Growth

Growth is regarded as one of the most fundamental and conspicuous characteristics of a living being. Growth can be defined as “an irreversible permanent increase in the size of an organ or its parts or even of an individual cell”. Generally, growth is accompanied by metabolic processes (both anabolic and catabolic), that occur at the expense of energy. For example, the expansion of a the leaf is growing. 

Characteristics of plant growth  

Plant growth generally is indeterminate : 
  • Plant growth is unique because plants retain the capacity for unlimited growth throughout their life.
  • This form of growth wherein new cells are always being added to the plant body by the activity of the meristem is called the open form of growth 

Types of Growth 

Plant growth is of two types : 

  • Primary growth : 

Root apical meristem and shoot apical meristem are responsible for the primary growth of the plants and principally contribute to the elongation of the plants along their axis.

  • Secondary growth :

In dicotyledonous plants and gymnosperm, the lateral meristems (vascular cambium and cork cambium) are responsible for secondary growth and contribute to the increase in the girth of the organs (root & stem).







Growth is Measurable 


At the cellular level, growth can be measured by measuring the increase in the amount of protoplasm but it is very difficult to measure directly, so growth is measured by a variety of parameters, they are:
(a) Increase in fresh weight 
(b) Increase in dry weight 
(c) Increase in the surface area:- Growth in a dorsiventral leaf 
(d) Increase in the number of cells:- One single maize root apical meristem can give rise to more than 17,500 new cells per hour. 
(e) Increase in size of cells:-
Watermelon may increase in size by up to 3,50,000 times. 
(f) Increase in length:- Growth of a pollen tube

Growth can be measured by Auxanometer.


Phases of Growth  

The period of growth is generally divided into three phases, namely: I. Meristematic phase (Cell Division) 
II. Elongation phase 
III. Maturation phase


Meristematic phase

The constantly dividing cells, both at the root apex and the shoot apex, represent the meristematic phase of growth. The cells in the meristematic region are characterized by :
a. Cells are small in size with abundant plasmodesmata connections. b. Intercellular spaces are absent, if present, then very small. 
c. Cells are rich in protoplasm and possess large conspicuous nuclei.



Elongation phase 

 The cells proximal to the meristematic zone represent the phase of elongation. Cells in this region are characterized by : 
(a) Increased vacuolation 
(b) Cell enlargement 
(c) New cell wall deposition



Maturation phase

The cells more proximal to the phase of elongation represent the phase of maturation. Cells of this zone, attain their maximal size in terms of wall thickening and protoplasm modifications.


Growth Rate 

a) Arithmetic growth: 

In arithmetic growth, only one daughter cell among the two further divides while others differentiate and become mature (stop dividing). Example: Root & Shoot elongation at a constant rate.
It is mathematically expressed as 
Lt = L0 + rt 
Where  
• Lt - length at time 't‘ 
• L0 - length at time 'zero‘
• r - growth rate/elongation per unit time. 
The arithmetic growth rate shows a linear growth curve
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b) Geometric Growth: 

• In this growth, both the progeny cells following mitotic divisions retain the ability to divide and continue to do so. 
• Example:- Early embryonic development/division in the zygote, Division in a unicellular organism.
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It is mathematically represented as 
W1 = W0e rt  
• Where; 
• W1 - final size (Weight, height, number, etc.) 
• W0 - initial size at the beginning of the period. 
• r - growth rate 
• e - base of natural logarithms. 
• A sigmoid curve is a characteristic of living organisms growing in a natural environment. It is typical for all cells, tissues, and organs of a plant. 

  • Here r = relative growth rate and is also the measure of the ability of the plant to produce new plant material is referred to as the efficiency index. 
  • Quantitative comparisons between the growth of the living system can also be made in two ways: 
  • a) Absolute growth rate: Measurement and comparison of total growth per unit time are called the absolute growth rate. 
  • b) Relative growth rate: 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.

Conditions For Growth 


1. Water: The plant cells grow in size by cell enlargement which in turn requires water. Turgidity of cells helps in extension growth. 
2. Oxygen: Oxygen helps in releasing metabolic energy essential for growth activities. 
3. Nutrients (macro and micro essential elements): Nutrients are required by plants for the synthesis of protoplasm and act as the source of energy. 
4. Temperature: In addition, every plant organism has an optimum temperature range best suited for its growth. Any deviation from this range could be detrimental to its survival. 
• Environmental signals such as light and gravity also affect certain phases/stages of growth.


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