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Preferential states of longitudinal tension in the outer tissues of Taraxacum Officinale (Asteraceae) peduncles
Author(s) -
Niklas Karl J.,
Paolillo Dominick J.
Publication year - 1998
Publication title -
american journal of botany
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.218
H-Index - 151
eISSN - 1537-2197
pISSN - 0002-9122
DOI - 10.2307/2446340
Subject(s) - biology , anatomy , tension (geology) , xylem , ultimate tensile strength , peduncle (anatomy) , botany , composite material , materials science
We tested Wilhelm Hofmeister's hypothesis that the outer layers of herbaceous stem tissues are held in a preferential state of longitudinal tension by more internal stem tissues that are held in a reciprocal state of compression. We measured (1) the biaxial stiffness of dandelion peduncles that were barometrically inflated with a Scholander pressure bomb, and (2) the stiffness and mechanical behavior of different layers of tissues that were surgically manipulated as longitudinal strips placed in uniaxial tension. Hofmeister's hypothesis predicts that stems will shorten and expand in girth as their volume transiently increases (due to barometric or hydrostatic inflation), that they will longitudinally rupture when excessively inflated, and that the principal stiffening agents in their outer tissues will be aligned in the longitudinal direction with respect to stem length. Our experiments confirmed these predictions: (1) the longitudinal strains observed for inflated peduncles were negative and smaller than the circumferential strains such that stems contracted in length and expanded in girth, (2) peduncles longitudinally ruptured when excessively inflated, (3) surgical experiments indicated that the epidermis was stiffer in longitudinal tension than any other immature peduncle tissue and was as stiff as any other tissue region in mature stems, and (4) microscopic analyses showed that the net orientation of cellulose microfibrils in the cell walls of the outer region of stem tissues was parallel to stem length. A strong positive correlation existed between the tensile stiffness of tissues and the net orientation of cell wall microfibrils. A biomechanical model for the dandelion peduncle wall was constructed based on the thermal behavior of a cross‐ply unidirectional composite material whose adjacent plies contain orthogonally oriented stiffening agents. This model accurately predicted the magnitudes of the tensile stiffness of different tissue regions, the proportional thickness of these regions, and the formation of a bulge in the epidermis and a crease on the inner surface of the stem wall when the innermost region of stem tissues was transversely cut. For each of its variants possessing an outer ply with longitudinally aligned stiffening agents (i.e., a 0° ply) and containing at least one internal ply with transversely aligned stiffening agents (i.e., a 9° ply), the model predicted a state of preferential longitudinal tension in the 0° ply (or plies) and of preferential longitudinal compression in the 9° ply as posited by Hofmeister's hypothesis.