(phyllotaxy) The arrangement of leaves on a stem. In most plants the leaves are inserted singly and spirally on the stem. Sometimes two or more leaves are formed at each node giving an opposite or whorled arrangement (see illustration). The phyllotaxis of a given plant may be described by giving the angle of divergence between successive nodes. This varies between species but if many primordia are being formed it is normally not less than 137.5°, the so-called Fibonacci angle. Phyllotaxy may also be described by the *genetic spiral or by * parastichies. Many theories have been advanced to account for the regular nature of phyllotaxis (see available space theory , repulsion theory ).  |
