Can plants calm stressed teenagers? The Plants & Flowers Foundation Holland staged indoor gardens in exam gyms to find out, and to reframe what plants are for. Picture the typical exam venue: a gymnasium emptied of everything but rows of desks, the squeak of sneakers replaced by the scratch of pens, fluorescent light flattening the room. This spring, the Plants & Flowers Foundation Holland set out to disrupt that sterile ritual. During the Dutch national secondary-school exams, it filled the gym halls of four schools with carefully chosen houseplants, turning the most pressure-laden rooms of the academic year into something closer to a conservatory. The selection wasn't random greenery for atmosphere. Working with creative agency Gardeners , the foundation chose species it associates with calm and concentration, then gave them a visible place in the hall.…