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Urology => Reproductive System => Plant Propagation
Plant Propagation
INTRODUCTION
Plant Propagation, growing new plants from seeds or from parts of existing plants. Plant propagation occurs in nature to ensure survival and spread of species. It is also used commercially to produce seeds and plants for agriculture, horticulture, and forestry. Plant propagation includes sexual propagation, which involves the union of sperm and egg to form seeds, and asexual propagation. Asexual propagation, also known as vegetative propagation, is the growing of new plants from a leaf, stem, or root of a single parent plant. These two forms of plant propagation transmit genetic information between plants of the same species. Genetic engineering transfers genes from one organism to another that may or may not be of the same species to introduce desirable traits into an organism. These so-called transgenic plants can then be propagated by sexual or asexual propagation.
SEXUAL PROPAGATION
In nature, sexual propagation begins when water, wind, insects, birds, or small mammals carry pollen randomly between plants. In flowering plants, this transfer of pollen enables the male sex cells, or sperm, of one flower to fertilize the female sex cell, or egg, of a second flower.
The egg is located at the base of the flower in a structure called an ovule, found within the ovary. Depending on the species, an ovary contains one, several, or many ovules. The ovaries of peach and avocado flowers, for example, have one ovule, while those of watermelon and cantaloupe have many.
As the fertilized egg (or eggs) within the ovule begins to develop into an embryonic plant, it produces a variety of hormones that stimulate the outer wall of the ovule to harden into a seed coat. Other biochemical changes in the ovule produce a starchy substance that will be used as a food supply. In this way, the ovule ripens into a seed-a structure containing an embryonic plant and its food supply surrounded by a seed coat. The ovary, which houses the ovule or ovules, is also stimulated by hormones, which cause its tissues to enlarge into a fruit. The fruit contains the ripened ovules, or seeds.
In certain nonflowering plants such as pine, spruce, and fir trees, cones, not flowers, hold the ovules. The young cone, which averages about 2.5 cm (about 1 in) in length, is composed of numerous soft, green, leaf-like scales arranged in a spiral, with two ovules on the upper surface of each scale. Wind transports pollen to the cone scale, where it enters the ovule and fertilizes the egg. The fertilized egg develops into a seed that lies exposed on the cone scale.
Seeds, whether from flowering or nonflowering plants, typically contain a random combination of the genetic material from the sperm and egg of the parents and are therefore genetically unique. Plants grown from a seed typically have different traits from their parents-they might be weaker, with poor root development or little resistance to disease, or stronger, with good roots and successful resistance to disease. A population with diverse traits is more likely to survive changes in the environment such as emerging diseases, new predatory insects, or climate changes. While environmental fluctuations may kill some plants, those with the necessary adaptations are able to survive, reproduce, and carry on the species. In this way, traits that help plants survive in different environments get passed down from generation to generation, resulting in the evolution of new species with traits different from those of their ancestors.
Sexual propagation also helps plants spread geographically because it produces fruits and seeds that animals such as squirrels and birds consume. The seeds pass through an animal's digestive system and are deposited at a distance from the parent plants. It is often advantageous for seeds to germinate far from their parents so that parents and offspring are not competing for light, water, and nutrients.
Seed companies take advantage of naturally pollinated plants by harvesting seed from the company fields for sale to farmers, home gardeners, and nursery workers. Seed companies, geneticists, and other researchers also use artificial pollination, or hand pollination, to control the transfer of pollen between plants. In artificial pollination, a paintbrush or other delicate tool is used to transfer pollen between selected plants. The physical transfer of pollen in this way is called a cross, and the plants that result are also referred to as crosses. Some researchers use artificial pollination to develop new agricultural crops that combine the best traits of both parents. A cross between wheat and rye, for example, produced triticale, a vigorous pasture grass that resists many diseases and insects, and whose seeds are used to make a nutritious flour. Geneticists also use artificial pollination to study how particular traits are inherited within a species-to determine, for example, if the trait for large seeds is dominant or recessive .
ASEXUAL PROPAGATION
Asexual propagation is the production of new plants from the leaves, stems, or roots of a single parent plant. Asexual propagation, which does not require pollination or fertilization, is a rapid method of propagation. It ensures that all of the parent's genetic material survives even if the parent dies, and it creates offspring, known as clones, with the same traits as the parent plant. Asexual propagation is advantageous when plants are well adapted to a particular environment. Several methods of asexual propagation occur in nature. They have been adapted for commercial use for rapid propagation and to obtain plants that are hard to grow from seeds.
° Cuttings and Layering
For many plant species, a leaf, section of stem, or piece of root cut from a plant and lightly covered in soil, peat moss, or another growth medium develops a new, independent plant by generating the missing parts. Stimulated by hormones called auxins, a partially buried leaf or piece of stem, for example, develops roots on the buried portion, and a piece of root forms stems and leaves above the soil. Cuttings and layering are widely used for perennial plants, plants that grow back from the same roots year after year. Commercially, cuttings are the most important source for perennials, such as new fruit trees; conifers, including pine and spruce; a variety of shrubs, roses and honeysuckle, for example; and many florist blooms.
In layering, a new plant develops from a stem that is still attached to the parent plant. In nature, the stem simply arches over and spreads out on the ground. The parts of the stem that are in secure contact with the soil develop the roots, stems, and leaves of a new plant. The flexible stems of trailing blackberries, black raspberries, and several other species spread rapidly in this way.
Growers typically layer species that propagate naturally in this way. Layering is also used for plants such as filberts and Muscadine grapes that cannot be propagated easily by other methods. If plants do not layer naturally, a grower can induce them to layer by pinning the stems to the ground. Growers also layer plants by cutting them to the ground during the winter and covering the new spring shoots with soil, a technique called mound layering. This causes new roots to form along the buried lower portion of the new shoots, and new stems and leaves then grow up through the mound of soil. In a few weeks the new plants are cut away from the parent and planted in a garden or nursery. This technique enables a grower to produce many plants from a single parent that has desired traits. Apple trees, currants, and gooseberries are commonly propagated in this way. In air layering, often used with house plants, stems are partially cut and peat moss or sphagnum moss is wrapped around the wound, which is then covered in plastic wrap. A single plant develops from the place where the stem was cut.
° Grafting
In grafting, a freshly cut section of stem with buds, called a scion, is joined to another plant called the stock. The upper stem of the stock is severed and the scion is joined to the lower stem. The scion is securely attached to the stock, and the tissues of the two plants grow into each other, forming a single plant. The scion produces the stems, leaves, and flowers on the new plant and the stock provides the root system.
Grafting combines desirable qualities from one species, such as disease resistance or the ability to grow in waterlogged soils, with desirable qualities of another, such as the ability to produce high quality fruit. Grafting is often used to make fruit trees more vigorous and productive. Bud grafting is a form of grafting in which a single bud cut from a stem is grafted onto the stock. It can be carried out more rapidly than other forms of grafting and is used widely in the nursery industry to propagate hundreds or thousands of plants in a relatively short amount of time. In nature, roots of oak trees of the same species commonly graft together, hastening the spread of diseases such as oak wilt, a fungal disease that kills a variety of oak trees.
° Agamospermy
In agamospermy, also known as apomixis, a seed develops directly from tissues of the ovule rather than from a fertilized egg. Depending on the species, a fruit may or may not be produced. The plants that develop by agamospermy are clones of the mother plant. Agamospermy occurs in nature in species such as dandelions and blackberries, enabling them to spread rapidly since they can bypass pollination and fertilization.
° Tissue Culture
Also called micropropagation, tissue culture is the production of plants under sterile laboratory conditions. A variety of tissue culture techniques are used to propagate plants. In one method, growers remove a tiny piece of leaf or stem from a plant and place it in a sterile test tube on a gel-like medium enriched with hormones and nutrients. A yellow-brown mass of cells called callus develops from the piece of plant. Small chunks of the callus are separated, and each piece is placed in a petri dish with a hormone and nutrient mix that stimulates the development of the callus pieces into plants. The young plants are removed from the petri dish and placed in pots with soil, or into the ground, where they grow to maturity.
Tissue culture enables researchers and growers to rapidly generate numerous clones year-round in greenhouses. In nature, strawberry plants typically produce their fruits in summer. Commercially grown strawberries, however, are propagated throughout the year by tissue culture, providing consumers with a steady supply of strawberries for every season. Tissue culture is also used to produce plants free of viruses, fungi, and bacteria, and to propagate species such as Douglas fir and rhododendron, which are difficult to grow commercially from cuttings, layering, or grafting.
° Propagation from Stems and Roots
Some plants produce special underground stems such as tubers, bulbs, and corms that enable them to reproduce asexually. Like all stems, these structures have buds, or nodes, from which new stems branch. Tubers are swollen, fleshy stems with several buds called eyes that produce new plants; an example of a tuber is the potato. Bulbs, such as those found in onions, lilies, hyacinths, and tulips, are short, wide, teardrop-shaped underground stems surrounded by scaly leaves. Corms, such as crocuses and gladioli, are similar, but lack the scaly leaves. Both bulbs and corms make clumps of new bulbs or corms, called offsets, which can be divided and buried in the soil to generate new plants. Irises and ferns produce rhizomes, fleshy stems that grow horizontally beneath the soil, with new plants developing from the tip of the rhizome and from each node on the stem. Stolons, specialized stems found in strawberries and many lawn grasses, are similar to rhizomes but are usually thinner and grow on top of the soil. They also produce new plants at the tip and from the nodes.
GENETIC ENGINEERING
In research laboratories, genetic engineering and plant propagation are often combined. Scientists remove genes from a bacterium, fungus, animal, or plant and insert them into a plant egg or embryo. This gene transfer creates plants with genetic combinations that typically would not occur in nature. The inserted genes may code for traits such as increased protein content, insect and disease resistance, or fruit that keeps longer without spoiling after harvest. Researchers then propagate the engineered plant-through tissue culture or another propagation method-in numbers large enough to study its characteristics. Genetically engineered plants pose risks and benefits. The traits and behaviors of these plants are difficult to predict and they may jeopardize natural plant communities by pollinating native plants and introducing new genes. If a gene that delays fruit ripening, for example, is added to a natural plant community, the fruit may ripen too late for the birds or other animals that depend on it for food. This, in turn, means that seeds will not be dispersed, a necessary event for the perpetuation of many species. Despite the risks associated with genetic engineering, the potential to rapidly propagate plants tailored for desirable characteristics may provide untold benefits for farmers, foresters, horticultural workers, and consumers alike.
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