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

Abstract

The hypothesis is proposed that nodes of hollow plant stems act as spring-like joints by storing strain energy when stems are bent and releasing this energy to elastically restore the original postures of stems when bending forces are removed. This hypothesis was tested by subjecting stem segments consisting of four nodes and three intervening hollow internodes to axial compressive loads and by determining the natural frequencies of vibration of their nodes. Compression tests were used to determine the critical load required to produce elastically recoverable deformations for each of a total of 115 stem segments of the grassArundinaria técta(Walt.) Muhl. Each segment was observed to flex at or very near its nodes while internodes appeared to act as rigid bars. The natural (fundamental) frequencies of vibrations of the nodes of these stem segments were subsequently determined and equalled those predicted by engineering theory assuming that nodes behave as spring-like joints. The data from resonance frequency tests were then used to calculate the spring constants of stem segments (i.e. the force required to produce a unit deflection in stems). These constants were found to agree with those predicted by theory provided that nodes acted mechanically as spring-like joints. The transverse septa of the nodes of 20 randomly selected stem segments were perforated with a needle and the spring constants of the impaired nodes were remeasured and compared with those of the same stems before surgical manipulation. On average, nodal spring constants were reduced by 35%. This reduction agreed with the prediction that the perforation of septa would significantly reduce the ability of nodes to store strain energy. Collectively, these results are interpreted to support the hypothesis that septate nodes can store and release strain energy. The hypothesis is discussed further in light of the behaviour of a physical model which shows that nodal ‘diaphragms’ can substantially stiffen a hollow cylindrical structure, although they are neither essential for the storage of strain energy nor the subsequent elastic restoration of the model's shape once bending loads are removed.