Plant Growth and Development – Class 11

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1. Introduction

Have you ever wondered how structures like roots, stems, leaves, flowers, fruits, and seeds develop in a plant and that too in a specific order? By now, you already know the words like seed, seedling, plantlet, and mature plant. You must have noticed that trees keep growing in height and thickness as time passes. But even though the tree continues to grow, its leaves, flowers, and fruits remain limited in size. Also, they appear and fall from the tree at specific times, and sometimes this happens again and again. Have you ever thought why a plant first grows its leaves and stems (vegetative phase) before it starts to produce flowers (flowering phase)? Every part of the plant is made of different kinds of tissues, and it is important to ask if the structure of a cell, a tissue, or an organ is connected with the function it performs. Is it possible for their structure and function to change? All the cells in a plant originally come from a single cell called the zygote. So, how and why do different cells in the plant look and act differently even though they all come from the same starting cell?

The word development includes two main processes: growth and differentiation. For now, it is important to know that the development of a plant from a zygote (fertilised egg) happens in a very specific and well-organised way. As the plant develops, it forms a complex body that gives rise to roots, leaves, branches, flowers, fruits, and seeds. Eventually, the plant completes its life and dies. The very first step in plant growth is seed germination. A seed germinates when the environment provides the right conditions for it to grow. If these favourable conditions are missing, the seed does not grow and enters a resting stage where its growth stops for some time. When the right conditions return, the seed starts its metabolic activities again and begins to grow.

In this chapter, you will also learn about the various factors that affect and control the plant’s developmental processes. These factors are of two types — some are intrinsic (internal) to the plant, and others are extrinsic (external), which means they come from the outside environment.

2. Progressive and Irreversible Increase in Plant Body Dimensions

Growth is considered one of the most important and easily noticeable features of all living organisms. But what exactly is growth? It can be described as a permanent and irreversible increase in the size of an organ, any of its parts, or even a single cell. This means that once something grows, it cannot go back to its original size on its own. Usually, growth happens along with metabolic processes, which include both anabolic (building up) and catabolic (breaking down) reactions. These processes use energy to help in growth. For example, when a leaf expands, it is a form of growth. But now think about something different: if a piece of wood swells when you put it in water, can that be called growth? No, because this swelling is temporary and can go back to the original size, so it does not fit the definition of permanent growth.

2.1 Unlimited and Continuous Expansion in Plants

Plant growth is special because, unlike animals, plants can continue to grow without limits throughout their entire life. This continuous growth ability comes from special areas in the plant body called meristems. The cells in meristems have the unique ability to divide again and again, and also to maintain themselves. But the new cells produced by the meristem eventually stop dividing, and instead, they develop into different parts of the plant body. This type of growth, where new cells are constantly being added by meristem activity, is known as open form of growth. But think for a moment — what if the meristem stops dividing? What would happen to the plant’s growth in that case? Can this kind of stoppage ever happen?
In your earlier classes, you have already learned about the root apical meristem and the shoot apical meristem. These meristems are mainly responsible for the primary growth of plants. They help in making the plant grow longer along its main axis — that is, from root to shoot. You also know that in dicotyledonous plants and gymnosperms, some other meristems appear later in life. These are called lateral meristems, and the main ones are the vascular cambium and cork cambium. These lateral meristems are responsible for increasing the thickness (girth) of plant organs like the stem and root where they are active. This type of thickening of the plant body is called secondary growth.

2.2 Increase in Plant Size Can Be Quantified

At the cellular level, growth mainly happens because of an increase in the amount of protoplasm in the cell. However, it is very hard to measure protoplasm directly, so instead, we measure other things that are usually proportional to the amount of protoplasm. This is why growth is measured using different parameters. Some of these include the increase in fresh weight, dry weight, length, area, volume, and number of cells. You might be surprised to learn that the apical meristem of a single maize root can produce over 17,500 new cells every hour. On the other hand, a cell in watermelon can increase in size by up to 3,50,000 times. In the first example, growth is shown by an increase in cell number, while in the second example, it is shown by an increase in cell size. When a pollen tube grows, its growth is measured by the increase in its length. In contrast, the growth of a dorsiventral leaf is measured by the increase in its surface area.

2.3 Stages in the Process of Growth

The time during which a plant shows growth is usually divided into three phases: the meristematic phase, the elongation phase, and the maturation phase. To understand these stages better, we can look at the root tips. The cells at the root apex and shoot apex keep dividing continuously, and they are part of the meristematic phase. These meristematic cells have a lot of protoplasm, and they contain large, clearly visible nuclei. Their cell walls are thin, made of cellulose, and are primary in nature. These cells also have many plasmodesmatal connections, which help in communication between cells. The cells located just behind (proximal to) the meristematic region enter the elongation phase. In this phase, cells show increased vacuolation, they grow larger, and they deposit new cell walls. Moving further back from the root tip, the cells enter the maturation phase. In this phase, the cells reach their full size, their walls thicken, and their protoplasm changes to take on special functions. Most of the tissues and cell types you have already studied in earlier classes belong to this maturation phase.

2.4 Speed at Which Growth Occurs

The term growth speed refers to how much growth takes place in a specific amount of time. In simple words, it is the increase in size or number over time. This rate of growth can be calculated using mathematical expressions. A plant or any of its parts can produce more cells in different ways. The growth rate can follow two main patterns: arithmetic growth and ge
In arithmetic growth, when a cell divides through mitosis, only one of the daughter cells keeps dividing, while the other one stops dividing and starts to mature and perform its function. A simple example of arithmetic growth is a root that grows at a constant speed. If you make a graph using the length of this root over time, you will get a straight line, also known as a linear curve. The formula for arithmetic growth is written as:
- Lt = L₀ + rt,
  where Lt is the length at time t, L₀ is the initial length, and r is the growth rate or elongation per unit time.
Now let’s understand geometrical growth. In this type of growth, at the start, the plant grows slowly — this slow start is called the lag phase. After that, the growth becomes very fast, which is known as the log phase or exponential phase. In this case, both daughter cells produced by mitosis keep dividing, which results in rapid growth. But if there is limited nutrient supply, the growth will start to slow down again, entering the stationary phase. If you plot this entire pattern of growth over time, the graph will show an S-shaped curve, called a sigmoid curve. This kind of curve is typical of all living plants, their cells, tissues, and organs, especially when they grow in a natural environment.
The formula for exponential growth is:
W₁ = W₀ × eʳᵗ,
where W₁ is the final size (could be weight, height, or cell number), W₀ is the initial size, r is the growth rate, t is the time, and e is the base of natural logarithms. In this formula, r is also called the relative growth rate, and it measures how well a plant can produce new material. This is also called the efficiency index. So, the final size (W₁) of a plant part depends on its initial size (W₀).
There are also two ways to compare growth between living systems. The first is called the absolute growth rate, which means measuring and comparing how much total growth takes place in a specific time. The second is called the relative growth rate, which means measuring how much a plant grows in a certain time, relative to its starting size. For example, imagine two leaves, A and B, which are of different sizes. After some time, they both grow larger and become A₁ and B₁, and even if they show equal increase in area, one of them may have a higher relative growth rate. This is because it had a smaller starting size, so its growth compared to its original size is greater.

2.5 Essential Factors Required for Growth

Can you think about what basic needs are necessary for plant growth? This list would definitely include water, oxygen, and nutrients, as these are very important for growth to happen. Plant cells grow bigger mainly through cell enlargement, and this process needs water. When cells take in water, they become turgid (swollen), and this turgidity helps the plant parts to stretch and grow. So, a plant’s ability to grow and develop is very closely connected to its water status. Water also acts as the medium where enzymes can work, and these enzymatic activities are needed for all growth processes. Oxygen plays another important role — it helps in releasing the metabolic energy that is essential for growth activities. Plants also need nutrients, including both macronutrients and micronutrients. These nutrients are required to make protoplasm (the living part of cells) and to serve as a source of energy.
Apart from this, every plant has a specific optimum temperature range where it grows best. If the temperature goes too high or too low compared to this range, it can harm the plant or even stop its growth. Also, environmental signals like light and gravity can affect different stages of growth in a plant.

3. Cell Specialization, Reversal, and Re-acquisition of Function

The cells that come from the root apical meristem, shoot apical meristem, and cambium later go through changes so that they can carry out specific functions. This process, where the cells become mature and specialized, is called differentiation. During differentiation, a cell goes through many structural changes, both in the cell wall and in the protoplasm. For example, when a plant forms tracheary elements (which help in water transport), the cells lose their protoplasm completely. These cells also develop strong, elastic secondary cell walls made of lignocellulose, which help them carry water over long distances, even when there is very high tension. You can try to connect these anatomical structures with the functions they perform in different parts of the plant.
Plants also show a very interesting feature. Sometimes, even fully mature cells that have already differentiated and lost the ability to divide can start dividing again under certain conditions. This special ability is known as dedifferentiation. A good example of this is when interfascicular cambium or cork cambium forms from already mature parenchyma cells. When this happens, the newly formed meristematic tissues start dividing again, and they produce cells that eventually stop dividing and become mature to perform specific tasks. This process is called redifferentiation. In a woody dicot plant, there are many tissues that are formed through redifferentiation — you can try listing them as examples.
Now think about how a tumour is formed. It is an abnormal growth of cells. In plant tissue culture, when parenchyma cells are made to divide in a controlled laboratory setting, what would you call them? These are also examples of dedifferentiated cells.
we mentioned that plant growth is open, meaning it can continue for a long time (also called indeterminate growth) or can stop after a certain point (determinate growth). Now, we can also say that differentiation is open in plants. This is because the same meristem can produce different types of cells or tissues depending on what part of the plant they become. The final structure and function of a cell or tissue depends a lot on its position in the plant. For example, cells that are located away from the root apical meristem become root cap cells, and those that are pushed to the outer edge of the root become the epidermis. You can also think of other examples where a cell’s position decides what type of cell it becomes in a plant organ — this is a clear sign of open differentiation.

4. Overall Progression in Structure and Function of an Organism

Development refers to all the changes that happen in an organism during its entire life cycle, starting from the germination of the seed until the plant reaches senescence, which means the final stage of its life. It includes all stages and transformations that the plant undergoes from the beginning to the end of its life.

A diagram that shows the sequence of processes involved in the development of a plant cell. This diagram does not just apply to individual cells but can also be understood in relation to the development of tissues and organs in higher plants. These steps include various processes that help a cell change and mature throughout its life.

Plants can grow in different ways depending on their environmental conditions or the stage of their life cycle. They may develop different structures based on these situations. This ability of plants to change their form in response to different conditions is called plasticity. A good example of this is seen in plants like cotton, coriander, and larkspur, where the leaves of young (juvenile) plants have a different shape than those of mature plants. This change in leaf shape over time is known as heterophylly.

In some plants, like buttercup, the shape of the leaves changes depending on whether they are growing in air or in water. This is another type of heterophyllous development, but in this case, the change is not due to age but due to environmental conditions. This example also shows the concept of plasticity, where the same plant can develop different forms of leaves depending on its surroundings.

The developmental process in a plant cell involves a series of steps. It begins with cell division, followed by plasmatic growth, elongation (expansion), differentiation, maturation, and finally death or senescence. This complete cycle turns a meristematic (actively dividing) cell into a mature cell, as shown in Figure 13.8.

So, in plants, growth, differentiation, and development are deeply connected processes. In general, development is seen as the combined result of growth and differentiation. These processes are controlled by both internal (intrinsic) and external (extrinsic) factors. The intrinsic factors include genetic material inside the cells and chemical signals like plant growth regulators that work between cells. On the other hand, the extrinsic factors are from the outside environment, such as light, temperature, water, oxygen, and nutrients.

5. Chemical Substances Controlling Plant Growth and Development

5.1 Distinctive Features or Traits

Plant growth regulators (PGRs) are small and simple molecules that can have very different chemical structures. Some of these include indole compounds like indole-3-acetic acid (IAA), adenine-based molecules like kinetin (N6-furfurylamino purine), carotenoid derivatives like abscisic acid (ABA), terpenes like gibberellic acid (GA3), and even gases like ethylene (C2H4). In scientific writings, these substances are sometimes called plant growth substances, plant hormones, or phytohormones. Even though their names may differ, they all refer to the same category of compounds that regulate various processes in plants.

These plant growth regulators are generally put into two main groups depending on what they do in the plant. The first group is made up of growth-promoting regulators. These help with important processes like cell division, cell enlargement, pattern formation, tropic movements, flowering, fruit development, and seed formation. Because of these roles, they are often called plant growth promoters. The main examples of these are auxins, gibberellins, and cytokinins.

The second group of PGRs is involved in helping plants respond to damage or stress, which could be caused by living organisms (biotic) or non-living factors (abiotic). These regulators also slow down growth and are responsible for actions like dormancy (when growth temporarily stops) and abscission (the falling of leaves, flowers, or fruits). The main example of this type of PGR is abscisic acid (ABA). There is also a gaseous plant hormone called ethylene, which has a unique role. It can act as a growth promoter or inhibitor, but in most cases, it is considered a growth inhibitor.

5.2 Uncovering the Existence of Growth-Controlling Substances in Plants

It is quite interesting that the discovery of all the five main types of plant growth regulators (PGRs) happened by accident. The story began with Charles Darwin and his son Francis Darwin, who made an important observation while studying canary grass. They noticed that the coleoptile (a protective covering over the young shoot) bent towards light when exposed from one side. This movement towards light is called phototropism. After doing several experiments, they found out that the tip of the coleoptile was responsible for sending a signal that made the whole structure bend toward the light. Later, a scientist named F.W. Went was able to isolate a substance from the coleoptile tips of oat seedlings, which was responsible for this bending. This substance was named auxin, the first known plant growth hormone.

Another major discovery came from a strange disease in rice plants called ‘bakanae’ or foolish seedling disease. The rice seedlings affected by this disease grew too tall and weak. In 1926, a scientist named E. Kurosawa found that even when rice seedlings were treated with sterile (germ-free) liquid from the Gibberella fujikuroi fungus, they showed similar abnormal growth. Later, the active compound in the fungus was identified as gibberellic acid, a type of gibberellin, which is now known as a growth-promoting PGR.

Further progress was made by F. Skoog and his team, who studied pieces of tobacco plant stems. They found that a mass of undifferentiated cells, called callus, would only grow and multiply when auxins were present along with certain substances like vascular tissue extracts, yeast extract, coconut milk, or even DNA. In 1955, a scientist named Miller and his colleagues were able to identify and crystallize the active substance responsible for promoting cytokinesis (cell division). They named this new growth substance kinetin, which is a type of cytokinin.

In the mid-1960s, three different groups of scientists independently discovered three separate substances that inhibited plant growth. These were called inhibitor-B, abscission II, and dormin. However, later studies proved that all three were actually the same chemical compound. This single substance was eventually named abscisic acid (ABA), and it became known as a major growth-inhibiting PGR.

The final discovery was made much earlier, in 1910, by H.H. Cousins. He noticed that ripe oranges gave off a gaseous substance that caused unripe bananas nearby to ripen faster. This gas was later identified as ethylene, a gaseous plant growth regulator that influences fruit ripening. Ethylene is now widely known for its role in many plant processes, including senescence and abscission.

Now that we understand how these five major categories of PGRs—auxins, gibberellins, cytokinins, abscisic acid, and ethylene—were discovered, we can go on to learn about their effects on plant physiology in the next section.

5.3 Impact of Growth Substances on Plant Functions

1. Auxins

Auxins are a type of plant growth regulator named from the Greek word ‘auxein’, which means “to grow.” The first auxin was actually isolated from human urine. The term “auxin” usually refers to indole-3-acetic acid (IAA), along with other natural and synthetic compounds that help control plant growth. Auxins are made mostly in the growing tips (apices) of stems and roots and are then transported to the areas where they act. Examples of natural auxins include IAA and indole butyric acid (IBA). Some commonly used synthetic auxins are NAA (naphthalene acetic acid) and 2,4-D (2,4-dichlorophenoxyacetic acid). These compounds are widely used in agriculture and horticulture.

Auxins play many important roles in plants. They help in root formation from stem cuttings, which is a common method for propagating plants. They also help in flowering, like in pineapple plants, and in preventing early fruit or leaf drop. However, they promote the falling (abscission) of older leaves and fruits. In many plants, the main growing shoot (apical bud) stops the side buds (axillary buds) from growing, a phenomenon called apical dominance. If the tip of the shoot is removed (a process called decapitation), the lateral buds start growing. This technique is commonly used in tea gardens and for making hedges.

Auxins can also cause parthenocarpy, which means fruit formation without fertilization, such as in tomatoes. They are also used as herbicides. For example, 2,4-D kills broad-leaved weeds (dicots) but does not harm monocot crops like grass. Gardeners use it to keep lawns weed-free. Auxins also influence xylem formation and help in cell division.

2. Gibberellins

Gibberellins are another group of growth-promoting plant hormones. More than 100 gibberellins have been found in different organisms like fungi and higher plants. These hormones are labeled as GA1, GA2, GA3, and so on. The most well-known type is Gibberellic acid (GA3), which was the first to be discovered and is still the most studied. All gibberellins are acidic in nature. They produce several important effects in plants. For example, they help increase the length of the plant stem, which is used to make grape stalks longer. In apples, they help the fruits grow bigger and better in shape. Gibberellins also delay senescence, which means fruits can stay longer on the tree and have a longer market life.

In the brewing industry, GA3 is used to speed up the malting process, which is essential for making beer. In sugarcane, which stores sugar in its stem, spraying gibberellins makes the stems longer, which can increase the yield by up to 20 tonnes per acre. In young conifer plants, gibberellins can make the plant mature faster, so they start producing seeds earlier. Gibberellins also help in bolting, which is the rapid stem growth before flowering, seen in plants like beet and cabbage.

3. Cytokinins

Cytokinins are a group of plant hormones that mainly help in cell division (cytokinesis). They were first discovered from a substance called kinetin, which was isolated from autoclaved herring sperm DNA. Interestingly, kinetin itself does not occur in plants, but it led scientists to search for natural cytokinins. This search led to the discovery of zeatin, found in corn kernels and coconut milk. Since then, many natural and synthetic cytokinins have been identified. These hormones are made in areas where cell division is active, such as root tips, developing buds, and young fruits.

Cytokinins help in producing new leaves, forming chloroplasts in leaves, promoting the growth of side shoots, and encouraging the growth of adventitious shoots. They also counter the effect of auxins by reducing apical dominance. Another important role of cytokinins is to mobilize nutrients, which helps in delaying the aging (senescence) of leaves.

4. Ethylene

Ethylene is a gaseous plant hormone that is produced in large amounts by aging plant tissues and ripening fruits. It has many effects on plant development. It causes horizontal growth in seedlings, swelling of plant stems, and the creation of an apical hook in dicot seedlings. Ethylene promotes the aging and shedding (senescence and abscission) of plant parts like leaves and flowers. One of its most important roles is in fruit ripening, where it increases the respiration rate in fruits, a phenomenon called respiratory climactic.

Ethylene can break seed and bud dormancy, help in peanut seed germination, and cause potato tubers to sprout. It also causes elongation of stems and leaf stalks in deep-water rice plants, which helps the plant parts stay above water. Ethylene promotes root growth and formation of root hairs, increasing the plant’s ability to absorb water and nutrients. It is also used to trigger flowering and synchronize fruit development in crops like pineapple. In mango, ethylene helps in flower formation.

Ethylene is used extensively in agriculture because it regulates many plant activities. A compound called ethephon is commonly used as a source of ethylene. When sprayed on plants, ethephon gets absorbed and slowly releases ethylene gas. This helps in ripening fruits like tomatoes and apples, and in removing extra flowers and fruits in crops like cotton, cherry, and walnut. It also encourages the development of female flowers in cucumbers, increasing their yield.

5. Abscisic Acid (ABA)

Abscisic acid (ABA) is known for its role in leaf fall (abscission) and dormancy. However, it also affects many other aspects of plant growth and development. ABA works mostly as a plant growth inhibitor and slows down plant metabolism. It prevents seeds from germinating too early and helps plants conserve energy under stress. ABA causes the stomata (tiny pores on leaves) to close, helping the plant survive drought or other harsh conditions, which is why it is often called the “stress hormone”.

ABA is also important in seed development, maturation, and dormancy, especially during dry periods. By keeping seeds dormant, ABA helps them avoid harsh conditions like dryness or extreme temperatures. Often, ABA and gibberellins (GAs) work against each other. While GAs promote growth, ABA slows it down when needed.

6. Combined Action of PGRs

Every stage of a plant’s growth, development, and differentiation is controlled by one or more plant growth regulators (PGRs). Sometimes, they help each other (called synergistic effects), and sometimes they oppose each other (called antagonistic effects). Many important events in a plant’s life, such as seed dormancy, leaf fall (abscission), aging (senescence), and apical dominance, happen due to the interaction of multiple PGRs.

It is important to remember that PGRs are only one part of the internal control system in plants. Along with genes (genomic control) and external environmental factors, PGRs play a key role in controlling how a plant grows. Factors like light and temperature, which are external (extrinsic), also influence plant growth by affecting hormone activity. Some major plant events like flowering, vernalisation, dormancy, germination, and movement of plant parts are regulated by both environmental signals and plant hormones.

6. Brief Overview of Key Concepts

Growth is a very noticeable feature in all living things. It means a permanent increase in things like size, area, length, height, volume, and number of cells. This increase happens because of the rise in protoplasmic material, which is the living part inside cells.
In plants, the regions where growth happens are called meristems. These are special tissues found at the tips of roots and shoots, known as root and shoot apical meristems. Sometimes, a third type of meristem, called the intercalary meristem, also helps in increasing the length of the plant body. In big plants, this kind of growth keeps happening and is called indeterminate growth.
After the cells divide in the apical meristems, the type of growth that happens can be either arithmetic or geometrical. But growth does not continue at the same fast rate all the time. It usually slows down as time passes. In general, there are three main phases of growth – the lag phase (slow start), the log phase (rapid growth), and the senescent phase (decline in growth).
When a cell stops dividing, it starts to change its structure to do a specific function. This change is called differentiation. During differentiation, a cell develops special parts that help it do its job. The same rule applies to the development of cells, tissues, and organs.
Sometimes, a differentiated cell can go back to its earlier state; this is called dedifferentiation. Later, it can again become a specialized cell, which is known as redifferentiation. In plants, this process is open, which means that cells keep having the ability to change based on need. That is why, in plants, development (which includes both growth and differentiation) is flexible.
This flexibility in development is called plasticity. It means plants can change their growth and development depending on the situation around them.
Plant growth and development are controlled by both internal and external factors. The internal factors are mainly chemicals inside the plant known as plant growth regulators (PGRs). These are of five main types – auxins, gibberellins, cytokinins, abscisic acid, and ethylene.
These PGRs are made in different parts of the plant. They control many processes like differentiation and development. A single PGR can have many physiological effects (body functions) on a plant. Also, different PGRs can sometimes do similar things. They can work together (synergistic effect) or oppose each other (antagonistic effect).
Apart from internal factors, external factors also affect plant growth and development. These include things like light, temperature, nutrients, oxygen levels, and gravity.

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Plant Growth and Development – Class 11 | NEET Notes