CHAPTER: 13
Physiology
(Physical Aspect) of Growth & Development
Plant
Growth, Development & Differentiation:
Plant
Growth:
• Plant
Growth may be defined as an increase in size, morphology and weight.
• F.W.
Went (1962) define growth as an increase in cell number, size, and
developmental differentiation.
• Growth
involves an increase in dry mass, duplication of protoplasm, cell
multiplication, and a permanent increase in volume.
• Growth
is a process in which the organism passes in orderly stages of development through
time and the internals necessary to encompass the various critical stages.
• Growth
, development, and differentiation in the plant is the outcome of highly
organized and specialized physiological process which are guided by the growth
substances to bring change.
• Growth
is the permanent and irreversible increase in volume and weight about by cell
division, cell elongation and increase in protoplasm.
• Increase
of organic matter or formation of new protoplasm is basis to the phenomenon of
growth.
Elements
of Growth:
- Growth is outcome of specialized and organized
physiological biochemical reactions.
- Permanent and irreversible increase in volume
and weight.
- Cell division and cell elongation in
meristematic zones.
- It can be measured with the help of auxanometer
.
- Growth zones are limited .(It occurs only in
meristematic tissues)
Development:
• The
morphological changes which accompanied the growth in size is called
development.
• “It
may be defined as the qualitative process as by which undergo progressively and
orderly change in structure and physiology during their entire course of life.”
• It
is a sequential series of changes taking place in the plant.
• It
can be controlled by the factors as genetical hormone and environment.
• Exact
and direct measurement can not be taken.
Differentiation:
• It
is the establishment of localized quantitative differences in biochemical
metabolic processes and structural organization in cells, tissues and organs.
• It
is the structural and functional modification of unspecialized cells to specialized
cells.
• These
differentiation is brought about by growth regulators.
• They
are reversible.
• Do
not lose the nucleic material and have full genetic potentiality (totipotency)
• It
cannot be measured
Ø However,
the growth , development and differentiation are often linked process of
organisms.
Types
of Growth( Importance of Meristem in Growth):
• The
growth may be of two types:
- Limited growth: The organs like leaves ,
flowers, and fruits have limited growth. These do not possess meristem
- Unlimited growth: the organs like stem and
roots have unlimited growth. These possess meristem.
Ø In
unicellular plants there is overall growth and not confined to any specific
region but in multicellular plants growth is restricted to specific regions
having meristematic cells.
Ø On
the basis of their position in the plant body (higher plants) meristem are
divided into three main categories:
- Apical Meristem
- Intercalary Meristem
- Lateral Meristem
- Apical Meristem:
• Found
at root and shoot apex.
• Activity
of these meristems results increase in
length in plants.
- Intercalary Meristem:
• Found
above the nodes.As a result of
activity of these meristems , increase in length takes place.
- Lateral Meristem:
• These
meristem are made up of cells which divide in radial direction only.
• They
form laterally placed new cells towards the centers and periphery.Cork Cambium
and vascular cambium are the examples of lateral meristem. Increase in girth
ofshoots and roots take place.
Phases
of Growth:
• In
meristems, mitosis occurs which result in two cells resembling the parent
cells.
• Some
of these newly formed cell divide again and again forming large number of new
cells while in some others the capacity of mitosis is reduced and they starts
to elongate and ultimately they get differentiated into vrious type of cells
depending upon the function that they have to perform and thus, they become
part of the mature organ
• The
phases of growth are as follows:
- Cell
Division 2. Cell
Elongation 3. Cell
Differentiation
- Cell Division:
• It
is the first stage of growth. A cell becomes metabolically active at the time
of cell division.
• Its
genetic material replicates and the cell divides mitotically into daughter
cells.
- Cell
Elongation:
• It
is the second stage of growth, during this state the daughter cells formed as a
result of mitosis enlarge.
• During
cell enlargement, new cell wall materials are synthesized in large quantities
to cope with the enlargement of the cells.
- Cell
Differentiation:
• Different
tissues are formed as a result of differentiation of newly formed cells.
• The
form of the cells changes and some cells are differentiated into xylem
tracheids and trachea and some other into sieve tubes and companion cells.
Course
of growth/ Growth Curve (Grand Period of Growth):
• Growth
is slow at first (Lag phase), then gain speed (Log Phase) and eventually slows
down (decrease in growth rate).
• The
total time during which this course of growth take place is called as the Grand
Period of Growth.
• In
this system, if the rate of growth of a plant or a part of it is measured and
is plotted against time , a typical S- Curved is obtained. This curve is known
as growth curve.
• The
course of growth can be divided into four parts:
- Lag
Phase
- Log
Phase (Exponential or Linear Phase)
- Decreasing
/ Stationary / Steady Phase
- Death
phase or Senescent Phase
- Lag Phase:
• It
is an initial growth phase. During this phase, little growth occurs. Events
that occurs in this phase include germination and vegetative growth.
• The
increased in size or weight is very slow or negligible.
- Log
Phase (Exponential or Linear Phase):
• It
is usually a short phase, and there is rapid growth.
• The
rapid growth rate continues when the plant develop its photosynthetic
mechanisms.
• Therefore
, this phase is also called exponential phase or Grand Period of Growth.
• In
all cases of growth, the log phase declines and the rate of growth begins to
decrease.
• The
point at which this occurs is known as the inflection point.
• This
phase is called the decreasing phase.
• The
plant activities during this phase include flowering, fruiting, and seed
filling.
- Decreasing
/ Stationary / Steady Phase:
• In
this phase, the rate of growth declines again after the log phase.
• In
this phase, the flow of food to the vegetative organs is finally ceased.
• Plant
activities in this phase include cell maturation.
- Death
phase or Senescent Phase:
• After
a period of maximum growth, there is a period of negative (senescence) before
the death of the plants.
1.
Nutrient supply
• Green
plants require several mineral ions and other essential elements for normal
growth and development. These nutrients come from the soil for manufacturing of
food. Growth ceases when the nutrient supply becomes limiting.
2.
Water
• Water
is a medium for all chemical reactions and physiological processes. Water is
essential for photosynthesis. Early growth which is due to the turgid
conditions of the cells requires plenty of water. Under conditions of water
scarcity, plant growth is severely impaired. On the other hand, excess of water
in the soil may cause water logging, resulting anaerobic conditions in the
roots whose growth is severely affected.
3.
Oxygen
• Oxygen
is necessary for cellular respiration in the plants. Food materials are broken
down in the process of respiration and energy is released in the form of ATP
molecules. This is the utilizable form of energy for the living cells. This
energy is used for various activities of the cell and directly takes part in
the growth processes.
4.
Temperature
• Plants
are influenced by variation of temperature of the soil in which they grow. Most
plant species prefer temperature in the range of 20°C to 40°C.
• Light
influences many physiological and growth processes of plants. Plants which can
grow well under bright, direct sunlight and grow poorly in shady conditions are
called photophilic plants. On the other hand, the plants capable of growing
best under low light conditions arc photophobic plants.
• Light
has great morphological effect on the leaves and stems of the plants. Plants
growing. in darkness are characterzied by long, succulent weak stems. The
leaves become underdeveloped, pale, yellowish, chorophyll deficient.
• This
is called etiolating variations in the intensity, quality and duration of light
variously affect seed germination and flowering in different plants. The effect
of light on reproductive growth is called Photoperiodism.
6.
Growth regulators
• These
are, otherwise, known as hormones. These have pronounced effect on the growth
of the plants.
