為什麼向日葵會各著太陽??

Sunflowers
Heliotropism

Sunflowers in the bud stage exhibit heliotropism. At sunrise, the faces of most sunflowers are turned towards the east. Over the course of the day, they move to track the sun from east to west, while at night they return to an eastward orientation. 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 and the blooming stage is reached.
Sunflowers in the blooming stage are not heliotropic anymore. The stem has frozen, typically in an eastward orientation. The stem and leaves lose their green color.
The wild sunflower typically does not turn toward the sun; its flowering heads may face many directions when mature. However, the leaves typically exhibit some heliotropism.

向日葵跟著太陽轉動，主要是因為在莖部的地方，含有一種叫做「植物生長素」的東西。這種物質一遇到光線照射，就會自動跑到背光的那一面去躲起來，不過它還有個非常重要的功能，就是刺激細胞生長，加速分裂繁殖。
太陽剛升起，向日葵莖部的生長素，便馬上躲到背光的一面去，刺激這裡的細胞繁殖，讓這裡生長更快，於是背光面就壓過向光面，所以就看到向日葵朝著太陽彎下腰來。一旦太陽慢慢朝西方落下時，向日葵也總是會跟著它移動。許多其他植物的葉子，也有和向日葵一樣的習性，在植物學上稱做植物的「向光性」。然而也有植物葉子會背著光，則稱做「負向光性」。


圖片參考：http://i201.photobucket.com/albums/aa187/emage_pro/d9faeb8c.jpg

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