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The
alive beings, need to adapt themselves to the environtment
that surrounds them to assure their survival. The animals
are characterized by their capacity to change place when
the environmental conditions demand to. The plants, in spite
of being rooted in the floor, also need to have strategies
that allow them the necessary displacements to survive.
The main ones are the following :
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Tropisms:
They are movements that experience the plants when they
need to adapt to some more favorable environmental conditions.
The movements take place for phenomena of vegetable growth,
with increase of the total mass of the plant, so, contrary
to the movements that take place in the animal Kingdom,
they cannot be reversed and they are completely involuntary.
Among these adaptations we have:
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Phototropisms: Or reactions of the plants
when they are stimulated by the light. The stem has positive
fototropism. On the contrary, the root shows negative fototropism.
A fototropism example is seen when we place a vegetable
in a room next to a window. This, little by little, will
go bending towards the light. These movements take place
because the plants possess some specialized receivers ,
called phototropins that activate the vegetable hormone
auxine. This phenomenon was discovered in 1880 by Charles
Darwin and later developed by Fritz Went. Both put the bases
of the importance of the hormones in the vegetable world
as regulators of most of the processes of the plants.
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Gravitropisms:
They
take place because of the gravitatory force . The root presents
positive gravitropism, that is to say it tends to grow in
the same directions than the force of the gravity, while
the stem presents negative gravitropism, because it looks
for the opposed direction. The gravitropism takes place
for the presence of amyloplasts in specialized cells. These,
by changing position inside the mentioned cells, generate
a descompensación of mass that triggers an unequal
growth that originates the tropisms
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See animation |
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Tigmotropisms
: Reactions of the plants when they are in contact with solid
objects. These movements allow certain plants to be able to
climb when clinging to other plants or surrounding objects
as plants with clambering
stems,plants with tendrils or plants with aerial roots.
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| Hidrotropisms:
Vegetable reactions in front of the stimulus of water. |
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Nastic
momements: They are movements of the plants that respond
to external stimuli, as the contact. These movements, contrary
to the previous ones don't go in the same direction of the
stimulus and they don't mean an increase of the vegetable
mass. We have such an example in the case of Mimosa pudica
which reacts to contact by closing its leaves.
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See film |
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| Circadian
responses : They are changes that experience the plants
triggered by environment and according to some predetermined
biological stimuli. Inside these we would have, for example,
the photoperidic and seasonal movements that are answers from
the vegetables to the variations of the solar light between
the day and the night and between some seasons and others.
These changes are manifested in the different position of
leaves, in the night closing of flowers, the blooming period,
the germination of seeds, etc. They are carried out because
of the photoreceptive proteins , called phytocromes, that
activate female hormones, being the auxins the most important
among them . Light and temperature seem to be the more influential
external stimuli for these changes. |
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