Re: how can a plant grow upside down?

Plants respond to gravity, termed gravitropism, so it would be unusual to grow 
them upside down. If you turned a potted plant 180 degrees, the shoot tips would 
start to bend upward again within a few hours. Parts of the stem that have 
already become woody could not bend but growing shoots could. The upside down 
plant would probably survive if the leaves received sufficient light. Lateral 
roots often grow roughly horizontally so being upside down would not be as much 
of a problem for roots. You would have to water the upside down pot from the 
bottom somehow. This would be an interesting experiment.

Scientists interested in growing plants in the space station and on space 
flights have sometimes used a pea plant, named ageotropum, that lacks the normal 
gravitropic response. The ageotropum pea roots do not respond to gravity at all 
while the shoots respond to gravity in the light but not in the dark.

Reference

Jaffe, M.J., Takahashi, H. and Biro, R.L. 1985. A pea mutant for the study of 
hydrotropism in roots. Science 230: 445-447.