Petra,
I don’t see mentioned above is to select the curve and do Edit>Control Points>Control Points On.
That will show the complexity of your curves. In general, Rhino works better when your curves are less complex. You can see here that some of your curves are very dense relative to others, even where the shape is similar.
You might want to try the Rebuild or RebuildCrvNonUniform commands on them to simplify.
One of the things you do not indicate is how flexible you are with your shapes. How precisely do they need to match what you have?
Some of your leaves come to points while others have curves at the end. If you can you might want to try bringing them all to a point.
Another alternative is to use the Fillet command to use arcs to round your curves.
Another command you might find useful is FAIR in conjunction with Analyze>Curvature>Curvature Graph On. The latter will give a graphical indication of changes in your curves. You can use the FAIR command to smooth waves out of your curves. The curvature graph will show how your progressing with that.
If you FAIR the curves then rebuild you can get a simpler curve than a rebuild without a FAIR.
Here you have a curve that waves (that may be what you want but if not . . . . )
After fair:
