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The earliest forms of life that are known to us date
back to about 3.5 billion years ago. These beings are called bacteria and
blue-green algae. Photosynthesis probably first developed in bacteria. The
earliest land plants developed a little more than 400 million years ago. The first
seed plants appeared about 350 million years ago and were referred to as seed
ferns because of their large fernlike leaves.

Plants have different types of growth responses to
the things around them. These responses are called tropisms. There are four
main types of tropisms: phototropism, geotropism, thigmotropism, and
chemotropism.

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Phototropism is the response to light direction,
geotropism is the response to gravity, thigmotropism is the response to water,
and chemotropism is the response to a particular chemical. One of the general
characteristics of plants is that they tend to grow continuously throughout
their lives. There are two aspects of plant growth: primary and secondary.
Primary growth takes place in young organs, which gives the result of the
increase in length of shoots and roots of the plant. Secondary growth refers to
the increase in the plant’s width.

Radiation refers to the transmission of energy in
wave form and to the transmission of streams of atomic particles through space.
Any energy that is emitted through the form of waves is called electromagnetic
radiation. All kinds of electromagnetic radiation follow the same physical
laws, all of them travel as fast as light, and when they fall on a surface they
release a pressure. (Global Industries, 1991)

There are three types of radiation: alpha, beta, and
gamma.                                                             

Alpha particles are helium-4 nuclei (two protons and
two neutrons). They interact with matter strongly because of their charges and
combined mass. Their usual velocities can only penetrate a few millimeters of
low density material. Therefore, this means that alpha particles do not penetrate
the outer layers of dead skin cells and do not damage live tissues. There are a
fewvery high energy alpha particles that are made up about 10% of cosmic rays
and they can penetrate the body and even thin metal plates.

Beta radiation can penetrate more than alpha
radiation can.  Beta radiation from
radioactive decay can be stopped by a few centimeters of plastic or a few
millimeters of metal. Sometimes beta radiation is used in radiotherapy to give
treatment to superficial tumors.

Gamma radiation occurs to get rid of an unstable
nucleus of excess energy. Both alpha and beta particles have an electric charge
and mass, and can probably interact with other atoms along their path. However,
gamma radiation is composed of photons, which do not have either mass or
electric charge, which makes it be able to penetrate much further through
matter than either alpha or beta radiation. Gamma rays can be stopped by a
thick or dense layer of material, and it doesn’t matter whether the material
has high or low density. (Nowicki)

Radiation can have a positive effect on plant growth
at low levels and harmful effects at high levels. Radiation can affect a
plant’s growth and sprouting. The amount of damage is proportional to the
radiation that is received by the plant. Because
of radiation, soil can become compact and can also lose the nutrients that are
needed for a plant’s growth. Intense radiation kills plants but in a different
way. Trees and shrubs change in their reactivity and sensitivity towards
radioactive substances. (Taylor, 2017)

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