PGR Flashcards
what is growth
Growth can be defined
as an irreversible permanent increase in size of an organ or its parts or
even of an individual cell. Generally, growth is accompanied by metabolic
processes (both anabolic and catabolic), that occur at the expense of
energy. Therefore, for example, expansion of a leaf is growth
what is th eunieque feature of plant growht
Plant growth is unique because plants retain the capacity for unlimited
growth throughout their life. This ability of the plants is due to the presence
of meristems at certain locations in their body. The cells of such meristems
have the capacity to divide and self-perpetuate. The product, however,
soon loses the capacity to divide and such cells make up the plant body.
This form of growth wherein new cells are always being added to the
plant body by the activity of the meristem is called the open form of growth
types of plant growth
Primary Growth – Root apical meristem and shoot
apical meristem are responsible for the primary
growth of the plants and principally contribute to
the elongation of the plants along the axis.
Secondary Growth – In Dicotyledonous plants &
gymnosperm the lateral meristem (vascular
cambium & cork cambium) are responsible for
secondary growth & contribute to the increase in
the girth of the organs (roots, stem).
what is growth at cellular level? is it easy to be measured?
Growth, at a cellular level, is principally a
consequence of increase in the amount of
protoplasm. Since increase in protoplasm is
difficult to measure directly, one generally
measures some quantity which is more or less
proportional to it.
what parameters are used to measure growth
Growth is measured by a variety of parameters :-
1.Increase in fresh weight
2.Increase in dry weight
3.Increase in length
4.Increase in area
5.Increase in volume
6.Increase in cell number
One single maize root apical meristem can give rise to more than 17,500 new cells
per hour (growth is expressed as increase in cell number).
*Cells in a watermelon may increase in size by up to 3,50,000 times (growth is
expressed as increase in cell size).
*Growth of a pollen tube is measured in terms of length.
*Increase in surface area denotes the growth in a dorsiventral leaf.
what are the phases of growth
- Meristematic
*Constantly dividing cells (at root apex and shoot apex)
*Cells in this region are rich in protoplasm, possess large conspicuous nuclei.
*Cell walls are primary in nature, thin and cellulosic.with abundant
plasmodesmatal connections. - Elongation
*Cells proximal to meristematic zone represent phase of elongation.
*Increased vacuolation, cell enlargement and new cell wall deposition are
the characteristics of cells in this phase. - Maturation
*More proximal to phase of elongation, lies the portion of axis which is
undergoing the phase of maturation.
*Cells of this zone, attain their maximal size in terms of wall thickening and
protoplasmic modifications.
what is growth rate? what is types ?
*Increased growth per unit time is termed as growth rate.
*Growth rate can be expressed mathematically.
*An organism or a part of organism can produce more cells in a variety of ways.
*Growth rate shows an increase that may be arithmetic or geometrical.
what is arithmetic growth
In arithmetic growth, following mitotic cell
division, only one daughter cell continues to divide
while the other differentiates and matures. The
simplest expression of arithmetic growth is
exemplified by a root elongating at a constant rate.
On plotting the length of the organ against time, a linear
curve is obtained.
Mathematically, it is expressed as :-
Lᵻ = L₀ + rt
Lᵻ = length at time ‘t’
L₀ = length at time ‘zero’
r = growth rate / elongation per unit time.
what is geometric growth
In most systems, the initial growth is slow
(lag phase), and it increases rapidly thereafter – at
an exponential rate (log or exponential phase). Here,
both the progeny cells following mitotic cell division
retain the ability to divide and continue to do so.
However, with limited nutrient supply, the growth
slows down leading to a stationary phase. If we plot
the parameter of growth against time, we get a typical
sigmoid or S-curve (Figure 15.6). A sigmoid curve
is a characteristic of living organism growing in a
natural environment. It is typical for all cells, tissues
and organs of a plant.
W1
= W0
ert
W1
= final size (weight, height, number etc.)
W0
= initial size at the beginning of the period
r = growth rate
t = time of growth
e = base of natural logarithms
how are qualitative growth comparisions made
Quantitative comparisons between the growth of living system can
also be made in two ways : (i) measurement and the comparison of total
growth per unit time is called the absolute growth rate. (ii) The growth of
the given system per unit time expressed on a common basis, e.g., per
unit initial parameter is called the relative growth rate. In Figure 15.7
two leaves, A and B, are drawn that are of different sizes but shows
absolute increase in area in the given time to give leaves, A1 and B1
. However,
one of them shows much higher relative growth rate.
what are the conditions for growth
The plant cells grow in size by cell enlargement which
in turn requires water. Turgidity of cells helps in extension growth. Thus, plant growth and further development is intimately linked to the water status of the plant. Water also provides the medium for enzymatic activities
needed for growth.
Oxygen helps in releasing metabolic energy essential
for growth activities.
Nutrients (macro and micro essential elements) are
required by plants for the synthesis of protoplasm and act as source of energy
In addition, every plant organism has an optimum temperature range
best suited for its growth. Any deviation from this range could be
detrimental to its survival. Environmental signals such as light and gravity
also affect certain phases/stages of growth.
define :
i) differentiation
ii) dedifferentiation
iii)redifferentiation
w/ examples
i)The cells derived from root apical and shoot-apical meristems and
cambium differentiate and mature to perform specific functions. This act
leading to maturation is termed as differentiation. During differentiation,
cells undergo few to major structural changes both in their cell walls and
protoplasm. For example, to form a tracheary element, the cells would
lose their protoplasm. They also develop a very strong, elastic,
lignocellulosic secondary cell walls, to carry water to long distances even
under extreme tension.
ii)Plants show another interesting phenomenon. The living differentiated
cells, that by now have lost the capacity to divide can regain the capacity
of division under certain conditions. This phenomenon is termed as
dedifferentiation. For example, formation of meristems – interfascicular
cambium and cork cambium from fully differentiated parenchyma cells
iii)While doing so, such meristems/tissues are able to divide and produce
cells that once again lose the capacity to divide but mature to perform
specific functions, i.e., get redifferentiated.
even differentiation ins plants is open, as in growth.elucidate
differentiation in plants is open, because cells/tissues arising out of
the same meristem have different structures at maturity. The final
structure at maturity of a cell/tissue is also determined by the location of
the cell within. For example, cells positioned away from root apical
meristems differentiate as root-cap cells, while those pushed to the
periphery mature as epidermis.
what is development
Development includes all changes that an organism goes through during its
life cycle from germination of the seed to senescence.
*It is also applicable to tissues/organs.
*Development is the Sum of two processes :- Growth and Differentiation.
*Development of a mature plant from a zygote (fertilized egg) follow a
precise and highly ordered succession of events.
*During this process roots, leaves, branches, flowers, fruits, and seeds are
produced and eventually they die.
what is plasticity
Plants follow different pathways in response to environment or phases of life to form
different kinds of structures, this ability is called Plasticity.
*Example-
1.Development Heterophylly in Cotton, Coriander and Larkspur. In such plant the
leaves of the juvenile plant, are different in shape from those in mature plant.
2.Environmental Heterophylly in Buttercup (Ranunculus). In this plant leaves are in
different shape produced in air from leaves those produced in water.
