Anatomy of Flowering plants

Tissue

A group of similar or dissimilar cells which perform a common function; the study of tissues is called histology

Two main types of plant tissue

Meristematic and Permanent

Meristematic tissue

A group of actively dividing cells that give rise to permanent tissues

Features of meristematic cells (4)

Small, spherical cells; tightly packed with no intercellular spaces; thin cell walls made of cellulose; dense granular cytoplasm with no or very small vacuoles

Classification of meristems by origin (3 types)

Promeristem, Primary meristem, Secondary meristem

Promeristem

Also called primordial meristem; originates from the embryo; consists of thin-walled isodiametric cells with dense cytoplasm and large nuclei; found at apices of embryonic shoots and roots

Primary meristem

Meristematic tissue present from the very beginning, derived directly from the promeristem, e.g. apical meristems, fascicular cambium, intercalary meristems of monocot stems

Secondary meristem

Meristematic tissue derived from permanent tissues that develops later in the plant's life, e.g. interfascicular cambium and cork cambium

Classification of meristems by position (3 types)

Apical meristem, Lateral meristem, Intercalary meristem

Apical meristem

Found at the tips of stems and roots; responsible for growth in length

Lateral meristem

Meristem lying along the sides of stem and roots (e.g. fascicular cambium, cork cambium); divides in the tangential plane causing increase in diameter (secondary growth)

Intercalary meristem

Found at the base of internodes and at the base of the leaf in plants like grasses and bamboo; responsible for increase in length of internodes and pedicel

Permanent tissue

Tissue made of cells that have stopped dividing, temporarily or permanently

Two types of permanent tissue

Simple tissue and Complex tissue

Simple tissue

A group of similar cells performing a common function; classified into parenchyma, collenchyma, and sclerenchyma

Complex tissue

A group of different kinds of cells that work together as a unit; main types are xylem and phloem

Parenchyma

Simple tissue of thin-walled, living, isodiametric cells; may be spherical, cubical, elongated, pillar-like, or irregular in shape, with moderate protoplasm

Chlorenchyma

Parenchymatous cells that develop chloroplasts and perform photosynthesis

Aerenchyma

Parenchymatous cells separated by numerous air spaces; found in floating hydrophytes and in petioles of Canna and banana

Prosenchyma

Parenchymatous cells that become long and tapering at the ends

Idioblasts

Secretory parenchymatous cells that store

1.alkaloids

2.tannins

3.oils and crystals of different types

Functions of parenchyma (5)

Storage of reserve food; slow conduction of water and food; provide turgidity in succulent plants; chlorenchyma manufactures food; aerenchyma aids aeration and buoyancy

Collenchyma

Living simple tissue thickened at the angles (due to cellulose/pectin deposition) where cells meet; absent in roots and monocot stems

Where is collenchyma found

As a complete cylinder just below the epidermis of young dicot stems, petioles, and in the corners of angular stems

Functions of collenchyma

Provides tensile strength; cells occasionally contain chloroplasts and carry out photosynthesis

Sclerenchyma

Simple tissue of dead, elongated cells with thick lignified walls, narrow and pointed at both ends

Two types of sclerenchymatous cells

Sclerenchymatous fibres and Sclereids (stone cells)

Sclerenchymatous fibres

Spindle-shaped, tapering, economically important dead cells, e.g. Madras hemp (Hibiscus cannabinus) and jute; used to make rope and linen

Sclereids (stone cells)

Dead, irregularly shaped cells derived from parenchyma, occurring singly or in groups; abundant in cortex/phloem of stems and roots and in coverings of seeds and nuts (e.g. the 'grit' in pears, shells of walnuts and coconuts)

Key difference: collenchyma vs sclerenchyma

Collenchyma cells are living with unevenly thickened cellulose/pectin walls; sclerenchyma cells are dead with uniformly thickened lignified/suberin walls

Complex tissue types (2)

Xylem and Phloem

Xylem cell types (4)

Tracheids, Xylem vessels (trachea), Xylem parenchyma, Xylem fibres

Tracheid

A single elongated cell, pointed at both ends, with lignified thick walls (mechanical strength); thickenings absent at some places forming pits that allow water to pass cell to cell; main vascular tissue of gymnosperms and ferns

Xylem vessel (trachea)

A multicellular, wide tube formed from many prosenchymatous cells joined end to end whose cross walls dissolve; characteristic of angiosperms; absent in most pteridophytes and gymnosperms

Xylem parenchyma

Parenchymatous cells associated with xylem, moderately thickened by lignin but not dead; store reserve food; may develop protrusions called tyloses that block xylem vessels

Tyloses

Balloon-like outgrowths of xylem parenchyma (or wood parenchyma) cells that project into and block the lumen of xylem vessels

Xylem fibres

Greatly thickened, long, slender, dead sclerenchymatous cells with pointed ends and very few pits; provide mechanical strength

Key difference: tracheids vs xylem vessels (trachea)

Tracheids are unicellular and short with narrow lumen and intact septa; vessels are multicellular, longer, with wider lumen and partly/wholly dissolved septa (transverse end walls)

Function of xylem

Conduction of water and minerals, and providing mechanical strength

Phloem cell types (4)

Sieve tubes, Companion cells, Phloem parenchyma, Phloem fibres (bast fibres)

Sieve tubes

Non-lignified, prosenchymatous cells arranged end to end with sieve pits/sieve plates on transverse walls; vacuolated cytoplasm without nucleus at maturity; conduct food

Callose

Colourless, insoluble carbohydrate deposited over the sieve plate in winter, forming a pad-like structure called callus that stops sieve tube activity (temporarily or permanently)

Companion cells

Thin-walled parenchymatous cells with dense cytoplasm and a conspicuous nucleus lying adjacent to sieve tubes; absent in Pteridophyta and gymnosperms (replaced by albuminous cells); control sieve tube activity via cytoplasmic connections

Phloem parenchyma

Normal parenchymatous cells associated with phloem that store reserve food and aid metabolic functions; absent in monocots

Phloem fibres (bast fibres)

Lignified, elongated tapering sclerenchymatous fibres associated with phloem that provide mechanical strength; economically important (used for cloth, ropes, cords)

Function of phloem

Conduction (translocation) of food (organic material) manufactured in leaves to other parts

Tissue system

One or more kinds of tissues working together as a unit to carry out a common function

Who classified tissue systems and when

Sachs, in 1875

Three types of tissue systems

Epidermal tissue system, Vascular tissue system, Ground (fundamental) tissue system

Epidermis

Outermost layer of the plant body (over stem, root, leaf); usually a single layer, but multilayered in Nerium, Indian rubber plant, banyan, etc.

Cuticle

A waxy layer covering the epidermis of aerial plant parts that prevents water loss; absent on root epidermis (epiblema/rhizodermis)

Stoma (plural stomata)

Small opening in the leaf epidermis surrounded by two kidney-shaped guard cells; regulates gas exchange and transpiration

Guard cells

Living epidermal cells surrounding the stomatal pore; unlike other epidermal cells they contain chloroplasts; walls are unevenly thickened (thicker on the inner side)

Stomatal apparatus

The stomatal pore, guard cells, and surrounding subsidiary (epidermal) cells together

Trichomes (hairs)

Outgrowths of epidermal cell walls that may be simple, branched, or star-shaped, e.g. cotton fibres of commerce are epidermal hairs of cotton seeds

Functions of epidermal tissue system (6)

Protects internal tissue from injury, parasites, cold/heat; reduces water loss via cuticle, wax, bloom, hairs, multiple epidermis; glandular hairs deter herbivores; stomata allow gas exchange; stomata regulate respiration/transpiration/photosynthesis; root hairs absorb soil water

Vascular tissue system

Complex tissue system consisting of xylem and phloem, usually grouped into vascular bundles; a strip of cambium may lie between xylem and phloem

Radial vascular bundle

Xylem and phloem lie on separate, alternate radii, often separated by conjunctive tissue; characteristic of roots; protoxylem always points toward the periphery (exarch)

Concentric vascular bundle

Xylem and phloem lie one surrounding the other in concentric cylinders; always a closed bundle (no cambium)

Hadrocentric (amphicribral) bundle

Concentric bundle type where xylem lies in the centre surrounded by phloem, e.g. in some ferns

Leptocentric (amphivasal) bundle

Concentric bundle type where phloem lies in the centre surrounded by xylem, e.g. monocot stems like Yucca and Dracaena

Conjoint vascular bundle

Xylem and phloem lie on the same radius

Conjoint collateral vascular bundle

Xylem and phloem lie on the same radius, xylem toward the centre and phloem toward the endodermis; characteristic of most angiosperm and gymnosperm stems

Open vascular bundle

Conjoint bundle with a strip of cambium present between xylem and phloem, e.g. dicots and gymnosperms

Closed vascular bundle

Conjoint bundle with no cambium between xylem and phloem, e.g. monocots

Conjoint biocollateral vascular bundle

Bundle with phloem present on both the inner and outer sides of the xylem, e.g. Cucurbitaceae (gourd family), tomato, potato

Ground (fundamental) tissue system

Forms the major, largely parenchymatous portion of the plant body between the epidermis and vascular bundles; components are cortex, pericycle, pith and pith rays

Hypodermis

Outer part of the cortex; collenchymatous in dicot stems (provides support before vascular tissue matures) and sclerenchymatous in monocot stems

General cortex

Region between hypodermis and endodermis made of large, thin-walled parenchymatous cells with intercellular spaces; may contain stone cells or fibres and can photosynthesize

Endodermis

Innermost layer of the cortex, made of barrel-shaped cells, often with a suberized/lignified Casparian strip on radial walls; regulates water movement into the stele

Casparian strip

A thickened (suberized/lignified) band on the radial, upper and lower walls of endodermal cells, named after discoverer R. Caspary (1865)

Passage cells

Thin-walled endodermal cells (near protoxylem) that remain unthickened, allowing water to pass from the cortex into the stele

Pericycle

Outermost layer of the stele, lying just inside the endodermis, usually single-layered and parenchymatous; gives rise to lateral roots

Functions of pericycle (5)

Gives rise to lateral roots; gives rise to adventitious roots from stems; in dicot roots gives rise to part of the vascular cambium (secondary meristem); sclerenchymatous pericycle gives mechanical strength; parenchymatous pericycle stores food

Stele

All tissues enclosed by the pericycle, collectively — including pericycle, pith, pith rays and vascular bundles

Pith (medulla)

Central region of the stem or root, usually composed of parenchymatous cells with intercellular spaces; conjunctive/intrastellar ground tissue

Pith rays (medullary rays)

Radial rows of parenchymatous cells that arise from the pith and extend to the pericycle, separating vascular bundles of dicot stems; conduct food and water radially and store food

Epiblema (piliferous layer/rhizodermis)

Outermost single layer of uncutinised cells of the root, bearing unicellular root hairs that increase surface area for water and mineral absorption

Root hair

Unicellular outgrowth of an epiblema cell that increases the surface area for absorption of water and minerals

Velamen

Specialized, spongy, dead cortical layer (3-5 cells thick) found on aerial roots of orchids, beneath the exodermis, that absorbs water vapour from the atmosphere

Exodermis (root)

Layer beneath the withered piliferous layer in older parts of the root; cells are slightly thickened and suberized; makes its appearance only after the epidermis loses its absorptive function; a protective layer

Order of tissues in a dicot root cross-section

Epiblema (piliferous layer), cortex, endodermis, pericycle, pith, vascular bundles (radial, exarch)

Dicot root xylem arrangement

Exarch (protoxylem points toward periphery); usually 2-7 (diarch to heptarch) xylem strands alternating with an equal number of phloem strands

Monocot root special feature

Polyarch xylem (seven or more xylem groups) and a well-developed, persistent pith; no secondary growth (except in Yucca and Dracaena)

Key difference: root vs stem epidermis

Root epidermis (rhizodermis) bears unicellular absorptive hairs and lacks cuticle/stomata; stem epidermis bears multicellular protective hairs and has cuticle/stomata

Key difference: monocot root vs dicot root (xylem groups)

Monocot root has 5-20 or more protoxylem groups (polyarch); dicot root has up to 5 protoxylem groups

Key difference: monocot root vs dicot root (pith)

Monocot root pith is well developed; dicot root pith is absent or poorly developed

Key difference: monocot root vs dicot root (secondary growth)

Monocot roots show no secondary growth (except Yucca, Dracaena); dicot roots undergo secondary growth

Dicot stem (sunflower) epidermis

Single layer of cells with numerous multicellular hairs and a cutinized outer wall

Dicot stem hypodermis

4-5 cell thick layer of collenchyma just below the epidermis, providing mechanical strength

Dicot stem vascular bundles

Conjoint, collateral, open (cambium present), arranged in a ring; xylem is endarch (protoxylem points toward the centre/pith)

Endarch xylem

Protoxylem lies toward the centre (pith) and metaxylem toward the periphery — arrangement typical of stems

Exarch xylem

Protoxylem lies toward the periphery and metaxylem toward the centre — arrangement typical of roots

Monocot stem (maize) vascular bundles

Conjoint, collateral, and closed (no cambium); scattered throughout the ground tissue; each surrounded by a sclerenchymatous bundle sheath

Monocot stem hypodermis

Made of sclerenchymatous cells (unlike collenchymatous hypodermis in dicot stems)

Key difference: dicot stem vs monocot stem (vascular bundles)

Dicot stem bundles are arranged in a ring and are open (cambium present); monocot stem bundles are scattered and closed (no cambium)