Storage Organs, Vegetative Reproduction And It Examples


    Annual plants survive the dry season as seeds only, for after germination, flowering and seed formation, the rest of the plant dies off. Cow pea (vigna) and maize are examples of these. In certain cases the whole germination - reproduction cycle lasts only a few weeks, but may be repeated many times in one season, e.g. (Ageratum conyzoides). Plants such as these are called ephemerals.

    Perennial plants. Woody perennial plants are trees and shrubs in which the trunk and branches persist and grow from year to year. Those which shed all their leaves in the dry season are called deciduous; e.g. flamboyant (Delonic) and silk cotton (Ceiba), while evergreen trees such as the mango and coconut, shed their leaves throughout the year.

    Herbaceous perennials are plants which do not die after producing flowers and seeds but persist from season to season, though with reduced growth during dry periods. In some cases the vegetation above ground may die off entirely in the drought, as in certain of the grasses of the dry savannah. In perennial plants such as the canna lily, some leaves and flowers may remain in regions where the dry season is less prolonged and interrupted by storms, or in gardens where plants are watered.

In either case, the structures such as bulbs, corms or rhizomes, which persist below ground usually contain a store of food which is used for the rapid production of a new flowering shoot.

 Vegetative propagation

    In the rainy season, the terminal and lateral buds of herbaceous perennials sprout and produce new shoots. The shoots formed from some of the lateral buds often develop their

Own adventitious roots and by the end of the season have become independent of the parent plant and other lateral shoots. Thus, new plants are produced from buds without pollination or fertilization being necessary. This is called asexual or vegetative reproduction and is illustrated by the accounts of the life cycles described below.

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Bulbs are condensed shoots with fleshy leaves. The stem is very short and never grows above ground. The internodes are short; the leaves are very close together and they overlap. The outer leaves are scaly and dry and protect the inner ones which are thick and fleshy with stored food. In the harmattan lily (Hippeastrum equestre) and crinum lily, the bulb is formed by the bases of the leaves which completely encircle the stem, and it is to these cylindrical leaf bases that the food is sent by the rest of the leaf above ground. In the onion bulb, all the storage leaves are cylindrical and are not part of the leaves appearing above ground; the latter are produced separately by a terminal bud. In the leaf axils of both types of bulb are lateral buds which can develop into new bulbs and shoots.

Life cycle. In the rainy season, adventitious roots grow out of the stem, and a terminal or lateral bud, according to the species, begins to grow above the ground, making use of the stored food in the fleshy leaves which consequently shrivel.

During the wet months some of the food made in the leaves of the lily is sent to the leaf bases, which swell and form a new bulb inside the old one. In the onion type of bulb, the food is not sent to the leaf bases but to the lateral buds between the circular

Scales, so that as the buds enlarge, two or more new bulbs are formed inside the old one. In either type of bulb, the shrivelled storage leaves of the old bulb become the dry, scaly leaves which surround the newly formed daughter bulbs. When the daughter bulbs of the onion or the lateral buds of the harmattan lily sprout in the following year, there appear two plants where the original single parent grew. This cycle of events may be repeated several times during the wet season or while the plants are being watered. Several species, e.g. zephyr lily (Zephyranthes) are sensitive to water and flower after rain following a period of drought.


Some plants store food not in special leaves or leaf bases but in the stem, which is very short and swollen, forming a corm.

When the foliage has died off, the leaf bases, where they encircle the short stem, form protective scaly coverings. A familiar corm is that of the ornamental cocoyam (Caladiumn), and the example illustrated in the cocoyam. 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 the ground.

Life cycle. The food stored in the corm enables the terminal buds to grow rapidly and produce leaves and flowers above ground. Later, 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.

Rhizomes And Stolons

In plants with rhizomes or stolons, 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 canna lily, the terminal buds turn up and produce leaves and flowers above ground. The old leaf bases form circular scales round the rhizome, which is swollen with food reserves. A stolon forms in a similar way but does not become swollen with food.

Life cycle. The life cycle of a rhizome is similar to that of a corm. 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. Adventitious roots grow from the nodes of the underground stem. Rhizomes and stolons branch as they grow underground and so can colonize wide areas of soil. Cutting or grazing does not destroy the plant since the Thizome remaining below the soil can produce new buds and shoots. Similarly, stolons and rhizomes will often survive after fires have burned off the surface vegetation. In favourable conditions, buds from the underground stems will quickly form fresh aerial shoots.

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Stem tubers

     The yam (Dioscorea spp.) is considered to be a stem tuber; that is an underground stem swollen with food. The shape and number of tubers produced by a plant varies according to the species of Dioscorea and the method of planting and propagation, e.g. the Chinese yam, IDioscorea esculenta, forms a large number of small yams, Dioscorea alata forms only one or two.

One of the commonest methods of propagation is to cut a large tuber into eight or more pieces, called setts, and plant them under the soil. Although no buds are visible either on or below the corky outer layer a bud soon develops in the cortex of the tuber segment. It sprouts a shoot and roots using the store of food in the sett. By the time the food reserve in the sett is exhausted there is an extensive vine with many leaves making food by photosynthesis. Some of this food is sent down the stem to the junction of root and shoot and at this point, one or more tubers grow down into the soil. The sugar sent down from the leaves is turned into starch and stored in the tubers which swell greatly in diameter as a result.

In September or October when the rains stop, the vine dies away and the yams are harvested. Sometimes before the end of the rainy season, a yam tuber is dug up carefully without damaging the shoot and the bottom 90 per cent of the tuber is cut off and eaten. The remaining top part of the tuber with the vine still attached 1s replanted and continues growth but in this case it produces several small "seed yams which are used as setts in the next growing period.

Root tubers

The roots of the sweet potato (Ipomoea batatas) and cassava (Manihot esculenta) are swollen near their tops with food reserves which are used during active growth early in the rainy season. Food made in the wet months goes to new roots which swell and make fresh tubers.

Since roots usually bear no buds, root tubers in general are not reproductive structures. Cassava is propagated by planting pieces of stem whose buds produce shoots. The leaves on these shoots send food down to adventitious roots which swell to form tubers. The root tubers of the sweet potato, however, do develop adventitious buds which grow into new shoots, resulting in vegetative propagation of the plant.

Advantages of vegetative reproduction

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

Vegetative reproduction does not usually result in the rapid and widespread distribution of offspring achieved by 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 underground food stores and buds, can still grow after their foliage has been destroyed by insects, fire, or man's cultivation, Those of them in the "weed' category are difficult to eradicate by physical methods, Since even a small piece of rhizome bearing a bud can give rise to a new colony.

Farmers and gardeners make use of vegetative propagation when they divide up the rhizomes, tubers or rootstocks at the end of the growing or flowering season. Each section will grow in the following year to make a separate plant. Grafting, layering and taking cuttings are artificial methods of vegetative propagation. In taking a cutting, a piece of stem is made to produce adventitious roots by pushing it into the ground so that some of its lateral buds are covered, e.g. in cassava cultivation.

Vegetative reproduction by cuttings or grafting is often the only way of propagating a crop plant, e.g. cultivated bananas, which are seedless, or of preserving the useful characteristics of the plant, which if grown from seed would give too wide a range of variability in its offspring.

Advantages of food storage

Food in the storage organs enables very rapid growth in favourable conditions, e.g. the rainy season, and acts as a reserve in periods of drought, often allowing limited growth to continue. Early growth enables the plant to flower and produce seeds before competition with other plants for water, mineral salts and light reaches its maximum.

In agriculture, man has exploited many of these types of plant and bred them for bigger and more nutritious storage organs for his own consumption, e.g. yam, cocoyam, cassava, Sweet potato.



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