Hi Gang, Scientists have long known that leaves can sense daylength, and somehow transmit this information to the shoot tip; the result is that plant shoots make flowers at the top of the plant at right time of year (and leaves, etc.). This principle was named long ago named "florigen." Florigen is hypothesized to be a substance that is made in leaves in response to long days (or short days), and which is then transported to the shoot tip where it causes flowers to develop. Though this mystery has important implications for gardeners (e.g., when will the cherry blossoms open), the story is important in many other ways. Flowers give rise to fruits and seeds; we depend upon correct flowering for grains (wheat, corn, rice, etc.) as well as fruits (apples, strawberries, and squash, etc.). This year scientists figured out a bit more about the puzzle; and it turns out that plants are amazing and clever creatures. Plants do make a florigen-like molecule in their leaves that is transported out to other parts of the plant; this florigen-like molecule is called FT. However, in the shoot tip cells there is another molecule called FD that must also be present to interact with FT. FT and FD work together and promote flowering, but the wonderful thing is that they have different roles. FT is made in the leaves and tells a plant "when" to flower. However, FD tells a plant "where" to make the flowers (i.e., at the shoot tip). The two molecules (FT and FD) work together in the shoot tip; in normal plants it is only when they are both together in the same place at the same time that flowers are made. Thus, even if FT travels to the roots or other leaves, flowers are not be made in those places because FD is not present. FD is only made in the shoot tip where it waits for FT to arrive, thence the two partners begin the floral process. The full story is not in, and surely there will be other exciting discoveries. However, it is impressive that plants are such finely tuned organisms. The leaves sense day length and when the days are long enough they send a signal out to the rest of the plant. But, it is only the shoot tip that is equipped to properly respond to the signal, and thus the shoot tip is where tiny flower buds are made. The plant keeps growing, and we may not actually see the flowers for weeks, by which time the shoot tip has grown on another few inches (or more), but the flowers are produced as tiny flower buds in the shoot tip and no where else. FT and FD activate certain proteins that then cause more proteins to be made, and so on. There are, of course, other molecules involved in this delicate timing dance including LFY, AP1, SOC1 and FLC. But, it is clear the FT and FD are part of a clever scheme to ensure that plants work correctly. Citations: Science, August 12, 2005, 309:1024-1025 (subscription may be required) http://sciencemag.org/cgi/content/… Max Planck Society Press Release http://mpg.de/english/illustrationsDocumentation/… The Human Flower Project (August 12, 2005) http://www.humanflowerproject.com/index.php/C7/P30 Molecular and Genetic Mechanisms of Floral Control (The Plant Cell, 2004) http://plantcell.org/cgi/content/… A Time to Grow, a Time to Flower (The Plant Cell, 2005) http://tinyurl.com/7ra3e/