English Ivy (Hedera helix) is a root climber which can climb up to 35 m in height on substrates of various compositions. Despite a vast literature on various aspects of Ivy, its climbing mechanism has not yet been analysed in detail. English Ivy develops adventitious roots in the nodal regions which, under the right conditions, act as climbing organs. To test their attachment properties we used a custom made, portable force-distance gauge on plants growing on house fineries and tree barks. We measured the normal force required for detaching the stem segments from their substrate. In ongoing studies the forces at which single roots fail are tested with custom made stationary micro-force-distance gauge. In a different test series we presented small plates made from various materials to English Ivy plants and investigated the attachment quality. For plants growing on tree bark up to 12.8 N per cm plant stem segment were necessary to detach the stem parts from their substrates. On our test substrate plates Ivy attached itself usually well on structured surfaces such as wood, cork and sponge rubber but was not able to adhere on smooth surfaces such as plain aluminium, glass and copper. Contrary attachment occurs on some smooth artificial substrates such as gelatine and Mylar foil. In these cases Hedera helix excretes a glue-like substance, which facilitates the attachment of the roots to smooth surfaces. Roundish excrescences found on the root hairs are likely to contain the adhesive.
The morphology was investigated using high resolution light microscopy, SEM as well as ESEM. Especially the root hairs - dimensions around 20-200 um in length - of the climbing roots are of interest for understanding the attachment mechanics of English Ivy. Three categories of root hairs, with respect to the shape, can be distinguished: (1) fresh turgescent root hairs, which are pillar like with a round tip, and two kind of dried root hairs, i.e. (2) a grooved variety and (3) a spiral one. Both types of dried root hairs exhibit shoe-horn like tips. Our data further indicate that in addition drying of the roots themselves leads to a substantial repositioning of the root hairs.
From these observations and studies of attached root hairs we deducted a three phase model for the attachment of English Ivy on smooth and structured surfaces. (1) The first phase of the attachment is characterized by chemical adhesion of the root hairs via the adhesive substance contained in the roundish excrescences. (2) The second phase is characterized by changes of shape of the root hairs. (2-1) On smooth surfaces mainly grooved root hairs are found with the groove averted from the substrate, and with sides and tip of the hairs bent upwards. Strings of glue connect the underside of the hairs with the surfaces. (2-2) On structured substrates spiral hairs were interlocked within gaps and fissures of the surface. The root hairs appear to grow into these notches and while drying out they shrink helically and attach to the substrate. The attachment is further strengthened by the shoe horn shape of the tip, acting as a hook, and the possibility of chemical adhesion. Due to the helically shortening of the root hairs the root is further attracted to the surface of the substrate. (3) After the root hairs are dried the root itself starts to shrink in length and diameter. As the roots are attached via the root hairs they pull the plant stems closer to the substrate, which we consider the third phase of Ivy attachment.