1. Introduction
Plant growth is fundamentally different from animal growth: it is localized in zones of undifferentiated cells called meristems, and it is continuous throughout the plant's life. This localized and continuous growth is the origin of the spiral shapes observed in stems, leaves and inflorescences.
2. Types of Meristems
- Shoot apical meristem (SAM): at stem tip, produces leaves and flowers
- Root apical meristem (RAM): at root tips, protected by the root cap
- Intercalary meristem: at the base of grass internodes
- Vascular cambium: lateral meristem producing wood and phloem
3. Spiral Growth
The tendril of a climbing plant is an example of active helical growth. It is produced by differential growth: one side of the tendril grows faster than the other, generating curvature. When the tendril touches a support, it winds around it forming a helix. The direction of winding (right or left) is often genetically determined.
4. Role of Auxin
Auxin (indole-3-acetic acid, IAA) is the main plant growth hormone. It is produced in the apical meristem and distributed downward by active polar transport. Asymmetric auxin distribution causes differential growth: the side with more auxin grows faster, bending the stem. This is the mechanism of phototropism (bending toward light) and gravitropism (bending downward).
5. Conclusion
The spiral growth of plants is a direct consequence of the meristematic localization of growth and the hormonal gradients that regulate it. It is not a mysterious property but the predictable result of well-characterized biochemical and biophysical mechanisms.