¨ 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
¨ The leaf is a system specialized for photosynthesis.
¨ Subsystems include tissues that bring gases, water, and nutrients to the
cells that carry out 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 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.
¨ 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
¨ Leaves take in carbon dioxide and give off oxygen during
¨ 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.