Download Plant Anatomy—Simple Tissues

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