Biological Foundations 112, Lecture 2

Primary Tissues, Part I

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

I.   Primary Tissues Differentiated from the Protoderm

     A.     Epidermis

              1.   Outermost layer of the stem

              2.   Secretes a thin layer of cutin on the external surface, called cuticle

              3.   Contains stomata composed of a pore and guard cells

                    a.   Pore - opening in the epidermis though which gases enter and exit the stem

                    b.   Guard Cells - the two cells that surround the pore

              4.   Produces trichomes on the external surface of the epidermis

II. Primary Tissues of the Ground Meristem

     A.   Three regions of the ground meristem

            1.   Cortex - region between the epidermis and the procambial strands

            2.   Pith Ray - region between the procambial strands

            3.   Pith - region in the very center of the stem

     B.   Parenchyma Tissue

            1.   Characteristics of parenchyma tissue

                  a.   Cells are isodiametric (all the sides are of equal length)

                  b.   The cell walls are thin and generally transparent

                  c.   The cells are not highly differentiated

            2.   Three kinds of parenchyma cells

                  a.   Parenchyma - parenchyma cells without chlorophyll

                  b.   Chlorenchyma - parenchyma cells with chlorophyll

                  c.   Aerenchyma - parenchyma cells without chlorophyll, surrounding air spaces

     C.   Collenchyma Tissue

            1.   Strengthening tissue

            2.   Living when mature and functioning

            3.   Cell walls never contain lignin

            4.   Elongated cells

            5.   Cell walls are thickened in the corners where several cells come together

            6.   Often found just under the epidermis

            7.  Example - the "strings" one experiences when eating celery

     D.   Sclerenchyma Tissue

            1.   Characteristics of sclerenchyma

                  a.   Strengthening tissue

                  b.   Dead when mature and functioning

                  c.   Cell walls always contain lignin

                  d.   The lignin generally stains red on the microscope slides

            2.   Two types of sclerenchyma cells

                  a.   Sclerids - star-shaped and stone-like cells

                  b.   Fibers - elongate and thin cells

     E.   Secretory Cells

           1.   Generally have the morphology of parenchyma cells

           2.   Generally secrete resins and nectar

     F.   Transfer Cells

           1.   Generally have the morphology of parenchyma cells

           2.   Transfer cells occur where solute is transferred from one tissue to another

           3.   The bundle sheath and border parenchyma in the leaf mesophyll are good examples
                 of transfer cells

           4.   Surface of the plasmalemma is folded to increase surface area of plasmalemma

           5.   Mitochondria aggregate along the plasmalemma to provide energy for transfer

III. Primary Tissues of the Procambium

     A.   The Procambium differentiates into the primary Vascular Tissues

     B.   Three regions of the procambium

            1.   Primary Phloem - carries products from photosynthesis to all parts of the plant

            2.   Residual Procambium - procambial tissue that never differentiates

            3.   Primary Xylem - carries water and minerals upward in the plant

     C.   Primary Phloem

            1.   Differentiates four kinds of cells

                  a.   Sieve-tube Member

                  b.   Companion Cells

                  c.   Fibers

                  d.   Parenchyma

            2.   Sieve-tube Member

                  a.   The conducting cell of the phloem

                  b.   Conducts the products of photosynthesis to all parts of the plants

                  c.   Length of time for functioning

                        1.   1-3 years in dicots

                        2.   Life of the plant in perennial monocots (palm trees)

                  d.   When mature and functioning:

                        1.   Contains no nucleus

                        2.   Has only thin layer of cytoplasm around the periphery of the cell

                        3.   Living when mature and differentiated

                        4.   Contains a Sieve Plate - cross walls at each end of the cell that contains
                              holes like a sieve

                  e.   Development of the sieve-tube member

                        1.   Nucleus disintegrates

                        2.   Plastids lose most of their internal membranes

                        3.   Mitochondria become small

                        4.   Cytoplasm reduced to a thin peripheral layer

                        5.   Callose forms around the sieve plate

                        6.   P-protein occupies central portion of cell

                  f.    Function of callose

                        1.   A carbohydrate substance

                        2.   Forms around the sieve plate

                        3.   Changes the aperture of the holes in the sieve plate

                        4.   Thus, regulates the flow of material through the sieve tube

                  g.   Function of P-protein

                        1.   An acronym for phloem-protein

                        2.   A "spider web" of protenaceous filaments within the sieve-tube member

                        3.   When the sieve-tube element loses pressure from an injury, the p-protein
                              piles up against sieve plate of the depresurized end of the cell.  The p-protein
                              in this condition acts like a blood clot and stops the loss of material bleeding
                              from the broken sieve-tube element. 

            3.   Companion Cell

                  a.   Is always immediately adjacent to a sieve-tube element

                  b.   Appears to control the sieve-tube element

                        i.   Plasmodesmata occurs in the cell wall between the companion cell and the
                             sieve-tube element

                        ii.  Endoplasmic reticulum extends from the nucleus of the companion cell through
                            the plasmodesmata to the cytoplasm of the sieve-tube element

                        iii. Thus a protoplasmic connection between the companion cell and the
                            sieve-tube element which allows for control of the callose that surrounds the
                            holes in the sieve plate.

                  c.   Companion cell and Sieve-tube element are Sister Cells

                        i.   A single procambial cell divides once to form two sister cells

                        ii.  One of the sister cells becomes a sieve-tube element

                        iii. Other sister cell becomes the companion cell

                        iv. Thus the two cells are called sister cells because they originate from a single

        4.   Fibers  - fibers are strengthening cells in the primary phloem

        5.   Parenchyma  -  parenchyma carries the food products out laterally into the stem

PUC Home Page | Gilbert Muth Home Page | Botany Syllabus Home Page

E-mail Gilbert Muth