Branching morphology and biomechanics of ivy (Hedera helix) stem-branch attachments

Hedera helix is a striking example of a plant with morphological traits and growth habits that vary between juvenile and adult phases. The present study focuses on its branching morphology and variations with age and change in growth habit, based on conspicuous stem-branch attachments previously described in related Araliaceae species.

We decorticated and morphologically analyzed 300 samples of ramifications from prostrate, climbing and self-supporting axes of H. helix. For bending experiments, 103 specimens with the self-supporting growth habit were collected.

Ramifications of H. helix exhibited a so-called "finger-like" branching morphology with abaxial branch lobes and varying degrees of fusion of woody strands. Three categories of woody strand coalescence were defined. Biomechanical experiments in which the branches of stem-branch attachments were bent revealed two main modes of failure, breaking failure in (1) the attachment region and (2) the side branch.

Coalescence of woody strands in H. helix ramifications results from accumulation of secondary xylem with age, influenced by mechanical stimuli causing specific loading situations during different growth habits. Mechanical experiments showed the tendency toward failure in the side branch with increasing fusion of woody strands, affected by the diameter ratio of the side branch to the main axis. Of specific interest is the comparison of H. helix branching with tropical Araliaceae, which do not show the described coalescence of woody strands to this extent. Fracture toughness of self-supporting H. helix axes with merged stem-branch attachment regions are comparable to other self-supporting plant species, despite anatomical and ontogenetic differences.