Elodea rooting

I have some lovely stems of Elodea growing. (Olga you should get some from
me) It grows like crazy with lotsa light. The trouble is, it doesn't
seem to want to take root. The tank I have it in has only gravel for
substrate. I should put a bit in the soil/vermiculite/gravel tank and
see but lacked energy due to a recent flu bout.

Does Elodea need a soil substrate to root properly? 

Will it be fine growing as it is, without roots?

Steve     in Vancouver  with       rain          again. -sigh-