Ask mr Brain...all will be explained

PUBLISHED : Saturday, 24 July, 1999, 12:00am
UPDATED : Saturday, 24 July, 1999, 12:00am

Why do leaves change colour in autumn? There are scientific explanations as to why leaves change colour but there are also many myths and folk tales surrounding this phenomenon. Each year, hundreds of Americans go to New Hampshire just to admire the beauty of the autumn foliage.

According to Native Americans, in autumn, the Great Bear constellation (the Plough) was killed and its blood dripped on to some leaves, making them red. While its meat was cooking, fat dripped down and coloured other leaves yellow.

Even today, scientists are not 100 per cent sure what causes leaves to change colour. The primary factor is the photoperiod - the amount of sunlight a region receives during the course of a year.

As autumn comes, the days become shorter. The decrease in sunlight intensity, shorter hours and cooler temperatures reduce the growth and food-making processes that normally flourish during spring and summer.

Plants rely on sunlight for energy and nutrients. Leaves contain chlorophyll, which absorbs energy from sunlight to make sugar and carbohydrates through photosynthesis. Several weeks before leaves change colour, a hardened cell layer forms at the base of the leaf due to decreased photosynthesis. This layer blocks nutrients and moisture flow to the leaf, inhibiting further production of chlorophyll.

The chlorophyll begins to break down as the tree absorbs nutrients into the trunk and eventually the roots. Since chlorophyll reflects green light waves, the green colour starts to diminish. The yellow/orange pigments, which have existed in the leaf since spring but were not visible because of the large amount of chlorophyll, now become apparent.

Some leaves have more vibrant colours because they had an abundant amount of rainfall and sunlight during the summer. Also, the red, orange, yellow and gold colours of a tree depend on the acidity of plant cell fluids. It is also possible that leaves directly exposed to the sun may turn red, while leaves in the shade turn yellow. Some trees change colour before others because they are unhealthy. If a tree's bark is ruptured or it is situated close to roads where it is exposed to harmful exhaust fumes, its leaves will drop quicker.

Why is Venus' atmosphere made of gases? Venus' atmosphere is created from the elements found deep inside it. Volcanoes on the planet give out lots of gases during eruptions. These gases build up in the atmosphere and are altered by the sun. Venus' atmosphere is about 96 per cent carbon dioxide. In 1972, scientists discovered that the clouds over Venus were made of droplets of sulphuric acid.

Which is the biggest museum? The Smithsonian Institution in Washington DC, is the biggest one. But it is really a lot of different museums all under the name Smithsonian. It includes the American History, the National Portrait Gallery and Museum of African Art.