The leaves are the principal organs of photosynthesis in the vascular plants. The cuticle surrounds the epidermis of the leaf to reduce water loss, while gases pass through pores called stomata. Beneath the upper epidermis of the leaf is a layer of elongated palisade cells. The palisade cells contain numerous chloroplasts where photosynthesis takes place. Below the palisade cells is the spongy mesophyll, an arrangement of loosely packed cells that also contain chloroplasts for photosynthesis. The air spaces around the cells permit efficient gas exchange to take place during photosynthesis.
Bundles of vascular tissues extend through the leaf and form its veins. The vascular tissue supplies water and nutrients to the photosynthetic cells, and the products of photosynthesis are conducted away from the cells through the phloem. Vascular tissue also runs through the petiole, the stalk that connects the leaf to the node of the stem. The broad, flat portion of the leaf is the blade.
One of the most important activities in the leaf is the opening and closing of the stomata. These pores regulate the rate of gas exchange, which regulates the rate of photosynthesis. The opening and closing of a stoma is regulated by osmotic pressure within a pair of guard cells. Guard cells are thicker on their inner sides than on the outside, so when the cells are swollen with water, they bow outward, opening the stoma. The pressure exerted on the guard cells to open is called turgor pressure. Scientists believe that a low concentration of carbon dioxide and an accumulation of potassium ions in the guard cells instigate their opening. ATP provides the energy for opening and closing the guard cells.
When the stomata are open, the carbon dioxide needed for photosynthesis enters the leaf, while the oxygen gas produced in photosynthesis leaves the leaf. The water produced during photosynthesis also leaves through the stomata. This water loss is called transpiration.