Lab_1-Intro_to_the_Plant_Body-Spring2024.pdf

Introduction to the Plant Body Introduction You have probably learned that roots anchor a plant and absorb water and minerals, leaves trap solar energy in the process of photosynthesis, and stems support the leaves, conduct water, and transport organic molecules. Roots, stems, and leaves are the org...

Introduction to the Plant Body Introduction You have probably learned that roots anchor a plant and absorb water and minerals, leaves trap solar energy in the process of photosynthesis, and stems support the leaves, conduct water, and transport organic molecules. Roots, stems, and leaves are the organs that comprise the architecture of most plants and will be the focus of this laboratory. You will see how the structure of these organs reflects their functions and how to recognize the differences between them. What you should be able to do after this lab 1. 2. 3. 4. Define some terms and basic concepts related to the plant body. Understand the functional anatomy of roots, stems, and leaves. Provide some examples of how plants adapt their bodies to the environment. Apply your understanding of the plant body to determine what plant part is represented by various common produce. Part I. Roots The common bean (Phaseolus vulgaris) will be used to familiarize you with the external structure of the plant body. OBTAIN a young plant from the back bench to examine. The root system (in most plants, the belowground portion) includes all the roots, while the shoot system (the aboveground part) includes all the stems and leaves. Note that the root system of the bean consists of one main root, the primary root, or tap root, and many lateral roots. The primary root has its origin in the embryo; lateral roots arise from deep within the primary root or other lateral roots. The typical plant root is underground and provides the plant with a firm anchoring in the soil. Roots also function as the primary source of water and mineral absorption, and are often storage sites for plants. Roots may be enlarged for the storage of starch, an energy reserve, and in arid regions, the root may store water. Root systems are organized as either tap or fibrous. A tap root system has a single main root, sometimes enlarged for water and/or food storage, and many smaller lateral roots. A fibrous root system has several main roots of equal size. Additionally, some plants have specialized roots that are adapted to perform function other than the major jobs expected of ‘normal’ roots. For instance, some plants have roots that come out of the stem above the soil line to provide extra support for the shoot system, called prop roots. 1 See FIG 1. Root Architecture below and Fig 9.4 on p. 130 in your atlas. LABEL the tap Architecture. root system, fibrous root system, and prop roots on FIG 1. Root FIG 1. Root Architecture 2 EXAMINE the root system of your bean plant. QUESTION #1: What type of root system does it have? EXPLAIN: Tap roots can extend to significant depths and often reach the water table. Fibrous roots, on the other hand, are shallow but spreading. How can these two different root systems both achieve the same result of securing a water supply? What are their alternative strategies? On FIG 2. Bean Plant, LABEL the tap root and a lateral root. There are several functional regions at the tips of roots. The root cap is the shieldshaped region of cells at the very end of a root. It protects underlying tissues as the root grows through the soil. It is also the part of the root that detects direction so that roots grow down, in the direction of gravity. Immediately above the root cap lies the apical meristem in the region of cell division, a site of actively dividing cells that accounts for the growth of roots and replaces cells in the root cap as they are worn off. Moving upwards, the next region is the region of elongation, an area of rapid root growth as cells elongate. The region of maturation is the final region in which FIG 2. Bean Plant 3 embryonic cells differentiate into specialized tissues that make up the anatomy of a root. It is marked by the appearance of root hairs, fine extensions of the epidermis. Root hairs are the sites where water and minerals are first absorbed into the root. VIEW the regions of a radish root tip on display under a dissecting microscope. EXAMINE the seedling through the petri dish lid. LOCATE the shield-shaped root cap (FIG 3) at the very tip of the root. Immediately behind the root cap lies the region of cell division. Next LOCATE the region of elongation. The regions of cell division and elongation are the sites of root growth. Still further up, the region of maturation is indicated by the appearance of root hairs. Each root hair is an extension of a single epidermal cell and is extremely fragile. On FIG 3. Root Regions below, LABEL the root cap, region of cell division, region of elongation, region of maturation, root apical meristem, and root hairs (see Fig 9.18 in atlas). FIG 3. Root Regions 4 Part II. Stems Unlike roots, stems are divided into nodes and internodes. Nodes are the areas of the stem to which one or more leaves are attached. An internode is the portion of stem between two successive nodes. EXAMINE your bean seedling to find nodes and internodes. Go back to FIG 2. Bean Plant, and LABEL a node and internode. Branching of the shoot system takes place through the formation of buds, which arise in the axils (the upper angles formed at the nodes by the leaves and stems) of the leaves. In many plants these axillary buds (also called lateral buds) develop protective scales and become dormant for a period of time. In others, such as beans, the buds develop directly into adult shoots, some of which give rise to flowers. Flowers are part of sexual reproduction in plants and develop into fruits containing seeds. EXAMINE the bean fruit. QUESTION #2: Where are the seeds? Roots, stems, and leaves are commonly referred to as the vegetative parts of the plant, and flowers are referred to as reproductive parts. We will be exploring flowers, fruits, and seeds in more detail later in the semester. QUESTION #3: Why are the leaves and stems of the plant green? Part III. Leaves Leaves are the primary photosynthetic organs for most plants. Leaves are also important players in gas exchange and in the water stream that flows upwards from the soil and evaporates from the leaves. Leaves come in two types: broad, like an oak leaf, or needle, like a pine needle. Most broad leaves are composed of three parts: a blade, a petiole, and a pair of stipules (FIG 4). See Fig 9.63 on p. 142 in your atlas. The blade is the flat, expanded portion of the leaf. The petiole is the stalk that supports the blade, attaching it to the stem. In some plants, the petiole is absent; when this happens, the leaf is said to be sessile. Stipules are paired structures that may be found at the base of leaves. Their presence in many species is temporary and they are often shed early in the growing season. Stipules vary greatly in appearance, with some modified into spines, commonly called thorns. Others, such as those of the sycamore tree, look like miniature versions of the leaves. 5 Leaves are further classified on the basis of composition, arrangement, and venation (FIG 4). The appearance of the blade is important in determining leaf composition. If a blade is undivided, the leaf is classified as simple. However, if the blade is divided into separate pieces or leaflets, the composition of the leaf is said to be compound. In some leaves, even the leaflets are subdivided! Sometimes it is difficult to tell whether you are looking at a simple leaf or a leaflet. The position of the axillary bud (FIG 4) is a good clue as to whether the leaf is simple or compound. Axillary buds are only found in the axil of a leaf. Leaflets do not have axillary buds. There are two types of compound leaves. When the leaflets occur in a featherlike pattern, it is a pinnately compound leaf. If all the leaflets arise from a single point, it is a palmately (think hand) compound leaf (FIG 4). Arrangement pertains to how leaves are distributed on a stem. If there is only one leaf at a node, the leaf arrangement is called alternate. If the number of leaves at a node is upped to two, the arrangement is opposite. With more than two leaves at a node, the arrangement is whorled (FIG 4). FIG 4. Leaf Morphology 6 Leaf venation is another easily recognizable characteristic. Venation refers to the pattern of the veins running across the blade and can be parallel or net. Parallel venation is as it sounds: most of the main veins in the leaf run parallel to one another. Those leaves with highly branched veins have net venation. In netted venation, smaller and smaller veins branch off the larger veins in such a way that they resemble a net. There are two types of net venation: palmately netted, where several major veins radiate out from a single point (usually near the attachment to the petiole), or pinnately netted, in which major veins branch off along the entire length of the midrib, the main vein of a leaf. QUESTION #4: How can you distinguish a leaf from a leaflet? Choose three sample branches from the back lab bench. In the area to the right SKETCH each leaf and its attachment to the stem. LABEL the blade, petiole, axillary bud, and stipules (if present). NOTE the leaf composition, arrangement, and venation. BRANCH A Composition: ___________________ Arrangement: ___________________ Venation: ___________________ BRANCH B Composition: ___________________ Arrangement: ___________________ Venation: ___________________ BRANCH C Composition: ___________________ Arrangement: ___________________ Venation: ___________________ 7 Part IV. Environmental Adaptations Plants have adapted to live in a wide variety of environments. The roots, shoots, and leaves of plants have all adapted to meet the particular challenges of the environments in which they live. Desert plants must manage extreme temperature and drought conditions, rainforest plants deal with low nutrient and light levels, and prairie plants have to be able to survive fire. EXAMINE the photos of leaf sections on display to see how they have adapted to different environments. Start by examining the “typical” leaf. The shoots of plants are covered with a layer of wax called the cuticle. QUESTION #5: What is the function of the cuticle? The top and bottom of the leaf have a solid layer of living cells called the epidermis. QUESTION #6: This layer would be analogous to your __________________. In the center of the leaf is a layer of cells under the upper epidermis that that are packed tightly side by side to create a layer called the palisade mesophyll. QUESTION #7: These cells have many chloroplasts and their main function is _____________________________. Below the palisade mesophyll is a layer of random-looking cells surrounded by air spaces called the spongy mesophyll. QUESTION #8: These cells are also photosynthetic and the air spaces between them allow the _______________ ________ _______________ needed for photosynthesis throughout the leaf. Stomata are openings in the epidermis that allow gasses to diffuse in and out of the leaf. The stomata have specialized cells called guard cells that open and close these pores. Finally, locate a vascular bundle. These structures contain cells that transport water and organic molecules like sugar throughout the plant and make up the veins you observed above. 8 Now that you are familiar with the basic structure of a leaf, let’s look at some ways leaves adapt to different environments. Start with a leaf section from a xerophytic plant, the type that lives in deserts. COMPARE it with the typical leaf, note any differences and speculate on how the change helps the plant adapt to a dry environment. Cuticle _______________________________________________________________ Epidermis _______________________________________________________________ Mesophyll ______________________________________________________________ Placement of stomata _______________________________________________________________ Look at the cross section of a water lily leaf. These leaves float on the surface of a pond. COMPARE it with the typical leaf, note any differences and speculate on how the change helps the plant adapt to floating on water. Cuticle _______________________________________________________________ Epidermis _______________________________________________________________ Mesophyll _______________________________________________________________ Placement of stomata _______________________________________________________________ 9 One interesting adaptation allows plants to change the shape of their leaves as needed. Some plants like corn have specialized cells called bulliform cells located in the epidermis. The plant can fill these cells with water so that they are turgid or remove the water so that they are flaccid. Look at the images of corn leaf cross sections on display. Find the bulliform cells. Also see the figures on p. 146 in your atlas. QUESTION #9: What shape is the leaf when the cells are turgid? QUESTION #10: What shape is the leaf when the cells are flaccid? QUESTION #11: Under what conditions might a plant want to fold its leaves? Part V. Supermarket Botany EXAMINE some familiar vegetables to test your knowledge of plant anatomy. Don't rely on common, but often erroneous, assumptions when assessing the anatomy of a vegetable. LOOK for solid anatomical evidence before drawing a conclusion. Many vegetables have been greatly modified from their wild ancestors by selective breeding and can appear different from what is considered typical for a root, stem or leaf. EXAMINE the vegetables on display from the produce section of the grocery store. Use your knowledge of plant anatomy to determine which plant organ they are, and COMPLETE TABLE 1. 10 TABLE 1. Which plant part do you eat? Vegetable Asparagus Broccoli Scientific Name Daucus carota Celery Apium graveolens Cucumber Cucumis sativus Green Bean Phaseolus vulgaris Lettuce Lactuca sativa Potato Solanum tuberosum Radish Raphanus sativus Turnip How can you tell? Asparagus officinalis Brassica oleracea var. botrytis Carrot Tomato Edible plant organ Solanum lycopersicum Brassica rapa var. rapa 11 Introduction to the Plant Body: Terms you should know Root system Stipule Shoot system Simple Tap root Alternate Lateral root Opposite Prop root Whorled Tap root system Parallel venation Fibrous root system Net venation Root Cap Cuticle Region of Cell Division Epidermis Region of Elongation Palisade mesophyll Region of Maturation Spongy mesophyll Root apical meristem Stomata Root hairs Guard cells Node Vascular bundle Internode Bulliform cells Bud Xerophyte Axil Palmate Vegetative parts Pinnate Reproductive parts Compound Blade Sessile Petiole 12