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In the 18th century,

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Swedish botanist Carolus Linnaeus
designed the flower clock,

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a timepiece made of flowering plants

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that bloom and close 
at specific times of day.

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Linnaeus's plan wasn't perfect,
but the idea behind it was correct.

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Flowers can indeed sense time,
after a fashion.

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Mornings glories unfurl their petals
like clockwork in the early morning.

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A closing white water lily 
signals that it's late afternoon,

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and moon flowers, as the name suggests,
only bloom under the night sky.

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But what gives plants 
this innate sense of time?

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It's not just plants, in fact.

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Many organisms on Earth
have a seemingly inherent awareness

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of where they are in the day's cycle.

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That's because of circadian rhythms,

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the internal timekeepers
that tick away inside many living things.

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These biological clocks allow organisms
to keep track of time

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and pick up on environmental cues
that help them adapt.

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That's important, because the planet's
rotations and revolutions

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put us in a state of constant flux,

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although it plays out in a repetitive,
predictable way.

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Circadian rhythms incorporate various cues

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to regulate when an organism 
should wake and sleep,

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and perform certain activities.

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For plants, light and temperature
are the cues which trigger reactions

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that play out at a molecular scale.

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The cells in stems, leaves, and flowers
contain phytochromes,

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tiny molecules that detect light.

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When that happens, phytochromes
initiate a chain of chemical reactions,

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passing the message down 
into the cellular nuclei.

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There, transcription factors trigger
the manufacture of proteins

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required to carry out 
light-dependent processes,

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like photosynthesis.

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These phytochromes not only sense
the amount of light the plant receives,

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but can also detect tiny differences

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in the distribution of wavelengths 
the plant takes in.

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With this fine-tuned sensing,

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phytochromes allow the plant
to discern both time,

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the difference between 
the middle of the day and the evening,

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and place, whether 
it is in direct sunlight or shade,

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enabling the plant to match
its chemical reactions to its environment.

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This makes for early risers.

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A few hours before sunrise,
a typical plant is already active,

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creating mRNA templates 
for its photosynthesizing machinery.

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As the phytochromes 
detect increasing sunlight,

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the plant readies 
its light-capturing molecules

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so it can photosynthesize
and grow throughout the morning.

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After harvesting their morning light,

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plants use the rest of the day
to build long chains of energy

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in the form of glucose polymers, 
like starch.

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The sun sets, and the day's work is done,

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though a plant is anything 
but inactive at night.

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In the absence of sunlight,

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they metabolize and grow,

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breaking down the starch from 
the previous day's energy harvest.

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Many plants have seasonal rhythms as well.

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As spring melts the winter frost,

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phytochromes sense the longer days
and increasing light,

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and a currently unknown mechanism
detects the temperature change.

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These systems pass the news
throughout the plant

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and make it produce blooming flowers

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in preparation for the pollinators
brought out by warmer weather.

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Circadian rhythms act as a link
between a plant and its environment.

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These oscillations come 
from the plants themselves.

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Each one has a default rhythm.

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Even so, these clocks 
can adapt their oscillations

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to environmental changes and cues.

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On a planet that's in constant flux,

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it's the circadian rhythms that enable
a plant to stay true to its schedule

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and to keep its own time.