Anisotomous dichotomy results from an unequal bifurcation of the original shoot apical meristem in Diphasiastrum digitatum (Lycopodiaceae).

PREMISE OF THE STUDY: Two types of dichotomy are recognized in Lycopodiaceae: isotomous (equal) and anisotomous (unequal). Anisotomous dichotomy (anisotomy) has been hypothesized to result from unequal growth of an equal bifurcation of the original shoot apical meristem (SAM). Diphasiastrum digitatum (Lycopodiaceae) exhibits anisotomy at various locations. We thus used D. digitatum to test this classic hypothesis about anisotomy. METHODS: Transverse areas of original and derived SAMs of anisotomy exhibited by the rhizome and the vertical aerial vegetative stem were measured using scanning electron microscopy. The difference between half of the original SAM and one derived SAM in terms of transverse area were compared using paired t-tests. KEY RESULTS: During the anisotomy exhibited by the rhizome SAM, 77.4% of the transverse area of the original rhizome SAM contributed to the derived rhizome SAM. During the first anisotomy exhibited by the vertical aerial vegetative stem SAM, 66.2% of the transverse area of the original vertical aerial vegetative stem SAM contributed to the derived vertical aerial vegetative stem SAM. During the second anisotomy exhibited by the vertical aerial vegetative stem SAM, 49.4% of the transverse area of the original vertical aerial vegetative stem SAM contributed to the derived vertical aerial vegetative stem SAM. Nonetheless, the shape of the two derived SAMs differed though they did not differ in size. CONCLUSIONS: In D. digitatum, anisotomy results from an unequal bifurcation of the original SAM. This finding sheds light on plant body architecture evolution as well as plant organ (megaphyllous leaf) evolution.

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

PREMISE OF THE STUDY: 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. METHODS: 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. KEY RESULTS: 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. CONCLUSIONS: 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.

The plastochron index: still useful after nearly six decades.

The plastochron index (PI) introduced by Erickson and Michelini in 1957 provides a solution to a long-standing problem, of how to measure time in growing plant populations, such that the occurrence of critical developmental events can be more readily detected, compared, and analyzed, than if chronologic time is used. The PI reduces the rather large variation associated with chronologic time in measuring such events by taking advantage of the growth characteristics of stem organs that repeat at regular intervals (the plastochron) and has found widespread application in botanical research. The original formulation and derivation of the PI and associated leaf plastochron index (LPI) is reviewed. Additional formulations that have been developed to overcome some of the limitations of the original PI formulation are examined. Major advancements that have been achieved in understanding the physiology, growth, and development of agriculturally important and current model plant species are reviewed to illustrate how various researchers have used the PI in such studies. Potential uses to which the PI and LPI might be applied in emerging frontiers of plant science are suggested. A searchable bibliography of most all the primary research studies that cite the original PI article is provided.

Anatomical basis for biophysical differences between Pinus nigra and P. resinosa (Pinaceae) leaves.

Differences in the flexibility of Pinus nigra and P. resinosa leaves can be used to discriminate these two similarly looking pine species from one another. When bent along the longitudinal axis, P. resinosa leaves snap, while P. nigra leaves appear flexible. This useful field test has had no known biophysical or anatomical explanation until now. Analysis of the first order mechanics of bending and buckling of the pine needles was used to elucidate any important anatomical differences between these two species that can account for their different biophysical behaviors when bent. Neither the cross section of the total leaf area nor the inner core area between the two species differed significantly. Differences in the pattern of cell wall thickening and lignification of the endodermal layer of the inner core of the leaves best explain the differences in bending behavior. Thus, subtle variation in anatomy can influence the biophysical properties of naturally occurring structures, which in turn could have important implications for the engineering of manufactured objects.

COMPARISON OF EARLY LATERAL VEIN FORMATION IN LINUM USITATISSIMUM LEAVES WITH THEORETICAL NETWORK MODELS.

The plastochron age of the Linum leaf that first exhibited lateral leaf vein divergences, the divergent leaf, increased through shoot ontogeny, but the size of the divergent leaf remained constant. There were progressive decreases in the plastochron and relative plastochron rate of leaf elongation, but no significant change in relative chronological rate of leaf elongation, through ontogeny. Thus, divergent leaves of similar sizes occupied different relative positions in the array of leaves on stems of different plastochron ages. These observations are partially consistent with theoretical network model predictions on early leaf vein development. The empirical data of this study suggest additional features of leaf development that should be incorporated into future simulation models for leaf vein development.