Growth and Its Phases | Biology Class 11 - NEET PDF Download

When we talk about growth, we are talking about a remarkable journey of change. Imagine a tiny seedling emerging from the soil, pushing its way upwards to become a towering tree, spreading its branches and leaves towards the sky. 

Growth 

What is Growth and Development in Plants? 

"Growth" is a fundamental biological process that refers to an increase in size, mass, and complexity of an organism over time by becoming more differentiated.  Growth can occur in various biological life forms including plants, animals, and microorganisms".

Differentiation refers to the process by which unspecialized or undifferentiated plant cells develop into specialized cell types with distinct structures and functions. This is a fundamental aspect of plant development and growth.

"Development" is sum of growth and differentiation. development process leads to the formation of a complex body organization in plants, including structures like roots, leaves, branches, flowers, fruits, and seeds.

Example: Seed germination

• Seed germination is the initial step in the plant growth process.
• Seeds germinate only under favorable environmental conditions that support growth.
• When these favorable conditions are not present, seeds enter a dormant stage.
• Once the favorable conditions return, seeds become metabolically active again, and the process of growth resumes.

Seed Germination

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Mnemonics-Plant-Growth-Development

Characteristics of Growth

(i) Plant Growth Generally is Indeterminate

• Plant growth is characterized by continual expansion throughout its lifespan.
• Specialized regions called meristems exist in specific locations within the plant's structure.
• Meristematic cells within these regions can undergo division and self-renewal.
• As meristematic cells differentiate, they lose their ability to divide and become integral parts of the plant's structure.
• This mode of growth, where new cells are consistently added through meristematic activity, is termed "open growth."
• Meristems primarily contribute to the lengthening of the plant along its central axis.
• In dicotyledonous plants and gymnosperms, additional types of meristems, such as the vascular cambium and cork cambium, become active at a later stage in the plant's life cycle.
• These lateral meristems are responsible for increasing the thickness or girth of the plant's organs, known as "secondary growth."

Locations of Root, Shoot and Vascular Cambium

(ii) Growth is Measurable

• Cellular growth in plants is primarily a result of an increase in protoplasm.
• Directly measuring protoplasmic increase is challenging, so scientists use various parameters to assess growth.
• Parameters used to measure growth include increase in fresh weight, dry weight, length, area, volume, and cell number.
• Different plant structures may require different parameters for measurement.
  (a) Length is often used for structures like roots and pollen tubes.
  (b) Area is relevant for dorsiventral (flattened) leaves.
  (c) Volume measurement is less common but applicable for certain structures.
  (d) Cell number can be used to assess growth in terms of cell division.
• The choice of measurement parameter depends on the specific plant structure and research objectives.
• Growth can be expressed differently; for example, some plants emphasize an increase in cell size, while others focus on cell number.
• These measurements are essential for plant science research, agricultural applications, and understanding plant development and physiology.

Phases Of Growth

The term "Phases of growth" refers to a concept that is often used to describe the process of development and maturation in various organisms or systems. It signifies that growth is not a continuous or uniform process but occurs in distinct stages or phases, each of which is marked by specific and characteristic changes. These changes can be related to various aspects, including physiology, structure, and function.

Phases of Growth

The period of growth in plants is generally divided into three phases:
(a) Meristematic Phase: 

• This is the initial phase of growth, where cells are constantly dividing. Both the root apex (bottom part of the root) and the shoot apex (top part of the stem) contain meristematic cells. These cells are rich in protoplasm and possess large, conspicuous nuclei. 
• Their cell walls are primary in nature, thin, and cellulosic, with abundant plasmodesmatal connections. This phase is primarily responsible for increasing the number of cells and building the plant's basic structure.

(b) Elongation Phase: 

• Cells in the region proximal (just next, away from the tip) to the meristematic zone represent the phase of elongation. 
• During this phase, cells undergo significant changes, including increased vacuolation (formation of vacuoles), cell enlargement, and the deposition of new cell walls. 
• These changes result in the elongation of the plant's organs, such as roots and shoots.

(c) Maturation Phase: 

• Further away from the apex, i.e., more proximal to the phase of elongation, lies the portion of the plant axis that is undergoing the phase of maturation. 
• In this phase, cells attain their maximal size in terms of wall thickening and protoplasmic modifications. 
• Growth RateThis phase is responsible for the development of specialized tissues and cell types. Most of the tissues and cell types studied in earlier classes, such as xylem and phloem, represent this phase of growth.

Growth Rate

Growth rate is a fundamental concept used to quantify the increase in size, mass, or number of cells in an organism or a part of an organism over a specific period of time. It is a key parameter in the study of growth and development, and it can be expressed mathematically to provide a quantitative measure of how quickly an organism is growing.

"Growth rate represents the rate at which a biological entity, such as an organism, a tissue, or a population of cells, increases in size, mass, or numbers per unit of time. It measures the change in a particular characteristic (e.g., length, weight, cell count) over a defined time interval". 

There are two types of growth patterns seen in biological systems.

(i) Arithmetic growth
(ii) Geometrical (exponential) growth

Types of Growth Rate(i) Arithmetic growth
In arithmetic growth, after cell division, only one of the daughter cells continues to divide, while the other differentiates and matures. This type of growth is characterized by a constant rate of increase.

Example: An example given is the elongation of a root, where it grows at a constant rate.

Mathematically, it is expressed as Lt=L0+rt, where Lt= length at time 't', L0= length at time '0' and r=growth rate/elongation per unit time .

(ii) Geometrical (exponential) growth

Geometrical growth begins slowly (lag phase), then increases rapidly at an exponential rate (log or exponential phase) due to all progeny cells retaining the ability to divide. Eventually, growth slows down as nutrient supply becomes limited, leading to a stationary phase.

Example: Geometrical growth is a typical pattern for living organisms in natural environments and is seen in various cells, tissues, and organs of a plant.

Mathematically, Geometrical growth rate is expressed as W1 = W0ert.
Phases of Growth Rate

Growth Curve

A growth curve in plants is a graphical representation of how various aspects of a plant's development change over time. It is a valuable tool in plant biology, agriculture, horticulture, and ecology to study and understand the growth and development of plants.

Sigmoid Growth Curve

(a) Lag phase

• The lag phase is the initial phase of bacterial growth in a culture.
• During this phase, bacteria are adjusting to the new environment, and their growth is slow or nearly stagnant.
• Bacterial cells in the lag phase are metabolically active but are primarily focused on preparing for rapid growth.
• They may be synthesizing enzymes and other molecules needed to utilize the available nutrients efficiently.
• The duration of the lag phase varies depending on factors such as the bacterial species, the initial conditions of the culture, and the availability of nutrients.

(b) Log phase 

• The log phase follows the lag phase in bacterial growth.
• In this phase, bacterial growth is exponential, with the number of bacterial cells doubling at regular intervals.
• Conditions in this phase are typically optimal for bacterial growth, with an abundant supply of nutrients and no significant factors limiting growth.
• Bacterial cells in the log phase are actively dividing and metabolically very active.
• This phase is characterized by a steep upward slope on a graph of bacterial growth over time when plotted on a logarithmic scale, hence the term "log phase."

(c) Steady or stationary phase 

• The stationary phase is the subsequent stage in bacterial growth after the log phase.
• During this phase, bacterial growth slows down and eventually reaches a plateau.
• Conditions become less favorable for bacterial growth due to factors such as nutrient depletion, accumulation of waste products, and limited space in the culture.
• In the stationary phase, the number of newly divided cells roughly equals the number of cells dying, resulting in a relatively constant total population.
• Bacterial cells in this phase might undergo changes in metabolism and gene expression to adapt to the limited resources and stressful conditions.

Measurement of growth

(2) By horizontal microscope

(3) By Crescograph (J.C. Bose): It magnifies growth as 10,000 times

Auxanometer

Conditions for Growth

• Water: Essential for cell enlargement, turgidity, enzymatic activities, and nutrient transport.
• Oxygen: Required for respiration, energy production, and metabolic processes.
• Nutrients: Macro and micronutrients for synthesis of essential molecules.
• Temperature: Optimal range for metabolic rates and physiological processes.
• Light: Necessary for photosynthesis and growth direction (phototropism).
• Gravity: Influences root and stem growth orientation (gravitropism).

Plant Differentiation 

Plant differentiation is the process by which the cells of the root, cambium, apical meristems, and shoot mature to carry out specific roles. Within the plant cell, a lot of structural modifications take place throughout this process. As an illustration, protoplasm is lost as the treachery components of a plant are developing.

Differentiation Process in Plants

The differentiation and development processes of plants are distinct from those of other kingdoms since they belong to different kingdoms. Distinct from how it happens in animals, plants differentiate and develop in different ways.

• Cells of the root system, shoot apical meristem, and the cambium matures through a process known as plant differentiation, which prepares them to carry out particular tasks. The process through which a cell transforms from one cell type to another is known as cellular differentiation.
• The primary result of this change is the formation of a certain type of cell. Different structural alterations to the cell wall and protoplasm occur during the differentiation processes. 
• For instance, the cell would shed its protoplasm to develop a tracheary element. In addition, they form a lignocellulosic cell wall that is extremely resilient, elastic, and robust to transport minerals and water under difficult circumstances. 
• The process through which the various cell types diverge from their precursor cells is another way to describe it. These essential cells, which come in a variety of forms in plants, are all in charge of the organs’ fundamental operations. 
• One type of cell can change into another under the right circumstances, depending on its functions. There are two different types of differentiation processes:

Dedifferentiation Process

The dedifferentiation process occurs when cells go through a process where they lose the ability to divide and then under specific circumstances get it back. For instance, meristems are created when parenchymal cells have finished developing. Similar to this, tumor cells are created when the body’s normal cells dedifferentiate.

• Differentiation is the process by which the cells generated from the cambium, root, and shoot apical meristems differentiate and mature to carry out particular activities.
• Cells go through some significant structural changes during differentiation, and they also produce lignocellulosic secondary cell walls, which are robust, elastic, and capable of transporting water over great distances.
• Dedifferentiation is the process through which differentiated live cells that have lost the ability to divide might do so again under specific circumstances.
• Dedifferentiation is the ability of differentiated cells in a specific area of the plant body to divide once again. It enables a section of the plant to generate new cells.
• Therefore, before the significant physiological or structural change, differentiated cells typically go through dedifferentiation. Functional cell types go back to their early stages of development during dedifferentiation. 
• Dedifferentiated cells thus act as many types of meristematic tissue in plants, such as the interfascicular vascular cambium, cork cambium, and wound meristem.

Redifferentiation Process

The cells split into new cells that can no longer divide but are mature enough to carry out particular tasks. In other words, after being dedifferentiated, a mature plant cell loses its capacity to divide. This condition is known as redifferentiation.

• The capacity for cell division and subsequent differentiation is lost when new cells are generated from dedifferentiated tissues that serve as meristems. They eventually develop to conduct certain plant body functions.
• Redifferentiation is the reversal of differentiated cells’ ability to divide. It enables functionally specialized cells in the plant body to be made up of differentiated cells.
• Usually, differentiated cells that have been treated with dedifferentiation to prepare the plant body for physiological or structural change return to their Redifferentiated state and carry out the intended function.
• For instance, after cell division, the dedifferentiated vascular cambium redifferentiates into the secondary xylem and phloem.
• The cells of the secondary xylem and secondary phloem, on the other hand, are unable to undergo additional cell division, and once they have reached adulthood, they perform a variety of tasks, such as conducting food and water while maintaining the structural integrity of the plant.