Humans and animals move through the interaction of nerves, muscles and tendons. We perceive changes in light or temperature through our eyes and skin. But how does this work with plants?

Life on earth has to deal with a wide variety of circumstances: light, dark, warm, cold, damp, dry... some conditions change regularly in the course of the day-night rhythm. To adapt to these regular changes, plants - like humans and animals - have developed an internal clock. This "circadian clock" runs at the molecular level in all plant cells and controls the 24-hour cycle. Even when a plant is placed in the dark, its day-night rhythm continues for several days: the clock continues to "tick".

A. The circadian clock allows plants to anticipate the alternation of day and night. B. Plant movements (often in response to day/night alternance), as seen here in the work of Charles Darwin (1880). C. The circadian clock involves complex cellular regulations.

How do plants tell the time?

Plants control the day-night rhythm through a complex system of regulators that maintain a 24-hour cycle. The regulators are proteins that are synchronised by environmental factors such as the light and temperature. The first regulator is activated in the morning and as its concentration increases, it begins to activate the "afternoon" regulator. This, in turn, inhibits the first regulator… and everything starts all over again in the morning. An entire field of biology, chronobiology, is devoted to understanding this circadian clock. Plants that behave according to a circadian rhythm that is adapted to the environment bind more carbon, grow faster and have better chances of survival than plants with a deviating this rhythm.

How do plants move without muscles?

There are different mechanisms how plants move their petals without any muscles at all. For example, they can regulate the fluid in their cells, causing them to expand or contract. Sometimes growth movements are also involved and in some flowers the difference in moisture between the inside and the outside is enough to move the petals. The speed at which flowers open varies from species to species. The flowers of the evening primrose (Oenothera spp.) take about 20 minutes to open, those of the ivy (Hedera helix) only 5.