CAPITULUM PHYLLOTAXIS AND NUMERICAL CANALIZATION IN MICROSERIS-PYGMAEA (ASTERACEAE, LACTUCEAE)

The positions at which floret primordia arise in developing capitulum buds of Microseris pygmaea D. Don have been mapped by computer-assiste d light microscopy. The primordia can be assigned positions along a ba sic phyllotactic spiral with a divergence angle of about 137.5-degrees . In addition, there are regular deviations from a spiral arrangement. Typically, the first 26 primordia in phyllotactic sequence are arrang ed in two concentric circles of 13 primordia with considerable deviati ons in the divergence angle and in the distances,between primordia alo ng a parastichy at positions 13 and 26. This arrangement can be simula ted by geometric models that include nearest neighbor packing, togethe r with spiral phyllotaxis. The circular arrangement of peripheral prim ordia at nearly equal radial distances from the center of the developi ng capitulum helps to explain the numerical constancy (canalization) o f peripheral structures, especially the constant number of 13 inner ph yllaries on heads with very different numbers of florets.