Roots and Shoots

Chapter 2

 

What makes up a plant body?

•Cells are grouped together in tissues

–e.g. xylem cells are arranged together to form a vascular tissue

•Tissues form organs

–e.g. vascular tissues plus epidermis and ground tissues for an herbaceous stem

•Organs form a body

–stem, leaves and roots form a plant

Three Major Plant Organs

•Roots

•Stems

•Leaves

 

•(above-ground parts called the “shoot”, stem and leaves)

 

•Other parts are modifications of one or more of these (see chapter 6)

 

Roots

Function of Roots

•Roots anchor plants firmly in the soil

–Roots form an extensive branching network that constitutes 1/3 of total dry weight of the plant

–Roots penetrate to depths from less than one inch up to 100 feet

•Roots absorb water and minerals

–Absorb water for the entire plant's needs

–Roots "mine" the earth for mineral elements, unlocking from soil its store of elements

•Some roots have additional functions such as storage of food or water (discussed later)

Root Origins

•In the seed, the embryonic root is called the radicle

–develops into the first root after germination

Root systems

•Monocots tend to have fibrous root systems (left)

•Dicots usually taproot systems (right)

 

Root hairs

•Root hairs are extensions of single cells (branch roots are many cells thick)

–increase the surface area of roots

 

Radish seedling with root hairs

Length and Surface Area of Root System

•Single rye plant grown in a pot with 0.6 m3 soil

–separate each root

–end to end = 644 kilometers long

–38,000 root hairs per cm2

•14 billion root hairs

•surface area = size of a football field

•more surface are to contact soil, more area to absorb water and minerals

 

Regions of a root tip

•root cap covers tip, tough cells secrete mucilaginous substance, protect inner cells

•apical meristem immediately behind root cap, zone of cell division

•zone of cell elongation behind apical meristem (newly divided cells expand in length)

•root hair zone further behind

 

Onion root tip

Radish seedling with root hairs

Lateral (or branch) roots

•bigger than root hairs, made up of many cells

Stems

Function of stems

•Support

–display the energy-capture system (the leaves) to receive sunlight

•Transport

–water and nutrients from soil to rest of plant

–food from leaves to rest of plant

Meristems

•Apical meristems in buds of shoots

•At apex (tip) of growing stems, more complex than in roots

•Gives rise to primary tissues, including

–leaf primordia (will become new leaves)

–axillary bud primordia (will become new axillary buds which will become new branches)

•Lateral meristems develop in woody stems to give rise to secondary tissues

 

Tip of a shoot

External Parts of a Woody Twig

Attachment of Leaves to Twig

•Alternate (one leaf)

•Opposite (two leaves)

•Whorled (3 or more leaves)

Special Regions on Stems

•Nodes

–area of stem where a leaf or leaves is (are) attached

•Internodes

–region of stem between nodes

•Axils

–angle between a petiole and the stem

 

Buds

•Axillary (or lateral)

–found in leaf axils (angle between leaf and stem)

•Terminal (or apical)

–found at tips of twigs

 

Scar Tissue on Twigs

•Terminal bud scale scars

–series of narrow scars encircling twig where bud scales had been attached

–can be used to age a twig

•Leaf scars

–remain where leaves have fallen off from twigs of deciduous or evergreen trees and shrubs

•Bundle scars

–mark the location of the vascular bundles within a leaf scar

 

Scar Tissue on Twigs

•Lenticels

–pores in stem to allow for gaseous exchange

–in woody stems with smooth bark, first appear as small specks, later stretch into horizontal lines of corky tissue

 

Leaves

Functions of leaves

•Photosynthesis

•Transpiration

–evaporation of water through pores in leaves (stomates)

Variations

•Sizes

–duckweeds (leaves 1 mm wide) to palms (6 meters long)

•Shapes

–flattened, tubular, feathery, cup-shaped, spine-like, needle-like

•Textures

–smooth, hairy, slippery, sticky, waxy, glossy

 

Leaf Parts

•Petiole (stalk)

–if petiole is present, called petiolate leaf

–if petiole is absent, called sessile leaf

•Blade

–broad, flat part of leaf with network of veins (vascular bundles)

•Stipules (optional)

–small, blade-like appendages at base of leaf

 

Simple and Compound Leaves

•look for bud at the base of the leaf

–never at the base of a leaflet!

 

 

Simple and Compound Leaves

•Simple leaf

–undivided blade

•Compound leaves (blade divided into leaflets)

–Pinnately compound

•leaflets in pairs along a central rachis

–Palmately compound

•leaflets attached at top of petiole

Arrangement of Veins (Venation)

•Reticulate (netted) veins (dicots)

Arrangement of Veins (Venation)

•Reticulate (netted) veins (dicots)

–Pinnately veined

•single main vein called the midrib, smaller veins branch off along midrib

–Palmately veined

•several veins fan out from base of blade like fingers from the palm of your hand 

Arrangement of Veins (Venation)

•Parallel veined (monocots)

–major veins all parallel to each other (but may be perpendicular to midrib)

Arrangement of Veins (Venation)

•Dichotomous venation

–veins fork evenly and progressively from base of blade

–special feature found in Ginkgo trees, a broad-leafed gymnosperm (more like conifers than other broad-leaf trees)

Grass and meristems

•Why do grass leaves keep growing after mowing or grazing?

•Intercalary Meristems (neither apical nor lateral meristem)

–Found in many plants which do not have a vascular cambium (e.g., grasses, bamboo)

–Occur near nodes and at the base of leaves

–Produce increases in the length of stems and/or leaves