The attachment strategy of English ivy: a complex mechanism acting on several hierarchical levels

Abstract

English ivy (Hedera helix L.) is able to grow on vertical substrates such as trees, rocks and house plaster, thereby attaching so firmly to the surface that when removed by force typically whole pieces of the climbing substrate are torn off. The structural details of the attachment process are not yet entirely understood. We studied the attachment process of English ivy in detail and suggest a four-phase process to describe the attachment strategy: (i) initial physical contact, (ii) form closure of the root with the substrate, (iii) chemical adhesion, and (iv) shape changes of the root hairs and form-closure with the substrate. These four phases and their variations play an important role in the attachment to differently structured surfaces. We demonstrate that, in English ivy, different mechanisms work together to allow the plant's attachment to various climbing substrates and reveal the importance of micro-fibril orientation in the root hairs for the attachment based on structural changes at the subcellular level.

Figure 1.

The different morphological levels involved in the attachment process. (a) Juvenile shoot of English ivy on tree bark with attachment roots. (b) Attachment roots with root hairs, SEM picture. Scale bar, 30 µm. (c) Group of root hairs, SEM image. Scale bar, 20 µm.

Figure 2.

Structural evidence for different phases in the attachment process of English ivy. (a) Tip of a root hair with spherical excrescences, which are most probably the containers of the gluing substance excreted by the plant in phase 3 of the attachment process. Scale bar, 5 µm. (b–d) Formation of the spoon-like shape at the tip of root hairs. Images of different root hairs taken at different developmental stages during the shape change. Scale bar, 10 µm. (b) Tip of a fresh root hair, ESEM image. (c) Tip of a root hair in the process of drying showing the first indications of a spoon-shaped tip, SEM image. (d) Tip of a dried root hair in completed spoon-like configuration, SEM image. (e) Single root hair, demonstrating the fibre orientation, SEM image. Scale bar, 20 µm. (f) Differential interference contrast picture of a stained single root hair. The darker lines visible on the root hair show micro-fibril orientation, The picture is a montage of 11 micro-images of one root hair, the lines drawn in black indicate the borders of the individual images. Scale bar, 50 µm. (g) Bar plot diagram of the mean fibre angle orientation versus the relative position on the longitudinal axis over the entire relative length of the root hair in steps of 5% ((0–5%), (5–10%), … (95–100%)). The error bars represent standard deviations. The diagram demonstrates the steep increase in cellulose micro-fibril angles in the last 5% of root hair length; number of tested root hairs: n = 11, pooled data. The box colours indicate two groups with significantly different cellulose micro-fibril angles with a p-value of 7.023 × 10⁻⁸ (Welch two-sample t-test; for details, see §2). The blue group consists of all 5% steps from the base of the root hair to 95% relative length of the root hair length. The orange group consists of the apical 5% of root hair length including the tip of the root hairs.

Figure 3.

Adaptation for smooth and structured climbing substrates. (a) Root hair attached to Mylar foil. Strands of glue can be seen to brace the root hair against the substrate, SEM image. Scale bar on overview 10 µm; scale bar on inset 5 µm. (b) Fresh root hair after first contact with the substrate, schematic drawing. (c) Dried root hair with strands of glue bracing it against the substrate, schematic drawing. (d) Spirally curled flattened root hair, SEM image. (e) Fresh root hair growing into a cavity in the substrate, schematic drawing. (f) Drying root hair anchored to protrusions inside the cavity, pulling the attachment root towards the substrate, schematic drawing.