Re: Do plant roots always grow downward?

No, roots do not always grow downward. Many lateral roots grow more nearly
horizontal. Even the main root may be forced to grow sideways if it encounters a
hard layer of soil or a rock. A few specialized roots may actually grow upward,
such as the knees of bald cypress and the pneumatophores of mangroves.

Roots have a mechanism, termed gravitropism to sense the direction of gravity.
Geotropism is an obsolete term for gravitropism but is still in use.
Gravitropism allows roots to orient themselves in the proper direction, The
first root that emerges from a seed, the radicle, usually grows toward gravity,
making it positively gravitropic. A seed planted upside down will send its
radicle downward. 

Search the madsci archives for several other answers on gravitropism and
geotropism. Google.com also has many websites on gravitropism.

References


Gravitropism


Roots