Which structure allow the flow of gases in and out of a leaf

The correct answer is stomata.

Explanation:

The leaf structure is adapted for gas exchange between plants and the environment. The cells in the spongy mesophyll have tiny pores, termed as stomata, present in the leaf surface. Most of the stomata are found in the lower epidermis. These tiny pores control gas exchange. Opening and closing of stomata depend on the guard cell turgidity.

Diffusion of oxygen, carbon dioxide and water vapor in and out of the leaf is highest when these stomata are open.

Thus, the correct answer is stomata.

blood enters the heart through two large veins, the inferior and superior vena cava, emptying oxygen-poor blood from the body into the right atrium of the heart. as the ventricle contracts, blood leaves the heart through the pulmonic valve, into the pulmonary artery and to the lungs where it is oxygenated.