It's a personification, a metaphor that gives human traits to non-human entities. Not everybody might be familiar with the absorption of water through osmosis. The process is made more understandable by comparing it with something that people know: drinking.

Then why do you water plants?

It's plural.

Plural weak inflexion (preceded by definite article Die), in the nominative. http://en.wikipedia.org/wiki/German_declension#Attributive_adjectives