We have grown vertically-aligned carbon nanotubes on (100) silicon substrates by means of a plasma enhanced chemical vapor deposition method. The growth of CNTs is achieved by a mixture of hydrogen and acetylene gases in a CVD reactor and a 2-5nm thick nickel is used as the seed for the growth. Following the growth of nanotubes on the silicon substrates, they are covered by a titanium-oxide layer and then the substrate is placed back into the original chamber to expose to a hydrogen plasma. Depending on the hydrogenation step, the nickel seed layer, which is placed on the tip side of the original nanotube, is expanded. The subsequent process in the same reactor leads to the growth of carbon nanotubes in a branched manner. Scanning electron microscopy has been used to investigate the results of such tree-like nanostructures.