1 tto
Asexual
reproduction
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1 Introduction
Reproduction is the process by which an organism produces new individuals
(offspring - nakomelingen). Cell division, which we studied in the last
chapter, is the basis of reproduction.
In humans and many other organisms reproduction involves two
individuals, the male and the female. We call this sexual reproduction.
However, some organisms can reproduce on their own without another
individual. We call this asexual reproduction. In this chapter, we shall look
at some of the methods by which organisms reproduce asexually.
2 Splitting in two
Paramecium is a single-celled organism which
lives in ponds and puddles. It reproduces by
splitting in two. We call this binary fission
(fission means 'splitting' and binary means
'two'). The process is just like cell division
described before: first the nucleus divides
into two, and then the cell splits across the
middle as shown in Fig 1. The two new
paramecia then grow, and after a day or so
each of them may split again.
The nucleus is the control centre of the cell
Figure 1. Paramecium reproduces
asexually
by dividing into two
because it holds the chromosomes. In
chromosomes all the characteristics of the
organism are described. When a cell divides it copies the nucleus and its
chromosomes so that the new cell will receive the exact same characteristics. As
a result, binary fission leads to two identical new cells, with the same
characteristics.
bacteria dividing
paramecia dividing
questions:
1. When it is warm and plenty of food is available, Paramecium may
split once every 24 hours. Dividing at this rate, how many paramecia
could be formed from a single paramecium after seven days?
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2. Why is it important that the nucleus is copied and passed on to the new
cells?
3. Explain that binary fission leads to identical offspring.
3 Budding
Yeast reproduces by budding. The cell sends out a small outgrowth which
gets larger and eventually breaks away from the parent cell (Fig 2a).
Meanwhile the nucleus divides into two. One of the two resulting nuclei
stays in the parent cell, and the other one moves
into the bud.
Sometimes the new cell starts budding before it has
broken away from the parent cell, thus giving rise to
a chain of cells as shown in Fig 2 b.
yeast cells budding
bacteria dividing
Figure 2. Yeast cells
question
budding.
4. Compare yeast cells budding and bacteria dividing.
What do you notice about the sizes of the cells during these processes?
4 Spores
A mushroom is also a fungus, but a very different
one from yeast. Mushrooms, like many other
fungi, reproduce asexually by forming millions of
spores. A spore is a tiny round cell enclosed
within a thick protective wall. The wall enables
the spore to withstand unfavourable conditions
Figure 3. The spore forming body
of a mushroom or toadstool.
such as frost (vorst) and drought (droogte).
A spore is formed by cell division and receives the same nucleus and
chromosomes as its parent. All offspring are therefore identical.
The spore is like a tiny speck of dust and is so light that it can float
through the air over a large distance. If it lands in a suitable (gunstige)
place it breaks open and gives rise to a new fungus.
Spores are formed by these three main groups of organisms: fungi (Fig.
3), mosses (mossen) and ferns (varens).
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questions:
5. As well as providing a method of reproduction, spores enable the species
to grow many new individuals over a large area.
Name two characteristic of spore formers that enable them to do this.
6. Some serious diseases of crop plants (landbouwgewassen) such as wheat
(graan) are caused by fungi. Once they get into the crop they are very
difficult to get rid of. Why do you think it is difficult to get the fungi out
of the soil?
5 Tubers
A tuber is a swollen part of the stem of a plant
located under the ground. It is mainly filled with
starch.
In plants, chloroplasts play an important role. In
the chloroplasts, plants make glucose from water,
sunlight and carbon dioxide. The plant needs
glucose as building blocks to grow and it gives the
Figure 4. A potato tuber.
plant energy (respiration).
During the season, the leaves of a potato plant produce more glucose
than the plant needs. The extra glucose is stored as starch in tubers.
Normally, plants grow stems and branches that reach out of the soil.
Potato plants, however, grow many underground braches as well. The ends
of these branches become thick when they are filled with starch. These thick
branch endings are called tubers: the potato’s (Fig. 4). In a potato cell, the
starch is stored in starch grains. You’ll study starch grains in experiment 1.
When the potato plant dies the potato’s stay alive under the ground.
When the winter has passed, a new plant will grow from each potato. In
spring (lente), the potatoes start to form shoots (uitschieters, stengels) and
roots (wortels). The nutrients necessary for growth and energy come from
the starch stored in the potato. Starch is turned into glucose which is respired
to give energy or turned into proteins, fats or carbohydrates which serve
as building blocks.
The starch in the potato is enough to make a new plant with leaves. Once
the leaves are formed, the new plant can make the all glucose it needs for
growth and energy.
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Since all potatoes are formed by cell division (growth), all new potato
plants are identical. Thus, the formation of tubers presents the potato plant
with two advantages: asexual reproduction and a way to survive the winter.
Experiment 1 Examining a potato tuber
a You may have heard people talk about
the 'eyes' of a potato. They are shown in
Fig 5. Can you see "any 'eyes' on your
potato tuber? Each one is a very small bud
which is capable of sprouting into a new
potato plant.
You will also see little black specks dotted
about over the skin of the potato. These
are little holes called lenticels which allow
air to get through the skin so that the
tuber can breathe.
b Cut a potato tuber in two. With a knife,
scrape away a little of the white pulp and
put it on a slide. Add a drop of dilute
iodine solution to the pulp, and cover it
with a cover slip.
c Look at the potato pulp under the microscope. Do you see a lot of dark
blue, egg-shaped objects? These are starch grains.
d Keep another potato in the window of your laboratory. You may find that
eventually leafy shoots start growing out of it (Fig. 6).
questions:
7. By what process is the starch stored in the potato tuber produced?
8. Where in the cell is the starch stored?
9. What will the starch be used for after the winter (two purposes)?
10. Forming spores is sometimes better than making tubers.
Name two advantages of forming spores over forming tubers.
11. What are the two advantages of potatoes to the potato plant?
12. What makes potatoes so suitable (geschikt) as a food for humans?
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6 Bulbs
The most well known bulbs (de bekendste bollen)
are the tulip bulb and the onion.
A bulb is really a kind of bud (een knop) in which
the leaves are filled with glucose. A regular bud is a
collection of leaves wrapped around each other at
the side of a branch waiting to grow into a branch
with leaves (Fig. 7).
Like a tuber a bulb contains a store of food
(glucose, sugar). However, its structure is more
complicated (Fig. 8). The glucose is stored in thick
Figure 7. A regular bud.
leaves (“fleshy leaves”). In the middle of the bulb, the
young plant is already
present (“bud”). At the
bottom of the bulb we
find the stem to which
the leaves are attached.
Each bulb has a
number of lateral bulbs
(Fig. 8), which will be
filled with glucose during
the season. When the
parent plant dies in
autumn, it will have
produced several new
bulbs.
Figure 8. The parts of a bulb.
In spring, each bulb
sprouts into a new
plant. Then, the glucose, present in the fleshy leaves, is used up for growth
and energy and the lateral bulbs will be filled with glucose again.
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Experiment 2 Examining a bulb
a Slice the bulb (daffodil or onion) lengthways (Fig 9).
b Make a schematic drawing of this
lengthways section indicating the papery
covering, the bud, the fleshy leaves, the
roots and the stem (see Fig. 8).
c A bulb is capable of producing new
bulbs. Look at the bulb, and see if you
can find small lateral bulbs attached
Figure 9. How to slice a bulb to see
to the side of the parent bulb.
its internal structure.
questions:
13. When you sliced your bulb in half lengthways, did you notice that it
consists of a series of 'scales' (rokken)? These are actually leaves. In what
ways do they differ from normal leaves? Explain the differences.
14. There are three kinds of leaves in the bulb. The brown ones around it for
protection. What are the functions of the other two kinds?
15. Some plants need to flower in early spring. That is important because if
they don’t flower early, the trees around them will take away all the
sunlight.
Often these plants form bulbs. How does producing bulbs help them to
solve this problem (hoe helpt het maken van bollen hen dit probleem op te
lossen).
16. A bulb will start to grow only if it is first subjected to a period of cold.
a) What is the advantage of sprouting áfter a period of cold?
b) Describe an experiment to find out how low the temperature has to be
before the bulb will produce a shoot.
17. Suggest three uses that humans make of bulbs. In each case give at least
one example.
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7 Cuttings
If a branch of a plant is cut off and stuck into some soil, with compost or
water, roots may grow out of it so that it becomes a new plant. This is
called a cutting.
Gardeners often take cuttings from plants which they particularly like
because the plants formed this way are identical to the parent plant
(because they are formed by cell division ...). However, not all plants will
grow from cuttings.
Experiment 3 Taking cuttings of Peace-in-the-home.
Peace-in-the-home (slaapkamergeluk – Fig.
10) is a plant from which it is particularly
easy to take cuttings. You will need a plastic
cup with some soil and a pair of scissors.
a Cut off a side branch from the Peace-inthe-home plant, cutting cleanly with a
sharp pair of scissors. This is your cutting.
b Put it in the soil. Cover the plastic cup with
sandwich bag.
Figure 10. Peace-in-the-home.
c Observe your cutting every week.
d After a few weeks several roots will have grown out. If the cutting formed
roots itself successfully you wilt have obtained a new Peace-in-the-home
plant. You can take it home then.
questions:
18. Why was it important to put a sandwich bag over the cup with the cutting?
19. Taking cuttings is an artificial way of reproducing plants, used by
gardeners.
In what circumstances might this kind of reproduction occur naturally?
20. Why is it best to take cuttings from branches which do have leaves and do
not have any flowers on them?
21. Some gardeners take cuttings by sticking a branch straight into the soil;
others put the branch in water until new roots start growing out and
then they stick it in the soil. What are the advantages and
disadvantages of each method?
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8 Runners
These are side branches of some
plants that grow along the surface of
the soil. Roots grow down from buds
(knoppen) on the runners. These
develop into new plants. Strawberry
plants reproduce in this way (see
Fig.11).
Figure 11. Strawberry runners.
9 Advantages and disadvantages of asexual reproduction
The most obvious advantage of asexual reproduction is that there is no need
for the organism to have a partner. When asexual reproduction occurs by the
formation of spores it occurs rapidly and enables the species to spread
quickly over a wide area. Spores, tubers and bulbs enable the plant to
survive the winter.
The main disadvantage is that the offspring are always exactly like the
parent. It is therefore impossible to produce new kinds of organisms by this
method. In other words asexual reproduction does not give rise to variety
(variatie – verschil). Variety can only be produced by sexual reproduction as
we shall see in the next chapter.
10 Importance of asexual reproduction to humans
Suppose a gardener has a plant with a particularly good feature. How could
the gardener produce a lot of plants with the same feature? Asexual
reproduction produces offspring which are exactly like the parent, so it would
be sensible to propagate the plant by asexual means, for example by taking
cuttings.
New varieties of useful plants with good features are created by sexual
reproduction and then propagated by asexual methods. This is particularly
important to farmers and market gardeners who produce commercially
important plants such as potatoes on a large scale.
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questions:
22. The fern (varen) is a plant which has two
methods of asexual reproduction: like the
mushroom it can form numerous spores, and
it also possesses an underground rhizome
(wortelstok) from which new plants grows up
each spring (Fig. 12). A rhizome is a long
horizontal root full of starch.
Figure 12. Ferns with a rhizome.
Suggest two advantages which the spores
have over the rhizome as a means of reproduction.
23. Seven methods of asexual reproduction are listed below. In each case,
write down the name of one organism which uses the method.
Binary fission, budding, spore formation, tuber and bulb formation and
runner and rhizome formation.
24. Make a table (6 columns) with in the first column the seven methods of
asexual reproduction. Above each subsequent column write down the
questions below. Finish off the table by putting crosses in the correct
boxes. Label the first column “method of reproduction”.)
a the method enables the organism to survive the winter
b the method produces only two offspring every time reproduction
occurs
c the method involves the formation of an underground structure
containing food
d the method enables the species to spread quickly over a wide area
25. A plant consists of roots, a stem and branches, leaves and flowers.
a) Out of which part are tubers formed?
b) Out of which part are bulbs formed?
c) Out of which part are rhizomes formed?
d) Out of which part are runners formed?
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Glossary
asexual reproduction
binary fission
cell division
nucleus
chromosomes
characteristics
multiply
budding
spores
offspring
identical
tuber
potato plant
branches
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starch grains
proteins
fats
carbohydrates
building blocks
lenticels
Bulbs
cutting
Runners
variety
rhizome
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Lists
Name the ways of asexual reproduction and examples of organisms using it.
Name advantages of asexual reproduction.
Name disadvantages of asexual reproduction.
Name the advantages of spore formation.
Name which parts of the plant are involved in each type of asexual
reproduction.
Name the parts of a bulb and their functions.
Name the advantages of taking cuttings.
What do you need to explain?
Explain how asexual reproduction leads to identical offspring.
Explain what the difference is between binary fission and budding.
Explain the function of the nucleus.
Explain how each way of asexual reproduction works.
Explain the life cycle of a potato plant.
Explain what the importance is of storing starch/glucose in tubers/bulbs.
Explain what variety is.
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