Flowering plants for S2

An Introduction to Vegetative Reproduction


Vegetative Reproduction

is a form of asexual reproduction in plants. It does not involve flowers, pollination and seed production. Instead, a new plant grows from a vegetative part, usually a stem, of the parent plant. However, plants which reproduce asexually almost always reproduce sexually as well, bearing flowers, fruits and seeds. Vegetative reproduction from a stem usually involves the buds. Instead of producing a branch, the bud grows into a complete plant which eventually becomes self-supporting. Since no gametes are involved, the plants produced asexually have identical genomes and the offspring form what is known as a clone. In some cases of vegetative reproduction, the structures involved also become storage organs and swell with stored food, e.g. potatoes.

The principal types of vegetative reproduction structures are bulbs, corms, rhizomes and runners.

Bulbs

Bulbs consist of very short stems with closely packed leaves arranged in concentric circles round the stem. These leaves are swollen with stored food e.g. onion. A terminal bud will produce next year’s flowering shoot and the lateral (axillary) buds will produce new plants.


Corms

Corms also have a short stem but in this case it is the stem itself which swells and stores food. The circular leaves form only papery scales. As with bulbs, the terminal bud grows into a flowering shoot and the lateral buds produce new plants.


Rhizomes

Rhizomes are stems which grow horizontally under the ground. In some cases the underground stems are swollen with food reserves e.g. iris. The terminal bud turns upwards to produce the flowering shoot and the lateral buds may grow out to form new rhizomes.


Runners

Runners are also horizontal stems growing from the parent plant, but they grow above ground. When their terminal buds touch the ground they take root and produce new plants.


Advantages of vegetative reproduction

Since food stores are available throughout the year and the parent plant with its root system can absorb water from quite a wide area, two of the hazards of seed germination are reduced. Buds are produced in an environment where the parent is able to flourish, but many seeds dispersed from plants never reach a suitable situation for effective germination.

Vegetative reproduction does not usually result in rapid and widespread distribution of offspring in the same way as seed dispersal, but tends to produce a dense clump of plants with little room for competitors between them. Such groups of plants are very persistent and, because of their buds and underground food stores, can still grow after their foliage has been destroyed by insects, fire, or cultivation. Those of them regarded as weeds are difficult to eradicate, since even a small piece of rhizome bearing a bud can give rise to a new colony (clone).

Bulbs - Snowdrop

In the snowdrop and daffodil, the bulb is formed by the leaf bases which completely encircle the short, conical stem. The part of the leaf above ground makes food by photosynthesis and sends it to the leaf bases which swell as they store the food. In the following year the stored food is used for the early growth of the bulb.

Life cycle of Daffodil
In the spring, adventitious roots grow out of the stem, and the leaves begins to grow above ground, making use of the stored food in the fleshy leaf bases which consequently shrivel. During late spring some of the food made in the leaves in the daffodil is sent to the leaf bases which swell and form a new bulb inside the old one.

Life cycle of Tulip and Onion
In these bulbs, the food is not sent to the leaf bases but to the lateral buds. As these buds enlarge they form two or more ‘daughter’ bulbs inside the old bulb. The leaves of the old bulb shrivel and dry out forming the dry scales which surround the daughter bulbs. In both cases, when the daughter bulbs grow, they form a clump, together with the parent bulb.

Corms - Crocus
Plants with bulbs store food in special leaves or leaf bases. Plants with corms store food in the stem, which is very short and swollen. When the foliage has died off, the leaf bases, where they encircle the short stem, form protective scaly coverings. A familiar corms is that of the crocus, and the wild arum corm is illustrated on p.1. Since the corm is a stem, it has lateral buds which can grow into new plants. The stem remains below ground all its life, only the leaves and flower stalk coming above ground.

Life Cycle of Corm
In Spring, the food stored in the corm enables the terminal bud to grow rapidly and produce leaves and flowers above ground. Later in the year, food made by the leaves is sent back, not to the old corm, but to the base of the stem immediately above it. This region swells and forms a new corm on top of the old, now shrivelled, corm. Some of the lateral buds on the old corm have also grown and produced new plants with corms.

Contractile Roots
The formation of one corm on top of another tends to bring the successive corms nearer and nearer to the soil surface. Adventitious roots develop from the base of the new corm. Once these have grown firmly into the soil, a region near their junction with the stem contracts and pulls the new corm down, keeping it at a constant level in the soil. Wrinkles can be seen on these contractile roots where shrinkage has taken place. Bulbs also have contractile roots which counteract the tendency in successive generations to grow out of the soil.

Rhizomes

In plants with rhizomes, the stem remains below ground but continues to grow horizontally. The old part of the stem does not die away as in bulbs and corms, but lasts for several years. In the iris, the terminal bud turns up and produces leaves and flowers above ground. The old leaf bases form circular scales round the rhizome, which is swollen with food reserves. Lateral buds grow into new rhizomes.

Life Cycle of Rhizome
The annual cycle of a rhizome is similar to that of a corm. In late spring/early summer, food from the leaves passes back to the rhizome, and a lateral bud uses it, grows horizontally underground, and so continues the rhizome. Other lateral buds produce new rhizomes which branch from the parent stem. The terminal buds of these branches curve upwards and produce new leafy shoots and flowers. Contractile, adventitious roots grow from the nodes of the underground stem and keep it at a constant depth.

Runners
Plants such as the strawberry have a very short stem, called a rootstock, with thin scale leaves,. Foliage leaves and flowers grow from the buds in the axils of the scale leaves. Some of the lower buds produce shoots which grow horizontally over the surface of the ground and bear scale leaves and buds. The terminal buds of these runners turn up and produce daughter plants some distance away from the parent, the new plants developing adventitious roots. Later, the runner shrivels away. The runner does not store food but conducts it from the parent plant to the daughters, until they are well developed.

Stem tubers - potato
In the potato plant, lateral buds at the base of the stem produce shoots which grow laterally at first and then down into the ground. These are comparable to rhizomes, as they are underground stems with tiny scale leaves and lateral buds. They do not swell evenly along their length with stored food.

Annual cycle - potato
Food made in the leaves passes to the ends of these rhizomes, which swell and form the tubers we call potatoes. Since the potato tuber is a stem, it has leaves and lateral buds; these are the familiar ‘eyes’. Each one of these can produce a new shoot in the following year, using the food stored in the tuber. The old tubers shrivel and rot away at the end of the season

Stolons
Blackberry stems form a rather different type of runner in which the main shoot forms the new individual. When the growing end of a shoot arches over and touches the ground, the terminal bud curves up, producing a new shoot which soon develops adventitious roots.

Grafting
A bud or shoot from one plant is inserted into a cleft or under the bark on the stem of a closely related variety. The rooted portion is called the stock; the bud or shoot being grafted is the scion. The stock is obtained by growing a plant from seed then cutting away the shoot. The scion is a branch or a bud cut from a cultivated variety with the required characteristics of flower colour, fruit quality, etc.

Rose plants grown from seed would produce a wide variety of plants, only a few of which would retain all the desirable features of the parent plant. Most of them would be like wild roses. Similarly, most of the apple trees grown from seed would bear only small, sour ‘crab-apples'. By taking cuttings and making grafts, the inbred characteristics of the plant are preserved and you can guarantee that all the new individuals produced by this kind of artificial propagation will be the same.

Cuttings
It is possible to produce new individuals from certain plants by putting the cut end of a shoot into water or moist earth. Roots grow from the base of the stem into the soil while the shoot continues to grow and produce leaves.

In some cases the cut end of the stem may be treated with a rooting 'hormone' to promote root growth. Evaporation from the shoot is reduced by covering it with polythene or a glass jar. Carnations, geraniums and chrysanthemums are commonly propagated from cuttings.

Tissue culture
Once a cell has become part of a tissue it usually loses the ability to reproduce. However, the nucleus of any plant cell still holds all the 'instructions' (genes) for making a complete plant and in certain circumstances they can be brought back into action. In laboratory conditions single plant cells can be induced to divide and grow into complete plants. One technique is to take a small piece of plant tissue from a root or stem and treat it with enzymes to separate it into individual cells The cells are then provided with particular plant 'hormones’ which induce cell division and, eventually the formation of roots, stems and leaves.

An alternative method is to start with a small piece of tissue and place it on a nutrient jelly (agar). Cells in the tissue start to divide and produce many cells forming a shapeless mass called a callus. If the callus is then provided with the appropriate ‘hormones’ it develops into a complete plant.