Anomalous Secondary Growth

Not every dicot or monocot that undergoes secondary growth follows the single, continuous vascular-cambium-plus-cork-cambium pattern described in the sections above. Departures from that normal pattern are called anomalous secondary growth, now more often referred to as cambial variants - genuinely abnormal patterns of secondary thickening that occur in some dicots and, more unusually, in a few monocots. Three broad kinds of cambial variant are commonly recognised. The first is the appearance of extra, successive cambial rings outside the first one, seen in plants such as Boerhavia and Amaranthus - once the original cambial ring has been active for a time, additional rings of meristematic tissue arise further out (typically from pericyclic or conjunctive tissue) and each new ring in turn produces its own concentric band of secondary xylem and phloem, so a cross-section shows several concentric rings of vascular tissue rather than one. The second is interxylary (or included) phloem, in which patches of phloem are formed embedded within the secondary xylem itself, rather than lying only outside it as in normal secondary growth - a pattern found in genera such as Leptadenia. The third, and the only way secondary growth occurs in monocots at all, is seen in a handful of arborescent (tree-forming) monocots such as Dracaena and Yucca, which entirely lack a vascular cambium of the dicot kind but instead develop an anomalous, extrastelar secondary meristem in their ground tissue, outside the primary vascular bundles; this meristem cuts off new, scattered vascular bundles inward and parenchyma outward, letting these otherwise cambium-less monocots slowly thicken and, over many years, take on a genuinely woody, tree-like habit. NOTE ON SOURCE: the textbook page carrying this section's own worked examples and figures could not be extracted (an image-only stretch of the source, with the sec …