Aquatic fl owering plants form a relatively young plant group

on an evolutionary timescale. The group has developed over

the past 80 million years from terrestrial fl owering plants that

re-colonised the aquatic environment after 60-100 million

years on land. The exchange of species between terrestrial

and aquatic environments continues today and is very intensive

along stream banks. In this chapter we describe the physical

and chemical barriers to the exchange of plants between

land and water.