Secondary growth in stems

Secondary growth in stems

Secondary growth Some of the meristematic cells in plants with secondary growth keep their meristematic state and become cells of the cambium. The addition of secondary vascular tissue and periderm = increase the girth or diameter of the plant

Annuals, biennials and perennials. Description of the time for growth Woody perennials produce flowers only when they become adults. Woody plants in temperate regions are deciduous.

Secondary growth in roots and stems Secondary xylem and phloem provide physical continuity between primary xylem and phloem of roots and stems Leaves have minimal, if any, secondary growth, generally is restricted to strengthening vein tissue.

Origin of the secondary growth Secondary growth originates from two tissues: Vascular cambium and cork cambium. Both of them are lateral meristems

The vascular cambium Meristematic cells of vascular cambium are highly vacuolated. Two types of cells: Fusiform Initials (elongated) è axial elements = vertical conduction Ray Initials (isodiametric) è ray parenchyma = radial conduction and storage

Are radial (lateral) transport system cells ray initials in xylem and phloem = parenchyma axial (vertical) transport system cells In xylem and phloem = sieve-tube member companion cells tracheids vessels fibers Can be storied or nonstoried

Cambium exhibits seasonal dormancy. This contributes to the growth rings common in wood. In many tropical species cambium is always active, and wood lacks distinctive rings.

Cambium exhibits seasonal dormancy. The inner lighter toned ring is that laid down during early spring growth, and is known as spring wood. This is relatively soft and the cells ( tracheids ) are thin walled and carry sap. The outer ring, sometimes being quite dark is usually a harder band and is laid in summer; it is called summer wood. These Tracheids are thick walled and provide rigidity and stability to the bole. Lignin in the wall

Development of secondary vascular tissues Fascicular cambium Interfascicular cambium fascicle = vascular bundle

Periderm cork = phellem cork cambium = phellogen cork cortex = phelloderm periderm Cork Cu'cle epidermis phelloderm cortex

Lenticels Lenticels are portions of periderm with intercellular spaces Present in stems, roots and fruits Commonly appear below of a stoma or a group of stomata

Transversal Bark Xylem ray Outer bark Inner bark With phloem Tangen*al surface Xylem ray Radial surface Bark refers to all the tissues outside the vascular cambium Dark area= heart wood Lighter part of the wood= sapwood

Aging of bark Increment in the girth è stress on the older tissues of the bark. The older phloem gets crushed with the lateral expansion of the stem. New periderm layers separate the newer phloem from the older phloem The older phloem and periderm layers are eventually sloughed off

External features of woody Aging stems of bark Branches and twig are small extensions of the stem of certain angiosperms Scales are transformed and reduced leaves. They cover and protect the embryonic parts. Twig Leaf scar

1. Terminal bud 2. Internode 3. Terminal bud-scale scars 4. Lenticel 5. Lateral bud 6. Node area 7. Stem Len'cel Lateral bud Leaf scar Bundle scars 1 2 3 4 5 6 7 Terminal bud- scale scares

Plants with secondary protective tissues

The manner in which new periderm is formed the kind of tissues isolated determine the surface of the bark bark

Cork Quercus suber After the first cork is removed from the epidermis, new cork cambium is formed in the cortex

Conifer wood Lack of vessels Reduced wood parenchyma Long tapering tracheids Resin ducts in the parenchyma cells

Conifer wood Transverse radial tangen'al Early wood Late wood Ray Early wood Late wood Resin duct ray

Pit-pairs of Pinus tracheids Torus border Torus border

Reaction wood In conifers = compression wood dense wood lignin lower side of branches In angiosperms = tension wood soft wood cellulose upper side of branches