The growth and development process in plants is uniquely different. The plant grows in size and height over a period of time. They also follow certain periodical events and go through different phases in their life cycle. Cell division, cell enlargement and cell differentiation are some of the different processes that occur during the growth phase of plants. Plant growth is measurable. In plants, growth takes place normally by dividing cells that help in increasing the size of cells or tissue. Meristematic cells are the cells that divide continuously. Let us learn to know more about the phases of growth in plants and bacteria.

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Growth in Plants: Different Phases

In the multicellular plant’s growth takes place in three distinct phases:
1. Meristematic phase or phase of cell division
2. Elongation phase or phase of cell elongation
3. Maturation phase or phase of differentiation

Fig: Different Types of Phases in Growth in Plants

1. Meristematic or formative phase or phase of cell division is the phase where active cell division takes place in the root apex, shoot apex, and other regions having meristematic tissue, thereby increasing the numbers.
The features shown by the cells of this phase are:
i) The cells have dense protoplasm.
ii) The cells have a large prominent nucleus.
iii) The cell walls are primary in nature.
iv) The cell walls contain plasmodesmata.
v) The cells have high respiration rates.

2. Elongation phase is the one in which the cells elongate by changing their size of vacuoles, the mass of protoplasm, etc. This phase is situated just next to the meristematic zone, away from the growing tip.
The features shown by the cells of this phase are:
i) The cell increased vacuolation.
ii) The enlargement of the cell takes place due to increased turgor pressure and an increase in plasticity. The enlargement of the cell takes place in all directions.
iii) Deposition of the new cell wall takes place.

3. Maturation or differentiation is the phase in which there is a distribution of functionality to the newly formed cells. This phase is lying just behind the zone of elongation.
The features shown by the cells of this phase are:
i) The cells attain maximum thickening of their walls.
ii) The protoplasmic modifications are maximum as per the need of the cells.
iii) The new cell wall materials are deposited over the old ones.
iv) The cells, once differentiated, remain unchanged till their death.

Fig: Phases of Growth in Plants

Growth: Measurement

The rate of growth can be measured in various ways. Such as
i) Increase in length: It is used in the case of stem, root and pollens.
ii) Increase in volume: It is used in the case of fruits.
iii) Increase in diameter: It is used in the case of fruits and tree trunks.
iv) Increase in fresh or dry weight: Fresh weight is measured in fruits, bulbs, corms, roots, etc., and dry weight is used for actual measurement of growth. Organs are dried in an oven at \({110^ \circ }{\rm{C}}\) for several hours.
v) Increase in the number of cells produced: It is used in bacteria, yeast and many algae.
vi) Increase in the surface area: It is used in the case of flat organs like leaves.

Direct Method

It is the most simple method in which the length of the growing part is measured using a scale at regular intervals.

1) Horizontal Microscope

In this method, the linear growth of the plant is measured. Here, a point is marked near the plant’s growing tip (stem or the root tip) and is focused by a hori­zontal microscope that slides over a graduated vertical stand. At the regular interval, the same point is again focused either by rais­ing (in case of stem tip) or lowering (in case of root tip) the horizontal microscope. The difference of the initial and final readings on the graduated vertical stand measures the overall increase in the height of the plant.

Fig: Horizontal Microscope

2) Auxanometer:

It is an instrument used to measure the total growth rate at a specific time and overall growth pattern.

i) Arc Auxanometer

a) It is the most simple and used to measure the vertical growth of the plant.
b) It consists of a small pulley to the axle attached to a long pointer sliding over a graduated arc.
c) A thread, one end of which is tied to the stem tip and the other end to weight, passes over the pulley tightly.
d) As soon as the stem tip increases in length, the pulley moves, and the pointer slide over the graduated arc.
e) The actual increase in length of the stem is then calculated by knowing the length of the pointer and the diameter of the pulley.

Fig: Arc Auxanometer

ii) Pfeffer’s Auxanometer

a) It is a complex instrument.
b) It consists of a compound pulley with a small and a large wheel, both having the same axle.
c) A thread that is tied to one end of the stem tip and the other end to weight passes over the smaller wheel tightly.
d) Another thread whose both ends are tied with weights tightly passes over the larger wheel. A pointer is attached near one end of this thread which remains in touch with a rotating cylindrical drum covered by smoked paper.
e) As soon as the stem tip increases in length, the wheels of the pulley move so that the pointer also moves downward and traces a special white marking on the smoked paper that gives an idea of the growth in the stem tip.
f) The actual increase in length of the stem can be calculated by knowing the radii of larger and smaller wheels and the rate of rotation of the drum.

Fig: Pfeffer’s Auxanometer

3) Bose’s Crescograph:

Highly sensitive apparatus based on the system of compound levels that uses a series of clockwork gears and a smoked glass plate to record the movement of the tip of a plant.

Fig: Crescograph

Plant: Growth Rates

The organisms or part of an organism can produce more cells in a variety of ways. The Growth rate is defined as the increase in growth per unit time (growth/time). The growth rate can be expressed mathematically. There are two major growth rates:
1. Arithmetic growth rate is followed in mitosis division in certain organs, where one of the two cells formed differentiate and the other continues division.

Fig: Arithmetic Growth Rate

The simplest expression of arithmetic growth is exemplified by a root elongating at a constant rate. On plotting the length of the organ against time, the graph obtained is straight-like with a standard slope. It is a linear curve.

The mathematical expression is:
\({{\rm{L}}_{\rm{t}}} = {{\rm{L}}_0} + {\rm{rt}}\)
\({{\rm{L}}_{\rm{t}}}\) is the length at time \({\rm{t}}\)
\({{\rm{L}}_0}\) is the initial length at time \({\rm{t = 0}}\)
\({\rm{r}}\) is the growth rate

Fig: Constant Linear Growth, A Plot of Length L Against Time t

2. Geometric growth rate is shown by various organisms, where the initial rate is low, and the final rate is also low.

Fig: Geometric Growth Rate

The graph obtained when we plot a geometric growth is S-shaped and is called a sigmoid curve, which has three phases:
i) The lag phase is the initial slow phase.
ii) The log or exponential phase is the phase of maximum growth.
iii) The stationary phase is the phase of retardation of growth.

Fig: The Sigmoid Curve

The mathematical expression is:
\({{\rm{W}}_{\rm{1}}}{\rm{ = }}{{\rm{W}}_{\rm{0}}} \cdot \,{{\rm{e}}^{{\rm{rt}}}}\)
\({{\rm{W}}_{\rm{1}}}\) is the final size, number, or height
\({{\rm{W}}_{\rm{0}}}\) is the initial size, number, or height
\({\rm{r = }}\) growth rate; \({\rm{e = }}\) base of natural logarithms; \({\rm{t = }}\) time
The final size \({W_1}\) depends more on the initial size and not much on the growth rate \({\rm{r}}\)
Some cells, organs can show the two types of growth successively.

Fig: Stages During Embryo Development Showing Geometric and Arithmetic Phases

The following diagram displays the various stages of embryo development, showing both geometric and arithmetic phases.
The dark blue blocks represent the cells capable of division, while light blue blocks represent the cells that have lost the capacity to divide.
The growth of the living system can be compared quantitatively in two ways:
i) The measurement and comparison of total growth per unit time are known as absolute growth rate.
ii) The relative growth rate is the growth that takes place in a unit of time from a given initial measurable quantity.

Bacterial Growth Curve

In unicellular organisms, growth and reproduction are synonymous. Bacteria are easy to grow in the laboratory, so their growth is studied. The entire process of the cell cycle can take as little as \(20\) minutes for an active culture of E. coli bacteria. Bacteria can be grown in a closed system known as batch culture in which no nutrients are added, and most waste is not removed, following a reproducible growth pattern. A growth curve is a graphical representation of the rate of growth with respect to the number of individuals. The growth curve of a bacterial culture is represented by the logarithm of the number of live cells plotted as a function of time. In bacteria, a typical sigmoid curve is observed. The growth curve in bacteria shows four phases. These are:
1. Lag phase
2. Log phase or acceleration phase
3. Stationary phase
4. Death phase

Fig: Bacterial Growth Curve

1. Lag phase

a) The lag phase is the initial phase, also called an adaptation period, where the bacteria adjust to their new conditions and prepare themselves for the next phase of growth.
b) It is mainly characterised by very slow growth.
c) The bacteria do not multiply in this phase but prepare the necessary and essential factors, ribosomes, proteins and enzymes that are required for conducting important physiological and molecular processes.
d) The duration of the lag phase based on conditions that the bacteria came from and the condition of the bacterial cells themselves and depends on the species and genetic makeup of the cells, the composition of the medium, and the size of the cells in the original inoculum.
e) Actively growing cells transferred from one type of media into the same type of media will have the shortest lag period with the same environmental conditions. 
f) Damaged cells will have a long lag period since they must repair themselves before they can engage in reproduction.

2. Log phase

a) The logarithmic (log) growth phase is also known as the exponential growth phase because the cells are actively dividing by binary fission. Their number increases exponentially, i.e. predictable doublings of the population take place, i.e. the total number of population can be derived by \({\rm{2 n}}\), where \({\rm{n}}\) is the number of times bacteria has multiplied or the number of generations. It also shows balanced growth.
b) For any given bacterial species, the generation time under specific growth conditions (nutrients, temperature, pH, and so forth) is genetically determined. This generation time is called the intrinsic growth rate.
c) During the log phase, the relationship between time and number of cells is not linear but exponential; the plot is straight.
d) Cells in the log phase show a constant growth rate and uniform metabolic activity. For this reason, cells in the log phase are used for industrial applications and research work.
e) The log phase is also the stage where bacteria are the most susceptible to the action of disinfectants and common antibiotics that affect protein, DNA, and cell-wall synthesis.

3. Stationary phase

a) Stationary phase or steady-state growth where the rate of growth remains almost static due to some limiting factors such as accumulation of Waste products, nutrients or chemicals are gradually used up, lack of physical space. In addition, gradual depletion of oxygen begins to limit aerobic cell growth.
b) At this point, the number of new cells being produced by the cell division is equal to the number of cells dying off or growth has entirely ceased, resulting in a flattening out of growth on the growth curve.
c) During the stationary phase, cells are also prone to synthesise secondary metabolites or metabolites produced after active growth, such as antibiotics. Cells that are capable of making an endospore will activate the necessary genes during this stage in order to undergo the sporulation process.
d) In certain pathogenic bacteria, the stationary phase is also associated with the expression of virulence factors, products that contribute to a microbe’s ability to survive, reproduce, and cause disease in a host organism.

4. Death phase

a) It is the last phase and sometimes called the decline phase, where the culture medium accumulates toxic waste and nutrients are exhausted, cells die in greater and greater numbers.
b) The cells collected from this phase fail to show growth when transferred to a fresh medium
c) Soon, the number of dying cells exceeds the number of dividing cells, leading to an exponential decrease in cells.
d) It has also been shown that \({\rm{100 \% }}\) cell death is unlikely for any cell population, as the cells mutate to adapt to their environmental conditions, however harsh.

Summary

The phases of growth in plants are divided into three phases. These are the meristematic phase or phase of cell division, elongation phase or phase of cell elongation and maturation phase or phase of differentiation. The plants show arithmetic and geometric growth rate. It can be expressed mathematically. The sigmoid curve represents the integrated sum of the curves for each growing organ and cell and presents the changing size of these parts. The growth curve in bacteria shows four phases. These are the lag phase, log phase or acceleration phase, stationary phase and death phase.

Frequently Asked Questions (FAQs) on Phases of Growth

Frequently asked questions related to phases of growth is listed as follows:

Q. What are the four stages of bacterial growth?
Ans: The four stages of bacterial growth are lag phase, log phase, stationary phase and death phase.

Q. What are the three phases of growth?
Ans: The three phases of growth are the phase of cell division, phase of cell elongation and phase of differentiation.

Q. What is the initial slow phase of geometric growth called?
Ans: Lag phase is the initial slow phase of geometric growth.

Q. What is an Auxanometer?
Ans: Auxanometer is the instrument used to measure the growth in plants.

Q. What is cell maturation?
Ans: Maturation or differentiation is the phase in which there is a distribution of functionality to the newly formed cells.

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