Secondary Growth of Stems

                                                                                                                                                              

I.             Secondary stem growth in angiosperms (introduction)

                A.            Requirements for secondary growth

                                1.             Secondary thickening meristems: found mostly in dicots - some monocots

                                                a)            Vascular cambium (like procambium) - between primary xylem & phloem

                                                                1)            Fascicular cambium - forms from within vascular bundles

                                                                2)            Interfascicular cambium - origin. fr. parenchyma between bundles

                                                b)            Cork cambium - forms from cells of cork cambium

                                                                1)            Cork (phellem) to the outside; phelloderm (parenchyma) to the inside

II.            Secondary growth of tissues (in detail)

                A.            Secondary phloem and secondary xylem (secondary vascular tissues)

                                1.             The bulk of tissues in woody stems of dicots are secondary vascular tissues

                                2.             How secondary vascular growth occurs

                                                a)            Begin at vascular cambium (fusiform and ray initials)

                                                b)            Primary phloem gets pushed out and primary xylem gets pushed in

                                                                1)            Cells orient up and down (vertically) the stem

                                                c)             New cells are located closest to the vascular cambium

                                                d)            Circumference increases because cells divide perpendicular to as well as parallel to the stem surface

                                                                1)            Ray cells - develop horizontally

                                                                2)            Function in growth and transport of minerals

                                3.             Where are secondary phloem and secondary xylem found?

                                                a)            New external cells mature into secondary phloem

                                                                1)            Primary phloem gets crushed

                                                b)            New internal cells mature into secondary xylem

                                                c)             Xylem can be seen as "rings" on a tree

                B.            Cork cambium

                                1.             Forms in the cortex (or sometimes from the epidermis)

                                2.             Produces cork cells (phellem) to the outside and parenchyma (phelloderm) to the inside

                                3.             Cork cambium, cork (phellem), and phelloderm make up the PERIDERM (bark)

                                4.             Lenticles - specialized regions of periderm functioning in gas exchange

                                5.             Inner bark - phloem that has been pushed outward - peeled away, it exposes the most recently formed secondary phloem (where sap can be found)

                                6.             Knots - come from wounds and broken branches

III.          Wood - secondary xylem - THE GROWTH RINGS

                A.            Heartwood and sapwood

                                1.             Heartwood - central darkened core (cross section)

                                                a)            Cells are dead and often pigmented

                                                b)            Composed of tracheids and vessel members

                                                                1)            Tracheids - simple and primative

                                                                2)            Vessels - efficient and advanced

                                                c)             Contains tyloses - ingrowths of adjacent parenchyma into vessels

                                                                1)            Good wood for storage containers (whiskey & pickles)

                                2.             Sapwood - periphery of the wood

                                                a)            Includes living xylem and parenchyma

                                                b)            Functions in water and nutrient transport

                B.            Wood of conifers (gymnosperms)

                                1.             Softwood - composed mostly of tracheids and has smaller and fewer rays

                                2.             Vascular cambium

                                                a)            Fusiform initials - produce tracheids to the inside

                                                b)            Ray initials - produce ray cells

                C.            Cuts of wood

                                1.             Tangential (section) - longitudinal, but does not pass through pith

                                2.             Radial (section) - longitudinal and does pass through pith (radius)

                                3.             Transverse cut or cross (section) - at a right angle to longitudinal section