help in decreasing width along curve

hi guys i have been trying to recreate the venation of a branching system, i already have the curves of this system thanks to hoopsnake and the logic follow in these video  http://vimeo.com/25604611  , now the problem is when wanting to have a varible width along these curves, right now i can control the width of the pipe component in a relation with the length of the curve, but these does not respet the logic behind any venation system since it do not follow the path of the growing system, what i want is that in the beginning of the branching system the curves start with certain width and while growing this width need to be decreasing.

i am also trying to make some surface from the closest lines to create some kind of spacial containers i already got some surfaces from this branching but it seems to messy, i would like it to be more logic but my head is already smoking so any sugestions are totally welcome
here is attached my gh definition and the rhino file