PLANT ANATOMY I:
Plant Organs Are Comprised of Tissues
Objectives:
1) Describe the relationship between a tissue and organ.
2) Identify a variety of root, stem, and leaf modifications and explain the adaptive significance of each.
3) Describe the 3 primary tissue types, give examples of each, and be able to locate the tissues in a variety of plant organs.
4) Explain primary growth (elongation) and development of plant shoots in terms of meristems, nodes, internodes, and bud and leaf primordia.
INTRODUCTION
The basic body plan of plants consists of 3 organ systems: roots, stems, and leaves. In this lab, we will examine a variety of modifications of these organ systems that enable plants to cope with a range of habitats and environmental conditions. In addition, some of the modifications you will see enable plants to perform unique functions that other plants are incapable of doing – e.g., eating animals!
The materials provided in lab represent just some of the organ modifications possible that enable plants to live in a variety of different environmental conditions and habitat types. In our examination of roots, stems and leaves, we will emphasize structures and modifications typical of the group of plants known as “angiosperms” or Phylum Anthophyta (flowering plants). Angiosperms account for approximately 235,000 of the 260,000 known species of plants and are considered the most evolutionarily “advanced” of any of the plant groups. In later labs, we will explore the structural and functional attributes of other kinds of non-angiosperm plants, such as mosses, ferns, and gymnosperms.
Angiosperms are subdivided into 3 groups: monocots, dicots, and “non-monocot paleoherbs”. However, this third group constitutes less than 3% of angiosperm species, and since traditional botany primarily emphasizes the differences between monocots and dicots, they are often excluded in most discussions of angiosperm structure and classification. Thus, in our examination of the anatomy of roots, stems, and leaves, we will only emphasize differences between monocots and dicots without specifically addressing the non-monocot paleoherbs.
When you think of flowering plants, the examples that come to mind will likely be dicots. For example, roses, oaks, beans, cabbages, and maples are all dicots. Monocots include plants such as grasses, sedges, rushes, and members of the lily/onion, orchid, and iris families. Monocots and dicots differ structurally in a number of respects. For example, the names “monocot” and “dicot” refer to the number of cotyledons, or “seed-leaves” borne on the seedlings of each group – monocots have 1 cotyledon; dicots have 2. Cotyledons are embryonic leaves that store starches to support early growth of the seedling. Cotyledons usually fall off after true leaves become established and start photosynthesizing. Other structural differences are indicated below:
Monocots / DicotsNumber of cotyledons in seed/seedling / 1 / 2
Pattern of venation in leaves / parallel / netted
Arrangement of vascular bundles in stem / scattered / ring
Flower parts in multiples of / 3 / 4, 5
Number of pores/furrows in pollen / 1 / 3
Some of the differences between monocots and dicots will become apparent to you as you examine the gross structure and tissue arrangements of the roots, stems, and leaves provided. The pictures below represent cross sections of monocot and dicot roots and stems. Note the arrangement of vascular tissues in each.
PLANTS HAVE 3 ORGAN SYSTEMS: ROOTS, STEMS, LEAVES
Despite the dramatic difference in appearance of a massive oak compared to a single duckweed plant, the body plan of plants consists of just 3 organ systems – roots, stems, and leaves. Other anatomical features, including fruits, flowers, and cones are really just modifications of one of these three organs. The basic structure and function of each is considered below.
1) ROOTS
Roots function primarily to anchor the plant to the substrate and to absorb water and dissolved minerals and ions. Some roots are further modified for storage of carbohydrates, such as starch, that can later be used by the plant as food. There are two basic kinds of root systems in angiosperms: taproot and fibrous root systems.
Tap root system. Tap roots are found in most dicots. They are distinguished by having a single primary (usually enlarged) root called the "taproot" which derives from the stem. From this primary root several other secondary, or lateral roots emerge.
Fibrous root system. In a fibrous root system, typical of monocots, there is no primary root. Instead, all the roots derive from the stem, are approximately equal in size, and tend to occupy shallower layers of soil. Fibrous root systems are especially well-suited for anchoring the plant in unstable substrates.
Root Modifications:
· Adventitious and prop roots (e.g., maize): These roots arise higher on the stem to assist in anchorage and support.
· Pneumatophores (e.g.,mangroves): The uneven particle size of soil naturally creates gaps and pockets that allow for gas exchange in the roots of most plants. However, plants that grow in submerged or aquatic habitats must cope with very low or no oxygen at the root surface. Pneumatophores are roots which grow upwards, above the surface of water for the purpose of aerating root cells. Why do roots need oxygen?
· Velamen roots (e.g., orchids): Epiphytes (such as orchids and bromeliads) grow upon the tops of other plants in a forest canopy - their roots never touch the ground. As a result, water is not taken up from the soil, but must be absorbed from the humid environment and fiercely conserved. The multiple layers of epidermis in velamen roots reduce water loss and aid in absorption of water.
· Fleshy roots (e.g., beet, carrot, radish, and sweet potatoes): Fleshy roots are especially well adapted for the storage of food materials, such as starches.
2) STEMS
Stems function primarily as support for leaves and for the conduction of food and water between roots and leaves. Stems are divided into nodes (the part of the stem to which leaves are attached) and internodes (the region between nodes). As the stem elongates from the apical meristem, leaf and bud primordia arise which define the node. Leaf primordia mature into leaves, bud primordia develop into buds which give rise to new branches of stem.
Stem Modifications:
· Tendrils (e.g., grape): Tendrils are modifications of shoot (of stem or leaf origin) that aid in the support of the main stem by wrapping onto and "climbing" existing structures.
· Cladophylls (e.g., Asparagus, Epiphyllum): Although they often resemble leaves, cladophylls are actually short branches that are modified for photosynthesis.
· Thorns (e.g., Bouganvillia, hawthorn): Modified branches on the stem that serve as defensive structures.
· Storage stems (e.g., potato tubers, onion bulbs, gladiolus corms, rhizomes in iris, ginger): These stems are specialized for food storage and typically have high levels of starches.
· Runners/Stolons (e.g., strawberry, grasses, spiderplant): Stolons and runners are stems that grow horizontally along the substrate and facilitate vegetative propagation. Clones of the original parent plant arise at nodes along the runner.
3) LEAVES
The external morphology of leaves is highly variable, but the primary function of leaves is usually photosynthesis. The leaf generally consists of two portions: the blade (the flattish, expanded part of the leaf) and the petiole (the stem-like appendage that attaches the leaf to the stem). Leaves that lack a petiole are referred to as sessile.
Leaves can be described as simple or compound. Blades of compound leaves are sub-divided into leaflets. Sometimes leaves are so highly modified it is difficult to determine if they are a leaf, leaflet, or neither! Two good indicators of a leaf are:
1) buds are found in the axis of a leaf, and
2) leaves extend from the stem in various planes while leaflets all lay on the same plane.
Stomata (pores, singular stomate) occur on the upper and lower surfaces of the leaf to accommodate gas exchange. The size of the opening of a stomate is regulated by guard cells which contract and expand with changing conditions of humidity and/or solute concentration. Surfaces of leaves are often covered with fuzz or hairs called trichomes which help reduce water loss and sometimes play a role in defense.
Leaf Modifications:
· Spines (e.g., cacti): Leaves modified for physical defense and for water conservation.
· Bracts (e.g., Bouganvillea, bird of paradise, poinsettia): Bracts tend to resemble flower petals and are used primarily for attracting pollinators or dispersers.
· Carnivorous leaves (e.g., sundew, pitcher plant, Venus flytrap): Leaves modified for the capture and digestion of insects - often found in nitrogen-poor habitats, such as bogs.
· Tendrils (e.g., peas): Tendrils are modifications of shoot (of stem or leaf origin) that aid in the support of the main stem by wrapping onto and "climbing" existing structures.
· Plantlets (e.g., Kalanchoe): Plantlets are simply small clones of plants that arise from leaf notches as a means of vegetative propagation.
PLANT ORGANS ARE CONSTRUCTED FROM 3 TYPES OF TISSUES
Although the architecture varies considerably among roots, stems, and leaves, all plant organs are comprised of 3 basic tissue types: dermal tissue, vascular tissue, and ground tissue. A tissue is simply an assemblage of cells that work together as a structural or functional unit. In this lab, we will examine the general arrangements of those tissues among different plant organs. You will find that the distribution of tissues varies somewhat among different plants as well as among different organ systems. For example, one of the key differences between monocots and dicots has to do with the arrangement of vascular tissues in leaves and stems. In next week’s lab, we will continue our examination of plant tissues by exploring some of the different cell types that form these three plant tissue sytems.
1) DERMAL TISSUE
The dermal tissue system is derived from the protoderm* and consists of 2 tissue types:
· Periderm. Periderm includes cork and cork-producing tissues and is only found in plants with secondary growth.
· Epidermis. The epidermis constitutes the outmost layer of cells of roots, stems and leaves. Epidermal cells are usually covered by a waxy layer, called a cuticle that serves for waterproofing the cells.
2) VASCULAR TISSUE
Vascular tissues, derived from the procambium*, are the tissues responsible for transporting substances from one point in the plant body to another. As is the case with animals, the vascular tissues are responsible for the movement of dissolved gases, sugars, and minerals throughout the organism.
In plants, there are 2 types of conducting tissues:
· Xylem. Xylem is the tissue specialized for the conduction water and dissolved minerals. Cells of the xylem are dead at maturity and act as the structural medium for the movement of water. Individual xylem cells have multiple holes in the walls to allow for the passage of water between adjacent cells. The bulk of wood consists of xylem tissue.
· Phloem. Phloem conducts dissolved sugars throughout the plant. The main conducting cells of phloem, called “sieve elements”, are alive at maturity, but are highly modified for carrying out the function of transport. For example, many of the organelles you might expect to find in a living plant cell, such as nuclei, ribosomes, Golgi apparati, cytoskeleton, and vacuoles are absent from the sieve elements in order to make more space for fluid transport. The small amount of protoplast in sieve elements is distributed in a thin layer along the periphery of the cell wall. In some cases, you may be able to observe smaller cells called "companion cells" distributed among the sieve elements. Companion cells contain all the organelles you would expect to find in a typical plant cell and carry out many of the necessary life functions for the sieve elements which lack organelles.
3) GROUND TISSUE
Derived from the ground meristem*, ground tissues are distributed between and among the dermal and vascular tissues of roots, stems, and leaves. Ground tissues include a wide variety of tissue types that perform a multitude of functions within the plant. Some of the more familiar ground tissues include the following:
· Cortex – the ground tissue distributed between the epidermis and vascular tissues in roots and stems. Cortex is usually modified for storage, especially of starches. In herbaceous green stems, cortex often contains chloroplasts and thus is further modified for carrying out photosynthesis.
· Pith – like cortex, pith is generally modified for starch storage, but is only found to the interior of the vascular tissues in stems and roots.
· Mesophyll - the ground tissue of leaves found between the two layers of epidermis. Leaf mesophyll is the tissue most adapted for photosynthesis and thus is generally rich with chloroplasts.
In addition to the functions indicated above, ground tissues also have an important role in wound healing and the regeneration of damaged or dead cells throughout the plant.
PLANT TISSUES ARE DERIVED FROM MERISTEMS
Unlike animals, most plants grow (increase the number and size of their cells) throughout their lifetime. This ability to grow indeterminately is due to a special form of tissue known as a “meristem”. A meristem is a region of tissue that is perpetually embryonic – that is, it is relatively undifferentiated and divides continuously, giving rise to all new cells and tissues that make up the plant body. Can you think of an analogy to the plant’s meristem in animals/humans?
There are several meristems distributed throughout the plant body which are named for their location and/or for the tissue systems that are derived from them.
1) APICAL (PRIMARY) MERISTEMS are located in the tips (or, apex) of the growing roots and shoots of all plants. Primary meristems produce primary tissues and account for all the primary growth of plants. Primary growth is growth that contributes to elongation of the plant (height and root lengthening) and for the establishment of its basic organ structure (roots, stems, leaves).
2) LATERAL (SECONDARY) MERISTEMS are organized as concentric cylinders (think of a can inside a can inside a can) within the plant body and are responsible for secondary growth. Secondary growth contributes to the thickening of a plant structure, or increases in girth. With few exceptions, lateral meristems tend to be present only in woody plants, such as trees, shrubs, and some vines.