Reproduction is the biological process by which new organisms are produced to ensure the perpetuation of the species. Reproductive methods are grouped into two main groups: asexual reproduction and sexual reproduction.
Asexual reproduction is when an organism produces a genetically identical offspring without the contribution of genetic material from another organism. Sexual reproduction is when a genetically dissimilar offspring is produced through the fusion of two gametes, one from each parent organism, during the process of fertilisation. Gametes are reproductive cells that contain half the number of chromosomes as a normal body cell. The zygote produced during fertilisation contains genetic material from both parents and is therefore genetically different from them.
Bacteria reproduce by cell division or fission. Any bacterial cell can divide into two and each daughter cell becomes an independent bacterium. In some cases, this cell division can take place every 20 minutes so that, in a very short time, a large colony of bacteria can be produced. This is one reason why a small number of bacteria can seriously contaminate our food products. This kind of reproduction, without the formation of gametes (sex cells) is called asexual reproduction.
Fungi have sexual and asexual methods of reproduction. In the asexual they produce single-celled, haploid spores. These are dispersed, often by air currents and, if they reach a suitable situation, they grow new hyphae, which develop into a mycelium.
Penicillium and Mucor are examples of mould fungi that grow on decaying food or vegetable matter. Penicillium is a genus of mould fungi that grows on decaying vegetable matter, damp leather and citrus fruits. The mycelium grows over the food, digesting it and absorbing nutrients. Vertical hyphae grow from the mycelium and at their tips, produce chains of spores. These give the colony a blue-green colour and a powdery appearance. The spores are dispersed by air currents and if they reach a suitable substrate, grow into a new mycelium.
Mucor feeds, grows and reproduces in a similar way to Penicillium, but Mucor produces spores in a slightly different way. Instead of chains of spores at the tips of the vertical hyphae, Mucor forms spherical sporangia, each containing hundreds of spores. These are dispersed on the feet of the insects or by the splashes of rain drops.
The gills on the underside of a mushroom or toadstool produces spores. Puffballs release clouds of spores.
Asexual reproduction in flowering plants (vegetative propagation)
Although all flowering plants reproduce sexually, many of them also have asexual methods (also called ‘vegetative propagation’) as described below. When vegetative propagation takes place naturally, it usually results from the growth of a lateral bud on a stem which is close to, or under, the soil. Instead of just making a branch, the bud produces a complete plant with roots, stem and leaves. When the old stem dies, the new plant is independent of the parent that produced it.
An unusual method of vegetative propagation is shown by Bryophyllum.
Stolons and rhizomes
The flowering shoots of plants such as the strawberry and the creeping buttercup are very short and, for the most part, below ground. The stems of shoots such as these are called rootstocks. The rootstocks bear leaves and flowers. After the main shoot has flowered, the lateral buds produce long shoots, which grow horizontally over the ground. These shoots are called stolons (or ‘runners’) and have only small, scale-leaves at their nodes and very long internodes. At each node there is a bud that can produce not only a shoot, but roots as well. Thus a complete plant may develop and take root at the node, nourished for a time by food sent from the parent plant through the stolon. Eventually, the stolon dries up and withers, leaving an independent daughter plant growing a short distance away from the parent. In this way a strawberry plant can produce many daughter plants by vegetative propagation in addition to producing seeds.
In many plants, horizontal shoots arise from lateral buds near the stem base, and grow under the ground. Such underground horizontal stems are called rhizomes. At the nodes of the rhizome are buds, which may develop to produce shoots above the ground. The shoots become independent plants when the connecting rhizome dies.
Many grasses propagate by rhizomes; the couch grass is a good example. Even a small piece of rhizome, provided it has a bud, can produce a new plant.
In the bracken, the entire stem is horizontal and below ground. The bracken fronds you see in summer are produced from lateral buds on a rhizome may centimeters below the soil.
Bulbs and corms
Bulbs such as those of the daffodil and snowdrop are very short shoots. The stem is only a few millimetres long and the leaves which encircle the stem are thick and fleshy with stored food.
In spring, the stored food is used by a rapidly growing terminal bud, which produces a flowering stalk and a small number of leaves. During the growing season, food made in the leaves is sent to the leaf bases and stored. The leaf bases swell and form a new bulb ready for growth in the following year.
Vegetative reproduction occurs when some of the food is sent to a lateral bud as well as to the leaf bases. The lateral bud grows inside the parent bulb and next year will produce an independent plant.
The corms of crocuses and anemones have life cycles similar to those of bulbs but it is the stem, rather than the leaf bases, which swells with stored food. Vegetative reproduction takes place when a lateral bud on the short, fat stem grows into an independent plant.
Daffodil bulb; vegetative reproduction
In many cases the organs associated with asexual reproduction also serve as food stores. Food in the storage organs enables very rapid growth in the spring. A great many of the spring and early summer plants have bulbs, corms, rhizomes or tubers: daffodil, snowdrop and bluebell, crocus and cuckoo pint, iris and lily-of-the-valley and lesser celandine.
Potatoes are stem tubers. Lateral buds at the base of the potato shoot produce underground shoots (rhizomes). These rhizomes swell up with stored starch and form tubers . Because the tubers are stems, they have buds. If the tubers are left in the ground or transplanted, the buds will produce shoots, using food stored in the tuber. In this way, the potato plant can propagate vegetatively.
Stem tubers growing on a potato plant and a potato tuber sprouting
Agriculture and horticulture exploit vegetative reproduction in order to produce fresh stocks of plants. This can be done naturally, e.g. by planting potatoes, dividing up rootstocks or pegging down stolons at their nodes to make them take root. There are also methods that would not occur naturally in the plant’s life cycle. Two methods of artificial propagation are by taking cuttings and by tissue culture.
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 practice, the cut end of the stem may be treated with a rooting ‘hormone’ (a type of auxin) to promote root growth, and evaporation from the shoot is reduced by covering it with polythene or a glass jar. Carnations, geraniums and chrysanthemums are commonly propagated from cuttings.
Once a cell has become part of tissue it usually loses the ability to reproduce. However, the nucleus of any cell in a plant still holds all the ‘instructions’ 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 small pieces of plant tissue from a root or stem and treat it with enzymes to separate it into individual cell. 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. Cells in the tissue start to divide and produce many cells, forming shapeless mass called a callus. If the callus is then provided with the appropriate hormones it develops into a complete plant see Figure below.
Using the technique of tissue culture, large numbers of plants can be produced form small amounts of tissue and they have the advantage of being free from fungal or bacterial infections. The plants produced in this way from clones, because they have been produced from a single parent plant.
Asexual reproduction in animals
Some species of invertebrate animals are able to reproduce asexually.
Hydra is a small animal, 5-10 mm long, which lives in ponds attached to pondweed. It traps small animals with its tentacles, swallows and digests them. Hydra reproduces sexually and digests them. Hydra reproduces sexually by releasing its male and female gametes into the water but it also has an asexual method, which is shown in Figure below.
Asexual reproduction in Hydra
(a) a group of cells on the column start dividing rapidly and produce a bulge
(b) the bulge develops tentacles
(c) the daughter Hydra pulls itself off the parent
(d) the daughter becomes an independent animal
The advantages and disadvantages of asexual reproduction
The advantages and disadvantages of asexual reproduction discussed below are in the context of flowering plants. However, the points made are equally applicable to most forms of asexual reproduction.
In asexual reproduction no gametes are involved and all the new plants are produced by cell division from only one parent. Consequently they are genetically identical; there is no variation. A population of genetically identical individuals produced from a single parent is called a clone. This has the advantage of preserving the ‘good’ characteristic of a sucessful species from generation to generation. The disadvantage is that there is no variability for natural selection to act on in the process of evolution.
In agriculture and horticulture, asexual reproduction (vegetative propagation) is exploited to preserve desirable qualities in crops: the good characteristics of the parent are passed on to all the offspring. With a flower such as a daffodil, the bulbs produced can be guaranteed to produce the same shape and colour of flower from one generation to the next. In some cases, such as tissue culture, the young plants grown can be transported much more cheaply than, for example, potato tubers as the later are much heavier and more bulky. Growth of new plants by asexual reproduction tends to be a quick process.
In natural conditions in the wild it might be a disadvantage to have no variation in a species. If the climate or other conditions change and a vegetatively produced plant has no resistance to a particular disease, the whole population could be wiped out.
A plant that reproduces vegetatively will already be growing in a favourable situation, so all the offspring will find themselves in a suitable environment. However, there is no vegetative dispersal mechanism and the plants will grow in dense colonies, competing with each other for water and minerals. The dense colonies, on the other hand, leave little room for competitors of other species.
As mentioned before, most plants that reproduce vegetatively also produce flowers and seeds. In this way they are able to colonise more distant habitats.
The store of food in tubers, tap roots, bulbs, etc. enables the plants to grow rapidly as soon as conditions become favourable. Early growth enables the plant to flower and produce seeds before competition with other plants (for water, mineral salts and light) reaches its maximum. This must be particularly important in woods where, in summer, the leaf canopy prevents much light from reaching the ground and the tree roots tend to drain the soil of moisture over a wide area.
Summary of advantages and disadvantages of asexual reproduction
|quick||little variation ⇢ adaptation to environment is unlikely|
|only one parent needed||offsprings inherit bad characteristic (e.g.: resistance from a disease)|
|no gametes needed||lack of dispersal ⇢ competition (nutrients, water, light)|
|all good characteristics passed on to the offspring|
|no dispersal (potato tubers) ⇢ grow in some favourable environment as parent|
|store large amounts of food ⇢ rapid growth|