Plants have a number of factors that guide their growth. While the best known of these is phototropism, in which plants seek to grow towards the light, equally important is geotropism (AKA gravitropism), in which plants orient themselves using gravity. This allows buried seeds, roots, and rhizomes to grow sprouts up toward the surface, and extend roots downwards. Sprouts are strongly geotropic until they break free of the soil, at which point phototropism becomes the driving force; however, geotropism is active in all parts of the plant throughout its life.
There are a lot of rarely used technical terms around geotropism. Roots are positively geotropic, growing in the direction of gravity, while sprouts and stems are negatively geotropic. Rhizomes are generally diageotropic, growing horizontal to the pull of gravity, while branches are often plagiotropic, growing up at an angle1.
Geotropism is primarily due to the plant hormone auxin (also active in phototropism), which sinks in the plant's vascular system; the areas containing the most auxin grow the fastest, so in branches the side of the meristem most towards the ground grows faster, resulting in the tip of a new branch to point upwards.2 In roots growth is stimulated in the cells containing less auxin, causing the tip to move downwards. Roots are also more limited in their geotropic response, suddenly tapering off their response once the root reaches about a 40° angle from vertical.
However, there is another organelle that helps in geotropism; statoliths are small, dense amyloplasts3 that collect in the root cap, helping it to effectively redistribute auxin in response to sedimentation of the statoliths, vaguely akin to how the otoliths in our inner ear help us maintain balance. This process is not fully understood, but may possibly have developed to make up for the comparatively low levels of auxin in root tissue.
1. Note the spelling change from -geo- to -gio-. Plagiotropic is a general term that does not refer specifically to gravity, but simply growing obliquely to the direction of stimulus.
2. However, this process is usually superseded in stems by phototropism. Auxin is also attracted by shade; as the auxin collects in the shaded side of the stem, that side grows faster, pushing the tip of the stem toward the light.
3. Statoliths exist only to be small things pulled on by gravity; you can think of them as small marbles. In animals statoliths are made of calcium, but in plants they are made of starch.