Helianthus
Helianthus () is a genus comprising about 70 species of annual and perennial flowering plants in the daisy family Asteraceae commonly known as sunflowers. Except for three South American species, the species of Helianthus are native to North America and Central America. The best-known species is the common sunflower (Helianthus annuus). This and other species, notably Jerusalem artichoke (H. tuberosus), are cultivated in temperate regions and some tropical regions, as food crops for humans, cattle, and poultry, and as ornamental plants. The species H. annuus typically grows during the summer and into early fall, with the peak growth season being mid-summer.Several perennial Helianthus species are grown in gardens, but have a tendency to spread rapidly and can become aggressive.  On the other hand, the whorled sunflower, Helianthus verticillatus, was listed as an endangered species in 2014 when the U.S. Fish and Wildlife Service issued a final rule protecting it under the Endangered Species Act. The primary threats are industrial forestry and pine plantations in Alabama, Georgia, and Tennessee. They grow to 1.8 metres (6 feet) and are primarily found in woodlands, adjacent to creeks and moist, prairie-like areas.The common sunflower is the national flower of Ukraine, cultivated there for several centuries.

Description
Sunflowers are usually tall annual or perennial plants that in some species can grow to a height of 300 centimetres (120 inches) or more. Each "flower" is actually a disc made up of tiny flowers, to form a larger false flower to better attract pollinators. The plants bear one or more wide, terminal capitula (flower heads made up of many tiny flowers), with bright yellow ray florets (mini flowers inside a flower head) at the outside and yellow or maroon (also known as a brown/red) disc florets inside. Several ornamental cultivars of H. annuus have red-colored ray florets; all of them stem from a single original mutant. While the majority of sunflowers are yellow, there are branching varieties in other colours including, orange, red and purple.
The petiolate leaves are dentate and often sticky. The lower leaves are opposite, ovate, or often heart-shaped. The rough and hairy stem is branched in the upper part in wild plants, but is usually unbranched in domesticated cultivars.This genus is distinguished technically by the fact that the ray florets (when present) are sterile, and by the presence on the disk flowers of a pappus that is of two awn-like scales that are caducous (that is, easily detached and falling at maturity). Some species also have additional shorter scales in the pappus, and  one species lacks a pappus entirely. Another technical feature that distinguishes the genus more reliably, but requires a microscope to see, is the presence of a prominent, multicellular appendage at the apex of the style. Further, the florets of a sunflower are arranged in a natural spiral.Variability is seen among the perennial species that make up the bulk of those in the genus. Some have most or all of the large leaves in a rosette at the base of the plant and produce a flowering stem that has leaves that are reduced in size. Most of the perennials have disk flowers that are entirely yellow, but a few have disk flowers with reddish lobes. One species, H. radula, lacks ray flowers altogether.
Overall, the macroevolution of the Helianthus is driven by multiple biotic and abiotic factors and influences various floral morphology.Helianthus species are used as food plants by the larvae of many lepidopterans.

Growth stages
The growth of a sunflower depends strictly on its genetic makeup and background. Additionally, the season it is planted will have effects on its development; those seasons tend to be in the middle of summer and beginning of fall.  Sunflower development is classified by a series of vegetative stages and reproductive stages that can be determined by identifying the heads or main branch of a single head or branched head.

Phylogeny
The geographical history of this genus accounts for its evolutionary history, with its levels of genetic variation across its gene pool increasing as new hybrids are created both for commercial use and in the wild. As a result, sunflower species are also experiencing a genetic bottleneck as a result of selective breeding for industrial use.

Facing the Sun (heliotropism)
Before blooming, Helianthus plant heads tilt during the day to face the Sun. This movement is referred to as heliotropism, which continues for a short time when flower buds form and young Helianthus heads track the Sun. At night, the flower heads reorient their position and face East in anticipation for the sunrise. Sunflowers move back to their original position between 3am and 6am, and the leaves follow about an hour later.By the time they are mature and reach anthesis, Helianthus generally stop moving and remain facing east, which lets them be warmed by the rising sun. Historically, this has led to controversy on whether or not Helianthus is heliotropic, as many scientists have failed to observe movement when studying plants that have already bloomed.This is notably different from heliotropism in leaves, as the moving mechanism for leaves exists in the pulvinus. Since flowers do not have pulvini, the movement is caused by increased growth rate of the stems. The growth rate accumulation of the stem on the east side of the stem gradually pushes the flower from east to west during daytime. This matches with the Sun as it rises from the east and falls in the west. At night, the growth rate is higher in the west side of the stem that gradually pushes the flower from the west side back to the east side. In addition, it is not actually the whole plant that changes its direction to face the Sun, but the flower itself that bends to be illuminated by the Suns rays.The heliotropic movement is caused by growth on the opposite side of the flower, driven by accumulation of growth hormones during Sun exposure.Heliotropism persists on cloudy days when the sun is not shining brightly, meaning that the movement is endogenous as a trained and continuous process. However, flower movement does not occur during long periods of rain or clouds. It also does not occur in a growth chamber when exposed to 16 hours of light or in greenhouses, suggesting that the plants require a directional, moving light source. Helianthus can also discriminate between different types of light. When exposed to different light frequencies, the hypocotyls will bend toward blue light but not red light, depending on the quality of the light source.
It is the circadian rhythms and the differences of the stem growth rate that work together and cause the heliotropism of the Helianthus. This is important for attracting pollinators and increasing growth metabolism. Future studies are needed for identifying the exact physiological basis and cellular mechanism for this behavior.

Taxonomy
Helianthus is derived from Greek ἥλιος hēlios "sun" and ἄνθος ánthos "flower", because its round flower heads in combination with the ligules look like the Sun.There are many species recognized  in the genus:

Formerly included
The following species were previously included in the genus Helianthus.
Flourensia thurifera (Molina) DC. (as H. thurifer Molina)
Helianthella quinquenervis (Hook.) A.Gray (as H. quinquenervis Hook.)
Helianthella uniflora (Nutt.) Torr. & A.Gray (as H. uniflorus Nutt.)
Pappobolus imbaburensis (Hieron.) Panero (as H. imbaburensis Hieron.)
Viguiera procumbens (Pers.) S.F.Blake (as H. procumbens Pers.)

Uses
The seeds of H. annuus are used as human food. Most cultivars of sunflower are variants of H. annuus, but four other species (all perennials) are also domesticated. This includes H. tuberosus, the Jerusalem artichoke, which produces edible tubers.
There are many species in the sunflower genus Helianthus, and many species in other genera that may be called sunflowers.

The Maximillian sunflower (Helianthus maximiliani) is one of 38 species of perennial sunflower native to North America. The Land Institute and other breeding programs are currently exploring the potential for these as a perennial seed crop.
The sunchoke (Jerusalem artichoke or Helianthus tuberosus) is related to the sunflower, another example of perennial sunflower.
The Mexican sunflower is Tithonia rotundifolia. It is only very distantly related to North American sunflowers.
False sunflower refers to plants of the genus Heliopsis.

Ecology
Sunflowers have been proven to be excellent plants to attract beneficial insects, including pollinators.  Helianthus spp. are a nectar producing flowering plant that attract pollinators and parasitoids which reduce the pest populations in nearby crop vegetation.  Sunflowers attract different beneficial pollinators (e.g., honey bees) and other known insect prey to feed on and control the population of parasitic pests that could be harmful to the crops.  Predacious insects are first attracted to sunflowers once they are planted.  Once the Helianthus spp. reaches six inches and produces flowers it begins to attract more pollinators.  Distance between sunflower rows and crop vegetation plays an important role in this phenomenon, hypothesizing that closer proximity to the crops will increase insect attraction.In addition to pollinators of Helianthus spp., there are other factors such as abiotic stress, florivory, and disease which also contribute to the evolution of floral traits. These selective pressures, which stem from several biotic and abiotic factors are associated with habitat environmental conditions which all play a role in the overall morphology of the sunflowers’ floral traits.An ecosystem is composed of both biotic (which are living elements of an ecosystem such as plants, animals, fungi, protists, and bacteria), and abiotic factors (non-living elements of an ecosystem such as air, soil, water, light, salinity and temperature).It is thought that two biotic factors can explain for the evolution of larger sunflowers and why they are present in more drier environments. For one thing, the selection by pollinators is thought to have increased the sunflower’s size in a drier environment. This is because in a drier environment, there are typically less pollinators. As a result, in order for the sunflower to be able to attract more pollinators, they had to increase the morphology of their floral traits in that they had to increase their display size. Another biotic factor that can explain for the evolution of larger sunflowers in drier environments is that the pressure from florivory and disease favors smaller flowers in habitats that have a more moderate supply of moisture (mesic habitat). Wetter environments usually have more dense vegetation, more herbivores, and more surrounding pathogens. As larger flowers are typically more susceptible to disease and florivory, smaller flowers may have evolved in wetter environments which explains the evolution of larger sunflowers in more drier environments.

Gallery
See also
Fermat's spiral
Phyllotaxis
Stegocintractia junci


== References ==