The
World
Of Plants
2A Introducing plants
The uses of plants
The range of uses of plants is enormous. Not only do we eat a
lot of plants as food, we also use plants as raw materials and as
a source of medicines.
Food examples– rice, wheat and potatoes
Raw materials examples– cotton, jute and pine
Medicine examples – poppy, foxglove and willow tree
An example of a commercially used plant
Although wood is not the most important building material now,
it still is required in large quantities. Forestry is still and
important industry in Scotland.
Land is ploughed
Seedlings planted in nurseries
Seedlings planted out aged 2-4 years
Fertiliser (if required) delivered by helicopter
Forest first thinned out after 20 years
Forest thinned from then on every 5 years
Forest finally harvested after 40m -50 years
The importance of plant conservation
There is a huge variety of plants in the world and we exploit
many of them. There are probably many species of plants that
we still have not discovered and many that are in danger of
extinction. New scientific techniques also mean that plants
that were previously thought to be of little use can now be
investigated. Therefore it is important that a wide variety of
plants be maintained not only for the present use but also as
potential sources of new foods or medicines in the future.
2B
Growing plants
In order to reproduce flowering plants produce seeds. These
seeds contain nearly everything that is required to start the
growth of a new plant.
The structure of the seed
shoot
food store
root
seed coat
The seed coat protects the seed.
The food store provides food for the plant embryo until it has
its own leaves and can make its own food. The root and shoot
are part of the embryo plant which will develop into a mature
plant. We eat seeds directly like peanuts, beans and sweet corn
or we eat foods that have been made from seeds eg wheat
ground to make flour.
Germination
Seeds that have been in Egyptian pyramids for thousands of
years are still capable of growth. Seeds can be dormant but
when the conditions are right, they can burst into life and
produce a new plant.
Germination is when a seeds takes in water and the plant
embryo starts to develop into a mature plant that can
photosynthesise.
In order for a seed to germinate it must have the right
conditions which are a warm temperature, water and oxygen.
Usually, as seen above, seeds have an optimum temperature at
which they will germinate best. The optimum temperature for
these seeds is 350C.
Structure of the flower
Flowers of different plants are built to the same design
although they may not be exactly alike
Pollen
Pollen grains are made in the anthers of the stamens and
contain the flower’s male gametes or sex cells.
Pollination
Once pollen has been released from the anthers it has to reach
the stigma and then the ovules. Pollination is the transfer of
pollen grains from an anther to a stigma.
Self pollination is when a flowers’ pollen grains land on the
same flower’s stigma or another flower on the same plant.
Cross pollination is when pollen grains from one plant are
transferred to the stigma of another plant.
In order for pollination to be successful pollen grains must land
on a flower of the same species.
Wind pollinated and insect pollinated plants
These two methods of pollination have resulted in major
difference between the two kinds of plants and pollen.
Feature
Wind pollinated
plant
Amount and type of Lots, light so easily
pollen produced
carried by wind
Position and size of Hang outside the
stamens
flower to catch the
wind
Position of stigma
Hang outside the
plants; easier to
catch pollen
Scent
None
Colour of petals
Green
Nectar
none
Insect pollinated
plant
Less, sticky or with
hooks
Inside flower
where insects can
brush against them
Inside flower
where insects can
brush against them
Usually to attract
insects
Bright colour to
attract insects
Encourages insects
to come to flower
Wind pollinated flower
Fertilisation
Once the pollen grain has landed on the stigma, the male sex
cell inside has to get to the female sex cell in the ovule. To do
this it has to grow a pollen tube down through the style to the
ovary.
The pollen grain lands on the stigma and a pollen tube grows all
the way down to the ovary. The male gamete travels down the
pollen tube until it reaches an ovule where it fuses with the
female gamete. After fertilisation, the fertilised ovule
becomes a seed and the wall around it hardens to become the
seed coat. Meanwhile the ovary develops into the fruit.
Seeds can either be fleshy or dry.
Tomato
Walnut
Seed dispersal
The seed or seeds formed after fertilisation must be
dispersed (scattered) as far away as possible from the parent
plant otherwise the young plants would be competing with each
other and the parent plant for water , light and space.
There are three ways that seeds may be dispersed. They are
wind
animal internal
animal external
Asexual Reproduction
Some plants can reproduce new plant without forming seeds. It
involves only one parent and there are no sex cells. It is called
asexual reproduction. During this process the parent plant
produces new cells which eventually separate from the parent
and become new independent plants. Pollination and
fertilisation are not necessary.
Runners
Spider plant
wild strawberry
Some plants reproduce using structures called runners. These
are side shoots which grow out from the parent plant. Buds
form at intervals along these side shoots. These get their food
and water from the parent plant until they grow their own
roots. Eventually they become separated from the parent
plant.
Tubers
Many garden plants die down in the autumn but the following
spring they grow again in the same place. During the summer,
they form underground storage organs which fill up with food
such as starch. This organ remains dormant during the winter
but in spring a new plant will grow out of it. This is also a way
of reproducing because a plant produces many such storage
organs, each of which can form a new plant in winter. The most
commonly known tuber is the potato.
Sexual reproduction versus sexual reproduction
Asexual Advantages
Because plantlets get food and
water from parent they grow
quicker
Plantlets will be growing in an
area which is suitable
No time lost over pollination
and germination
Sexual Advantages
A set of characteristics from
each parent gives lots of
variation
The seeds get scattered over
a wider area
Clones
A clone is a group of organisms which have exactly the same
genetic information. So the potatoes formed from one tuber
will all be a clone of the original potato.
Artificial propagation
Gardeners make use of a plant’s ability to reproduce asexually.
Instead of growing seeds they take a small section of stem,
root or leaf. Under the right conditions these will grow into a
whole plant.
Cuttings
This is the most common method of artificial propagation.
A short piece of stem with leaves is cut with a slanting cut to
allow the maximum amount of water to enter the cutting. Most
of the leaves are stripped to cut down water loss. The cutting
can be put straight into soil or dipped in a rooting powder and
then put into soil.
Grafting
Grafting is a special method of propagating fruit trees and
rose bushes.
A cutting is made of the desired plant and it is fitted to a
healthy wild version of the same plant. The join is bound for
awhile until the graft has healed.
The commercial Advantages of artificial propagation
1.
2.
3.
You can make multiple copies of a plant far faster than
growing from seed.
You can make many copies of popular varieties.
Food plants can come into production quicker.
2C
making Food
Animals get their energy from food. Green plants get their
energy from the sun and make their own food in a process
called photosynthesis.
The word photosynthesis comes from two Greek words, ‘photo’
meaning light and ‘synthesis’ to make so these two words tell us
that green plants use light energy to make their own food.
This can be written as an equation:
Chlorophyll
Carbon dioxide + water
Raw materials
light
oxygen + glucose
waste
Product
useful
product
The useful product glucose has three possible uses:
1. used straight away for energy
2. converted to starch and stored. This starch can be detected
using iodine.
3. converted to cellulose and become part of the cell wall.
Iodine is used to detect the presence of starch in the leaves.
A brown colour means there is no starch. A black colour means
there is starch present.
What will happen if you deprive plants of the necessary
conditions to make their food?
We can set up experiments:
No CO2
Leaves or whole plants without CO2 are missing an important
raw material for photosynthesis so when stained with iodine
the leaves stay brown showing that no starch has been
produced.
No light
Leaves or whole plants kept in the dark are missing the source
of energy to make photosynthesis work, so when stained with
iodine a leaf kept in the dark will stay brown.
No starch has been made
No chlorophyll
Some plants have areas which contain chlorophyll and are green
while other areas of the plant have no chlorophyll and are
white or pale yellow.
When tested with iodine the green areas turn black and the
white areas stay brown, showing that chlorophyll is essential
for photosynthesis.
The transport system of plants
The transport systems of plants do two jobs. One is carry raw
materials of photosynthesis and the other is to carry the
products.
How does water get to the leaves?
One of the jobs of the plant’s transport system is to carry
water and minerals from the roots to le leaves, to be used in
photosynthesis.
Xylem vessels – these are the tubes in the plants that carry
water. They are hollow because there is no living material in
them. The walls are made of lignin which is sometimes in spirals
or ring shapes.
Phloem – these tubes carry food from the leaves where it is
made during photosynthesis to every part of the plant.
Unlike xylem, phloem is a living tissue. It is composed of sieve
tubes, each connected to the next by a perforated wall. They
contain cytoplasm but have no nucleus. Therefore they have to
have another cell alongside them, the companion cell, to control
its function.
Together the xylem and phloem make up a plant’s vascular
tissue. In the stem the xylem and phloem are arranged on
vascular bundles around the outside edge. This helps support
the plant. In the root the xylem and phloem are to be found in
the centre. Being in the centre makes the root more flexible as
it grows through the soil.
Root
stem
How does carbon dioxide get into the leaves?
Leaves take in carbon dioxide from the air and release oxygen
back into the air. Leaves have special structure for this.
Leaf structure
Spongy mesophyll
Air space
Vascular
bundle
Guard cell
Stomatal behaviour
There are tiny pores on a leaf which allow water to escape
from the surface. These pores are called stomata. Gases can
enter or leave through these pores.
The stomata are mostly found on the lower epidermis of the
leaf.
Carbon dioxide gas enters the leaf and oxygen gas leaves
through the stomata.
Elaodea bubbler experiment
When green plants photosynthesise, they produce oxygen. In
aquatic plants, this oxygen is given off as bubbles, that can be
easily seen. The Canadian pondweed, Elodea , has large spaces
in its stem . When the stem is cut and the plant placed in a
strong light, bubbles of oxygen can be seen coming from the
cut end. The number of bubbles produced every minute gives us
an idea of how fast the plant is photosynthesising.
Oxygen gas
A lamp acts as a source of light but unfortunately also provides
heat. To allow the light through but not the heat, a sheet of
glass or a large beaker of water is placed between the light
and the test tube with the Elodea. Sodium bicarbonate is
added to the water to provide carbon dioxide.
Limiting factors
The rate of photosynthesis is limited by the availability of
essential materials or factors. A limiting factor is a factor
which if in short supply will hold up photosynthesis. Light,
carbon dioxide and temperature are the main limiting factors
for photosynthesis.
When experiments are done the limiting factors graph for
light looks like this:
While the graph is increasing the amount of light available is
limiting the rate of photosynthesis. When the graph levels off,
even if you carry on increasing the light, the rate does not
change. This means that some other factor, carbon dioxide or
temperature, is now limiting the rate of photosynthesis.