I've read that it isn't recommended to use hormones with this plant, but I'm curious to try some hormones in laboratories. Root, stem, leaves, any of them would be nice, but I'd rather start with stems.
You could try an equilibrated balance of auxin (indol-acetic acid) and cytokin (zeatin). These hormones will stimulate cell proliferation, without any cell differenciation (in equivalent proportion). You can also use gibberelins, which are commonly known as plant growth hormones. Gibberelin treatment will enhance the length and thickness of your plant material.
You should look in cell culture litterature to find the best concentrations for these substances.
Thanks Bertrand for your reply! I'm also planning to do micropropagation with those lovely plants, I'm on my way to search any applicable protocol and/or also try to use some similar protocols for the micropropagation. I might start this Thursday, it's so exciting! :)