Question

The number of stomata and epidermal cells in ${ 1 }{ mm }^{ 2 }$ the area of the abaxial surface of the leaves of A, B, C, and D plants is given below. Identify the plant having the least stomatal index :

Plant	Number of stomata	Number of epidermal cells
A	20	100
B	40	160
C	50	300
D	70	560

(A) B
(B) C
(C) D
(D) A

Answer 1

Stomatal index (I) is defined as the average number of stomatal cells per square millimeter of the epidermis of the leaf. In other words, it's the percentage of the number of stomatal cells present to the entire number of epidermal cells. It is often calculated by a given formula, ${ I }{ = }\dfrac { S }{ E+S }{ \times }{ 100 }$, where ‘I’ is the stomatal index. ‘S’ is the number of stomata and ‘E’ is the number of epidermal cells.

Complete answer:
As you know that the Stomatal index is calculated by ${ I }{ = }\dfrac { S }{ E+S } { \times }{ 100 }$.
- A: ${ I }{ = }\dfrac { 20 }{ 100+20 } { \times }{ 100 }$ ${ = }{ 16 }$
- B: ${ I }{ = }\dfrac { 40 }{ 160+40 } { \times }{ 100 }$ ${ = }{ 20 }$
- C: ${ I }{ = }\dfrac { 50 }{ 300+50 } { \times }{ 100 }$ ${ = }{14 }$
- D: ${ I }{ = }\dfrac { 70 }{ 560+70 } { \times }{ 100 }$ ${ = }{ 11 }$
It is clear from the above calculations that the least stomatal index is present in the plant D.
So, the correct answer is ‘plant D’.

Additional Information: In biology, a stoma, also called a stoma is a pore, found within the epidermis of leaves, stems, and other organs, that controls the speed of gas exchange. The pore is bordered by a pair of specialized parenchyma cells referred to as guard cells that are liable for regulating the dimensions of the stomatal opening.
This term is usually used to refer to the complete stomatal complex, consisting of the paired guard cells and therefore the pore itself, which is mentioned because of the stomatal aperture. Air enters the plant through these openings by gaseous diffusion and contains ${ C }{ O }_{ 2 }$ which is employed in photosynthesis and oxygen which is employed in respiration. Oxygen produced as a by-product of photosynthesis diffuses bent the atmosphere through these same openings. Also, water vapor diffuses through the stomata into the atmosphere during a process called transpiration. The degree of stomatal resistance is often determined by measuring the leaf gas exchange of a leaf.

Note: Stomatal density is completely different from the stomatal index. Stomatal density tells us about the number or you can say the number of stomata present in the leaves of the plants.