Biological Foundations 112, Lecture 9


Secondary Growth in Roots

Chapter 31, pages 668-691
Plant Anatomy-2  10:29-14:44


I.   Secondary Growth in Roots

     A.   Formation of vascular cambium

            1.   Forms from the residual procambium between the xylem and phloem

            2.   Forms from the pericycle at the tips of the protoxylem poles.

            3.   Finally all parts link up into a single wavy ring

     B.   Activity of vascular cambium

            1.   When the ring is complete the vascular cambium of the root acts just like the vascular
                  cambium of the stem

            2.   The vascular cambium in the region of the primary phloem is more active than the
                  other vascular cambium until the ring of vascular cambium become circular and
                  not wavy

            3.   Old root has annual rings just like a stem

            4.   Not possible to tell an old root from an old stem

     C.   Cork cambium

            1.   Also forms a cork cambium as does the stem

            2.   Originates in the pericycle in the root (originates in the cortex in the stem)

            3.   In the old root it will form in the outer regions of the living phloem as is the case for
                  the stem

     D.   Lateral root formation

            1.   Lateral roots originate from cells in the pericycle at the tip of a xylem pole

            2.   Grows out through the cortex and epidermis by crowding and dissolving the cells
                  ahead of the root cap

            3.   Lateral roots emerge with a root cap and continue to grow and elongate just like any
                  other root

II. Mycorrhizae

    A.   Roots of plants in symbiosis with a fungus

    B.   Two types of mycorrhizal roots

          1.   Ectotrophic

                a.   Fungus external or on surface of root or cortical cells

                b.   Occurs in pines, birches, willows, and oaks

                c.   Drastic change occurs in root shape

                      (1)  0.5 cm long

                      (2)  No root cap

                      (3)  Produces a simple monarch root

                      (4)  Fungus squeezes in between the cell walls of the cortex and forms a covering
                            sheath or mantle around the root

          2.   Endotrophic

                a.   Fungus enters and actually penetrates the cortical cells

                b.   No mantle forms over the root

          3.   More efficient in mineral absorption

          4.   Not absolutely essential for the growth of the plant

          5.   Mycorrhizae may be beneficial in addition by:

                a.   Secreting hormones

                b.   Secreting antibacterial agents

III. Bacterial Nodules

      A.   Fix nitrogen by changing N2 (molecular nitrogen) to NH4 (ammonium ion)

      B.   NH4+ is one of the forms of nitrogen that plants like

      C.   Rhizobium is the bacterium responsible for the fixing of nitrogen in the roots of legumes

      D.   Infection process:

             1.   Root hairs become infected with Rhizobium

             2.   Thread of infection passes through the root hair to the epidermis to the cortex

             3.   Bacteria divide and stimulate cortex to grow into a nodule


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