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April 27, 2009

Plant Anatomy 101: Sunflowers

This summer I am participating in the Great Sunflower Project and my sunflower seeds finally came this weekend! I am going to plant them tonight when I get home from work. Let the fun begin! Since I will be spending so much time growing, cultivating, and watching my sunflowers I thought I would be a good idea to learn about them a little more:

Sunflowers are annual plants that can grow up to 9 feet tall! They are native to the Americas and were first cultivated in what is now Mexico, New Mexico, and Arizona around 3000 B.C.E. (Some archaeologists suggest that sunflowers were even cultivated before corn by Native Americans.) Sunflowers were used by many indigenous American peoples as a symbol for sun deities.

Sunflowers exhibit heliotropsim, meaning that the flowers and/or leaves follow the sun throughout the day starting in the East in the morning, moving toward the West throughout the day, and then returning to the East at night. Domesticated sunflowers exhibit heliotropism in both the flower buds and the leaves, however they will only do this during the bud stage. This motion is performed by motor cells in the pulvinus, a flexible segment of the stem just below the bud. As the bud stage ends, the stem stiffens as the blooming stage is reached. Sunflowers in the blooming stage no longer exhibit heliotropism as the stems are "frozen," usually in an Eastward direction. Typically, wild sunflowers do not exhibit heliotropism in their bud heads, but their leaves often do to some degree. During the blooming stage, wild sunflowers will face any direction when mature.

Sunflowers are pseudanthiums. A pseudanthium (Greek for false flower), or flower head, is a kind of plant in which several florets (small flowers) are grouped together to form a large flower-like structure on top of a stem. A sunflower's ray florets on the outside of the flower head are sterile and come in yellow, maroon, orange, or other colors ("petals"). The disc florets (the florets on the inside of the head) are traditionally called sunflower seeds when mature. These sunflower "seeds" however are acutally the whole fruits of each floret (akenes, similar to the fruits on a strawberry). The inedible husk is the wall of the fruit and the seed is inside the kernel.

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1 comment:

Tim Brown said...

One of the most amazing things about sunflowers (and many other flowering plants), is that if you count the number of seeds/buds in the opposing spirals on the head of the flower, they are usually successive numbers in the Fibonacci sequence. The Fibonacci sequence comes from adding each number to its predecessor (i.e. 1,2,3,5,8,13...34,55... etc). In the case of sunflowers, they usually have 21 and 34 or 34 and 55 seeds/flowers in each of the opposing spirals. Count the parts in the spirals on a pine cone, they probably will be 5 & 8. Scientists still don't totally understand why this is so but it pops up all over the place in nature. For instance, the ratio of each Fibonacci number to the previous number in the series (i.e. 34/21, etc...) converges on the ratio of the sides of the golden rectangle which, in addition to being popular in art, also perfectly describes the ratio of shell parts in animals like the chambered nautilus! (Try wikipedia for more info on any of the terms above).