With the decrease of the diurnal period, the drop in temperature and the increase in humidity, we observe the fall of the first leaves, we know that the autumnal equinox is coming.
What are equinoxes?
The word equinox is derived from Latin (aequinoctiu = equal night; aequale = equal + night = night). Indicates the instant when, at noon, in a location on the equator, there is a point where the sun's rays have an incidence perpendicular to their tangent. At this point both hemispheres are lit equally, which means that the day and night periods are of the same duration. On the 23rd of September we will have the opportunity to witness this phenomenon.
But why is it at this time of year that leaves fall?
Just as many animals hibernate in cold periods to ensure their survival in periods when there is less food available and extreme climatic conditions, terrestrial plants (lower and higher) also have their mechanisms, which ensure the flora biodiversity that Man has. available.
With the decrease in light, the leaves fall and the plants enter a period of dormancy, which ends another cycle. This is a defense mechanism and energy saving so that in spring, when all the ideal conditions are met, we can be presented with the spectacle that we all enjoy, the sprouting of the first buds and the blooming of the first flowers, forming carpets of indescribable color and an unparalleled beauty.
Why do only some plants lose their leaves?
Plants that lose their leaves, called deciduous plants, such as oak, sycamore, apple, vine, etc., do so deliberately to protect themselves from the cold and snow of winter and periods of less light. This way the leaves are not burned, the distribution of water and necessary nutrients is not compromised.
Pine and fir are examples of plants that do not lose their leaves, they are called perennials or evergreen plants. 
How is leaf fall determined?
The loss of leaves is gradual, no plant loses all its leaves at the same time, it works almost like a faucet that turns off and lets out less and less water until it stops completely. A scar tissue is formed (a scar) that gradually interrupts the passage of water and mineral nutrients from the stem to the leaf, the petiole ("leaf foot") begins to dry and with the help of the wind, or just the force of the gravity, the sheath (base of the petiole) detaches from the branch and the leaf falls off.
Why do the leaves show different colors before falling off?
The colors of the leaves and flowers of plants are determined by substances (pigments) present in their biochemical composition, which absorb certain bands of visible light and reflect the rest. The color we see is reflected light, which has a complementary color to that absorbed by the plant.
To capture light, plants have a green pigment called chlorophyll, stored in chloroplasts and present in greater amounts in the leaves. When this pigment is no longer present, we observe other pigments “hidden” behind the green (yellow, orange, red) that allow us to observe the diversity of colors of the leaves before they fall.
In autumn, the leaves fall and the arrival of spring is prepared with renewed energy.
Text from: scallop daisy
Biologist
Science in the Regional Press – Ciência Viva
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