what is the function  of the white columns of the spinal cord. ascending and descending tracts.

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White matter in general is bundles of axons of neurons (parts that send/carry signals away from the nerve cell), each axon wrapped by a special kind of fat called myelin.  This myelin acts as an insulation that greatly speeds up the speed of the signal.  Unmyelinated axons send signals at speeds of around 1 m/sec (other factors can change this), while myelinated axons can sometimes reach speeds upwards of 100 m/sec.So, the "white columns" (parallel bundles of myelinated axons making up part of the outer portions of the spinal cord) are essentially high-speed communication lines.  Ascending tracts would be for neurons sending signals up to the brain, while descending tracts would be carrying signals down from the brain to the body.