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.