Guttating oat grass
A closeup of oat grass (tack oats; Avena sativa) early one morning showing guttation. The water droplets at the tips of a few of the leaves are caused by xylem fluid being pushed up and out of the leaves due to root pressure.
This guttation was not a forced demonstration; I found it occurring naturally one cool, humid morning. I like how it's more subtle than most demonstrations.