Biological Foundations 112, Lecture 3

Primary Tissues, Part II

Chapter 31, pages 668-691
Plant Anatomy-1   9:59-19:05
Plant Anatomy-3 19:43-30:30

I.   Primary Tissues of the Procambium

     A.   Primary Xylem

            1.   Differentiates four kinds of cells

                  a.   Tracheids

                  b.   Vessels

                  c.   Fibers

                  d.   Parenchyma

            2.   Tracheids

                  a.   One of two types of conducting cells in the xylem

                  b.   Dead when fully mature and functioning

                  c.   Contains lignin

                  d.   Morphology of the tracheid cell

                        (1)  Very long and thin cell

                        (2)  Ends of cell with long oblique cross wall

                   e.   Contain pits

                         (1)  Structure of the cell wall

                                i.   Middle Lamella

                                ii.  Primary Cell Wall

                                iii. Secondary Cell Wall

                         (2)  Structure of a simple pit

                                i.   Pits are depressions in the cell wall that are circular to oval where the
                                     secondary cell wall is the portion of the cell wall that is hollowed out
                                     to make the pit

                                ii.  Pits are adjacent to one another on either side of the cell wall

                                iii.  Thus, pits come in pairs and are called pit pairs

                                iv.  The pit membrane is composed of the primary wall and middle lamella                                            of the adjacent cell walls of the pit pairs

                                v.   U-shaped in morphology

                                vi.  Found in parenchyma, collenchyma, fibers, and sclerids

                         (2)  Structure of the Bordered Pit

                                i.    Similar to the simple pit

                                ii.   The secondary cell wall forms a border over the pit membrane leaving a
                                      small opening called a pore

                                iii.   In the center of the pit membrane is a knob of secondary tissue called a

                                iv.   The torus can swell to the extent of plugging the pore in the border and
                                       prevent water loss during water stress in the plant

                                v.    In tracheids, vessels, and xylem fibers

            3.   Vessel Elements

                   a.   One of the two conducting cells of the xylem, the other one is the tracheid

                   b.   Dead when mature and functioning, contains lignin

                   c.   Morphology of the vessel element

                         (1)  Thick cell walls

                         (2)  Cross walls

                                i.    Many times are missing in the mature cell

                                ii.   At most will contain large holes or just bars of tissue to keep the end of
                                     the cell from collapsing

                                iii.  Will be almost horizontal in some to slanted in others but not as slanted
                                     as in tracheids

                         (3)  Cells are much shorter than tracheids

                         (4)  Cells are much wider than tracheids

                         (5)  Contain characteristic secondary wall thickenings (annular, reticulate, pitted,
                                scleriform, etc.)

            4.   Fibers

            5.   Parenchyma

                  a.   Used for lateral transport

                  b.   Used for food storage

     B.   Residual Procambium

            1.   Thin layer of procambium that remains undifferentiated between the xylem and

            2.   Remains meristematic

            3.   Destined to become the vascular cambium

II.  Pith and Pith Rays

      A.   Pith - large parenchyma cells in the center of the vascular bundle ring

             1.  Used for food storage

             2.  Used for waste storage

      B.   Pith rays - large parenchyma cells between the vascular bundles

             1.   Used for food storage

             2.   Used for lateral transport of food and water

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