Bring Your Text to Lab!!! Vascular Plant Anatomy: Flowering Plants Objectives: 1. To observe what the basic structure of vascular plants is, and how and where this form originates. 2. To begin to understand how form and function are connected and how plants work. I. INTRODUCTION TO FORM AND FUNCTION A. General Anyone who has grown plants realizes the importance of providing proper amounts of water, nutrients, and light. Unless you re the proverbial "green thumb", it may appear that plants are decidedly temperamental about getting these necessities in the proper amounts. How do you suppose that plants that grow unattended meet these needs in an unpredictable natural environment? Only cursory observation is required to reveal the specialized features of plants that enable them to satisfy these needs and survive the inclement environmental periods. Such features are adaptations acquired through evolution. Much of the organization of the plant is related to adaptations for obtaining necessary levels of water, nutrients and light. Examples of these adaptations can be found from the molecular and cellular levels, through organs, tissues, and the entire plant organization. In fact, understanding many adaptations requires knowledge of how these levels are integrated in the plant. For example, the necessity of the plant to carry out a minimum level of photosynthesis is related to the organization of specialized photosynthesizing organelles in specialized cells, and often in organs specialized to this task, like leaves. Bring Your Text to Lab!!!
Many other cell types in the leaf are specialized to support the principal cells that carry out photosynthesis. In the most basic sense, one can even argue that roots are organized to absorb water and nutrients that support the photosynthetic process, and that both stems and roots provide the physical support for maintaining the principal photosynthetic organs in a favorable light environment. Most stem and root tissue can be best understood when viewed from this perspective. Secondary functions of plant organs, however, are quite important and account for many variations in forms at all levels. These functions include: protection from desiccation, carbohydrate storage, protection from herbivores, protection from pathogen invasion, and even protection from fire result in variations of the basic cell, tissue, and organ forms. These adaptations have developed according to the environmental pressures that have molded individual species in preceding generations. The combination of adaptations causes each species to be unique and distinctive, a concept that also could be appreciated from the taxonomy and field botany laboratory. In this and the next lab, we will examine these adaptations and their related functions. The roots, stems, and leaves, as other organs, consist of groups of tissues that carry out the processes essential to life. A tissue is a group of cells with some similarity in structure, function or position. Cells that carry out a particular function usually have very similar characteristics adapted to that particular function. When they are first formed, plant cells are relatively unspecialized. They acquire their unique forms as they age, a process called differentiation. Growth in plants is a nearly constant process terminated only by death (indeterminate growth compared to determinate growth seen in animals). Within plants are tissues that "specialize" in growth. That is, tissues whose cells remain undifferentiated and retain the ability to divide. These tissues, called meristems, occur at strategic positions where growth is required to support the plant's continuing quest for light, water, and other essentials from the environment. As these cells divide they give rise to cells that specialize through differentiation to form the other tissues in the plant. As you work though the labs, examine each organ or tissue: 1) with the view of locating these basic tissue types, 2) examine the variations of cell types within these tissues, and their relative positions to one another, 3) remind yourself of the functions, or if the function is not clear, speculate about the function as you examine the forms, and 4) draw everything you see (preferably) in color. 2
B. Tissue Types. Although there would appear to be a vast array of tissue types in the plant, there are only three basic types, with almost unlimited variation on them: 1. Dermal: Dermal tissue is adapted almost exclusively for protection against desiccation or external agents such as pathogens. 2. Vascular: Vascular tissue is adapted primarily for physical support and translocation or movement of materials within the plant. 3. Ground: Ground tissue is the most varied in function and form, but is generally associated with functions of synthesis and accumulation of organic compounds. 3
II. Methods Exercise #1 PLANT MORPHOLOGY As you begin your investigation of the structure and function of plants, you need an understanding of the general shape and form of the whole plant. In this exercise you will study a bean (or other plant), identifying basic features of the three vegetative organs: roots, stems, and leaves. In following exercises you will investigate the cellular structure of these organs, mostly in cross sections. Examine the plant and identify the following: SHOOTS: a. Nodes are regions of the stem from which leaves, buds and branches arise and which contain areas of meristematic tissues (areas of cell division). b. Internodes are regions of the stem between the nodes. c. Terminal buds are located at the tips of stems and branches. They enclose the short apical meristem, which gives rise to leaves, buds, and all primary tissue of the stem. Only stems produce buds. d. Axillary or lateral buds are located in the leaf axils at the nodes; they may give rise to lateral branches. Leaves consist of flattened blades attached at the node of a stem by a stalk, or petiole. ROOTS: a. Primary and secondary roots. The primary root is the first root produced by the plant embryo and may become a long taproot. Secondary roots arise from meristematic tissue deep within the primary root. Root tips consist of a root apical meristem that gives rise to a root cap and to all the primary tissues of the root proper. A short distance from the root tip is a zone of root hairs (specialized epidermal cells), the principal site of water and mineral absorption. 4
Exercise #2 PRIMARY MERISTEMS Plants produce new cells throughout their lifetime as a result of cell divisions in meristems. Tissues produced from apical meristems are called primary tissues, and this growth is called primary growth. Primary growth occurs along the plant axis at the shoot and root tips. Away from the zone of active cell division, new cells elongate and differentiate for specific functions resulting in, for example, parenchyma and epidermis. Examine a prepared slide of a longitudinal section through an apical bud of Elodea ( or Anachris). Use low power for an overview and then increase magnification. Follow the cells down from the apical meristem and note changes. (Draw what you see.) 5
Exercise #3 ROOT ANATOMY There are slides of roots in cross sections and longitudinal sections available in lab. Look at each under the microscope and draw what you see. Make sure you can identify epidermis, cortex, endodermis, stele (vascular cylinder), pericycle, xylem and phloem. Where do the branch roots arise? Exercise #4 STEMS ***At the beginning of lab take a sample of the plant provided as directed by your TA, and put the stem in Toluidine stain.*** Now, make thin cross sections and longitudinal sections of the stem through an internode and a node. Place the sections in a drop of water on a clean glass slide. a. The dermal tissue system consisting mainly of epidermis and possibly cuticle. b. The ground tissue system, background tissue that fills the spaces between the epidermis and the vascular tissue. The cortex (between the epidermis and vascular bundles) and the central pith function as storage and support. c. The vascular system composed of discrete vascular bundles, a continuous system of xylem and phloem providing transport and support. As always make accurate drawings of what you see. The dicot root you looked at earlier had the vascular tissue together in the center of the root. The dicot stem has vascular tissue in discrete bundles arranged in a ring. There is a transition zone that occurs at ground level. What do you think the vascular tissue looks like there? The vascular tissue branches off into the petiole and blade of the leaf. Try making a section of a petiole. 6
Leaves are the organs especially adapted for photosynthesis. The thin blade portion provides a very large surface area for the absorption of light and the uptake of carbon dioxide through the stomata. The leaf is basically a layer of parenchyma cells (the mesophyll) between two layers of epidermis. The loose arrangement of parenchyma cells within the leaf allows for a great surface area for the rapid exchange of gases. In this lab study you will examine the structure of the leaf in cross section as well as looking at the surface of leaves and the cells that are contained therein. There are also fixed slides of Coleus stems available to view and draw. Exercise #5 Epidermal Peel of a Leaf Make an epidermal peel from the lower surface of the leaf material provided. Place it carefully in a drop of water on a clean microscope slide. Notice how some cells are very different than others in the epidermis. These pairs of cells are the guard cells of the stomata (which is some plants may have between 30,000 and 50,000 guard cells/square inch). Describe them with both a picture and in your own words. Remember that the opening or pore between the guard cells is the stomata. What is the function of the stomata? If possible, make a peel from a leaf that has been kept in the dark. Any differences? Why? Note any internal structures in the guard cells. What might their function be? Make a peel from the upper surface of the leaf. Note the distribution of stomata. Are there as many stomata in the upper surface as in the lower? Why or Why not? What is going on here? How do you think water lily stomata are distributed? Exercise #6 Leaf Anatomy Look at cross sections of a lilac (Syringa ) leaf under medium power. Note the cuticle, epidermis (upper and lower), mesophyll (palisade and spongy), air spaces, and stomates. How are form and function related in the leaves that you have examined? And as always, draw what you see. 7