Charles and Francis Darwin concluded from their experiments on ...

[xclash]

Charles and Francis Darwin concluded from their experiments on phototropism by grass seedlings that the part of the seedling that detects the direction of light is the
A) tip of the coleoptile.
B) part of the coleoptile that bends during the response.
C) base of the coleoptile.
D) cotyledon.
E) phytochrome in the leaves.

[clippers!]

I chose A and got the answer, hope that helps.