Light micrograph of a seed grain of Setaria pumila, Yellow Bristle-grass. Yellow Bristle-grass is an upright annual growing 25 to 45 cm high, sometimes taller. The seedhead is a cylindrical spike, 2.5 to 10 cm long. It consists of many densely packed spikelets, with each spikelet bearing a single seed. Microscopic contrast technique : Incident lighting. Magnification 30x when printed 10 centimetres wide.