Which of the following facilitates opening of stomatal aperture?
A. Longitudinal orientation of cellulose microfibrils in the cell wall of guard cells.
B. Contraction of outer wall of guard cells.
C. Decrease n turgidity of guard cells.
D. Radial orientation of cellulose microfibrils in the cell wall of guard cells.

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Hint: Transpiration is the loss of water in the form of vapours from the aerial parts of the plant. The loss of water is so much that it affects the water level in the soil and gets reduced and can lead to the death of the plant.

Complete step-by-step answer:
Stomata are the tiny apparatus found on the epidermis of leaves and young green stems. Each stoma is covered by two specialized epidermal cells which are called guard cells. They differ from epidermal cells in their shape which is kidney or bean shaped and in the presence of chloroplasts.
> Longitudinal orientation of cellulose microfibrils in the cell wall of guard cells: - This is defined as the opening of the stoma which is aided due to the orientation of the microfibrils but these microfibrils are not longitudinally oriented.
> Contraction of outer wall of guard cells: - When the osmotic concentration of guard cell increases then the water moves inwards, so stomata gets open and water moves inside. Due to contraction of guard cells, the stomata get closed.
> Decrease and turgidity of guard cells: - The change in turgidity of the guard cells are responsible for the immediate cause of opening and closing of the stomata. When turgidity increases within the two guard cells and opening of each stomatal aperture or pore occurs.
> Radial orientation of cellulose microfibrils in the cell wall of guard cells: - Opening of the stoma which is aided due to the orientation of the microfibrils in the cell walls of the guard cells. Cellulose microfibrils are oriented radially, making it easier for the stoma to open rather than longitudinally.

Hence, the correct answer is option (D). Radial orientation of cellulose microfibrils in the cell wall of guard cells.

Note: Guard cells are covered by one or more modified epidermal cells known as subsidiary cells or accessory cells. The lower surface of a dorsiventral or dicotyledonous leaf has a larger number of stomata while in isobilateral or monocotyledonous leaf they are about equal on both surfaces. In monocots, guard cells are ellipsoidal or dumb-bell shaped known as gramineous stomata or poaceous stomata.