Leaves often twist on their petioles and, in response to illumination, become perpendicularly oriented to a light source. In fact, many plants have solar-tracking leaves with the blades oriented at right angles to the sun throughout the day. Some scientists have referred to solar-tracking movements as heliotropisms, but, unlike phototropic responses of stems and roots, growth is not involved.
It has been widely reported that sunflowers exhibit heliotropic movements and face the sun throughout the day. This is not true, except, perhaps, when the plants are very young; sunflowers face east as they develop and remain facing in that direction until the seeds are mature and the plant dies.
Strictly speaking, the twisting of petioles that facilitates heliotropic movements should be called phototorsion, because motor cells (in pulvini) at the junction of the blade and the petiole control the movement (see turgor movements, page 210).
If one views a tree from directly overhead or observes a vine growing on a fence, it is perhaps surprising to note how little overlap of the leaves occurs. Each leaf is oriented so that it receives the maximum amount of light available (Fig. 11.18).
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