Question

Why does the upper surface of leaf is smooth and the lower surface of the leaf is rough?

Answer 1

As the primary objective of leaves are to allow for photosynthesis so that the plant will have a source of food, the leaves will need to have a green pigment called chlorophyll as chlorophyll allows photosynthesis to take place as it converts light energy, in other words, sunlight, into chemical energy which is of use to the plant.

As the upper surface of the leaf is more exposed to the sun than the underside of the leaf, there will be more chlorophyll on the upper surface of the leaf so as to maximise the amount of sunlight that the chlorophyll receives. This results in a more intense green on the upper surface then on the underside of the leaf.