Always on the bright side: the climbing mechanism of Galium aparine

Abstract

Galium aparine is a herbaceous climbing plant that attaches to host plants mainly via its leaves, which are covered by hooked trichomes. Although such hooks are found on both leaf surfaces, the leaves of G. aparine are mainly positioned upon the leaves of supporting plants and rarely beneath. In order to understand the mechanism underlying this observation, we have studied structural and mechanical properties of single leaf hooks, frictional properties of leaf surfaces, turgor pressure in different leaf tissues and bending properties of the leaves in different directions. Abaxial and adaxial leaf hooks differ significantly in orientation, distribution, structure and mechanical properties. In accordance with these differences, friction properties of leaves depend on the direction of the applied force and differ significantly between both leaf surfaces. This results in a ratchet mechanism. Abaxial leaf hooks provide strong attachment upon the leaves of adjacent plants, whereas adaxial hooks cause a gliding-off from the underside of the leaves of host plants. Thus, the leaves of G. aparine can function as attachment organs, and simultaneously orient themselves advantageously for their photosynthetic function. Further adaptations in turgor pressure or concerning an anisotropy of the flexural stiffness of the leaves have not been found.

Figure 1.

(a) Tangle of Galium aparine shoots in a hedgerow, (b) detail of a flowering shoot.

Figure 2.

Diagram of the experimental set-up in contact separation force experiments with single leaf hooks on (a) adaxial and (b) abaxial leaf surfaces. Hooks were pulled with a Kevlar loop at different angles relative to the leaf blade until the hooks gave way or until the loop slipped off. Plus symbols (+) denote traction in the direction of hook orientation; minus (−) symbols denote traction against the direction of hook orientation.

Figure 3.

Experimental set-up for friction experiments with leaves of Galium aparine. (a) Leaves (1) were attached to a load cell force transducer (2) and moved horizontally over a test surface (3) by a step motor device (4). (b) Schematic drawing of the experimental situation at the interface during friction measurements, where the abaxial leaf surface (grey) is moved over the test surface (hatched) in the direction (arrow) of the orientation of leaf hooks.

Figure 4.

Representative force–time curves obtained during the traction of the abaxial side of a single leaf over the VELCRO Vel-Loop surface in the direction of the orientation of trichome hooks: (a) 150 s and (b) 510 s after cutting from the plant. Force peaks indicate ‘temporary hooking’ of single leaf hooks with the substrate. The 10 maximum force peaks of each test run are numbered according to their absolute values (highest value of the 10 maximum force peaks = 1, lowest value = 10). The speed of motion was 3.1 mm s⁻¹.

Figure 5.

Leaf hooks of Galium aparine. (a) Lateral view of a leaf: hooks on the abaxial surface (AB) of the leaves are curved towards the leaf base (LB), hooks on the adaxial surface (AD) are curved towards the leaf tip (LT). (b) Distribution of hooks: adaxial hooks (AD) are distributed evenly over the surface area, abaxial hooks (AB) occur exclusively on midrib and leaf margins (cryo-scanning electron microscope (SEM) micrograph). (c) Adaxial hook, stained with acridine orange, revealing the lignification of the tip in fluorescence microscopy. (d) Adaxial hook, SEM. (e) Freeze fracture at the base of an adaxial hook. (f) Fluorescence of an entirely lignified abaxial hook, stained with acridine orange. (g) Abaxial hook, cryo-SEM micrograph. (h) Abaxial hook, stained with toluidine blue; scale bar, 100 µm.

Figure 6.

Box-and-whisker diagram of the contact separation forces obtained for single leaf hooks. The ends of the boxes define the 25th and 75th percentiles, with a line at the median and error bars defining the 10th and 90th percentiles. Results marked with the same letter do not differ significantly. Hooks could not be pulled at an angle of 90° in the direction of their curvature owing to slipping of the Kevlar. Plus symbols (+) indicate traction in the direction of hook orientation; minus symbols (−) indicate traction against the direction of hook orientation. Unfilled bars, adaxial; filled bars, abaxial.

Figure 7.

Friction forces generated by abaxial and adaxial leaf surfaces 150 s after cutting, pulled over different artificial test surfaces in the direction of hook orientation (cf. figure 3). Results marked with the same letter do not differ significantly. Error bars represent standard deviations of the mean. Unfilled bars, adaxial; filled bars, abaxial.

Figure 8.

Mean values of friction forces generated by abaxial and adaxial leaf surfaces at different times after leaf cutting given in percentage of the value obtained at the first run 150 s after cutting. Data, obtained for VELCRO Vel-Loop and the Spurr resin moulds of both the abrasive papers and the foam plastic were pooled, as there was no significant difference between the friction force generated with these test surfaces. Results marked with the same letter do not differ significantly. Error bars represent standard deviations of the mean. As the value obtained at the first experimental run is set as 100% for all test surfaces, no standard deviation is given for this value. Unfilled bars, adaxial; filled bars, abaxial.

Figure 9.

Failure of abaxial hooks after the fifth friction test on the same leaf. (a) Abaxial leaf hook with broken hook-shaped tip; (b) abaxial leaf hook entirely pulled out of the leaf surface.