Leaf Structure

¨       Leaves of a plant are its main organs of photosynthesis

¨       The blade is a thin flattened section used to collect sunlight.

¨       A petiole is a thin stalk that attached the blade to the stem.

¨       Leaves have three main tissues:

·         Dermal = epidermis covered by the cuticle (waterproof barrier)

·         Vascular = xylem and phloem in the veins

·         Ground = parenchyma and sclerenchyma

Leaf Functions

¨       The leaf is a system specialized for photosynthesis.

¨       Subsystems include tissues that bring gases, water, and nutrients to the cells that carry out photosynthesis.

Photosynthesis

¨       The mesophyll is ground tissue that makes up the bulk of the leaves and the site of photosynthesis.

¨       The carbohydrates produced move into phloem vessels and carry it to the rest of the plant.

¨       Palisade mesophyll is just under the upper epidermis and absorbs light that enters the leaf.

¨       Spongy mesophyll is a loose tissue with many air spaces that connect with the exterior through stomata.

¨       Stomata are porelike openings on the underside of the leaf that allow carbon dioxide and oxygen to diffuse into and out of the leaf.

¨       Guard cells control the opening and closing of stomata by responding to changes in water pressure.

Transpiration

¨       The spongy mesophyll needs to be kept moist so that gases can enter and leave the cells easily.

¨       Transpiration is the loss of water through its leaves.

¨       Lost water is replaced by water drawn into the leaf through xylem vessels.

Gas Exchange

¨       Leaves take in carbon dioxide and give off oxygen during photosynthesis.

¨       The gases are exchanged through the air spaces in the spongy mesophyll and the exterior by opening their stomata.

¨       If stomata were kept open all the time, water loss would be too great and the plant would not survive.

¨       Plants keep their stomata open just enough to allow photosynthesis to take place but not so much that they lose excess water.

¨       Guard cells control the stomata by opening and closing in response to water pressure within the guard cells.

¨       Guard cells respond to conditions in the environment, such as wind and temperature, and maintain homeostasis within a leaf.

¨       Stomata are open during the daytime, when photosynthesis is active, and closed at night, when open stomata would lead to water loss.

¨       Stomata may even be closed during bright sunlight under hot, dry conditions in which water conservation is a matter of life and death.