Propagation of Crops Without True Seeds

AUGUST E. KEHR, FRED P. ESHBAUGH, AND DONALD H.SCOTT.

Moss of the plants in our fields and gardens usually are grown from true seeds. Many other plants must be propagated in another way.

To do that, we cut small sections of living stem or root tissue from the parent plant. We handle the cut sections in one of several hundred different ways. In due time we have new plants exactly like the original ones. This seedless method of increasing plants is asexual, or vegetative, propagation, which is essentially the development of entire plants from buds.

Plants so grown are literally and actually "chips off the old block." They therefore have the qualities of the original plant.

We use vegetative propagation for two basic reasons.

First, some plants do not commonly produce seeds and hence must be propagated vegetatively. The seedless grape is one example. Another is garlic. No one in this country has ever produced true seed of garlic, although its near relative, the common onion, almost always is grown from seed. Other plants that produce flowers but seldom, if ever, a seed are horseradish, pineapple, some palms, sugarcane, some bamboos, bananas, and certain sterile hybrid plants. Still other plants, such as the common ground-ivy (Hedera helix), rarely produce flowers. Vegetative propagation is the most economical way to increase them.

The second reason is that many seed-bearing plants do not breed true that is, seedlings that grow from their seeds do not closely resemble the parents. We could plant seeds of the Delicious apple, or Peace rose, or Meyer zoysia grass, and get seedlings that definitely are apples, roses, and zoysia grasses, but we would lose the identifying characteristics that made the originals famous. In short, when we buy new plants of the Delicious apple or another named variety, we want them to be identical with the parent plants and with each other to the extent that we can distinguish no differences in them.

Our sole purpose in propagating named varieties of many plants woody ornamentals, tree fruits, small fruits, grasses, ornamental perennials, white potatoes, sweetpotatoes, bulbs is to keep intact the variety features that we prize.

Valuable breeding stocks of alfalfa and other field crops often are maintained vegetatively to insure the exact genetic makeup in the resulting seed and thus prevent the unavoidable change that occurs when they are increased by true seeds.

We choose and plant named varieties of certain plants in our fields, gardens, and orchards primarily because we know they have certain desirable features, but the only way we can be reasonably certain of getting plants absolutely true to variety name is by propagation of stem or root tissues of the original parent plant.

VEGETATIVE PROPAGATION has its limitations.

For the same reason that it is a highly desirable means of maintaining the status quo in a variety, the usefulness of vegetative propagation is restricted to this purpose, and consequently it cannot be used to develop new varieties of plants. It is always necessary to resort to sexual reproduction in order to obtain true seed with which to develop new varieties of crops that commonly are propagated vegetatively.

To create a new variety of white potato, for example, we must allow the potato plant to grow, form flowers, pollinate the flowers, and then allow the seedballs to develop. Occasionally when weather conditions are suitable and the seedballs are formed naturally, the event is so unusual that many gardeners are bewildered to see "small tomatoes" growing on their potato vines.

Improvement of plants that normally are propagated vegetatively by pollination and formation of true seed is exceedingly slow. For white potatoes, about 1 million seedling plants must be grown in order to obtain one selection worthy of being named as a new variety. From that point onward, it must be maintained vegetatively.

New forms or varieties of plants can appear naturally but very rarely! in vegetatively propagated plants as the result of sports. Another name for sports is mutations. Sports and mutations are plants or parts of a plant that abruptly show a noticeably different appearance from the rest of the plant.

Such a change, if it is the result of a true mutation, may be maintained by vegetative means once it has occurred.

Examples of changes in a sport or mutation are a deeper red in a red apple, an unusual color or shape in a flower, a different leaf color in a shade tree, double flowers instead of singles, and similar noticeable changes.

For some unknown reason, mutations, which, as we said, are rare in most plants, occur so often in some sweetpotatoes that it is difficult to maintain a variety unchanged even by vegetative propagation.

WHEN WE increase plants by vegetative reproduction, our aim is to stimulate the growth of buds on "borrowed" roots and stems or to encourage the formation of entirely new buds and roots from primordial first-formed tissue.

There are essentially two types of buds of importance in vegetative propagation true and adventitious buds.

A true bud is a growing point that usually is formed at the base of a leaf and along the stem of a plant.

True buds occasionally may remain inactive and are made active only by some unusual stimulus. For example, an apple tree, if cut, may grow new shoots from the stump when the inactive true buds are stimulated to grow.

True buds may develop into leaves, flowers, or stems. Inactive true buds are "reserves" and function only when active true buds are killed or removed or do not grow for some other reason.

Adventitious buds are formed from embryonic or undifferentiated tissue cells in the roots, stems, or leaves. Given the proper stimulus, these cells form buds that in turn may form either roots or sprouts, depending on their location.

Theoretically, any living cell in the plant maybe stimulated to form a bud and ultimately a new plant. In fact, fully developed carrot plants have been grown experimentally from a single root cell of a carrot.

Such development, however, usually is easiest from the cambium (a soft formative tissue just under the outer bark, which is the growing layer in most stems); the pericycle (undifferentiated cells surrounding the central cylinder); or rarely from phloem tissue (the tissue in which food materials are transported within the plant).

Adventitious buds in a sense are emergency buds, which insure the perpetuation of the plant when it is in danger of dying for lack of true buds.

The structure of the plant largely determines the method of vegetative propagation that we can use. Thus in propagating plants classified as monocots pineapple, sugarcane, and bamboo, for example it is difficult or impossible to stimulate adventitious buds. We achieve vegetative increase by stem tillers or offshoots or from inactive or latent buds at the nodes of the stems.

In the plants classified as divots, bud formation is not so restricted because the cambial and other undifferentiated tissues are distributed more widely. It is rather easy therefore to stimulate the growth of adventitious buds in most dicotyledonous plants.

True stems are used oftenest in asexual propagation methods. They include cuttings, grafting, and budding. Other modified stems are used tubers, stolons, rhizomes, tillers, bulbs, and corms.

Stems differ from true roots in certain basic ways. We all know that roots always grow down toward the center of the earth, but stems, even though they are underground tend to grow upward. The distinction between roots and stems that is of greatest importance in vegetative propagation is that stem buds are formed on the surface while roots have no surface buds.

Bud growth from roots is adventitious and is always initiated from the internal tissue.

VEGETATIVE PROPAGATION may be applied to both underground and aboveground parts.

Below-ground parts would include roots, tubers, stolons, rhizomes, tillers, bulbs, corms, and mycelium. (Mycelium is the threadlike vegetative growth of fungi that develops underground more or less as roots do for most plants. In mushrooms mycelium is commonly called spawn.)

Above-ground parts involve stems for cuttings, layering, grafting, and budding.

Plants often are increased directly from true roots by two primary methods, root suckers and cuttings of whole roots.

A true root sucker is formed when an adventitious bud develops from the internal root tissue and continues growing into a stem. Root suckers are sometimes produced naturally near the base of the parent plant and form dense, close-knit clumps, which easily can be lifted and separated.

In plants like the raspberry, the root suckers occur at some distance from the parent plant, and we get new plants merely by cutting the connecting roots.

Two variations of the usual root-cutting method are used commercially induced suckering and undisturbed whole-root cutting.

Induced suckering is useful in species that rarely produce root suckers. By this method, suckers form in response to partial girdling or cutting of an exposed root in such a way that the partly severed root receives some nutrients from the parent plant.

Undisturbed whole-root cutting merely goes one step further. The exposed root is severed completely, and the cutoff portion is left in the soil.

Polyethylene plastic film has recently come into common use for rapid rooting of softwood and hardwood cuttings. Methods employed are simple.

Cuttings are prepared in the usual way in lengths of 2 to 6 inches, depending on the material to be propagated. The stem is cut off at an angle of 45 degrees just below a node. One-half to one-third of the lower leaves are removed. Unusually large leaves are cut back to one-half their length to save space and reduce loss of moisture.

Cuttings must be kept fresh and turgid from the time they are removed from the plant until they are placed in the rooting medium. Freshness may be preserved by placing cuttings in a polyethylene bag or between layers of damp paper or cloth. Cuttings root more quickly if they are treated with a rooting compound such as Hormodin #2 or #3 before they are inserted in the rooting media.