Those in possession of a good memory may remember last fall’s post about a local field of sunflowers. Although these were not planted in the same field this year a much larger and adjacent area was put into sunflower cultivation this spring. I have been watching the crop closely and although it had a difficult start, the plants are now doing very well and flowers began to develop a couple of weeks ago. Lots of folks believe that a sunflower will twist, over the course of a day, to keep its inflorescence perpendicular to the sun as it moves, east to west, across the sky. Although solar tracking (also known by the more milifluous term heliotropism) by leaves is a fairly common phenomenon it is not, contrary to popular belief, often observed of flowers. Of course, plants do not have anything like the boney joints about which movements occur in vertebrate animals, or anything analogous to the non-boney skeletons observed among invertebrates. Instead, plants have highly specialized motor cells which pump ions into tissues which surround a flexible part of the stem called the pulvinus. This action causes cells of the pulvinus to swell and, depending on which cells are affected, the stem twists or bends in consequence. [These movements are similar to those seem in a Weather Stick. Have you ever seen one of these in action? They were once used by Native Americans to predict weather change in the days before the advent of the mercury barometer. If you’re interested in what makes such a thing useful as a predictor of weather check out a very nice explanation at New Potato Technologies.] Although immature sunflowers may demonstrate some degree of tracking, the flowers of mature plants do not move and the position of the inflorescence is fixed in the direction of the rays of the rising sun. Click the image and migrate to a larger version with higher resolution.


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