The Stem

Lab 9 The stem Slides -Prepared: 1- Coleus Stem L.S 2- Coleus stem C.S 3- Cucurbita stem C.S 4- Aristolochia stem C.S 5- Helianthus old stem C.S 6- Zea stem C.S 7- Tila stem C.S 8-Liriodendron stem C.S 9- Pinus Stem C.S (Resin ducts) Fresh specimens: 1- Germinated seed 2- Geranium branch with leaves or Mint stem. 1 3Lab.No.9 The Stem - Stems support photosynthetic leaves and reproductive structures above the ground, supply water and minerals to shoots through xylem and photosynthetic products to roots through phloem. -Stems are more complex in structure than the roots because of the complexity of vascular system. Stem vary in structure so may be classified into types depending on: - Spatial relationship of vascular and non vascular tissues - The relative amount of secondary growth 1) Apical Meristems of vegetative shoots Exercise 1 examine a prepared slide of Coleus stem tip l.s Longitudinal section and notice the following: - The apical meristem (region of initiation of tissues and organs of shoot) at shoot tip ((in all angiosperms including our example Coleus) the meristematic cells are arranged into two zones in the apex of shoot; tunica zone (smaller cells, single layer or more, divide anticlinally) and Corpus zone (larger cell, divide in all planes) (Tunica- corpus organization)) - leaf primordia (horn like structures) at different stages of development, appear close to each other because the internodes are not yet extended in the youngest portion of the shoot Leaf primordia have different arrangements (spirally, whorls, opposite) depending on species, but in case of Coleus plant , leaves arranged in opposite pairs (decussate; each two are perpendicular to the other two) -axillary buds, each with small leaf primordia - Protoderm which will differentiate into epidermis -Procambium which will differentiate into vascular system - Ground meristem which will differentiate into cortex - Rib meristem which will differentiate into pith 2 Exercise 2: Examine a fresh germinated seed, notice the shoot with leaf primordia and also notice the young root ( examine using dissecting microscope), try to make a longitudinal section in shoot, put on a slide and examine using the compound microscope. 2) Nodal Anatomy Studying nodes reveals the association between the vascular bundles of the leaves and the vascular system of the stem. -Node: part of stem where leaves grow -leaf trace: extension of vascular bundle of stem bend toward the leaf to be continued with that of the leaf - leaf gap (lacuna) region of parenchyma in the vascular cylinder of a stem located above the level where a leaf trace diverges toward the leaf - Different plants have different numbers of leaf traces per leaf and different numbers of leaf gaps per node. Nodes could be unilacunar, multilacunar, - a leaf trace may consists of more than one bundle -in most vascular plants, only one trace is related to each gap. Exercise 3: make a serial section using a razor blade in a fresh Pelargonium branch at node region and internode region above and below the node, try to make sections thin as much as you can, examine those sections using compound microscope (you may not put the cover slip) notice: 1) the change in position of leaf traces at different levels of stem 2) how many leaves occur at each node? 3) how many traces does each leaf have? 4) how many leaf gaps face each leaf? 5) what type of node is represented by this plant? 3 4) Primary structure of stems Exercise 4 To study the primary structure of the stem, examine a prepared slide for Coleus stem c.s and notice the following: -tissue regions are the epidermis, cortex, starch sheath, vascular bundles separated by interfascicular region (narrow region1-2 layers in thickness) and pith. -complex vascular region, the vascular tissue is made up of discrete strands of various sizes which may be very close together or widely separated . - a delimitation of tissue regions that is less precise than in roots - the starch sheath occupies the same position of the endodermis (next to phloem)( For starch storage) -collenchyma may or may not be present beneath the epidermis; if present the cortex would be classified as a complex tissue. htaehs hcrats Types of stem A) Stems of herbaceous dicot plant Different patterns of procambial distribution in meristems results in different patterns of vascular tissue organization giving different types of herbaceous dicot stems. 1) Procambium remains as separate strands and give rise to separate (discrete) vascular tissue. a- no cambium develops, only primary vascular tissue in the mature stem. As in Ranunculus stem b- Cambium develops, secondary tissue formed 1- Only fascicular cambium develops 4 Exercise 5 Examine a prepared slide for Cucurbita stem as an example develops 2- Both fascicular and interfascicular cambium 2-a) Interfascicular cambium form only ray parenchyma Exercise 6 Examine a prepared slide for Aristolochia stem c.s as an example 5 2-b) Interfascicular cambium have activity similar to that of fascicular cambium, so both form complete cylinder of secondary tissue Exercise 7 Examine a prepared slide for Helianthus stem c.s. as an example on stem with interfascicular cambium that has same activity as fascicular cambium It can be studied as an example on a herbaceous dicot plant too. Notice the following -epidermis and stomata in young stem, epidermis is replaced by periderm in old stem, cork cambium (phellogen) arises from the subepidermal layer. -cortex of parenchyma with abundance of intercellular spaces, tannin inside cells in old stems, pitting may be visible in cortical cells. -perivascular fibers or bundle caps arise from the same part of the procambium that gives rise to primary phloem. -proacmbium forms a continous cylinder or ring in youg stem, with certain parts wider than others. Wider parts become vascular bundles and the narrow parts become the interfascicular areas. -collateral vascular bundles, procambium lies between the primary xylem and primary phloemin young stem. In old stem, fascicular cambium is formed between the secondary xylem and secondary phloem in the vascular bundle -Interfascicular cambium develops and forms a continuous cylinder with the fascicular cambium and both forms a continuous cylinder of secondary vascular tissue. -pith of parenchyma cells. 6 B) Stems of herbaceous monocot plants Stems of monocots characterized by distribution of the vascular bundles and the general lack of cambial activity (not in all monocots) Vascular bundles are distributed in two basic arrangements: -In two circles, one circle with smaller bundles near the periphery, the other circle with larger bundles deeper in stem. - scattered vascular bundles throughout the stem. Exercise 8 Examine a prepared slide of Zea mays stem c.s. Notice the following: -thick walled epidermis -no delimitation between of cortex and pith so the tissue is referred to ground tissue (parenchymatous) -subepidermal parenchyma cells are sclerefied. -Vascular bundles are scattered throughout the ground tissue (atactostele). The vascular bundles are primary collateral and enclosed in sclerenchymatous bundle sheath (closed vascular bundles) -the earlier centermost xylem (protoxylem) usually includes 1-4 or more cells, the inner protoxylem is destroyed during elongation to form a cavity (lacuna) [seen in mature vascular bundles]. -the later outer xylem is metaxylem [two large metaxylem vessels could be recognized,with other metaxylem elements between them] -phloem occurs external to metaxylem, the outermost protoxylem is crushed, metaphloem is intact and made up of large-diametered sieve tube elements and smaller companion cells. -no cambium is present. All procambial cells have formed primary xylem and primary phloem. 7 C) Stems of woody dicot plants -Differ in presence or absence of vessels and amount of secondary xylem or wood. 1) Stems of woody dicots with vessels Exercise 9 Examine a prepared slide for Tilia stem c.s or Liriodendron stem c,s and notice the following: -periderm containing several layers of tannin-filled cork cells. The cells containing protoplasts with nuclei are located beneath cork cells are phellogen (cork cambium) cells. - complex cortex composed of collenchyma and parenchyma -phloem is divided into two wedges *wedges containing sieve tube elements, companion cells, phloem parenchyma cells and phloem fibers which occur as bands alternating with bands that contain other phloem components, also this type of wedges contain narrow undilated phloem rays. * wedges of dilated phloem ray parenchyma. -the outermost fiber bundles are primary phloem fibers -secondary xylem contains vessels, tracheids, fibers, wood parenchyma cells, wide and narrow rays. Primary xylem occurs next to pith. -the pith occupies the center and it is made up of thick-walled, tannin-containing cells next to primary xylem and is called perimedullary zone (medullary sheath), large mucilage cells could be present in the pith. Tilia stem c.s 8 Liriodendron stem c.s 2) Stems of woody dicot vines Exercise 10 Examine a prepared slide for Aristolochia stem c.s as an example (this slide was mentioned as an example on stem with interfascicular cambial activity to produce ray parenchyma) During your study notice the following: -wide periderm -subepidermal collenchyma -perivascular fibers -Phloem with crushed protophloem elements -cambium lying between xylem and phloem -xylem -wide ray parenchyma formed by interfascicular cambium -crushed parenchymatous pith due to presence of twisting vine. 9 D) Stems of conifers Exercise 11 Examine a prepared slide of Pinus stem c.s and notice the following: -periderm -cortex with large resin canals lined with epithelial cells that secrete resinous material into the canals -Phloem beginning closely to inner side of resin canals, crushed , non-functional phloem occurs next to the resin canals, functional phloem next to cambium -several layers of secondary xylem, the primary xylem occurs in the form of wedges protruding into the pith -pith occupying the center and made up of parenchyma cells. 10