A tropism is a growth movement whose direction is determined by the direction from which the stimulus strikes the plant.
Stems are positively phototropic.
Roots are negatively phototropic. Stems are negatively gravitropic while roots are positively gravitropic. The adaptive value of these tropisms is clear. Stems growing up and toward the light will be able to expose their leaves so that photosynthesis can occur. The Darwins used grass seedlings for some of their experiments.