Angiosperms IV

Primary Growth:

Roots and Stems

Roots and Root Systems

• Root Systems are often classified as either:– TAP ROOT SYSTEMS

(found most commonly in eudicots)• have prominent tap root

and smaller branch roots

Root Systems

– FIBROUS ROOT SYSTEMS (common in monocots)• many similar sized roots

in an extensive network

Functions of Root Systems

• Anchorage• Storage• Absorption• Conduction• Reproduction (occasionally)

Typical Eudicot Root

YoungRoot (cont.)

• Above the region of the meristem are the regions of

elongation and region of

maturation

Meristem

RootHairs

Single epidermal cells with a long extension

Structure of an Older Dicot Root (cont.)

Functions of Root Structures

• ENDODERMIS– innermost layer of the cortex – functions in water regulation– cells are “belted” with a suberized layer

called the Casparian strip– waterproofs the layer so water must

move through the cells (not the walls) into the stele

Endodermis

Functions of Root Structures (cont.)

• PERICYCLE– source of cells for

formation of branch (lateral) roots

– involved in formation of cambial layers in woody roots

– endogenous growth of branches

Functions of Root Structures (Cont.)

• PHLOEM– transport of nutrients

• XYLEM– protoxylem (first to mature

in the root)– metaxylem (last to mature)– water and mineral

conduction

• CAMBIUM (with pericycle cells will form vascular cambium in woody root)

Root Modifications• Storage tap roots (radish, jicama,

carrots)

Root Modifications (cont.)

• Fascicled storage roots (yams, sweet potatoes, cassava)

Casava for saleSweet Potatoes

Sweet Potato vs.Yam

• Impomea batatas (sweet potato)• Diascorea batata (yams)

Root Modifications (cont.)

• Prop roots (corn)

RootModifications

(cont.)

• Aerial roots (Spanish moss, orchids)

Aerial roots on an orchid

RootModifications

(cont.)

• Pneumatophores for gas exchange on white mangrove plants

Eudicot Stem Structure (cont.)

• Leaf primordia with trichomes

• Axillary (lateral) buds– give rise to

branches– exogenous

development

Structure of a Eudicot Stem• Apical Meristem

(shoot apex, shoot tip)

– composed of tunica and corpus layers

– tunica gives rise to protoderm

– corpus gives rise to procambium and ground meristem

Older Eudicot Stem (xs)

• Epidermis• Collenchyma

or fibers• Cortex• Vascular

bundles• Pith

Eudicot Stem Vascular Bundle

• Bundle cap (composed of fibers)

• Phloem• Fascicular

cambium• Xylem

Monocot Stem

• Scattered vascular bundles (atactostele)

• Surrounded by parenchyma cells (ground tissue)

Monocot Vascular Bundle

• Sheath of fiber cells

• Phloem with obvious companion cells

• Large metaxylem vessels

• Air space

Stem Modifications

• Tendril (may also be a leaf modification)

• Rhizome• Stolon or runner• Cladophyll

• Thorn• Prickle• Spine (leaf modification)• Tuber• Corm vs. bulb (leaf modification)

Tendrils• Leaf or stem modifications for

attachment

Rhizomes• Underground,

horizontal stems

Stolon or Runner• Above ground horizontal stems

– Seen in strawberry plants

Thorns• Thorns are modified branch systems

CrategusLocust tree

Spines• Spines are modified leaves

Prickles• Epidermal structures found in roses and other

plants

Tuber• Underground, storage stem (a white

potato)