Ryan Page, of King's High School, asks :-
When you lay a plant on its side why does it still grow up?
Matthew Turnbull, a plant physiologist at the University of Canterbury, responded.
Like many living things, plants respond to the direction of a number of environmental signals so that they can organise their activities efficiently. These growth responses are called tropic responses. It is important for the stems of plants to grow upwards (in the general direction of the sun's rays) and for roots to grow downwards (towards the ground!). Not surprisingly then, the two most important stimuli plants respond to are light and gravity.
You might think that the answer to your question is that plants are simply growing toward the light - but you would be wrong! While stems and leaves do use light as a signal to orient themselves in space, the stimulus for the response you have asked about is actually gravity! What makes the stem bend upwards is growth away from the direction of the force of gravity - a negative gravitropic response. One simple piece of evidence that light is not involved here is that you can get a plant to bend upwards in the dark. Although there is still much argument, it seems the way plants sense gravity is by the movement of starch granules within the cell. These granules move towards the source of gravity when the plant is tipped on its side, resulting in the increased growth of cells on the bottom surface of the horizontal stem - causing it to bend upwards.
Scientists have been interested in tropic response for well over 200 years and, despite much study, we remain baffled by the basic physiological mechanisms. With modern techniques progress is being made, some of it with the help of space programmes which enable biologists to study the effects of gravity (or lack of it) in orbiting spacecraft.