Inside Roots and Leaves

Chapter 4

 

Root Cap

Thimble-shaped mass of parenchyma

Protects apical meristem

Lubricates root as it moves through the soil

Mucilage provides favorable habitat for beneficial bacteria

No equivalent structure in stems

Root cap cells being constantly lost and replaced

Perception of gravity takes place in root cap cells

special organelles shift in cell when root is turned

Region of Cell Division = Apical Meristem

Cell divisions take place at edges

Cells divide every 12-36 hours

Quiescent center (infrequent cell divisions here)

Cells produced are cuboidal in shape, large nucleus and few, if any, small vacuoles

Region of Elongation

Cells become elongated as cell differentiation proceeds

Cells assume their mature size and shape in this region

Region of Maturation

Cells fully mature and functioning in this region

Also called the root-hair zone

Root hairs emerge in this zone

Absorption of water and nutrients from soil occurs only in this zone

Radish seedling with root hairs

Tissue Regions in a Root Herbaceous dicot

Epidermis

thin cuticle or none at all

not waterproof like epidermis in stem and leaf

Cortex

Parenchyma cells

Located between epidermis and stele

Functions in food storage

 

 

Stele

Endodermis

The inner boundary layer of cortex

Unlike epidermis, this layer is mostly waterproof

Casparian bands (or strip)

creates barrier to passage of water and mineral elements

suberin in cell walls

Passage cells

cells without suberin

water with solutes (nutrients and other substances dissolved in water) can pass though if cell membrane allows

 

 

Pericycle

Retains meristematic capability

Produces branch roots and portion of vascular cambium

Lateral or Branch Roots

Form from pericycle

Push their way through cortex to the surface of root

Primary vascular tissue

Primary Xylem

form spoke-like arms or ridges that point toward the pericycle (dicots only)

Primary Phloem

forms in discrete patches between the xylem arms

 

Monocot roots

Vascular tissue in ring surrounding pith in monocot roots

Secondary tissue

Vascular cambium may form from parts of pericycle and between xylem and phloem.

Seconadary xylem and phloem are produced by vascular cambium.

Older, woody roots may lose endodermis, cortex and epidermis, contain mostly rings of xylem with thin layers of phloem and cambium, very similar to woody stems.

 

 

Leaves

Internal Structure of Leaves Overview

Epidermis

Mesophyll

Veins

Epidermis

Single, colorless layer of cells covering entire leaf

Cuticle present on epidermis

waxy substance called cutin

 

Stomates (or stomata) present in epidermis

Glands

Often secrete sticky substances

Occur as depressions, protuberances, and hairs or tips of hairs

 

Stomates

Tiny pores in epidermis

Can occur on both upper and lower epidermis

Distribution of stomates variable

Dicots: more in lower epidermis

Monocots: equal distribution on both surfaces

Aquatic plants (e.g., water lily), on upper epidermis only

 

Guard cells

Two kidney bean shaped cells surround the stomate (the opening)

Regulate size of stomate

Allow air and water vapor to enter or exit leaf

air is source of CO2 for photosynthesis

water vapor exits leaf in transpiration

guard cells prevent too much water loss on dry days

 

 

Mesophyll in dicots

Palisade

tightly packed, chloroplast-filled parenchyma cells in layer which gets most direct sunlight

Spongy

loosely arranged parenchyma; air spaces between cells for CO2 to move around

 

Mesophyll in monocots

Leaves are held closer to vertical

Neither side gets more sunlight

Therefore, no differentiation between palisade and spongy mesophyll.

It's all just mesophyll.

 

Veins (Vascular Bundles)

Xylem and phloem

xylem toward upper epidermis

Bundle sheath

layer of parenchyma and/or fibers which surround veins

gives support to hold leaf upright

 

 

Arrangement

Reticulate veins (Dicots)

Parallel veins (Monocots)

 

Conifer leaves

Abscission

The process by which leaves are shed

Abscission Zone = region near base of petiole

Protective layer

develops closest to the stem

suberin lines cell walls to seal wound (form leaf scar)

Separation layer

cells in at base of petiole swell and become gelatinous

pectins in middle lamella broken down by enzymes