Elongating shoots tend to grow upward or towards light, elongating roots downward and away from light. Such kinds of directional growth are called tropisms so that, for instance, roots may be said to be positively gravitropic and negatively phototropic. One side of the shoot or root is stimulated to elongate more rapidly than the other (owing to differences in turgor pressure), resulting in curvature.

This definition is abridged from A – Z of tree terms: A companion to British arboriculture.
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