Plants carry out a number of processes that are essential to their survival. Internal water and sugar transport are largely carried out within the vascular system. The upward flow of water through the xylem is "pulled" by transpiration, while the flow of organic nutrients (sugars) through the phloem is "pushed" by turgor pressure from sources to sinks. These processes ensure that the entire plant receives water and food even though these materials are brought in or produced only in certain parts of the plant-- the roots and leaves, respectively.
Plant hormones determine the timing and occurrence of many of the processes of the plant, from germination to tissue growth to reproduction. These hormones are divided into five classes, each with its own function within the plant. The classes are composed of auxins, cytokinins, gibberellins, inhibitors, and ethylene. Auxins play a role in phototropism, the growth of a plant toward a light source. This growth, described by the acid growth hypothesis, results from the rapid elongation of cells on the dark side of the plant.
Plants respond to stimuli in other ways as well. In thigmotropism, parts of a plant respond to touch by thickening or coiling. In gravitotropism, roots and shoots grow up or down depending on their orientation with respect to gravity. Turgor movements, accomplished by rapid changes of the turgor pressure in selected cells, allow the plant to move quickly and reversibly in response to stimuli. And with photoperiodism, many plants respond to seasonal daylight shifts by timing their flowering according to the length of nighttime.
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