Activity of Vascular Cambium

When the vascular cambial ring is active, its initials divide periclinally (parallel to the stem's surface) and cut off new daughter cells on both faces at once - toward the inside and toward the outside. The cells cut off on the inner face differentiate into secondary xylem, while the cells cut off on the outer face differentiate into secondary phloem; at intervals around the ring, the cambium also cuts off narrow horizontal bands of parenchyma that pass radially through both the secondary xylem and secondary phloem, forming the rays. Crucially, the cambium does not divide equally on both faces - over a growing season it produces many more layers of xylem than of phloem, so a cross-section built up over successive periods of activity shows first-formed, second-formed, third-formed (and so on) xylem layers stacking up rapidly on the inner face, while only one or two comparably-aged phloem layers accumulate on the outer face in the same span. Because new secondary xylem and secondary phloem keep being laid down season after season, the stem …