1
00:00:06,960 --> 00:00:08,411
In the 18th Century,

2
00:00:08,411 --> 00:00:11,228
Swedish botonist Carolus Linnaeus
designed the flower clock,

3
00:00:11,228 --> 00:00:13,126
a timepiece made of flowering plants

4
99:59:59,999 --> 99:59:59,999
that bloom and close 
at specific times of day.

5
99:59:59,999 --> 99:59:59,999
Linnaeus' plan wasn't perfect,
but the idea behind it was correct.

6
99:59:59,999 --> 99:59:59,999
Flowers can indeed sense time,
after a fashion.

7
99:59:59,999 --> 99:59:59,999
Mornings glories unfurl their petals
like clockwork in the early morning.

8
99:59:59,999 --> 99:59:59,999
A closing white water lily 
signals that it's late afternoon,

9
99:59:59,999 --> 99:59:59,999
and moon flowers, as the name suggests,
only bloom under the night sky.

10
99:59:59,999 --> 99:59:59,999
But what gives plants 
this innate sense of time?

11
99:59:59,999 --> 99:59:59,999
It's not just plants, in fact.

12
99:59:59,999 --> 99:59:59,999
Many organisms on Earth
have a seemingly inherent awareness

13
99:59:59,999 --> 99:59:59,999
of where they are in the day's cycle.

14
99:59:59,999 --> 99:59:59,999
That's because of circadian rhythms,

15
99:59:59,999 --> 99:59:59,999
the internal timekeepers
that tick away inside many living things.

16
99:59:59,999 --> 99:59:59,999
These biological clocks allow organisms
to keep track of time

17
99:59:59,999 --> 99:59:59,999
and pick up on environmental cues
that help them adapt.

18
99:59:59,999 --> 99:59:59,999
That's important, because the planet's
rotations and revolutions

19
99:59:59,999 --> 99:59:59,999
put us in a state of constant flux,

20
99:59:59,999 --> 99:59:59,999
although it plays out in a repetitive,
predictable way.

21
99:59:59,999 --> 99:59:59,999
Circadian rhythms incorporate various cues

22
99:59:59,999 --> 99:59:59,999
to regulate when an organism 
should wake and sleep,

23
99:59:59,999 --> 99:59:59,999
and perform certain activities.

24
99:59:59,999 --> 99:59:59,999
For plants, light and temperature
are the cues which trigger reactions

25
99:59:59,999 --> 99:59:59,999
that play out at a molecular scale.

26
99:59:59,999 --> 99:59:59,999
The cells in stems, leaves, and flowers
contain phytochromes,

27
99:59:59,999 --> 99:59:59,999
tiny molecules that detect light.

28
99:59:59,999 --> 99:59:59,999
When that happens, phytochromes
initiate a chain of chemical reactions,

29
99:59:59,999 --> 99:59:59,999
passing the message down 
into the cellular nuclei.

30
99:59:59,999 --> 99:59:59,999
There, transcription factors trigger
the manufacture of proteins

31
99:59:59,999 --> 99:59:59,999
required to carry out 
light-dependent processes,

32
99:59:59,999 --> 99:59:59,999
like photosynthesis.

33
99:59:59,999 --> 99:59:59,999
These phytochromes not only sense
the amount of light the plant receives,

34
99:59:59,999 --> 99:59:59,999
but can also detect tiny differences

35
99:59:59,999 --> 99:59:59,999
in the distribution of wavelengths 
the plant takes in.

36
99:59:59,999 --> 99:59:59,999
With this fine tuned sensing,

37
99:59:59,999 --> 99:59:59,999
phytochromes allow the plant
to discern both time,

38
99:59:59,999 --> 99:59:59,999
the difference between 
the middle of the day and the evening,

39
99:59:59,999 --> 99:59:59,999
and place, whether 
it is in direct sunlight or shade,

40
99:59:59,999 --> 99:59:59,999
enabling the plant to match
its chemical reactions to its environment.

41
99:59:59,999 --> 99:59:59,999
This makes for early risers.

42
99:59:59,999 --> 99:59:59,999
A few hours before sunrise,
a typical plant is already active,

43
99:59:59,999 --> 99:59:59,999
creating MRNA templates 
for its photosynthesizing machinery.

44
99:59:59,999 --> 99:59:59,999
As the phytochromes 
detect increasing sunlight,

45
99:59:59,999 --> 99:59:59,999
the plant readies 
its light-capturing molecules

46
99:59:59,999 --> 99:59:59,999
so it can photosynthesize
and grow throughout the morning.

47
99:59:59,999 --> 99:59:59,999
After harvesting their morning light,

48
99:59:59,999 --> 99:59:59,999
plants use the rest of the day
to build long chains of energy

49
99:59:59,999 --> 99:59:59,999
in the form of glucose polymers, 
like starch.

50
99:59:59,999 --> 99:59:59,999
The sun sets, and the day's work is done,

51
99:59:59,999 --> 99:59:59,999
though a plant is anything 
but inactive at night.

52
99:59:59,999 --> 99:59:59,999
In the absence of sunlight,

53
99:59:59,999 --> 99:59:59,999
they metabolize and grow,

54
99:59:59,999 --> 99:59:59,999
breaking down the starch from 
the previous day's energy harvest.

55
99:59:59,999 --> 99:59:59,999
Many plants have seasonal rhythms as well.

56
99:59:59,999 --> 99:59:59,999
As spring melts the winter frost,

57
99:59:59,999 --> 99:59:59,999
phytochromes sense the longer days
and increasing light,

58
99:59:59,999 --> 99:59:59,999
and a currently unknown mechanism
detects the temperature change.

59
99:59:59,999 --> 99:59:59,999
These systems pass the news
throughout the plant

60
99:59:59,999 --> 99:59:59,999
and make it produce blooming flowers

61
99:59:59,999 --> 99:59:59,999
in preparation for the pollinators
brought out by warmer weather.

62
99:59:59,999 --> 99:59:59,999
Circadian rhythms act as link
between a plant and its environment.

63
99:59:59,999 --> 99:59:59,999
These oscillations come 
from the plants themselves.

64
99:59:59,999 --> 99:59:59,999
Each one has a default rhythm.

65
99:59:59,999 --> 99:59:59,999
Even so, these clocks 
can adapt their oscillations

66
99:59:59,999 --> 99:59:59,999
to environmental changes and cues.

67
99:59:59,999 --> 99:59:59,999
On a plant that's in constant flux,

68
99:59:59,999 --> 99:59:59,999
it's the circadian rhythms that enable
a plant to stay true to its schedule

69
99:59:59,999 --> 99:59:59,999
and to keep its own time.