Biological Foundations 112, Lecture 5

Monocot Stems

Chapter 31, pages 668-691
Plant Anatomy-2  28:03-33:32

I.   Herbaceous Monocot Stems

     A.   Differences between monocots and dicots in stem structure:

            1.   Monocots

                  a.   Procambial strands scattered throughout the ground meristem

                  b.   No vascular cambium

                  c.   Vascular bundles closed to secondary growth

            2.   Dicots

                  a.   Procambial strands in a ring in the ground meristem

                  b.   Vascular cambium present

                  c.   Vascular bundles open to secondary growth

      B.   General structure of monocot stems

             1.  Epidermis

             2.  Strengthening tissue, either collenchyma or fibers, just under the epidermis

             3.   Ground parenchyma

             4.   Vascular bundles scattered throughout the ground parenchyma

             5.   No pith, pith ray, or cortex regions, only ground parenchyma

             6.   Stomata usually occur in the epidermis

      C.   Detailed look at the monocot stem cross section of monocot stem, vascular bundle

             1.   Vascular bundles

                    a.   The general morphology is like a "human face"

                          (1)  The eyes, nose, cheeks, and mouth make up the xylem

                          (2)  The forehead makes up the phloem

                          (3)  The hair and and the beard make up the bundle cap and bundle sheath

                    b.   Phloem

                          (1)   Sieve tube members

                          (2)   Sieve plates

                          (3)   Companion cells

                    c.   Xylem

                          (1)   Vessels

                                  i.   Usually two per bundle

                                  ii.  Make up the "eyes" of the bundle

                                  iii. Usually quite large and pitted

                          (2)   Air Space - in the position of the "nose"

                          (3)   Tracheids - cells over the "bridge of the nose"

                          (4)    Parenchyma - cells in the "cheek" areas

                    d.   Bundle Cap - cluster of fibers in the "hair" region

                    e.   Bundle Sheath - a layer of fibers that surround the vascular bundle, the
                                                      "beard" region

             2.    Primary growth in monocots

                    a.   All of the above tissues and cells differentiate from the primary meristem tissues

                          (1)  Protoderm - differentiates into the epidermis

                          (2)  Ground Meristem - differentiates into the ground parenchyma

                          (3)  Procambium - differentiates into the vascular bundle

                    b.   In each of the vascular bundles, all the procambium differentiates so that there is
                          no residual procambium left to further differentiate into a vascular cambium

              3.   Secondary growth in monocots

                    a.   Does not occur

                    b.   There are isolated cases of a form of secondary growth in monocots that is
                          quite different from dicots.  The details of this are greater than we want to
                          pursue in this class. Thus, for the purposes of this class, secondary growth does
                          not occur in monocots.

              4.   Intercalary Meristem

                    a.   Dormant meristem tissue at the base of the internode

                    b.   Causes intercalary growth - growth in the length of the stem

                    c.   There is always a dominant terminal meristem.  When the terminal meristem is
                          destroyed (as in cutting of grass in your lawn) the most terminal intact
                          intercalary meristem becomes the dominant meristem and the grass begins to
                          grow again.

II.  Modified Stems

      A.   Rhizome - underground root-like stem (perennial rootstock)

      B.   Tuber - very fleshy underground stem (potato)

      C.   Bulb - underground stem with many scales covering it (onion)

      D.   Corm - underground solid bulb-like structure that contains no scales

      E.    Stolon - above ground "runners" (strawberry plants)

      F.    Thorn - stickers or thorns on above ground stems (rose bushes)

      G.   Cladophylls or Cladodes

             1.   Leaf-like stems that contain node and internodes

             2.   May bear flowers, fruits, and temporary leaves

             3.   Examples:  Rucus, Asparagus, Smilax, and various cactus

III. Stem Branching Patterns

      A.   Dichotomous branching - repeated forking

      B.   Monopodial or Excurrent branching - a single main trunk the entire length of the tree
                                                                       with whorled branching at each node, like a
                                                                       pine tree

      C.   Sympodial or Deliquescent branching - a short single main trunk with irregular
                                                                          branching out of the top of the short trunk, like
                                                                          an oak or sycamore tree

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