Hierarchical Functionally Gradient Structures of Bamboo, Barley, and Corn

Bamboo and certain other plants have excellent characteristics based on functionally gradient structures. Figure 1 shows a cross section of bamboo, corn, and barley culms. The collection of solid dots in bamboo is called the bundle sheath, which acts as fibers for the plant. These bundle sheaths are shown in the magnified photograph of corn and barley. These two plants are made of composite materials reinforced by fibers of the bundle sheath; the fiber strength is about 10 times that of the matrix. Furthermore, the fiber distribution is dense in the outer region and sparse in the inner region. It can be said that the fiber distribution forms a gradient structure. The fibers in barley are sparse in the narrow cross section and are different from the fibers in corn and bamboo. The number of honey-comb-shaped cells in barley is higher on the outside than the inside region. Therefore, barley also has a gradient structure. These relations of the microscopic gradient structure are displayed graphically by the cross-sectional photographs. Figures 1a and 1b present the fiber contents of bamboo and corn, respectively, with respect to radius measured from the outer radius. Figure 1c gives the gradient distribution of the cell number for barley with respect to radius. These gradient features probably relate to the bending-stress distribution caused by wind, rain, and snow—the so-called environmental loads.