Information

3: Stem, Leaf, and Root Anatomy - Biology


3: Stem, Leaf, and Root Anatomy

3: Stem, Leaf, and Root Anatomy - Biology

2) Vascular Tissue System
Function: Conduction of water, nutrients, sugars and hormones throughout the plant.
Tissues:
a) xylem - conducts water and nutrients up roots, stems and leaves.
b) phloem - conducts water, sugar, hormones, etc. down and up roots, stems and leaves
moves from where produced (called sources ) to where needed (called sinks ).

  • thin, non-lignified primary cell walls
  • filler, storage, protection, photosynthesis
  • examples: flesh of potato, lettuce leaf
  • unevenly thickened, non-lignified primary cell walls
  • support in growing tissues
  • example: strings in celery stalks

2 Types
a) fiber

  • evenly thickened, lignified (tough) secondary cell walls
  • dead at maturity
  • support in mature tissue
  • examples:
    fiber - bamboo cane
    sclereid - seed coat
    stone cell - pear fruit
    a) polysaccharide - a polymer or chain of sugars
      1) cellulose - forms a matrix of microfibrils (chains of b -1,4-linked glucose , see below)
      2) hemicellulose - filler between cellulose microfibrils (chains of misc. sugar)
      3) pectin - cementing agent or filler high in middle lamella and fruit (chains
      of galacturonic acid)

    3) plasmodesmata - tubular plasma membrane extensions through cell walls that connect
    adjacent cells.

      a) cytosol - much of the cytoplasm is a water solution of dissolved compounds
      b) organelles - specialized structures in cytoplasm, each with specific functions.
        1) nucleus - location of DNA and some of the RNA
          2) mitochondria - major site of respiration called the "power house" of the cell.
          3) plastid - double membrane-bound bodies for storage and photosynthesis
            a)leucoplast - colorless plastids
              1) amyloplast - starch storage (chains of a -1,4-linked glucose , seebelow)
              2) elaioplast - fat and oil storage
              a) tonoplast - membrane that surrounds the vacuole

            Base Pairing of Nucleic Acids between the double strands of DNA
            A - T (adenine-thymine)
            G - C (guanine-cytosine)
            Base Pairing of Nucleic Acids between DNA strands and RNA strands
            A - U (adenine-uracil)
            G - C (guanine-cytosine)

            meristem - discrete regions or groups of cells that possess continued cell division for the
            life of the plant or that organ.

            PLANTS EXHIBIT TWO TYPES OF GROWTH
            1) Primary Growth - growth in length that gives rise to primary (herbaceous) tissues
            called the primary plant body.

            lateral meristem - meristematic regions along the sides of stems and roots.

            2 Types of lateral meristems give rise to secondary growth
            a) vascular cambium or cambium - a sheet-like meristem between the bark and wood
            along the sides of woody stems and roots it gives rise to secondary xylem (commonly called wood ) on
            the inside and secondary phloem on the outside.
            b) cork cambium or phellogen - gives rise to the periderm (commonly called bark ).

            LEAVES
            1) photosynthesis site where primarily occurs
            2) regulate water loss e.g. by opening and closing stomata
            3) storage ex. carbohydrates and water in garlic, aloe vera
            4) support ex. tendrils on grape
            5) protection ex. spines on cacti bud scales
            6) attraction ex. bracts on poinsettia or dogwood
            7) propagation ex. bryophyllum with plantlets on leaves

            terminal bud - a bud at the tip of a stem responsible for terminal growth.

            axillary bud or lateral bud - buds along side the axis of a stem they were produced by the terminal bud during growth once they grow out and form a lateral stem they become terminal buds of the lateral branch.

            flower bud - a bud containing a floral meristem which develops into flowers usually larger than vegetative buds.

            leaf scar - a scar marking the former point of attachment of a leaf or petiole to the stem.

            internode - the part of the stem between nodes

            node - part of stem marking the point of attachment of leaves, flowers, fruits, buds and other stems.

            lenticel - rough areas on stems (and some fruits, ex. apple) composed of loosely packed cells extending from the cortex through the ruptured epidermis serve as "breathing pores" for gas exchange. Only occur on young stems.

            Mechanism of Opening
            a) open when guard cells are turgid (due to water uptake in response to potassium influx)
            b) closed when guard cells are flaccid (due to water loss in response to potassium efflux)

            Daily Cycle
            C-3 and C-4 Plants
            a) open during day
            b) closed during night
            CAM Plants
            a) open during night
            b) closed during day

            Designed for gas exchange
            a) CO2 in and 02 out for photosynthesis
            b) CO2 out and 02 in for respiration
            c) H20 out during transpiration

            MESOPHYLL
            Palisade parenchyma
            a) Contains 70-80% of the chloroplasts in the leaf.
            b) Specialized for photosynthesis - because it contains a large number of chloroplasts
            and it occurs towards the top side of leaf
            Spongy mesophyll
            a) Contains large air spaces
            b) Specialized for gas exchange - because of the large air space and more stomata occur in
            the epidermis of lower leaf surface


            Stem & Root Anatomy

            V ascular plants contain two main types of conduction tissue, the xylem and phloem. These two tissues extend from the leaves to the roots, and are vital conduits for water and nutrient transport. In a sense, they are to plants what veins and arteries are to animals. The structure of xylem and phloem tissue depends on whether the plant is a flowering plant (including dicots and monocots) or a gymnosperm (polycots). The terms dicot, monocot and polycot are summarized in the following table.

            Flower parts in 3's or multiple of 3's one cotyledon inside seed parallel leaf venation includes Lilium , Amaryllis , Iris , Agave , Yucca , orchids, duckweeds, annual grasses, bamboos and palms.

            Flower parts in 4's or 5's 2 cotyledons inside seed branched or net leaf venation contains the most species of flowering herbs, shrubs and trees includes roses ( Rosa ), buttercups ( Ranunculus ), clover ( Trifolium ), maple ( Acer ), basswood ( Tilia ), oak ( Quercus ), willow ( Salix ), kapok ( Ceiba ) and many more species.

            Gymnosperms include pines ( Pinus ), spruce ( Picea ), fir ( Abies ), hemlock ( Tsuga ) and false hemlock ( Pseudotsuga ). Some of the coniferous genera (division Coniferophyta) are the most important timber trees in the world. Since these species have several cotyledons inside their seeds, they are conveniently referred to as polycots.

            X ylem and phloem tissues are produced by meristematic cambium cells located in a layer just inside the bark of trees and shrubs. In dicot stems, the cambium layer gives rise to phloem cells on the outside and xylem cells on the inside. All the tissue from the cambium layer outward is considered bark, while all the tissue inside the cambium layer to the center of the tree is wood. Xylem tissue conducts water and mineral nutrients from the soil upward in plant roots and stems. It is composed of elongate cells with pointed ends called tracheids, and shorter, wider cells called vessel elements. The walls of these cells are heavily lignified, with openings in the walls called pits. Tracheids and vessels become hollow, water-conducting pipelines after the cells are dead and their contents (protoplasm) has disintegrated. The xylem of flowering plants also contains numerous fibers, elongate cells with tapering ends and very thick walls. Dense masses of fiber cells is one of the primary reasons why angiosperms have harder and heavier wood than gymnosperms. This is especially true of the "ironwoods" with wood that actually sinks in water.

            A recent article in Science Vol. 291 (26 January 2001) by N.M. Holbrook, M. Zwieniecki and P. Melcher suggests that xylem cells may be more than inert tubes. They appear to be a very sophisticated system for regulating and conducting water to specific areas of the plant that need water the most. This preferential water conduction involves the direction and redirection of water molecules through openings (pores) in adjacent cell walls called pits. The pits are lined with a pit membrane composed of cellulose and pectins. According to the researchers, this control of water movement may involve pectin hydrogels which serve to glue adjacent cell walls together. One of the properties of polysaccharide hydrogels is to swell or shrink due to imbibition. "When pectins swell, pores in the membranes are squeezed, slowing water flow to a trickle. But when pectins shrink, the pores can open wide, and water flushes across the xylem membrane toward thirsty leaves above." This remarkable control of water movement may allow the plant respond to drought conditions.

            S piral thickenings in the secondary walls of vessels and tracheids gives them the appearance of microscopic coils under high magnification with a light microscope.

            Magnified horizontal view (400x) of an inner perianth segment of a Brodiaea species in San Marcos showing a primary vascular bundle composed of several strands of vessels. The strands consist of vessels with spirally thickened walls that appear like minute coiled springs. Although this species has been called B. jolonensis by San Diego botanists for decades, it appears to be more similar to B. terrestris ssp. kernensis . This species contains at least 3 strands of vessels per bundle, while B. jolonensis only has one strand per bundle.

            T he water-conducting xylem tissue in plant stems is actually composed of dead cells. In fact, wood is essentially dead xylem cells that have dried out. The dead tissue is hard and dense because of lignin in the thickened secondary cell walls. Lignin is a complex phenolic polymer that produces the hardness, density and brown color of wood. Cactus stems are composed of soft, water-storage parenchyma tissue that decomposes when the plant dies. The woody (lignified) vascular tissue provides support and is often visible in dead cactus stems.

            Left: Giant saguaro ( Carnegiea gigantea ) in northern Sonora, Mexico. The weight of this large cactus is largely due to water storage tissue in the stems. Right: A dead saguaro showing the woody (lignified) vascular strands that provide support for the massive stems.

            P hloem tissue conducts carbohydrates manufactured in the leaves downward in plant stems. It is composed of sieve tubes (sieve tube elements) and companion cells. The perforated end wall of a sieve tube is called a sieve plate. Thick-walled fiber cells are also associated with phloem tissue.

            I n dicot roots, the xylem tissue appears like a 3-pronged or 4-pronged star. The tissue between the prongs of the star is phloem. The central xylem and phloem is surrounded by an endodermis, and the entire central structure is called a stele.

            Microscopic view of the root of a buttercup ( Ranunculus ) showing the central stele and 4-pronged xylem. The large, water-conducting cells in the xylem are vessels. [Magnified Approximately 400X.]

            I n dicot stems, the xylem tissue is produced on the inside of the cambium layer. Phloem tissue is produced on the outside of the cambium. The phloem of some stems also contains thick-walled, elongate fiber cells which are called bast fibers. Bast fibers in stems of the flax plant ( Linum usitatissimum ) are the source of linen textile fibers. Gymnosperms generally do not have vessels, so the wood is composed essentially of tracheids. The notable exception to this are members of the gymnosperm division Gnetophyta which do have vessels. This remarkable division includes Ephedra (Mormon tea), Gnetum , and the amazing Welwitschia of Africa's Namib Desert.

            P ine stems also contain bands of cells called rays and scattered resin ducts. Rays and resin ducts are also present in flowering plants. In fact, the insidious poison oak allergen called urushiol is produced inside resin ducts. Wood rays extend outwardly in a stem cross section like the spokes of a wheel. The rays are composed of thin-walled parenchyma cells which disintegrate after the wood dries. This is why wood with prominent rays often splits along the rays. In pines, the spring tracheids are larger than the summer tracheids. Because the summer tracheids are smaller and more dense, they appear as dark bands in a cross section of a log. Each concentric band of spring and summer tracheids is called an annual ring. By counting the rings (dark bands of summer xylem in pine wood), the age of a tree can be determined. Other data, such as fire and climatic data, can be determined by the appearance and spacing of the rings. Some of the oldest bristlecone pines ( Pinus longaeva ) in the White Mountains of eastern California have more than 4,000 rings. Annual rings and rays produce the characteristic grain of the wood, depending on how the boards are cut at the saw mill.

            Microscopic view of a 3-year-old pine stem ( Pinus ) showing resin ducts, rays and three years of xylem growth (annual rings). [Magnified Approximately 200X.]

            A cross section of loblolly pine wood ( Pinus taeda ) showing 18 dark bands of summer xylem (annual rings).

            A ngiosperms typically have both tracheids and vessels. In ring-porous wood, such as oak and basswood, the spring vessels are much larger and more porous than the smaller, summer tracheids. This difference in cell size and density produces the conspicuous, concentric annual rings in these woods. Because of the density of the wood, angiosperms are considered hardwoods, while gymnosperms, such as pine and fir, are considered softwoods.

            T he following illustrations and photos show American basswood ( Tilia americana ), a typical ring-porous hardwood of the eastern United States:

            A cross section of the stem of basswood ( Tilia americana ) showing large pith, numerous rays, and three distinct annual rings. [Magnified Approximately 75X.]

            A cross section of the stem of basswood ( Tilia americana ) showing pith, numerous rays, and three distinct annual rings. The large spring xylem cells are vessels. [Magnified Approximately 200X.]

            In the tropical rain forest, relatively few species of trees, such as teak, have visible annual rings. The difference between wet and dry seasons for most trees is too subtle to make noticeable differences in the cell size and density between wet and dry seasonal growth. According to Pascale Poussart, geochemist at Princeton University, tropical hardwoods have "invisible rings." She and her colleagues studied the apparently ringless tree ( Miliusa velutina ) of Thailand. Their team used X-ray beams at the Brookhaven National Synchrotron Light Source to look at calcium taken up by cells during the growing season. There is clearly a difference between the calcium content of wood during the wet and dry seasons that compares favorably with carbon isotope measurements. The calcium record can be determined in one afternoon at the synchrotron lab compared with four months in an isotope lab.

            M onocot stems, such as corn, palms and bamboos, do not have a vascular cambium and do not exhibit secondary growth by the production of concentric annual rings. They cannot increase in girth by adding lateral layers of cells as in conifers and woody dicots. Instead, they have scattered vascular bundles composed of xylem and phloem tissue. Each bundle is surrounded by a ring of cells called a bundle sheath. The structural strength and hardness of woody monocots is due to clusters of heavily lignified tracheids and fibers associated with the vascular bundles. The following illustrations and photos show scattered vascular bundles in the stem cross sections of corn ( Zea mays ):

            A cross section of the stem of corn ( Zea mays ) showing parenchyma tissue and scattered vascular bundles. The large cells in the vascular bundles are vessels. [Magnified Approximately 250X.]

            U nlike most monocots, palm stems can grow in girth by an increase in the number of parenchyma cells and vascular bundles. This primary growth is due to a region of actively dividing meristematic cells called the "primary thickening meristem" that surrounds the apical meristem at the tip of a stem. In woody monocots this meristematic region extends down the periphery of the stem where it is called the "secondary thickening meristem." New vascular bundles and parenchyma tissue are added as the stem grows in diameter.

            The massive trunk of this Chilean wine palm ( Jubaea chilensis ) has grown in girth due to the production of new vascular bundles from the primary and secondary thickening meristems.

            T he scattered vascular bundles containing large (porous) vessels are very conspicuous in palm wood. In fact, the vascular bundles are also preserved in petrified palm.

            Cross section of the trunk of the native California fan palm ( Washingtonia filifera ) showing scattered vascular bundles. The large cells (pores) in the vascular bundles are vessels.

            The trunk of a California fan palm ( Washingtonia filifera ) in Palm Canyon, Anza-Borrego State Park. The palm was washed down the steep canyon during the flash flood of September 2004. The fibrous strands are vascular bundles composed of lignified cells.

            Right: Cross section of the trunk of a California fan palm ( Washingtonia filifera ) showing scattered vascular bundles that appear like dark brown dots. The dot pattern also shows up in the petrified Washingtonia palm (left). The pores in the petrified palm wood are the remains of vessels. The large, circular tunnel in the palm wood (right) is caused by the larva of the bizarre palm-boring beetle ( Dinapate wrightii ) shown at bottom of photo. An adult beetle is shown in the next photo.

            A beautiful cutting board made from numerous flattened strips of bamboo ( Phyllostachys pubescens ) glued together. Through a specialized heating process, the natural sugar in the wood is caramelized to produce the honey color. Vascular bundles typical of a woody monocot are clearly visible on the smooth cross section. The transverse surface of numerous lignified tracheids and fibers is actually harder than maple.

            A 270 Million-Year-Old Petrified Tree Fern

            D uring the Carboniferous Era, approximately 300 million years ago, the earth was dominated by extensive forests of giant lycopods (division Lycophyta), horestails (division Sphenophyta) and tree ferns (division Pterophyta). Much of the earth's coal reserves originated from massive deposits of carbonized plants from this era. Petrified trunks from Brazil reveal cellular details of an extinct tree fern ( Psaronius brasiliensis ) that lived about 270 million years ago, before the age of dinosaurs. The petrified stem of Psaronius does not have concentric growth rings typical of conifers and dicot angiosperms. Instead, it has a central stele composed of numerous arcs that represent the vascular bundles of xylem tissue. Surrounding the stem are the bases of leaves. In life, Psaronius probably resembled the present-day Cyathea tree ferns of New Zealand.


            Stems in Plants: Function, Types and Anatomy

            The stem is the ascending part of the plant formed by the elongation of the plumule of the embryo. It bears leaves, branches and flowers.

            It is generally erect, strong and usually grows away from the soil (negatively geotropic). There are several plants in which the stem is weak and it either trails on the ground or twines around a support.

            Stems are differentiated into regions called nodes. Leaves and branches arise from nodes. The portion between the nodes is called the Internode.

            The growing apex of the stem is covered by numerous, tiny, developing leaves and is called the apical bud. Buds also arise in the axils of leaves they are termed axillary or Lateral buds. These buds give rise to branches or flowers.

            Plants have been classified on the basis of the height and strength of stem and their life- span. Herbs are small plants with a soft stem. Medium-sized plants with woody stems that branches profusely from the base and attain a bushy appearance are called Shrubs.

            Trees have a stout and tall trunk with profuse branching. Plants which complete their life cycle within one season are termed annuals such as agricultural crops (rice, groundnut etc.). Biennials complete their life cycle in two seasons (radish, cabbage).

            Plants that usually survive for a number of years and produce flowers and fruits during specific seasons are termed perennials (mango, apple etc.).Besides bearing branches, leaves and flowers, stems perform other functions such as presentation, vegetative propagation and storage of reserve food.

            Primary or main functions:

            1. It supports and holds leaves, flowers and fruits.

            2. Leaves are borne on stem in such a fashion that they are able to carry on the important function efficiently just like to receive the light and to carry on the gaseous exchange.

            3. The stem conducts the water and minerals from roots to leaves and fruits.

            4. Stem bears flowers and fruits in position to facilitate the processes of pollination and fertilization.

            Secondary or Accessory functions:

            There are three types of stem: Underground stem, Aerial stem and Sub- aerial stem.

            1. Underground stem:

            Stems of some plants remain in the ground and serve the function of perennation and storage of food. They produce aerial shoots annually. They resemble roots superficially but are distinguishable by the presence of scale leaves and buds at nodes. Such stem also act as a means of vegetative propagation. The modified underground stems are the following: (Fig. 4.6 i – iv)

            It is a thickened, prostrate, underground stem having distinct nodes and internodes, scaly leaves at the nodes, axillary and terminal buds present may be branched or un-branched sometimes adventitious roots also arise, e.g. Ginger.

            The underground stem becomes enlarged at the growing tips by the accumulation of stored food, commonly starch, tubers are produced e.g. Potato. The eyes of potato are nodes at each of which 1-3 buds are produced in the axils of small scaly like leaves.

            Bulb is a short underground stem with fleshy leaf base called scales. Stem is very much reduced and becomes disc like. The discoid stem in convex or conical in shape and bears highly compressed internodes. These node bear fleshy scales. On the upper side, disc bears terminal bud surrounded by number of leaves. The axillary buds are present between the axis of leaves. The adventitious roots are borne on the lower side of the disc. E .g. Onion.

            Corm is short, thick and un-branched underground stem with stored food material. It grows vertically and covered by thin sheathing leaf bases of dead leaves called scales. The corm bears buds at their nodes. These buds are responsible for giving off adventitious roots. Corm serves the functions of food storage, vegetative propagation and perennation. Corm is more or less rounded in shape or often somewhat flattened from top to bottom, e.g. Colocasia.

            2. Sub – aerial stems:

            Lower buds of the stem in some plants grow out into short, lateral branches. These are named according to their origin, nature and mode of reproduction (Fig. 4.7 i- iv):


            It grows prostrate in all directions above the soil level. Nodes bear scale leaves. It has a creeping stem with long internodes. On the lower sides, nodes bear adventitious roots. Runner develops from the axils of lower leaves of aerial stem which sends slender horizontal branches in the form of runners. When older parts of plant die, the branches separate from parent plant and form independent plants e.g. Doob grass.

            It is a slender lateral branch which appears from the lower part of main axis. This lateral branch grows aerially for some distance and becomes arched and finally touches the ground to give rise to new shoot with the help of its terminal bud. It also bears roots to get fixed with the soil e.g. Jasmine.

            Offset is more shorter and thicker. It is usually found in aquatic plants like water hyacinth and Pistia. It bears a cluster of leaves near the water or ground level and gives adventitious roots inside water or ground from all nodes, e.g. Pistia.

            Like the stolon the sucker is also a lateral branch but it grows obliquely upwards and gives rise to a new plants e.g. Mentha.

            These modified aerial stems perform unusual functions. Different forms of these stems are the following (Fig. 4.8 i -v):

            It is a leafless, spirally coiled branch formed in some climbers and helps them in climbing neighbouring objects they may be modification of axillary bud, e.g. Passiflora.

            Stem thorn is a hard, straight and pointed structure it is a defensive organ also helps in climbing originates from axillary or terminal bud, e.g. Duranta.

            It is a green, flattended or cylindrical stem which takes the form and function of leaf. It contains chlorophyll and is responsible for carrying on photosynthesis. It bears succession of nodes and intemodes at long or short intervals. Phylloclades are found in xerophytic plants where the leaves either grow feebly or fall off early or modified into spines e.g. Opuntia,

            Phylloclade with one or two internodes is called cladode e.g. Asparagus. In Asparagus cladodes are needle-like, slightly flattened green structures which appear in cluster in the axil of a scaly leaf. Main stem bears leaf spines at its nodes. A scale leaf is found just above the spine. Every branch on main stem bears only scale leaves. In the axil of scale leaves cluster of cladodes appear

            Bulbil is the modification of vegetative or floral bud. It is swollen due to storage of food. It can function as an organ of vegetative propagation e.g. Dioscorea.

            Anatomy of Stem:

            A thin transverse section of a young stem reveals the internal structure when observed under me microscope:

            Internal structure of Dicot Stem (Fig. 4.9):

            It forms the single-celled outermost layer of the stem. The outer wall of epidermal cells is cutinized. It bears multi-cellular hairs and a few stomata. It is protective in nature.

            Cortex lies below the epidermis. It is differentiated into three zones-

            It is formed of 4 to 5 cell thick layer of collenchymatous cells. These cells are living and contain chloroplasts.

            It lies below the hypodermis. It consists of a few layers of thin- walled parenchymatous cells with intercellular spaces. Some of the cells have chloroplasts and they are known as chlorenchyma.

            It is the innermost layer of cortex. It is made up of single row of compact barrel-shaped cells without intercellular spaces. Since the cells of endodermis contain starch grains, it is also known as starch-sheath. Casparian strips are distinctly visible in endodermal cells.

            It lies below the endodermis. It is formed of semilunar patches of sclerenchyma. The sclerenchyma cells are dead and rigid with their walls. Pericycie provides mechanical support to the plant and protects the vascular bundles.

            4. Vascular bundles:

            They are many in number and arranged in a ring enclosed by the pericycle. The vascular bundles are conjoint, collateral, open and endarch. Each vascular bundle is composed of Xylem, Phloem and Cambium.

            It is the innermost layer of vascular bundles and lies towards the centre of the stem. Xylem consists of vessels, tracheids, wood fibres and wood -parenchyma. The smaller vessels which He towards the centre comprise the protoxylem and the bigger ones which lie away from the centre are known as metaxylem.

            It lies below the pericycle and is composed of sieve tubes, companion cells and phloem parenchyma. The phloem cells store starch, protein and fats.

            It is a strip of thin-walled cells lying in between the phloem and xylem. The cambial cells consist of a single layer of meristematic cells.

            5. Pith or medulla:

            It is the central part of the stem, composed of parenchymatous cells with conspicuous intercellular spaces. Its main function is storage of food and transverse conduction of food materials.

            Internal Structure of Monocot Stem (Fig. 4.10):

            It is the outermost layer stem composed of square-shaped cells and it is interrupted by stomata. It is covered with cuticle and epidermal hairs are absent. It is protective in nature.

            It lies below the epidermis. It consists of two or three layers of sclerenchymatous cells. It is mechanical in functions, and provides support and strength to the stem.

            It consists of a mass of thin-walled parenchyma cells extending from below the hypodermis to the centre of the stem. It is not differentiated into definite tissues like cortex, endodermis, pericycle, etc., as in dicot stems. The cells of the ground tissue have intercellular spaces. The cells contain reserve food materials, vascular bundles are scattered in the ground tissue.

            Many vascular bundles remain scattered in the ground tissue. They lie closer to periphery. The peripheral vascular bundles are smaller than the central ones. Each vascular bundle is surrounded by a sheath of thick-walled sclerenchyma cells called the bundle-sheath. It provides protection and strength to the vascular bundles. Vascular bundles are conjoint, collateral, endarch and closed. Each vascular bundle is composed of xylem and phloem.


            Anatomy of Dicot Roots | Botany

            1. Outermost, single – layered epiblema consists of barrel shaped or rectangular cells.

            2. From some cells arise unicellular hairs.

            3. Few layered exodermis is present below epiblema. The cells are thin walled and without any intercellular spaces.

            4. Many layered cortex consists of thin walled, rounded, oval or polygonal cells.

            5. It is parenchymatous, with many intercellular spaces. The cells are filled with starch grains.

            6. Endodermis is the innermost cortical layer consisting of barrel shaped cells.

            7. The endodermal cells are thick-walled and contain casparian strips. A few thin walled passage cells are also present against the protoxylem.

            8. Single layered pericycle consists of thin walled, small cells, and lies immediately inner to the endodermis.

            9. These are radial and exarch.

            10. Four xylem strands alternate with the four phloem strands showing tetrarch condition.

            11. Protoxylem lies towards the periphery (i.e., exarch), and consists of annular and spiral vessels.

            12. Metaxylem vessels meet in the centre and consist of pitted and reticulate vessels.

            13. Four patches of phloem consist of sieve tubes, companion cells and phloem parenchyma.

            (a) 1. Presence of vessels in the xylem.

            2. Vessels have perforated end walls with scalariform and regularly arranged holes. (Angiosperms)

            (b) 1. Presence of unicellular root hairs.

            2. Radial vascular bundles.

            3. The xylem is exarch. (Root)

            (c) 1. Presence of tetrarch condition.

            2. Pith is absent. (Dicotyledones)

            2. Anatomy of Cicer – Root:

            It is circular in outline and reveals following tissues from out side with-in:

            1. It is the outermost layer consisting of many thin walled cells.

            2. From some of its cells arise unicellular hairs.

            4. It is very large, parenchymatous and well developed occupying the big part of the section.

            5. In this region there are present many intercellular spaces.

            6. Cortical cells are filled with starch grains.

            7. In older sections, few layered exodermis, consisting of thin walled compact cells, is present just below the epiblema.

            8. Endodermis is the ring like innermost layer of cortex made up of barrel shaped cells.

            9. Casparian strips are present in the endodermal cells.

            10. Some of the endodermal cells, particularly opposite to the protoxylem, are thin walled and have been termed as passage cells.

            11. Single – layered, ring like pericycle is present close to the endodermis on its inner side.

            12. It is also a compact layer of thin walled cell.

            13. The vascular bundles are 2 to 6 and radial, i.e., xylem and phloem on different radii alternating with each other.

            14. Xylem and phloem patches are equal in number.

            15. Xylem consists of protoxylem and metaxylem.

            16. Protoxylem is exarch and consists of small annular and spiral vessels.

            17. Metaxylem strands are big, present towards the centre and composed of large reticulate and pitted vessels.

            18. In some cases the metaxylem meet in the centre and thus obliterating the pith.

            19. Phloem composed of sieve tubes, companion cells and phloem parenchyma.

            20. In mature roots,.cambium also appears cutting the secondary structures.

            21. The parenchymatous cells in between xylem and phloem strands form conjunctive tissue.

            22. It is very small, parenchymatous and without any intercellular spaces. It gets reduced after the formation of secondary structures.

            (a) 1. Presence of vessels in the xylem.

            2. Vessels have perforated end walls with scalariform and regularly arranged holes. (Angiosperms)

            (b) 1. Presence of unicellular root hairs.

            2. Vascular bundles are radial and xylem is exarch. (Root)

            (c) 1. Vascular bundles are 2-6.

            2. Reduced pith. (Dicotyledones)

            3. Anatomy of Tinospora – Root:

            T. S. appears circular in outline and reveals following tissues from outside with-in:

            1. It consists of cork, cork cambium and secondary cortex which are also termed as phellem, phellogen and phelloderm, respectively.

            2. Cork is the outermost region of the section, consisting of dead cells which are rectangular in shape. It is few to many cells deep.

            3. Cork cambium is meristematic in nature and cuts cork on the outer side and secondary cortex towards inner side.

            4. Secondary cortex consists of thin walled parenchymatous, rounded or oval cells leaving many intercellular spaces. Cells are filled with many plastids.

            5. Endodermis is present in the form of a single layer in young stages but at maturity it is not observed due to the formation of periderm.

            6. Single layered pericycle, consisting of barrel shaped cells, is clearly observed in young roots.

            7. Vascular bundles arc radial, exarch and show the secondary growth due to the presence of cambium.

            8. Vascular tissues remain divided into many smaller groups with the help of broad medullary rays.

            9. Vascular tissue consists of primary phloem, secondary phloem, cambium, secondary xylem, primary xylem and medullary rays.

            10. Primary phloem is crushed and situated alternating with primary xylem groups.

            11. Secondary phloem is more developed below the primary phloem. Phloem consists of sieve tubes, phloem parenchyma and companion cells.

            12. Cambium is one to many celled thick, wavy and present in the form of a complete ring.

            13. Secondary xylem is well developed and consists of tracheids, xylem parenchyma and large vessels.

            14. Primary xylem group’s arc centrally located and face their groups towards periphery.

            15. Medullary rays are parenchymatous, multiseriate and separate the vascular tissue in small groups.

            3. Chlorenchymalous secondary codex.

            (a) 1. Presence of vessels in the xylem.

            2. Vessels have perforated end walls with scalariform and regularly arranged holes. (Angiosperms)

            (b) 1. Radial vascular bundles.

            2. Protoxylem is exarch. (Root)

            (c) 1. Vascular bundles are between 2-6.

            2. Presence of cambium and secondary growth.

            3. Reduced pith. (Dicotyledones)

            4. Anatomy of Ficus – Root (Family – Moraceae):

            It is circular in outline and reveals following tissues from outside with-in:

            1. It is composed of cork, cork cambium and secondary cortex.

            2. Cork is well developed and 6 to 8 or more layers are present. The cells are rounded, irregular or rectangular in shape and may be filled with tannin.

            3. Cork cambium or phellogen is well developed and meristematic in function.

            4. Secondary cortex or phelloderm is parenchymatous and the cells contain chlorophyll in young stages. The cells are rounded with many intercellular spaces in between.

            6. Endodermis is well-developed, single layered and present in the young root but it gels crushed due to the secondary growth.

            7. Crushed due to secondary growth.

            8. It is composed of crushed primary phloem, well developed secondary phloem, cambium, secondary xylem and primary xylem.

            9. Primary phloem is radial to primary xylem and present in the form crushed patches. The number of the patches are as many as the number of xylem groups.

            10. Secondary phloem ring is situated inner to the primary phloem. It consists of sieve tubes, companion cells and phloem parenchyma.

            11. Cambium is present in the form of a continuous ring. But opposite to protoxylem, it is consumed in the production of medullary rays.

            12. Secondary xylem is well developed and consists of large vessels, tracheids and xylem parenchyma.

            13. Primary xylem bundles are centrally located, two to six or rarely more in number and facing their protoxylem towards the periphery, i.e., it is exarch.

            14. It is very small, parenchymatous and present in the centre.

            2. Well developed rough-type of cork.

            4. Secondary parenchyma contains chloroplast.

            5. Cuticle is present in young roots.

            (a) 1. Vessels are present in the xylem.

            2. Vessels have perforated end scalariform and regularly arranged holes. (Angiosperms)


            1. Collenchymatous hypodermis is characteristics of
            (a) Hydrophytes
            (b) Monocot and dicot stem
            (c) Monocot stem
            (d) Dicot stem

            2. The lacunae in the vascular bundles of monocot stem is
            (a) Metaxylem
            (b) A mucilage canal
            (c) Lysigenous water cavity
            (d) A large sized protoxylem

            3. The protoxylem in its midrid bundle in a vertical section of a dorsiventral leaf
            (a) is not distinct
            (b) faces the ventral epidermis of the leaf
            (c) faces the dorsal epidermis of the leaf
            (d) is surrounded by metaxylem

            4. This is not a characteristic feature of anatomy of dicotyledonous root
            (a) Pith little or absent
            (b) Secondary growth
            (c) Radial vascular bundles
            (d) Vascular bundles 15-20

            5. Vascular bundles are scattered in
            (a) Dicot stem
            (b) Dicot root
            (c) Bryophytes
            (d) Monocot stem

            6. The correct situation of mesophyll in isobilateral grass leaf is shown by
            (a) Palisade along both the surface
            (b) Undifferentiated mesophyll
            (c) Palisade towards abaxial surface
            (d) Palisade towards adaxial surface

            7. Well-developed pith is found in
            (a) Monocot root and Dicot stem
            (b) Dicot root and Dicot stem
            (c) Dicot root and Monocot stem
            (d) Dicot and Monocot stems

            8. In monocot leaf
            (a) Veins form a network
            (b) Mesophyll is not differentiated into palisade and spongy parenchyma
            (c) Mesophyll is well differentiated into these parts
            (d) Bulliform cells are absent from epidermis

            9. In orchids, Velamen tissues is found in
            (a) Leaves
            (b) Root
            (c) Flowers
            (d) Shoot

            10. Polyarch and exarch vascular bundles occur in
            (a) Dicot stem
            (b) Monocot stem
            (c) Dicot root
            (d) Monocot root


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            Leaf Tissues

            Leaf tissues are composed of layers of plant cells. Different plant cell types form three main tissues found in leaves. These tissues include a mesophyll tissue layer that is sandwiched between two layers of epidermis. Leaf vascular tissue is located within the mesophyll layer.

            The outer leaf layer is known as the epidermis. The epidermis secretes a waxy coating called the cuticle that helps the plant retain water. The epidermis in plant leaves also contains special cells called guard cells that regulate gas exchange between the plant and the environment. Guard cells control the size of pores called stomata (singular stoma) in the epidermis. Opening and closing the stomata allows plants to release or retain gases including water vapor, oxygen, and carbon dioxide as needed.

            The middle mesophyll leaf layer is composed of a palisade mesophyll region and a spongy mesophyll region. Palisade mesophyll contains columnar cells with spaces between the cells. Most plant chloroplasts are found in palisade mesophyll. Chloroplasts are organelles that contain chlorophyll, a green pigment that absorbs energy from sunlight for photosynthesis. Spongy mesophyll is located below palisade mesophyll and is composed of irregularly shaped cells. Leaf vascular tissue is found in the spongy mesophyll.

            Vascular Tissue

            Leaf veins are composed of vascular tissue. Vascular tissue consists of tube-shaped structures called xylem and phloem that provide pathways for water and nutrients to flow throughout the leaves and plant.


            Function of the Leaf

            As one of the most important constituents of plants, leaves have several essential functions:

            Photosynthesis

            The primary function of the leaf is the conversion of carbon dioxide, water, and UV light into sugar (e.g., glucose) via photosynthesis (shown below). The simple sugars formed via photosynthesis are later processed into various macromolecules (e.g., cellulose) required for the formation of the plant cell wall and other structures. Therefore, the leaf must be highly specialized to combine the carbon dioxide, water, and UV light for this process. Carbon dioxide is diffused from the atmosphere through specialized pores, termed stomata, in the outer layer of the leaf. Water is directed to the leaves via the plant’s vascular conducting system, termed the xylem. Leaves are orientated to ensure maximal exposure to sunlight, and are typically thin and flat in shape to allow sunlight to penetrate the leaf to reach the chloroplasts, which are specialized organelles that perform photosynthesis. Once sugar is formed from photosynthesis, the leaves function to transport it down the plant via specialized structures called the phloem, which run in parallel to the xylem. The sugar is typically transported to the roots and shoots of the plant, to support growth.

            Transpiration

            Transpiration refers to the movement of water through the plant, and subsequent evaporation via the leaves. When the stomata open to accommodate the diffusion of carbon dioxide into the plant for photosynthesis, water flows out. This process also serves to cool the plant via evaporation of the water from the leaf, as well as regulate the plant’s osmotic pressure.

            Guttation

            Guttation refers to the excretion of xylem from the edges of leaves and other vascular plants due to increased levels of water in the soil at night, when the stomata are closed. The pressure caused at the roots results in the leakage of water from the xylem out of specialized water glands at the edges of leaves.

            Storage

            Leaves are a primary site of water and energy storage since they provide the site of photosynthesis. Succulents are particularly adept at water storage, as evidenced by the thick leaves. Due to the high levels of nutrients and water, many animal species ingest the leaves of plants as a source of food.

            Defense

            In general, the types of leaf can be divided into six major types, although there are also plants with highly specialized leaves:

            Conifer Leaf

            Conifer leaves are needle-shaped or in the form of scales. Conifer leaves are typically heavily waxed and highly adapted to colder climates, arranged to dispel snow and resist freezing temperatures. Some examples include Douglas firs and spruce trees. The images below illustrate this type of leaf.

            Microphyll Leaf

            Microphyll leaves are characterized by a single vein that is unbranched. Although this type of leaf is abundant in the fossil record, few plants exhibit this type of leaf today. Some examples include horsetails and clubmosses. The image below illustrates this type of leaf.

            Megaphyll Leaf

            Megaphyll leaves are characterized by multiple veins that can be highly branched. Megaphyll leaves are broad and flat, and generally comprise the foliage of most plant species. The image below illustrates this type of leaf.

            Angiosperm Leaf

            Angiosperm leaves are those found on flowering plants. These leaves are characterized by stipules, a lamina, and a petiole. The illustration below shows an example of an angiosperm leaves.

            Fronds

            Fronds are large, divided leaves characteristic of ferns and palms. The blades can be singular or divided into branches. The image below presents an example of a frond.

            Sheath Leaf

            Sheath leaves are typical of grass species and monocots. Thus, the leaves are long and narrow, with a sheathing surrounding the stem at the base. Moreover, the vein structure is striated and each node contains only one leaf. The image below presents an example of a sheath leaf.

            1. The primary function of a leaf is:
            A. Water evaporation for cooling
            B. Photosynthesis
            C. Provide shade to the shoot and root structures of the plant
            D. Transpiration

            2. Which of the following statements is TRUE regarding guttation:
            A. It typically occurs at night.
            B. It occurs when the stomata are closed.
            C. It results from increased water pressure in the soil.
            D. All of the above


            Modified Stems, Leaves and Roots

            Each plant organ originally evolved in the context of specific environmental imperatives related to terrestrial life. Roots anchor the plant and also absorb water and mineral nutrients. Leaves were adapted to optimize photosynthesis. Stems elevate the leaves, serve as a conduit from the roots to the leaves, and also generate new growth. However, each linage of plants have followed their own unique evolutionary path through time, and in many plant groups stems roots and leaves have become secondarily modified by natural selection in unusual and surprising ways.

            Modified Shoots

            These are examples of shoots (including both the stem and leaves) that have become secondarily modified.

            Bulbs: These shoots are an adaptation for interrupted life. for existence in environments with a dormant season due to annual rainfall variations or to life in the temperate climates.

            Thorns : The thorns of hawthorne are modified dwarf shoots.

            Tendrils : Tendrils of grape are modified flowering shoots. In this link you can see an ontolical mistake where a shoot is one part tendril and one part flower shoot.

            Rhizomes : While typically competition results in vertical stems, there are environments where horizontal stems are adaptive. These are termed rhizomes .

            Tubers : Like bulbs these are adaptations for interrupted life. Irish potatoes are tubers. It is clear that they are stems when you realize that the eyes are axillary buds.

            Corms : Like tubers and bulbs these are adaptations for intrrupted life. Often mistakenly called, bulbs, a corm is a solid underground stem, vertically oriented where tupically only one bud germinates at the end of dormancy.

            Stolons : These are like rhizomes inthat they are oriented horizontally. However, they are adapted to generate new plants that are clones of the original.

            Succulent/photosynthetic Stems : In extremely dry environments, often the leaf becomes reduced and non-photosynthetic, and the stem becomes the primary photosynthetic organ of the plant. A barrel shape provides a low surface to volume ratio which reduces water loss.

            Cladophylls: If a plant becomes genetically boxed in, like the plants in the preceeding link, and the environment becomes more mesic, the same set of factors that caused the generic leaf to become thin and flat will work to make barrel-shaped, succulent stems, thin and flat too.

            Succulence : In a dry environment, leaves may either become reduced, like in cacti, or, else, become succulent. Succulent leaves not only retain lots of moisture but also have a lower surface to volume ratio.

            Stipular Spines : Stipules can become modified into spiines for defense.

            Stipular Spines Modified for Mutualistic Ants : In bullhorn acacia, evolution has proceeded one more step. In this plant the stipular spines are hollow and house mutualistic ants. The leaves are further modified to provide yellow structures on the margins of the leaves which only function to feed the ants. The perioles bear glands that excrete nectar for the ants too.

            Spines for Defence : The non-photosynthetic leaves of cacti have become modified for defence.

            Tendrils : In peas the terminal leaflet of the leaf has become modified into a tendril for climbing.

            Leaves modified for Reproduction : Typically, leaves are determinately growthed. The grow, function and then die without sustaining new growth. But Kalanchoe has leaves that have become modified to generate new plants.

            Leaves Modified to Trap Animals a Great Source of Nitrogen :

            Bladderwort : This flowering plant grows below the water and is greatly reduced. It is often mistaken for green algae. Its leaves entrap small animals such as Daphnia and even larval fish.

            Pitcher Plants : These are adapted for life in marshy/boggy nitrogen poor soil. The leaves are passive but lethal to insects.

            Venus Fly Traps and Sundew : These are in the same family. They are adapted for life in wet nitrogen poor soils and both entrap insects.

            Movie of Venus Fly Trap : While both sundew and the venus fly trap respond actively to entrapped insects, the fly trap does so spectacularly!

            Succulent Petioles of Celery : Stalks of celery are actually petioles. These crist succulent petioles were derived from selective breeding.

            Storage : Like bulbs and tubers, this is an adaptation for interrupted life. Further, unlike stems and shoots, the primitive habit of roots are subterranean, where it is best to secure energy and propagative resources.

            Climbing : Some vines will form adventitious roots that assist th eplant in clinging to a substrate. Poison ivy has this habit.


            Watch the video: Plant Anatomy - Root, Stem and Leaf. Sprint. Biology. NEET 2020. Vani Maam Vedantu VBiotonics (December 2021).