Plant Growth
Mechanisms by which whole plants grow:
o How do plants grow?
Irreversible quantitative change in cell size & number. Not all cells contribute to growth. Growth is localised in plants – restricted to
embryonic regions. (Mature plants are surrounded by thick walls. Mechanical and vascular cells are non-living).
o Determinate and indeterminate growth:
A plan can grow throughout its life – indeterminate growth. Some plant organs cease to grow at a certain size (maturity – determinate
growth), e.g. flowers, fruit, leaves.
o Phyllotaxy:
Arrangement of leaves on plant stem. Arrangement can be opposite or spiral. Caused by regular changes in distribution of auxin in
meristem – a leaf primordium starts to develop in areas of the meristem where auxin is absent, as this develops it draws auxin from
adjacent areas in the meristem depleting it of auxin > initiation of another new leaf…etc.
o Plants grow from meristems:
Meristems are perpetually embryonic/ undifferentiated tissue and allow for indeterminate growth.
Meristems give rise to: initials (stem cells) which remain in the meristem & derivatives which become specialised in mature tissues.
Apical meristems are located at the tips of roots and shoots and in axillary buds of shoots. They elongate shoots and roots = primary
growth.
Produces almost all of the plant body in herbaceous plants (non-woody plants). In these plants, only the green soft growth (new) is the
result of primary growth.
o Types of primary meristem:
Apical meristems may differentiate into 3 kinds of primary meristem:
Protoderm – lies around the outside of the stem, develops into epidermis.
Procambium – lies just inside of the protoderm, develops into primary xylem and
phloem. Ground meristem – develops into the cortex and pith, produces the cork
cambium (secondary meristem).
o Lateral meristems thickness to woody plants = secondary growth:
Two secondary lateral meristems in plants – vascular cambium & cork cambium.
, Vascular cambium – adds layers of vascular tissue called secondary xylem (wood) and secondary phloem. Cork cambium – replaces
epidermis with periderm, thicker and tougher (secondary dermal tissue).
o Overview of primary and secondary growth =>
o Plants still senesce despite having meristems:
Flowering plants can be categorised depending on the length of their life cycle:-
Annuals – life completed in a year or less, e.g. staple food crops like rice and wheat, many ‘weeds’ and meadow flowers.
Biennials – 2 growing seasons required, flower and fruit in the second year, e.g. carrots, turnips, foxglove.
Perennials – long lived, e.g. trees, shrubs and some grasses.
Process of primary growth:
o Primary growth of roots:
The root tip is covered by a root cap – protects the
apical meristem as root pushes through the soil.
Secretes a polysaccharide slime that lubricates the soil.
Growth occurs just behind the root tip in 3 overlapping
zones: zone of cell division, zone of elongation and
zone of differentiation or maturation. =>
o Lateral roots arise from the
pericycle:
The pericycle is a cylinder of
parenchyma or sclerenchyma cells
that lies just inside the endodermis
and is the outer most part of the
stele of plants. Although it is composed of non-vascular parenchyma cells,
it is still considered part of the vascular cylinder because it arises from the procambium as do
the vascular tissues it surrounds.
Stele - In a vascular plant, the stele is the central part of the root or stem containing the tissues
derived from the procambium. These include vascular tissue, in some cases ground tissue
(pith) and a pericycle, which, if present, defines the outermost boundary of the stele.
Mechanisms by which whole plants grow:
o How do plants grow?
Irreversible quantitative change in cell size & number. Not all cells contribute to growth. Growth is localised in plants – restricted to
embryonic regions. (Mature plants are surrounded by thick walls. Mechanical and vascular cells are non-living).
o Determinate and indeterminate growth:
A plan can grow throughout its life – indeterminate growth. Some plant organs cease to grow at a certain size (maturity – determinate
growth), e.g. flowers, fruit, leaves.
o Phyllotaxy:
Arrangement of leaves on plant stem. Arrangement can be opposite or spiral. Caused by regular changes in distribution of auxin in
meristem – a leaf primordium starts to develop in areas of the meristem where auxin is absent, as this develops it draws auxin from
adjacent areas in the meristem depleting it of auxin > initiation of another new leaf…etc.
o Plants grow from meristems:
Meristems are perpetually embryonic/ undifferentiated tissue and allow for indeterminate growth.
Meristems give rise to: initials (stem cells) which remain in the meristem & derivatives which become specialised in mature tissues.
Apical meristems are located at the tips of roots and shoots and in axillary buds of shoots. They elongate shoots and roots = primary
growth.
Produces almost all of the plant body in herbaceous plants (non-woody plants). In these plants, only the green soft growth (new) is the
result of primary growth.
o Types of primary meristem:
Apical meristems may differentiate into 3 kinds of primary meristem:
Protoderm – lies around the outside of the stem, develops into epidermis.
Procambium – lies just inside of the protoderm, develops into primary xylem and
phloem. Ground meristem – develops into the cortex and pith, produces the cork
cambium (secondary meristem).
o Lateral meristems thickness to woody plants = secondary growth:
Two secondary lateral meristems in plants – vascular cambium & cork cambium.
, Vascular cambium – adds layers of vascular tissue called secondary xylem (wood) and secondary phloem. Cork cambium – replaces
epidermis with periderm, thicker and tougher (secondary dermal tissue).
o Overview of primary and secondary growth =>
o Plants still senesce despite having meristems:
Flowering plants can be categorised depending on the length of their life cycle:-
Annuals – life completed in a year or less, e.g. staple food crops like rice and wheat, many ‘weeds’ and meadow flowers.
Biennials – 2 growing seasons required, flower and fruit in the second year, e.g. carrots, turnips, foxglove.
Perennials – long lived, e.g. trees, shrubs and some grasses.
Process of primary growth:
o Primary growth of roots:
The root tip is covered by a root cap – protects the
apical meristem as root pushes through the soil.
Secretes a polysaccharide slime that lubricates the soil.
Growth occurs just behind the root tip in 3 overlapping
zones: zone of cell division, zone of elongation and
zone of differentiation or maturation. =>
o Lateral roots arise from the
pericycle:
The pericycle is a cylinder of
parenchyma or sclerenchyma cells
that lies just inside the endodermis
and is the outer most part of the
stele of plants. Although it is composed of non-vascular parenchyma cells,
it is still considered part of the vascular cylinder because it arises from the procambium as do
the vascular tissues it surrounds.
Stele - In a vascular plant, the stele is the central part of the root or stem containing the tissues
derived from the procambium. These include vascular tissue, in some cases ground tissue
(pith) and a pericycle, which, if present, defines the outermost boundary of the stele.
