*3.2. Results of the Stacking Angle Measurement*

The physical stacking angle images of the phloem and xylem were processed using MATLAB for grayscale processing, binarization, and extraction of the image boundary pixels. The slope of the boundary pixels was linearly fitted using the least squares method, and the angle between the stacking angle tangent and the horizontal line was defined as the stacking angle. The average of the two sides was taken as the stacking angle for the phloem, and the left degree was taken as the stacking angle value for the xylem; the image processing process is shown in Figure 10.

**Figure 10.** Physical stacking angle image processing: (**a**) original image of the phloem stacking angle; (**b**) grayscale processing of the phloem image; (**c**) binarization of the phloem image; (**d**) fitting phloem image using the least squares method; (**e**) original image of the xylem stacking angle; (**f**) grayscale processing of the xylem image; (**g**) binarization of the xylem image; (**h**) fitting xylem image using the least squares method.

The average value of the stacking angle of the xylem was determined to be 27.17◦, with a standard deviation of 1.27◦. The average value of the stacking angle of the phloem was 37.93◦, with a standard deviation of 2.29◦.
