Site author Richard Steane
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The BioTopics website gives access to interactive resource material, developed to support the
learning and teaching of Biology at a variety of levels.
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What is the difference between growth and development?
Growth is an increase in size of
basic parts of plants (due to production of extra cells), whereas development
is the production of new structures in the progress towards maturity - flowers,
fruits etc.
In what ways are plant hormones similar to animal hormones?
Both :
are "chemicals"
produced in one region
move - by diffusion not blood flow!
to another ("target") region
have major physiological effects.
Which parts of a plant are involved in "growing"?
meristems - cells dividing at tips
of roots and shoots.
If a plant responds by growing in a particular direction,
it is called a > tropism.
Positive tropisms are towards the stimulus, negative
tropisms are away from the stimulus.
A plant's response to light is called > phototropism.
Why do green plants need light?
> as energy for photosynthesis
Specifically, which parts of the green plant need light?
> leaves
If the shoot - the tip of a green plant's stem - is growing toward
light, what happens to these structures?
> leaves spread out sideways
What will be the advantage to the plant?
> maximise exposure gaining more
light for photosynthesis
In what way do the following parts of plants respond to gravity,
and what is the advantage (adaptive significance) to the plant?
roots > grow downwards
> into ground, towards water?
shoots > grow upwards
> towards light
The way that plants respond to the stimuli of light and gravity have been
explained in terms of a chemical called auxin. Auxin functions like a plant
hormone, but it may also be known as a plant growth substance.
Auxin is said to have different effects on stems and roots.
In shoots, what does auxin do?
> causes it to bend up
In shoots, where is auxin made?
> at the tip
When cuttings are taken, the shoot which is selected is removed from the
plant and disconnected from a supply of water and minerals from the roots.
It is therefore important to encourage the development of roots, to aid
the establishment of the "cutting" as an independent plant.
"Hormone rooting powders" contain the substance indoleacetic acid
IAA which has been identified as the same chemical as "auxin" which
causes curvature in roots and shoots. Usually the newly exposed surface of
the cutting is dipped into a powder containing the hormone, mixed with inert
powder, and then it is placed into a pot containing moist compost or soil,
which is gently firmed around the cutting and placed in a cool moist environment.
There are a variety of other chemically similar compounds which may be
used for this purpose.
Although there are a wide variety of chemicals which can be toxic to plants
or which damage their exposed parts, such as leaves and shoots, these are
of limited use due to problems of toxicity to humans and other animals, and
danger due to their oxidising nature. These substances may not affect parts
of plants which are underground, and there is a likelihood that new growth
will sprout again from these parts.
There are also many situations in which more specificity is required: killing
some plants but not others, and leaving soil fit for subsequent growth of
plants which are required.
Hormone weedkillers have a variety of advantages over more traditional
substances:
- They should be more toxic to plants than animals.
- They should be carried to underground plants and kill then too
- Some weedkillers have a greater effect on "broad-leaved weeds" (mainly
dicots) than on grasses and also cereal crops (monocots). For this reason
they are called selective weedkillers. These may be used to control
weeds amongst the grass on lawns, or to reduce competition from weeds in cereal
crops. Examples include compounds with complicated chemical names, popularly
abbreviated to 2.4-D and 2,4,5-T. Other compounds such as Dalapon are more
effective against monocots than dicots, so they could be used in the opposite
situation. In fact these weedkilllers cause sensitive plants to grow too
fast, damaging their cells so that their conducting tissue does not develop
properly, or they use up their food reserves and die.
What is the definition of a weed?
A plant growing in the wrong place
- i.e. not planted by a gardener or grower. In other words, a plant which
is able to compete and grow well in the prevailing conditions - probably in
its own natural habitat!
Why do the directions for "Lawn feed and Weed" say "Use
between April and September. Apply in fair weather conditions when the soil
is moist"?
In order to be effective, weather
conditions must favour fast plant growth. Otherwise there will be much less
chance of obtaining the weedkilling effect, not just a slow effect.
However this apparent economic improvement has been at the expense of
biodiversity in the countryside, as many "weed" species are food sources
and act as shelter for many species of wildlife. Many plants which used to
flower alongside crops and weeds, adding to the visual impact of the countryside,
have been adversely affected by herbicides sprayed onto fields. Similarly,
the survival of animal species, such as birds and butterflies, which add immeasurably
to the public enjoyment of the wider environment, are consequently threatened
more by the lack of food and protection than by the killing power of the
chemicals involved.
Recently, there have been a series of developments aimed at reversing this
trend, including "set-aside" where farmers are paid compensation in order
to leave sections of their farms fallow in order to revert to "natural" conditions
which should support normal fauna and flora. Regulations about maintenance
of hedges and borders around fields have also been relaxed in order to increase
the areas which can act as habitats for British wildlife.
The term pesticide is used to describe a chemical which
kills pests. What term describes the sub-category of this which is used to
kill weeds?
herbicide
What is meant by the terms flora and fauna?
flora covers all the plant life,
and fauna means the animal life, in an area
The ripening of fruits is known to be affected by a number of plant hormones.
Commercially, growers sometimes use chemical sprays to prevent fruit from
falling from trees. The substances used are auxins such as IAA or substances
such as 2,4-D which is also used as a herbicide. Depending on weather conditions,
this normally happens with a proportion of the fruit crop, leading to uncertainty
as to the likely productivity of the crop. The consistency of cropping and
ease of picking can thus be increased.
Normally, a flower which is pollinated develops into a fruit, containing
seeds derived from the fertilised egg cell. However, in some circumstances,
fruit develops without pollination taking place. In seedless varieties of
fruits this parthenocarpic fruit development occurs when other parts
of the fruit produce auxins which would otherwise come from the seeds.
Other chemical substances acting as growth regulators can be used to induce
flowering and alter the number and shape of fruit forming.
The gas ethene (ethylene) has also been shown to have a hormone-like
activity on fruit and is used to control the ripening of stored fruit such
as bananas and apples which are best picked and shipped before maturity.