Linnaeus's flower clock

Linnaeus's flower clock was a garden plan hypothesized by Carl Linnaeus that would take advantage of several plants that open or close their flowers at particular times of the day to accurately indicate the time.^[1][2] According to Linnaeus's autobiographical notes, he discovered and developed the floral clock in 1748.^[3] It builds on the fact that there are species of plants that open or close their flowers at set times of day. He proposed the concept in his 1751 publication Philosophia Botanica, calling it the horologium florae (lit. 'flower clock').^[4] His observations of how plants changed over time are summarised in several publications. Calendarium florae (the Flower Almanack) describes the seasonal changes in nature and the botanic garden during the year 1755. In Somnus plantarum (the Sleep of Plants), he describes how different plants prepare for sleep during the night, and in Vernatio arborum he gives an account of the timing of leaf-bud burst in different trees and bushes.^{[5] [6]} He may never have planted such a garden, but the idea was attempted by several botanical gardens in the early 19th century, with mixed success. Many plants exhibit a strong circadian rhythm (see also Chronobiology), and a few have been observed to open at quite a regular time, but the accuracy of such a clock is diminished because flowering time is affected by weather and seasonal effects. The flowering times recorded by Linnaeus are also subject to differences in daylight due to latitude: his measurements are based on flowering times in Uppsala, where he taught and had received his university education.

The plants suggested for use by Linnaeus are given in the table below, ordered by recorded opening time; "-" signifies that data are missing. ^[7]