Transport in Plants

From Tarzan of the Apes to George of the Jungle, in legions of comic strips and adventure movies, heroes have swung through the forest canopy on lianas—twining jungle vines. And when these heroes' exertions left them thirsty, they took a machete, chopped open the lianas, and drank the water found in the hollow stems. Lianas are a realistic source of water, in fact as well as fiction. Like all plants, these vines are continually moving water, along with dissolved solutes, from place to place in their bodies.

The water and minerals in a plant's xylem must be transported from the roots to the entire shoot system, all the way to the highest leaves and apical buds. Similarly, carbohydrates produced by photosynthesis in all the leaves, including the highest, must be translocated in the phloem to all the living nonphotosynthetic parts of the plant, such as roots, tubers, and internal stem tissues. Before we consider the mechanisms underlying these processes, we should have some idea about the magnitude of what they accomplish. Let's consider two questions: How much water is transported? And how high can water be transported?

In answer to the first question, consider the following example: A single maple tree 15 meters tall is estimated to have some 177,000 leaves, with a total leaf surface area of 675 square meters—half again the area of a basketball court. During a summer day, that tree loses 220 liters of water per hour to the atmosphere by evaporation from the leaves. To prevent wilting, the xylem needs to transport 220 liters of water from the roots to the leaves every hour. (By comparison, a 50-gallon drum holds 189 liters.)

The second question can be rephrased: How tall are the tallest trees? The tallest gym-nosperms, the coast redwoods—Sequoia semper-virens—exceed 110 meters in height, as do the tallest angiosperms, the Australian Eucalyptus regnans. Any successful explanation of water transport in the xylem must account for the transport of water to these great heights.

In this chapter, we will consider the uptake and transport of water and minerals by plants, the control of evaporative water loss from leaves, and the translocation of substances in the phloem.

Hollywood Vines Although the famous swinging-through-the-jungle scenes in the Tarzan movies of the 1940s were pure fiction and the vines the actors used were rope props, the use of lianas—heavy, twining vines of the tropical rain forests—as a source of drinking water is quite plausible.

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