Fig. 2
Radial sections showing the presence and absence of starch grains in parenchyma cells. A shows the variation of starch grain content in per unit area of ray parenchyma cells and B shows the variation of starch grain content per unit area of axial parenchyma cells. The numbers 1–12 correspond to the different growth rings of the xylem, with heartwood in 1-4th growth rings, transition zone in 5th growth ring and sapwood in 6-12th growth rings. C depicts the distance between the change in starch grain content and the appearance of heartwood color in a part of the whole xylem, where the heartwood color starts to appear at x = 0 and x = 2000 μm is where the starch grain content decreases abruptly, and the curve is the trend of starch grain change obtained by using the Boltzmann fitting method. The positive axis is in the direction of the cambium and the negative axis is in the direction of the heartwood