The outermost layer of cells i.e., usually the epidermis is not continuous at some places e.g., on the surface of leaves or green herbaceous stems.
The epidermis of the young shoot and leaves contains numerous minute pores called stomata. Each stomatal opening is surrounded by two semilunar cells known as the guard cells.
The term ‘stoma’ (plural-stomata) is applied to the stomatal opening plus the guard cells. The guard cells are living and contain chloroplasts. Their inner walls (wall towards opening) are thicker and outer walls thinner.

Fig. 6.4. Structure of stomata. (A) Stomatal pour is open. (B) Stomatal opening is closed

Function of guard cells: The guard cells regulate the opening and closing of the stomatal pore. The guard cells swell when water flows into them causing the stomatal pore to open. Because inflow of water in guard cells causes the stretching and bulging of outer thin walls in converse shape. This drags thick walls apart leading to opening of the pore. When there is outflow of water from guard cells, the outer thin walls come to their original position resulting in closure of stomatal pore.

The following activity can prove that chlorophyll is essential for photosynthesis.

Activity:

1. Take a potted plant with variegated leaves (leaves partly green and partly non-green), for example colens, croton, money plant.
2. Keep the plant in a dark place for 3 days. So that all the starch of leaves gets used
up.
3. Place the plant for 6 hours in sunlight.
4. Pluck a leaf from the plant. Mark the green areas of the leaf and trace them on a sheet of paper.
5. Boil the leaf first in water for a few minutes and then replace the leaf in a beaker containing alcohol. Place this beaker in a bigger beaker containing enough water. Heat till the alcohol begins to boil on a burner.
(Note: If you use a hot plate to heat the leaf in alcohol, there is no need to use water bath. Boil the leaf directly in alcohol on a hot plate. In case, you use burner, vapour of alcohol can catch fire from flame of the burner).
6. Boiling of the leaf in alcohol decolourizes the leaf.
7. Place decolourlized leaf in a petridish and pour .iodine solution on it.

Fig. 6.3. To show that chlorophyll is necessary for photosynthesis

8. Take out the leaf and rinse off the iodine solution. Observe the colour of the leaf and compare this with the tracing of the leaf done in the beginning iodine solution on it. The portion of the leaf which was previously green has turned into dark blue colour while the non-green portion turns yellow (colour of iodine solution). The dark blue colour indicates the presence of starch (starch with idone turns blue black) while the yellow portion does not show any starch.
This proves that only chlorophyll containing cells synthesize starch. Hence chlorophyll is necessary for photosynthesis.