Exam practice

Asexual reproduction needs only one parent and no gametes. The new individual grows from part of that one parent (a split cell, a bud, a piece, a spore), so no male gamete and female gamete have to meet, and there is no need for a male parent and a female parent.

A potato tuber is a swollen underground stem, a vegetative part of the plant, not a flower or seed. New plants grow from the buds in its eyes without any gametes fusing. Reproduction from a vegetative part (root, stem or leaf) is vegetative propagation.

A grafted plant bears fruit of the same good quality as the tree the scion came from, and it starts bearing in a few years instead of many, because the scion is already mature. The stock can be chosen for strong roots that resist disease or drought. So farmers get better fruit, sooner, and earn more.

It uses modern lab methods (germ-free culture media, controlled light and temperature, plant hormones) and needs only a tiny piece of the parent. From that piece it can produce hundreds of thousands of identical, disease-free plantlets of almost any plant in a short time, in any season, which no other method can do.

Reproduction is the biological process by which organisms produce new individuals of their own kind. Example: a hen lays eggs that hatch into chicks; a mango seed grows into a mango tree; an amoeba splits into two amoebae.

Fission, budding, fragmentation, regeneration, sporulation and parthenogenesis.
Budding: a small outgrowth, the bud, forms on the parent’s body. It grows and develops into a complete small organism, then breaks off and lives on its own. Example: yeast, hydra.
(You may describe any one; for example, binary fission: a single-celled organism such as an amoeba divides its nucleus and then its cytoplasm, splitting into two daughter cells.)

Draw a row of four hydras, as in Figure 5.3.

1. A small bump (bud) appears on the side of a hydra’s body, where cells divide rapidly.
2. The bud grows longer; its hollow inside is joined to the parent’s.
3. A mouth and a ring of small tentacles form at its tip.
4. The young hydra pinches off at its base, attaches itself nearby, and lives on its own.

Vegetative propagation
→ Natural: root; underground stem (sucker, rhizome, corm, bulb, tuber); aerial stem; bulbils; leaf
→ Artificial: cutting; layering; gootee; grafting; tissue culture
(Draw it as the tree in Figure 5.8.)

A bulbil is a small, fleshy bud full of stored food that forms at a node, at the base of a leaf or in place of a flower. It falls off and grows roots and a shoot to become a new plant. It is a unit of natural vegetative propagation. Example: agave (kettuke), githa (air yam), some lilies.

The stock is the lower part, with roots, usually a year-old seedling cut 20–30 cm above the ground. The scion is a twig with a few buds taken from a tree with good fruit.

Process: the stock and scion are cut so that their ends fit together closely, with their living layers touching. They are joined, tied, and sealed airtight with grafting wax or tape. After a few weeks the cut surfaces heal and the two grow into one plant.

Advantages: the plant bears fruit as good as the scion’s parent; it bears early; the stock can give strong, disease-resistant roots; many good plants can be made from one tree.

Layering is growing roots on a branch while it is still joined to the parent plant. A ring of bark 2–3 cm wide is removed from a low, soft branch (the layer), and that part is bent down and buried in moist soil, the tip left above ground. When roots have grown from the buried part, the branch is cut from the parent and planted. Example: jasmine, grapevine. (Draw the middle panel of Figure 5.13.)

• Farmers get plants that bear good fruit early: grafted mango and orange saplings start fruiting in 3–4 years, against 6–8 or more for a seedling.
• Good qualities are kept: a sweet, large variety of litchi copied by gootee gives the same fruit, which a seed can’t promise.
• Plants without seeds can be grown: banana (from tissue culture or suckers), pineapple, seedless grapes.
• Compared with natural propagation, people choose the best parent plant and the method, so the young are good copies, many are made at once, and they are planted where and when they are wanted, not left to chance.
• Many healthy plants quickly: tissue culture gives thousands of disease-free potato and banana plantlets for farmers.
• Consumers get more and better fruit, the nation needs to import less, and nurseries give jobs.

Both are self-pollination: the genes come from one plant.

Pollen can’t move by itself. For cross-pollination, and for geitonogamy, something must carry it from an anther to a stigma on another flower: wind, water, insects or birds. Without these agents, most flowers would get no pollen, and no fertilization, fruits or seeds would follow.

Sexual reproduction is the fusion of a male gamete with a female gamete. The male gamete (sperm, or the pollen’s male gamete) is made only by a male organ, and the female gamete (egg) only by a female organ. So both, whether in two separate individuals or in one bisexual organism, are needed to make a zygote.

In external fertilization the sperms have to swim to the eggs, and the eggs, with no shell, would dry up and die in air. So animals with external fertilization lay their eggs in water (fishes, most frogs), or in very wet places on land, such as damp moss or a foam nest (some frogs), where the sperms can reach them and the eggs stay moist. On dry land it can’t happen.

Pollination is the transfer of pollen grains from an anther to a stigma of a flower of the same species. Example: a honey bee visiting mustard flowers carries pollen from one flower to another (entomophily); the wind carries pollen from a maize tassel to the silks of a cob (anemophily).

Draw the three routes in Figure 5.19.

In flowering plants, two fusions happen in each ovule; this is double fertilization.
A pollen grain on the stigma grows a pollen tube down the style. Two male gametes form in it. The tube enters the ovule through the micropyle and releases them.

1. One male gamete fuses with the egg cell → zygote → embryo.
2. The other fuses with the two polar nuclei (secondary nucleus) → endosperm nucleus → endosperm, the food for the embryo.

Then the ovule becomes the seed and the ovary becomes the fruit.

Draw them as in Figure 5.22.

Ovum: corona radiata (outer ring of cells), zona pellucida (clear layer), plasma membrane, cytoplasm, nucleus, nucleolus.

Sperm: head (with acrosome and nucleus), neck, middle piece (with mitochondria), tail.

This is sexual reproduction, with internal fertilization and live birth (dogs are mammals).
Importance of gametes:

• A male gamete and a female gamete fuse to form the zygote, the first cell of every new individual.
• Each gamete carries half of a parent’s genes, so the young get a new mixture from both parents. That is why the four puppies look different from each other.
• This variation helps a species adapt to changes and makes evolution possible.
• Gametes keep the number of chromosomes the same in every generation: half + half = a full set.

Male (testes) → sperm
Female (ovaries) → ovum (egg)
sperm + ovum → fertilization (internal or external) → zygote → cell division → embryo → baby (born or hatched) → grows into an adult → makes gametes again.
(Draw it like Figure 5.23, with arrows.)

Gametogenesis is the making of gametes (sex cells) in the reproductive organs.

• In animals: sperms are made in the testes, and ova in the ovaries.
• In flowering plants: the male gametes are formed from the pollen made in the anthers (in many plants they form in the pollen tube), and the egg cell is made in the ovule, inside the ovary.

Internal: human, pigeon, tortoise, butterfly, bat, cow, dragonfly, whale.

External: fish (most fishes), frog.

• Animals that reproduce only sexually (cows, dogs, birds, humans) would have no young at all, and would die out in one generation.
• Plants that make only seeds (wheat, rice, mustard) would vanish; there would be no grains or most fruits to eat.
• Even organisms that can reproduce asexually would all be copies. With no variation, one new disease could wipe out all of them. Example: in the 1840s, Ireland’s potatoes, all grown from tubers and so nearly identical, were destroyed by one disease, and about a million people died in the famine that followed.
• With no variation, living things couldn’t adapt or evolve.

In a flowering plant the male gametes are carried in the pollen, and the egg is deep inside the ovule. The only way the male gametes reach the egg is by a pollen grain landing on the stigma and growing a pollen tube down to the ovule. If there is no pollination, no pollen reaches the stigma, no tube grows, and the male and female gametes never meet. So no fertilization, no seeds and (in most plants) no fruits form.

• The sperms are put directly inside the female’s body, so they are not lost in water, and many more eggs are fertilized.
• The fertilized egg is protected inside the mother (mammals) or by a shell (birds, reptiles), safe from drying, cold and animals that eat eggs.
• So, although fewer eggs are made, a much larger share of them survive to become adults. In external fertilization most eggs are never fertilized, or are eaten.

• Scent (with color and nectar) attracts insects and birds. As they move from flower to flower to feed, pollen sticks to them and rubs onto the next stigma, so cross-pollination happens.
• If the anthers and stigma of a flower ripen at different times, the flower can’t pollinate itself: when its pollen is ready its stigma isn’t, or the other way round. Its stigma must receive pollen from another flower, so cross-pollination is ensured, and the young get the benefits of variation.

• i.A: stigma. B: anther. C: ovary. D: ovule.
• ii.B is the anther, which makes the pollen. Without its own anthers the flower could not pollinate itself, so self-pollination and fertilization by its own pollen would stop. It could still be fertilized if an insect or the wind brought pollen from another flower (cross-pollination). Growers remove anthers on purpose (emasculation) when they want to cross two varieties.

A dry seed is dormant: with almost no water, its cells can’t carry out their chemical reactions. When soaked, it absorbs water through its micropyle and coat; the seed coat softens, enzymes become active, stored food is broken down for energy and growth, and the embryo starts to grow. So water breaks dormancy and starts germination.

Simal seeds are small and light and each is wrapped in fine, silky cotton. The cotton gives a lot of surface for very little weight, so the seed floats in the air and the wind carries it a long way, even several kilometers.

A seed holds an embryo, a new plant made by sexual reproduction, with food and a protective coat. It can stay dormant through bad seasons, travel to new places by dispersal, and germinate into a new plant that grows up and makes seeds of its own. So seeds link one generation to the next.

A seed is a ripened, fertilized ovule, containing an embryo (a baby plant), a food store, and a protective seed coat, that can germinate into a new plant. Examples: rice, maize, gram, bean, mustard, mango seed (the stone), pumpkin seed.

By wind, by water, by animals, by gravity, and by bursting of pods.
Wind: seeds that are small and light, with hairs, wings or air pouches, float in the air and are carried far by the wind. Example: simal, whose seeds float on silky cotton; maple, whose winged fruit spins as it falls. (Draw a seed with a tuft of hairs, or a winged maple fruit, with an arrow for the wind.)

Dormancy is the resting state of a ripe seed, in which it stays alive but does not grow, with its activity almost stopped.
It helps the seed to survive bad conditions (cold winter, dry season) and to wait until there is enough water, air and warmth to germinate and grow; it also gives time for the seed to be dispersed. For people, it means seeds can be stored safely from one season to the next.

Draw the bean in Figure 5.31.

1. The seed absorbs water through its micropyle and swells; the seed coat softens and splits.
2. Stored food in the cotyledons is broken down and used for energy (with oxygen) and growth.
3. The radicle comes out first and grows down, forming the root and root hairs.
4. The hypocotyl grows into a hook and pulls the cotyledons out of the soil (in bean); they turn green.
5. The plumule grows up into the stem and the first true leaves. The young plant is now a seedling and starts making its own food.

Air gives the seed oxygen. The cells of the germinating embryo respire, using oxygen in their mitochondria to break down stored food, and the energy released is used to grow. Without enough oxygen (in waterlogged soil, or under water) a seed can’t germinate, and often rots.

• It produces the next generation, keeping the plant’s kind going.
• Good germination means good crops: more food and income for farmers and the nation.
• Sprouted grains and pulses are easier to digest and more nutritious.
• Germination of the seeds of trees and grasses renews forests and pastures and keeps biodiversity.

• i.To cover the seeds with soil, which keeps them moist and warm, gives them air in the loose soil, and saves them from birds.
• ii.The dry part lacked water. Without enough moisture, the seeds couldn’t absorb water, their dormancy didn’t break, and they did not germinate (or germinated late).
• iii.Irrigate the field evenly (or sow just after rain), soak the seeds for a day before sowing, keep the soil loose and moist but not waterlogged, sow at the right time and depth, and use good seeds.

• i.A: plumule. B: radicle. C: testa (outer seed coat). D: tegmen (inner seed coat). E: hilum. F: cotyledon.
• ii.The plumule grows into the shoot system: the stem and leaves.
• iii.Nearly all do: a seed has one cotyledon in monocots (maize) and two in dicots (bean). (The dust-like seeds of orchids are a rare exception: their tiny embryo has none.) The cotyledons store food for the germinating embryo (in dicots) or pass food to it from the endosperm (in monocots), protect the plumule and radicle, and in some seeds come up as the first green leaves.

When a seed sprouts, its enzymes break stored starch and protein into simpler sugars and amino acids, which are easier to digest. Some vitamins, such as vitamin C, are made, and others increase. Substances that block the absorption of minerals are reduced. And sprouts are soft, cook quickly, and taste fresh. So sprouted gram, moong or kwati are more nourishing and easier on the stomach than the same seeds dry.