The diagram given below shows the internal structure of the lamina and midrib of a leaf Study the same and answer the questions that follow:

(i) Is the above section that of an isobilateral or dorsiventral leaf ? Give reason to support your answer.

(ii) Name the parts 1, 2, 3 and 4 indicated by the guidelines.

(iii) What is the function of the part labelled 3?

(iv) Explain how part 3 performs the function mentioned by you. 

(i) A dorsiventral leaf. This is so because stoma is present only on the ventral surface of the leaf.

(ii)
1 - Palisade mesophyll
2 - Xylem vessel / xylem
3 - Guard cells
4 - Substomatal airspace

(iii) Guard cells regulate the opening and closing mechanism of the stomata.

(iv) During daytime, the guard cells perform photosynthesis thus the turgor pressure increases owing to the glucose manufactured during the process. Water is drawn in from the adjoining cells due to endosmosis, resulting in the opening of the stomata. During nighttime or low-sunshine periods, the glucose manufactured is transformed into starch and transported to various other parts of the plant. Thus, water is drawn out of the guard cells due to exosmosis, resulting in the closing of the stomata.