Responses of Hollow, Septate Stems to Vibrations: Biomechanical Evidence that Nodes Can Act Mechanically as Spring-like Joints

The hypothesis is proposed that nodes of hollow plant stemsact as spring-like joints by storing strain energy when stemsare bent and releasing this energy to elastically restore theoriginal postures of stems when bending forces are removed.This hypothesis was tested by subjecting stem segments consistingof four nodes and three intervening hollow internodes to axialcompressive loads and by determining the natural frequenciesof vibration of their nodes. Compression tests were used todetermine the critical load required to produce elasticallyrecoverable deformations for each of a total of 115 stem segmentsof the grassArundinaria técta(Walt.) Muhl. Each segmentwas observed to flex at or very near its nodes while internodesappeared to act as rigid bars. The natural (fundamental) frequenciesof vibrations of the nodes of these stem segments were subsequentlydetermined and equalled those predicted by engineering theoryassuming that nodes behave as spring-like joints. The data fromresonance frequency tests were then used to calculate the springconstants of stem segments (i.e. the force required to producea unit deflection in stems). These constants were found to agreewith those predicted by theory provided that nodes acted mechanicallyas spring-like joints. The transverse septa of the nodes of20 randomly selected stem segments were perforated with a needleand the spring constants of the impaired nodes were remeasuredand compared with those of the same stems before surgical manipulation.On average, nodal spring constants were reduced by 35%. Thisreduction agreed with the prediction that the perforation ofsepta would significantly reduce the ability of nodes to storestrain energy. Collectively, these results are interpreted tosupport the hypothesis that septate nodes can store and releasestrain energy. The hypothesis is discussed further in lightof the behaviour of a physical model which shows that nodal‘diaphragms’ can substantially stiffen a hollowcylindrical structure, although they are neither essential forthe storage of strain energy nor the subsequent elastic restorationof the model's shape once bending loads are removed. Plant stems; nodes; internodes; strain energy; elastic buckling; Brazier buckling; biomechanics