Plants lay down because the stem isn't strong. It works like this - plants need wind to knock them around a little. Plants respond by strengthening the stem and will stand upright. without getting some excercise by wind, believe it or not, plants remain weak and will fall over. It's that simple. The wind must be on them from the time they are very young. Also if a plant's light is too far away, the stem will stretch, become weak, and the plant will also fall over.