Transpiration Organs

The stomata are bounded and controlled by two kidney-shaped cells called guard cells. Water evaporates from the leaf cells and goes into the air through these ventilation sites by a process referred to as transpiration. Normally, during lighthours, the stomata are open (and losing water). At nighttime, the cells close, and water is retained. To open, potassium ions (K+) are pumped into the guard cells, and water follows by osmosis, which causes an increase in internal pressure, called turgor pressure.Aspres-sure increases, the water pushes against and stretches the guard-cell walls, bowing the cells outward. The filling and stretching of the guard cells opens the stoma.

Stomata close when water stress (lack of water in the plant) occurs, which can result from insufficient water in the soil or excessive transpiration rates. The most likely physiological mechanism for stomatal closing involves the hormone abscisic acid (ABA). The effects of water stress seem directly to trigger the release of ABA. The exact mechanism is unclear, but in some way ABA causes K+ ions to move out of the guard cells and, again, water pas

Pictures Monocotyledonous Plants

Monocotyledon leaf

Parts of a Leaf

Margin

Leaf blade (lamina)

Leaf bud

Margin

Leaf blade (lamina)

Leaf Blade

Lateral vein

Leaf bud

Midrib

Sublateral vein

Dicotyledon leaf

Midrib

Lateral vein

Sublateral vein

Monocotyledon leaf

Dicotyledon leaf

Cross-Section of Leaf Tissue

Upper epidermis Palisade mesophyll Vein j— Spongy mesophyll Lower epidermis

Upper epidermis Palisade mesophyll Vein j— Spongy mesophyll Lower epidermis

Lower Epidermis Leaf

sively follows. When the guard cells lose their water, they become limp and close, sealing the stomatal opening, thus greatly reducing transpiration.

Transpiration of water at the leaf surface may be affected by several factors. Wind blowing across the surface carries off water molecules, leaving room for more water molecules to take their place; an increase in temperature does the same thing. Loss of water may be slowed by opposite conditions. In rainy or foggy conditions when the air is already saturated with water, water loss from leaves is lower. Water loss also occurs slowly in cool conditions, such as those prevailing at night. An average-sized birch tree will typically lose 17,260 liters of water through transpiration in a single growing season. One acre of grass lawn may lose 102,200 liters of water in a single week.

Water transpired into the air can affect rainfall patterns: It is likely that 50 percent of rainfall in the Brazilian rain forest originates from transpired water. Plants that grow in arid conditions have developed specialized leaves to decrease the amount of water lost by transpiration. Many of these plants have leaves that are small and thick, so that surface area is reduced. The stomata may be housed in deep pits, away from wind's evaporative force. During especially dry periods, some plants even shed their leaves to reduce water loss. Others carry on an alternate form of photosynthesis that allows the stomata to remain closed during all or part of the day.

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  • deodato
    How do marine plants transpire?
    7 years ago
  • Lodovico
    Do marine plants have stomata?
    6 years ago
  • niklas
    What is organ transpiration?
    3 years ago

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