Lateral Meristem and Secondary Growth in Plants: Vascular Cambium & Cork cambium

Secondary growth

• occurs in all gymnosperms, some monocots, and many dicots

• responsible for increase in girth/diameter of the stem

• produce by two lateral meristem: - vascular cambium, cork cambium/phellogen

vascular cambium

• nature of cells: meristematic parenchyma cells

• Location:

  • between the primary xylem and the primary phloem of each vascular bundle

  • and in rays of ground tissue between the bundles

• Types:

  • Fascicular/ intrafascicular cambium

  • Interfascicular cambium

Fascicular/ Intrafascicular cambium

• located within the vascular bundle

• compose of cells called fusiform initials

  • produce new vascular tissues

  • Secondary xylem- towards the pith

  • Secondary phloem- towards the cortex

• Function of sec. vascular tissues:
  - vertical conduction of water, minerals and products of photosynthesis

Interfascicular cambium

• located in the rays between vascular bundles

• compose of cells called ray initials

  • produce radial files of parenchyma cells known as xylem rays and phloem rays

• Function of xylem and phloem rays:

• lateral transport of water and nutrients

• storage of starch and other resources

Dicot: Tilia sp.

common name: linden, lime or bass wood)

dicot: Aristolochia sp.

common name: dutch man’s pipe or pive vine

gymnosperm: Pinus sp.

common name: pine tree

deciduous trees

• reaching typically 20 to 40 meters (65 to 130 ft)

• occurrence: Europe, north america and asia

• lifespan; few hundred years (up to 1,000 years old in some specimens)

Aristolochia flower

• vine native to the eastern united states

• ecological importance: cultivated in gardens as larval host for the pipevine swallowtail, battues philenor

• medical importance: natural source of aristolochic acid mutagen and carcinogen causing urological cancers and permanent kidney failure)

cork cambium

• a cylinder of meristematic tissue that forms from the outer cortex of the stem

• also known as the phellogen

• produces:

  • cork cells on the exterior- phellem

  • parenchyma cells on the interior-phelloderm

Periderm= phelloderm+phellogen +phellem

protective tissue that replaces the epidermis after secondary growth is initiated

importance of the cork cells

• mature cork cells secrete suberin (a waxy substance) in their cell walls and then die

• cork cells function as a barrier to protect the stem from physical damage and from pathogens

• unlike the vascular cambium which can grow in diameter via multiplicative growth, the cork cambium is fixed in size

• after a few weeks, the cork cambium loses meristematic ability

secondary xylem or wood

• components: tracheids, vessel elements, fibers, sclereids and parenchyma

• function: for support and conduction

variations in sec. xylem structure: gymnosperms

• contain only tracheids; fibers and parenchyma cells (sparse or absent)

• xylem rays are uniseriate except near resin ducts

dicots

• all types of cells present: vessels, tracheids, xylem ray, parenchyma cells and fibers

• with water storing parenchyma cells adjacent to vessel elements

• fibers provide maximum strength

heartwood

• the xylem in the center of the tree

• non functional

• serves as storage site of waste products from the plant

• these products help preserve the wood tissue

sapwood

• functional xylem closest to the cambium

• typically ligther in color (white) than the heartwood

hardwoods (such as oak and hickory) are?

are dicots and have tracheids, vessels, and fibers in their xylem

softwoods (such as pine and redwood) are?

• gymnosperms

• the xylem is composed mostly of tracheid’s and there are no vessels or fibers

• softwoods typically grow more rapidly than hardwoods

spring wood

wood formed by the vascular cambium during the moist days of the spring

• made of larger thin walled cells

summer wood

wood made during the drier days of the summer

• made of smaller cells with thicker walls

annual rings or tree rings

• concentric rings of secondary xylem and indicate annual growth layers

• appear as alternation light bands of early wood and dark bands of late wood, defining the annual growth layers

secondary phloem

function:

• long distance transport of organic nutrients

• a constituent of the bark and is responsible for protecting the secondary plant body

• with sclerenchyma fibers that offer flexibility and strength

• rich in secretory tissues e.g laticifers (latex) and resin glands (resin)

Secondary Phloem in Conifers

Cell types: A. sieve cells with albuminous cells – for vertical transport of nutrients B. non conducting parenchyma cells – storage C. fibers – provide strength and support D. phloem rays – single layer of parenchyma cells – for lateral transport of nutrients

Secondary phloem of dicots

Cell types: A. sieve tube elements and companion cells - for vertical transport of nutrients B. non conducting parenchyma cells - storage C. fibers - provide strength and support D. phloem rays - compose of single or multilayered parenchyma cells for lateral transport of nutrients

Bark

• consists of the periderm + the phloem • all tissues outside the cambium, including the phloem – Mature bark may consist of alternating layer of crushed phloem and cork
Types: A. Scale bark – overlapping scales or shell e.g. Pinus B. Ring bark – formed as entire cylinders around the stem e.g. Vitis C. Wound bark – suberized cells separate dead tissues from inner healthy cells

Scale bark

overlapping scales or shell e.g. Pinus

Ring bark

formed as entire cylinders around the stem e.g. Vitis

Wound bark –

– suberized cells separate dead tissues from inner healthy cells

Lenticels

• Area with loosely arranged cells and abundant small intercellular spaces • Function: – For gas exchange • Distribution: - on stems, roots and some fruits e.g. apples and pears • Origin : - beneath the stomata (it replaces the stomata in plants with secondary growth)