I dont think you need to know that, but ill try,
Triodes use NPN or PNP Semiconductors, using this only allows flow of current in one direction. Since the middle element [NPN - the P is middle] is only direction the current can flow out of, if that is connected to a circuit, the electrons will literally be PULLED through from either N type, getting rid of the depletion zone, and hence, because the electrons are being forced and pulled through the Middle elemeent, the signal is amiplified..
Its in your book...