The period of growth is generally divided into three phases, namely, meristematic, elongation and maturation.
(i) Meristematic Zone. The constantly dividing cells, both at the root apex and the shoot apex, represent the meristematic phase of growth. The cells in this region are rich in protoplasm, possess large conspicuous nuclei. Their cell walls are primary in nature, thin and cellulosic with abundant plasmodesmatal connections.

(ii) Elongation Zone. The cells proximal (just next, away from the tip) to the meristematic zone represent the phase of elongation. Increased vacuolation, cell enlargement and new cell wall deposition are the characteristics of the cells in this phase.

(iii) Maturation Zone. Further away from the apex, i.e., more proximal to the phase of elongation, lies the portion of axis which is undergoing the phase of maturation. The cells of this zone, attain their maximal size in terms of wall thickening and protoplasmic modifications.

Abscisic Acid. It acts as a general plant growth inhibitor and an inhibitor of plant metabolism. ABA inhibits seed germination. ABA stimulates the closure of stomata in the epidermis and increases the tolerance of plants to various kinds of stresses. Therefore, it is also called the stress hormone.
ABA plays an important role in seed development, maturation and dormancy. By inducing dormancy, ABA helps seeds to withstand desiccation and other factors unfavourable for growth. In most situations, ABA acts as an antagonist to GAs.

Ethylene. Ethylene is a simple gaseous PGR. It is synthesised in large amounts by tissues undergoing senescence and ripening fruits. Influences of ethylene on plants include horizontal growth of seedlings, swelling of the axis and apical hook formation in dicot seedlings.
Ethylene promotes senescence and abscission of plant organs especially of leaves and flowers. Ethylene is highly effective in fruit ripening. It enhances the respiration rate during ripening of the fruits. This rise in rate of respiration is called respiratory climactic.
Ethylene breaks seed and bud dormancy, initiates germination in peanut seeds, sprouting of potato tubers. Ethylene promotes rapid internode/petiole elongation in deep water rice plants. It helps leaves/upper parts of the shoot to remain above water. Ethylene also promotes root growth and root hair formation, thus helping the plants to increase their absorption surface.
Ethylene is used to initiate flowering and for synchronising fruit-set in pineapples. It also induces flowering in mango. Since ethylene regulates so many physiological processes, it is one of the most widely used PGR in agriculture. The most widely used compound as source of ethylene is ethephon. Ethephon in an aqueous solution is readily absorbed and transported within the plant and releases ethylene slowly. Ethephon hastens fruit ripening in tomatoes and apples and accelerates abscission in flowers and fruits (thinning of cotton, cherry, walnut). It promotes female flowers in cucumbers thereby increasing the yield.

Gibberellins. Gibberellins are another kind of promotery PGR. There are more than 100 gibberellins reported from widely different organisms such as fungi and higher plants. They are denoted as GA₁, GA₂, GA₃ and so on. However, Gibberellic acid (GA₃)was one of the first gibberellins to be discovered and remains the most intensively studied form. All GAs are acidic.
They produce a wide range of physiological responses in the plants. Their ability to cause an increase in length of axis is used to increase the length of grapes stalks. Gibberellins, cause fruits like apple to elongate and improve its shape. They also delay senescence. Thus, the fruits can be left on the tree longer so as to extend the market period. GA₃ is used to speed up the malting process in brewing industry.
Sugarcane stores carbohydrate as sugar in their stems. Spraying sugarcane crop with gibberellins increases the length of the stem, thus increasing the yield by as much as 20 tonnes per acre.
Spraying juvenile conifers with GAs hastens the maturity period, thus leading to early seed production. Gibberellins also promotes bolting (internode elongation just prior to flowering) in beet, cabbages and many plants with rosette habit.

Photoperiodism. Flowering in certain plants depends not only on a combination of light and dark exposures but also their relative durations. This response of plants to periods of day/night is termed photoperiodism. It is also interesting to note that while shoot apices modify themselves into flowering apices prior to flowering, they (i.e., shoot apices of plants) by themselves cannot perceive photoperiods. The site of perception of light/ dark duration are the leaves.
It has been hypothesised that there is a hormonal substance(s) that is responsible for flowering. This hormonal substance migrates from leaves to shoot apices for inducing flowering only when the plants are exposed to the necessary inductive photoperiod.
The significance of photoperiodism is in regulating flowering in plants. Flowering is an important step towards seed formation and seeds are responsible for continuing the generation of a plant. So, photoperiodism has an important role to play in evolution.
Vernalisation. There are plants for which flowering is either quantitatively or qualitatively dependent on exposure to low temperature. This phenomenon is termed vernalisation. It prevents precocious reproductive development late in the growing season, and enables the plant to have sufficient time to reach maturity. Vernalisation refers specially to the promotion of flowering by a period of low temperature.
Some important food plants, wheat, barley, rye have two kinds of varieties: winter and spring varieties. The 'spring' variety are normally planted in the spring and come to flower and produce grain before the end of the growing season. Winter varieties, however, if planted in spring would normally fail to flower or produce mature grain within a span of a flowering season. Hence, they are planted in autumn. They germinate, and overwinter come out as small seedlings, resume growth in the spring, and are harvested usually around mid-summer.
Another example of vernalisation is seen in biennial plants. Biennials are monocarpic plants that normally flower and die in the second season. Sugarbeet, cabbages, carrots are some of the common biennials. Subjecting the growing of a biennial plant to a cold treatment stimulates a subsequent photoperiodic flowering response.

Geometric Growth. In most systems, the initial growth is slow (lag phase), and it increases rapidly thereafter - at an exponential rate (log or exponential phase). Here, both the progeny cells following mitotic cell division retain the ability to divide and continue to do so. However, with limited nutrient supply, the growth slows down leading to a stationary phase. If we plot the parameter of growth against time, we get a typical sigmoid or S-curve. A sigmoid curve is a characteristic of living organism growing in a natural environment. It is typical for all cells, tissues and organs of a plant.

The exponential growth can be expressed as
W₁ = W_o ert
W₁ = final size (weight, height, number etc.)
W₀= initial size at the beginning of the period
r = growth rate
t = time of growth
e = base of natural logarithms
Here, r is the relative growth rate and is also the measure of the ability of the plant to produce new plant material, referred to as efficiency index. Hence, the
final size of WI depends on the initial size, W₀.
Quantitative comparisons between the growth of living system can also be made in two ways:
(i) measurement and the comparison of total growth per unit time is called the absolute growth rate.
(ii) The growth of the given system per unit time expressed on a common basis,
e.g., per unit initial parameter is called the relative growth rate.

(a) Arithmetic Growth. In arithmetic growth, following mitotic cell division, only one daughter cell continues to divide while the other differentiates and matures. The simplest expression of arithmetic growth is exemplified by a root elongating at a constant rate.
Mathematically, it is expressed as
L_t = L₀ + rt
L_t = length at time 't'
L₀= length at time 'zero'
r = growth rate/elongation per unit time

(b) Geometric Growth. In most systems, the initial growth is slow (lag phase), and it increases rapidly thereafter - at an exponential rate (log or exponential phase). Here, both the progeny cells following mitotic cell division retain the ability to divide and continue to do so. However, with limited nutrient supply, the growth slows down leading to a stationary phase. If we plot the parameter of growth against time, we get a typical sigmoid or S-curve. A sigmoid curve is a characteristic of living organism growing in a natural environment. It is typical for all cells, tissues and organs of a plant.

The exponential growth can be expressed as
W₁ = W_o ert
W₁ = final size (weight, height, number etc.)
W₀= initial size at the beginning of the period
r = growth rate
t = time of growth
e = base of natural logarithms
Here, r is the relative growth rate and is also the measure of the ability of the plant to produce new plant material, referred to as efficiency index. Hence, the
final size of WI depends on the initial size, W₀.
Quantitative comparisons between the growth of living system can also be made in two ways:
(i) measurement and the comparison of total growth per unit time is called the absolute growth rate.
(ii) The growth of the given system per unit time expressed on a common basis,
e.g., per unit initial parameter is called the relative growth rate.
(c) Sigmoid growth curve
The growth of living organisms in their natural environment is characterised by an S-shaped curve called sigmoid growth curve. This curve is divided into three phases – lag phase, log phase or exponential phase of rapid growth, and stationary phase. 
  
Exponential growth can be expressed as:
W₁ = W₀eⁿ
Where,
W₁ = Final size
W₀ = Initial size
r = Growth rate
t = Time of growth
e = Base of natural logarithms
(d) Absolute and Relative growth rates:
1. Absolute growth rate is total growth per unit time. In the given representation, the absolute growth rate for both leaves is 5 square centimeters per given time.
2. Relative growth is the rate of growth with respect to the initial size. Here, the relative growth rate for leaf A is higher as its surface area increased by 100% while the surface area of leaf B increased by only 10%.

The cell which are derived from root apical meristem and shoot and other meristems undergo certain changes to attain maturity to perform specific functions. This process which leads to maturation of cells is termed as differentiation. During this, the cells undergo a few major structural changes both in their cell walls and protoplasm to perform some specific functions throughout their life.
(i) In plants, some living differentiated cells, e.g. parenchyma can regain the capacity to divided mitotically under certain conditions. A dedifferentiated tissue can act as merisstem, e.g. interfascicular vascular cambium,cork cambium and wound meristem.
(ii) The product of dedifferentiated cells or tissues which lose the ability to divide is called redifferentiated cells and the term is known as redifferentiation. Secondary xylem and secondary phloem from interfascicular vascular cambium, secondary cortex are the examples of tissues.

The factor which prove the phytohormones act synergistically or antagonsitically are
(i) Cell division is promoted by both auxins and cytokinins acting synergistically.
(ii) Auxins and cytokinins act antagonistically in controlling apical dominance. Auxins cause apical dominance, while cytokinins promotes shoot bud formation.
(iii) Auxins and cytokinins act antagonistically in controlling apical dominance. Auxins cause apical dominance, while cytokinins overcome same.
(iv) Senescence is prevented by auxins and cytokinins,while it is stimulated by absciscis acid.
(v) The activity of cambium and fruit growth seems to be promoted by auxins, gibberellins and cytokinnins, the same is inhibited by abscisic acid.
(vi) The dormancy of seeds and buds is mostly due to abscisic acid and the same is broken by gibberellins.
(vii) Cytokinins cause opening of stomata, while abscisic acid results in their closure.

Plant growth regulators are small, simple molecules secreted in minute quantities, that influence various physiological functions in plants. They are of diverse chemical composition.
(i) Indole compounds [such as-indole-3-Acetic Acid (IAA)]
(ii) Adenine derivatives[such as kinetin, 6-furfuryl amino purine]
(iii) Derivatives of carotenoids {such as Abscisic Acid (ABA)]
(iv) Terpenes[such as gibberellic acids]