The tip of the shoot is the receptor. The effector responds the stimulus detected by the receptor. They communicate by means of plant hormones.
Auxin is produced constantly by the cells at the tip of the shoot. It diffuses downwards, and makes cells below the tip increase in length. It is needed for plant growth.
Auxin concentrates on the shady side of a plant.
You use agar jelly for partial permeability and mica for substances to not be able to diffuse through it all when investigating phototropism.
Radicle will always grow downwards regardless of whicever way up a seed is planted.
