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ASEXUAL REPRODUCTION AND ARTIFICIAL PROPAGATION



Asexual reproduction differs from sexual reproduction in that it does not require 2 parents, and that special cells are not required. This also means that the special mechanisms necessary to bring together sex cells, and permit fertilisation, then to support development of the fairly helpless stages, from zygote to independent organism, are not required.

Although we often think of plants when we consider asexual reproduction, it must be remembered that there are many animals which reproduce asexually . The simplest organisms (Amoeba, bacteria, etc) may rely on asexual reproduction alone; but many organisms are able to use both methods, according to the requirements of their life-cycle.

Try to think of some animals that reproduce asexually (they may also use sexual methods):
   >
    Hydra, greenfly, stick insects

Asexual reproduction in plants is also called vegetative reproduction because it simply involves the growth of parts which eventually become detached to form new plants. Since the new parts are produced by ordinary cell division, they are direct copies of the original, with no input from another individual as in sexual reproduction. Consequently, asexual reproduction does not introduce variation, unlike sexual reproduction (flowers and their products: seeds). However, it follows that asexual reproduction may be seen as a means of maintaining continuity.
It is also a fairly quick method of increasing numbers.

Because many of these parts of plants sometimes contain stored food in a concentrated form, they may have another function in the plant's life-cycle: perennation, i.e. survival through the unfavourable months of the year. It also explains why we find it easy to use some plants ("vegetables") for food.

What chemical form will the stored food be in?
   > starch

What is the name of the process of nuclear division which takes place during the growth of these parts which then function independently?
   >
mitosis
List at least 5 types of structure which are used in asexual reproduction.
   > runners      > tubers      > rhizomes

   > corms       > bulbs       > plantlets etc

Which example of vegetative reproduction does not involve storage of food as well?
   > runners

Which examples of vegetative reproduction do not contribute to Man's food supply?
   > iris, gladioli

The same principle can also be used by Man in order to grow more plants, perhaps starting from only one plant, a process called artificial propagation. This is especially useful if you want to produce many copies of a plant which has some desirable characteristics, such as flowers of a different colour, or fruits with a particular taste or quality.

One fairly easy way of doing this is to take cuttings of a plant, by removing part of a plant, which then eventually grows into a whole plant on its own.

Which parts of a plant can be used for cuttings?

   > shoot (tip)      > root      > leaf

Refer to the section on taking tip cuttings, taken from a gardening book.


Taking cuttings
When taking cutting from tips of shoots, it is customary to select stems or sideshoots of the plant which are young but not flowering, then to cut them off, strip off the lower leaves and trim them cleanly just below a leaf node.

Why do gardeners select a "young" shoot?

   > It is vigorous, probably with no virus or other infection

What is the importance of selecting a "non-flowering" shoot?

   > All resources go into growth, they are not diverted into seeds, etc

Why are the lower leaves stripped off?

   > They will eventually be under the soil, and may rot in water

   > This will reduce water loss by transpiration

What is the reason for trimming below a leaf node?

   > This is where roots will (most likely) grow out from.

Cuttings may be placed in water until roots have formed, or directly into a pot of soil, when they are usually covered with a polythene bag and kept out of direct sunlight.

What is the purpose of the polythene bag?

   > To reduce water loss by evaporation/transpiration (also warmer, and protects against pests)

Why is it unnecessary if a cutting is put directly into water?

   > Cut surface give xylem direct access to water

Why not keep them in complete darkness?

   > The plant needs light to photosynthesise, make organic food for growth)


Grafting is a somewhat similar process used by gardeners to propagate plants, especially fruit trees. A piece of one plant (it could be as small as a bud) is joined onto the top of another plant, and it continues to grow and eventually produce flowers and fruit. Sometimes more than one variety of apple, for instance, may be grafted onto one tree.

The part onto which the graft is attached is called the rootstock, and it may have some desirable feature, such as strong roots, or resistance to disease, and some apple trees are specially "dwarfed" in this way to make them easier to harvest! The rootstock is not expected to produce flowers or fruits, so its own buds are removed.

"Suckers" are long thin shoots forming as outgrowths from the root regions of fruit trees, especially if damaged during mowing of grass in orchards.
Why do you think these are looked on as undesirable?

   > They divert the effort of plant growth, taking nutrients etc away from fruit formation   
   > The rootstock may be a different (unsightly/unproductive?) variety

Why do you think damage to the roots causes suckers to form?

   > It is like taking root cuttings! Dormant side buds may be stimulated.

Suppose that you have two varieties of the same species of plant: one with brightly coloured flowers but no scent, and the other with poorly coloured flowers but a good scent.
How would you go about the process of combining the bright colour with the scent in one plant? Give some details of the practicalities of the method. (Not necessarily in this unit)

   > Cross-pollinate variety A with B (and B with A?). Prevent self-pollination, or access by other pollen (remove stamens/ shield flowers with paper bags?). Collect seeds, germinate them and grow on seedlings. Screen the resulting plants. Some will probably have neither of the desired characteristics! THIS IS SEXUAL REPRODUCTION.

When you are successful, how would you make more plants of your new variety, so as to sell for a vast profit?

   > Take cuttings or propagate using some other form of ASEXUAL REPRODUCTION.

List some characteristics that would be useful, in combination with others, in a crop plant, i.e. one which is used in the production of food for Man.
   > Heavy yield   > High in protein   > Good flavour
Others: pest/disease resistance, easy to harvest, withstands weather, e.g. drought resistance/ short so not easily blown over, enhanced vitamin/mineral content etc.

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