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Linnaeus's Three Groups of Flowers:

Meteorici:

Flowers which change their opening and closing times according to weather conditions.

Tropici:

Flowers which change their times for opening and closing with the length of the day.

Aequinoctales:

Flowers which have fixed times for opening and closing.

Only Aequinoctales are suitable if you want to use flowers to see what time it is.

A detailed account of this in English can be found in F. W. Oliver's translation of Anton Kerner's "The Natural History of Plants" (1895).
   
How To Make Your Own Flower Clock:

First, you'll need to select some flowers that open and close at different times.

Try to find plants that grow well in your area, ones that flower at the same time of year.

Make a small circle (about a foot in diameter) in some outdoor soil.

Plant the flowers in order around the outside of the circle so you can read them like a clock's face. 

Divide the circle into 12 sections to represent each hour of the day.

When they bloom, you'll have your own flower clock.

Be forewarned that the plant timing varies in accordance to biological factors such as light, air, soil, season, temperature, and latitude.

Also, replanting will have to be done from time to time due to life cycle of the plants that are chosen. 

Plants can be sensitive--so don't put them under stress.

How to Make a Flower Clock

Keeping track of the passing of time as well as the seasons has existed for thousands of years. The Chinese, Japanese, Romans, Egyptians, Aztec, Hopi and Navajo Indians, and many others developed various forms of calendars. The one thing that was common among these calendars was their close tie with nature.

According to Chinese lore, two trees grew at the Emperor's Court. One tree produced a new leaf every day for fifteen days as the moon waxed, and then it shed one leaf every day for fifteen days, as the moon waned. In this way the months were measured. On the other side of the garden was a tree that produced leaves every month for six months, and then shed leaves every month for six months. And so, the passage of years were counted.

In the 4th century B.C., Alexander the Great's scribe, Androsthenes, noted that the leaves of certain trees opened during the day and closed at night.

In England during the 18th century Tragopogon partensis (Jack-go-to-bed-at noon) was considered so reliable, that boys working in the fields based their lunchtime on the movement of this flower.

In 1729, the French astronomer Jean Jacques d'Ortous deMairan noticed that his heliotrope plant's leaves opened during the day and folded at night (first known experiment on biological rhythms).

He did not go any further in his research, as his true love was astronomy. However, he did encourage others to continue researching the subject.

A Swedish botanist by the name of Carl Linnaeus from University of Uppsala noticed that the flowers of different species opened and closed at certain times each day.

It is said that in 1748 he decided to plant a flower clock, so that those who visited him could look at it and tell what hour it was. Others say that Linnaeus probably never actually planted a 'flower clock', but instead based his conclusion on his observations.

He published his findings in "Philosophica Botanica" in 1751.

Keeping track of the passing of time as well as the seasons has existed for thousands of years. The Chinese, Japanese, Romans, Egyptians, Aztec, Hopi and Navajo Indians, and many others developed various forms of calendars. The one thing that was common among these calendars was their close tie with nature.

According to Chinese lore, two trees grew at the Emperor's Court. One tree produced a new leaf every day for fifteen days as the moon waxed, and then it shed one leaf every day for fifteen days, as the moon waned. In this way the months were measured. On the other side of the garden was a tree that produced leaves every month for six months, and then shed leaves every month for six months. And so, the passage of years were counted.

In the 4th century B.C., Alexander the Great's scribe, Androsthenes, noted that the leaves of certain trees opened during the day and closed at night.

In England during the 18th century Tragopogon partensis (Jack-go-to-bed-at noon) was considered so reliable, that boys working in the fields based their lunchtime on the movement of this flower.

In 1729, the French astronomer Jean Jacques d'Ortous deMairan noticed that his heliotrope plant's leaves opened during the day and folded at night (first known experiment on biological rhythms).

He did not go any further in his research, as his true love was astronomy. However, he did encourage others to continue researching the subject.

A Swedish botanist by the name of Carl Linnaeus from University of Uppsala noticed that the flowers of different species opened and closed at certain times each day.

It is said that in 1748 he decided to plant a flower clock, so that those who visited him could look at it and tell what hour it was. Others say that Linnaeus probably never actually planted a 'flower clock', but instead based his conclusion on his observations.

He published his findings in "Philosophica Botanica" in 1751.

Linnaeus observed that once bees found the flowers they preferred, they would return to them at the same time every day.

This is one example of time-linked behavior that is not fully understood by science.

What is known is that biological timing plays an important role in the survival of plants and animals.

Nearly all-living organisms have a biological clock, with the exception of organisms such as bacteria or blue-green algae.

The bean plant leaves for example are horizontal at noon and vertical after midnight.

This phenomenon can also be linked to the fact that certain medications or treatments for human illnesses are more effective at different times of the day.

Lord Bacon was a keen observer of nature and stated that the "Chickweek," (sometimes mistaken for the Pimpernel) "when it expanded fully no rain will disturb the summer day, but if it entirely shuts up, veiling the tiny white flower with its green mantle, let the traveler put on his great-coat and the ploughman with his beast of draught, expect rest from their labour."

There are other plants that are natural barometers, such as:

If the flowers of the Sow-thistle keep open all night-- rain will certainly fall the next day.
The stalks of the Dandelion are closely together in wet weather, dispersing only on dry days.
The buds of the Anemone will curl up its petals when rain approaches.

In 1932, Erwin Bunning demonstrated that plants and insects behaved according to circadian rhythms, whether or not they were in continuous light or darkness. His work showed that an internal clock is genetically inherited.

In the 1950's Gustav Kramer and Klaus Hoffmann proved the existence of a Biological Clock.

They demonstrated a bird's internal clock reoriented itself by using the direction of the sun (similar to using a compass).

However, their internal clocks are influenced by climate and stress.

The Biological Clock of Some Flowers:

2 a.m.:
Common Morning Glory (opens)
Night-Blooming Cereus (closes)

3 a.m.
Imperial Morning Glory (opens 3-4 a.m.)

4 a.m.
Yellow Hawkweed (opens)
Dogrose, Chicory, Yellow Goats-Beard (opens 4-5 a.m.)

5 a.m.
Buttercups, Poppy (opens)
Dandelion, Morning Glories, Wild Roses (opens 5-6 a.m.)

6 a.m.
Spotted Cat's Ear (opens)
Flax, Potatoes (open 6-7  a.m.)

7 a.m.
African Marigold, Lettuce, White Water Lily ( opens)

8 a.m.
Mouse-Ear Hawkweed, Scarlet Pimpernel (opens)
African Daisies, Nolana (open 8-9 a.m.)
Dandelion (closes 8-9 a.m.)

9 a.m.
Calendula (Field marigold), Catchfly (opens)
Coltsfoot, Gentians, Sandworts (opens 9-10 am)
Prickly Sow Thistle (closes)

10 a.m.
Common Nipplewort (closes)
Star-of-Bethlehem (opens 10-11 a.m.)
California Poppies (open 10a.m.-1 p.m. only in sunlight)

11 a.m.
Star-of-Bethlehem (opens)

Noon
Goatsbeard, Blue Passion flower (opens)
Morning Glories (closes)

1 p.m.
Carnation (opens)
Childing Pink (closes)

2 p.m.
Afternoon Squill (opens)
Scarlet Pimpernel, Water Lily (closes)
Chicory, Dandelion, Poppy, Potatoes, Sandworts (closes 2-3 p.m.)
Dandelion (closes 2-5 p.m.)

3 p.m.
Hawkbit (closes)
Calendula, Spider plant (closes 3-4 p.m.)

4 p.m.
Purple Hawkweed (opens 4 p.m.)
Four O' Clocks(opens 4-7 p.m.)
Small bindweed, Allyssum (closes)
California Poppies, Cat's Ear (closes 4-5 p.m.)

5 p.m.
Night-Flowering Catchfly (opens 5-6 p.m.)
Chicory, White Water Lily (closes)
Coltsfoot (closes 5-6 p.m.)

6 p.m.
Showy Evening primrose, Goatsbeard, Moonflowers (opens)
White water lily (closes 6-7 p.m.)

7 p.m.
White campion (opens)
Fig-marigold (opens 7-8 p.m.)
Iceland poppy (closes 7 p.m.)
Daylily, Dogrose (closes 7-8 p.m.)

8 p.m.
Night flowering cereus (opens 8-10 p.m.)
Catchfly, Dandelions, Daylilies (closes  8-9 p.m.)

9 p.m.
Flowering Tobacco (opens 9-10 p.m.)

References:

Annual Review of Physiology, Volume 55, Copy. 1993, by Annual Reviews Inc.

Biological Clocks, Cells know the time by Loes Pihlajamaa-Glimmerveen

Fredericksburg Herb Farm Family Farm Newsletter Vol. 5 No. 4 page 3

The Linnean Society of London Burlington House, Piccadilly, London

Omaha Sunday World-Herald 58(32): 8-E; May 6, 1923. A nature editorial (possibly Miles Greenleaf).

The Science Museum of Virginia; Explore Life Processes and Biological Clocks; Center for Biological Timing

The Washington Post Company; A Garden to Set Your Clock October 21, 2002 by Joel M. Lerner