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On Transpiration
IN THE TRANSPIRATION CYCLE, plants move water from soil through roots and stems to leaves and into the surrounding air. The cycle begins when water enters roots in a process called osmosis. Osmosis is the push that water gives as it seeks to make plant roots as full of water as the damp soil around them. Water enters roots through permeable cell membranes. Simple Summary Plants take in water from soil through their roots. This water moves up through roots, into stems and leaves. At the same time, heat from the sun evaporates water, pulling it through stems and up into the leaves at the top of the plant. Water vapor escapes from plants into the air through tiny holes on the undersides of leaves.When a root's epidermis is holding as much water as it can, this water slowly filters its way between cells towards an inner layer of dermal cells (the endodermis), where osmosis works again to transport water through cell membranes and into the xylem at the root's core. Xylem are the little pipes that carry water from roots through stems to leaves. Osmosis through epidermal and endodermal cells is called root pressure. In low growing plants, root pressure is strong enough to move water into stems and all the way to leaves. Taller plants require a second source of motion to bring water to their leaves. This is supplied by transpirational pull. By means of osmosis, water moves from xylem into leaf cells. When these cells become saturated, they release water into leaves' internal spaces, where it is vaporized by heat. This water vapor is then released through the stomata, tiny pores on the undersides of leaves, which open to allow carbon dioxide to enter leaves for photosynthesis. Water vapor that has exited through the stomata is replaced by more water, pulled by the mesophyll cells from the xylem. Because water travels in long chains of connected molecules, this pull is felt throughout the plant, all the way to the roots. THE WATER CYCLE: Together, root push and transpirational pull are strong enough to move significant amounts of water from soil to leaf surface, where it becomes available for evaporation and precipitation (rain and snow), which returns it to soil. In addition to water, transpiration also moves nutrients from soil and roots to stems and leaves and helps moderate plant temperatures. On a dry, sunny day, a typical, 50-foot deciduous tree may transpire as much as 50 gallons of water every hour, cooling itself and the surrounding air. Conversely, a cactus will transpire hardly any water, having adapted to its desert climate by minimizing moisture loss through tanspirational pull. Sources Brian Capon, Botany for Gardeners: An Introduction and Guide. Timber Press, Inc: Portland, Oregon, 1990 |
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