Page 9 - Biology_Form_3

Biology for Secondary Schools

autonomic or spontaneous movement. factors like light, moisture and gravity
The other type is paratonic or induced (phototropism and gravitropism). When
movement controlled by external factors a plant seedling is placed horizontally, the
such as light, water, chemicals, touch auxin hormone accumulates more on the
and gravity. Example of self-controlling side of the plant that faces downwards,
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movement in plant is the growth of shoots towards the direction of gravitational force.
and roots. The lower side of the seedling shoot where

Paratonic movement is divided into tropic auxin hormone concentration is high will
movement or tropism and nastic movement. grow faster than the upper part. This will
result into bending of seedling shoot upward

Plant hormones and hence growth against gravity. Auxin
also regulates root formation, flower and
Task 1.2 fruit development, and the differentiation
Search for information from the library of vascular tissues. Likewise, it helps to
and internet sources about the plant maintain apical dominance, where the main
hormones then write short notes. stem grows more vigorously than the side
branches. Auxins play a crucial role in the
Plant hormones or plant growth hormones plant’s response to stress and environmental
also known as phytohormones or plant changes. This makes the plant survive harsh
growth regulators are chemical substances environmental conditions such as drought.
that regulate plant growth. Plant hormones
are signal molecules produced within the Cytokinin hormone
plants and usually occur in low concentration.
They control all cellular activities related Cytokinins are plant hormones that
to growth and development in the plants. primarily promote cell division and growth,
Auxins, abscisic acid, gibberellins and especially in the shoots and roots. They
cytokinin are examples of plant hormones. regulate the development of new tissues
and organs by stimulating cytokinesis (the
Auxin hormone division of cells). Cytokinin also work in

Auxins are also called growth promoters conjunction with auxins to control the
balance between root and shoot growth,
because they influence plant growth and influencing the formation of branches and
assist in producing a response which results leaves. They also delay senescence (aging)
into growth. It promotes cell elongation, of plant tissues by promoting protein
particularly in stems and roots. Normally, the synthesis and inhibiting the breakdown
higher concentration of auxins in the shoots of chlorophyll, thus enhancing the plant’s
promotes the growth of shoots while low overall vitality. Cytokinin also play a role
concentration of auxin hormone in the roots in nutrient distribution, seed germination,
promotes growth of roots. The concentration and the formation of flowers and fruits.
of auxin hormone influences the direction
of growth in response to environmental

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