1. Internal structure of a leaf
A leaf consists of following layers.
Cuticle
Upper epidermis
Palisade mesophyll
Spongy mesophyll
Lower epidermis
Pores also known as stoma /stomata

2. Cuticle The outer thick waxy covering of the plants and leaves.
Cuticle protects plant from drying out by reducing water loss.
Does not let oxygen or carbon dioxide pass through it.

3. Upper epidermis
Upper epidermis consists of a single layer of cells covered by cuticle. Sometime contains pores which allow gas or water molecule to pass through it.

4. Palisade mesophyll
Just beneath the epidermis there are column shaped cells containing chloroplast are known as Palisade mesophyll .
Most photosynthesis takes place here.

5. Spongy mesophyll
The second type of mesophyll tissue is the spongy mesophyll which is composed of closely packed irregular shape cells surrounded by air spaces. These spaces allow carbon dioxide to e stored in the leaf for photosynthesis. There are fewer chloroplast because they are under side of the leaf.

6. Lower epidermis
Below the spongy mesophyll is the lower epidermis. We will see vascular bundle here and stomata.

7. What are stomata or stoma?
The pore like openings in the underside of the leaf leaf is called stoma (singular) and stomata (plural). Each stoma consists of two guard cells which control the opening and closing of the stomata

8. What is the function of stoma /stomata ?
Stomata allow carbon dioxide and oxygen and water vapor diffuse into and out of the leaf by opening and closing .

9. Structure of stomata
Stomata consists of two guard cells. When there is high water pressure guard cells open. Low water pressure in the guard cells cause stomata to close.

10. Do you think stomata are open all the time?
Plants keep their stomata open just enough to allow photosynthesis to take place but not so much that they lose excessive amount of water.

11. What would happen if the stomata were kept open all the time?
If the stomata were kept open all the time, water loss due to transpiration would be so great that few plants would be able to take enough water to survive.

12. What is the time frame of opening and closing stomata?
In general, stomata are open during the day time when photosynthesis are active, and closed at night.
However, stomata may be closed even in the bright sunlight under hot, dry conditions in which water conservation is a matter of life and death.