PLANT HEIGHT AND THE PROPERTIES OF SOME HERBACEOUS STEMS

The volume fraction (VF) of lignified tissues and density-specific stiffness (the quotient of the Young's elastic modulus E and bulk tissue-density p) were determined for the stems of a total of 76 herbaceous species and correlated with plant height X and stem diameter D to determine the influence of stem anatomy and physical properties on the allometry of plant height. Among the 76 bryophyte, pteridophyte, dicotyledonous and monocotyledonous species examined, H increased as the 1.53-power and VF increased as the 1.73-power of D, indicating that the stems of taller species were disproportionately more slender and lignified than those of shorter species. In contrast, E/p increased as the 0.98-power of VF, showing that stem stiffness relative to weight per unit volume of tissues varied in near direct proportion to the degree of stem lignification. The isometric scaling of E/p with respect to VF resulted from the inverse relation between E and p, although the correlation for this relation was poor (r(2) = 0.59). Based on D and E/p, stem critical buckling height H-crit (i.e. the maximum height to which a stem can grow before it elastically buckles under its own weight) scaled as the 1.19-power of D. Since H proportional to D-1.53, the safety-factor against elastic buckling decreased roughly as the 1/3-power of D (i.e. H-crit/H proportional to D-0.34). Despite significant anatomical differences among stems, most of the 76 herbaceous species had stems consisting of a rigid rind surrounding a comparatively incompressible core of tissues.