The ontogeny, phyllotactic diversity, and discontinuous transitions of Diphasiastrum digitatum (Lycopodiaceae)

Fibonacci phyllotactic patterns in seed plants are well documented, but whether such predominance holds true for lower vascular plants is relatively unknown. We investigated Diphasiastrum digitatum (Lycopodiaceae) phyllotaxis throughout its ontogeny to extend our knowledge of pattern frequency of lower vascular plants and to measure quantitative variables associated with discontinuous phyllotactic transitions. These investigations allowed us to test whether the same mechanisms inherent in shoot apical meristem (SAM) development of seed plants are applicable to early-diverged lower vascular plants SAM development.

Divergence angle, plastochron ratio, leaf insertion angle, circumferential ratio, radial ratio, half conic angle, area, circumference, and circularity of the shoot apical meristem were compared among different phyllotactic patterns and different meristem types observed throughout D. digitatum ontogeny, using scanning electron microscopy.

Fibonacci patterns were not predominant during six stages of D. digitatum ontogeny. In all five cases of discontinuous transition associated with strobili formation, divergence angle was the only variable that has changed consistently.

The predominance of non-Fibonacci patterns/series in D. digitatum is inconsistent with the prediction of interpretive model of phyllotaxis. We hypothesize this is because its SAM, due to its frequent dichotomy, is not circular and primordia initiation is restricted spatially and temporally at the beginning of pattern formation. Change in divergence angle associated with discontinuous transitions is most likely due to the change of the location of new auxin maxima, due to the change of SAM shape and size.