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Comparative Study
. 2015 Dec 2;10(12):e0143850.
doi: 10.1371/journal.pone.0143850. eCollection 2015.

Rose Prickles and Asparagus Spines--Different Hook Structures as Attachment Devices in Climbing Plants

Friederike Gallenmüller  1 , Amélie Feus  1 , Kathrin Fiedler  1 , Thomas Speck  1
Affiliations
Comparative Study

Rose Prickles and Asparagus Spines--Different Hook Structures as Attachment Devices in Climbing Plants

Friederike Gallenmüller et al. PLoS One. .

Abstract

Functional morphology and biomechanical properties of hook structures functioning as attachment devices in the leaning climbers Rosa arvensis, Rosa arvensis 'Splendens', Asparagus falcatus and Asparagus setaceus are analysed in order to investigate the variability in closely related species as well as convergent developments of hook structure and properties in distant systematic lineages (monocots and dicots). Prickles and spines were characterised by their size, orientation and the maximum force measured at failure in mechanical tests performed with traction forces applied at different angles. In Rosa arvensis and Rosa arvensis 'Splendens' three types of prickles differing largely in geometrical and mechanical properties are identified (prickles of the wild species and two types of prickles in the cultivar). In prickles of Rosa arvensis no particular orientation of the prickle tip is found whereas in the cultivar Rosa arvensis 'Splendens' prickles gradually gain a downward-orientation due to differential growth in the first weeks of their development. Differences in mechanical properties and modes of failure are correlated to geometrical parameters. In Asparagus falcatus and Asparagus setaceus spines are composed of leaf tissue, stem tissue and tissue of the axillary bud. Between species spines differ in size, orientation, distribution along the stem, tissue contributions and mechanical properties. The prickles of Rosa arvensis and its cultivar and the spines of the studied Asparagus species have several traits in common: (1) a gradual change of cell size and cell wall thickness, with larger cells in the centre and smaller thick-walled cells at the periphery of the hooks, (2) occurrence of a diversity of shape and geometry within one individual, (3) failure of single hooks when submitted to moderate mechanical stresses (Fmax/basal area < 35 N/mm²) and (4) failure of the hooks without severe stem damage (at least in the tested wild species).

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Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Fig 1
Fig 1. Types of mechanical tests.
Mechanical tests were performed with a) traction applied via a Kevlar loop resulting in a combination of bending and shear stresses (Rosa prickles and Asparagus spines), b) traction parallel to the stem applied at the base of the hook via perforated aluminium plates (Rosa arvensis ‘Splendens‘ type II prickles, resulting in pure shear stresses) and c) traction perpendicular to the stem and applied to the whole Rosa prickle, resulting in pure tension stress.
Fig 2
Fig 2. Directions of traction forces in the mechanical tests.
Traction force was applied with a Kevlar loop. 1: parallel to the axis and oriented to the apex, 2 and 4: with an angle of +/-45°, 3: with an angle of 90°, 5: parallel to the axis and oriented to the stem base.
Fig 3
Fig 3. Analysis of prickle and spine geometry.
Measured parameters are h (height), l (length of prickle base), w (maximum width of prickle base), v (distance of vertex from prickle base) and the angle between t (inner tangent) and the stem.
Fig 4
Fig 4. Morphology of different types of long axes and prickles in Rosa arvensis and Rosa arvensis ‘Splendens‘.
a) long axis of Rosa arvensis, b) long axis of Rosa arvensis ‘Splendens‘, Type I, c) long axis of Rosa arvensis ‘Splendens‘, Type II.
Fig 5
Fig 5. Development of prickles in the different types of long axes of Rosa arvensis ‘Splendens‘.
a) type I, b) type II. In the younger stages (day 1–7) the glandular margins of adjacent leaf petioles lie flat against the axis (stem apex to the right).
Fig 6
Fig 6. Morphology of stems and spines in Asparagus falcatus.
a) shoot tip with spines lying flat against the axis, b) same shoot tip 5 days later with emerging axillary buds, c) transition zone with spines elevated at different angles, d) zone near the apex with fully developed spines erected at higher angles (≤ 80°).
Fig 7
Fig 7. Morphology of axes and spines in Asparagus setaceus.
a) shoot tip with young, not yet fully developed spines, b) middle segment with fully differentiated spine.
Fig 8
Fig 8. Transverse section of a Rosa arvensis ‘Splendens‘ stem with prickle (type I).
The section is stained with FCA. x: xylem, ph: phloem, s: sclerenchyma, cp: cortical parenchyma, e: epidermis, co: cork, pt: lignified prickle tissue.
Fig 9
Fig 9. Spine of Asparagus falcatus.
a) Side-view of a spine with split leaf structure and emerging axillary bud, black lines indicate section planes of cross sections b) and c), b) and c) cross sections at different planes (from different specimens) both stained with toluidine blue (box: close up from a different, unstained cut), d) longitudinal section stained with toluidine, red lines indicate modes of failure observed in the mechanical tests, a: failure of the tip, b: failure of the whole spine). l: leaf tissue, s: stem tissue, ls: lateral shoot tissue.
Fig 10
Fig 10. Spine of Asparagus setaceus.
a) Side-view of a spine, lines indicate section planes of cross sections b) and c), b) and c) cross sections at different planes (from different specimens), b) stained with FCA, c) stained with toluidine blue (box: close up from a different cut), d) longitudinal section stained with toluidine blue. l: leaf tissue, s: stem tissue, ls: lateral shoot tissue, co: cork layer.
Fig 11
Fig 11. Schematic interpretation of the spine structures in Asparagus falcatus and Asparagus setaceus.
The interpretation of the spine structures (a): Asparagus falcatus, b): Asparagus setaceus) is derived from longitudinal sections at different planes in different specimens. l: leaf tissue, s: stem tissue (including thick-walled lignified cells), ls: lateral shoot tissue, e: epidermis, co: cork layer.
Fig 12
Fig 12. Maximum force and maximum force/basal area measured at failure of the tested rose prickles.
In the illustrations of force direction the stem apex is situated on the left and the stem base on the right. x indicates that testing with a traction force in this direction was not possible due to slippage of the Kevlar loop. Numbers below indicate the number (n) and percentage (%) of samples having failed in the mechanical tests. In long axes type II of Rosa arvensis ‘Splendens‘ prickles were additionally tested in tension, with the prickle tested perpendicular to the stem, and in shear with the prickle tested parallel to the main axis of the stem (on the right).
Fig 13
Fig 13. Modes of failure of rose prickles submitted to mechanical tests.
Pictures, schematic drawings with typical crack location indicated by a red line and typical force-displacement-diagrams are shown. The initial displacements with almost zero force in the force-displacement diagrams represent the first testing phase when the Kevlar loop gets straightened before the force is exerted on the prickle. a) failure within the prickle (picture Rosa arvensis), b) failure just above the cork layer (picture R. arvensis), c) failure just beneath the cork layer (picture R arvensis) and d) failure within the cortical parenchyma of the stem (picture Rosa arvensis ‘Splendens‘ type II).
Fig 14
Fig 14. Maximum forces and maximum forces/basal area measured at failure in Asparagus spines.
Maximum forces at failure were measured in Asparagus falcatus and A. setaceus with traction forces parallel to the stem and with an angle of 45° (toward the apex). Numbers below indicate the number (n) of samples (100% in all tests) having failed. (***: p ≤0.001, Student’s t-test and Mann-Whitney U-test).
Fig 15
Fig 15. Modes of failure in spines of Asparagus falcatus.
a) longitudinal cut, the red lines indicate failure at the base (1), and failure at the tip (2), respectively, b) typical appearance of a spine having snapped off at the base.
Fig 16
Fig 16. Typical mode of failure in spines of Asparagus setaceus.
a) longitudinal cut, red line indicating failure along the cork layer (failure at the tip not shown), b) typical appearance of the fracture area left by failure along the cork layer. The arrow indicates the small spot of damaged stem tissue being characteristically for this mode of failure.
See this image and copyright information in PMC

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