Plant Biology: Anatomy of Vegetative Parts - Stems (PDF)

Plant Biology. Supplementary material for 2nd practical work ANATOMY OF VEGETATIVE PARTS STEMS Objectives: at the end of this lab the successful student will be able to: 1. Identify the structures in a stem....

Plant Biology. Supplementary material for 2nd practical work ANATOMY OF VEGETATIVE PARTS STEMS Objectives: at the end of this lab the successful student will be able to: 1. Identify the structures in a stem. 2. Distinguish between primary tissues and secondary tissues. 3. Identify the structures and give the function of vascular cambium and cork cambium. 4. Identify the cross section of the dicot stem woody and herbaceous. 5. Identify the cross section of the monocot stem. 6. Know the composition of wood and its annual rings. During this lab: 1. Studies on theoretical part. 2. Learning diagnostic futures of monocot and dicot stems. Identify the structures in a Woody and Leafy stem  Woody twig consists of an axis with attached leaves. o Alternately or oppositely arranged.  Leaves attached at a node.  Stem region between nodes is an internode.  Leaf has a flattened blade and is usually attached to the twig by a petiole. o Axillary bud located in axil (between petiole and the stem).  Terminal bud often found at twig tip.  Stipules - paired appendages at the base of a leaf. Often remain throughout leaf life span. o Deciduous trees and shrubs have dormant axillary buds with leaf scars left after leaves fall.  A lenticel is an airy aggregation of cells within the structural surfaces of the stems, roots, and other parts of vascular plants. It functions as a pore, providing a medium for the direct exchange of gasses between the internal tissues and atmosphere. The shape of lenticels is one of the characteristics used for tree identification. 1  Structure of Leafy stem Lenticels Distinguish between Primary tissues and Secondary Tissues  Apical meristem is dormant before the beginning of the growing season. o Protected by bud scales and by primordia.  When a bud begins to expand, apical meristem cells undergo mitosis and three primary meristems develop o Protoderm - gives rise to epidermis. o Procambium - produces primary xylem and primary phloem cells. o Ground meristem - produces tissues composed of parenchyma cells.  Pith  Cortex 2  Longitudinal section of a coleus leaf Coleus (Plectranthus barbatus) Identify the structures and give the functions  Narrow band of cells between the primary xylem and primary phloem may become vascular cambium. o Cells produced by the vascular cambium become components of secondary xylem and secondary phloem.  In many plants, a second cambium, cork cambium, arises. o Produces cork cells and phelloderm cells. Identify the cross section of the Dicot Stem  Herbaceous dicotyledonous stems o In general, annuals are green, herbaceous plants.  Most monocots are annuals, but many dicots are also annuals. 3  o Herbaceous dicots have discrete vascular bundles composed of patches of xylem and phloem.  Procambium produces only primary xylem and phloem, but vascular cambium arises later and adds secondary phloem and xylem to the vascular bundles.  Woody dicotyledonous stems o Vascular cambium of a typical broadleaf tree produces relatively large vessel elements of secondary xylem (spring wood).  Xylem produced next has smaller or fewer elements, and is referred to as summer wood.  One year’s growth of xylem is called an annual ring. o Vascular cambium produces more secondary xylem than phloem, thus bulk of a tree trunk consists of annual rings of wood.  Examining rings can determine the age of a tree, and provide some indications of climatic conditions. o Vascular rays consist of parenchyma cells that function in lateral conductions of nutrients and water.  Xylem ray  Phloem ray Woody dicot stem 4 Identify the cross section of the Monocot stem  The stems of most monocots have neither a vascular cambium nor a cork cambium and thus produce no secondary vascular tissues or cork. o Xylem and phloem exist in discrete vascular bundles. Secondary meristem produces only parenchyma cells to the outside and secondary vascular bundles to the inside. Comparison of a monocot vs. dicot stem. Note particularly the arrangement of the vascular bundles.  Monocot stems have many vascular bundles arranged in a complex pattern within the grownd tissues.  Diocot stems have one ring of vascular bundles located in the periphery of grownd tissues. Know the composition of wood and its annual rings  Older, darker wood at the center is called heartwood, while the lighter, still-functioning xylem closest to the cambium is called sapwood. o Formed at roughly the same rate as heartwood.  Softwood - xylem consists primarily of tracheids; no fibers of vessel elements.  Cone-bearing trees.  Bark - Refers to all the tissues outside the cambium, including the phloem. o Mature bark may consist of alternating layers of crushed phloem and cork. 5  Secondary growth of stems (see pictures above) a. A dicot herbaceous stem before secondary growth begins. b. Secondary growth has begun. Epidermis was replaced by cork. Vascular cambium produces secondary xylem and phloem. c. A three-year-old stem which cork cambium produces new cork. The primary phloem and cortex evetually disappear, and only the second phloem (within the bark) produces by vascular cambium will be active that year. The secondary xylem, also produced by vascular cambium, builts up to become the annual growth rings. d. section of woody stem shows tissues at higher magnification Works cited: Kellogg Community College http://academic.kellogg.edu 6