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Chapter 9

Chapter 9

Minerals Absorbed Plants

Figure Q.14 A barrel cactus (Ferrocactus). The stems store, in the form of organic acids, carbon dioxide taken in at night; the carbon dioxide is then released for use in photosynthesis during the day.

leaf at 30°C (86°F), for example, is about twice as great as it is for the same leaf at 20°C (68°F). The various adaptive modifications of leaves and their surfaces and the availability of water to the roots also may play important roles in influencing the amount of water transpired. Leaf modifications are discussed in Chapter 7.

If a cool night follows a warm, humid day, water droplets may be produced through structures called hydathodes at the tips of veins of the leaves of some plants. This loss of water in liquid form is called guttation (Fig. 9.15). Minerals absorbed at night are pumped into the intercellular spaces surrounding the vessels and tracheids of the xylem. As a result, the water potential of the xylem elements is lowered, and water moves into them from the surrounding cells. In the absence of transpiration at night, the pressure in the xylem elements builds to the point of forcing liquid water out of the hydathodes in the leaves. Although the droplets resemble dew, the two should not be confused. Dew is water that is condensed from the air, while guttation water is literally forced out of the plant by root pressure. As the sun strikes the droplets in the morning, they dry up, leaving a residue of salts and organic substances, one of which is used in the manufacture of commercial flavor enhancers (e.g., the monosodium glutamate in products such as Accent). In the tropics, the amount of water produced by guttation can be considerable. In taro plants, used by the Polynesians to make poi, a single leaf may overnight produce as much as a cupful (about 240 milliliters) of water through guttation.

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