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II. SIMPLE TISSUES Bot 404--Fall 2004 A. Introduction to Tissues (DIAGRAM—allow a full page) B. Definitions Adaxial = facing the axil; upper surface of leaf Abaxial = facing away from the axil; lower surface of leaf Periclinal = parallel to closest surface; usually used in reference to plane of cell division Anticlinal = perpendicular to nearest surface; usually used in reference to plane of cell division C. Parenchyma 1. General Characteristics -most common cell type, fundamental tissue -evolutionarily the oldest (original) plant cell type -great diversity of size, shape, metabolism and function -cells usually more or less isodiametric but can be elongated or lobed -complete, living protoplast -all have a thin primary wall (a thin secondary wall may also occur in one subtype) and are living at maturity -can transform themselves, that is, become meristematic again -sclerified parenchyma can become sclerenchyma under certain conditions 2. Subtypes (not mutually exclusive) a. Synthetic—metabolically active cells that synthesize various products (e.g., chlorenchyma, meristematic cells, secretory parenchyma) i) chlorenchyma: many chloroplasts, conspicuous vacuole, extensive intercellular spaces, near surfaces (although can be deeper inside the plant); tissue in which photosynthesis takes place ii) meristematic cells: undergo regular division to produce new cells; vacuoles small, usually several per cell; in an apical or lateral meristem or other meristematic region; typically small, isodiametric iii) secretory parenchyma: accumulate and secrete a variety of products; often densely cytoplasmic and palisade-like; part of cell wall often hydrophobic, with transfer areas thin and permeable b. Storage—intercellular spaces smaller, intermediate-sized cells, deeper in the plant, no chloroplasts, can have a thick wall; spaces decrease because gas and water vapor exchange is not needed; amyloplasts for starch storage, enlarged vacuole for water storage, thick walls for storage of hemicelluloses c. Structural—provide support, air spaces may be integral to this i) aerenchyma: exaggerated intercellular spaces; characteristic of plants growing in low-oxygen environment, e.g., mud or water but spongy mesophyll can be included here; provides an internal oxygen supply; individual cells distorted (lobed) in one form; light in weight but strong ii) sclerified: cells not greatly elongated, 2° wall developed but relatively thin, but cell still living at maturity d. Transport—responsible for moving materials over short or long distances i) transfer: parenchyma with wall ingrowths that increase surface area of the plasmalemma; involved in short-distance transport of solutes between cells (e.g., phloem loading, secretion) ii) conducting: sieve elements for long-distance transport e. Boundary—interfaces between plant parts or plant and environment i) epidermis: boundary between plant and environment; often hydrophobic for water retention or control of water flow; secretes cuticle; resistant ii) endodermis: layer that isolates water conducting tissues from other parts of the plant; typically in roots but also occasionally in stems or leaves D. Collenchyma 1. Characteristics -irregularly thickened 1° walls, several patterns of thickening -wall consists of alternating layers of pectin and cellulose, basically soft and pliable, rich in water; sometimes does not stain exactly like a 1° wall -living at maturity with a complete protoplast, may have chloroplasts -can become meristematic (involves removal of wall thickening) -isodiametric to elongated, end walls flat to tapering -always occurs as a homogenous mass of cells (not as individual, scattered cells) -usually found just below the epidermis in herbaceous stems, young stems of woody plants, and petioles; often as isolated strands, but may occur as a continuous cylinder -in older stems, a 2° wall can be laid down (e.g., lignin) so it becomes sclerenchymatous (the primary wall can also become lignified) 2. Function -provides flexible (plastic) support by position in organs and structure of cell wall -environmental factors such as wind, stress can cause collenchyma formation to begin earlier and become more massive -why thickening is irregular is not known E. Sclerenchyma 1. General Characteristics -cells of this tissue always have a 2° wall (same constituents as 1º wall + lignin), therefore pits are present -3 sublayers (S1, S2, S3); deposition of S1 can begin while cell is still elongating in areas where growth has ceased; S2 and S3 laid down after growth ceases -2° wall is usually thick -often dead at maturity but can retain protoplast and can live for many years -2 main subtypes based on function: conducting (xylem) and non-conducting (fibers and sclereids) -intrusive growth: older central or middle part stops growing but the apices continue and insert themselves between walls of other cells; parts that have developed through intrusive growth lack pits (DIAGRAM) 2. Function of non-conducting sclerenchyma -provides rigid but elastic support (degree of “hardness” depends upon thickness of 2° wall and amount of lignin deposited) -resistance to/protection from animals (herbivory, oviposition, etc.) due to indigestibility of 2º walls 3. Sclereids -short or elongated, but if elongated then branched, if short, variable in form (M, pp. 68-69; E, p. 73) -usually with thick 2° wall that is strongly lignified -occur in various parts of the plant, either in layers or clusters but often as isolated, scattered cells = idioblasts (“crazy cells”) in stems, leaves, fruits -proposed as protection against herbivory or for inflexible protection -subtypes—various names applied to various kinds, but don’t need to know a) astrosclereids—more or less star-shaped, often associated with vein endings [no pits in the arms (branches) of sclereids due to intrusive growth] b) brachysclereids—small, isodiametric, extremely thick walls with pit channels (if branched, are known as ramiform pits); e.g., stone cells 4. Fibers -long and narrow, unbranched, tapered end walls -thick 2° wall, not highly hydrated -often rather sparse pits (original connections severed), often slitlike -narrow lumen that is dead at maturity (usually) -xylary vs. extraxylary: i) xylary fibers: in xylem, believed to be derived from tracheids, a type of conducting sclerenchyma (M, Fig. 5.15); either as libriform fibers (thick walls, simple narrow pits) or fiber-tracheids (thinner walls, pits with a chamber and oval inner aperture); fiber-tracheids intermediate between tracheids and libriform fibers, range of intermediates possible ii) extraxylary fibers: in tissues other than xylem, especially often associated with phloem (phloem fibers, perivascular fibers, pericyclic fibers, bundle cap); no features of tracheids but do have thick walls, simple pits with circular inner apertures; heavy lignification gives “hard” fibers (typical of monocots), little or no lignification gives “soft” fibers (typical of dicots, e.g., linen) -some fibers remain living: a) multinucleate fiber in tobacco; b) septate fibers which have a thin 1° wall dividing the cell into compartments, each section with its own nucleus; combine mechanical support and storage functions