HORTICULTURE

Asexual Propagation

Asexual propagation utilizes the vegetative parts of a plant to grow new plants. Stems or roots are more commonly used, but leaves can be used as well. Each vegetative cell has the inherent ability to reproduce an entire plant that is genetically identical to the one from which the cell originated.

When many plants are reproduced asexually from a single plant, the group of new plants is termed a clone. Each individual plant within the clone is referred to as a ramet. Plants of horticultural importance that are propagated almost totally by asexual means are termed clonal vari­eties. They are a type of cultivar. Some plants have been perpetuated by horticulurists for so many years that their original parental origins have become lost in time. They depend totally on humans for survival in their present form.

Some of the techniques used by professional horticulturists to prop­agate plants asexually do little more than allow the plants to grow and reproduce normally. Other methods of propagation are totally depen­dent on the efforts of the horticulturist for their success. Techniques such as grafting and budding may replace the root system of a plant with a hardier one or create a plant having multiple colors of flowers on a single stem. Such plants are artificial contrivances, but nevertheless important in the industry. Still other propagative techniques are basi­cally natural in origin but subject to manipulation by horticulturists. The following list summarizes various asexual techniques.

Runners

These are stems that grow along the ground and form new plants at one or more of their nodes. The strawberry plant is an example likely to be familiar to most readers. The spider plant (Chorophytum como - sum variegatum) is an example of an ornamental plant (Figure 5-3). Propagators need only separate the new rooted plants from the parent and transplant them.

Stolons

Stolons are aerial shoots that take root after coming into contact with the soil. Numerous grasses spread in this fashion, as do several of the shrubby dogwoods (Cornus sp.). As with runners, the propagators sim­ply separate the rooted shoot from the parent and transplant.

figure 5-3. The spider plant gets its common name from the appearance created by its runners and new plants. (Courtesy Rodney Jackson, Photographer)

Sucker shoots

Certain plants produce new shoots from adventitious buds that develop on the roots (Figure 5-4). The black locust (Robinia pseudoacacia) is a prolific sucker producer. Suckers can be cut from the parent root system and transplanted.

Bulbs and similar organs

Vegetative structures termed either bulbs, corms, tubers, tuberous roots, or rhizomes are modified stem or root tissues that store food during dormant periods. Seasonal perennials like tulips, daffodils, cro­cuses, and gladioli exemplify these structures. The organs form as part of the root system and are easily collected and separated by propagators (Figure 5-5).

Layering

In layering, roots develop on a stem that is still attached to the par­ent plant. Runners are characteristic of naturally layered plants. Some plants root better this way because food production and water uptake are not reduced or totally severed as when a cutting is taken. The rub­ber plant (Ficus elastica) and many temperate woody ornamentals can be reproduced by layering. There are various ways of layering plants depending on the species involved. In general, the stem is partially sev­ered, a rooting hormone is applied, and the cut stem is buried in soil or wrapped in a cool, moist medium promotive of root formation (Figure 5-6). The illustration shows a peat pot split and wrapped in plastic around the cut stem. Using moistened sphagnum moss is equally effec­tive and probably more common. Once new roots have formed within the plastic cocoon, the new plant can be separated from the parent and potted for continued growth.

Tubers. These are fleshy enlargements on under­ground stems. Each eye is a new shoot and will produce a new plant if the tuber is cut apart.

Example: Potato

Cuttings

When segments of roots, leaves, or stems are cut off a plant and placed under appropriate environmental conditions, new shoots and roots form, and eventually a new plant is produced. Propagation by stem cuttings is the most common means of reproducing plants asexually. Chrysanthemums, geraniums, and woody shrubs are among the many important horticultural plants perpetuated by cutting propagation (Figure 5-7).

Grafting

When grafting with compatible species, the upper portion of one plant can be joined with the lower portion of a different plant, and they will fuse to become a single new plant expressing characteristics of the two from which it originated. Roses are commonly grafted to produce a plant having showy flowers and a hardy root system. Many woody orna­mentals are propagated by grafting (Figure 5-8).

Budding

This is a type of grafting in which buds of one plant are implanted into the stem of another compatible species. Roses are the major ornamen­tals utilizing this propagative technique (Figure 5-9). Fruit and nut trees are the most common plants grafted in this manner.

Division of the crown

As shrubs and herbaceous perennials mature, the crown (junction of the roots and shoots) enlarges and can be divided into root-shoot units that will then grow into separate plants. Even in plants whose shoots die back at the end of the growing season, the crown divisions will regenerate plants as long as adventitious shoot buds exist (Figure 5-10).

Tissue culture

Small sections of meristematic shoot tissue or callus tissue are surgically removed from the parent plant, placed into balanced nutrient media,

and grown under carefully controlled environmental conditions (Figure 5-11). Through mitosis and tissue differentiation, new plants result that are copies of the plant from which they originated. Orchids and foliage plants are commonly reproduced in this way. A major benefit of this method is to ensure that the propagated plants are disease-free by culturing with active apical meristem (meristmatic) tissue. Such tis­sue usually grows faster than any disease-causing agents that might be infecting a parent plant and that could be transported into the new plants through other propagative techniques. Tissue culture is only suc­cessful if done under aseptic conditions.

Apomictic embryos

In certain cases, an embryo can be produced without meiosis and fer­tilization. The resulting seed is thus asexually produced. Several grass species propagate in this manner, but it is of greater significance with fruit crops than with ornamentals.