Asexual reproduction

In asexual reproduction, a single parent produces offspring. The book shows several ways:

In fission, the nucleus divides first, then the rest of the cell. Amoeba can split in any direction. Leishmania, which causes kala-azar, splits in a fixed direction. The malaria parasite Plasmodium splits into many cells at once: multiple fission. ("Daughter cells" just means the new cells; it has nothing to do with being female.)

Regeneration is different from reproduction. Cut a Planaria into pieces and each piece regrows a whole body; Hydra can do this too. But this is repair after damage, so organisms don't depend on it to multiply. Planaria can reproduce in other ways.

Not every cell can make a new organism: your skin cell can't. Spores are special. In bread mould, they form inside round sporangia at the tips of threads. Each spore has a thick wall that protects it until it lands on something moist, like bread, where it starts to grow.

Vegetative propagation grows new plants from roots, stems or leaves. The leaf of Bryophyllum has buds along its notches, each of which can grow into a plant. Sugarcane, roses and grapes are grown by layering or grafting. Why do gardeners like it?

• plants flower and fruit sooner than plants grown from seed;
• plants that make no seeds (banana, orange, rose, jasmine) can still be grown;
• the new plants are just like the parent: take a cutting from a tree with sweet mangoes, and you get sweet mangoes.

Tissue culture goes one step further. Cells from the growing tip of a plant are put in a special nutrient medium, where they divide into a lump called a callus. With hormones, the callus grows into tiny plantlets. Many disease-free plants can be grown from one parent this way, as is done for ornamental plants.