Like all organs of a plant, leaves consist of epidermal, ground, vascular, and meristematic tissues. The leaf's primary function is that of photosynthesis, thus the structure of leaves must promote the acquisition of ingredients to make sugar – water from the roots, CO2 from the air, and sunlight to power the chemical reactions of the process.
The earliest stage of leaf development is a small bulge near the shoot apex called a leaf primordium that consists of a few hundred cells. Continued cell division and cell expansion results in the growth of the leaf. The vascular tissue (xylem and phloem) forms in strands called veins. Xylem forms on the upper side of a vein, and phloem on the lower side. Veins are supported by a layer of parenchyma cells called the bundle sheath. Leaves are covered by clear epidermis containing stomata that regulate gas exchange and a waxy cuticle to reduce water loss. Horizontal leaves have majority of stomata on lower surface, vertical leaves have equal distribution. Opening and closing of each stomatal pore is controlled by a change in turgor pressure in the two surrounding guard cells.
Along the upper side of the leaf are one or more layers of long cells called palisade mesophyll cells, which are densely packed (i.e., lack intercellular spaces), are arranged in columns, and perform as much as 90% of the leaf's photosynthesis. Palisade cells contain many chloroplasts and are therefore specialised for light absorption and photosynthesis.
Along the lower side of the leaf are spongy mesophyll cells, which are irregularly shaped and green due to chloroplasts they contain. Spongy mesophyll cells are separated by large intracellular spaces connected to stomata. The arrangement of mesophyll cells promotes the movement of water vapour and CO2 and oxygen gases into and out of a leaf.