Process Of Gaseous Exchange In Plants

Question 1

Anatomy of typical dicot and monocot leaf

Answer 1

Epidermis is usually a single cell layer.
In between the upper and lower epidermis, there is a ground tissue called the mesophyll.
This tissue consists of parenchyma cells, specialized for photosynthesis.

In dicot leaves,
stomata are mainly found in the lower epidermis.
The mesophyll consists of two distinct layers called palisade and spongy.
Palisade mesophyll consists of elongated cells that are arranged in one or more layers.
This can be found in the upper part of the leaf, just beneath the upper epidermis.
The spongy mesophyll can be found between the palisade layer and lower epidermis.
They are loosely arranged with many air spaces.
Spongy mesophyll cells have less chloroplasts than in palisade mesophyll cells.
The vascular tissue of the leaf is continuous with vascular tissue of the stem.
Veins in the leaf is highly branched (net like venation) in the mesophyll layer.
Each vein is protected by a bundle sheath layer.

In monocot leaves,
stomata are present in both lower and upper epidermis.
Mesophyll is not differentiated into palisade and spongy layers.
Chloroplasts are abundant in all mesophyll cells.
Veins are parallelly arranged (parallel venation).

Question 2

Define stomata

Answer 2

Stomata are pores surrounded by guard cells in the epidermis of the leaves and stems of plants which can open and close.

Question 3

Structure of stomata

Answer 3

Stomata are pores surrounded by guard cells in the epidermis of the leaves and stems of plants which can open and close.
Guard cells are modified epidermal cells which have a distinct shape and are the only epidermal cells that contain chloroplasts.
Guard cells are typically bean shaped in angiosperms.
The guard cell walls are unevenly thickened.
The inner cellulose wall is thicker and less elastic than the outer wall. Some of the cellulose microfibrils are radially arranged to form inelastic hoops around guard cells.
Guard cells regulate the diameter of the stomata by changing shape, widening or narrowing the gap between the pair of guard cells.

Question 4

Role of ABA in stomatal closure in drought

Answer 4

• ABA is produced in roots and leaves in response to water deficiency.
• Production of ABA leads to close the stomata by removal of K+in guard cells.
• This prevents the wilting of the plant.

Question 5

Factors affecting stomatal action

Answer 5

• Stomata open during day and mostly closed at night. Light stimulates accumulation of K+ in guard cells.
• Decrease in CO2 concentration in substomatal cavity lead to open stomata
• Internal clock in the guard cells controlling their daily rhythm of opening and closing of stomata.
• Environmental stresses such as drought, high temperature and wind can cause stomata to close during the day time.