Biological Foundations 112, Lecture 4


Secondary Growth

Chapter 31, pages 668-691
Plant Anatomy-2   0:00-10:29
Plant Anatomy-2 14:44-33:32


I.   Introduction

     A.   Primary growth is responsible for:

            1.   Growth in length of the stem

            2.   Produces the basic tissue pattern in the stem

     B.   Secondary growth is responsible for:

            1.   Growth in girth of the stem

            2.   Produces secondary (new and more) vascular tissues

            3.   Provides a continuous connection of meristem cells between the primary tissues of  
                  the roots and the primary tissues of the shoots.

            4.   This is why some trees (California Redwoods and Bristlecone Pines) to live to such a
                  great age (several thousand years)

II.  Development of the Vascular Cambium

      A.   The function of the vascular cambium is to produce secondary growth, thus the vascular
             cambium must be formed before secondary growth can occur

      B.   Two regions of the primary stem contribute to the vascular cambium

             1.   Fasciscular cambium - the meristem cells within the vascular bundle

             2.   Interfascicular cambium - the meristem cells between the vascular bundles

      C.   The differentiation of the fascicular cambium

             1.   Not all of the procambium in the vascular bundle differentiates into xylem or phloem

             2.   This undifferentiated procambium is called the residual procambium

             3.   The residual procambium is 2-4 cells wide and remains meristematic

             4.   The residual procambium will be called vascular cambium when the
                   vascular cambium begins to divide to form secondary tissue

             5.   Thus, fascicular cambium is the regions of the vascular cambium that originated
                   within the vascular bundle

      D.   The differentiation of the interfascicular cambium

             1.   This portion of the vascular cambium originates in the pith rays between the vascular
                   bundles

             2.   A band of parenchyma cells, about 2-4 cells wide de-differentiate and become
                   meristematic

             3.   When these cells begin to divide they are called vascular cambium and are referred
                    to as interfascicular cambium because of where they originated

      E.   The vascular cambium

            1.   Becomes vascular cambium when the fascicular and interfascicular cambium join to
                  form a complete cylinder around the stem

            2.   As soon as the cylinder is formed the vascular cambium becomes active by dividing
                  on both the inner and outer surface of the vascular cambium surfaces.

            3.   Activity of the vascular cambium

                  a.   New xylem cells are formed inwardly and attached to the previously produced
                        xylem

                  b.   New phloem cells are formed outwardly and are attached to the previously
                        produced phloem

                  c.   Two kinds of vascular cambium cells exist:

                        (1)  Ray initials

                               i.  Produces the radial (lateral) transport system cells

                               ii.  In xylem and phloem these are the parenchyma cells

                        (2)  Fusiform initials

                               i.   Produces the axial (vertical) transport system cells

                               ii.  In xylem and phloem these are the sieve-tube member, companion cells,
                                   tracheids, vessels, and fibers

                  d.   Generally the xylem-producing cells are more active than the phloem-producing
                        cells

                  e.   The cambium is active from the spring to the fall and is inactive in the winter

                  f.    The yearly activity of the cambium produces the annual rings in the xylem

II.  Periderm (cork)

      A.   Function of the periderm:

             1.   Increase in diameter of the stem occurs with the activity of the vascular cambium

             2.   This causes the protective epidermis to crack and split open

             3.   Thus, a need for a meristematic layer at the outer edge of the phloem for the internal
                   protection of the stem

             4.   Thus a layer of cork cambium forms outside of the phloem.

             5.   The cork cambium forms a layer of waxy cork cells

             6.   The cylinder of cork cambium increase in diameter as the stem increases in diameter

      B.   Formation of  the cork

             1.   In the young stem (1 year old or less)

                   a.   Cortical cells just under the epidermis become meristematic

                   b.   Produces a layer 1-2 cells thick of cork cambium called phellogen

                   c.   Phellogen produces a layer of cork cells 4-6 cells thick external (toward the
                         epidermis) to the phellogen

                   d.   Phellogen produces a single layer of cells, phelloderm, internal (toward the
                         xylem and phloem) to the phellogen

             2.   Structure of the cork cells

                   a.   Cells are flattened

                   b.   Cell walls contain suberin, a waxy substance

             3.   In old stems (more than 1 year old, generally 3-4 years)

                   a.   A new phellogen forms because the former phellogen dies

                   b.   The new phellogen forms in the outer region of the still-living phloem

                   c.   New phellogens will form about every one to four years depending upon the
                         species of tree

      C.   Bark

             1.   As the layers of cells outside the vascular cambium die, they are sloughed off as
                   bark

             2.   In the young stem the bark contains:  epidermis, cork, cork cambium, phelloderm,
                   cortex, and phloem

             3.   In the old stem the bark contains:  cork, cork cambium, phelloderm, and phloem

III. Other Points about Woody Dicot Stems

      A.   Microscopic Sectioning

             1.   Cross Section

             2.   Radial Section - longitudinal section on the diameter of the stem

             3.   Tangential Section - longitudinal section on the tangent (perpendicular to the
                   radius) of the stem

      B.    Wood - xylem tissue

      C.    Softwood - wood with only tracheids in it

      D.    Hardwood - wood with both tracheids and vessels in it

      E.    Heartwood - wood in the center of the tree, no longer conducting

      F.    Sapwood - wood at the periphery of the stem, actively conducting


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