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Questions and answers provided by Science Line's Dial-a-Scientist on 0345 600444

Q What tells trees to shed their leaves for winter?

A In many trees leaf fall seems to be brought about by shorter days and falling temperatures toward the end of the growing season. Chlorophyll, the green pigment in plants, is lost; yellow and orange pigments called carotenoids become more conspicuous and, in some species, anthocyanin pigments accumulate. These changes cause the autumn colours of leaves.

There are some indications that day length triggers leaf loss in deciduous trees: some researchers think the lack of daylight breaks down a couple of plant hormones which are important in controlling leaf fall. Artificially increasing the level of two hormones, called gibberellins and auxins, in the autumn can halt leaf fall and preserve the leaves' greenness.

Q The sea is made of rain, the rain is made from the sea; so where did the first raindrop come from?

A When Earth first formed 4.6 billion years ago, much of its atmosphere was derived from volcanic out-gassings containing water vapour. This cooled and condensed to form drops of water - which would have collected in basins on the land, creating large lakes and oceans. So rain came before seas.

Q Why is the sea salty?

A The early oceans were probably not salty. Erosion of local rocks, by rivers which flowed into them, provided the salts. All fresh water in rivers contains traces of salts and minerals, but these get concentrated in the oceans because they are left behind when water evaporates (to form clouds). This water falls as rain which eventually washes more minerals into the sea. But if you measure the sea's salt content now, and work out how salty it should have been after billions of years of minerals being washed into it, it is much less salty than expected. Someone tried to calculate the age of the Earth from how salty the seas were and got a figure close to 5,000 years - so something else is happening.

Q How many teeth does a great white shark have?

A The front teeth of sharks are constantly replaced by rows of teeth growing behind them. The adult great white shark has an average of 30- 34 teeth in its front row. Each tooth has a new one ready to swing into place, so the actual number of teeth in the shark's mouth is 60-68, of which only half are in use.

You can visit the technoquest World Wide Web site at http://www.campus.bt.com/ CampusWorld/pub/ScienceNet. Questions for this column can be submitted by e-mail to ex17@cityscape.co.uk

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