Secondary Growth in Roots
In some woody eudicots the thickening and strengthening of the root system is important in supporting the trunk. Most dicot roots possess at least a small amount of secondary thickening (Fig. 3.5), with the exception of a few herbaceous species such as Ranunculus (Fig. 3.3). In contrast, secondary growth in roots is extremely rare in monocots, even among arborescent or woody species that possess a secondary thickening meristem. A notable exception is Dracaena, in which a limited region of secondary tissue is formed.
As in the stem, secondary vascular tissues of the root are produced by a vascular cambium. This initially develops in the regions between the primary xylem and phloem, then in deriv-atives of cell divisions in the pericycle next to the xylem poles. Since cambial activity proceeds in this sequence, the xylem cylinder soon appears circular in transverse section (Fig. 3.5). Further pericyclic cell divisions result in a secondary cortex, and in many cases a periderm forms, particularly where secondary growth is extensive. The epidermis splits and is sloughed off together with the primary cortex and endodermis. Root secondary xylem usually resembles that of the stem in the same plant, but may differ in several respects. For example, in Quercus robur stem wood is ring porous, with earlywood vessels markedly larger than latewood vessels, but root wood is diffuse porous, with vessels of relatively consistent sizes across each growth ring. As with trunk wood, root wood of individual taxa often exhibits identifiable characteristics.