Lab Exercise 4: Primary Growth and Tissues in Stems

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Mariah Fowler
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1 Lab Exercise 4: Primary Growth and Tissues in Stems Tissues of the plant body can be classified in a variety of ways: functionally (based on the tissue function, e.g. vascular tissue ), morphologically (based on cellular characteristics, e.g. parenchyma tissue, living thin- walled cells), and developmentally (based on their origin, from primary or secondary meristems). In this lab you will examine some primary meristems, the tissues they produce, and how those tissues are organized in the stems of plants. You will also examine the external morphology of stems. After completing this exercise, you should be able to: Understand how primary and secondary growth contribute to the overall growth of a plant. identify the main mature primary tissues in a herbaceous dicot stem and in a monocot stem (eg. Xylem, phloem, cortex, pith, ground tissue) describe how vascular bundles are arranged in a typical dicot stem cross- section, and how monocot stems differ. describe the cell types present in phloem (i.e., sieve tube members and companion cells) and xylem (i.e. vessel elements, fibers). Be able to identify, from a photo or diagram, the three primary meristematic tissues in a stem tip (protoderm, procambium, and ground meristem), and explain which mature tissues will develop from each. Explain what is meant by primary growth and primary tissues. define or explain the following, and know their importance/function in plants: apical bud, apical meristem, axillary bud, node, alternate leaves, opposite leaves, cortex, pith, xylem, phloem, vessel element, sieve tube member, companion cell, protoderm, procambium, ground meristem, node, internode, axil, epidermis, dicot, monocot.

2 I. The Stem Tip and Primary Growth Primary growth refers to an increase in the length of a stem or root. Both stems and roots increase in length only at their tips, through the activity of primary meristems. This involves two processes: cell division and cell elongation. At the very tip of the stem or root is a region of active cell division, the apical meristem, where cells are increasing in number. Below this is a region of cell elongation (where cells are increasing in size/length) and finally a zone of cell maturation (where cells are maturing and differentiating into different cell types (collenchyma, parenchyma, and sclerenchyma) and tissues. As the processes of elongation and differentiation occur, the immature tissues are referred to as primary meristematic tissues. Three primary meristematic tissues are defined: Protoderm: cells that will develop/differentiate to form the dermal tissue (epidermis). Ground meristem: cells that will develop/differentiate to form the ground tissue (cortex, pith).procambium: cells that will develop/differentiate to form the vascular tissue. Obtain a prepared slide of a longitudinal section of Coleus stem tip. Here you will be able to observe the three primary meristematic tissues and early stages of cell elongation and differentiation. (see Atlas p 135, fig 9.28) Also examine the potted coleus plant, if available, and relate the topics discussed here to the living plant. 1. Locate the leaf primordia and axillary bud primordia, the rudimentary structures that will develop into leaves and axillary buds (sing. = primordium). Your knowledge of stem morphology and examination of the coleus plant should help you identify which is which (what is the relative position of leaves to axillary buds). 2. Locate the dome- like apical meristem, which appears dense and dark red due to the concentration of small, actively dividing cells with relatively large nuclei. 3. Immediately beneath the apical meristem, three primary meristematic tissues develop. These are (1) the protoderm, which will develop into the epidermis, (2) the procambium, which gives rise to primary vascular tissue, and (3), the ground meristem, which will mature into the pith and cortex in dicots. Cortex is the tissue between the epidermis and the vascular bundles in the stem, while pith is the tissue in the center of the stem. 4. Notice that traces of the procambium extend out into the leaf primordia, as well as down the stem. These procambial tissues mature into the conducting tissues of the plant, xylem and phloem, which are continuous throughout the plant.

3 Figure 4.1) Longitudinal section of coleus stem tip. Label the following: A) B) C) D) E) apical meristem leaf primordium axillary bud primordium protoderm cortex F) procambium G) ground meristem H) leaf vascular trace I) pith J) epidermis II. Primary Tissues in Stems A. Corn (Zea mays), a monocot. 1. Examine a prepared slide of a cross section of a Zea mays stem at low power or with a dissecting scope (to get an overall view of how the stem tissues are organized). The vascular tissues (xylem and phloem) can be seen in distinct clusters of cells called vascular bundles. These are like the veins of the plant, and form a continuous system that distributes water and nutrients throughout the plant body.

4 Using the circle below to represent the stem cross section, show the distribution of the vascular bundles in corn. Do not show any cellular details; simply use small circles to show where vascular bundles are located. This arrangement of stem vascular bundles is typical of monocots, the group of flowering plants that includes grasses (of which corn is one species), bananas, pineapples, palm trees, lilies, orchids, etc. Because of this random arrangement of vascular bundles, no distinction is made between cortex and pith; it is simply referred to as ground tissue. It develops from the ground meristem. How do you know that the tissues in this stem cross section are mature, and are not primary meristematic tissues? 2. Now examine a single vascular bundle at high power using a compound microscope. To many observers, a vascular bundle resembles a monkeyface. a) Bundle sheath: The red- stained cells that outline the vascular bundle are sclerenchyma fibers, which form a bundle sheath. Remind yourself of the characteristics of sclerenchyma cells and fibers from a previous lab. On some slides the bundle sheath cells may not be completely differentiated. b) Primary phloem: The green- stained forehead of the monkeyface is the primary phloem, in which you should be able to distinguish two types of cells. The larger cells are the sieve tube members and the smaller are companion cells. c) Primary xylem: The eyes and nose of the face are large vessel elements of the primary xylem. Note the thickened, red- stained cell walls and lack of cellular contents in these water conducting cells. There may also be a few smaller vessels present. Some of the larger vascular bundles will also have a round or irregularly- shaped airspace that forms a mouth for the monkeyface; this airspace is a result of stretching and tearing of tissues that occurs as surrounding cells elongate.

5 d) Examine the outer region of the corn stem (near the epidermis). The vascular bundles here are smaller and closer together. There are relatively more sclerenchyma fibers, particularly just below the epidermis. These tissues contribute strength and mechanical support to the corn stem. Figure 4.2: Vascular bundle of corn stem (cross-section) Label : a) bundle sheath (sclerenchyma fibers) b) ground tissue c) primary phloem d) vessel element B. Sunflower (Helianthus sp.), a dicot. 1. Examine a prepared slide of a cross section of Helianthus stem. Note that our slides contain sections from young and old regions of the stem. For now, concentrate on the smaller, young section, to examine the primary tissues. As with corn above, use ovals to show the distribution of vascular bundles in the stem. Notice that Helianthus has a hollow stem there is an empty area, called the pith cavity, at the center of the pith tissue. This ring- like arrangement of vascular bundles is characteristic of herbaceous (non- woody) stems of dicots, the other major group of flowering plants. Other examples of dicots are daisies, poppies, apple trees,

6 buttercups, tomatoes, oaks, etc. The ring of vascular bundles divides the ground tissue into two regions: cortex and pith. What seems to be the most abundant type of tissue in the young Helianthus stem? Is coleus a monocot or a dicot? 2. Examine a Helianthus vascular bundle at low, medium, and high power. a) Since we looked at these sections last week, you should recognize the sclerenchyma fiber caps that lie between many of the vascular bundles and the epidermis. b) Toward the center of each bundle you should see green- stained primary phloem. Try to distinguish sieve tube members and companion cells in this tissue. c) The primary xylem is adjacent to the primary phloem, on the interior side of the vascular bundles. This arrangement of xylem and phloem, with xylem to the inside and phloem to the outside is important to recognize and is characteristic of stems. Vessel elements are visible, as are smaller parenchyma cells which may eventually develop into fibers, vessel elements, or other cell types as the stem continues to mature. Figure 4.3. Cross section of Helianthus stem. Label the following: A) epidermis. B) cortex. C) sclerenchyma fibers. D) primary phloem. E)primary xylem. F) vessel element. G) pith. H) Pith cavity.

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