KnowHow Team explains: Leaves are nature’s food factories. They use sunlight to turn water and carbon dioxide into glucose, which is used as food for energy and as a building block for growth. The process by which the food is prepared is called photosynthesis. The pigment chlorophyll helps in trapping the sunlight and also imparts the green colour to the leaves.

As autumn sets in, days become shorter and this is how trees “know” that winter is approaching. During winter, there is not enough light or water for photosynthesis. The trees live off the food they had stored during the summer. They thus begin to shut down their food-making factories. The green chlorophyll disappears from the leaves, giving way to yellow and orange hues. Small amounts of these colours have been in the leaves all along; we just couldn’t see them because of the green mask.

The plants we’re talking about here are called deciduous trees. These trees can “sense” that nights are getting longer as autumn approaches. The word deciduous comes from the Latin words meaning “to fall or cut”. The phenomenon wherein plants “decide” to lose their leaves by sensing the length of nights, is called photoperiodism (from the Greek words for a light/time system).

Typically broad, thin and without any thick covering, deciduous leaves are vulnerable to damage. The fluid in the cells is usually a thin, watery sap that freezes readily. This means that the cells cannot survive winter where temperatures fall below freezing. Tissues unable to overcome winter must therefore be sealed off and shed to ensure the plant’s continued survival.

The question was sent by Satish Kumar via email