Spontaneous and rapid growth of tubes occurs from crystals of a polyoxometalate upon addition of an aqueous solution containing an organic cation. Application of a potential through a multietectrode system allows the growth direction of tubes through 90 degrees or 180 degrees and direct increase or decrease in the cation concentration reliably alters the outer diameter of the growing tube. These robust, self-growing, micrometre-scale tubes offer application opportunities in the growth of microfluidic devices and the self-assembly of metal oxide based semipermeable membranes for diverse applications.