Heliotropism is the
diurnal motionDiurnal motion is an astronomical term referring to the apparent daily motion of stars around the Earth, or more precisely around the two celestial poles. It is caused by the Earth's rotation on its axis, so every star apparently moves on a circle, that is called the diurnal circle...
of plant parts (flowers or leaves) in response to the direction of the sun.
Heliotropism was first described by
Leonardo da VinciLeonardo di ser Piero da Vinci was an Italian polymath, scientist, mathematician, engineer, inventor, anatomist, painter, sculptor, architect, botanist, musician and writer....
(along with
gravitropismGravitropism is a turning or growth movement by a plant or fungus in response to gravity. Charles Darwin was one of the first Europeans to document that roots show positive gravitropism and stems show negative gravitropism. That is, roots grow in the direction of gravitational pull and stems...
) in his botanical studies. The term "heliotropism," though, was introduced in the early 1800s by
A. P. de CandolleAugustin Pyramus de Candolle also spelt Augustin Pyrame de Candolle was a botanist. The author abbreviation used in citing plant names he published is "DC."....
, for the growth of the stem tip towards light, which is now called
phototropismPhototropism is directional growth in which the direction of growth is determined by the direction of the light source. In other words, it is the growth and response to a light stimulus. Phototropism is most often observed in plants, but can also occur in other organisms such as fungi. Phototropism...
. The term heliotropism is now used only for solar tracking.
Floral heliotropism
Heliotropic flowers (like the
Alpine ButtercupRanunculus is a large genus of about 400 species of plants in the Ranunculaceae. It includes the buttercups, spearworts, water crowfoots and the lesser celandine ....
,
Ranunculus adoneus, an alpine plant) track the sun's motion across the sky from East to West. During the night, the flowers may assume a random orientation, while at dawn they turn again towards the East where the sun rises. The motion is performed by motor cells in a flexible segment just below the flower, called a
pulvinusA pulvinus is a joint-like thickening at the base of a plant leaf or leaflet that facilitates nyctinastic and thigmonastic movements. It consists of a core of vascular tissue within a flexible, bulky cylinder of thin-walled parenchyma cells...
. The motor cells are specialized in pumping
potassiumPotassium is the chemical element with the symbol K , atomic number 19, and atomic mass 39.0983. Potassium was first isolated from potash...
ionAn ion is an atom or molecule where the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge...
s into nearby tissues, changing their
turgor pressureTurgor pressure or turgidity is the main pressure of the cell contents against the cell wall in plant cells and bacteria cells, determined by the water content of the vacuole, resulting from osmotic pressure, i.e. the hydrostatic pressure produced by a solution in a space divided by a semipermeable...
. The segment flexes because the motor cells at the shadow side elongate due to a
turgor rise. Heliotropism is a response to blue light.
Some solar tracking plants are not purely heliotropic: in those plants the change of orientation is an innate circadian motion triggered by light, which continues for one or more periods if the light cycle is interrupted.
Leaf heliotropism
Leaf heliotropism is the solar tracking behavior of plant leaves. Some plant species have leaves that orient themselves perpendicularly to the sun's rays in the morning (
diaheliotropism), and others have those that orient themselves parallel to these rays at midday (
paraheliotropism). Floral heliotropism is not necessarily exhibited by the same plants that exhibit leaf heliotropism.
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