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

Young Root (cont.) Above the region of the meristem are the regions of elongation and region of maturation Meristem

Root Hairs 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 sale Sweet Potatoes

Sweet Potato vs.Yam Impomea batatas (sweet potato) Diascorea batata (yams)

Root Modifications (cont.) Prop roots (corn)

Root Modifications (cont.) Aerial roots (Spanish moss, orchids) Aerial roots on an orchid

Root Modifications (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 T endril (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 Crategus Locust tree

Spines Spines are modified leaves

Prickles Epidermal structures found in roses and other plants

Tuber Underground, storage stem (a white potato)