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 sun 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 hēlios "sun" and ánthos "flower", because its round flower heads in combination with the ligules look like the Sun. There are many species recognized in the genus: Helianthus agrestis Pollard – southeastern sunflower – Florida, Georgia Helianthus ambiguus Britt. – Wisconsin, Michigan, Ohio, New York Helianthus angustifolius L. – swamp sunflower – Texas, northern Florida to southern Illinois, Long Island, New