Plant Height and the Properties of Some Herbaceous Stems

Abstract

The volume fraction (VF) of lignified tissues and density-specific stiffness (the quotient of the Young's elastic modulus E and bulk tissue-density ρ) were determined for the stems of a total of 76 herbaceous species and correlated with plant height H 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 /ρ 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 /ρ with respect to VF resulted from the inverse relation between E and ρ, although the correlation for this relation was poor (r2 = 0·59). Based on D and E/ρ, stem critical buckling height Hcrit (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 ∝ D1·53, the safety-factor against elastic buckling decreased roughly as the 1/3-power of D (i.e. Hcrit /H ∝ 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.