NEET Biology Notes Plant Growth and Development
Growth can be defined as an irreversible permanent increase in size of an organ or its parts or even of an individual cell. It is accompanied by metabolic processes (both anabolic and catabolic), that occur at the expense of energy. 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.
In plants (dicotyledonous plants and gymnosperms), the lateral meristems, vascular cambium and cork cambium are the meristems that cause the increase in the girth of the organs in which they are active. This is known as secondary growth of the plant. Growth in plants can be measured by the parameters like increase in fresh weight, dry weight, length, area, volume and cell number. The growth of a pollen tube is measured in terms of its length, an increase in surface area.
Phases of Growth
In plants, the period of growth is generally divided into three phases :
- Meristematic Phase or Lag Phase
The constantly dividing cells, both at the root apex and the shoot apex, represent the meristematic phase or phase of cell formation or cell division. The cells in this region are rich in protoplasm and possess large conspicuous nuclei.
- Elongation Phase or Log Phase
The cells of proximal to the meristematic zone represent the phase of elongation. Increased vacuolation, cell enlargement and new cell wall deposition are the characteristics of the cell in this phase.
- Maturation Phase or Senescence Phase
Towards more proximal to the phase of elongation, lies the portion of axis, which undergoes the phase of maturation. Most of the tissues and cell types represent this phase.
Measurement of Growth
At cellular level, growth is the consequence of increase in the amount of protoplasm. As it is difficult to measure the increase in the protoplasm directly, so it is generally measured by measuring some quantity of it that is more or less proportional to it. Hence, growth can be easily measured by a variety of parameters such as dry and fresh weight, length, area, volume or cell number.
Hence, growth can be expressed in terms of increase in the cell number, e.g. single root apical meristem in maize give rise to more than 17,500 new cells per hour. It can be also expressed as an increase in the size of the cell, e.g. cells in watermelon increases is size by about 3,50,000 times per hour. Growth can be also measured in terms of its length, e.g. length of pollen tube and can be also measured in terms of surface area, such as in a dorsiventral leaf.
- Growth Rate
The increased growth per unit time is termed as growth rate. The growth rate shows an increase that may be arithmetic or geometrical.
- Arithmetic Growth
In following mitosis, only one daughter cell continues to divide, while the other differentiates and matures. A linear curve is obtained in this type.
Mathematically, it is expressed as
Lt =L0 + rt
Lt = Length at time t
L0 = Length at time ‘zero’
r = growth rate/elongation per unit time
- Geometrical Growth
The initial growth is slow (lag phase) and it increases rapidly thereafter, i.e. at exponential rate (log phase). Here, both the progeny cells following mitosis retain the ability to divide and continue to do so. However, with limited nutrient supply, the growth slows down leading to stationary phase. A sigmoid curve is obtained in this pattern.
S-curve of growth is typical of most living organisms in their natural environment. It also occurs in cells, tissues and organs of plants.
- Conditions of Growth
The essential requirements of growth in plants are water, oxygen, nutrients, temperature and light.
Plant cells grow in size by cell enlargement, which in turn require water. Turgidity of cells help in extension of growth. Water also provides medium for enzymatic activities needed for growth.
These are required by plants for the synthesis of protoplasm and act as a source of energy.
Every plant also has an optimum temperature range best suited for its growth.
It is essential for aerobic respiration and hence availability of energy for biosynthetic activity.
Environmental signals such as light and gravity also affect certain phases/stages of growth.
Differentiation, Dedifferentiation and Redifferentiation
The three processes that are associated with the specialisation of cells in different organisms including plants are as follows:
- Differentiation It is a permanent localised qualitative change in size, biochemistry, structure and function of cells, tissues or organs,
e.g. in plants palisade parenchyma, tracheid, guard cells, root cap, fibre, trichome are differentiated cells.
- Dedifferentiation It is the process of despecialisation of differentiated cells, so that they regain the capacity to divide and form new cells. In plants, formation of meristems, interfascicular vascular cambium, cork cambium and wound cambium are the examples of dedifferentiation.
- Redifferentiation It is the structural, chemical and physiological specialisation of cells being derived from dedifferentiated meristematic cells. In plants, secondary phloem, secondary xylem, cork cells and secondary cortex are some tissues formed through this process.