Plant Propagation

B

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.

C

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.

D

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.

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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.

E

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.

IV

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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