Walking in the woods on a warm summer evening , I have observed
burdocks and other broad-leaved plants appearing to shrink into the
ground ,quite rapidly.I figure it is some sort of water conservation
or loss of turgor pressure . The phenomenon is quite startling ! Any
explanation ?

Hello, and thanks for the question. A long time ago, I got a degree in
plant ecology, so this is a bit of a "busman's holiday"

You were right, in a way, in thinking it's to do with turgor, but the
over-riding reason for the movement is the plant's natural circadian
rhythm.

Nyctinastic (NIK-tuh-NAS-tik) movements are plant movements in
response to the daily cycle of light and dark.

This movement is caused by the nightly loss of turgor pressure in
certain cells, which triggers a plant hormone to cause Hydrogen ions
to be pumped out of the cells at the base of leaflets and petioles.
Water then moves out of the cells by osmosis. As the cells get
"drier", the plant's leaves, and in some plants stems, move and, in
severe cases, even seem to shrink.

The effect can be magnified after hot dry days, when the plant's are
trying to conserve water as much as possible at a time when
transpiration ( the plant's loss of water through its leaves ) is very
high. . If cells are already under water stress, the effects of the
osmosis above will be more severe, causing the effect to be magnified.

Nyctinastic movements occur in many broad-leaved plants, with large
effects often seen in, among others, bean plants, honeylocust trees,
and silk trees.

(Some plants, like Mimosa and the Venus Fly Trap, use extremely fast
loss of turgor (by pumping pottasium ions -very- fast out of cells) to
respond to touch very quickly. This fast response is known as a
Thigmonastic movement.)

Most of the above came from my course notes from a series of Botany
lectures on plant movement from Glasgow University, but there’s a nice
run down of the different types of movement shown in plants here:
"Plant Movements"
( http://www.bisonscience.org/abbas/Reading/Chap33/plantmovements.htm
)

And scientists are still working on defining the exact mechanism by
which the plants "clock" operates

You can see the abstract of a recent article here:
Bioorganic Chemistry of Nyctinastic Leaf-movement using synthetic
probe compounds, Current Organic Chemistry, Volume 6, No. 6, 2002  Pp.
493-505
(http://www.bentham.org/coc/coc6-6%20.htm##link1 )


Hope that’s what you were after, but if you need any clarification,
just ask.

Willie-ga

Google search used
nyctinastic plant movement

Thanks..you know how some puzzles seem to "bug" you .I even asked a
biology graduate student the question on his wedding day ( in the
reception line ), but he was more into zoology and had never noticed
it .That day was exceptionally hot for an outdoor wedding and his
father had cut down all the burdocks.I will have to observe my honey
locusts and bean plants more closely.Thank you very much.