Figure 5.

Distribution of rays, such as shown in Fig. 4A, recorded in a large photomontage of tangential sections of hybrid poplar cambium. The montage was scanned and the vertical height was foreshortened. The spindle-shaped ray areas were then traced (stippled regions). Clear areas contain fusiform cambial initials. Sometimes rays had recently undergone binary fission due to intrusive growth of fusiform cells; the two sister rays are outlined within a single oval. Ray cell complexes align on 3 evident parastichies (red lines). Occasionally, a parastichy terminates due to the introduction of a new ray complex arising from fission of a pre-existing ray complex. Circumferential expansion of the cambium accompanied by growth and division of the fusiform cells lying between the sister ray complexes and elsewhere causes the ray complexes to move apart.