Reproduction

Reproduction may be defined as the formation of new organisms. It is important as it is responsible for the continuation of species, preventing extinction.

There are two types:

• Sexual

• Asexual

Asexual reproduction

The production of offspring without the use of gametes.

This means that only one parent is involved. The offspring is instead produced from mitosis instead of fertilisation. Since there are no gametes which promotes genetic variety (no sperm cell and egg cell are the same), all offspring are genetically identical to the parent and each other (clones). The parent organism divides into two or a part of the parent develops into a new organism.

Types of asexual reproduction

1. Binary Fission

This is the main type of asexual reproduction undergone by organisms such as the amoeba. The nucleus divides first, then the cell membrane and cytoplasm follow. This produces two identical daughter cells.

2. Budding

In budding, a small outgrowth from the body of the parent organism occurs. This happens in yeast, jellyfish, sea anemones, and flatworms.

3. Spore Formation When stuff gets “moldy” it has green stuff on it. This green stuff is actually reproductive spores of a fungus. These spores are released from spore bearing structures which go saprophytically (as in on dead and decaying organisms) on the bread. The spores then land on other bread slices and start growing, which spoils the bread even more. The spores come from the air, and they try to find somewhere suitable to land. In this case, that’s the bread.

4. Vegetative Propagation In vegetative propagation, a plant grows, then a portion of the plant falls off or detaches itself from the parent plant and then a new plant grows from the detached part of the plant. The growth of the detached part of the plant into a new and fully functional plant is only possible by mitosis. Since mitosis creates genetically similar cells, the daughter plant produced is genetically identical to the parent plant.

There are two types of vegetative propagation:

1. Natural Vegetative Propagation

2. Artificial Vegetative Propagation

Natural vegetative propagation

In natural vegetative propagation, there is no interference from humans. There are some parts to natural vegetative propagation:

1. Runners/stolons: a runner is essentially a stem that grows from the parent stem and develops into a new plant by sending roots into the soil and sending up new shoots. The stolon/runners are called adventitious roots because they constantly try to explore new areas.

2. Leaves: leaves and adventitious roots (called platelets) grow from notches in the leaf, when they reach a certain size they fall off and form new plants

3. Storage organs: bulbs (e.g. onions and tulips), corms (e.g. garlic, banana), rhizomes (e.g. ginger). Tubers (e.g. stem tubers - irish potato, & root tubers- sweet potato, cassava), taproots (e.g. carrot) These can produce new plants using the food that they store.

Artificial vegetative propagation

1. Cuttings Part of a stem is cut near the root. The end is buried in soil. This cut end produce new roots.

2. Part of one plant (scion) is cut and inserted into a slit on another plant (stock). This joint is then sealed up and growth takes place.

3. Tissue culture: part of a plant (a tissue) is removed, placed in a medium of nutrients. These cells divide by mitosis to form new cells. The cells are stimulated to differentiate and grow into a plant (a clone is developed)

Disadvantages of Asexual Reproduction

1. No variation: so some organisms may not survive as they cannot adapt to a changing environment

2. Poor quality may be passed from one generation to the next

3. Overcrowding might occur resulting in competition Advantages of asexual reproduction

1. No energy and time wasted to find a mate

2. Thrive in a stable environment

3. Many offsprings are produced quickly

4. If the parent has good genes/traits, they are passed on to several future

5. If many resources are available, they can be exploited.

Sexual reproduction

Flowering plants produce flowers for sexual reproduction. A flower consists of an extended tip, the receptacle, which usually bears four whorls of modified sepals (when the flower comes into bloom), stamen and carpel. Most flowers are hermaphrodites meaning they have both:

• female reproductive parts (carpels) consisting of ovules containing gametes

• male reproductive parts (stamen, producing pollen grains containing male gametes. Consists of the filament and an anther) Insects can use pollen to male honey. Diagram of a flower The flower has a stalk, called the pedicel.

It has a nectary at the base of the flower to attract pollinators (birds, bees, etc. that would have the means to transport the pollen) for pollination. In the male reproductive system, the filament holds the anther in the best position so it can deliver pollen to the stigma.

The stigma is sticky to collect said pollen grains. The style holds the stigma (it is similar to the filament, and the fallopian tube) in the right position. There are dotted markings on the flower called honey guides that guide the pollinators to the nectary.

Pollination

Pollination is the process by which pollen moves from the anther of a flower to the stigma of the flower. There are two types of pollination:

• cross-pollination

• Self-pollination

Self pollination is the transfer of pollen from the anther of a plant to the stigma of the same flower or a different flower on the same plant.

Cross-pollination is the transfer of pollen from the anther of one plant to the stigma of a different plant of the same species. Agents of pollination The way flowers become pollinated varies between different types of plants.

The way they get pollinated, i.e. their agent of pollination, determines how the flower looks. • some agents of pollination include: wind, insects, animals (birds) Observing this picture, you’ll notice:

• A has vibrant, red flowers while B has dull green flowers

• The stigma and style of B are feathery

• The filament and anther of A are located inside the flower, while they are located outside the flower in B This is because A is an insect-pollinated flowers (designed to attract insects to carry pollen) while B is a wind-pollinated flower (pollen is carried to a different flower via wind).

Fertilization

Ferilization is the process in which the male gamete (pollen) and the female gamete (ovum) meet to make a diploid zygote.

Steps:

1. The pollen grain lands on the stigma

2. A pollen tube, with two male nuclei (from the pollen) and a tube nucleus is formed

3. This pollen tube extends through the style by secreting digestive enzymes that digest a pathway

4. The pollen tube ends up growing through the ovary wall, through the micropyle of the ovule, and bursts. The two male nuclei are released as the pollen tube disintegrates

5. One of the two male nucleus joins with the ovum inside the ovary, while the other male nucleus forms with the two polar nuclei which are fertilised to form the endosperm (which then forms the cotyledon

After fertilization:

• Each ovule forms a seed

• zygote divides by mitosis forming the embryo, which develops into 3 parts

• The plumule (embryonic shoot)

• The radicle (embryonic root)

• One/two cotyledons (provide food for the zygote)

The difference between endospermic seeds and non-endospermic seeds is that the endosperm remains endospermic seeds, but is absorbed by cotyledons in non-endospermic seeds. Fruits Each ovary wall develops into a fruit.

A fruit contains one or more seeds (depends on the number of ovules in the original ovary). The shape and structure of the fruit are similar to the ovary. The stigma, style, stamens and petals wither and drop off after fertilization so the plant can develop into a fruit. The sepals may or may not do the same (in some cases they don’t, e.g. eggplant).

The purpose of this fruit is to protect the developing seeds that form after fertilization, but they can also be eaten.

The wall of the fruit (known as the pericarp) has 3 layers:

• Exocarp (outer) layer

• Mesocarp (middle) layer

• Endocarp (inner) layer

There are also two types of fruits:

• succulent: the one or more layers of pericarp is fleshy and juicy -> mango, tomato, cucumber

• dry: the pericarp is thin and dry -> pigeon peas

N.B. A fruit has two scars- one where it was attached to the parent plant, and one where the style used to be. The seed has one scar, known as the hilum, where the seed was attached to the fruit.

Seed dispersal

Seeds disperse so there are never too many of the same plant in the same area- eliminating any competition for natural resources (water, minerals, oxygen).

There are 4 main types of seed dispersal

Dispersal By Animals! Succulent fruits are colorful, scented and juicy, attracting animals who eat said fruits. The seeds of these fruits are spit out and deposited in other areas. Other seeds are also ingested and egestef unharmed.

• e.g. orange

Also, some dry fruits have hooks, attaching themselves to animals fur. • castor oil fruit

Dispersal by wind: Some fruits develop wing like characteristics which help them fly in the wind. Dispersal by water: Some exocarps are waterproof, so they are able to rise above and float on water surfaces. They can then be transported. • E.g. coconut

Dispersal by mechanical means: Some fruits split open along specific lines lf weakness and eject their seeds. • e.g. pigeon peas