Asexual reproduction: one parent, exact copies

No living thing lasts forever. What carries a species on is reproduction, making new individuals of the same kind. It comes in two broad forms, and this page covers the one that needs only one parent.

Asexual reproduction is common in single-celled life such as bacteria, amoeba and yeast, in simple many-celled animals such as hydra and sponges, and in a huge number of plants. Tap through the gallery to see the main ways it is done.

Start in the kitchen. A potato or a piece of ginger forgotten in a dark corner sends up shoots, because both are fleshy underground stems, and no seed is needed. Farmers plant sugarcane from pieces of stem, and a money plant cutting roots in a pot of soil. Bryophyllum goes further: tiny plantlets form in the notches of its leaves, then drop off and grow on their own (Fig. 11.1).

Vegetative propagation is the name for all of this: a new plant grows from a root, stem or leaf, the parts that grow rather than the flowers. With a single parent, each new plant has the same genetic information as the plant it came from.

Gardeners and scientists noticed long ago that plants copy themselves so readily, and they turned it into a tool. If you have a plant with the perfect fruit or the most beautiful flower, vegetative propagation lets you make hundreds more that are exactly the same, quickly. This has changed how farms and gardens are run and lets farmers grow a favoured crop on a large scale.

For Activity 11.1, find a gardener or a farmer and watch how they do it. The three methods below are the ones to look for. Pick one and step through it.

Questions to ask while you watch a cutting being made: Is the plant pruned once its growing season is over? How long is a typical cutting? Count its nodes and internodes. Write the answers in your notebook.

Tissue culture takes propagation down to a few cells. A tiny piece of the growing tip of a shoot, the apical meristem, is placed on a nutrient medium and its cells multiply into a mass that grows shoots and roots. Watch it below.

Banana growing is the big success story. Healthy plantlets, mass produced this way, now reach farmers in bulk; they keep virus-infected stock out of the fields and give high yields. Each plantlet still has only one parent, so this is asexual reproduction too.

Plants are not the only ones. Yeast is a single-celled fungus, the same one that makes dough rise. Activity 11.2 lets you catch it reproducing. Step through the set-up below.

Spot the small, round bumps growing out of some yeast cells. Each bump is a bud, a copy of the cell in the making. It swells and, in time, becomes a yeast cell of its own.

Hydra, a tiny many-celled animal of ponds, does something similar on a bigger scale. At one spot on its body, cells divide again and again and form a bulge, a bud. The bud grows its own tentacles, then breaks away and lives on its own (Fig. 11.7). A single hydra often carries several buds at once. This process is called budding.

Leave cooked food out on a hot day and by the next it smells and may sprout fuzzy patches; damp walls collect black ones. Both are microorganisms, often fungi. Where do they come from? For centuries the common belief was that they simply appeared out of the food or the wall itself. Careful experiments by Louis Pasteur settled it: no living thing arises without a living parent. Three things followed from his work: the idea that germs cause disease, firmer support for cell theory (new cells only come from existing cells), and the habit of sterilising food and instruments.

So where does bread mould come from? Here is Activity 11.3, sped up. Drag the slider through the days, then put a bit of the mould under the microscope.

Under the microscope you see thin threads, the hyphae, with a round sac at the tip of some of them, packed with tiny round spores. In Rhizopus the spores form inside such a sac; in Aspergillus they form on a swollen tip (a vesicle) at the end of a long hypha (Fig. 11.8).

Fresh bread had no mould. The spores came from the air: they were floating about, landed on the moist bread, and started growing and multiplying fast. A single colony sends out millions of spores. Each weighs almost nothing and is usually a single cell, so air currents carry it far. It waits for moisture and food, then germinates into a new mould.

Budding, spores and vegetative propagation look nothing alike, yet one kind of cell division runs them all. One cell in, two cells out, each holding exactly the parent's set of chromosomes: that is mitosis.

So nothing in the genetic information changes from parent to offspring, and genetically identical offspring like these are called clones. Copying this way is quick, so numbers shoot up when food, warmth and moisture are plentiful.