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Fig. 16.1-4. Radial section of secondary phloem of pine. This radial section shows xylem on the left and phloem on the right; between the two labels is the vascular cambium. Examine the ray between the two arrows. In the vascular cambium at this point is a set of ray initials that has produced the xylem ray (with ray tracheids visible along the top) as well as the phloem ray (in the phloem, the uppermost cells differentiate as tall parenchyma cells, not as ray tracheids). This section shows only a little of the phloem ray; why? Notice the stacks of parenchyma cells on the right side of the micrograph, aligned with the ray those are part of the same ray. As the sieve cells collapse, the ray is thrown into undulations, so a straight cut with the microtome knife will not be able to follow the undulating ray. The section ends up having just short segments of the ray.