The transpiration pull is initiated by the movement of water from the leaves to the
surrounding air. By what process does water vapor leave the stomata? (A) Active transport, (B) oxidation, (C) evaporation, or (D) condensation.
This diagram shows the cross section through a leaf. The orange structures represent the xylem vessels, which are responsible for
transporting water up the stem of the plant, from the roots to the leaves. Represented in pink is the phloem tissue, which transports sugars up and down the
stem of the plant. We can also see a stoma, plural stomata, which is a pore on the underside of the leaf
that opens and closes in response to changing environmental conditions. For example, stomata, like the one we can see here, are open in the light. This allows lots of carbon dioxide to diffuse into the leaves for photosynthesis.
Now let’s turn our attention to the movement of water out of the leaves. When water reaches the leaf, it exits the xylem vessels and moves through the spongy
mesophyll cells and into the air spaces by osmosis. In the air spaces, the water changes state and turns from a liquid into a gas called
water vapor. Because there is a higher concentration of water vapor inside the leaf than outside,
the water vapor moves out of the leaf through the stomata by diffusion. This process is known as evaporation.
We have therefore determined that the correct answer is (C). The process by which water vapor leaves the stomata is evaporation.