Key Stages of Plant Growth and Their Biological Importance
Growth is an essential, natural and irreversible factor. As living beings, every plant grows but unlike humans, plants have unique characteristics of growth. They grow throughout their life. Plant growth takes place in a systematic manner. This systematic mannerism of growth is defined as development in plants.
Plant growth is an irreversible and permanent increase in the size of the plant parts, organs, tissue, entire plant or an individual cell. Growth is the most fundamental characteristic of living plants, similar to any other living organism. Plant growth is also accompanied by metabolic processes such as catabolism and anabolism that require spending energy.
Growth in plants is an open form of growth i.e. it is indeterminate and some cells are continuously dividing and helping in the increase in the size of cell or tissue. Cells that continuously divide are called meristematic cells.
Plant growth is measurable since an increase in protoplasmic content takes place. Also, an increase in fresh or dry weight takes place, an increase in length & size and an increase in the number of cells occur which can be all measured.
Three Phases of Growth in Plants
The period of growth in plants are divided into three phases, namely-
Meristematic or Formative Phase-
The tip region of the root of a plant consists of meristematic cells which are continuously dividing and are isodiametric (with no spaces between the cells), and that zone is called the meristematic zone; here, cells in the meristematic zone have rich protoplasm with the large and conspicuous nucleus. Cell walls of these cells are primary in nature, thin, and consist of cellulose. These cells have white plasmodesmata connection.
The cells in this region of the plant keep dividing vigorously. The meristematic region of the plant generally comprises a large nucleus. The cells present to stimulate the primary phase of growth in plants. These are present in the roots and shoots of every plant.
Elongation Phase-
The cells present next to meristematic cells are present in the elongation phase. These cells elongate due to enlargement of the vacuole present in the cell and hence that zone is called the zone of elongation. The cell enlargement and new cell wall formation take place in the cells present in the elongation phase. Modifications like larger vacuoles take place in the cells here.
Maturation Phase-
The next cells found after the elongation zone are present in the maturation phase and they attain the maximum size. They do not divide further after reaching the maturation phase. The zone of maturation is found as a mass differentiation phase where cells differentiate to perform special functions in the plants.
Knowing these three phases of cell growth, we can say that cell division, cell enlargement and cell differentiation are the different processes that occur in these phases. The first two phases lead to an increase in the plant size whereas the third phase results in bringing maturity to the cells. During differentiation, the cells undergo structural changes in the protoplasm and the cell wall.
Arithmetic and Geometric Growth Pattern
Plant Growth Rate- It is defined as growth per unit time. The two types of growth in plants are-
Arithmetic Growth in Plants:
When plant cells undergo mitotic division, two cells are formed from one cell. Out of these two cells, one continues dividing and the other gets differentiated forming different structures. The former cell again divides and forms two cells again and then one continues dividing and there is one more that is differentiated. Now, we have two differentiated cells from one cell initially. Similarly, a pattern is formed and a gradual increase in size of the plant takes place.
We can represent this phase in the form of a linear graph with respect to height of the plant vs. time.
To calculate Arithmetic Growth curve results, the Formula is as Follows:
Lt = L0 + rt
Where, L= length of the plant
Lt = length at time ‘t’ or after time ‘t’
L0 = Length at initial time
r= rate of growth
t= time interval when we are calculating the plant growth rate
Geometric Growth in Plants
In this case, the cell divides by mitotic division and two cells are formed. Both the cells again divide to form two more cells for each. Thus, now the number of cells gets doubled at each stage. The number of cells will be like 2, 4, 8, 16, 32, 64, ...and so on. Unlike arithmetic growth stages, here no differentiation of cells take place. The cell growth rate is very fast here in the geometric phase and when the growth comes at its peak, it later starts becoming slow and later becomes steady due to some limiting factors. The geometric plant's growth can be represented in the form of the following S-shaped graph having a Sigmoid Growth curve in plants.
Lag Phase- It is the slower phase where cell division is slow.
Log Phase- Here, the growth is very fast and known as the exponential phase too. A steep curve is formed.
Steady or Stationary Phase- Here, the growth is steady or stationary and becomes constant.
Formula that Represents this Sigmoid Growth Curve is as Follows:
Wt= W0 * ert
W0 = size at the initial time
Wt= size at time t or after time t
r= growth rate
t= time period
e= base of natural logarithm
Growth is a natural process and unlike human beings plants have different forms of growth. The process which helps the growth to the fullest is the particular process known as the process which plants perform in the presence of sunlight, water and organic compounds in order to produce food in the form of ATP. This in turn helps the plant to grow and provides energy to each and every cell of its structure.
Characteristics of plants growth
Some of the characteristics of plant growth are mentioned below:
Plant Growth is Indeterminate: Plants have meristems which are present in roots and shoots both. Meristems provide a proper response for primary growth. Due to the presence of meristems only, the plants grow indefinitely throughout their life. Meristems have the capability to divide and self propagate. This is because plant growth is said to be indeterminate.
Plant Growth is Measurable: Growth simply indicates a regular increment in protoplasm. Growth in plants is measured by the increment of cell number, area, volume, length, etc. Usually, the growth in plants is hard to measure, this is why the increment in the quantity of plant growth is measured accordingly using protoplasm increment. The increment of plant growth and protoplasm shares a proportionality.
FAQs on Phases of Growth in Plants Explained
1. What are the three main phases of growth in plants as per the NCERT syllabus?
Plant growth is primarily characterised by three distinct phases that occur in specific regions of the plant body, such as the root and shoot apices. These phases are:
Meristematic Phase: This is the phase of cell division, where new cells are continuously added. The cells in this region are small, have dense protoplasm, and a large nucleus.
Elongation Phase: Following cell division, the newly formed cells increase in size. This phase is marked by vacuolation, new cell wall deposition, and an increase in cell dimensions.
Maturation Phase: In this final phase, the elongated cells differentiate to attain their final size, shape, and function, forming specialised tissues and organs.
2. How do the cellular characteristics change during the meristematic, elongation, and maturation phases of growth?
The cellular characteristics undergo significant changes across the three growth phases. In the meristematic phase, cells are isodiametric with thin cellulosic walls, dense protoplasm, and prominent nuclei. As they enter the elongation phase, the cells enlarge, vacuoles appear and merge to form a large central vacuole, and the cell wall thickens with new material deposition. Finally, in the maturation phase, the cells achieve their maximum size and develop specific structures for their function, such as the thickening of walls in xylem vessels or the formation of root hairs on epidermal cells.
3. What is the importance of the sigmoid (S-shaped) growth curve in understanding plant development?
The sigmoid or S-shaped curve is important because it provides a realistic model of growth for a plant or organ in a natural environment. It illustrates the typical progression of growth over time, which includes:
Lag Phase: An initial period of slow growth as the plant adapts to its environment.
Log (Exponential) Phase: A period of rapid, exponential growth where cell division and enlargement are at their maximum.
Stationary Phase: A final phase where growth slows down and eventually stops as the plant reaches maturity or as limiting factors restrict further growth.
This curve helps in analysing and comparing the growth rates of different plants under various conditions.
4. How does a plant's growth phase differ from its life cycle stage?
It is a common misconception to confuse growth phases with life cycle stages. Growth phases (meristematic, elongation, maturation) describe the cellular processes of increasing in size and complexity at a micro-level. In contrast, life cycle stages (e.g., seed germination, vegetative growth, flowering, fruiting) describe the entire macroscopic developmental sequence of a plant from a seed to producing its own seeds. The three growth phases occur continuously during the vegetative life cycle stage.
5. If you observe a root tip, where would you find the zones corresponding to the three phases of growth?
The three phases of growth are spatially separated along a root tip, creating distinct zones. The region of meristematic activity is located at the very apex, protected by the root cap. Just behind this is the region of elongation, where the cells rapidly increase in length, pushing the root tip through the soil. Proximal to the elongation zone is the region of maturation, which is easily identified by the presence of numerous fine root hairs. This is where cells have differentiated into permanent tissues.
6. What are the essential environmental factors required for optimal plant growth?
For optimal growth, plants require a balanced supply of several key environmental factors. The primary ones include:
Water: Essential for cell turgidity, photosynthesis, and nutrient transport.
Light: Provides the energy for photosynthesis, the process that produces the plant's food.
Temperature: Plants have an optimal temperature range for metabolic activities; extremes can slow or halt growth.
Nutrients: Macro and micronutrients obtained from the soil are crucial for building plant tissues and facilitating enzymatic reactions.
Oxygen: Required for cellular respiration to release energy for growth.
7. Why is the growth in plants described as being "open" and "indeterminate"?
Plant growth is unique and is described as open and indeterminate for specific reasons. It is called 'open' because plants continuously add new cells, tissues, and organs throughout their life, thanks to the activity of meristems. This leads to the 'indeterminate' nature of their growth, meaning that unlike animals which have a genetically determined final body size, plants can theoretically grow indefinitely as long as conditions permit. This is why you can find trees that are hundreds of years old and are still growing.
