Phases of Growth in Plants

Growth is an essential, natural and irreversible factor. As living beings, every plant grows but unlike humans, plants have unique characteristics of growth. They grow throughout their life. Plant growth takes place in a systematic manner. This systematic mannerism of growth is defined as development in plants.

Plant growth is an irreversible and permanent increase in the size of the plant parts, organs, tissue, entire plant or an individual cell. Growth is the most fundamental characteristic of living plants, similar to any other living organism. Plant growth is also accompanied by metabolic processes such as catabolism and anabolism that require spending energy.

Growth in plants is an open form of growth i.e. it is indeterminate and some cells are continuously dividing and helping in the increase in the size of cell or tissue. Cells that continuously divide are called meristematic cells.

Plant growth is measurable since an increase in protoplasmic content takes place. Also, an increase in fresh or dry weight takes place, an increase in length & size and an increase in the number of cells occur which can be all measured.

Three Phases of Growth in Plants

The period of growth in plants are divided into three phases, namely-

Meristematic or Formative Phase- 

The tip region of the root of a plant consists of meristematic cells which are continuously dividing and are isodiametric (with no spaces between the cells), and that zone is called the meristematic zone; here, cells in the meristematic zone have rich protoplasm with the large and conspicuous nucleus. Cell walls of these cells are primary in nature, thin, and consist of cellulose. These cells have white plasmodesmata connection.

The cells in this region of the plant keep dividing vigorously. The meristematic region of the plant generally comprises a large nucleus. The cells present to stimulate the primary phase of growth in plants. These are present in the roots and shoots of every plant.

Elongation Phase-

The cells present next to meristematic cells are present in the elongation phase. These cells elongate due to enlargement of the vacuole present in the cell and hence that zone is called the zone of elongation. The cell enlargement and new cell wall formation take place in the cells present in the elongation phase. Modifications like larger vacuoles take place in the cells here.

Maturation Phase-

The next cells found after the elongation zone are present in the maturation phase and they attain the maximum size. They do not divide further after reaching the maturation phase. The zone of maturation is found as a mass differentiation phase where cells differentiate to perform special functions in the plants.

Knowing these three phases of cell growth, we can say that cell division, cell enlargement and cell differentiation are the different processes that occur in these phases. The first two phases lead to an increase in the plant size whereas the third phase results in bringing maturity to the cells. During differentiation, the cells undergo structural changes in the protoplasm and the cell wall.

Arithmetic and Geometric Growth Pattern

Plant Growth Rate- It is defined as growth per unit time. The two types of growth in plants are-

1. Arithmetic Growth in Plants:

When plant cells undergo mitotic division, two cells are formed from one cell. Out of these two cells, one continues dividing and the other gets differentiated forming different structures. The former cell again divides and forms two cells again and then one continues dividing and there is one more that is differentiated. Now, we have two differentiated cells from one cell initially. Similarly, a pattern is formed and a gradual increase in size of the plant takes place. 

We can represent this phase in the form of a linear graph with respect to height of the plant vs. time. 

To calculate Arithmetic Growth curve results, the Formula is as Follows:

Lt = L0 + rt 

Where, L= length of the plant

Lt = length at time ‘t’ or after time ‘t’

L0 = Length at initial time

r= rate of growth

t= time interval when we are calculating the plant growth rate

2. Geometric Growth in Plants

In this case, the cell divides by mitotic division and two cells are formed. Both the cells again divide to form two more cells for each. Thus, now the number of cells gets doubled at each stage. The number of cells will be like 2, 4, 8, 16, 32, 64, ...and so on. Unlike arithmetic growth stages, here no differentiation of cells take place. The cell growth rate is very fast here in the geometric phase and when the growth comes at its peak, it later starts becoming slow and later becomes steady due to some limiting factors. The geometric plant's growth can be represented in the form of the following S-shaped graph having a Sigmoid Growth curve in plants. 

• Lag Phase- It is the slower phase where cell division is slow.

• Log Phase- Here, the growth is very fast and known as the exponential phase too. A steep curve is formed.

• Steady or Stationary Phase- Here, the growth is steady or stationary and becomes constant.

Formula that Represents this Sigmoid Growth Curve is as Follows:

Wt= W0 * ert

W0 = size at the initial time

Wt= size at time t or after time t

r= growth rate

t= time period

e= base of natural logarithm

Growth is a natural process and unlike human beings plants have different forms of growth. The process which helps the growth to the fullest is the particular process known as the process which plants perform in the presence of sunlight, water  and organic compounds in order to produce food in the form of ATP. This in turn helps the plant to grow and provides energy to each and every cell of its structure.

Characteristics of plants growth

Some of the characteristics of plant growth are mentioned below:

• Plant Growth is Indeterminate: Plants have meristems which are present in roots and shoots both. Meristems provide a proper response for primary growth. Due to the presence of meristems only, the plants grow indefinitely throughout their life. Meristems have the capability to divide and self propagate. This is because plant growth is said to be indeterminate.

• Plant Growth is Measurable: Growth simply indicates a regular increment in protoplasm. Growth in plants is measured by the increment of cell number, area, volume, length, etc. Usually, the growth in plants is hard to measure, this is why the increment in the quantity of plant growth is measured accordingly using protoplasm increment. The increment of plant growth and protoplasm shares a proportionality.