UNIT 2 PART 2

MERISTEMATIC TISSUES

• Permanent regions of growth in plants "

• Region where cells actively divide

Apical meristems, Lateral meristems, and Intercalary meristems

3 types of meristematic tissue

Apical Meristems

• Meristematic tissues found at, or near, the tips of roots and shoots which increase in length as the apical meristems produce new cells→ primary growth

• Also responsible for embryo leaves and buds

 protoderm, ground meristem, and procambium

3 primary meristems of apical meristems

Lateral Meristems

increase in width or girth on plants

• Produce tissues that increase the girth of roots and stems→ secondary growth

Vascular cambium

produces secondary tissues that function in support and conduction; extends throughout the length of roots and stems in perennial and many annual plants is in the form of a thin cylinder of mostly brick-shaped cells

Cork cambium

in the form of a thin cylinder that runs the length of roots and stems in woody plants; lies outside the vascular cambium, just inside the outer bark which it produces

Intercalary Meristems

• Develop at intervals along stems, where like the tissues produced by apical meristems, their tissues add to stem length

• Ability to divide and produce new cells but they are situated between regions of mature tissue

• Function only for a short time, eventually completely differentiate to mature tissues

• Located at positions in stems where leaves have emerged and responsible for elongation in grass shoots and leaves

PARENCHYMA TISSUES

• Composed of parenchyma cells (most abundant of the cell types)

• Function as storage and for basic metabolism

• Primary sites for metabolic functions such as photosynthesis, respiration, and protein synthesis

• Capable of dedifferentiate and redifferentiate

CHLORENCHYMA

Chloroplast-containing parenchyma cells for photosynthesis

AERENCHYMA

prominent intracellular spaces for improved gas-exchange capacity of the tissue

TRANSFER CELLS

highly convoluted, non-lignified cell walls; occurs in areas of high solute transport; specialized for short distance transport of solutes

COLLENCHYMA TISSUES

• Have living cytoplasm and remain alive a long time

• Walls are generally thicker and more uneven in thickness than parenchyma cells

• Often occur just beneath the epidermis

• Support growing regions of shoots and therefore common in expanding leaves, petioles, and elongating stems

• Differentiation is influenced by mechanical stress

SCLERENCHYMA TISSUES

• Cells that have thick, tough, secondary walls, normally impregnated with lignin • Dead at maturity and function in support

• Two forms: sclereids and fibers

SCLEREIDS

relatively short, have variable shapes, and usually occur singly or in small groups

FIBERS

long, slender cells typically occurring in strands; found in association with a number of different tissues in roots, stems, leaves, and fruits

Xylary fibers

fibers in xylem

Extraxylary fibers

occur in tissues other than the xylem

XYLEM

important component of the plumbing and storage systems of a plant; chief conducting tissue throughout all organs for water and minerals absorbed by the roots

XYLARY CELLS

conducting cells in xylem (tracheids and vessel elements)

TRACHEIDS

most primitive; water conducting cells in most woody, non-flowering plants such as pine; have bordered pits

VESSEL ELEMENTS

evolutionary advanced tracheids; facilitate more rapid transport of water than the tracheids

Primary xylem

differentiates from the procambium in the apical meristem

protoxylem

xylem in the elongating regions of plants

Secondary xylem

differentiates from the vascular cambium; commonly known as wood; cells are more abundant and occurs in different frequencies

PHLOEM

conducts dissolved food materials produced by photosynthesis throughout the plant

SIEVE TUBE MEMBERS

facilitate faster solute movement; associated with specialized cell called companion cell

companion cell

regulates the loading and unloading of carbohydrates from sieve tube members

SIEVE CELLS

more primitive; occur in non-flowering plants; most delicate cells in plants; associated with specialized parenchyma cells called albuminous cells

SIEVE ELEMENTS

conducting cells of the phloem

PHLOEM

solutes move in all directions; move under a positive pressure

Primary phloem

differentiates from the procambium and extends throughout the primary body of a plant

protophloem

phloem in the elongating regions of plants

metaphloem

non-elongating regions

Secondary phloem

differentiates from the vascular cambium; constitute the inner layer of bark

EPIDERMIS

• Protective layer covering all plant organs

• Primarily made up of parenchyma or parenchyma-like cells + specialized cells involved in the movement of water and gases in and out of plants, secretory glands, various hairs, cells in which crystals are isolated, and others that greatly increase absorptive parts of roots

cutin

Secrete a fatty substance called _____ within and on surface of the outer walls

cuticle

cutin forms a protective layer called the ___________

PERIDERM

• Replaces the epidermis in woody plants after the cork cambium begins producing new tissues

• Constitutes the outer bark and composed of cork cells which are dead at maturity

Suberin

: fatty substance that makes the cork cells waterproof and helps protect the phloem and other tissues beneath the bark from drying out, mechanical injury, and freezing temperature.

Lenticels

pockets of tissue that protrude through the surface of the periderm; function for gas exchange between the air and interior of stem

SECRETORY CELLS AND TISSUES

• Derived from parenchyma

• Occurs in various places of plants

• Secrete substances such as nectar (flowers), oils (citrus, mint, and many leaves), mucilage (glandular hairs of sundews and other insect-trapping plants, latex (members of plant families such as Spurge), and resins (secreted by cells lining tube-like ducts that form networks through certain plant species)

ROOTS

• Anchor trees firmly in the soil usually through an extensive branching network that constitutes about 1/3 of the total dry weight of the plant

• Absorb water and minerals

• Store large amounts of energy reserves

• Conduct water and dissolved nutrients to and from the shoot

Radicle or primary root

a part of the embryo (immature plantlet) that grows out when a seed germinates

Taproot

composed of large taproot and smaller branch roots; common to conifers and many dicots; function for storing large reserves of food usually carbohydrates (fleshy type) or reaching water deep into the ground (long type)

Fibrous

• Extensive mass of similarly sized roots

• Characterized by short-lived radicle that is later replaced by a mass of adventitious roots

• Common to most monocots (including grasses)

• May co-exists in mature plants together with the taproot system

Adventitious

• Roots that form on organs other than roots; attached to the radicle and continuous with it

• In many plants serve as primary means of vegetative reproduction

• In most monocots→ begin growing soon after the seed germinates

• Form in all sorts of places on plants including leaves, petioles, and stems

• May develop in both dicots and monocots

• Controlled by hormones such as auxin

auxin

adventitious roots are controlled by hormones called __________

ROOT CAP OR ROOT TIP

• Thimble-shaped structure that covers the tip of roots; senses light and pressure exerted by soil particles

• Cells are derived from the root cap meristem

• Has its own meristem that pushes cells forward into the cap

COLUMELLA CELLS

contain amyloplasts that are responsible for gravity detection; respond to light and pressure from soil particles

PERIPHERAL CELLS

columella cells that are pushed to the periphery of the root cap; secrete large amounts of mucigel

mucigel

slimy substance made by dictyosomes

QUIESCENT CENTER

• 500-1000 seemingly inactive cells

• Cells are arrested in the G1 phase and divide only about once every 15-20 days

• They function to replace the meristematic cells of the root cap meristem

• Differentially sensitive to environmental perturbations such as radiation

• Also important in organizing the patterns of primary growth in the root

ZONE OF CELLULAR DIVISION

• Dome-shaped apical meristem behind the root tip

• Made up of small, densely cytoplasmic cells; consists of cells that divide every 12-36 hours

ZONE OF CELLULAR ELONGATION

• Merges with apical meristem and usually extends 1 cm or less from the tip of the root

• Cells become several times their original length and also somewhat wider→ cells elongate by 150-fold by filling their vacuoles with water

• Tiny vacuoles merge and grow until 1 to 2 large vacuoles are formed.

ZONE OF CELLULAR MATURATION/DIFFERENTIATION

• Root hair zone

• Root hairs form only in the maturing, non-elongating region of the root→ increase the root’s absorptive surface area

• Root hairs are not separate cells rather tubular extensions of specialized epidermal cells

CORTEX

From the ground meristem; occupies the largest cross-sectional area of the root

HYPODERMIS

outermost layer; suberized protective layer→ prominent in roots growing in arid soils or near the soil’s surface

STORAGE PARENCHYMA CELLS

thin-walled; contain starch and separated by large intracellular spaces

ENDODERMIS

innermost layer; packed tightly together and lack intracellular spaces; impregnated with Casparian strip (lignin + suberin)

STELE

• All the tissues inside the cortex; consists of the pericycle, vascular tissues, and sometimes parenchymatous pith

PERICYCLE

outermost layer; produces the branch roots (secondary or lateral roots); different from the endodermis since it is a cylinder of parenchyma or sclerenchyma cells that surround the ring of the vascular bundles in the stele whereas the endodermis is a cylinder of cells that separate the cortex from the stele

• Temperature

• Other organisms

• Light

• Gravity

• Genetic differences

• Stage of plant development

• Soil properties (moisture and air, soil texture, nutrients, other factors)

FACTORS CONTROLLING THE GROWTH AND DISTRIBUTION OF ROOTS

FOOD STORAGE ROOTS

Roots that are enlarged and store large quantities of starch and carbohydrates which may later be used for extensive growth

WATER STORAGE ROOTS

• Characteristic of those that grow in arid regions or in those areas where there may be no precipitation for several months of the year

• Pumpkin Family (Cucurbitaceae)

PROPAGATIVE ROOTS

• Adventitious buds (buds appearing in places other than stems) along the roots that grow near the surface of the ground→ grow into aerial stems called suckers which have additional rootlets at their bases→ can be separated from original root and grow individually

• Rice-paper plants, Horseradish, Cherries, apples, pears, and other fruit trees

PNEUMATOPHORES

• Also known as respiratory or knee roots; project above the mud and have small openings (lenticels) through which air enters, passing through the soft spongy tissue to the roots beneath the mud

AERIAL ROOTS

• Can function to support plants in a high wind • Aid the plants in climbing • Orchids (velamen roots), corn (prop roots), Banyan trees, Ivies (adventitious roots), Virginia creeper

CONTRACTILE ROOTS

• Some herbaceous dicots and monocots

• Can pull the plant deeper into the soil

• Contract by shrinking their cortical cells

• Lily bulbs, Dandelions

BUTTRESS ROOTS

• Produced at the base of the trunk, giving them great stability

PARASITIC ROOTS

• Plants with no chlorophyll and have become dependent on chlorophyllbearing plants for their nutrition

• Parasitize their host plants via peg-like projections called haustoria which develop along the stem in contact with the host→ penetrate outer tissue and establish connection with the water-conducting and food-conducting tissues

• Dodders, pine-drops, Mistletoe