higher seed cost. In general, the higher yield offsets this disadvantage.

    • F1 hybrids mature at the same time when raised under the same environmental conditions. This is of interest for modern farmers, because all ripen at the same time and can be harvested by machine. Traditional varieties are often more useful to gardeners because they crop over a longer period of time, avoiding gluts and food shortages.

Floral formulas

A floral formula is a way to represent the structure of a flower using specific letters, numbers, and symbols. Typically, a general formula will be used to represent the flower structure of a plant family rather than a particular species. The following representations are used:

Ca = calyx (sepal whorl; e. g. Ca5 = 5 sepals)

Co = corolla (petal whorl; e. g., Co3(x) = petals some multiple of three )

Z = add if Zygomorphic (e. g., CoZ6 = Zygomorphic with 6 petals)

A = androecium (whorl of stamens; e. g., A∞ = many stamens)

G = gynoecium (carpel or carpels; e. g., G1 = monocarpous)

x: to represent a 'variable number'

∞: to represent 'many'

A floral formula would appear something like this:

Ca5Co5A10 - ∞G1

The four main parts of a flower are generally defined by their positions on the receptacle and not by their function. Many flowers lack some parts or parts may be modified into other functions and/or look like what is typically another part. In some families like Ranunculaceae, the petals are greatly reduced and in many species the sepals are colorful and petal-like. Other flowers have modified stamens that are petal-like, the double flowers of Peonies and Roses are mostly petaloid stamens. Flowers show great variation and plant scientists describe this variation in a systematic way to identify and distinguish species.

Specific terminology is used to descried flowers and their parts. Many flower parts are fused together; fused parts originating from the same whorl are connate, while fused parts originating from different whorls are adnate, parts that are not fused are free. When petals are fused into a tube or ring that falls away as a single unit, they are sympetalous (also called gamopetalous). Petals that are connate may have distinctive regions: the cylindrical base is the tube, the expanding region is the throat and the flaring outer region is the limb. A sympetalous flower, with bilateral symmetry with an upper and lower lip, is bilabiate. Flowers with connate petals or sepals may have various shaped corolla or calyx including: campanulate, funnel form, tubular, urceolate, salverform or rotate.