MadSci Network: Agricultural Sciences |
Dear Ms. Donelan's 3rd Grade Class:
Hello, nice to hear from you all! I read through your list of items you
wanted to plant your seeds in. Some of them would probably count as having
'no nutrients', namely the rocks and styrofoam. The rest, however,
probably would contain substances which might help plants grow, e.g. clay
is a type of soil, and some plants flourish in it, a fruits and vegetables
may also contain some of the vitamins and hormones which would also helpe
the plant grow.
However, I think some of the media are quite unusual, i.e. the shirt and
crickets. I have no idea what would turn out with them, though, so perhaps
you could try. Other stuff with virtually no nutrients and that could
support a growing plant would be powdered glass. You might want to ask Ms
Donelan about that :).
Let me elaborate a bit on what nutrients plants need. Nutrients are the
'food' of the plant, which help them grow by being the raw material of the
parts of the plant. As you know, plants are made up of cells, which
consist of water, proteins (which have many, many functions in the plant,
e.g. as enzymes and other 'building blocks'), fats, salts, carbohydrates
(like sugar), chlorophyll (the green stuff in leaves which converts carbon
dioxide and water with light into oxygen and sugar), and other stuff like
DNA (which gives
'instructions' on how the plant will grow).
But, for all of these to form, the plant must obtain nutrients (usually
from the soil and atmosphere) like oxygen, carbon, and hydrogen from the
air and water. These four components alone make up 96% of the plant's
mass. Imagine! So much of a plant used to be air and water. Other
important nutrients are nitrogen, which comes from salts called nitrates
in the soil; potassium, calcium, magnesium, phosphorus, and sulphur from
salts also found in the soil; and other so-called 'trace elements' or
'micronutrients' like iron and boron, which are only needed in very small
amounts, but still important to the plant. Nutrients are found in the
environment and have to be taken in by them through their roots or
leaves.
Plants also need chemicals called 'hormones' which act like 'reminders' to
the plant, affecting how it grows. For example, ethene
(C2H4) is produced by most plants, especially the
fruits, which ripen faster when gassed with ethene. It can also cause the
leaves of a plant to fall off (abscission), among other effects. Likewise,
other hormones which are more complicated can cause a plant to grow
faster, to flower, and so on. Hormones are produced by the plant
itself.
Now on to germination. Before a seed can germinate, it may require special
'stimuli' for it to begin to sprout. These stimuli (an event which causes
something else to happen) are different from plant to plant, so one might
not work for the other. If a seed is to grow, it must be 'viable', this
usually means that the embryo inside must be alive. This is why scientists
doing experiments like yours will use many seeds and take the results on
the whole, rather than just relying on one seed alone since it may not be
viable, giving a false result.
Some seeds are 'dormant', that is, they may have a seed coat that is thick
and tough, and need to be scratched or rubbed away before it can take up
nutrients and grow. Nature does this for the seed when it is scratched by
rocks, soil, animals, rain, and other such things. Some fleshy fruits,
like apples and tomatoes, contain chemicals in the fruit (don't worry,
they are safe for humans, at least) which prevent the seed from
germinating while inside the fruit, so it must be outside the fruit before
a new plant can grow. Imagine what would happen if it did not!
Other seeds, especially those living in cold climates, need a wet period
followed by a cold period before they can germinate. Why do you think this
is so? Think about the place such seeds grow in.
Another very important factor to take into account is the temperature.
Most plants will germinate well when the seeds are kept at about 20-30
degrees centigrade. Other substances needed when the seeds germinate are
air and water. These are also 'nutrients', if you use a strict definition,
but of course, you cannot do your experiment without them or the seeds
will surely not germinate!
So, to sum up, there are many things involved, all which must be just
right, before a plant can germinate and grow. This is why gardening is so
fun, and also so difficult :). Radish seeds are quite easy to work with
though. I hope you didn't find my explanation too long or complex, I hope
Ms Donelan won't complain about it :Þ. Well, I hope this would be
informative for you all, and good luck with your project! I'm sure it'll
turn out well if you all are careful.
Keep up your interest in science!
Thiam Hock "Radish" Tan
References:
Stern, Kingsley R., Introductory Plant Biology. Boston, McGraw-
Hill. 8th ed., 2000.
Hessayon, D.G., The Garden Expert. London, Expert Books. 2nd ed.,
1993.
Leow, Atomic, A Guide to Hydroponics. Singapore, S'pore Science
Centre. 2nd ed., 1995.
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