Vegetative propagation

Most higher plants reproduce by seeds or spores or develop a new plant from part of themselves asexually by vegetative propagation. There are only a few annual plants that reproduce vegetatively, but most herbaceous perennials do reproduce in this way, as well as relying on seeds for propagation.

Most plants that reproduce vegetatively are outstandingly successful in terms of numbers and areas colonized.

Plantlets growing from the edge of Bryophyllum sp. (left) drop to the ground and root to form new plants. In the same way, the purple bulbils developing in the papery bracts of crow garlic or Allium vineale (right) eventually fall to the ground, where they start growing. By these means the plants can multiply without relying on seed dispersal.

Advantages and disadvantages

Vegetative reproduction involves no exchange of genetic material and consequently every new plant is identical to the parent. The lack of potential variability of the offspring, and the absence of the dispersal mechanisms that operate in seed production, are the two major disadvantages of vegetative propagation.

On the other hand, asexual reproduction uses less energy than sexual, and in a harsh environment this factor may be of vital importance. Whereas seeds must carry their own food supply to provide the developing seedling with sufficient energy to become established, the vegetatively developing plant has abundant resources at its disposal from the parent plant. The new plant is thereby not as vulnerable to the problems of competition as a seedling is.

A seed is also vulnerable during its dispersal and may land in an area unsuitable for its growth—this rarely occurs in vegetative propagation. Seeds and seedlings are further disadvantaged in that any damage to them results in death or, at least, a malformed plant. But the vegetatively produced plantlet can withstand damage more easily because it has adequate supplies from the parent for repair.

Colonization of a wide area by vegetative means is rather slow because these plants tend to form clumps—such as buffalo grass (Buchloe dactyloides). But competition within the area is minimized because few other plants can grow within the clumps. Such clumps may be extremely persistent and capable of regrowth even after complete removal of the foliage by feeding animals or fire, and so the chances of survival of the species are increased.

The vegetative mechanism

Vegetative propagation occurs as a plant reproduces itself from a part of its stem or root. The Canadian pondweed (Elodea canadensis), for example, can reproduce itself from small fragments of its stem. This ability of a cell or group of cells to produce new ones with a function different from that of the parent cells is called cell totipotency.

In higher plants the meristem includes cambium cells. These are small, undifferentiated cells that have not developed a specific function. When properly stimulated they produce the appropriate new part.

Organs of propagation

In higher plants the meristems are also found in special organs, or certain parts of a plant. The organs have two functions—to survive nongrowing periods such as winter or dry seasons, and propagation. Organs of propagation can be underground food or water stores, but are also the aboveground parts of plants.

Many perennial plants grow from a persistent stem base, such as lupines (Lupinus spp.l. As the plant ages, the stem base increases in diameter, and the older center portions die off, leaving the newer outside parts as independent plants.

Some plants develop thick horizontal underground stems called rhizomes, such as irises [Iris spp.), Kentucky bluegrass (Poa pra-tensis), and some ferns, such as bracken (,Pteridium sp.). Growth is from a terminal bud, but branches also form at intervals, each of which produces a new root. The older parts of the rhizome then die off, leaving the branches independent. Rhizomes produce photosynthetic leaves that grow up through the soil. These aerial shoots are short-lived and die back after one season. The rhizome then overwinters on its food reserves, allowing the plant to survive.

Another mechanism of vegetative reproduction is the production of lateral stems or runners that periodically produce new plants. Strawberries (Frageria spp.) reproduce in this way.

Strawberries (Fragaria spp.) propagate by sending out runners from the main stem. These shoots root in the ground and form new plants. The plant has flowers and can reproduce sexually, and they develop into fruit, which may be eaten by animals, though it may be some time before the seeds are excreted and can germinate.

The flattened compact base of a stem, which grows underground, is called a corm and is found in plants such as crocuses (Crocus spp.). These stems reproduce at their tips on the surface where buds produce new corms directly; but in other plants, such as Montbretia, the rhizomes underground may develop corms at their tips.

In the potato (Solanum tuberosum) tubers form at the end of rhizomes that develop from the base of the stem. Shoots from the buds, or “eyes,” of an underground tuber also become tuberous themselves. These tubers are underground swollen stems and can produce new stems, leaves, and roots with new tubers.

Tubers are the underground swollen parts of plants that serve as food and water stores. They may become separated by the death of the parent plant and grow into a new plant, the shoots and roots of which may develop tubers and may be tuberous. Root tubers may involve the whole root, as in some orchids, or just the root tip, as in dahlias (Dahlia spp.). Roots themselves may generate stems if they become damaged or separated from the stem.

The underground stem, or rhizome, of the cowslip (Primula veris) acts as a food store throughout winter, but also develops new stems in the spring that bear flowers. After a few years the oldest section of the rhizome dies, and the plants are separated.

Bulbs, which can be likened to underground buds, frequently produce new bulbs by the growth of an axillary bud within the parent bulb. Bulb scales are fleshy leaves found on bulbs such as onions (Allium spp.). These scales store sugar or starch for the plant’s future use.

The swollen stem base (corm) of the crocus (Crocus sp.) is a food source for the plant during winter. It lasts one growing season. A new corm is formed by a thickening of the base of the new stem, just above the old corm. A tulip (Tulipa spp.), left, can reproduce by a bulb as well as by sexual reproduction. Bulbs differ from corms in that they are formed from layers of scale leaves that develop underground around the stem. The stem enlarges at its base to form the new bulb and the outermost scale leaves die off.