Structure, attachment properties, and ecological importance of the attachment system of English ivy (Hedera helix)

Abstract

Root climbers such as English ivy (Hedera helix) rely on specialized adventitious roots for attachment, enabling the plants to climb on a wide range of natural and artificial substrates. Despite their importance for the climbing habit, the biomechanical properties of these specialized adventitious roots compared with standard roots and their performance in the attachment to different host species or inert substrates have not been studied. Here organs and tissues involved in the attachment are characterized and their significance in regard to a broader functional and ecological aspect is discussed. Depending on the substrate, the root clusters show different types of failure modes at various frequencies, demonstrating the close interaction between the climber and its substrates. With a Young's Modulus of 109.2 MPa, the attachment roots are relatively stiff for non-woody roots. The central cylinders of the attachment roots show a high tensile strength of 38 MPa and a very high extensibility of 34%. In host trees naturally co-distributed with English ivy, a 'balanced' occurrence of failure of the attachment system of the climber and the bark of the host is found, suggesting a co-evolution of climber and host. Maximum loads of root clusters normalized by the number of roots match those of individually tested attachment roots. In comparison with most subterranean roots the properties and structure of the attachment roots of English ivy show distinct differences. There exist similarities to the properties found for roots of Galium aparine, suggesting a trend in not fully self-supporting plants towards a higher extensibility.

Fig. 1.

Basal modes of failure. Schematic drawing of shoot segments of English ivy with one attachment root each. The arrows indicate the direction of the force during the tests. (A) Failure of the substrate; (B) failure of the root; (C) failure of the shoot. ar, attachment root; cc, central cylinder; st, substrate; rb, root cortex; s, shoot. (This figure is available in colour at JXB online.)

Fig. 2.

Tensile testing of individual attachment roots. (A) Schematic drawing of the test set-up. The arrow indicates the direction of displacement. (B) Mounted sample, scale bar: 2 mm. ap, aluminium platelet; ca, cyanoacrylate adhesive; ct, compression–tension load cell; ir, isolated root; lt, linear table.

Fig. 3.

Comparison of failure modes of attachment between different substrate groups. The different modes of failure are shown on the x-axis and the relative frequencies of the failure modes per substrate are shown on the y-axis as a percentage. (A) Results from tests on mortar (pooled data for two types of mortars). (B) Results from tests on cork. (C) Results from tests on tree barks (pooled data for all bark species).

Fig. 4.

Comparison of failure modes of attachment between the seven tested tree barks. The different species of tested tree barks are shown on the x-axis and the relative frequency of the failure modes is shown on the y-axis. Descriptions above the columns indicate the level of bark structuring.

Fig. 5.

Schematic drawings of force application in various testing set-ups and typical force–displacement curves of mechanical tests in which root failure occurred. (A) Test of root clusters. The load is applied to a shoot segment of English ivy, normal to the surface. (A.1) Typical force–displacement curve with a slightly convex force increase followed by a sudden and complete failure of the sample, (A.2) Typical force–displacement curve for samples that showed preliminary failure events in the decreasing part of the curve before complete failure. (A.3) Typical force–displacement curve with a stepwise force decline showing several preliminary failure events after root strength is reached. (B) Tensile test of complete individual attached roots; the load is applied normal to the substrate. (C) Test of complete individual attached roots; the load is applied parallel to the substrate. (D) Test of isolated intact roots with cortex; the load is applied parallel to the longitudinal axis of the root. (E) Test of isolated central cylinders; the load is applied parallel to the longitudinal axis of the root. ar, attachment root; cc, central cylinder; cs, substrate; is, ivy shoot segment; rc, root cortex. (This figure is available in colour at JXB online.)

Fig. 6.

The root strength per root and isolated central cylinders experimentally measured or calculated, respectively (in the case of root clusters), for the different test set-ups. (A.1) Root cluster tests with root failure mode only (category A.1), calculation of root strength per single root. (B) Load applied normal to the substrate. (C) Load applied parallel to the substrate. (D) Tensile test on an intact isolated attachment root with cortex. (E) Tensile test on an isolated central cylinder. Lower case letters on top of A.1–D indicate statistical differences (pair-wise U-tests with Šidák correction). (This figure is available in colour at JXB online.)