Fissures are cracks in the rice kernel.
During milling, kernels with fissures tend to break, causing lower head and total percentages. Rice is subject to fissuring as it dries below a critical moisture level, either in the field or after harvest. The critical moisture is believed to differ among varieties, although exact data are not available for most varieties. When the kernel dries it shrinks and when it gains moisture it expands. Apparently above the critical moisture the kernel is pliable enough to allow some expansion and contraction without damage.