Here is a quote from Dune (book 1):
The man carried Paul’s banner on its staff – the green and black banner with a water tube in the staff – that already was a legend in the land. Half pridefully, Paul thought: I cannot do the simplest thing without its becoming a legend. They will mark how I parted from Chani, how I greet Stilgar – every move I make this day. Live or die, it is a legend. I must not die. Then it will be only legend and nothing to stop the jihad.
Could anyone explain to me why there is a water tube in the staff for Paul's banner? What is it for?