Are you talking about American waterweed (Elodea canadensis)? If so you want to plant the the bottom of where you trimmed it into the pot. The top is the part where the very tip of the plant has leaves growing on it.
Heres a pic
In your pots i suggest the botom layer to be gravel, 1/4 the way up, the next layer to be filter wool again 1/4 the way up the the rest of the way up with gravel again, or sand. If gravel it must be under 1-2mm in diamater. What do you mean cutit in half, you mean seperate or actualy slice it?. If your seperating it i see no problem but there is not a good succes rate, about 70%, jsut cut the tip of and plant that (about 5-10cm).
Stipe as per the copyright laws and the TOS for the site, I deleted the picture. You either need to post a link to the person's picture or have thier permission to post the picture.
Ed
(Message edited by Ed on February 15, 2006)