Ground tissues constitute the majority of nonwoody plant organs. They arc diverse in size and shape, and dicy perform a variety of functions. The diree ground tissues are parenchyma, collcnchyma, and sclerenchyrma (tig. 3.1). Of diese, parenchyma shows the greatest diversity' in form and function.
Parenchyma consists of thin-walled, living cclls that occur in different regions of the plant body. Parenchyma cells can be almost any size or shape; however, thev are frequently described as a 14-sidcd polygon. They may
FIGURE 3.1 GROUND TISSUES. (A) PARENCHYMA CELLS. THE MOST COMMON CELL TYPE IN PLANTS. HAVE THIN PRIMARY WALLS. (6) COLLENCHYMA CELLS HAV^ UNEVENLY THICKENED PRIMARY WALLS. (C) SCLERENCHYMA CELLS HAVE THICKENED SECONDARY WALLS.
occur singly or as groups of cells, thereby forming a tissue. Parenchyma cells have many diverse functions. In leaves, parenchyma cells are photosynthetic; in underground stems and roots, parenchyma cells arc used for storage. In nonwoody plants, parenchyma cells make up the majority of the plant body.
Collenchyma consists of living, elongated cells diat develop an unevenly thickened primary wall. These cells occur as filaments or in bands just below the epidermis in leaves, stems, and flowers. The walls, which are flexible and plastic, provide support in young, activclv growing plant organs.
Sclercnchyma consists of cells with thick, lignificd. secondary walls and variable shapes that usually have no living protoplasm when mature. Lignin, a unique component of cells with secondary walls, is a strong, complex polymer that increases wall strength, water impermeability, and resistance to decay. Sclercnchyma provides support and mechanical protection to mature plant parts. Pits are easily seen in the walls of some scle rcnchvma cclls. Sclercnchyma is subdivided into two groups: sclcreids, which are small, irregularly shaped cells, and fibers, which are elongated cclls. Sclcreids may occur in a complete layer or tissue in a seed coat (as a walnut shell), or thev may occur as clusters of cclls or even individual cells that arc randomlv distributed.
When sclcreids form a seed coat, they have a protective function; when they occur in leaves, they provide mechanical support. In other instances, the function is unknown. The shape of sclcreids is variable, but they are seldom long and thin. By contrast, libers arc thick-walled and very elongated sclercnchyma cclls. The scc-ondarv wall is usually highly lignificd. Fibers may occur singly, but more commonly they are groups of cclls. Fibers may be associated with the vascular bundles in the leaves and stems. They also comprise a significant part of the wood in many plants. Many plant fibers have commcrciaJ applications in the manufacture of fabrics and rope.
Dermal tissues form the outer covering of plants. They arc responsible for the environmental interactions in plants. The two types of dermal tissues are the epidermis and the periderm. The epidermis consists of flattened cclls that make up the outermost layer of young plants and the non-woody parts of older plants. In leaves and stems, the epidermal cclls secrcte cutin. a waxlike material diat prevents water loss from the plant. The layer of cutin is known as die cuticle. In some leaves, the curiclc is so thick that the leaf has a shiny, waxy appearance. In succulent plants, such as cacti, a thickcncd cuticle is one of the adaptations that helps them survive in arid conditions.
Trichomcs are hairlike structures that occur in the epidermis of many plants. Although individual trichomcs are microscopic, they may be abundant enough to give leaves a hairy or fiizzy appearance. Trichomcs, which may be branched or tinbranchcd, vary greatly in size and shape. Some trichomcs help prevent water loss in a plant, while other trichomcs arc glandular. Glandular trichomcs arc able to secrete various chemicals, such as nectar, enzymes, or aromatic compounds, that impart an aroma or sccnt to ccrtain plants. You may recall the aroma of the herbs we used for asexual propagation in Laboratory Topic 2. The essential oils that provide the aroma and flavor to herbs are found in glandular trichomes.
We find numerous pores in the epidermis of leaves and green stems; these pores arc called stomata (sing., stoma). The pores allow for exchange of gases, such as carbon dioxide, oxygen, and water vapor, between the plant and the environment. Kach stomata! pore is regulated by a pair of cclls called guard cclls, w hich occur on cither side of the opening (fig. 3.2). Guard cells, which arc sausage-shaped in many plants, are the only epidermal cclls that contain chloroplagts.
The periderm replaces the epidermis on plants that have secondary growth. For example, periderm that develops from the cork cambium makes up the outer bark of mature trees. We will look at the cells that compose die periderm in Laboratory Topic 15.
Vascular tissues arc conducting tissues. The vascular tissues occur in all parts of the plant and arc easiest to see as the veins in leaves. The tw o types of vascular tissue are xylcm, which conducts water and minerals upward from the roots, and phloem, which conducts organic materials throughout the plant. Both xyiem and phloem are composed of several different types of cells.
Plant Tissues—'Tm Fabrics of Our Lives 27
The water-conducting cells of the xvlcm are tracheitis and vessel elements (tig. 3.3/ih At maturity, these cells have secondary walls and lack cytoplasm. Tracheitis are elongated cells with tapering ends and many prominent pits in the side walls. Within the xylem, tracheids overlap, and water can pass from one trachcid to another through the pits in adjoining walls. Trachcids also function in support. In contrast to the long, thin trachcids, vessel elements are shorter and wider. Thcv often have horizontal end walls with large openings (fig. 3.3a). Vessel elements are attached end to end at these large openings to form long, pipelike vessels. Water readily moves
Xylem parenchyma cell
Phloem parenchyma cefl
Phloem parenchyma cefl
"Sieve plate Companion cell Sieve-tube member
FIGURE 3.3 (A) XYLEM. TRACHEIDS AND VESSEL ELEMENTS ARE THE CONDUCTING CELLS. ARROWS INDICATE THE DIRECTION OF WATER FLOW. (B) PHLOEM. COMPANION CELLS LOAD SUGARS INTO THE SIEVE-TUBE MEMBERS FOR TRANSPORT.
through the vessels, passing from one vessel element to the next through the large openings. Like the tracheids, the side walls have numerous pits. In addition to the conducting cells, xylem also contains fibers and parenchyma cells. The fibers provide additional support, while the parenchyma cells arc involved in storage and other metabolic activities. Primary xylem originates from the apical mcristcm, and secondary xylem originates from the vascular cambium. In trees, the secondary xylem can become very extensive: it makes up the tissue we call wood. Wc will look at the cells in wood in today's lab; xylem is examined in more detail in Laboratory Topics 4 and 15.
In the phloem, the cells involved in transporting organic materials arc known as sieve tube members ifig. 3.36). Sieve-tube members are elongate, living cells with thin primary walls. The end walls are often horizontal and have several to many large pores. These regions with pores are called sieve plates; cytoplasmic connections occur betw een adjacent sieve-tube members through die pores in the sieve plate. Organic materials are transported through these cytoplasmic connections from one sieve-tube member to another. Reside each sieve-tube member is a companion cell that is physiologically and dcvclopmen-tallv related to the sieve-tube member.
The companion cell functions in loading or unloading organic material into the sieve-tube member before and after transport. The phloem also contains fibers and parenchyma cells. Primary phloem is produced by the apical nicristem, and secondary phloem by the vascular cambium. In this exercise, we will look at sieve-tube members; phloem is examined in more detail in Laboratorv Topic 4.
Materials Needed for Exercise A
Dissecting microscope and compound microscope
Dropper bottle containing phloroglucinal-HCI stain. This is a useful stain for studying plant structure because it stains lignified walls red. You must be careful when using this stain because it is prepared with hvdrochloric acid.
Dropper bottle containing toluidinc blue stain
Glass slides, covcrslips, single-edged safety razor, and dissecting needles
Pieces of potato, onion, celery, pear, a geranium leaf, and one odier leaf type
Prepared slide of Cucurbiui stem
Prepared slide of macerated angiospcrm wood
Prepared slide of Sambueus (elderberry) stem
Prepared slide of Scdum epidermis
1. Freehand section of a piece of potato. Your instructor will demonstrate how to make freehand sections of plant tissue with a razor blade. Hold a wedge of potato in one hand (fig. 3.4) and a single-edged razor blade in the other hand. Make a thin cut at right angles to the long axis of the plant tissue, and placc the section in a drop of distilled water. Add a cover-slip. Examine with the compound microscope at low (10 x objective) and high (40 x objective) power. The parenchyma cells arc filled with starch grains (Icucoplasts or amyloplasts) that almost obscure the shape of die cells. Draw these parenchyma cells in the following space:
4. Mashed section of Pyrus (pear). Obtain a very small section of pear fruit. Using dissccting needles, mash it on the slide. Add a drop of phloroglucinol-l IC1 stain and then a covcrslip. (Be careful when adding this stain because it is prepared using hydrochloric acid.) What is the shape of the sclcrcids in pear? How arc they arranged? Draw a few of the sclereids in die following space:
sec the trichomes? Some of the leaves (such as the geranium leaf) may contain glandular trichomes. See if you can locate a glandular trichome. In the following space, draw the trichomes, and indicate whether they are glandular or nonglandular:
Macerated wood is wood that has been rreatcd with chemicals to dissolve the middle lamella berween the cells. This allows the cclls of the wood to separate. Examine a prepared slide. Can you lind vessels, tracheitis, fibers, and parenchyma cells? Draw the types of cclls you sec in the following space:
7 Fresh specimens of leaf trichomes. Obtain a geranium leaf and one other leaf as provided by your instructor. Examine the leaves with the dissecting microscope. Take a small part of a leaf and examine its edge under your compound microscope. Can you
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