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W Texas | Heliotropism is very common in plants, but more easily appreciated in the sunflower, for obvious reasons.
The base of the distal branches have a thick area called a Pulvinus, which responds to sunlight and allows stem to flex.
When the stems become woody, they cannot flex and heliotropism ceases.
Basically, the flower requires bee pollination, which is improved by vividly displaying itself all day. After pollination, it stays out of the way to allow leaves to get all the sun, as they are more efficient at photosynthesis. So the flower faces east, which is the part of the day with the least efficient light source, ensuring maximum total daily light interception for the leaves. | |
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Sunflowers and traveling with the sun.
