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Chapter 23 CELLS AND TISSUES OF THE PLANT BODY Plant growth occurs through the activity of the meristems o Undifferentiated cells 0 Retain the ability to divide The two functions of the meristems are 1 To increase the length of shoots and roots 2 To give rise to the cells that will produce the mature tissues of the primary plant body APICAL MERISTEMS Apical meristems are found at the tip of roots and stems and are responsible for the eXtension of the plant body Initials are cells that divide and produce one body cell the derivative and another cell that remain in the meristem Derivative cells divide near the root tip and produce three primary tissues that remain meristematic for some time before becoming differentiated These meristematic tissues are the protoderm ground meristem and procambium The protoderm ground meristem and procambium are partly differentiated tissues capable of cell division Activity of the initials and the three primary meristems constitutes the primary growth of the plant that will produce the primary plant body Plants continue to grow throughout their entire lifetime They have indeterminate growth GROWTH MORPHOGENESIS AND DIFFERENTIATION Development is the sum total of events that progressively forms an organism s body Development occurs in response to instructions contained in the genetic information that an organism inherits from its parents Environmental factors in uence the pathway followed during development Three processes are involved in development growth morphogenesis and differentiation 1 Growth is the increase in body size due to cell division and cell enlargement It is irreversible 0 Cell division alone does no imply growth It may increase the number of cells without increasing the volume 0 Most plant growth is due to cell enlargement 2 Morphogenesis is the development of form and structure in the plant 0 The way in cells divide and enlarge determines the shape and form of the plant part morphogenesis 3 Differentiation is the process by which cells that have the same genetic constitution become different from one another in form and function due to the eXpression and repression of different genes Cellular differentiation depends on the control of gene expression 0 Some genes are eXpressed in some cells and others not as a result cells produce different proteins and have different functions The fate of a plant cell is determined by its nal position in the developing organ 0 On aspect of plant cell interaction is the communication of positional information from one cell to another 0 Determination mean progressive commitment to a speci c course of development that brings about a weakening or loss of capacity to resume growth 0 Competency refers to the ability of a cell to develop in response to a speci c signal such as light INTERNAL ORGANIZATION OF THE PLANT BODY Tissues are group of cells that are structurally andor functionally distinct The principal tissues of plants are grouped together into larger units called tissue systems based on their continuity throughout the plant body 0 Ground or fundamental tissue system 0 Vascular tissue system 0 Dermal tissue system The precursors of these tissues in the primary meristem are ground meristem procam bium and protoderm respectively Within the plant body the various tissues are distributed in characteristic patterns depending on the plant part or plant taxon or both The vascular tissue is embedded within the ground tissue with the dermal tissue forming the outer covering Plant tissues composed of only one type of cell are called simple tissues and those made of two or more types of cell are called complex tissues The ground tissues parenchyma collenchyma and sclerenchyma are simple tissues Xylem phloem epidermis and periderrn are complex tissues 1 GROUND TISSUE SYSTEM 0 Composed ofthree simple tissues Parenchyma tissue parenchyma cells Collenchyma tissue collenchyma cells Sclerenchyma tissue sclereids bers The major functions of the ground tissue are photosynthesis storage secretion and to provide exible and rigid structural support a Parenchyma cells have thin primary walls polyhedral cells function in photosynthesis storage and secretion remain alive at maturity They retain the ability to divide at maturity and are important in the healing of wounds and regeneration In the primary plant body parenchyma cells occur in the pith cortex leaf mesophyll and in the esh of fruits Parenchyma cells also occur in the vertical strands of primary and secondary vascular tissues and in xylem rays Transfer cells are parenchyma cells with wall ingrowths that are involved in the movement of large amounts of solutes over short distances The ingrowths increase the surface of the plasma membrane They are found in the xylem and phloem of small veins and in the leaf traces of the nodes the placenta endosperm and other reproductive structures in glandular tissues nectaries salt glands carnivorous plant digestive cells b Collenchyma cells have an unevenly thickened nonligni ed primary wall provide exible structural support in soft nonwoody organs cells elongated remain alive at maturity They occur as strands beneath the epidermis in stems and petioles and bordering the veins in eudicot leaves They can continue to develop thick exible walls while the organ is still elongating and growing c Sclerenchyma cells have primary and thick ligni ed secondary walls provide rigid support to organs that have stopped elongating and growing cells usually die at maturity Sclerenchyma cells may form continuous masses or tissues may be found in clusters or individually among other cells They may develop in the primary and secondary plant bodies Two types of sclerenchyma cells are recognized o Fibers are long slender and tapered cells that occur in bundles or strands o Sclereids are short and often cubical or of variable shape Sclereids make up the seed coats of many seeds the shells of nuts and the stone endocarp or stone fruits 2 VASCULAR TISSUE SYSTEM 0 Composed oftwo complex tissues Xylem tracheids vessel elements parenchyma cells bers Phloem sieve tube members companion cells parenchyma cells bers Vascular tissue system conducts materials throughout the plant body and provides support a Xylem conducts water and minerals from the roots to all parts of the plant it also supports the plant and stores food and other organic substances Together with the phloem the xylem forms a continuous system of vascular tissue extending throughout the body The xylem is derived from the procambium in the primary plant body During secondary growth xylem is derived from the cambium There are two tracheary elements that make the xylem tracheids and vessel elements Tracheids and vessel elements are the conducting cells and are dead at maturity both have wall pits on their sidewalls for lateral transport Xylem vessels arise from individual cylindrical cells oriented end to end At maturity the cytoplasmic contents die The secondary walls of the xylem vessels are deposited in spirals and rings and are usually perforated by pits At maturity vessel elements have perforations which are areas lacking primary and secondary walls These perforations or holes occur on the end walls of the vessel elements The result is the xylem vessel a continuous nonliving duct Xylem also contains tracheids These are individual cells tapered at each end so the tapered end of one cell overlaps that of the adjacent cell Like xylem vessels tracheids have thick ligni ed walls and at maturity no cytoplasm Their walls are perforated so that water can ow from one tracheid to the next There are thin membranes in the pit that prevent air bubbles from passing to the adjacent tracheid Vessels are the principal waterconducting cell in angiosperms Many angiosperms also have tracheids in addition to vessels The xylem of ferns and conifers contains only tracheids Water owing from tracheid to tracheid must pass through the pit membrane 7 the thin modi ed primary walls of the pit pairs Water can ow relatively unimpeded from vessel element to vessel element through the perforation Air bubbles formed during the freezing and thawing can potentially obstruct the ow of water for the entire length of the vessel The tracheary elements of the primary xylem have a variety of secondary wall thickenings Secondary wall is deposited in the tracheary elements during the period of cell elongation in the procambium Secondary wall of the rstformed tracheary elements of the earlyformed primary xylem called protoxylem are deposited in the form of rings or spirals These rings allow the cell to elongate after the cells have differentiated Elements with annular and helical thickenings will develop in the lateformed primary xylem called metaxylem and in the secondary xylem In the metaxylem and in the secondary xylem the secondary cell walls of the tracheids and vessels cover the entire primary walls except the pit membranes and at the perforation of the vessel elements Programmed cell death apoptosis results in the elimination of the protoplast in the tracheary elements The xylem tissue also contains parenchyma cells that store various substances Xylem parenchyma commonly occurs in vertical strands but in the secondary xylem they are also found in the rays Xylem may also contain bers some of which are living at maturity and serve a dual function of storage and support Sclereids are sometimes found in the xylem b Phloem is the principal food conducting tissue in vascular plants In addition to sugars phloem transports many other substances including amino acids lipids micronutrients hormones proteins and RNA some of which act as signaling molecules There is piimary and secondary phloem The rst formed piimary phloem the protophloem is often stretched and destroyed du1ing elongation of the organ The p1incipal conducting cells of the phloem are the sieve elements The protoplasts of adjacent sieve elements are interconnected through sieve areas Two types of sieve elements are recognized sieve cells and sieve tube elements 0 Sieve cells are found only in gymnosperms o Sieve tube cells are found only in angiosperms o Sieve elements are va1iable in seedless vascular plants and are simply called sieve elements In sieve cell 0 The pores are narrow o The sieve areas are rather uniforms in structure on all walls 0 Sieve areas are concentrated on the overlapping walls of the long and slender sieve cells In sieve tube elements The sieve areas in some walls have larger pores than those in other walls The part of the sieve area bearing the larger pores is called the sieve plate Sieve plates are generally located on the end walls but they may occur anywhere in the cell The sieve tube elements are arranged end to end in longitudinal se1ies called sieve tubes The presence of sieve plates is a distinguishing characteiistic of the sieve tube elements Sieve elements have only piimary wall Callose a polysacchaiide of glucose is deposited in the pores of injured sieve elements wound callose Callose is also deposited on the walls of senescent sieve elements and is called de nitive callose Sieve elements remain alive at matuiity sieve elements lack nucleus vacuoles Golgi complex Iibosomes and cytoskeleton Plasma membrane and endoplasmic reticulum remain ER is particularly abundant near the sieve plates At maturity the plasma membrane ER some plastids and mitochondria remain distributed along the wall of the sieve elements Angiosperms with the exception of some monocots have proteins forming what is known as quotslimequot or P proteins Its function has not been determined 0 P proteins form in young sieve elements and become distributed along the walls of the cell 0 P proteins form plugs in injured vessels Sieve tube members are the conducting cells companion cells regulate the metabolism of the sieve tube members Both are derived from the same mother cell Companion cells contain all the organelles found in plant cells Companion cells move sugars amino acids informational molecules ATP and other substances into and out of the sieve elements There are numerous cytoplasmic connections plasmodesmata between the companion cells and the sieve tube members Albuminous cells are parenchyma cells found in the phloem of gynmosperms They are not derived from the same mother cell that gives rise to sieve cells It is thought that albumimous cells perform the same function as companion cells Albuminous and companion cells die when their associate sieve element dies Parenchyma cells are also found in the phloem and are associated with the storage of a variety of substances Fibers and sclereids may also be present in the phloem and help in supporting the plant body Dermal tissue system is the outer protective covering of herbaceous plants and the young tender parts of woody plants the primary plant body The epidermis covers leaves oral parts fruits seeds stems and roots until they have undergone considerable secondary growth DERMAL TISSUE Dermal tissue system is the outer protective covering of herbaceous plants and the young tender parts of woody plants the primary plant body EPIDERMIS The epidermis covers leaves oral parts fruits seeds stems and roots until they have undergone considerable secondary growth Epidermis usually consists of a single layer of parenchyma cells with guard cells and trichomes secretes the waxy cuticle gas exchange occurs through the stomata It is made of parenchyma type cells Plant cuticle is composed of a structural polymer cutin that is embedded in a complex mixture of highly hydrophobic soluble materials called waxes Cuticular waxes are complex substances made of lipids and esters and that vary from species to species Wax may form a smooth sheet or rodlike deposits on the surface of the epidermis These upward extensions of wax are called epicuticular wax The stomata are surrounded by the guard cells which contain chloroplasts in contrast with other epidermal cells that typically lack chloroplasts Guard cells are associated with epidermal cells that usually differ in shape from other epidermal cells these cells are called subsidiary cells Root hairs are involved in water and mineral absorption Trichomes have a variety of functions secretion of protective chemicals provide a barrier to insect attack secretion of salts in some species absorption of water in epiphytes etc PERIDERM Periderm replaces the epidermis in stems and roots having secondary growth It forms the outer bark of woody plants It is a secondary protective tissue Loosely arrange peridermal cells form lenticels that contribute to aeration of the stem The periderm consists mostly of cork or phellem Cork cells are dead at maturity and lled with suberin a waterproof substance The periderm also includes cork cam bium or phellogen and the phelloderm or living parenchyma The cork cambium forms cork on its outer surface and phelloderm on its inner surface Cork parenchyma functions as a storage tissue Chapter 17 SEEDLESS VASCULAR PLANTS EVOLUTION OF VASCULAR PLANTS Plants had an aquatic ancestor probably a Coleochaetelike alga of the Chlorophyta Plant evolution shows a tendency toward greater independence from water as they progressively occupied the land Air is drier than water and less buoyant Land plants had to develop adaptations to conserve and transport water with its solutes absorb water from the environment support itself facing the direction of sunlight solve reproductive problems like fertilization and nourishment of embryo and dispersal of offspring 1 Dominant sporophyte and reduced gametophyte In bryophytes the gametophyte is dominant generation Water is required for fertilization Pollen and embryo sac are much reduced gametophytes The occupation of the land by the bryophytes was undertaken with emphasis on the gamete producing generation which requires water for fertilization 2 Development of uidtransport system the xylem and phloem o Aquatic plants take water throughout their entire body 0 On land soil is the water reservoir the air is dry in comparison to cells 3 The ability to synthesize lignin 0 Early land plants were small and probably stayed upright by means of turgor pressure 0 Lignin adds rigidity to the cell wall and allows the plant to reach greater heights 4 Development of apical meristems o Bryophyte sporophyte growth is subapical and unbranched o It allows the sporophyte to branch many times 5 Ability to produce many sporangia 0 Only one sporangium is produced the bryophyte sporophyte o The many branches of vascular plants became capable of bearing many sporangia 6 More diverse plant body through the development of roots stems and leaves 0 Roots for absorption storage and anchorage o Stems for support above ground transport and growth toward the light 0 Leaves for photosynthesis 7 Evolution of seeds 0 Provides the embryo with food and protection 0 Dispersal of the species to new locations ORGANIZATION OF THE PLANT BODY The plant body consists of a root system and a shoot system 0 Root system absorbs and anchors 0 Shoot system is involved in photosynthesis and its supporting activities Cells are organized into tissues and tissue systems There are three tissue systems in plants which occur in all organs of plants and are continuous from organ to organ It reveals the basic unity of the plant body 1 Dermal tissue provides the protective covering of the body 2 Vascular tissue transports materials within the plant body it is embedded in the ground tissue 3 Ground tissue lls in the spaces between the other tissues and stores water and food Primary growth is produced by the activity of the apical meristems The primary tissues that arise from the apical meristems are part of the plant body the primary plant body Secondary growth results from the activity of lateral meristems 0 Vascular cambium produces vascular tissue every year 0 Cork cambium forms the periderm which is mostly cork tissue 0 Secondary vascular tissue and periderm constitute the secondary plant body The periderm eventually replaces the epidermis in older portions of the plant The transporting cells of the vascular tissue are 0 Tracheids and vessel elements in the xylem also called tracheary elements 0 Sieve tube members in the phloem Sieve tube members have a thin cell wall and do not preserve well as fossil Tracheids have a distinctive ligni ed wall thickening and preserve well in the fossil record The earliest vascular plants of the Silurian and Devonian have tracheids that conducted material and provided support Vessel elements are more specialized than tracheids and are found only angiosperms and gnetophytes Vessels probably evolved from tracheids in different groups of plants and are an example of convergent evolution The primary vascular tissue and associated ground tissue eXhibit three basic arrangements 1 Equot W 3 The protostele a solid core of vascular tissue surrounded by ground tissue The siphonostele a pit surrounded by vascular tissue 0 It has characteristic leaf gaps associated with leaf traces The eustele a system of strands surrounded the pith and separated from one another by ground tissue For a more complete description of other types of steles visit httpwwwbotanv hzwzii edn fclllfv webbhnt701 m n u quot39htm Roots are derived from the subterranean portion of the ancient stems Roots have retained many of the ancient structural characteristics no longer present in stems From an evolutionary perspective there are two kinds of leaves 0 Microphylls are single veined leaves associated with leaf gaps and evolved as super cial outgrowth of the stem Microphylls are associated with protosteles and are found in the lycophytes Megaphylls have a compleX venation pattern and evolved from a branch system Megaphylls are associated with siphonosteles and eusteles In ferns megaphylls are associated with leaf gaps The Telome Theory proposed by Zimmerman eXplains the development of megaphylls from a branch system REPRODUCTIVE SYSTEMS All vascular plants are oogamous They have alternation of heteromorphic generations Homosporous plants produce one kind of spores heterosporous plants produce two kinds of spores Homosporous plants produce bisexual gametophytes that bear antheridia and archegonia Homospory is found psilophytes sphenophytes some lycophytes and most ferns o The gametophytes of homosporous ferns are functionally unisexual They do not selffertilize Heterospory is the production of two types of spores in two types of sporangia Heterospory is found in the Selaginellaceae Isoetaceae and aquatic ferns Heterospory was already common in the Late Devonian about 370 million years ago 0 Microsporangia produce microspores and give rise to microgametophytes male 0 Megasporangia produce megaspores and give rise to megagametophyte female 0 These gametophytes are much reduced in size and develop within the spore wall The gametophytes of homosporous plants are relatively large and independent of the sporophyte u u The gametophytes of psilophytes and some 39 I are on endomycorrhizal fungi for nutrition 39 1 and 39 39 they depend Other species are photosynthetic There is an overall trend toward reduction of the gametophyte In angiosperrns the megagametophyte is reduced to seven cells and the microgametophyte to three cells and two of them are sperms Archegonia and antheridia have been lost in the lineage leading to angiosperms Some gymnosperms have archegonia but lack antheridia All of the seedless vascular plants have motile sperms and depend on water for fertilization Gymnosperms and angiosperms the seed bearing plants depend on pollination prior to fertilization Pollination is the transfer of pollen to the vicinity of the megagametophyte Pollen grains produce a pollen tube through which motile sperms cycads and Ginkgo swim or nonmotile sperm are transferred to the egg PHYLA OF SEEDLESS VASCULAR PLANTS There are three phyla of extinct seedless vascular plants Rhyniophyta Zosterophyllophyta and Trimerophytophyta The genera Rhynia Zosterophyllum and Trimerophyton are members of these phyla The earliest known go back about 425 million years ago and most went extinct by the end of the Devonian about 370 million years ago These three groups were the dominant vegetation from the midSilurian to the midDevonian 425 to 370 million years ago For the most part they were relatively simple plants 18 in to 36 inches tall They had the following characteristics 1 Naked photosynthetic stems 2 Terminal sporangia some lateral 3 No roots or leaves 4 They were all homosporous 5 They had protosteles P 39 39 p39 39 p39 and r ms are more complex groups that were dominant from the Late Devonian through the Carboniferous from about 370 to 290 million years ago Seed plants arose starting in the Late Devonian period about 380 million years ago and evolved many new lines by the Permian 290248 million years ago Gymnosperms dominated the land oras throughout the Mesozoic until about 100 million years ago Angiosperrns appeared in the fossil record about 125 million years ago It became the dominant group about 30 7 40 million years ago and has remained so until the present Phylum Rhyniophyta The Rhyniophytes are a group of early land plants originally described from the Rhynie Chert Scotland Rhynia appeared in the midSilurian record about 425 million years ago It became extinct in the midDevonian about 380 million years ago Seedless produced spores Dichotomous branching Terminal sporangia Homosporous Plant body was not differentiated into roots stems and leaves Underground rhizome with rhizoids Protostele consisting of a core of xylem surrounded by one or two layers of phloem cells Some had conducting cells similar to hydroids rather tracheids they are called protracheophytes There is evidence of isomorphic alternation of generations Examples Rhynia Cooksom39a A glaophyton Fig 1712 is the reconstruction of the plant Aglaophyton major by D S Edwards 1986 The original name given by Kidson and Lang was Rhynia major but Edwards showed that the central strand did not consist of tracheids with secondary wall thickenings So the plant had to be transferred to another genus In fact A glaophyton cannot be reckoned to belong to the Tracheophyta and does have affinities to the mosses Aglaophyton major may represent an intermediate stage in the evolution of vascular plant known as protracheophytes The transverse section of the stem resembles that of Rhynia gwynne vaughanii but it is larger The size of the plant is estimated at a maximum of 18 cms The stem diameter is between 15 and 6 ms Phylum Zosterophyllophyta Fossils of zosterophylls have been found in rocks from the Early and Middle Devonian 408 to 370 million years ago 0 Aerial stems were covered with cuticle and had stomata only on the upper branches Zosterophyllum dichotomously branched but frequently lateral branches further differentiated into one aXis that grew upward and another downward o The lower branch may have function as a root 0 Some Zosterophylls were naked or had small spinelike enations o The sporangia were globose or reniform and borne laterally on short stalks Zosterophylls were homosporous o Xylem matured from the periphery toward the center centripetal differentiation 0 They are considered to be the ancestral group that gave rise to the Lycopods the sporangia of both groups are borne laterally and are similar in shape and the xylem in both groups differentiates centripetally Phylum Trimerophytophyta This phylum probably evolved directly from the rhyniophytes Trimerophytes were larger and more compleX plants than the rhyniophytes or zosterophyllophytes Trimerophytes appeared in the Early Devonian about 395 million years ago and had become eXtinct by the end of the mid Devonian about 20 million years later 7 a relative short period of eXistence They lacked leaves Lateral branches forked dichotomously several times They were homosporous Some branches were vegetative while others bore elongated sporangia Vascular strand was more massive than that of the rhyniophytes The xylem differentiated centrifugally Phylum Lycopodiophyta There are about 15 genera of lycophytes and approximately 1200 living species This evolutionary line eXtends back into the Devonian but was most prevalent in the wet swamps of the Carboniferous period Molecular and morphological evidence indicate that they split up into two evolutionary lines before 400 million years ago in the Early 7 Mid Devonian o The lycophyte clade includes the modern lycophytes o The euphyllophytes clade includes whisk ferns horsetails and seed plants The eXtinct lycophytes include very large woody trees that did not survive in the drier climate at the end of and after the Carboniferous age In the Carboniferous some lycophytes were forestforming trees more than 35 meters tall Woody lycophytes became eXtinct before the end of the Paleozoic era 248 million years ago The second and the surviving group of Lycopods are the small and herbaceous plants Lycophyta remains became the largest coal deposits of all geologic time Lycophyta are characterized by 0 microphyllous leaves 0 a special spore producing body called a strobilus o the presence of true vascular stems roots and leaves There are three prominent orders of Lycophyta o Lycopodiales or club mosses o Selaginellales or Spike mosses o Isoetales or Quillworts Family Lycopodiaceae A family of about 400 species mostly tropical The taxonomic boundaries of the genera are not well understood and as many as 15 genera may eventually be recognized Seven of these genera are represented in North America 0 Sporophyte with true leaves stems and roots 0 Dichotomous branching rhizome from which aerial branches and roots arise o Stems and roots are protostelic or siphonostelic Leaf gaps absent 0 Leaves are microphyllous and spirally arranged sometimes opposite or whorled o Sporophylls modi ed fertile microphylls sometimes grouped into strobili strobilus cluster of 11 u u l r o In Huperzia and Phlegmariurus the sporophylls are similar to ordinary microphylls and are interspersed among the sterile microphylls 0 One sporangium per sporophyll near the base and on the adaXial side Homosporous u u u Hr Gametophyte bisexual either green or 39 structures depending on the genus Gametangia antheridia and archegonia may require 6 to 15 years to mature Selffertilization is rare Bi agellated sperm requires water to reach the archegonium Family Selaginellaceae There are about 700 species in this family most of them tropical Selaginella is the only genus in the family Plants herbaceous annual or perennial sometimes remaining green over winter Stems leafy branching dichotomously regularly or irregularly forked or branched Stems and roots protostelic sometimes with many protosteles or meristeles siphonostelic or actino plectostelic Protostele held in place by trabeculae Rhizophores modi ed lea ess shoots producing roots present or absent geotropic borne on stems at branch forks throughout or con ned to base of stems Leaves microphylls on 1 plant dimorphic or monomorphic small with adaXial ligule near base singleveined rarely veins forked Strobili sometimes illde ned terminal cylindrical quadrangular or attened Sporophylls fertile leaves monomorphic or adjacently different slightly or highly differentiated from vegetative sterile leaves microsoporophylls and megasporophylls Sporophylls and microphylls with ligule Sporangia shortstalked solitary in aXil of sporophylls opening by distal slits Spores of2 types plants heterosporous megaspores l24 large microspores numerous hundreds very small Gametophytes unisexual Gametophytes develop inside the spore wall endosporic development gt Microgametophyte lacks chlorophyll At maturity it consists of a single prothallial cell and an antheridium Antheridium produces many of bi agellated sperms Microspore wall ruptures to liberate the sperms Megagametophyte multicellular Megagametophyte protrudes through a rupture in the spore wall Archegonia develop in the eXposed area VVV VVV Sperms require water to swim to the archegonia The suspensor develops and pushes the developing embryo deep into the female gametophyte Family Isoetaceae A family of one genus Isoetes ca 150 spp found worldwide especially in temperate areas A second genus Stylites is sometimes recognized Some authors put fossil representatives in the genus Isoetites which is known from as early as the upper Triassic Plants tufted grasslike perennial evergreen aquatics to ephemeral terrestrials Underground stem brown cormlike lobed Roots arising along central groove separating each rootstock lobe simple or dichotomously branched containing eccentric vascular strand and surrounding lacuna Leaves linear simple spirally or distichously arranged dilated toward base tapering to apeX containing 4 transversely septate longitudinal lacunae a central collateral vascular strand and frequently several peripheral brous bundles Each leafis a potential sporophyll Ligule inserted above sporangium on each sporophyll 1 l I l and not alike Megasporophylls and microsporophylls usually borne in alternating cycles hardened scales and phyllopodia occasionally surround leaves Sporangia solitary adaXial embedded in basal cavity of leaf velum thin ap eXtending downward over sporangium partly to completely covering adaXial surface of sporangium Megasporangium with several to hundreds of megaspores Microsporangium with thousands of microspores Megagametophytes white endosporic exposed when megaspore opens along proximal ridges archegonia l to several indicated by quartets of brownish neck cells Microgametophytes 9celled endosporic antheridium releasing 4 multitailed spermatozoids Phylum Trimerophytophyta A fossil phylum that seems to have evolved from rhyniophytes It might be the ancestor of ferns progynmosperrns and perhaps horsetails They rst appeared in the early Devonian about 395 mya and became eXtinct by the end of the mid Devonian at about 370 mya Trimerophytes lacked leaves and roots most of the plant body consisted of branching stems that were photosynthetic throughout their length Vascular tissue was present forming a solid central bundle in the center of the stem or protostele Their protostele was more massive than that of rhyniophytes They had a band of thickcellwall cells in the cortex Xylem differentiated centrifugally like in rhyniophytes T 39 p39 branched l 39 l quot quot that means that the branching was unequal forming a main stem or aXis with several smaller lateral branches gt Rhyniophytes branched dichotomously stems always branched into two equal branches Lateral branches typically branched dichotomously and were often shortened to form bushy quotwebsquot of small closely spaced branches Some trimerophytes also bore enations on the main stems giving them a super cially quotthomyquot appearance Trimerophytes bore sporangia at the tips of branches like the rhyniophytes but unlike the super cially similar zosterophylls from the same time period The spindleshaped sporangia produced only one type of spore trimerophytes were homosporous Spores released from the sporangia would have germinated into gametophytes but no fossil trimerophytes gametophytes have been identi ed and we do not know whether trimerophytes like their later relatives the ferns sphenopsids and seed plants had small inconspicuous gametophytes Trimerophytes varied in size from a few centimeters to nearly a meter tall large trimerophytes were among the largest plants of the Early Devonian Phylum Psilophyta This phylum is also known as Psilotophyta It includes two living genera Psilotum and Tmesipteris from tropical and subtropical regions of the world 0 Sporophyte with a dichotomously branching aerial and subterranean stem system True roots lacking u uu Underground stems with rhizoids and with a fungal 39 39 an Aerial stems lacking leaves but with scalelike or larger leaflike structures enations Gametophytes bisexual subterranean resembling a small piece of rhizome Gametophyte with a symbiotic fungus Some have a vascular tissue The sperm is multi agellated and requires water to swim to the archegonium Homosporous Sporangia a 2 or 3chambered synangium borne at the apeX of small sidebranches appear to be arranged along the sides of the major stems of the plantsporophyte Sporophyte remains attached to the gametophyte in the early stages by a foot and derives nourishment from the gametophyte Phylum Sphenopsida Sphenopsids eXtend back to the Devonian and reached their maXimum development in the Carboniferous 380280 mya A family of one eXtant genus Equisetum ca 15 species of nearly worldwide distribution in damp habitats such as riverbanks lakeshores and marshes Michigan is a center of diversity for the genus with nine native species The sporophyte of Equisetum is differentiated into an underground rhizome that bears adventitious roots and an upright photosynthetic stem with whorls of microphylls Tough perennial herbs with jointed ridged aerial stems with distinct nodes Stems rough accumulating silica and metals and compleX anatomically The aerial stems contain a large central pith region which in mature plants is hollow Surrounding the pith cavity are discrete bundles of vascular tissue this arrangement of conducting tissue is known as a eustele The bundles contain both xylem and phloem and are marked by the presence of large canals known as carinal canals under the ridges which also function in water conduction External to the vascular bundles is another set of canals the vallecular canals or cortical canals under the valleys These canals line up with the depressions between the ribs on the surface of the stem Most fossil sphenophytes had very similar stem morphology Leaves are small whorled nonphotosynthetic fused together to various degrees and adpressed to the stem Sporangia borne on peltate sphorangiophores aggregated into a strobilus Strobili consist of tightly packed appendages called sporangiophores Homosporous spores with elaters Gametophytes green epigeal bisexual or unisexual male gametophytes smaller than female with rhizoids Archegonia develop before antheridia to increase the probability of crossfertilization Sperms are multi agellated and require water to swim to the egg Several eggs on a single gametophyte may be fertilized Phylum Pterophyta The fossil record of ferns extends back into the Carboniferous There are 11000 species of ferns in the world Most species are tropical gt There are about 380 species of ferns in USA and Canada gt Costa Rica has over 1000 species gt Philippines has over 950 species Sporophyte is differentiated into true roots stem rhizome and leaves megaphylls The stele varies from a protostele to a complex dictyostele The xylem is composed of tracheids Leaves or fronds develop in curled position at the apex of the rhizome stem and uncoil as they mature a condition called circinate vernation Leaves usually differentiated into stipe petiole and blade with a central rachis or vein Leaf gaps are conspicuous Most ferns are homosporous a few aquatic genera are heterosporous Sporangia normally develop on the abaXial dorsal lower side or on the margin of the leaf Gametophytes are inconspicuous and arise directly from spores Gametophytes are bisexual in homosporous ferns and unisexual in heterosporous ferns Sperms are agellated and coiled Sporangial development is important in the understanding of the evolutionary relationship of vascular pl ants l Equot The eusporangiate sporangium eusporangia has the sporangial wall developing from several super cial cells and the sporogenous tissue develops from internal cells of the sporophyll The sporangial wall is several cells thick The inner wall cells may be compressed and stretched so that the sporangial wall may appear to be made of a single layer of cells at maturity The sporangia are large and contain a large number of spores The leptosporangiate sporangium leptosporangia develops entirely from the periclinal division of a single super cial cell By a precise pattern of division a stalk and a globose capsule are formed A nutritive tapetum develops inside the capsule The inner mass of cells develops into spore mother cells that undergo meiosis The remnant of the tapetum is deposited around the spores and gives them their characteristic ornamentation The annulus is made of unevenly thickened cells The number of spores produced is a multiple of 2 between 16 and 512 in homosporous species I 39 39is l of all vascular plants except the leptosporangiate ferns There are ve orders of ferns often recognized by pteridologists gt Ophioglossales Marattiales Filicales Marsileales and Salviniales The Ophioglossales and Marattiales are eusporangiate The other three are leptosporangiate Order Ophioglossales There are three genera in the Ophioglossales o Ophioglossales are eusporangiate 0 Two genera Botrychium and Ophioglossum are widespread in the world 0 A single leaf is produced each year 0 Each leaf consists of two parts a vegetative portion or blade and a fertile segment o The vegetative portion is dissected in Botrychium and undivided in Ophioglossum 0 Their gametophyte is subterranean with many rhizoids and associated to an endophytic fungus o Ophioglossum reticulatum has the highest number of chromosomes of any known organism 1260 Order Marattiales This is an ancient group of eusporangiate ferns with fossil members eXtending back into the Carboniferous 0 They are large ferns with dissected fronds 0 There are siX living genera with about 200 species Order Filicales There are about 35 families 320 genera and 10500 species in this order 0 The Filicales are leptosporangiate which differentiate them form Ophioglossales and Marattiales o The Filicales are homosporous While the quot 39 and 39 39 39 39 are 39 o Megaphylls vary from entire to highly dissected pinnate gt Stipe rachis pinnae pinnules o Uncoiling of young leaves is in uenced by auXin a plant hormone o Scales called paleae and hairs of different kinds often cover the rhizome and young fronds o Sporangia may cover the dorsal surface of the blade or may be grouped into sori sorus o A sorus may or may not be protected by an indusium o Spores give rise to free living bisexual gametophytes called a prothallus o The gametophyte is chlorophyllous heartshaped with rhizoids o Gametangia develop on the underside of the prothallus 0 Water is required for fertilization to occur 0 In some ferns from different parts of the world produce gametophytes that persist inde nitely Without forming sporophytes 0 These gametophytes reproduce asexually by outgrowths called gemmae Order Marsileales and Salviniales Both orders are heterosporous leptosporangiate ferns The sporophyte grows in mud or submerged with leaves oating on the water surface Marsileales produce beanshaped drought resistant sporocarps Sporocarps germinate to produce a chain of sori each bearing megasporangia and microsporangia Their gametophytes are very specialized There are three genera of Marsileales and about 72 species There are two general of Salviniales Salvinia and Azolla These are small oating plants Azolla produces their spores in sporocarps Azolla leaves are modi ed to accommodate laments of Anabaena azollae nitrogen xing cyanobacteiia Salvinia produces a whorl of three leaves tow of which are chlorophyllous and undivided and the third is divided and hags down the surface of the water resembling a root Sterile leaves have water repellent hairs The submerged leaf produces a sporocarp with a single sorus The rst sporocarps formed produce megasporangia later ones produce microsporangia Gametophytes are very small Megagametophyte produces a single archegonium Microgametophytes germinate inside the sporocarp and produce two anthe1idia Each anthe1idium releases four motile sperms Chapter 24 THE ROOT STRUCTURE AND DEVELOPMENT FUNCTIONS OF THE ROOT Anchorage Absorption of water and minerals Storage of food Production of some hormones and secondary metabolites Conduction of absorbed water and ions and hormones and secondary metabolites kIIJBWNt I ROOT SYSTEMS Taproot system Produced by gymnosperms eudicots and magnoliids The primary root originates in the embryo derived from the embryonic radicle It grows downward and is called then a taproot It produces lateral roots This system generally penetrates deep into the soil Fibrous root system It is found in monocots The primary root originates from the embryonic radicle It is short lived Adventitious roots develop from the stem This system is usually shallow Feeder roots are those laterals that are actively engaged in absorption of water and minerals Most of the absorption of water and minerals takes place in the upper 15 cm of soil the area richest in organic material Desert plants can have very long taproots that penetrate deep into the soil 0 A mesquite bush near Tucson AZ had roots 533 m or 175 feet deep into the soil 0 Acacia and TamariX trees found in Egypt grow roots 30 m deep into the soil Plants maintain a balance between its shoot and root systems In young plants the absorbing surface of roots far exceeds that of photosynthesizing parts As the plant ages the roottoshoot ratio decreases Damage to the roots decreases the shoot production Damage to the shoot decreases the availability of food and hormones for the roots ORIGIN AND GROWTH OF PRIMARY TISSUES Root growth is a continuous process that stops only during very adverse conditions of low temperature and heavy drought Roots follow the path of least resistance through the soil Rootcap The tip ofthe root is covered by a rootcap that produces mucigel The rootcap is a thimblelike mass of living parenchyma cells that covers the apical meristem behind it and helps the root to penetrate the soil As the root grows it pushed the rootcap forward and the peripheral cells are sloughed off The sloughed off cells and the cap is covered with a slimy mucigel that lubricates the path of the root through the soil New cells produced by the apical meristem are added to the cap to replace the ones that have been sloughed off The mucigel is a hydrated polysaccharide secreted by the rootcap cells The mucigel accumulates in the Golgi vesicles that release the mucigel into the cell wall and eventually leaks to the outside The rootcap also plays a role in gravitropism The central column of cells or columella contains starch grains or amyloplasts which probably act as gravity sensors Apical organization Longitudinal les or lineages of cells emanate from the apical meristem of the root The meristematic initials and its derivatives are polyhedral cells with large nuclei and dense cytoplasm This region is called promeristem initials derivatives 1 Closed type organization consists of three layers of the apical meristem 0 Each of three regions 7 rootcap vascular cylinder and cortex 7 is interpreted as having its own initials The lower one gives rise to the rootcap The middle layer gives rise to the corteX and the protoderm The upper layer produces the procambium Each region rootcap cortex and vascular bundle is thought to have its own set of initials 2 Open type organization o All regions or at least the cortex and the rootcap arise from a group of initials The activity of the meristematic initials decreases as the plant develops and divisions become infrequent This region is called the quiescent region of the meristem Cell divisions occur in cells a bordering the quiescent initials The quiescent region helps to repopulate the bordering meristematic regions surrounding it in case of injury The quiescent center plays an essential role in organization and development of roots httpWWW binlnoie 39J b quot bonlinelibrarvwebbBOT3 l lPlantCell v quot quotquot 39 3htm httpWWW sbs nte as 39 39 wc39ulab Regions ofgrowth and development The apical meristem and the region around it where most or the cell division occurs is called the region of cell division The region of cell elongation is immediately behind the region of cell division It usually s a few millimeters in length The elongation of cells results in most of the increase in root length There is no elongation beyond this area Only one small portion of the root is pushed through the soil at a given time In the region of maturation or differentiation is where most of the primary tissues mature 0 Root hairs are produced here 0 This region is also called the quotroot hair zonequot There is gradual transition from one zone to the next Some cells may begin to elongate and differentiate in the region of cell division whereas others do it the region of elongation The protoderm ground meristem and procambium can be distinguished in very close proximity to the apical meristem These will differentiate into epidermis cortex and primary vascular tissues respectively PRIMARY STRUCTURE There are three tissue systems in the root epidermis dermal tissue system cortex ground tissue system and vascular bundle vascular tissue system Epidermis Absorption of water and minerals is facilitated by root hairs o A 4monthold rye plant was estimated to have 14 billion root hairs with an absorbing surface of 401 m2 and an endtoend length of over 10000 km Root hairs are short lived They are found in the region of maturation Production of root hairs occurs in the region of elongation at a rate that matches the rate at which the older root hairs are dying off at the upper end of the of the root hair zone The walls of the epidermal cells offer little resistance to the passage of water and minerals even in those species with a thin layer of epidermal suberin The mucigel produced by the rootcap is favorable to nitrogen xing bacteria The rhizosphere is the layer of soil bound to the root by mucigel and root hairs It contains a variety of microorganisms and sloughed rootcap cells Typically the root hairs do not develop into Ectomycorrhizae Cortex The cortex represents the ground tissue system in most roots The cortex occupies the greatest area of the primary body of most roots The plastids of cortical cells store starch these cells are devoid of chloroplasts Roots that undergo signi cant secondary growth shed their cortex early eg eudicots magnoliids and gymnosperms In monocots the cortex is retained for life The parenchymatous cells may develop secondary wall and become ligni ed There are numerous intercellular air spaces in the cortex Substances moving across the cortex may follow the symplast pathway moving from cell to cell through the plasmodesmata or an apoplastic pathway through the spaces in between cells and spaces in the cell walls The innermost layer of the cortex is the endodermis and is characterized by the Casparian strip in the radial and transverse walls anticlinal walls which are those perpendicular to the surface of the root The Casparian strip is made of a suberized the primary wall and middle lamella The Casparian strip is impermeable to water and ions and prevents their passage via the apoplast into the vascular tissue All substances entering and leaving the vascular cylinder must pass through the protoplasts of the endoderrnal cells This happens by passing through the plasma membrane or by moving through the symplast formed by the numerous l 39 39 39 the 39 39 39 cells with l l 39 of neighboring cells of the cortex and vascular cylinder In roots that undergo secondary growth the cortex and endodermis are shed early Old roots mostly transport water and minerals absorbed by the younger roots they are connected to In older roots that conserve the endodermis cortex a suberin lamella consisting of alternating layers of suberin and wax is deposited internally in the endodermal cells Opposite the protoxylem of the root some endodermal cells may remain thin walled and allow the passage of materials These cells are called passage cells The suberin lamella does not necessarily prevents the passage of water and minerals As long as the endodermis remains alive their plasmodesmata remains providing symplastic passage for water and minerals To roots of many angiosperms develop a suberized outermost layer of cortex under the epidermis called the exodermis The exodermis probably reduces the loss of water to the soil and minimizes the attachment of the root by microorganisms Vascular cylinder The vascular cylinder consists of vascular tissue and the nonvascular pericycle The pericycle originates from the procambium and completely surrounds the vascular tissue The pericycle is made of parenchyma which may develop secondary cell wall as they age Important functions of the pericycle are 0 Lateral roots originate in the pericycle o Gives rise to the cork cambium in roots with secondary growth The pericycle often produces more pericycle The center of the vascular cylinder is occupied by the xylem The phloem is found in between the quotarmsquot of xylem The rst xylem to develop is called the protoxylem and is located next to the pericycle The metaxylem is found to the inside and center of the cylinder and it matures after the protoxylem Some monocot roots have pith considered by some botanist to be part of the vascular cylinder because of its possible procambial origin