Which two organs of a plant interact to cause seedlings to grow upward

There's tropism and stimulus, for starters. Gravity, light, and touch can all be used as stimuli. So it is debatable... If the plant is reaching upwards towards a window that is above it... It would then be light, and the plant's response would be to reach upward.

Hope this helps and if it does, don't be afraid to give my answer a "Thanks" and maybe a Brainliest?  

b) the inside picture would look at a very high temperature in a very low ph