> If I have a cutting of a plant inside a cup of water for a long period, it
> starts to develop roots, but if I have the same cutting in a very wet soil,
> it starts to go rotten.
> Why this difference?
The water has its oxygen content constantly replenished from the surface
contact with air. The wet soil (mud) rapidly becomes anaerobic, since oxygen
diffuses into it very slowly and is used up by soil bacteria. Plant tissues,
roots and all, need oxygen to survive. Without oxygen they will die and rot.
Plants that are adapted to growing in swamps or underwater have modifications
that cause oxygen to be available to their roots. For example, many have air
channels from the top into the roots.
For more info on oxygen in submerged roots, there's an interesting article
on the Tropica web page, www.tropica.dk. Tropica is a company that specializes
in growing tropical aquatic plants. I don't have any connection with them.