Behavior of Minimum Free Energy distribution in plants. (A) Cumulative distribution of MFE of real and background precursors of three different lengths: 95-105 nt, 180-190 nt, and 260-270 nt. The Bgr indicates background precursors (from maize genome). (B) Cumulative distribution of MFE of background precursor of size 260-90 nt from five plant species. The source species does affect the nature of distribution, and they tend to cluster into monocots and dicots. While the differences in distributions within the group were insignificant (at confidence level <0.05), those from two groups were highly significant. (C) Same as in A, but the MFE has been normalized by its length. For clarity the distribution of only two values of length have been shown. (D) Empirical distribution of log-odds score of MFE and its best fit theoretical approximation by a modified sigmoid function. The probabilities for background precursors, used in computing log-odds, were average of that of three monocots, namely Z. mays, S. bicolor, and O. sativa.