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Introduction to Growth, Types of Growth, Phases of Plant Growth, Parameters of Plant Growth, Practice Problems and FAQs

Introduction to Growth, Types of Growth, Phases of Plant Growth, Parameters of Plant Growth, Practice Problems and FAQs

You are growing and you know that it is a characteristic of living beings. Now look around and analyse how many things around you have the ability to grow and develop? Yes, you are correct, all plants and animals. But do you think the chair in your classroom will be able to grow? No! Then what about a pen or table? No! 


                                                       Fig: Growth

How did you start your life? Yes, you started your life as a zygote. This zygote underwent cell division and cell differentiation. This led to the formation of well differentiated organs and finally you were formed. 



 Fig: Growth in plants

In the same way, life of a plant like an angiosperm also starts from a zygote which is present inside a seed. This zygote later develops into a plant with well differentiated organs like roots, shoots, stems, leaves, buds and flowers. This whole process is termed as development which is divided into two phases in plants such as growth and differentiation. Let’s take a deep dive into the details of growth and development in plants in detail in this article. 

List of contents:

Growth

It is defined as the irreversible increase in size of a cell or organ of an individual. Every living organism exhibits growth in their life and hence, it is one of the characteristic features of living beings. Examples include growth of zygote into a plant.

Growth is often referred to as an increase in size or weight of a cell, organ or organism. But increase in size or weight can occur without growth also. For example, absorption of water by a flaccid cell. Hence growth is called an irreversible increase in dry weight, volume or mass of a cell, organ or organism. 

Growth in plants

Growth in plants mainly occurs with the help of meristematic cells. 

Meristematic cells

In plants, the meristematic cells which are actively dividing cells are responsible for growth. These cells have thin cellulose cell walls. The shape of the cells may vary. They can be polygonal, oval, round or rectangular. They possess a large nucleus and a few small vacuoles in the cytoplasm. The new cells formed from the meristematic cells by division may or may not retain the capacity to divide based on the location. The cells which have lost the capacity to divide and enter into the G0 phase of a cell cycle make up the plant body. 



                     Fig: Meristematic tissue

Types of meristematic cells

Meristematic tissue is of different types based on the location as follows: 

Apical meristem

It is responsible for primary growth in plants. They mainly help in elongation of the plant parallel to its axis. It is found in the tips of the roots (root apical meristem) and shoots (shoot apical meristem) of the plants.

1

Intercalary meristem

It is present at the internodes, base of leaves etc. It promotes the growth of the plants by elongation at the nodes and internodes present at the stems and leaves. This also helps in primary growth of the plants. In grasses, this meristematic tissue helps in regeneration of the tissues removed by the grazing herbivores like goats, cows etc. 



              Fig: Intercalary meristem

Lateral meristem

They are present on the lateral side of the root and stem of a plant. It results in secondary growth in plants. Examples include vascular cambium and cork cambium. They increase the girth in plants. 

1

Germination

It is the first step in plant growth. The process of development of an embryo embedded in the seed into a plant is called germination. Seeds consist of reservoirs of nutrients called cotyledons or seed leaves which develop into embryonic leaves. During germination hypocotyl remains in the soil along with the cotyledon. But the epicotyl region emerges out of the soil, irrespective of the type of germination.



GIF: Germination of a seed

Types of germination

Based on the emergence of cotyledon, germination can be of two types as follows: 

Epigeal germination

In this type of germination, the whole cotyledon emerges out of the soil and grows into a plant. The region below the cotyledon is called hypocotyl whereas the region above the cotyledon is called epicotyl. In this type of germination, both epicotyl and hypocotyl emerge out of the soil. The upper region of the hypocotyl is bent or curved here. 



                                          Fig: Epigeal germination

Hypogeal germination

In this type of germination, the whole cotyledon stays beneath the soil and grows into a plant. That means this type of germination is characterised by the retaining of the cotyledons along with the hypocotyl underground or in the soil. The epicotyl region emerges out of the soil here. Pea, mango, and rice are examples of hypogeal germination.



                              Fig: Hypogeal germination

Viviparous germination

In this type of germination, the seed can only germinate if it is attached to the parent plant body. For example, the germination in mangrove trees.



                                    Fig: Mangrove tree

Types of growth

On the basis of meristematic activity, growth can be of two types as follows:

  •  Intermediate growth
  • Determinate growth.

Indeterminate growth

The type of growth in which the meristematic cells continue to divide throughout life is known as indeterminate growth. It is seen in cork cambium, intercalary meristem, root apical meristem, vascular cambium, and shoot apical meristem. It is also called the open type of growth.



          GIF: Growth in shoot apical meristem

Determinate growth

The type of growth in which the meristematic cells stop dividing at a certain point in life is known as determinate growth. It is seen in fruits and leaves of plants.



                              Fig: Fruits (Determinate growth)

Phases of plant growth

Growth of plants occurs in three phases as follows:

  • Formative phase
  • Elongation phase
  • Maturation phase

Formative phase

Growth of plants that occurs mainly due to mitotic cell division of meristematic cells is known as the formative phase. It is also known as the meristematic phase. Dense cytoplasm and thin cell wall of meristematic cells help in the rapid increase in the number of meristematic cells. Main sites for the formative phase of growth are root tips and shoot tips. Rapid division of cells consumes a lot of energy and hence results in a high respiration rate during the formative growth phase. For example, the apical meristems of maize divide at the rate of 17,500 cells per hour.



                                                Fig: Plant cells in formative phase

Elongation phase

The cells which are not capable of dividing further undergo elongation or enlargement. This phase is also known as the enlargement phase. The size of the cell increases due to increase in size of the vacuole. Rate of respiration is comparatively lower in this phase than in the formative phase. Deposition of the new cell wall occurs in the cells. Cell enlargement can occur in all directions. Maximum elongation is observed in conducting tissues and fibres. For example, the cells of watermelon enlarges up to 3,50,000 times during the elongation phase.



                                      Fig: Plant cells in elongation phase

Maturation phase

During this phase, cells lose the ability to divide and attain a particular function. Cell wall of plant cells thickens in this phase. Suberin and cutin are deposited in the cork cells. Sieve tube cells lose their nucleus during this phase of growth.



   Fig: Plant cells in maturation phase

Parameters of plant growth

The following parameters are taken into consideration when the growth is measured.

  • Increase in surface area
  • Increase in volume
  • Increase in diameter
  • Increase in weight 
  • Increase in number of cells

Increase in surface area

It is used to measure growth in flat organs in plants like leaves. The leaf of a plant is placed on the graph at regular time intervals in order to measure growth.



                Fig: Measurement of increase in surface area

Increase in volume

It is used to measure growth in fruits normally. Here the fruit of a plant is dipped in water and an increase in the level of water is taken into consideration. This indicates the growth in volume.



            Fig: Measurement of increase in volume

Increase in diameter

It is used to measure growth in cylindrical or globular organs. For example, the diameter of a plant organ can be measured using a vernier calliper or measuring tape.



                                       Fig: Measuring growth in diameter

Increase in weight

Weight of the plant can also be used as a measure to determine growth in plants. This weight measurement can be of two types as follows:

Fresh weight

Weight of a plant including the water content is known as the fresh weight.

Dry weight

Weight of a plant excluding the water content is known as the dry weight. Water content of a plant tissue is removed by drying it up in an oven at 110oC for several hours.



            Fig: Plant material with water content and without water content

Increase in number of cells

As the number of cells increases by mitotic division, it results in growth of plants. It is mainly observed in algae or plant culture in laboratories.

Types of growth on the basis of increase in cells

Growth can be of two types based on the increase in number of cells as follows: 

Arithmetic growth

In this type of growth, a cell divides into two cells which have the following fate: One cell stays meristematic and divides further, whereas the other cell attains maturity and stops dividing. It happens as shown in the below diagram, here the cells increase from 1 to 2, 3, 4, 5 and so on. Increase in the number of cells here occurs in arithmetic progression and the rate of growth remains constant.



                                 Fig: Arithmetic growth

Graphical representation

If plotted on a graph, arithmetic growth can be represented as a straight line where the X-axis represents the number of cells and Y-axis represents the time.



                     Fig: Graph of arithmetic growth

Mathematical expression

Equation for a straight line can be represented in the following way:

y = mx + c

Similarly, the slope of the graph plotted for arithmetic growth is represented by the following equation:



        Fig: Slope of linear graph of arithmetic growth

Gt = rt + G0

Where,

Gt = Growth at time ‘t’ 

G0 = Growth at time ‘zero’ 

r = Growth rate or change in parameter or Change in time

On plotting the length of the organ at different times, a linear curve is obtained. 

Lt = rt + L0

Where,

Lt = Length at time ‘t’ 

L0 = Length at time ‘zero’ or at the beginning

r = Growth rate or elongation per unit time

Geometric growth

In this type of growth, a cell divides into two meristematic cells. Here the rate of growth is not constant. It is commonly seen in unicellular organisms. Here every cell divides. The newly formed daughter cells grow and divide or repeat the process. But when there is a reduction in nutrient supply, the growth slows down in this type of growth.



                                                      Fig: Geometric growth

Graphical representation

If plotted on a graph, geometric growth can be represented as an exponential growth curve where the X-axis represents the number of cells and Y-axis represents the time.



       Fig: Geometric growth Curve

Mathematical expression

The equation for exponential growth can be represented as:

W1 = W0ert

where,

W1 = Size at time t (weight, height, number etc.)

W0 = Initial size

r = Growth rate

t = Time of growth 

e = Base of natural logarithm

Sigmoid Curve

Exponential growth can only happen in an ideal situation when the nutrients are unlimited, which is not possible in real life. In the real world, nutrients are limited and a sigmoid or S shaped curve is formed. It is a characteristic of every living organism growing in a natural environment. But the sigmoid curve of the individual cells, tissues, organs, or organisms may vary. For example, when one cell is entering into the stationary phase, the other may be entering into the lag phase. 

Phases of sigmoid curve

The following are the different phases of the sigmoid curve:

Lag phase

The growth is very slow in this phase although the resources are available in abundance. Only a few cells or individuals are available for growth in this phase. 



                                                      Fig: Lag phase

Log phase or exponential phase

In this phase, the growth rate is increased as the number of cells or individuals and resources are in abundance. After some time, resources start to decrease due to heavy consumption by the growing cells. By the end of this phase, there is a drastic decrease in resources and cell division is halted.



                                                   Fig: Log phase

Stationary phase

In this phase, growth becomes saturated as the resources are limited by this stage.



                                                Fig: Stationary phase

Arithmetic + geometric growth

The embryo of plant and animal exhibits geometric growth initially and later switches to arithmetic growth. Initially, there is rapid growth in the embryos of plants and animals which decreases later when some cells differentiate and lose the ability to divide.


 
   GIF: Human embryo and plant embryo

Growth rate

The pace at which plant growth occurs is known as growth rate.

Absolute growth rate

It is defined as the growth per unit time. It can be calculated by using the following formula:

AGR = Final parameter - Initial parameter/Time

Relative growth rate

It is defined as the growth per unit time per initial parameter. It can be calculated by using the following formula:

RGR = Final Parameter - Initial parameter/Time x Initial parameter

Comparison of absolute and relative growth rate

The comparison of absolute and relative growth rate can be done by the following experiment. 



                            Fig: Comparison of absolute and relative growth rate

For example, two leaves A and B of different sizes of area are taken into consideration. They show exact absolute increases in the area in a given time. But one of them shows a much higher relative growth rate. Here both A and B have grown by 5 cm2 in one day. But the initial size of A was 5 cm2 while that of the leaf B was 50 cm2. Here their absolute growth is the same, but the relative growth is faster in leaf A. 



                           Fig: Increase in the surface area by leaf A and leaf B

Factors affecting plant growth

The following factors affect the plant growth: 

Water

It contributes to growth by helping in cell enlargement and acts as a medium for enzymatic activities carried out during growth. Deficiency can result in reduction in growth. 

Oxygen 

It acts as a medium for the release of metabolic energy required for growth. It is essential for aerobic respiration. 

Nutrients

It acts as a source of energy for growth and aids the synthesis of protoplasm of new cells. Nutrients rich in nitrogen can increase synthesis of protoplasm. 

Temperature 

Temperature has several roles in growth and development. Certain seeds germinate when the temperature is favourable for germination. Enzymes can function effectively only in a certain range of temperatures. A temperature range between 28°C - 30°C is considered as optimum for proper growth in plants. Higher temperatures like 45°C or above result in excessive transpiration and denaturation of enzymes. Lower temperatures also affect the activity of the enzymes. 

Gravity

It decides the direction of growth of the shoots and roots in plants. 

Light 

Light is one of the crucial requirements for photosynthesis in plants which ultimately results in providing energy to plants for growth and development. It is required for tissue differentiation and synthesis of pigments. Plants growing in the absence of light shows etiolation. It shows symptoms like weak long stems due to the elongation of internodes, much smaller leaves and a pale yellow colour due to chlorosis. It decides the direction of growth of shoots in plants.



                                      Fig: Factors affecting plant growth

Practice Problems

Q 1. Which phase of growth is shown in the following diagram?


a. Log phase
b. Lag phase
c. Stationary phase
d. Decline phase

Answer: The given diagram represents the lag phase. The growth is very slow in this phase although the resources are available in abundance. Only a few cells or individuals are available for growth in this phase. Hence, the correct answer is option b.

Q 2. Match the following phases of growth with its description and choose the correct option.

Phase of growth

Description

A - Formative Phase

  1. Cells lose the ability to divide and attain a particular function

B - Elongation Phase

  1. Cells which are not capable of dividing further undergo elongation or enlargement

C - Maturation phase

  1. Occurs mainly due to mitotic cell division of meristematic cells

 
a. A - 1, B - 2, C - 3
b. A - 2, B - 3, C - 1
c. A - 3, B - 2, C - 1
d. None of the above

Answer:

Phase of growth

Description

A - Formative Phase

3. Occurs mainly due to mitotic cell division of meristematic cells

B - Elongation Phase

2.Cells which are not capable of dividing further undergo elongation or enlargement

C - Maturation phase

  1. Cells lose the ability to divide and attain a particular function

Hence, the correct match is A - 3, B - 2, C - 1. So option c is correct.

Q 3. What is growth? What are different types of growth?

Answer: Growth is defined as the irreversible increase in size of a cell or organ of an individual. On the basis of meristematic activity, growth can be of two types such as intermediate growth and determinate growth.

  • Indeterminate growth: The type of growth in which the meristematic cells continue to divide throughout life is known as indeterminate growth. It is seen in root apical meristem, shoot apical meristem etc.
  • Determinate growth: The type of growth in which the meristematic cells stop dividing at a certain point in life is known as determinate growth. It is seen in fruits and leaves of plants.

 Q 4. Which factors affect plant growth?

Answer: There are several factors that affect plant growth as follows:

  • Water: It contributes to growth by helping in cell enlargement and acts as a medium for enzymatic activities carried out during growth.
  • Oxygen: It acts as a medium for release of metabolic energy required for growth
  • Nutrients: It acts as a source of energy for growth and aids the synthesis of protoplasm of new cells.
  • Temperature: It has several roles in growth and development. Certain seeds germinate when the temperature is favourable for germination. Enzymes can function effectively only in a certain range of temperatures.
  • Light: It is one of the crucial requirements for photosynthesis in plants which ultimately results in providing energy to plants for growth and development.

 Q 5. What are the different phases of plant growth?

Answer: Growth of plants occurs in three phases: formative phase, elongation phase and maturation phase.

Q 6. What is the growth rate?

Answer: The pace at which plant growth occurs is known as growth rate.

Q 7. What is arithmetic growth?

Answer: In this type of growth, a cell divides into two cells which have the following fate. One cell stays meristematic and divides further while the other cell attains maturity and stops dividing. Increase in the number of cells occurs in arithmetic progression. Rate of growth remains constant here.

Q 8. How does the formative phase contribute to the growth of plants?

Answer: Growth of plants that occurs mainly due to mitotic cell division of meristematic cells is known as the formative phase. It is also known as the meristematic phase. Dense cytoplasm and thin cell wall of meristematic cells help in the rapid increase in the number of meristematic cells. Main sites for the formative phase of growth are root tip and shoot tip. Rapid division of cells consumes a lot of energy and hence results in a high respiration rate during the formative growth phase. For example, apical meristems of maize divide at the rate of 17,500 cells per hour.

FAQs

Q 1. Which are considered as the common stages of the plant life cycle?
Answer: The plant life cycle consists of the following four stages. They are the seed, sprout, small plant, and adult plant stages. The seed that is planted in the soil will grow into a small sprout first by utilising water. 

Q 2. Which are the 4 stages of seed germination?

Answer: The seed germination in plants involves the following steps:

  • Imbibition is the first step. This allows the entry of water into the seed.
  • This water then activates enzymes that help in the growth of the plant.
  • The seed then grows a root to access the water present underground.
  • Now the seed grows shoots which grow towards the sun.
  • The shoots now grow leaves and begin photomorphogenesis.



                            Fig: Seed germination

Q 3. Differentiate between sapling and seedling?
Answer: Saplings are the young trees. They have a diameter of 1 to 5 inches. Saplings may be bare root, balled and burlapped, or containerized. Seedlings are also young trees with a diameter of up to 1 inch, but are usually bare root.

Q 4. Write down the main difference between geometric and exponential growth?
Answer: The main difference between exponential and geometric growth is that geometric growth is discrete whereas exponential growth is continuous. In geometric growth, normally a fixed number is multiplied to x. In exponential growth, a fixed number is normally raised e to the x.

Related Topics

Types of growth (Arithmetic and Geometric), Sigmoid curve, Growth rate, Factors affecting plant growth, Practice Problems and FAQs 

Differentiation, dedifferentiation and redifferentiation, Practice Problems and FAQs 

Abscisic acid: Discovery and Physiological effects, Practice Problems and FAQs 

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