Functional Morphology And Structure Of The Attachment Systems In Climbing Plants

Since the Charles Darwin era,naturally occurring attachment systems in climbing plants have intrigued researchers.Afterwards,unfortunately,there are no more works carrying out deeply.In recent years,the basic scientific questions behind the attachment systems of climbing plants are put forward in terms of materials science and bionics,so as to promote the development of bio-inspired adhesive products effectively.Prior to the potential application-oriented researches,we should comprehensively understand the structure and mechanism of the botanic attachment systems.In this dissertation,the tendril-climber Parthenocissus tricuspidata and the root-climber Syngonium podophyllum are systematically studied with regards to their basic morphology and structures,respectively.The main contents and results are as follows:1.The attachment system of Parthenocissus tricuspidata(1)The morphological characterization and celluar structures of adhesive pads in different growth stages are compared by light and electron microscopes.Young pad is homogeneous axisymmetric,and stomas can be found on its surface.The dorsal(non-attached)and ventral(attached)epidermis of mature pad are significantly different,but both are rich in mucilage.Old pad is lignified highly,and flattened with its periphery cells rolled spirally.In combination with SEM and TEM techniques,we confirm that the intracellular mucilage is scattered in the form of the micro-spheric aggregations.Once secreted to the extracellular space,the mucilage coalesces into a gel with high mobility and can fill into any micro or nano gaps on the substrate.Adhesive pads are alternatively adhered to the supports,which can not develop normally on the transparent tape,glass,aluminum,copper substrates.When the surrounding environment is fairly wet,aerial roots will emerge on the nodes of stem,further strengthening the anchorage of stem to the support.(2)Three-dimensional visualization of the attachment system of Parthenocissus tricuspidata is first realized by means of synchrotron radiation X-ray imaging.The anatomic details are demonstrated in nondestructive manner,and the internal characteristics are also unraveled.The high X-ray-attenuation inclusions are identified as calcium oxalate raphide crystals by histochemical methods.The 3D distribution and quantification of crystals suggest that the crystals are concentrated close to the substrate,promoting the stability of adhesive pads.The mature adhesive pad achieves the structural and functional optimization in the arrangement of tissues.The mucilage-containing epidermis,the crystal-containing cortex and the xylem-developed vascular cylinder constitute a hierarchical reinforcement mechanism,avoiding the occurrence of the internal failure of adhesive pads effectively and ensuring the durability to the maximum extent.2.The attachment system of Syngonium podophyllum(1)The arrangement of aerial roots aesthetically follows a regular pattern.There are two types of aerial roots at each node: several skototropic clasping roots for the surface climbing and a single feeding root for the water or nutrition uptaking.The number and size of aerial roots are closely relevant to the climbing environment.The growth dynamic of clasping roots shows that the growth rate is fast and the length of root increases exponentially in the early stage.In the latter stage,the growth rate tends to be stable,and the length of root increases linearly.After reaching a certain length,the root stops growing.Aerial roots can normally develop on any inorganic and organic substrates,but the attachment strength changes with the texture and roughness of the substrates.On the tree substrate,to the attachment unit of aerial root,the average adhesion force is 11.3 N,and the maximum is 22.5 N.To a single 1-cm-long aerial root,the average axial shearing stress is 1.9 N,and the maximum is 5.8 N.(2)The microscopic structures and mechanism of the attachment system of aerial roots are revealed through light microscope and scanning electron microscope.The adhesion of aerial root is a time-oriented process from weak to strong.The root cap can secrete mucus,inducing the initial weak adhesion and serving as a lubricant to reduce friction with the substrate when root elongation.The side of aerial root toward the substrate develops large amounts of unicellular root hairs,directly responsible for the strong adhesion process.The mucilage discharged from root hairs is primarily confirmed as a composite of polysaccharide and protein by the histochemical technique.The secretion and curing of mucus directly promote the interfacial adhesion of root hair cells and substrate.From the structural perspective,the extreme extension of root hairs on the substrate increasing contact adhesion area,the formation of special structures on the tip of root hairs generating mechanical interlocking with the substrate,and a variety of deformation of root hairs adjusting the interaction of the root–substrate,consistently ensure the validity and stability of the attachment.(3)The unique helical-crack root hairs are found for the first time in the attachment system of aerial roots.Different from the common spirally curled root hairs after dehydration,they are generated since the root hair cell wall ruptures spirally into a ribbon-like structure.Scanning electron microscopy and light microscopy confirm the universality and particularity of the helical ribbon-like root hairs.The point of rupture initiation occurs randomly.The direction of all helices is left-handed.The formation of helical-crack structures of root hairs involves the crack initiation,the crack propagation and the final rupture.The root hair cell wall is of multilayer structure,and cellulose microfibrils are arranged spirally on the inner and outer wall,which is the structural basis of the formation of helical-crack root hairs.From the functional perspective,the helical-crack root hairs serving as energy-dissipating units retard the failure of adhesion under high winds and loads.(4)The structural transition of root hairs from a tube to a helical-crack ribbon is achieved through a creative glue-pulling method,suggesting the external force can directly result in the formation of helical-crack root hairs.In virtue of the micro-nano mechanical testing system,the real-time dynamic process of structural transition of a root hair under tensile force is recorded.The initial cracking force of a single root hair is only 80 ?N,and the final breaking force is 920 ?N.