Black sand????

[biocupid]

There is an experiment which is carried out to test the effect of the auxins on the phototropism...i was curious why the stem of the plant is immersed into a beaker half containing fine black sand and a light bulb is set up at the right hand side of the room....pls explain...i am dying for the answer

[MrMistery]

probably to keep that region of the stem from being hit by light