On Mon, 19 Apr 2004 12:04:11 +0100, Andy Hunt wrote:

So how do plants know which way is 'up', then, when they are germinating in
the dark . . . ? Does this not imply some sort of perceptive and
decision-making capability, albeit on a fairly mundane level?

Gravity, mostly.

Imagine a white T-shirt you've just soaked in a colourful dye. Hang the
shirt up and the dye will run to the bottom creating a gradient. in the
same way, there will be a gradient of various hormones throughout the plant
from tip to root, from leaf tip tip stem etc.

To (over) generalise a bit, the hormones responsible (a group called auxins
IIRC) for root growth also sink to the bottom of the plant, through the
cells. Thus roots grow at the bottom (where there is more root-growth
hormone), and shoots grow a the top (where there is less root-growth
hormone....).

You can show this by cutting small cubes of (certain) plant tissue and
whichever way you put them down they will grow roots at the bottom. Turn
them round, then they'll start growing roots from the new bottom surface.
As roots tend to grow first, it's possible in fact to create a cube of
plant tissue with roots growing out of all six sides, if you're bored.


--
Tim C.