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.
