The Oxalis Triangularis is a great example of a plant that
exhibits photonastic behaviour, that is one that
produces a mechanical response to changes in light levels.
As opposed to (photo)tropic movements which require direct
light, nastic responses are due to changes in diffuse
levels.
The reaction is made possible by changes in the osmotic
potential of cells of the pulvinus at the base of the leaf,
a chemo-mechanical process. In addition, the plant also
exhibits interesting growth behavior in the formation of the
canopy; the larger leafs occurring at the top of the canopy,
but organised in such a way that they allow light
penetration to the smaller leaves below.
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