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Secondary Growth in Stems and Roots • Review of introduction to secondary growth • Secondary Growth in Stems – Initiation Vascular Cambium – Annual Growth in Stems – Initiation of initial and subsequent Cork Cambia • How the Vascular Cambium produces cells – Fusiform and Ray Initials – Additive and Multiplicative Divisions • Secondary Growth in Roots – Initiation of Vascular Cambium – Initial and subsequent cork cambia Secondary growth (can be identified by radial alignment of cells) • Formed at a secondary meristem • (re-differentiated from “mature” non-dividing cells) • Responsible for increase in girth (diameter) • Meristems called cambia and are located laterally on stems and roots (less often in leaves) • Two lateral meristems are common in seed plants – Vascular Cambium (Xylem & Phloem) – Phellogen = cork cambium; produces cork or the bark) Changes in Stem Structure with primary and secondary growth Growth Rings Represent cones of secondary tissue 1. Vascular cambium (V.C.) is initiated in layer of Residual Procambium between the primary xylem and primary phloem in the bundle (Fascicular V.C.) 2. Vascular cambium is initiated in ground tissue between primary bundles of the stele (Interfascicular V.C.) 3. Is Bifacial – producing Secondary Xylem (wood) to the inside and secondary Phloem to the outside. 4. Consists of Fusiform and Ray Initials Vascular Cambial Activity at the Cellular Level of Organization One, Two, & Three year old stems 1. Growth rings 2. Increase in Girth (thickness of wood) 3. Increase in Size of Cambial Cylinder Diagramatic Representations of Stem Growth Page 618 textbook Vascular cambium of Pinus (Tangential Section) Fusiform and Ray Initials Additive Divisions a. Divisions are Periclinal b. Add new cells to radial files c. Increase thickness of wood & phloem Diagramatic Representations of Stem Growth Additive Divisions Multiplicative Divisions a. Divisions are anticlinal b. Add new cells to vascular cambium c. Increase circumference of vascular cambium Cell divisions in the vascular cambium Multiplicative = anticlinal division-adds to the width of the vascular cambium to keep up with the growth in diameter Additive = periclinal division-produces the new xylem and phloem cells Periderm 1. 2. 3. 4. • • Protective tissue Secondary origin Found in bark-or the bark system Consists of 3 parts Phellogen = cork cambium = lateral meristem Phellem = cork-produced to outside-cells suberized and sometimes also lignified • Phelloderm = parenchyma produced toward the inside-usually thin-walled cells Stages in periderm formation ep pl ph pd ep pl ph ph=phellogen pd=phelloderm pl=phellem ep=epidermis Fate of the epidermis and cuticle is to be sloughed off the outside of the stem as phellem (cork) is produced Periderm in x.s. Phellem cells can contain tannins Can deter insects and other animals from feeding on the bark Phellogen can start to form in one of several places 1. Just under the epidermis (in hypodermis) 2. In the epidermis 3. In the outer cortex 4. In the secondary phloem In older stems there are several periderms “Bark” • Informal –used for trees or shrubs for everything outside of the vascular cambium • Includes secondary phloem • Any primary tissues that still might be there in the inner cortex • The periderm • Tissues still outside of periderm –epidermis and outer cortex • Term often used inconsistently or to mean several things to different people • Some use “bark” just to mean phellem lenticels Lenticel= A specialized region of the periderm containing intercellular spaces allowing for gas exchange (O2 + CO2) Lenticel in cross section Interruption in epidermis- a part of the periderm in which the phellogen is more active than elsewhere The phellogen bends inward here Usually forms beneath a stoma Lenticel allows for gas exchange Note: cuticle still present outside Found in both stems and roots Vary in size and shape Secondary Growth in Roots 1. 2. Basically Similar to that in Stems, but Vascular cambium and Periderm are initiated differently Salix root x.s. Vascular Cambium initiated between protoxylem ribs Periderm initiated in pericycle Periderm Cortex and Epidermis Meristematic Growth in Stems Meristems and their Derivitives Secondary meristems