Root Growth

When the root cambium produces secondary xylem and secondary phloem, the secondary xylem, formed medial to the cambium, lies in contact with the primary xylem. The secondary phloem, deposited peripheral to the cambium, pushes all tissues external to it outward, until the small patches of primary phloem are soon pushed quite away from their original positions between the points of primary xylem (see figure 33-7). Another cambial layer, the cork cambium, develops in the pericycle cells; this layer of cells produces the outer bark. The primary tissues lying outside of the cambium, having no way of increasing, fracture and slough away.

As a root ages, it becomes more like a stem. The original primary xylem remaining at its center serves as a clue tot he root's identity, however. Another difference between roots and stems relates to the origins of branch roots and branch stems. Branch roots have their origin in the pericycle; thus, their origin is endogenous. Branch stems, on the other hand, derive from superficial tissues; their origin is therefore exogenous.

Figure 33-7 Cross section of a root exhibiting a small amount of secondary growth. In depositing secondary xylem, the cambium moves outward to form a circle. This pushes the primary phloem outward, where its remnants are represented four small islands. The primary xylem is unaffected. The secondary phloem is thin and compressed. The pericycle is still intact. A new cambium, the phellogen, takes form and produces a small amount of cork. The cortex and epidermis begin to slough away.

Figure 33-8 Branch roots have their origin in the pericycle.