SEED EMBRYOGENESIS Formation of the Embryo; * In most flowering plants, first division of the zygote is assymetrical with regard to the long axis of the zygote * With this division the polarity of the embryo is established; A) Upper chalazal pole consisting of a small apical cell which gives rise to most Of the mature embryo B) Lower micropylar pole consisting of a large basal cell, produces a stalk like Suspensor that anchors the embryo at the micropyle, the opening in the Ovule through which the pollen tube enters * In some angiosperms, polarity is already established in the egg cell & zygote Primary Meristems: * When first formed, the embryo proper consists of a mass of relatively undifferentiated cells Protoderm – the future epidermis, formed by periclinal (parallel to surface) divisions In the outermost cells of the embryo proper * Subsequent vertical divisions result in the distinction between the procambium & ground meristem * ground meristem is the precursor to ground tissue, ground tissue refers to any tissue other than vascular, peridermal, or epidermal in origin * Ground meristem surrounds the procambium, the precursor to the vascular tissuesxylem & phloem * The protoderm, ground meristem, and procambium are the 3 primary meristems Embryos Develop In A Sequence of Developmental Stages: Globular Stage – refers to when the embryo proper is spherical * The development of cotyledons may begin either during or after the time at which the procambium becomes discernable Heart Stage – in eudicots, the globular embryo assumes a two lobed form, in monocots It becomes cylindrical * During the transition of the globular stage & cotyledon emergence, partitioning of the axis into the shoot meristem, hypocotyls, embryonic root, and root meristem occurs Torpedo Stage – when the cotyledons & axis elongate, primary meristems extend along with them * In monocots at this time, the single cotyledon often most prominent structure * In eudicots, the SAM arises on one side of the cotyledon & is completely surrounded by a sheathlike extension from the base of the cotyledon * Suspensors play a role in the development of the embryo proper; A) Angiosperm suspensors are metabolically active, provide embryo proper with Nutrients & growth regulators, especially gibberrelins B) Suspensors are short lived, undergoing apoptosis at torpedo stage of development The Mature Embryo & Seed:
Epicotyl - the upper portion of the axis of an embryo/seedling, above the cotyledons & Below the next leaf/ves Plumule - The first bud of an embryo, the portion of the young shoot above the cotyledons Hypocotyl – Portion of the seedling between the cotyledon/s & the radicle Radicle – The embryonic root * Hypocotyl rot axis refers to the situation in which a radicle cannot be distinguished from in the embryo * In eudicots with a large amount of endosperm the cotyledons are thin and membranous which serves to absorb food from the endosperm during growth * In many eudicots, most or all of the food storing endosperm & perisperm if present is absorbed during embryonic development Seed Coat – the outer layer of the seed, developed from the integuments of the ovule, provides protection to the seed * The micropyle is often visible on the seed coat as a small pore Hilium – The scar left on a seed after separation of the seed from the furniculus (stem), it Is commonly associated with the micropyle
Cells & Tissues Of The Plant Body: Apical Meristems- found at the tips of all roots and stems & are involved primarily in Extension of the plant body * The primary meristems; protoderm, procambium, & ground meristem are initiated at embryogenesis & extended by activity of apical meristems * Tissue systems are initiated during the development of the embryo * Ground tissue consists of 3 tissue types: parenchyma, schlerenchyma, collenchyma parenchyma by far the most common * Vascular tissue consists of 2 conducting tissues; xylem & phloem * Dermal tissue represented by the epidermis & later by the periderm in plant parts that undergo a secondary increase in thickness * The principal differences in plant species patterns depend largely on the relative distribution of vascular & ground tissues * In stems of eudicots; may form vascular system of interconnected strands embedded within the ground tissue A) The region formed internal to the strands is called the pith, and the region External to them is the cortex B) In the roots, vascular tissues may form a solid cylinder, or stele, surrounding The cortex * In the leaf the vascular system typically forms a system of vascular bundles (veins) embedded in photosynthetic ground tissue – mesophyll simple tissues – composed of one cell type complex tissues – composed of two or more cell types * Ground tissue, parenchyma, collenchyma, & schlerenchyma all simple tissues, xylem and phloem & epidermis all complex tissues Parenchyma Tissue: * Involved in photosynthesis, storage & secretion
* In the primary plant body, parenchyma cells commonly occur as continuous masses in cortex & pith of stems & roots, in leaf mesophyll, and in the flesh of fruits * Also occurs as vertical strands of cells in the primary & secondary vascular tissues, also as horizontal rays in the secondary vascular tissues * Living at maturity, parenchyma cells are capable of cell division * Although their walls are mostly primary, some have secondary walls * Due to meristematic abilities, parenchyma cells with only primary walls play an important role in wound healing transfer cells – a specialized parenchyma cell with wall ingrowths which increase the surface area of the plasma membrane- apparently functions in the short distance transport of solutes A) Occur in association with the xylem & phloem of small or minor veins in Cotyledons & in the leaves of many herbaceous eudicotyledons B) Also common in glandular structures where intensive short distance solute Transfer is needed Collenchyma Tissue: * Like parenchyma, collenchyma are living at maturity * Collenchyma tissues commonly occur in discrete strands or as continuous cylinders beneath the epidermis in stems & petioles (leaf stalks) * Also commonly found bordering the veins in eudicots * Collenchyma cells typically elongated, most distinctive feature is their unevenly thickened, nonlignified walls * They develop these thick, flexible walls while the organ structure is still elongating, making these cells well adapted for the support of young, growing organs Schlerenchyma Tissue: * Strengthen & support plant parts no longer elongating * They may form continuous masses or occur as small groups, or individually among other cell types * May develop in any parts of the primary & secondary plant bodies and often lack protoplasts at maturity * Principal characteristic is their thick, often lignified secondary walls fibers – type of schlerenchyma cell, long & slender, commonly occur in strands or bundles, may or may not be lignified schlereids – schlerenchyma cell with a thick lignified secondary wall, having many pits, variable in form but typically not very long, may or may not be living at maturity Vascular Tissues: The Xylem: * Principal water conducting tissue, also involved in conduction of minerals, support, and food storage * Together with the phloem, the xylem forms a continuous system of vascular tissue extending throughout the plant body * In the primary plant body, the xylem is derived from procambium, during secondary growth, derived from the vascular cambium
tracheary elements – generic term for water conducting cells, two types tracheid – elongated, thick walled conducting & supporting cell of the xylem, has tapering ends & pitted walls without perforations, found in nearly all vascular plants vessel element – cells that form a tubelike structure composed of elongate cells placed end to end & connected by perforations, found in nearly all angiosperms & few other vascular plants * Both cell types lack protoplasts at maturity, both have pits in their walls * Vessel elements contain perforations- areas lacking primary & secondary wall * Tracheids, lacking perforations, are a less specialized cell type than vessel elements * Vessel elements appear to have evolved independently in several groups of vascular plants * Vessel elements more efficient at conducting water than tracheids * Vessel elements less safe than tracheids, which allow water to flow via pits but block even the smallest of air bubbles * Xylem tissue also associated with parenchyma cells that store various substances, xylem parenchyma commonly occur in strands * Fibers are also commonly found in the xylem The Phloem: * Principal food conducting tissue in vascular plants, may be primary or secondary in origin * As with the primary xylem, primary phloem frequently stretched & destroyed during the process of elongation of the organ sieve elements – generic term for conducting cells of the phloem, two main types; sieve cell – a long, slender sieve element with relatively undifferentiated sieve areas & tapering end walls that lack sieve plates, commonly found in the phloem of gymnosperms sieve tube elements – found primarily in angiosperms, typically associated with a companion cell, form a sieve tube by interconnecting end to end by sieve plates sieve plate – highly differentiated area of the wall of sieve tube elements sieve areas – clusters of pores through which the protoplasts of adjacent elements are interconnected * Sieve plates may occur anywhere on the cell wall, but typically located on end walls callose – a polysaccharide composed of spirally wound chains of glucose residues typically deposited at sieve areas & sieve plates of senescing leaves * Unlike tracheary elements, sieve elements have living protoplasts at maturity * The protoplasts of mature sieve elements are unique – unlike the protoplasts of the tracheary elements which undergo a total breakdown, that of sieve elements undergoes a selective breakdown at specific times companion cell – a specialized parenchyma cell associated with a sieve tube element in angiosperms phloem, arises from the same mother cell as the sieve tube element to which it is associated with A) These cells have numerous cytoplasmic connections (plasmodesmata) with
Sieve tube elements & because of their resemblance to secretory cells ( high ribosome population, numerous mitochondria)- believed to play a role in delivery of substances to sieve tube elements B) With the absence of a nucleus in a mature sieve tube element, these substances Would include DNA, proteins, ATP, etc. Albuminous cells – homologous to companion cells except they are in gymnosperms, parenchyma in origin, not derived from the same parental tissue as the sieve tube Element * Other parenchyma cells occur in the primary & secondary phloem, they are largely concerned with storage of various substances Dermal Tissues: The Epidermis: * Constitutes the dermal tissue of leaves, floral parts, fruits, seeds, and stems until they undergo considerable secondary growth * Epidermal cells quite variable structurally & functionally * The bulk of epidermal cells are compactly arranged to give mechanical protection * Walls of aerial epidermal cells are covered with a cuticle, composed mostly of cutin and wax * Interspersed among the flat, tightly compacted cells which typically lack chloroplasts, are chloroplast containing guard cells stomata – a minute opening bordered by guard cells in the epidermis of leaves & stems through which gas exchange occurs guard cells – pair of cells surrounding a stoma, changes in turgor pressure causes their contractions allowing the stoma to open & close subsidiary cells – typically epidermal cells varying in shape due to their association with guard cells * Trichomes have a variety of functions; hariness results in increased reflectance of solar radiation, lowers leaf temps, and lowers rates of dessication * Epiphytic bromeliads utilize trichomes for absorption of water & minerals * Saltbrush Atriplex utilizes trichomes to secrete salts from the leaf tissue preventing lethal accumulations Periderm Tissue: * The periderm commonly replaces the epidermis in stems & roots having secondary growth * Although cells normally arranged compactly, portions (lenticels) are loosely arranged & provide aeration of internal tissues of roots & stems * The periderm consists largely of protective cork – nonliving cells with walls that are heavily suberized at maturity * The periderm also consists of cork cambium – lateral meristem in origin, it produces cork on the outside surface of the plant & phelloderm on the inside, common in stems & roots of woody angiosperms & also in gymnosperms cork – secondary tissue produced by the cork cambium, composed of polygonal cells nonliving at maturity, with suberized cell walls which are resistant to the
passage of gases & water vapor phelloderm – tissue formed inwardly by the cork cambium, a living parenchyma tissue * Origin of cork cambium is variable dependant on the plant species & plant part
The Root: Structure & Development * The first structure to emerge from the developing seed is the embryonic root, enabling enabling the seedling to become anchored in the soil * Foods manufactured in photosynthesizing portions of the plant move through the phloem to the storage tissues of the root & brought back when needed * Hormones like cytokinins & gibberellins are synthesized in meristematic regions of the roots & transported upward via the xylem to aerial plant parts Root Systems: * The primary root is the first to emerge from the embryo taproot system – in gymnosperms, magnoliids, and eudicotyledons, primary root grows directly down , lateral roots branch from the taproot * Older lateral roots are found nearest the base of the root, younger nearest the tips fibrous root system – in monocots, primary root usually short lived, main root system is developed from adventitious roots arising from the stem A) No one root is more prominent than the other * Taproot systems penetrate deeper than fibrous systems * In a growing plant, a balance is maintained between the total surface area available for the manufacture of food & area available for absorption of water & minerals * Damage to the root system reduces the shoot growth by lack of water, minerals, and root produced hormones * Reduction in shoot system limits root growth by decreasing the availability of carbohydrates & shoot produced hormones The Root: Rootcap – a thimble-like mass of cells that covers & protects the growing tip of a root Mucigel – a slime sheath covering the root, lubricates as it grows A) mucigel is a highly hydrated polysaccharide, most likely a pectin secreted by The outer cells of the rootcap B) this substance accumulates in Golgi vesicles which fuse with the plasma membrane * In addition to protection of the meristem, the rootcap plays an important role in regulating the gravitropic response columella – site of perception of gravity in the rootcap, contains many starch filled amyloplasts, believed to be “gravity sensors” * Apart from the rootcap, the most striking feature of the root apex is the arrangement of longitudinal files of cells that emanate from the apical meristem * Apical meristem composed of small, many sided cells with dense cytoplasm & large nuclei * Two main types of apical organization found in roots of seed plants:
1. Closed Type – the rootcap, vacular cylinder, and cortex are traceable to individual layers of cells in the apical meristem, with the epidermis being of rootcap or cortex origin 2. Open Type – rootcap & cortex are minimum, they converge on a common group of cells, all regions have common initials * Three regions to the root: 1. Region of Division – includes apical meristem & nearby portion of the root 2. region of Elongation – not very delimited from area of dividion, usually only a few mm in length 3. Region of Maturation – most of the cells of the primary tissues are mature, root hairs are produced in this region Primary Root Structure: * Compared to that of the stem, the internal structure of the root is relatively simple root hairs – tubular outgrowths of the epidermal cells of the root, greatly increases the water absorbing surface area * Root hairs are short lived & confined to the region of maturation * A thin cuticle has been identified on the epidermis in the absorbing parts in some roots, while others contain suberin * Mucigel found to have a favorable environment to beneficial N-fixing bacteria rhizosphere – layer of soil bound to the root by the mucigel & root hairs, contains variety of microorganisms & sloughed rootcap cells * Mycorrhizae found on the roots of nearly all vascular plants The Cortex: * Occupies the largest area of the primary body of most roots * Plastids of cortical cells commonly contain starch & typically lack chlorophyll * Gymnosperms & eudicots that undergo considerable secondary growth shed their cortex early & cortical cells remain parenchymous * In monocots the cortex is retained for the life of the root & many of the cortical cells develop secondary walls that become lignified * Reguardless of differentiation cortical tissues contain pores for aeration * Cortical cells have numerous contacts with one another via plasmodesmata, connecting their protoplasts endodermis – innermost layer of the cortex, compactly arranged arranged cells with no spaces for air exchange, single layer of cells forming a sheath around the vascular region in roots & some stems Casparian Strips – a character of the endodermis, bandlike region of the primary wall containing suberin & lignin, found in the anticlinal walls- perpendicular to surface * Casparian Strips stops water & solutes , hence all substances entering/leaving must pass through the protoplasts of the endodermal cells passage cells – endodermal cells of root that retain a thin wall & Casparian Strip when other cells develop thick secondary cell walls exodermis – in roots of many angiosperms, a second compact layer of cells with Casparian Strips * Development is quickly followed by deposition of suberin in middle lamella & in some
species deposition of an additional cellulosic layer * This second wall further prevents water loss & provides protection from attack by microorganisms