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Chapter 27
Plant Growth
Growth: Is an irreversible increase in the dry mass of an organism and is accompanied by
an increase in cell number.
Apical meristems
Growth and development in plants are brought about by active cell division and
differentiation in certain areas of the plant. These regions are called meristems. In a
growing plant, the principal meristems are located at the tips of the stems and roots where
they form the apical meristems or growing points.
The cells at the tip of a growing stem or root are in a state of active mitotic division. Further
back, the cells develop vacuoles, expand and later become differentiated to perform specific
functions.
During the growth of a shoot or root, a new cell produced in a meristem by mitosis becomes
elongated, vacuolated and finally differentiated. Differentiation is the process by which the
unspecialised cell becomes altered and adapted to perform a special function as part of a
permanent tissue.
Study the above diagram and make sure you understand what is happening. Be prepared to
explain the stages in the formation of both xylem and phloem tissue.
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Experiment: An investigation into the growth of a young root.
Complete by making reference to the diagram below.
Protection of meristematic tissue (growing points)
In the stem, the apical meristems give rise to leaf primordia on either side. The function of
the leaf primordia is to protect the delicate meristematic tissue.
In the root, the root cap protects the meristematic tissue at the tip of the root.
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Lateral meristems and secondary thickening
Whereas annual plants die after 1 year’s growth, perennial plants continue to grow year
after year. As these plants become larger, more xylem tissue is needed to give extra support
and transport additional water. The development of this extra xylem each year makes the
stem (and root) of a woody perennial (eg a tree) increase in thickness. This thickening is
achieved by a process of secondary growth (secondary thickening). The secondary xylem
is produced by a lateral meristem called cambium.
Refer to diagram sheet (Secondary thickening in a woody stem) whilst reading the
following notes.
In both the stem and the root, the cambium cells (located between the xylem and phloem in
the vascular bundles) divide rapidly to form a ring of cambium tissue separating the xylem
and the phloem. This is achieved by radial division; the xylem is situated on the inside of
the cambium ring, the phloem on the outside.
The cells of the cambium ring now divide tangentially to form secondary xylem tissue on the
inside and secondary phloem on the outside. Much more xylem is formed than phloem, with
the result that the phloem tissue, together with the cambium ring, gradually gets pushed
outwards. Radial division of the cambium cells still occurs in order to keep pace with its ever
increasing circumference.
Annual rings
Using information from p 224 New Torrance explain how annual rings are formed. Refer to
previous diagram for illustration.
How do weather and other conditions during growth affect the thickness of annual rings?
The pattern of lines which shows up when wood is cut
lengthwise (longitudinally) is called the grain of the timber
and is a result of cutting through annual rings lengthwise.
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Secondary thickening in a woody stem and the formation of annual rings
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