Outlines of Previous Lecture Primary Growth of Roots and Shoots Apical meristems – herb (entire body), woody (younger parts) Roots – root caps, zones of division, elongation, maturation Vascular Tissues – Stele : Xylem from the center, Phloem in wedges Monocots – Xylem and phloem alternating rings and central parenchyma cells Ground Tissues – Cortex, Endodermis; lateral roots from pericycle Shoots – Dome shaped apical meristems Two leaf primordia Axillary buds form lateral shoots Epidermis, vascular tissue and lateral shoots from axillary buds Dicots – xylem faces the pith, phloem to cortex, ring form Monocots – vascular bundles scattered Leaves – Parenchyma, collenchyma, sclerenchyma Palisade mesophyll, spongy mesophyll

Secondary Growth of Plant Body

Woody plants 20% of all dicot species (oaks, maples) 5% of all monocot species All gymnosperms (pines, firs, redwoods)

Secondary growth - the growth in thickness Secondary growth commences where primary growth has stopped older regions of all gymnosperm species and many eudicots, but rarely in monocots lateral meristems stems and roots of woody plants, but rarely in leaves The secondary plant body vascular cambium – xylem (wood) and phloem cork cambium – cork cells (tough and thick cells)

Vascular Cambium and Secondary Vascular Tissue Cylinder of meristematic cells one cell thick Increases in circumference lays down successive layers of secondary xylem to its interior and secondary phloem to its exterior develops from undifferentiated cells and parenchyma cells that regain the capacity to divide

Secondary growth of dicot stems

Vascular cambium produces secondary vascular tissue in woody stems and roots

Vascular cambium in Roots typical gymnosperm or woody eudicot root, forms in segments between the primary phloem, the lobes of primary xylem, and the pericycle, eventually becoming a cylinder

Vascular cambium in stem gymnosperm or woody eudicot stem the vascular cambium forms in a layer between the primary xylem and primary phloem and in the ground tissue between the bundles The meristematic bands within and between the vascular bundles unite to become a continuous cylinder of dividing cells Fusiform initials Ray initials

The Vascular Cambium (Cont.) Fusiform initials tapered (fusiform) ends oriented parallel to the axis of a stem or root. produce elongated cells tracheids, vessel elements, and fibers of the xylem, sieve–tube members, companion cells, parenchyma, and fibers of the phloem

Ray initials shorter cells oriented perpendicular to the stem or root axis vascular rays—radial files consisting mainly of parenchyma cells

The Vascular Cambium (Cont.) Vascular rays living avenues move water and nutrients between the secondary xylem and secondary phloem store starch and other organic nutrients xylem ray – located in the secondary xylem phloem ray – located in the secondary phloem

Layers of secondary xylem (wood) accumulated over the years tracheids, vessel elements, and fibers Gymnosperms – tracheids Angiosperms – tracheids and vessel elements Both types of cells - thick, lignified walls that give wood its hardness and strength Early wood (Spring wood) Tracheids and vessel elements that develop early in the growing season, typically in early spring (have relatively large diameters and thin cell walls) Late wood (Summer wood) Tracheids and vessel elements produced later in the growing season, during late summer or early fall (thick–walled cells, add more support)

Secondary xylem & tree rings

springwood summerwood

Heartwood Sapwood The older layers of secondary xylem no longer transport water and minerals (xylem sap) Closer to the center of a stem or root Sapwood The outer layers of secondary xylem still transport xylem sap Only the youngest secondary phloem, closest to the vascular cambium, functions in sugar transport As a stem or root increases in circumference, the older secondary phloem is sloughed off

heartwood sapwood

Cork Cambium and the Production of Periderm Stem – arises in the outer cortex Roots arises in the outer layer of the pericycle During the early stages of secondary growth, epidermis pushed outward split, dry, and fall off the stem or root Replaced by two tissues produced by the first cork cambium Phelloderm – thin layered parerchymatous cells, forming interior of the cork cambium Cork cells – accumulate to the exterior of the cork cambium Periderm A cork cambium and the tissues it produces

Periderm Impermeable to water and gases Lenticels – small, raised areas in periderm, more space between cork cells Living cells of woody stem or root may exchange gases with the outside air Unlike the vascular cambium, cells of the cork cambium do not continue to divide; no increase in its circumference

Outer bark (periderm)

Periderm (outer bark)

Periderm

Lenticels Lenticels

Lenticels

thickening of a stem or root splits the first cork cambium, loses its meristematic activity and differentiates into cork cells A new cork cambium forms to the inside, resulting in another layer of periderm As this process continues, older layers of periderm are sloughed off, evident in the cracked, peeling bark of many tree trunks

Bark includes all tissues external to the vascular cambium (outward direction), secondary phloem (produced by the vascular cambium), the most recent periderm, and all the older layers of periderm