Abstract
The external breaking forms of elongated internodes in rice culms were observed, and were classified into three patterns basically, namely, the broken type (Fig. 1), the split type (Fig. 2) and the separated type (Fig. 3). The broken type was the most frequent in the lower internodes both in the breaking test by hands and in the lodged culms by natural force like as wind and rain (Table 1). Histological observations on the breaking parts by free hand sections indicate that the breakings seem to begin from the epidermis and cortical fibers and extend to parenchyma. In fold parts of the broken type failure, the cortical fiber cells are bent remarkably and divided longitudinally in each other, but are not cut off transversely (Figs. 4-7). In split parts of the broken and the split type failure, most of the epidermal and cortical fiber cells are divided in middle lamella. The parenchymatous cells in breaking parts are broken irregularly (Figs. 8-10). A dynamic consideration is attempted to clarify the mechanism which causes the three patterns of the breaking forms in the lower internodes. In a bent internode, bending stress and shearing stress seem to cause as illustrated in Figs. 11 and 12. Therefore, the compressive stress acting on the concave and the tensile stress acting on the convex seem to cause the failure of the broken type and the separated type, respectively. The shearing stress, on the other hand, acting on the transverse plane seem to cause the failure of the split type (Figs. 11 and 12).
